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The increase in energy valorization of paper sludge and biomass waste through incineration results in a rise in waste paper fly ash (WPFA), often perceived as hazardous and requiring specific treatment. In this study, natural carbonation technology at a pilot scale was employed to mitigate the hazardous nature of WPFA by enhancing the stabilization of metal and metalloid trace elements (MMTE), particularly barium (Ba) and lead (Pb) leaching. Natural carbonation of WPFA was found to be optimal at a water/solid ratio of 0.3 L/kg under natural temperature and humidity conditions. Batch leaching tests based on thermodynamic equilibrium were used to assess MMTE solubility concerning pH in both non-carbonated and carbonated WPFA at natural pH. After 7 days of natural carbonation, the leaching concentration of Ba and Pb was below the legal limit in France. The concentration of Ba and Pb in carbonated samples decreased by 98.5% and 98%, respectively. Analyses using thermogravimetry, X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy demonstrated the increased formation and quantity of calcite (CaCO3) with longer carbonation times, along with the formation of new minerals as BaCO3 and PbCO3, confirming the immobilization of these elements post-carbonation. The leaching behavior and release potential of carbonated WPFA were further evaluated using the four-stage sequential extraction procedure proposed by the European Commission's Bureau of Reference (BCR). The speciation of Ba and Pb shifted significantly from the soluble substance (F1) at 17.6% and 14.8%, respectively, to the carbonate fraction (F2) at 0.49% and 0.02% after 90 days of carbonation. The percolation water collected during the carbonation process adheres to discharge standards into the sea. [ABSTRACT FROM AUTHOR]

The increase of energy valorization of paper sludge waste and biomass due to their high calorific value by incineration leads to an increase of waste paper fly ash (WPFA) quantities, which are in most cases considered as hazardous, and must be dealt with accordingly. The Alternative options for the management of WPFA, such as stabilization/solidification, vitrification or sintering, are not fully developed or either costly. In the present study, accelerated carbonation technology was used to reduce the hazardous nature of WPFA by improving the stabilization of metallic and metalloids trace elements (MMTE), especially Barium (Ba) and Lead (Pb) leaching. The accelerated carbonation of WPFA was found to be optimal at a water/solid ratio of 0.3 L/kg under controlled temperature and humidity conditions. The use of water saturated with 25% sodium bicarbonate (NaHCO 3) improved the carbonation of WPFA and the stabilization of Ba and Pb. Batch leaching tests based on thermodynamic equilibrium was used to evaluate the solubility of the MMTE as a function of pH and at the natural pH of non‑carbonated and carbonated WPFA. Thermogravimetry, X-ray diffraction, scanning electron microscopes and energy dispersive spectroscopy analyses were performed to investigate the evolution of carbonation over time and its effect on the chemical and mineralogical properties of WPFA. After 30 days of accelerated carbonation, the leaching concentration of Ba and Pb was below the French legal limit. The leaching concentration of Ba and Pb from carbonated samples decreased 99% and 99.5% respectively. The leaching and release potential behavior of carbonated WPFA were further evaluated using the four-step sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR). The speciation of MMTE underwent significant transformation, shifting predominantly from the soluble fraction to the carbonated fraction as a result of the carbonation reaction. [Display omitted] • Accelerated carbonation with NaHCO 3 of WPFA has been performed. • Ba, Pb and Cl leaching concentration decreased effectively after carbonation. • After 15 days of accelerated carbonation, the WPFA becomes inert. • Parameters L/S = 0.3 l/kg, 20 °C/65 °C with NaHCO 3 improves carbonation. • Ba and Pb was precipitated as carbonate BaCO 3 and PbCO 3 [ABSTRACT FROM AUTHOR]

