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Abstract Over the past few decades, climate change has become a significant problem, the ramifications of which are of concern around the world. Global warming is a long-term process caused by phenomena such as increased levels of carbon dioxide (CO 2) concentration in the atmosphere, increases in rainfall, changes in relative humidity and changes in temperature. Thus, this will aggravate the degradation process of reinforced concrete (RC) structures, leading to the decrease of durability, safety and serviceability. Regarding the durability of concrete structures, carbonation-induced corrosion is definitely a significant and costly source of degradation, directly related to climatic parameters such as CO 2 , temperature and relative humidity. In Paraguay, the lack of control during the construction of buildings often leads to weak structures highly vulnerable to the attack of harmful external agents. Therefore, this paper seeks to provide an analysis of the correlation between real degradation data and a numerical carbonation model for concrete structures in the country. According to the Intergovernmental Panel on Climate Change (IPCC) reports, climatic parameters will increase in the future. For this reason, an analysis of the decreasing performance of these structures caused by climate change will be addressed in this paper. This research aims to choose the most accurate mathematical model that can be applied in Paraguay for the prediction of the service life of concrete structures subjected to carbonation-induced corrosion. For that purpose, parameters and variables of the selected model have been defined and subsequently validated through carbonation tests results carried out in a set of RC structures built in the country. The results of this paper should be considered as a first stage for the further formulation of maintenance strategies, which is highly necessary for Paraguay. [ABSTRACT FROM AUTHOR]

Recognition of the rising amount of atmospheric CO 2 has brought renewed interest in understanding the effects of carbonation on reinforced concrete performance. In laboratory testing, the specimens must be preconditioned to effectively study carbonation. This paper studied the influence of several preconditioning schemes on the carbonation profiles of cement paste specimens subjected to accelerated carbonation tests. The evolution of microstructure and moisture during carbonation were investigated accordingly. Bulk of the work was based on an extended X-ray attenuation method (XRAM), which relied on X-ray computed tomography (CT). A novel method was introduced to evaluate the extent of damage due to drying. Based on extent of damage, the paper recommends standard-cured specimens for carbonation tests as compared to water-cured specimens. Also, when comparing between oven drying and mass balancing, the latter was shown to be more suitable, as the inner moisture distribution becomes more uniform after this drying protocol, and less fluctuation of humidity will occur during carbonation. [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]

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]

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]

Some jointed plain concrete pavements in the midwestern region of the United States of America have exhibited damage at the joints. This damage manifests itself in cracking and spalling in a small section (approximately 100 mm wide) along joints. These cracks may be due to either freeze-thaw cycling of concrete with a high degree of saturation or chemical reactions that occur between the deicing salt and the cementitious matrix. For example, deicing salts (e.g., CaCl 2 MgCl 2 ) may react with the cementitious matrix leading to the formation of calcium oxychloride. The formation of calcium oxychloride leads to matrix damage that results in the cracking and spalling of the concrete. The objective of this paper is to document the effect of carbonation on the potential for calcium oxychloride formation in a cementitious matrix. This paper examines ground hydrated cement paste powder made of different ordinary portland cements and portland cement containing fly ash, slag or silica fume. Each of these materials are tested for their potential to react with CaCl 2 after exposure to different levels of carbonation. The results indicate that the potential of calcium oxychloride formation decreases with the increase in the degree of carbonation. For long carbonation durations or higher degree of carbonations, there is no calcium oxychloride formation even if the calcium hydroxide is not totally carbonated. It appears that this is due to the fact that carbonation reaction creates a calcite barrier around the remaining calcium hydroxide. The calcite barrier reduces the potential for reaction between calcium hydroxide with the deicing salt. This work indicates that in addition to previous work that has shown that calcium oxychloride formation can be reduced with the use of supplementary cementitious materials and topical treatments (sealers), carbonation is a factor that should be considered in determining the amount of calcium oxychloride that can form in a cementitious matrix. [ABSTRACT FROM AUTHOR]

