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• Evolution of linear attenuation coefficient (LAC) regarding carbonation may vary via different CT. • X-ray energy level used for CT scan can reverse the LAC relationship between calcium carbonate and calcium hydroxide. • Hygral equilibrium and polymorphs of carbonation product can also affect the LAC of carbonated areas. A discussion on a recently published article authored by Kim et al. [Effect of carbonation on cement paste microstructure characterized by micro-computed tomography. Constr. Build. Mater. 263 (2020) 120079] is presented. [ABSTRACT FROM AUTHOR]

The disposal of steel slag in landfills not only squanders precious landfill resources but also results in significant environmental contamination. Steel slag can be used to replace cement and natural aggregates in concrete. Nonetheless, utilizing untreated steel slag directly presents a substantial hazard to the structural characteristics and durability of concrete. To avoid this problem, study on accelerated carbonation of steel slag as construction materials has been extensively conducted. This review paper begins by introducing the classification and physicochemical properties of steel slag; it then describes the typical approaches of steel slag carbonation, including direct carbonation (dry carbonation, wet carbonation) and indirect carbonation (pH-swing, microbial carbonation), along with their processes and carbonation mechanisms. Furthermore, based on the reviewed literature, the influences of various reaction parameters such as reaction time, liquid-to-solid ratio, temperature, additives, CO 2 concentration, pressure, pH on carbonation effectiveness are analyzed. Also, the impact of carbonated steel slag on the micro-pore structure, stability, workability, and mechanical properties of concrete is discussed. Finally, the challenges and future work of using carbonated steel slag as construction materials are provided. • The current papers on steel slag classification, physicochemical properties, and methods for accelerated carbonation are reviewed. • The traditional carbonation methods, as well as recently proposed microbial carbonation methods are analyzed. • The factors influencing the efficiency of steel slag carbonation are summarized. • The impact of carbonated steel slag on concrete properties are concluded. • Insights into accelerated carbonation methods for steel slag and future work are provided. [ABSTRACT FROM AUTHOR]

For the development of unburned aggregate, accelerated carbonation technology is deemed to be a significant curing method. In this paper, accelerated carbonation was applied in preparing magnesium slag unburned aggregate (MSUA). The effects of curing time on physical property, phase evolution, CO 2 absorption, microstructure and stability of MSUA were explored by employing XRD, TGA, MIP and SEM-BSE characterization techniques. According to the results, with the increase of carbonation time, the mechanical strength and apparent density of MSUA prepared by accelerated carbonation technology enhance, while the water absorption rate declines. The predominant form of the carbonation product is calcite (CaCO 3), accompanied with amorphous silica gel. The amount of CO 2 absorption has positive correlation with time, exhibiting an absorption rate of 18.5 % at 168 h of carbonation. On the other hand, with 3 h CO 2 curing of MSUA, the alkali-silica reaction (ASR) expansion can be completely suppressed. As observed from MIP analysis, the increase of carbonation time reduces the porosity of aggregates. SEM analysis also reveals that gel pores are filled with carbonation products, leading to a denser structure of the aggregate matrix, which is possibly a crucial factor for the suppression to ASR expansion. Based on these findings, MSUA is demonstrated to have enormous potential in substituting natural aggregates and reducing carbon emissions. • This study mainly the preparation of unburned aggregate by employing accelerated carbonation and cold granulation method. • By changing the carbonation time, the mechanical properties and durability were improved, and ASR expansion was suppressed. • Magnesium slag unburned aggregate has significant ecological benefits, reducing the consumption of natural resources. [ABSTRACT FROM AUTHOR]

White mud (WM) is a solid waste produced in the process of papermaking and it remains to be effectively utilized. It contains a large amount of CaCO 3 and residual alkali metal ions. This paper aims to explore the effect of WM on carbonation resistance of alkali activated slag. The changes in mechanical properties and microstructure of alkali activated white mud/slag (AAWS) after natural carbonation and accelerated carbonation were analyzed. The results indicate that the mechanisms of natural carbonation and accelerated carbonation are different. The carbonation depth achieved in 90 days of natural carbonation can be reached in just 14 days of accelerated carbonation. Samples with 15 wt% WM blended possesses the smallest carbonation depth and the highest compressive strength. WM can effectively mitigate the natural carbonation of AAWS by dissolving Mg2+ and Ca2+. Mg2+ can migrate to increase the solubility of calcite, promote the generation of hydrotalcite to absorb carbon dioxide, and reduce the carbonation degree. Ca2+ will migrate after the carbonation and decalcification of the gel product and reduce the decalcification sensitivity. This study offers a sustainable approach for improving the carbonation resistance of AAWS. • WM inclusion promotes the generation of hydrotalcite for CO 2 absorption. • The migration of Mg2+ from WM increases the solubility of calcite and promotes the formation of aragonite and vaterite. • Ca2+ dissolved from WM reduces the decalcification sensitivity of AAWS. [ABSTRACT FROM AUTHOR]

Highlights • The variations in binder/water ratio can have a counterproductive effect on the mortar carbonation process. • The carbonation velocity in lime mortar is not related to the type of lime. The carbonation process slows down in most mortars at 90 days. • The flexural and compression resistance of traditional lime mortars increases gradually and in relatively short periods of time (around 90 days). • The type of Liesegang patterns observed in the lime mortar is directly related to the characteristics of the carbonation process of binder. • SPL mortars have a higher volume of macropores than CL mortars that are characterized by a higher microporosity. Abstract In recent years, numerous studies have been published on the technology of industrial aerial lime mortars. These papers refer to the incidence of carbonation process and mechanical behavior in its properties as construction material, of interest for its application in Cultural Heritage conservation. Among other issues, the importance of the type of aggregate (composition and granulometry) and relation binder/water (B/W) in its properties are highlighted. There are fewer papers in which the type of lime is about, especially on the basis of its manufacturing process (traditional or industrial), on the presentation of the product (powder or putty), or referred to the different types of mortars (prepared in situ or pre-dosed). In this paper, the behavior of aerial lime mortars elaborated with traditional lime has been determined and a comparative study has been carried out with pre-dosed mortars of powder (mCL) and in putty (mSPL) lime for its application in render mortar (B) and plaster mortar (F). The control parameters have been mechanical resistances-elasticity, ultrasound velocity, carbonation process and distribution pore size and surface area by mercury porosimetry. It is concluded that the behavior of traditional lime mortars, both in putty and powder, depends on the B/W ratio, the type of aggregates and the lime content. The importance of the application of the product in the building as the behavior of the mortar is very sensitive to kneading water content it shows; aspects that define the plasticity of the mortar. It is also concluded that the carbonation of traditional lime mortars is not slow and that, under optimal dosages conditions, can be finished before 90 days. It is also verified that a higher carbonation rate does not imply better properties of the mortar in its long-term construction. The utility of ultrasound velocity test as a non-destructive test and easy to apply on building for the quality control of traditional lime mortars and its consistence values it highlights, as is a macroscopic property of easy control during the elaboration of the product. [ABSTRACT FROM AUTHOR]

The heterogeneity of a cement-based material results in a random spatial distribution of carbonation depth. Currently, there is a lack of both experimental and numerical investigations aiming at a statistical understanding of this important phenomenon. This paper presents both experimental and numerical supercritical carbonation test results of cement mortar blocks. The carbonation depths are measured along the carbonation boundary by the proposed rapid image processing technique, which are then statistically studied by calculating, e.g., their probability density and power spectral density (PSD). The results indicate that the distribution of the carbonation depth can be approximately represented by a lognormal distribution function and the PSD has quantitative correlation with some of the statistic parameters used in the simulations. In particular, the effects of the autocorrelation lengths and the coefficient of variation of porosity, which are used to define the random porosity field, on the irregularity of carbonation depth are analyzed numerically in details and validated by experimental results. The study has shown that using a random field of porosity with due consideration of spatial correlation and variance, the irregularity of carbonation depth can be realistically captured by the numerical model. The numerical results confirm that lognormal distributions represent the random nature of carbonation depth well and the average and variance of the irregular carbonation depth increase with the increase of carbonation time, autocorrelation length and coefficient of variation of porosity. The study also offers a potential method to numerically calibrate some of the statistic parameters required by a numerical carbonation model through comparing the PSD with that from experimental tests. Overall the methodology adopted in the paper can provide a foundation for future investigations on probability analysis of carbonation depth and other similar work based on multi-scale and -physics modelling. [ABSTRACT FROM AUTHOR]