This study focused on the problem of recycling construction and demolition wastes as well as industrial waste by-products into sustainable concrete materials. By combining addition of three different industrial waste by-products, an environmentally friendly recycled aggregate self-compacting concrete (RA-SCC) mix was put forward. Carbonation resistance and microstructure of SCC incorporating high volumes of industry by-products and recycled aggregates were studied in this paper. A total of 9 RA-SCC with 50%, 75% supplementary cementitious materials (SCMs) and different SCM combinations were prepared, which consist of binary mix with fly ash (FA), ternary mix with FA and ground granulated blast-furnace slag (GGBS), and quaternary mix with FA, GGBS, and silica fume (SF). An exposure period of 3, 7, 14, and 28 days was adopted for accelerated carbonation tests. The test results indicate that, as RCAs content increased, RA-SCC carbonation depth increased. The carbonation depth of SCC with 100% RCAs increased by 49.35% compared to that of normal SCC. In addition, carbonation resistance of RA-SCC mixes with a given compressive strength decreased with increasing SCM content. However, the loss in carbonation resistance was greatly mitigated by the combined addition of different SCMs, on account of the substitution of cement with industrial by-products and natural aggregates with RCAs. The combined addition of different SCMs enhanced the carbonation resistance of RA-SCC mixes. The test results were validated through microstructural analysis using scanning electron microscopy (SEM), and X-ray diffraction (XRD). A comparison between predictions calculated using existing models and measured carbonation depth was implemented. By taking into account the effect of RCAs and SCMs, a modified carbonation model in conjunction with fib carbonation model was put forward. [ABSTRACT FROM AUTHOR]

The induction of accelerated carbonation curing (ACC) as carbon capture, utilization, and sequestration (CCUS) technique helps in achieving the net zero target. ACC is one of the potential techniques to sequester the CO2 in concrete. However, the usage of ACC in reinforced concrete is still debatable. This paper investigates the effect of ACC on mechanical, permeation, mineralogy, hydration and microstructural characteristics of concrete with the long-term performance of reinforced concrete using linear polarization resistance (LPR) and carbonation depth. In order to study the effect of ACC duration on the properties of concrete, the specimen was subjected to 6, 12, and 24 hours (h) of carbonation curing. The results were compared with conventional water-curing specimens. ACC at 12 h provided the best results for tested properties. The results were varied by statistical analysis using ANOVA and stoichiometry. Based on the best 12-h duration, 2.13 kg of CO2 can be sequestered in 1 m3 (0.89 kg/t) of concrete. [ABSTRACT FROM AUTHOR]

The objective of this paper is to present the methodology used to validate the threshold levels of durability indicators from data acquired from existing reinforced concrete structures that may present corrosion problems resulting from carbonation or chloride ingress. This study was conducted in the framework of the French National Project PERFDUB in close collaboration with the MODEVIE ANR project. Durability tests form the basis of a performance‐based approach for the durability of concrete structures, and in order to set the threshold level for each of these durability indicators, it is important to take into account the experience feedback that can be obtained from the actual behavior of existing structures. This paper presents the investigations carried out on eight reinforced concrete structures, all of them having an age well in excess of 50 years, and the main data gathered on some relevant durability and lifetime indicators. Based on all these data, predictive models of carbonation and chloride penetration are applied in order to validate the threshold levels to be adopted for the durability indicators, and several other conclusions are drawn. [ABSTRACT FROM AUTHOR]

Normally, the acidic impurities in hemihydrate phosphogypsum (HPG) must be neutralized when HPG is utilized, and a little amount of calcium hydroxide (CH) is the best choice. In this paper, the effects of excessive CH (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% of HPG) for carbonation curing on the performance of hardened HPG paste were studied. According to the results of macro tests and microanalyses of XRD, TG, SEM-EDS, MIP and N2 physisorption, it could be verified that CaF2, Ca3(PO4)2 and a large amount of nanoscale CaCO3 crystals were produced as a result of neutralization and carbonation, and the compressive strength and the water resistance of carbonated HPG + CH paste were significantly improved due to the effects of nanoscale CaCO3 crystals on pore refinement and the coverage on the surfaces of gypsum crystals of the hardened paste. Therefore, this study suggests a feasible and green method for recycling HPG/PG, with the collaborative effects of neutralization, performance enhancement and reductions in CO2 emissions. [ABSTRACT FROM AUTHOR]

The durability of concrete structures, which are designed for long- -term use is predominantly determined by the resistance to chemical influences, i.e., the concrete's ability to protect the reinforcement steel. The carbonation and the chloride ingress into concrete are the most significant causes of steel corrosion and the potential failure of the structure. The primary goal is to ensure that any significant damage does not occur during the structure's service life, primarily achieved by selecting an adequate thickness of the concrete cover. The issue is approached through calculations based on performance analyses, and the use of appropriate models for these chemical phenomena. The paper provides a brief overview and the methodology for analysing the impact on the durability of concrete structures in accordance with the leading international normative documents. The emphasis is on the recent changes introduced in second generation of European Eurocode standards. The consequences of the analysed phenomena are presented through the results of field tests conducted at salt factories, coke industries, and thermal power plants, and through laboratory tests. The tests were performed in order to develop a rapid prediction method for the measure of chloride ingress into concrete without the stimulating chloride ion migration by electricity, as an alternative to standardized tests. [ABSTRACT FROM AUTHOR]