Accurate prediction of concrete carbonation is important for the correct assessment of both durability and environmental impact of reinforced concrete (RC) structures. Loading applied to a RC structure and concrete cracking caused by this loading may significantly affect the concrete carbonation process. However, so far these factors have received little attention of researchers, especially this concerns ‘green’ concretes, i.e. concretes in which Portland cement (PC) is partially replaced by supplementary cementitious materials such as fly ash (FA) and ground granulated blast-furnace slag (GGBS). Thus, the aim of the study presented in the paper was to experimentally investigate the influence of static loading and associated concrete cracking on carbonation of RC elements made of PC concretes and ‘green’ concretes containing significant amounts FA and GGBS. For this purpose, six concrete mixes with two water/binder (w/b) ratios (0.40 and 0.55) and different proportions of PC, FA and GGBS were prepared. The mixes were used to cast twelve RC beams (100 × 120 × 900-mm) and a larger number of 100-mm concrete cubes. The beam specimens were loaded in four-point bending to produce flexural cracks of maximum width of either 0.1 mm or 0.3 mm. The loaded beam specimens along with unloaded cube specimens were then placed into a carbonation chamber and subject to accelerated carbonation for 120 days. After that the carbonation depths in the beams and cubes were measured. Results of the tests show a significant effect of load induced stresses (both tensile and compressive) on the carbonation resistance of the concretes, especially of ‘green’ concretes. The influence of cracking on concrete carbonation was also observed and discussed in the paper. [ABSTRACT FROM AUTHOR]

Concrete carbonation has an important influence on chloride penetration. The chloride distribution profiles found in carbonated concrete are often different from those found in uncarbonated concrete even when they have the same exposure conditions. In this paper we present an experimental study on chloride penetration in carbonated concrete specimens when they are subjected to stationary, splash, submerged, and pressured exposure conditions. Free chloride concentration profiles were measured in the specimens to examine the effect of carbonation and exposure condition on chloride diffusion. In addition, material characterisations were carried out using x-ray radiography imaging tests and thermogravimetric analysis to understand the influence mechanism of concrete carbonation on chloride penetration in concrete. The experimental results showed that the chloride distribution curve in carbonated concrete exhibits a second peak under marine exposure conditions. The exposure condition can significantly affect the position and value of the second peak point in the profile. It is also shown that concrete carbonation can drastically change the distribution of pores in concrete, leading to a sudden change in porosity, which is the reason for the carbonated concrete to have the zone-dependent chloride variation profiles. • The chloride distribution curve in carbonated concrete exhibits a second peak under marine exposure conditions. • The exposure condition can significantly affect the position and value of the second peak point in the profile. • The different carbonation zones can be identified based on the distribution curve of the calcium carbonate. • Change in porosity caused by carbonation leads to a zone-dependent chloride variation profiles. [ABSTRACT FROM AUTHOR]

Ammonium carbonate was used as the carbonation accelerator for air-hardening lime mortars in this study. Various analytical methods, including XRD, IR, TGA-DSC and SEM were employed to access the phase transformation, degree of carbonation and microstructural features of the lime-based mortars. The paper explores the correlation between the mechanical strength, microstructure of cured lime mortars and the composition/carbonation degree with the crystallization behaviors in Ca(OH)2/CaCO3 binders. The results indicate that ammonium carbonate significantly accelerates the carbonation reaction and increases the early mechanical strength of air-hardening lime mortars. The mechanical strength of these modified lime mortars can be comparable to that of natural hydraulic lime (NHL). The aggregation behaviors of nano-CaCO3 under kinetic control, are believed to contribute much to the compact pore-structures and robust mechanical strength of lime mortars. This viable accelerating carbonation method holds promise for enhancing the properties of various lime-containing building materials in conservation and modern buildings. • Ammonium carbonate is effective in accelerating the carbonation of aerial lime. • Lime mortars modified by ammonium carbonate rival natural hydraulic lime (NHL). • The mechanical strength is highly related to carbonation degree and pore structure. • Nano-CaCO 3 play pivotal role in compacting the pore structure of lime mortar. [ABSTRACT FROM AUTHOR]

This paper presents a critical review of existing studies on the effects of using sea-sand and/or seawater as raw materials of concrete on the properties of the resulting concrete, including its workability, short- and long-term strength as well as durability. It has been shown by existing research that concrete made with sea-sand and seawater develops its early strength faster than that of ordinary concrete, but the former achieves a similar long-term strength to the latter. Existing studies have also shown that the use of sea-sand and seawater may have a significant effect on chloride-induced steel corrosion but has only a negligible effect on the carbonation process of concrete. Strong evidence exists that a combination of mineral admixtures for the concrete and reinforcement with fiber reinforced polymer (FRP) can effectively solve the durability problem associated with the abundance of chloride ions in sea-sand seawater concrete (SSC). Such use of SSC also offers a good opportunity for the incorporation of recycled coarse aggregate (RCA) in concrete, particularly those that have been chloride-contaminated, as has been demonstrated by some preliminary research. The current understanding of the behavior of SSC, as summarized in the present paper, provides a solid basis for further research in the area to enable the wide use of SSC in concrete construction worldwide, particularly when combined with FRP as the reinforcing material. [ABSTRACT FROM AUTHOR]

Corrosion of reinforcement and concrete deterioration induced by aggressive media could severely reduce the bearing capacity of structures. The durability of alkali-activated materials (AAMs) in aggressive environments, such as carbonation, chloride penetration and sulfate attack, have been a research focus worldwide. Reaction products and microstructures of AAMs are different from ordinary Portland cement (OPC), therefore the corrosion mechanisms and assessments are different. This paper reviews factors influencing water absorption and permeability of AAMs, effect of gel composition and exposure environments on carbonation, chloride penetration and chloride migration test methods, and sulfate resistance in high-calcium and low-calcium alkali-activated systems. There remains a large space in these aspects to completely understand the deterioration of AAMs, as pointed out in the end. The perspectives suggested in this paper will be useful for future study on long-term durability of AAMs. [ABSTRACT FROM AUTHOR]

This paper developed a cost-effective and highly carbonatable industrial-grade γ-C 2 S binder sintered from limestone and sandstone, aiming to advance the industrialization of γ-C 2 S binder in construction fields. The carbonation behavior of the industrial-grade γ-C 2 S binder, including the evolution of carbonation kinetics, compressive strength, phase composition, and microstructure with carbonation duration was studied. Results showed that a temperature of 1400 °C and limestone-to-sandstone mass ratio of 2.1 can be regarded as the optimal sintering regime, which can secure a relatively complete self-pulverization of industrial-grade γ-C 2 S binder. The industrial-grade γ-C 2 S binder had a milder carbonation reaction, which can reduce the evaporation of water and lead to 48% greater compressive strength, compared to that sintered from analytically pure reagents. During carbonation, the proportion of γ-C 2 S phase significantly decreased, while the proportions of CaCO 3 and silica gel notably increased in the first 1 h, followed with a slight change in proportions of those phases with the carbonation further extending to 24 h. Similarly, the degree of carbonation rapidly increased to 36.3% in the first 1 h, followed with a slight increase to 43.7% at 24 h. The carbonation of industrial-grade γ-C 2 S, accompanied with the precipitation of CaCO 3 on γ-C 2 S phase surface and the bond of silica gel among CaCO 3 grains, contributed to the formation of nacre-bionic interlocking brick-mud microstructure. This led to the rapid enhancement of compressive strength with carbonation of industrial-grade γ-C 2 S. • Industrial-grade γ-C2S binder from limestone and sandstone was sintered. • Industrial-grade γ-C2S showed a greater compressive strength after carbonation. • Limestone-to-sandstone mass ratio of 2.1 sintered at 1400 °C was the optimal regime. • Nacre-bionic interlocking brick-mud microstructure was formed during carbonation. [ABSTRACT FROM AUTHOR]

Indirect carbonation of steel slag is an effective CO 2 sequestration strategy, which is of great significance for reducing CO 2 emissions of the iron and steel industry and achieving its green and low-carbon development. This paper proposed a process for high-temperature modification followed by indirect carbonation of steel slag, which enabled the separation and synergistic utilization of calcium and iron elements of the slag. The reactivity of various minerals in steel slag was initially studied at the level of ion coordination utilizing crystal structure theory. The results indicated that the O2- valence of β-Ca 2 SiO 4 was unsaturated, resulting in high reactivity. In contrast, the O2- valence of Ca 2 Fe 2 O 5 was relatively saturated, resulting in poor reactivity. The results of high-temperature modification and leaching experiments demonstrated that as the basicity of the steel slag decreased, the calcium component transformed into Ca 2 SiO 4 , while the iron component transformed into spinel minerals. When the basicity was too low, the Ca and Fe components recombined to form the poorly reactive Ca 3 Fe 2 (SiO 4) 3. As the basicity of the steel slag decreased, the Ca2+ leaching rate and the total iron content of leaching residue initially increased and then decreased. At a basicity of 2, both the Ca2+ leaching rate and the total iron content of leaching residue reached their maximum of 84.46% and 48.39%, respectively. This study significantly improved the Ca2+ leaching rate and the total iron content of leaching residue through high-temperature modification, thereby enhancing the CO 2 sequestration capacity of the steel slag and the recovery level of iron resources The above strategy would be of great significance for promoting the green and low-carbon development of the iron and steel industry. [Display omitted] • The method of modification to improve the Ca2+ leaching of steel slag was explored. • The total iron content of the leaching residue was first focused. • Ca leaching and Fe enrichment of steel slag were combined for the first time. • The carbonation capacity of each mineral phase in steel slag was investigated. [ABSTRACT FROM AUTHOR]

Carbonation cured calcium silicate binders can be considered an alternative material to reduce the carbon footprint of the construction industry. Carbonation at elevated temperatures is used to enhance carbonation reaction; however, it accelerates drying, which could result in drying shrinkage induced cracking. The effect of hydrogel on the microstructure and properties of carbonated wollastonite pastes was discussed in this paper. The teabag test and flow test showed that hydrogel absorption was notably higher in wollastonite paste than in OPC paste. The phase analysis based on Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), revealed calcite as the primary polymorph of calcium carbonate in the carbonated product. The micro-CT analysis showed crack formation in the control pastes while no or significantly fewer cracks were observed in the hydrogel modified pastes. The hydrogel modified pastes exhibited lower water loss and drying shrinkage compared to the control paste at early carbonation stage. The above results provided evidence that cracking was caused by drying shrinkage in the control carbonated pastes and hydrogel was effective in mitigating such cracking by lowering water loss at early stage of carbonation. The results of the nitrogen adsorption test indicated a higher specific surface area and pore volume in the hydrogel modified pastes. The hydrogel modified pastes exhibited a slightly reduced compressive strength due to the presence of hydrogel macrovoids in these pastes. • Hydrogels reduced drying shrinkage and cracking in carbonated wollastonite. • Calcite is the primary polymorph of calcium carbonate in the microstructure. • Hydrogel demonstrated significant water absorption in wollastonite. [ABSTRACT FROM AUTHOR]

The combined effect of carbonation and chloride ingress in concrete is studied in this paper. Based on the change of the pore structure and the chemical equilibrium, a comprehensive model is proposed for this problem. A coupled simulation of the transports of carbon dioxide, chloride ions, heat and moisture is carried out. Several sets of experimental data were compared with the prediction by the numerical model developed in this paper, for its verification. Parametric study shows that the differences between the combined mechanism and the independent mechanisms are significant in many aspects. [ABSTRACT FROM AUTHOR]

Large shrinkage and poor carbonation resistance are important reasons that limit the widespread use of alkali-activated slag. The expansion pressure of albite due to alkali-activated silica reaction in alkaline environments and the sequestration of CO 2 may solve the problems. In this paper, the effect of albite on shrinkage and carbonation resistance of alkali-activated slag were studied, and the mechanism of action between albite and alkali-activated slag was revealed. Firstly, alkali-activated slag slurry specimens with partial replacement of slag by albite were prepared with standard and carbonation curing. Then, the samples were tested for compressive and flexural strength as well as chemical shrinkage, autogenous shrinkage and drying shrinkage under standard and carbonation curing. Finally, the mechanism of interaction between albite and alkali-activated slag was analyzed by XRD, TG-DTG, FT-IR, SEM, MIP and 29Si MAS NMR. The results show that: 1) the influence of carbonation on the compressive strength and flexural strength of alkali-activated slag was reduced when mixed with albite. After carbonation curing for 28 days, the compressive strength of pure slag samples decreased by 50.67 %, while the strength of samples mixed with albite decreased by 34.51 %. 2) After adding albite, the chemical shrinkage, autogenous shrinkage and drying shrinkage of alkali-activated slag samples decreased by 33.33 %, 32.95 % and 46.76 %, respectively. 3) The amount of calcite due to carbonation curing was reduced. The content of calcite in the sample with albite was 65.49 % of that of pure slag sample under carbonation curing. 4) Albite can promote the degree of hydration and the generation of C-A-S-H gels of the alkali-activated slag. • Albite improves the hydration degree of alkali-activated slag. • Albite improves the carbonation resistance of alkali-activated slag. • Albite decreases the shrinkage of alkali-activated slag. [ABSTRACT FROM AUTHOR]

• Nesquehonite was used as a new nucleation seeds for reactive magnesia cement, and its performance was compared with hydromagnesite. • The nucleation effect of nesquehonite seeds was demonstrated, resulting in increased nesquehonite formation and carbonation degree. • Nesquehonite seeds combined with hydration agent can further promote the compressive strength of RMC through forming more amounts of nesqeuhonite crystals and amorphous hydrated magnesium carbonates. • Compared with the hydromagnesite seeds, the nesquehonite seeds showed a more significant increase in the CO2 sequestration capacity of RMC. Among all carbonation products of reactive magnesia cement (RMC), nesquehonite exhibited the highest CO 2 amount of sequestration per molar. Inducing the nesquehonite formation may improve the CO 2 sequestration capacity of RMC. In this paper, nesquehonite seeds were introduced to promote the formation of nesquehonite in RMC. The performances of RMCs with 0%, 0.5%, 1%, 1.5% and 2% nesquehonite seeds were studied and compared with that of RMC mixed with hydromagnesite seeds. It was found that the hydration of RMC could be facilitated by nesquehonite seeds, resulting in reduced setting times. The nucleation effect of nesquehonite seeds was demonstrated, resulting in increased nesquehonite formation and carbonation degree. Nesquehonite seeds combined with hydration agent can further promote the compressive strength of RMC through forming more amounts of nesqeuhonite crystals and amorphous hydrated magnesium carbonates. Compared with the hydromagnesite seeds, the nesquehonite seeds showed a more significant increase in the CO 2 sequestration capacity of RMC. [ABSTRACT FROM AUTHOR]

[Display omitted] • The environmental impact of RACB under different curing scenarios was assessed. • The contribution distribution of each RACB production stage was subdivided. • The effect of curing time on the environmental sustainability of RACB was supplemented. • The environmental advantages of accelerated carbonization instead of traditional curing were determined. Accelerated carbonization curing has been a technically feasible alternative to the conventional curing method in the production of recycled aggregate concrete blocks (RACB). However, the environmental impacts of the curing scenarios on production have not yet been studied. Therefore, the paper evaluated the environmental feasibility of curing methods to produce RACB based on the LCA framework. The system boundary from the cradle to the door was selected to compare the overall influence of accelerated carbonation curing methods (pressurized carbonation curing, wet carbonation curing) and traditional curing methods (standard curing, autoclave curing) on the environmental impact. CO 2 absorption was calculated by the Kim and Chae carbonation model combined with experimental carbonation depth. Environmental emission calculated from the inventory database was distributed to ten midpoint indicators and three endpoint indicators by the CML 2016 baseline and ReCiPe endpoint method. The distribution of contributions to the environmental impacts of the four production stages (raw material processing stage, raw material transportation stage, block manufacture stage, and block curing stage) was subdivided to facilitate decision-making and improve the management of the block plant. A sensitivity analysis ultimately complements the effect of curing time on the environmental sustainability of RACB production. The results showed that the traditional curing method had a more significant environmental impact, while accelerated carbonization curing had a relatively lower environmental impact. Compared with standard curing, the GWP value of pressurized and wet carbonization curing decreased by 33.3% and 52.8%, respectively. The 1 m3 RACB showed the lowest GWP value of 254.7 kg CO 2 -eq under wet carbonization curing and the highest GWP value of 539.5 kg CO 2 -eq under standard curing. The CO 2 absorption capacity of −73.0 kg CO 2 -eq and −99.3 kg CO 2 -eq was demonstrated by pressurized and wet carbonization curing, respectively. The ranking from all midpoint and endpoint indicators is standard curing > autoclaved curing > pressurized carbonization curing > wet carbonization curing. Contribution analysis based on midpoint indicators showed that the raw material handling stage has the highest percentage contribution. Cement is still the most significant contributor to the environmental impact, followed by significant energy consumption, while water consumption hardly negatively impacts the environment. The results of sensitivity analysis indicate that accelerated carbonation curing (especially wet carbonation curing) is less relevant to the variation of curing time, while conventional curing methods are strongly influenced by curing time. [ABSTRACT FROM AUTHOR]