This paper discusses the possibility of assessing the k-value of power plant fly ash using durability. Compressive strengths, activity indices, pressure water seepage depth and carbonation depth were determined for cement mortars with 10, 20 and 30% cement replacement by fly ash for ages of 7, 28 and 60 days. The k-values for each cement substitute were determined for each age using the relationship between water/cement ratio and compressive strength. Using the results of the depth of pressurized water seepage and depth of carbonation, the determined k-values were assessed and k-values that are safe for use of power plant fly ash in aggressive XC and XD environments were determined. [ABSTRACT FROM AUTHOR]

The current building industry is essentially changing due to implementation of new approach, which is predominantly motivated by the need to reduce its negative environmental impact. The paper is focused on the experimental evaluation of carbonation resistance in terms of accelerated tests performed on the concrete on the basis of blended binding system incorporating milled limestone. The used commercially produced cement was additionally modified by the limestone powder. The obtained results confirmed, that currently used design standards are convenient to real carbonation process, however further clinker replacement by the milled limestone causes significant loss of the durability of the hardened concrete. [ABSTRACT FROM AUTHOR]

The wet carbonation process of steel slag (SS) is envisaged to be an effective way to sequestrate CO2 and improve the properties of SS as a supplementary cementitious material. However, the carbonation process still struggles with having a low carbonation efficiency. This paper studied the effect of glycine on the accelerated carbonation of SS. The phase composition change of carbonated SS was analyzed via XRD, FT-IR, and TG–DTG. The carbonation process of SS is facilitated by the assistance of glycine, with which the carbonation degree is increased. After 60 min of carbonation, SS with glycine obtained a CO2 sequestration rate of 9.42%. Meanwhile, the carbonation reaction could decrease the content of free calcium oxide in SS. This significantly improves the soundness of SS–cement cementitious material, and the compressive strength of cementitious materials that contain carbonated SS with glycine is improved. Additionally, the cycling performance of glycine in the successive wet carbonation process of SS was investigated. Multicycle experiments via solvent recovery demonstrated that although the promotion effect of glycine was reduced after each cycle, compared with the SS–water system, the carbonation process could still be facilitated, demonstrating that successive wet carbonation via solvent recovery has considerable potential. Herein, we provide a new idea to facilitate the wet carbonation process of SS and improve the properties of SS–cement cementitious material. [ABSTRACT FROM AUTHOR]

The Intergovernmental Panel on Climate Change (IPCC), which is the United Nations body for assessing the science related to climate change, has recently recognized the natural carbonation process as a way of carbon offsetting with mortar and concrete. Accordingly, this activity could be recognized as a carbon removal process for which certification should be granted. The aim of the certification of carbon removal is to promote the development of adequate and efficient new carbon removal processes. Therefore, the main objective of this study is to provide reliable results on carbon dioxide uptake by cement-based materials in Spain. Yearly, greenhouse gas emissions are reported to the United Nations Framework Convention on Climate Change (UNFCCC) by each country, and the natural carbonation should be added up to the carbon accounting. Therefore, natural carbonation should be included in the IPCC Guidelines for National Greenhouse Gas Inventories, and such accounting information should be made available promptly to the national regulatory authorities. This paper provides the results of carbon dioxide uptake by Spanish cement-based materials from 1990 to 2020 by using an easy method of estimating the net carbon dioxide emissions (simplified method) considering the carbon dioxide released by the calcination during clinker production (process emissions). The outcome of this study reveals that there was 93,556,000 tons of carbon dioxide uptake by the mortar and concrete manufactured in Spain from 1990 to 2020. [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]

Reactive magnesium oxide cement (RMC) has been proposed as a sustainable alternative to ordinary Portland cement (OPC) due to its lower production temperature and ability to permanently sequester carbon dioxide (CO 2) through carbonation. However, mixtures with only RMC can have significantly reduced CO 2 absorption capacity due to limited CO 2 diffusion, thereby compromising the performance of RMCs. This paper demonstrates the adoption of cenospheres, an industrial waste, as a lightweight filler in RMC-based 3D printable composites to enhance carbonation and improve thermal insulation. This work investigates the effect of incorporating different dosages of cenospheres on the rheological properties, compressive strength, thermal conductivity, and microstructural development of the RMC mixtures using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The study reveals that substituting 30 % of RMC with cenospheres not only improves the printability and carbonation of the composite but also results in a 16 % reduction in density, achieving 1.72 g/cm³, and reduces the thermal conductivity by 59 % compared to the control without cenospheres to 0.63 W/(m·K). The findings provide valuable insights for developing lightweight 3D printable RMC-based composites. [ABSTRACT FROM AUTHOR]