The behavior upon exposure to CO 2 of novel cementitious materials prepared with a low clinker cement CEM II/C-M (S-LL) is investigated in this paper. A simplified carbonation model considering the dissolution of a combined Ca-containing hydrate phase (CH, C-S-H, ettringite, monocarbonate, and Si-hydrogarnet) and the precipitation of calcite according to the evolution of phase assemblages calculated with thermodynamic modeling is applied to simulate accelerated carbonation tests in the laboratory. The dissolution rate of the Ca-containing hydrate phase is assumed to be determined by the water saturation degree, the partial pressure of CO 2 , and the fraction of its remaining volume compared to the initial one, which provides a way to predict the precipitation rate of calcite. Two mass conservation equations are considered in the model: water migration through the connected pores and diffusion of carbon dioxide in gaseous phases. Selected physical variables representative of the reactive transport properties are quantified from experimental measurements, including dynamic vapor sorption tests and drying experiments. The model response is compared to experimental data in terms of portlandite profiles from thermogravimetric analysis, and carbonation depth measurements using a pH indicator after specific times of accelerated carbonation. These experiments were also performed on a reference mortar prepared with the CEM II/A-S cement. All experimental results were simulated with the simplified carbonation model and the results compared to the response of the novel material. • Carbonation response of a novel low clinker CEM II/C-M (S-LL) cement is studied. • A simplified carbonation — drying model is coupled with thermodynamic calculations. • Transport properties are identified via DVS tests and drying experiments. • Model is validated against experimental portlandite profiles and carbonation depth. • Performance of the novel CEM II/C-M (S-LL) is compared to existing CEM II/A-S. [ABSTRACT FROM AUTHOR]

Highlights • Aerated steel slag blocks are prepared by accelerated carbonation. • C 4 AČH 11 formed during the hydration process of steel slag-aluminum system. • C 4 AČH 11 converted to aragonite, vaterite, calcite, and amorphous aluminum hydroxide by carbonation. • The formation and carbonation reactions of C 4 AČH 11 can be produced thermodynamically. Abstract The present paper discusses investigated the foaming behaviors and microstructure of three aerated steel slag block (ASSB) samples prepared with three different foaming agents (namely animal protein, hydrogen peroxide, aluminum powder) and by accelerated carbonation. The composition and microstructure of the ASSB samples before and after accelerated carbonation were characterized using X-ray diffraction (XRD), Rietveld method, thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FT-IR) and field emission scanning electron microscopy (FE-SEM). It is an advisable method for foamed steel slag products to use the aluminum powder and accelerated carbonation technology. The aluminum powder favored a good foaming behavior for steel slag owing to (i) a high porosity and low dry density, (ii) a high hydration degree of minerals in steel slag, including brownmillerite, larnite, and lime, which was confirmed by the analysis of mineral compositions using Rietveld method, and (iii) the interlaced lamellas of monocarboaluminate and calcium silicate hydrate (C-S-H) gel strengthen the bubbles wall, which was confirmed by morphology analysis using FE-SEM. After accelerated carbonation, the monocarboaluminate shifted to aragonite, vaterite, calcite and amorphous aluminum hydroxide (Al(OH) 3), which was confirmed by XRD, TGA and FT-IR. The network skeleton formed by aragonite crystals improved the compressive strength of ASSB prepared by aluminum powder. In addition, the reactions of the formation and carbonation of monocarboaluminate phase were proved to be thermodynamically possible in this work. [ABSTRACT FROM AUTHOR]

Highlights • Time to carbonation-induced steel corrosion cover cracking is investigated. • Steel mass loss to initiate surface cracking increases with increase in cover depth. • Blended cement concretes required higher steel mass loss to cause cover cracking. • Higher w/b ratio concretes required longer times to cover cracking. • An empirical relationship for time to cover cracking is proposed. Abstract This paper reports on a study that was aimed at developing a coherent empirical relationship for the time to cracking of the cover concrete in a reinforced concrete structure, subjected to reinforcing steel corrosion that is induced by carbonation in an inland, urban environment in South Africa. In particular, the study considered the influence of cover depth and the quality of concrete in the cover zone on the extent of corrosion at the time of cover concrete cracking. The results of the study were then used to develop an empirical relationship to estimate the time to cover cracking and the associated extent of reinforcing steel corrosion. Test concretes were prepared using three binder types: plain Portland cement (PC – CEM I 52.5N), 70/30 PC/FA (fly ash) and 50/50 PC/GGBS (ground granulated blastfurnace slag) at relatively high w/b ratios of 0.60 and 0.95. 13.2 mm crushed granite stone and granite crusher sand were used as aggregates. RC prism specimens were prepared using 20 mm diameter high yield deformed steel bars placed at cover depths of 12 mm, 20 mm or 30 mm. Sufficient 100 mm companion concrete cube specimens were prepared for permeability, porosity tests and carbonation depth measurement. The results indicate that, as the cover depth increases, a higher extent of corrosion is required to initiate cracking at the concrete surface. Also, at the same cover depth, the higher w/b ratio concrete, which was more porous, more permeable and had lower tensile strengths, required more steel corrosion to initiate surface cracking. Blended cement concretes required a higher extent of steel corrosion to initiate cover cracking than the PC concretes. [ABSTRACT FROM AUTHOR]

Highlights • New ecological concretes made with carbonated aggregates. • Measurement of mechanical and thermal properties of eco-concretes studied. • Prediction and validation of thermal conductivity with HAC model. Abstract This paper presents a study on the effect of carbonated-aggregates on mechanical properties and thermal conductivity of eco-concretes. These carbon negative eco-aggregates are produced by accelerated carbonation where industrial by-products or incinerators' waste are combined with carbon dioxide (CO 2). These manufactured aggregates have particular characteristics giving them a certain interest in the field of civil engineering such as in replacement of natural aggregates in concrete. Morphological, physical, mechanical and thermal characterizations were performed on eco-aggregates in order to use them in the production of concrete. Then, concretes based on these carbonated-aggregates were formulated. In the experimental program, different mixtures were prepared by partially replacing natural aggregates by carbonated ones resulting in concretes with unit weights varying between 1750 and 2290 kg/m3. For all produced concrete mixes, the water to cement (w/c) ratio was kept constant. Compressive strength, modulus of elasticity, thermal conductivity, water absorption and sorptivity were measured. Results have been compared to control concrete mixes produced using ordinary aggregates and expanded clay. The results show that the tested eco-concretes have sufficient mechanical strength and interesting thermal conductivity. Therefore, they could be used for construction projects. [ABSTRACT FROM AUTHOR]

Highlights • The detrimental effect of carbonation on the physical sulfate attack on concrete is disclosed. • A larger carbonation depth leads to greater concrete scaling by Na 2 SO 4 crystallization. • The negative effect of concrete carbonation on the concrete durability is extended. Abstract Concrete carbonation is regarded as a significant cause of reinforcement corrosion in buildings. This research paper will present another detrimental effect of concrete carbonation on the concrete durability. Concrete cylinders placed in an accelerating carbonation chamber for 10 and 20d were partially immersed in a 10% Na 2 SO 4 solution. After 28, 60, 120, and 240d exposure, the mass loss of the concrete specimens was measured, and the damage mechanism checked by means of XRD analysis, NMR, ESEM and EDS. The results showed that a larger carbonation depth in the concrete caused more concrete scaling, and that sulfate crystals were also identified in the carbonated concrete. Due to their susceptibility to carbonation, blended concretes containing fly ash and slag showed more scaling than Portland concrete. Therefore, the "physical sulfate attack on concrete" can be more accurately described as the "physical sulfate attack on carbonated concrete." [ABSTRACT FROM AUTHOR]

Highlights • MgSO 4 crystallization distress on concrete combining the carbonation is confirmed. • Concrete carbonation plays a detrimental role in the physical sulfate attack on Portland cement concrete. • The factors accelerating the concrete carbonation, such as the large W/C ratio and the cement replacement by mineral admixtures, can promote the occurrence of physical sulfate attack on concrete. Abstract Based on the definition of physical salt attack, the Guide to Durable Concrete (ACI 201.2R) suggests that MgSO 4 cannot lead to crystallization distress on concrete. In this paper, concretes consisting of 30% fly ash (FA), 20% limestone powder (LP), 50% slag (SG) and pure Portland cement (PC) were acceleratingly carbonated for 20d before being partially immersed in a 10% MgSO 4 solution. After 150d of partial exposure, MgSO 4 crystallization was attributed to the damage of the outer layer of carbonated FA, SG, LP concrete specimens in the evaporation zone. The outer layer of FA and LP concretes used as reference had also become carbonated and detached due to MgSO 4 crystallization. The test results supported the idea that MgSO 4 crystallization distress can occur in concrete through carbonation, and that concrete carbonation should be considered when analyzing physical salt attack on concrete. [ABSTRACT FROM AUTHOR]

Highlights • 'Inorganic gel casting' feasible for highly porous glass-ceramics from vitrified MSWI bottom ash. • Hardening achieved with a low molarity alkaline solution, by formation of carbonate phases. • Hardened foams transformed into glass-ceramics by low temperature sinter-crystallisation. • Leaching of heavy metals from final product remaining under the thresholds specifications. Abstract The transformation of vitrified waste, such as bottom ash from municipal waste incineration, into cellular glass-ceramics is convenient, if the additional processing is simple and inexpensive. The present paper aims at presenting a possible route to achieve this goal, based on the recently proposed mechanical foaming of alkali-activated suspensions of waste glass powders, followed by sinter-crystallization at moderate temperatures (from 800 to 900 °C). Compared to previously studied glasses, in this experiment bottom ash-derived glass suspensions underwent progressive hardening at low alkali molarity and in limited times. The firing did not alter the open-celled structure that had developed upon low temperature foaming, owing to a significant crystallization. With an overall porosity of 80%, the optimized foams exhibited a remarkable compressive strength (>6 MPa). Finally, the process had no negative impact on the leaching of toxic elements, which remained well below the thresholds for inert materials. [ABSTRACT FROM AUTHOR]

Highlights • Proposed a residual strain-based effective CO 2 diffusion coefficient. • Established a theoretical carbonation model for fatigue-damaged concrete. • Verified the theoretical carbonation model by multiple experiments. • Identified the effect of fatigue damage degree on concrete carbonation. • Identified the effect of fatigue damage gradient on concrete carbonation. Abstract This paper presents the theoretical modeling of carbonation in fatigue-damaged concrete. First, a residual strain-based effective carbon dioxide diffusion coefficient in fatigue-damaged concrete was proposed. Based on that, the general carbonation equations were established for concrete with uniform compressive, gradient compressive and gradient tensile damage patterns. Then a numerical program was written to solve the established partial differential carbonation equations. Subsequently, the numerical carbonation model was validated by comparing the predicted carbonation depths with experimental results. Finally, parametric studies were conducted on carbonation in commonly used concretes with various fatigue damage patterns, which separated the effects of residual strains and residual curvatures. The parametric studies showed that fatigue damage did not alter the widely accepted proportional relationships between carbonation depths and square roots of exposure durations. Moreover, the effects of exposure conditions on carbonation of concrete were far more influenced by residual strains than by residual curvatures. [ABSTRACT FROM AUTHOR]

Highlights: • A green ECC is developed based on MgO and fly ash through accelerated carbonation. • Compression, tension, and multiple cracking characters are evaluated. • CO 2 uptake and materials sustainability of the newly developed ECC are assessed. • ECC with 1-d carbonation achieves both environmental and technical benefits. Abstract The use of reactive magnesium oxide (MgO) is widely recognized in carbonated concrete formulations associated with permanent sequestration of CO 2. Engineered Cementitious Composite (ECC) is an advanced fiber reinforced cement-based composite with high tensile ductility and intrinsically tight crack width. In this paper, we investigate an alternative binary binding system for ECC: reactive MgO and fly ash cured with an accelerated carbonation process. Compressive strength, density, carbonation depth, tensile performance and crack pattern of the carbonated reactive MgO-based ECC were investigated at various curing ages. In addition, the CO 2 uptake and materials sustainability, in terms of energy consumption, net CO 2 emission and cost of the newly developed ECC were assessed. The objective of this research is to further advance the application of reactive MgO and utilization of CO 2 in the construction industry through novel ECC material. It was observed that carbonation curing densifies the binding system, thus leading to an increase in both compressive and first cracking tensile strengths of ECC. The tensile strain capacity of the carbonated reactive MgO-based ECC achieved up to 6% with an average crack width below 60 μm after 1-day carbonation. Compared to conventional ECC (M45) and concrete, the 1-day carbonated reactive MgO-based ECC could reduce the net CO 2 emission by 65% and 45%, respectively. It is concluded that environmental and technical benefits could be simultaneously achieved for the 1-day carbonated reactive MgO-based ECC incorporated with 50% fly ash. The findings of this research shed light on further applications of reactive MgO cement in the precast industry. [ABSTRACT FROM AUTHOR]

In the original paper, the simple power function used correlates carbonation rate with air permeability in carbonated concrete rather than in non-carbonated concrete. However, the reported air permeability tests were conducted for non-carbonated concrete, which was not consistent with their theoretically-established function. This discussion modifies the original theoretical function through incorporation of a porosity-related term which connects air permeability in non-carbonated concrete with that in carbonated concrete. Consequently, a new analytical model to estimate carbonation resistance through air permeability in non-carbonated concrete is proposed and calibrated using experimental results reported in the original paper. [ABSTRACT FROM AUTHOR]

• Revealing the coupling effects on chloride ion transport caused by carbonation, FT cycles, loading states and levels, temperature, and saturation. • Modelling essentially the chloride ion transport under the coupling effects of carbonation, FT cycles, loading states and levels, temperature, relative humidity, saturation and concrete deterioration. • Revealing the degradation laws of flexural capacity of the multi-factor corroded beams. Reinforced concrete (RC) structures are often exposed to coinstantaneous deterioration effects such as carbonation, chloride corrosion, freeze–thaw cycles and external loads which causing deterioration of concrete and reinforcement. This paper discussed the effects of carbonation, freeze–thaw cycles and sustained loads on chloride ion transport and proposed a chloride ion transport model taking porosity as the main coupling variate. The attenuation law of the corroded beams under coupling corrosion factors was studied. Through chloride immersion tests, it was found that applying freezing and thawing cycles and sustained tensile stress can enhance chloride transport capacity by coarsening the pores; Carbonation and low compressive stress can repress the chloride ion transport by densifying concrete, but the compressive stress beyond about 0.7 times the compressive strength and tensile stress would enhance the penetrating capacity due to the damage of the internal microstructure of concrete. Then, a comprehensive and unified theoretical model of chloride ion transport considering the coupling of carbonation, freeze–thaw cycle, sustained loading and other factors was proposed and verified by taking porosity as the main coupling variable quantity. This model could essentially explain the effects of various factors on chloride ion transport and has good consistency with the test results. Finally, the flexural bearing capacity of the corroded beams was tested, and the failure characteristics, crack characteristics, ultimate bearing capacity, load–displacement relationship and strain distribution of the corroded beams under different corrosion conditions were researched. The results show that the flexural capacity of the corroded beams suffered the coupling effects of multiple corrosion factors degrades, and the bearing capacity decreases with the increase of the corrosion degree of carbonation-freezing thawing cycles; The continuous tensile stress applied to the test beams in the corrosion process intensifying the degradation of bending bearing capacity and ductility, the continuous compressive stress increases the flexural capacity and improves the ductility slightly, but excessive compressive stress significantly degrades the flexural capacity; When chloride ions are used as the third kind of corrosive substance for coupling corrosion, the tensioned or without-load corroded beams have obvious degradation in bending bearing capacity, while the corrosion beams bearing continuous compressive stress have no obvious degradation in bending bearing capacity. [ABSTRACT FROM AUTHOR]