The influence of limestone powder (primarily composed of calcite) on hydration and properties of concrete has been extensively researched. However, there is limited understanding of how different calcium carbonate (CC) polymorphs affect cement hydration due to the difficulty in formation of vaterite. Therefore, preparing calcite and vaterite from solid waste carbide residue (CS) by wet carbonation and examining influence of these two CC polymorphs (0–20 %) on cement hydration and properties are comprehensive investigated in the paper. The results reveal that pure cubic calcite can be obtained through CS carbonation, while high-purity spherical vaterite can be synthesized using glycine as a regulator. The specific surface area (SSA) of spherical vaterite (15.47 m2/g) is approximately three times that of calcite (4.8 m2/g) with a comparable particle size distribution (PSD). The addition of calcite reduces the flowability of cement, whereas vaterite enhances it. The cement mortar with calcite demonstrates a lower compressive strength, while the incorporation of vaterite results in a higher or comparable compressive strength in contrast to control sample. Compared with control cement hydration, new hydration products, like monocarbonate aluminate and hemicarbonate aluminate, are formed with the addition of both calcite and vaterite. The metastable vaterite exhibits increased reactivity. The research presents a novel approach for the valorisation of solid wastes through CO 2 sequestration, offering insights into the preparation of highly added-value products. [ABSTRACT FROM AUTHOR]

Determining the carbonation depth is of paramount importance in assessing the durability of cementitious materials as the carbonation process alters their near-surface physicochemical properties. This paper introduces a novel quantitative ultrasound (QUS) technique capable of nondestructively identifying the carbonation depth, regardless of the tortuosity of the carbonation front. A 2.5 MHz phased array transducer was employed to construct beamformed radio frequency images and measure two microstructure-dependent parameters – spectral slope (SS) and spectral intercept (SI) – based on backscatter and attenuation coefficients. The change in obtained SS and SI were then visualized as QUS images, which displayed the geometry of the carbonation front, correlating with images constructed using a phenolphthalein solution (1.6 mm difference in depth). Mineral compositions and porosities were analyzed to account for microstructural changes in each layer. This study opens an opportunity for nondestructive detection of carbonation depth, overcoming the limitations in conventional methods or regression-based approaches. • Quantitative ultrasound (QUS) technique is developed for detecting carbonation depth. • QUS images achieved based on scattering and attenuation phenomena confirm densification in carbonated layer. • Upon the QUS images, the measured carbonation depth shows great agreement with results from phenolphthalein solution. • Mineral compositions and porosities were evidenced by TG, MIP, XRD, and QXRD analyses. • The proposed method offers potential for visualizing ambiguous geometry of inclusions in cementitious materials. [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]

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]

Exploring the mineral compositions of steel slag and the microstructure evolutions of its carbonation products is very important for deeply understanding the carbonation mechanism of steel slag, which can promote the application of steel slag in carbon capture and storage. In this paper, the carbonation behaviors of various mineral phases in steel slag were investigated by using an ingenious in-situ observation method through combing laser scanning confocal microscopy (LSCM), backscattered scanning electron microscopy (BSEM) and focused ion beam-transmission electron microscope (FIB-TEM). It is shown that dicalcium silicate (C 2 S) and free lime (f-CaO) had high carbonation reactivity, while the calcium aluminoferrite with different Al/Fe ratios and mayenite (C 12 A 7) exhibited extremely poor carbonation reactivity. The carbonation reactivity of RO phase was positively correlated with its Mg/(Fe + Mn) ratio. RO phase with a Mg/(Fe + Mn) ratio below 0.5 owned almost no carbonation reactivity, while the Mg2+ would leach from RO phase with a Mg/(Fe + Mn) ratio higher than 2.0 during the carbonation process, inhibiting the precipitation of CaCO 3 , changing the micro morphology of CaCO 3 , and generating spheroidal aragonite. The ingenious in-situ observation method developed in this study provides a very visual information, which can be further extended to more fundamental research. • In-situ investigation on the carbonation behaviors of various mineral phases in steel slag was studied. • Calcium aluminoferrite with different Al/Fe ratio and C 12 A 7 exhibited extremely poor carbonation reactivity. • The carbonation reactivity of RO phase was positively correlated with its Mg/(Fe + Mn) ratio. • Mg2+ would leach from high Mg RO phase during the carbonation process, changing the micro morphology of CaCO 3. [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]