• Addition of GGBFS improves the carbonation resistance of calcined clay-GGBFS geopolymer. • The increase in activator modulus reduces the initial and final setting time. • GGBFS enhances the formation of amorphous phases (C-(A)-S-H/C-(N)-A-S-H) leading to better densification. • pH of hardened geopolymer pastes increases with increasing GGBFS content. • Increasing GGBFS content in binder results in lower porosity and higher compressive strength. Alkali-activated materials are obtained from the alkaline activation of a precursor rich in SiO 2 and Al 2 O 3 , which can be derived from by-products or industrial residues. Calcined clay is a potential candidate as a precursor for this type of material. Large amounts of suitable clays for alkali-activated materials are available around the world. This paper evaluated the performance of different alkaline-activated mortars using calcined clays (low grade) and ground-granulated blast-furnace slag (GGBFS), including physical and mechanical performance, reaction products phases, microstructure, and resistance to accelerated carbonation. The results indicate that the increase in GGBFS content up to 60% has a significant effect on the properties of the mortars, due to the effect of Ca-rich phases such as C-(A)-S-H/C-(N)-A-S-H, promoting the densification of the microstructure, and an increase in pH from 11.91 to 12.78 for specimens with 100% calcined clay and 60% GGBFS respectively, leading to a better carbonation resistance. On the other hand, adding GGBFS leads to a reduction in concrete initial and final setting time and lowers thermal stability compared to the reference calcined clay material. The increase in the modulus ratio of the activator from 1 to 1.5 also improves the mechanical properties and resistance to carbonation of geopolymer mortars with various binder compositions. [ABSTRACT FROM AUTHOR]

• A probabilistic analysis of the natural carbonation of RCA concretes was conducted. • RCA anticipated the failure probability by up to 100 months compared to NCA. • RCA requires an equivalent cover depth of up to 1.4 compared to NCA. Carbonation is a phenomenon that involves several uncertainties associated with concrete properties and environmental factors. However, most existing studies have adopted a deterministic approach to deal with this phenomenon, which can lead to misinterpretations. This study aimed to carry out a probabilistic analysis of natural carbonation with data collected for over 30 months of recycled coarse aggregate (RCA) concretes and natural coarse aggregate concretes (NCA), used as reference. The probabilistic analysis showed that the cover depth collected in field presented a normal distribution behavior, in this way, random variables were generated to sample cover depth values to be used in time-dependent Monte Carlo simulations. A computational algorithm was developed in MATLAB to evaluate the probability of the carbonation depth (x c) overcoming cover depths (c d) of 15-, 20-, 25-, 30-, 35-, and 40-mm, usual values in structural design. The simulations showed that the RCA anticipated the probability of failure (x c ≥ c d) by up to 100 months compared to natural coarse aggregate (NCA) concrete. The results indicated that the total replacement of NCA by RCA requires increases of up to 1.4 in cover depth to perform equivalently to the reference concrete (with NCA). Moreover, regardless of the type of aggregate and w/c ratio, with the cover depth established in the Brazilian standard for urban environments, the evaluated concrete did not reach the minimum service life of 50 years established for structural elements of reinforced concrete. The methodology applied in this paper can be used to predict the service-life considering failure due to carbonation using the statistics response of experimental data. [ABSTRACT FROM AUTHOR]

• This paper used ANN hybridized with swarm intelligence algorithms to predict the carbonation resistance of RAC. • A comparison between the performance of proposed prediction models was performed. • The variable importance analysis and parametric analysis of input variables were studied. This paper investigates the prediction of carbonation depth for recycled aggregate concrete (RAC) with machine learning models. Nine parameters including RAC intrinsic properties and environmental conditions were considered as input variables. A dataset comprising 593 test data was used to train, validate, and test machine learning models. Results show that the Random forest model shows superior performance than the Gaussian progress regression model and standalone artificial neural network (ANN) model. All ANN models hybridized with swarm intelligence algorithms outperform the standalone ANN model, especially for the ANN model hybridized with the whale optimization algorithm. All machine learning models show higher accuracy than the existing code models and statistical models. The variable importance analysis shows that the carbonation resistance for RAC was determined by both internal and external factors. Based on the parametric analysis, the robustness of the proposed machine learning models was further confirmed. [ABSTRACT FROM AUTHOR]

This paper addresses a design of a novel concrete with advanced carbonation resistance based on application of a structure modified Layered Double Hydroxides (LDHs) like material, including experiments and mechanism analysis. First of all, according to the concept of structural reconstruction and memory function of LDHs, three types of Mg-Al-LDHs with different structures are produced and evaluated. The thermodynamics analysis demonstrates that the structural modification of Mg-Al-LDHs could be triggered when the heating temperature is higher than 252.8 °C. Then, the carbonation resistance of the concrete with different structure modified Mg-Al-LDHs are tested and compared. The obtained experimental results demonstrate that the carbonation resistance of concrete with calcined Mg-Al-LDHs can be mostly improved, compared to the samples with other structure modified Mg-Al-LDHs. Moreover, several concrete carbonation depth prediction models are employed to evaluate the carbonation resistance of the concrete with structure modified Mg-Al-LDHs. It is proved that, compared to the random model and carbonation mechanism model, the multiple coefficient prediction model is more suitable to be used to predict the carbonation depth for the concrete developed in this study. Additionally, the microstructure development of the concrete with structure modified Mg-Al-LDHs is also discussed in this research. [ABSTRACT FROM AUTHOR]

This paper presents the results of research on the carbonation resistance of recycled aggregate concrete (RAC) and natural aggregate concrete (NAC) under loading. The correlation between axial stress/carbonation age and the carbonation depth was analyzed. Based on theoretical analysis and experiment results, a model to predict the carbonation depth of RAC under axial stress was established. The carbonation depths of NAC and RAC under the same experimental parameters were compared and results were discussed. The experimental results show that the carbonation depth of RAC first decreases and then increases with an increase in axial compressive-stress ratio, and consistently increases with a growing of axial tensile-stress ratio. The experimental data also demonstrate that the carbonation depth increases with the carbonation age, and that the carbonation resistance of RAC is lower than that of NAC under the same experimental conditions. [ABSTRACT FROM AUTHOR]

Carbon dioxide uptake during concrete carbonation has become an emerging problem, as more attention is paid to sustainable development. Therefore an advanced test methods has been adopted to measure process of carbonation. A Fourier Transformation Infrared Spectroscopy analysis was applied to define a progress of carbonation in Self-Compacting Concrete. Two types of samples were studied – a typical cubic samples and crushed concrete. Samples were kept in carbonation chamber for 56, 112 and 168 days in controlled conditions. A new sampling method is presented in the paper. Applying the proposed procedure a deeper progress of carbonation process is revealed, when compared with a standard drilling technique. Achieved findings were verified with Termogravimetric Analysis/Differential Thermal Analysis. [ABSTRACT FROM AUTHOR]

This paper presents an experimental work of a four-years study on the corrosion resistance of concrete made of sulfate contaminated aggregates. The study involved both carbonation and chloride induced corrosion. The studied parameters include effects of sulfate cation (sodium, calcium and magnesium sulfates) and sulfate content in addition to water-to-cement ratio, cement content and cement type. Concrete porosity and concrete compressive strength were used to analyze the corrosion resistance. From the test results, for carbonation induced corrosion, the four-years carbonation depth influenced by sulfate content and cation where magnesium sulfate yielded highest carbonation depth compared with those of studied cations. Using low water cement ratio, high cement content and use of Type V cement reduced the four-years carbonation depth. Also, carbonation rate was almost proportional with concrete porosity and concrete compressive strength. The porosity–carbonation rate and concrete compressive strength were sulfate cation and sulfate content dependent. For chloride induced corrosion in terms of steel reinforcement weight loss, the use of sulfate contaminated aggregate negatively affected the chloride induced corrosion. Low water cement ratio, high cement content and use of Type V Portland cement decreased the steel weight loss. Finally, there was a direct relation between concrete porosity/concrete compressive strength and steel weight loss but these relations were sulfate content and sulfate cation independent. [ABSTRACT FROM AUTHOR]

The aim of this paper was to investigate the effect of natural carbonation on the pore structure, and elastic modulus (E m ) of alkali-activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes after one year of exposure in the natural laboratory conditions. The chemical changes due to carbonation were examined by X-ray diffraction (XRD), scanning electron microscope/energy-dispersive X-ray (SEM−EDX) and attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR). Subsequently, the pore structure and E m of the degraded material were tested by mercury intrusion porosimetry (MIP), nitrogen (N 2 ) adsorption, and nanoindentation. The chemical degradation of alkali-activated pastes due to natural carbonation is showed to be dependent on the GBFS content and their pore structure development. It was found that the pure alkali-activated GBFS paste was not carbonated at all within the tested period due to fine gel pore structure. On the other hand, carbonation of the gel in the pastes consisting FA and GBFS generated significant mineralogical and microstructural changes. The extensive decalcification of the gel was reflected in the increase of nanoporosity. Consequently, the E m of the carbonated pastes decreased. This study suggests that the degradation of alkali-activated FA and GBFS pastes due to carbonation may be accurately evaluated through micromechanical properties measurements rather than only by testing alkalinity of the pore solution and corrosion of reinforcement such as commonly studied carbonation effect in the ordinary Portland cement (OPC)-based materials. [ABSTRACT FROM AUTHOR]

Durability and useful life of the structural concrete are related to the environmental conditions and degrading factors present in the environment. One of most concerning aggressive agents in the civil construction industry is carbon dioxide that penetrates into the pores of the concrete reacting with the interstitial hydrates reducing its pH, promoting depassivation of the steel armours inside the concrete, thus enabling to start the corrosion process. This paper produced grout proof-bodies, concretes with high fluidity used to fill the blocks in structural masonry, with different resistance to compression: 15 MPa, 20 MPa, 25 MPa. The specimens were submitted to a carbonation front in an accelerated chamber under controlled humidity, temperature and carbon dioxide. A numerical model was used aiming to estimate the forecast of the useful life by making a comparison of the values forecasted by the Brazilian and international performance rules. It was observed that the 15 MPa grout has a useful life forecast lower than that recommended by the standard, around 60% lower, while the 20 MPa and 25 MPa grout presented a satisfactory useful life. Hence, it was certified the importance of controlling the grout resistance and coverage in structural masonry works as a form to assure the desired durability to the structure. [ABSTRACT FROM AUTHOR]

The study detailed in this paper examined the influence of two types of metakaolin (MK1 and MK2) at high replacement rates on the carbonation rate of cement paste. “Flash” calcination produced MK1, which had a high amount of quartz mixed in, while “fluidized bed” calcination produced MK2, which was relatively pure. The metakaolins were incorporated into the cement paste at rates of up to 50% by weight of cement and cured for 90 days. With regard to durability, MK replacement increased water porosity and susceptibility to carbonation, while water adsorption and primary sorptivity decreased. Thermogravimetric analysis (TGA) and X-ray Diffraction (XRD) analysis of the hydration phases showed a decrease in portlandite (Ca(OH) 2 ) content with MK replacement, increasing carbonation susceptibility. Decreased portlandite showed a strong correlation with an increased carbonation rate, indicating that these are primary factors in metakaolin cement paste carbonation. With TGA deconvolution, MK1 significantly increased CSH content up to a replacement level of 20%, while replacement levels over 30% appeared to reduce CSH formation. MK2 replacement did not have as significant an effect on CSH formation, corresponding with the higher primary sorptivity and greater susceptibility to carbonation penetration of MK2. [ABSTRACT FROM AUTHOR]

The time-dependent reliability of carbonation behavior is investigated in recycled aggregate concrete (RAC). The gamma distribution of carbonation depth is tested with the data collected from previous investigations. The results show that the gamma distribution can be used to describe the carbonation depth. Thus, the gamma process is adopted to analyze the reliability of carbonation behavior with the maximum likelihood estimation (MLE) to determine the parameters of gamma process. With this methodology, four case studies are executed to investigate the influences of replacement ratio of recycled coarse aggregate (RCA), crushing procedure of RCA, water-to-cement ratio (w/c) and concrete cover depth on the reliability of carbonation behavior by calculating the probability density function (PDF) and failure probability. The results show that all these factors have an obvious influence on the reliability of RAC carbonation behavior. Finally, this paper concludes that the time-dependent reliability analysis by using gamma process is necessary because of its guidance in the optimization of mix design and determination of concrete cover depth which can further satisfy the durability design of RAC structures. [ABSTRACT FROM AUTHOR]

This paper presents a study of cellulose pulp fiber-cement composites subjected to supercritical carbonation at a very early age. The raw materials used for the creation of cementitious composite mixes include: Portland cement, limestone filler, and cellulose pulp. Composites were produced by a slurry vacuum dewatering process and subjected to curing conditions which studied the effect of supercritical carbonation concentration (0% and close to 100% of CO 2 ), the effect of cellulosic pulp (unbleached or bleached), and durability. All samples were subjected to mechanical, physical, and microstructural tests. The initial period of thermal curing (varying from 24 h to 48 h) and exposure time to carbonation (from 1 h to 2 h), did not have a statistically significant effect on the mechanical performance of the composites. However, the flexural test results of carbonated composites reinforced with bleached pulp showed a statistically significant improvement when compared to unbleached pulp reinforcement (24% increased average modulus of rupture). Thus, for cementitious composites cured with supercritical carbonation, the use of bleached cellulosic pulp, 24 h of thermal hydration and 1 h in an environmental chamber provided the optimal curing conditions and the most desirable properties in this study. [ABSTRACT FROM AUTHOR]

• Modifications of the microstructure due to carbonation seems to be limited to intra-granular porosity. • The segmented 3D image allows determining the macroscopic diffusivity tensor. • CO 2 diffusion coefficient depends mainly on the inter-granular porosity network. Recycled aggregates compacted in road layers are made of a majority of concrete aggregates which can mineralize atmospheric CO 2. Previous studies showed that carbonation modifies the microstructure of recycled concrete aggregates, but little information is available in the case of compacted recycled aggregates. Furthermore, carbonation kinetics depends on CO 2 diffusion through the compacted material, which is linked its microstructure. This paper presents an investigation of the evolution of both microstructure and CO 2 diffusion coefficient of compacted recycled aggregates during carbonation in two different environments by X-ray tomography. Segmented 3D images were analyzed to fully extract resolved pore structures. In addition, the CO 2 diffusion coefficient variations were investigated by calculating the macroscopic diffusivity tensor in the segmented 3D images. We show that the numerical results are very similar to previous experimental results from gas diffusion tests and that the diffusivity coefficients can be estimated by using only the larger pore network (pore size > 5.5 µm). These findings suggest that nondestructive imaging approaches, such as micro-Computed Tomography (µCT), are a relevant alternative to experimental characterization methods. [ABSTRACT FROM AUTHOR]

• This paper studied the microstructure and mechanical properties of CO 2 -cured steel slag brick in pilot-scale. • The CO 2 -cured steel slag bricks showed a better compressive strength and excellent volume and freeze-thaw stability. • The carbonation technology can turn steel slag into cementitious material and absorb CO 2. This paper reports a pilot study of beneficial CO 2 use in the manufacture of steel slag building bricks. The effects of steel slag fineness, mix proportion and carbonation time on the CO 2 uptake, compressive strength, volume stability and freeze–thaw resistance of steel slag bricks were systematically studied. The results showed that the CO 2 cured bricks containing 25 wt% steel slag reached the highest CO 2 uptake of 7.5% (kg/kg steel slag), and they presented the highest compressive strength of 27.7 MPa at 7 day, which meets the standard requirement of load bearing perforated brick. The steel slag brick after CO 2 cured for 30 min exhibited good volume stability under the steam curing testing conditions at 3 MPa, and its freeze–thaw resistance was shown excellent as well. In composition, C 2 S, C 3 S, Ca(OH) 2 , f-CaO and MgO presented in steel slag reacted with CO 2 and formed carbonates and silica gels; on the other hand, as expected, Ca 2 (Al,Fe) 2 O 5 and FeO remained much more stable. The results of this study have demonstrated that the carbonation technology can effectively turn steel slag into cementitious material, and the utilization of CO 2 has the feasibility of industrialization. [ABSTRACT FROM AUTHOR]