[Display omitted] • This study used synthetic pellets to amplify the fly ash carbonation reaction. • Physical encapsulation is the main mechanism for solidifying heavy metals. • Carbonation depth determined heavy metal solidification rate. • Shrinking core model can be applied to carbonation and leaching. • Leaching of heavy metals from carbonized fly ash can be divided into three stages. The physical encapsulation mechanism prevents the leaching of heavy metals by forming calcium carbonate-encapsulated fly ash through the reaction of carbon dioxide and calcium oxide. However, current research in this area primarily involves qualitative analysis, focusing on various methods to enhance the fixation effect of heavy metals. There is a lack of quantitative description regarding how physical encapsulation solidifies heavy metals, and the solidification mechanism of heavy metals is challenging to verify through macroscopic experiments. In this paper, a method for synthesizing small fly ash particles into pellets is proposed to quantitatively study the mechanism of physically encapsulating heavy metals. The data from the carbonation reaction and leaching reaction are fitted using the shrinking core model. The depth of carbonation increases with the lengthening of carbonation time, and the physical encapsulation strengthens the immobilization of heavy metals. The leaching of heavy metals after carbonation can be categorized into three stages based on the shrinking core model: external diffusion, internal diffusion, and chemical reaction control. Synthetic pellets enhance all three stages, especially the external diffusion, which is not detectable with fly ash powder. The fitting results of the experimental data for the carbonation reaction of the enlarged synthetic pellets and the shrinking core model are both above 0.9. This indicates that the primary barrier to leaching of heavy metals is the physical encapsulation formed by the carbonation reaction following the shrinking core model. The physical encapsulation serves as the primary mechanism for the solidification of heavy metals during the carbonation process. [ABSTRACT FROM AUTHOR]

Accelerated carbonation of recycled aggregates (RAs) enables fast carbon sequestration while improving engineering properties of prepared concrete. This paper presents a systematic experimental study on the fracture behaviour of concrete with carbonated recycled coarse and fine aggregates (RCA and RFA), in terms of fracture parameters and fracture process zone (FPZ). The relevant influencing mechanism was investigated by relating the macroscopic fracture responses to microstructural features. Results indicate that carbonated RFA mainly enhanced the initial fracture toughness of concrete, while carbonated RCA primarily increased its unstable fracture toughness. The enhancement of fracture energy was closely associated with carbonated RCA and RFA. The utilization of RFA resulted in the looseness and higher porosity of mortar matrix, which reduced the crack initiation resistance of concrete, mainly affecting its initial cracking stage. The incorporation of RCA reduced crack bifurcation and deflection of concrete, leading to a smaller width of FPZ, which made the initial descending segment of the tension softening curve (TSC) steeper and its tail segment shorter. The carbonation treatment of RAs improved the densification of the adhered old mortar and reduced the porosity of prepared concrete, rising the cracking resistance and inducing a wider FPZ. As a result, the initial descending segment of TSC became flatter and its tail segment tended to be longer. • Roles of carbonated recycled fine and coarse aggregates in concrete fracture are explored. • Fracture process zone is systematically characterized based on DIC and inverse analysis. • Fracture mechanism is discussed from both microscopic and macroscopic viewpoints. • Recycled fine and coarse aggregates respectively affect the crack initiation and propagation. [ABSTRACT FROM AUTHOR]

Carbonation depth involves complex physical process and interactions of multiple variables and is thus extremely complicated to predict in concrete structures. It is imperative to quantify this depth due to its vital role in the corrosion of rebars in recycled aggregate concrete (RAC). This paper developed a novel carbonation depth prediction model from a large database of 445 experimental results using artificial neural network (ANN). The relative importance and effect of the independent parameters in the carbonation depth are identified using Garson index and parametric analysis, respectively. Among all the architectures considered, the N 8-10-1 having 10 nodes in the hidden layer provided the best prediction in good agreement with experimental results. The model demonstrated superior performance relative to existing carbonation depth equations in the literature. Despite the presence of fuzziness in the data, the effect of each variable in the development of carbonation is explored in great detail. The model proposed here can provide a robust prediction of carbonation depth that can be used as a basis for assessing the structural health of recycled aggregate concrete structures. [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]