Ductility is an important factor used in the design of para seismic structures. In this paper, the effect of the carbonation and the type of cement (limestone cement (CEM II) and ordinary Portland cement (CEM I)) on ductility and compressive strength of concrete is investigated. From two concrete formulations, cubic samples of size 10 cm,10 cm and 10 cm) and beam samples of size (10 cm,10 cm and 40 cm) were made and subjected to the accelerated carbonation test over several periods for this purpose. The cubic samples were used for measurements of carbonation depths and compressive strength, beam samples were made for measurements of carbonation depths and displacement ductility. Based on these experiments, the results show that displacement ductility is reduced by carbonation. On the other hand, the compressive strength increases with the duration of exposure to carbonation. Moreover, for concrete formulated with Portland cement (CEM I), it is noted that the rate of reduction of the displacement ductility coefficient is 30% for the 7th day of exposure to accelerated carbonation for the carbonated samples compared to the control samples. However, the concrete samples formulated with calcareous additions cement (CEM II), the reduction rate of the displacement ductility coefficient is only 14% for the carbonated samples compared to the control samples. The results obtained show that the compressive strength, increases with the duration of exposure to carbonation for the two concretes and it is more important for the concrete with ordinary Portland cement, the rate of increase is about 12%. Hence, the carbonation decreases the service life of reinforced concrete structures mainly their ductility capacity and consequently their resistance against seismic actions. [ABSTRACT FROM AUTHOR]

In this paper, the supercritical carbonation process of cement-based materials is modelled by introducing a random porosity field to simulate the heterogeneous geometry of the carbonation profile. The suitability of two different random fields of porosity, based on the probability density function (PDF) and the ellipsoidal autocorrelation function (EAF) methods, are investigated, respectively, in simulating the distribution of porosity in cement mortar. After incorporating the above random fields into an established supercritical carbonation model, it is found that with some modifications, the EAF method with consideration of spatial correlation produces better simulation of the irregularities of the carbonation zones that have been observed from experimental results. It is also found that for given average porosity and coefficient of variation, the predicted average and maximum carbonation depths have much smaller coefficients of variation. The validated EAF supercritical carbonation model is then used in parametric studies that are conducted to assess the effect of various factors on the carbonation depth of the chemical process. [ABSTRACT FROM AUTHOR]

Supplementary cementitious materials (SCMs) like fly ash (FA) and blast furnace slag (BFS) are normally used to replace parts of Ordinary Portland cement (OPC) to reduce the cost and CO 2 emission. During the carbonation, a relatively high amount of C-S-H with low Ca/Si ratio will be carbonated in cement paste blended with SCMs. Therefore, it’s very important to figure out the influences of the carbonation of C-S-H on the pore structure of SCMs blended cement paste. In this paper, Mercury Intrusion Porosimetry (MIP) are used to determine the pore volume and size distribution of capillary pores. Results reveal that carbonation of most of the species of C-S-H results in increased porosity of cement paste. Total and effective capillary porosity of pastes blended with high amount of BFS increase after carbonation. This will bring adverse effects on the durability of blended cement concrete exposed to the carbonation. [ABSTRACT FROM AUTHOR]

In majority of exposure environments for concrete structures, there is a high probability of the cyclic occurence of both chloride ingress and carbonation. This paper reports a detailed investigation on the influence of carbonation on both the ingress and distribution of chlorides in three different types of concretes, by comparing results from exposure to chlorides, chlorides before carbonation and chlorides after carbonation. Concretes studied were of 0.55 water-binder ratio with 100% Portland Cement (PC), 70% PC + 30% pulverized fuel ash (PFA) and 85% PC + 10% PFA + 5% microsilica (MS) as binders. Chloride profiles were compared to assess the effects of all variables studied in this research. The effect of carbonation was quantified by measuring the consumption of hydroxyl ions (OH − ), air permeability and chloride migration coefficient. The results indicated that carbonation of concrete increases chloride transport, but the precise nature of this is dependent on the combined regime as well as the type of binder. In general, it was found that carbonation of chloride contaminated concretes results in a decrease of their chloride binding capacity, that is it releases the bound Cl − in concretes and pushes chlorides inwards, as has been established previously by other researchers. However, it is established in this research that the combined regimes detrimentally affect the service life of concrete structures, particularly when chloride induced corrosion is a concern. [ABSTRACT FROM AUTHOR]

Accelerated carbonation curing (ACC) is a new technique for curing of concrete that entails sequestering carbon dioxide (CO 2 ) gas into freshly cast concrete, resulting in the improvement of physico-mechanical properties and durability characteristics of concrete. This paper presents the results of an experimental study conducted to evaluate the effects of carbonation pressure and duration on the CO 2 uptake and evolution of strength of a concrete mixture. Concrete specimens were cured under six ACC pressures varying from 10 to 60 psi, applied for a duration of 1–10 h in a closed chamber. The effectiveness of varying ACC pressure and duration on the properties of concrete was assessed by measuring compressive strength gain, CO 2 uptake, morphology and mineralogy of concrete. It was noted that ACC at 60 psi (414 kPa) for 10 h resulted in the maximum strength gain and CO 2 uptake, leading to a post-ACC compressive strength of more than 200% of the pre-ACC strength, and a CO 2 uptake of about 11% by mass of cement. Finally, the analysis of variance of the experimental data indicated that the duration of ACC controls the concrete properties more than the pressure used for ACC. [ABSTRACT FROM AUTHOR]

In this paper, cyclic voltammetry technique, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky measurement combined with surface morphology observation were applied to investigate the electrochemical behavior of carbon steel (CS) and three Cr modified low alloy steels in the carbonated cement extract solution. Results show that pH plays an important role in the formation of passive films on steel rebars. In non carbonated cement extract solution, corrosion behaviors of CS and Cr modified steel vary slightly, and Cr addition brings down the amount of defects in passive film. In carbonated cement extract solution, pitting potential shifts negatively as pH value decreases. Polarization resistance declines and the density of the current carrier in the passive film increases. With Cr content increasing, corrosion current density declines; pitting potential, polarization resistance and passivation range are magnified. Besides, the effect of Cr is more significant in lower pH. The good-to-poor sequence of corrosion resistance is: 5Cr > 3Cr > 1.5Cr > CS, and 5Cr steel possesses the best corrosion resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] • The BSE-IA and EDS analysis were employed for characterize the ITZs of RAC. • Aggregate enhancements could take obviously strengthening effect on ITZs. • Soaking RCA in CEM-SF slurries had a better improvement effect on ITZs. • The Ca/Si atomic ratio of ITZs was reduced after enhancement of RCA. Recycled concrete aggregate (RCA) enhancement is an important method to improve the performance of recycled aggregate concrete (RAC). However, the quantitative studies on the change of microstructure of interfacial transition zones (ITZs) were few and far, especially after enhancement of RCA with different methods. This paper investigated the influence of carbonation enhancement (carbonation time, soaking in CH saturated solution and then carbonation) and chemical enhancement (respectively soaking in Na 2 SiO 3 saturated solution and cement-silica fume (CEM-SF) slurries) of RCA on the change of microstructure of ITZs (including the ITZ between old aggregate-new paste (ITZ 1) and ITZ between old paste-new paste (ITZ 2)) in RAC. The porosity, the un-hydrated cement particles (UH) content, the volume fraction of hydration products (VFHP) in ITZs and the width of ITZs were obtained by backscattered electron imaging analysis (BSE-IA) and energy dispersive spectrometer (EDS) analysis. Results showed that the carbonation enhancement of RCA could not effectively strengthen ITZ 1. However, soaking RCA in Na 2 SiO 3 saturated solution and CEM-SF slurries could decrease the porosity and the UH content, develop the VFHP, and reduce the width of ITZ 1 , then strengthening ITZ 1. Both carbonation and chemical enhancement could take obviously improvement on ITZ 2. Additional calcium sources could be added to RCA by soaking in CH saturated solution, improving the effect of carbonation enhancement of RCA. Soaking in CEM-SF slurries had a better improvement effect on ITZs. After RCA was soaked in CEM-SF slurries, the porosity in ITZ 1 and ITZ 2 decreased by 36.8% and 44.0%, the UH content of ITZ 1 and ITZ 2 decreased by 22.6% and 53.6%, the VFHP of ITZ 1 and ITZ 2 increased by 22.6% and 20.9%, and the width of ITZ 1 and ITZ 2 decreased by 42.9% and 50%. EDS analysis showed that the Ca/Si atomic ratio of ITZs was reduced after RCA enhancement, which generated more hydration products and then decreased the porosity and the width of ITZs. [ABSTRACT FROM AUTHOR]