This paper reviews the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behaviour of concrete. It identifies various influencing aspects related to the use of recycled aggregates, such as replacement level, size and origin, as well as the influence of curing conditions, use of chemical admixtures and additions, on carbonation over a long period of time. A statistical analysis on the effect of introducing increasing amounts of recycled aggregates on the carbonation depth and coefficient of accelerated carbonation is presented. This paper also presents the use of existing methodologies to estimate the required accelerated carbonation resistance of a reinforced recycled aggregate concrete exposed to natural carbonation conditions with the use of accelerated carbonation tests. Results show clear increasing carbonation depths with increasing replacement levels when recycled aggregate concrete mixes are made with a similar mix design to that of the control natural aggregate concrete. The relationship between the compressive strength and coefficients of accelerated carbonation is similar between the control concrete and the recycled aggregate concrete mixes. [ABSTRACT FROM AUTHOR]

Red mud, RM, is the main residue generated in the Bayer process for obtaining alumina. This paper gives an assessment on the penetration rate of aggressive substances such as chlorides and CO 2 through RM incorporated cement paste. Diffusivity values are significantly reduced in presence of a low RM content, which could contribute to an increase in the service life of the reinforcements. A study based on Mercury Intrusion Porosimetry (MIP) and Differential Scanning Calorimetry (DSC) was performed to clarify the decrease in the penetration rates. This paper emphasizes the relevance of performing an exhaustive microstructural analysis to have a better understanding of the changes induced not only due to the RM incorporation but also those produced after the chloride and CO 2 entrance. Regarding the chloride penetration, the high aluminium content of RM, able to trap chlorides to produce Friedel’s salt, is pointed as responsible for the reduction in the chloride diffusivity. On the other hand, the retention in the CO 2 penetration is assisted by the larger amount of C-S-H gel developed in the RM-mixtures along with the alkaline nature of the residue. A reasonable prediction of the carbonation progress can be obtained by measuring the weight increment as the experiment advances. [ABSTRACT FROM AUTHOR]

This paper discusses the results of a suite of tests designed to assess the structural and durability characteristics of hydraulic lime–pozzolan concretes. Specifically, this paper reports on the rate of strength development, elastic modulus, linear shrinkage and rate of carbonation of four hydraulic-lime–pozzolan concretes. The purpose of this investigation was to ascertain the technical feasibility of producing high strength concretes using hydraulic lime and pozzolans as an alternative binder to Portland cement. Results have demonstrated that 28-day compressive cube strengths of 35 MPa can be attained by water-cured lime–pozzolan concretes. The results are presented alongside comparable test results for Portland-cement (CEMI) and blastfurnace cement (CIII/A) concretes. Similarities and differences in material characteristics are discussed in terms of fundamental material properties and in terms of the emergent threats and opportunities for the potential development of these novel concretes. [ABSTRACT FROM AUTHOR]

Abstract This paper presents the evolution of mechanical and chemical properties of sodium silicate activated fly ash and metakaolin geopolymers in one year time scale. The initial curing protocols are considered, which were chosen based on the isothermal heat release properties of both reaction systems. Increasing initial temperature contributes to the cumulative heat release, strength development and zeolite formation of both geopolymers, especially the fly ash based, but results in slightly worse pore characteristics. The formation of zeolite is rapid, indicating a simultaneous process with geopolymerisation. During sample handling and aging, natural carbonation took place. Two kinds of carbonation processes are detected: reaction of CO 2 with mobile alkalis on the one hand and with framework oxygen atoms of the alunimosilicate gels on the other hand. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a study on the carbonation activated binders prepared from pure calcium silicate phases, which included tricalcium silicate (3CaO.SiO 2 , [C 3 S]), β-dicalcium silicate (β-2CaO.SiO 2 , [β-C 2 S]), γ-dicalcium silicate (γ-2CaO.SiO 2 , [γ-C 2 S]), tricalcium disilicate (rankinite, 3CaO.2SiO 2 , [C 3 S 2 ]), and monocalcium silicate (wollastonite, CaO.SiO 2 , [CS]). The overall study consisted of three experimental parts, with individual focus on the following issues: (i) reaction kinetics, (ii) mechanical performance at the microscale, and (iii) mechanical performance at the macroscale. Carbonation of calcium silicate phases was found to occur in two distinct stages, namely: phase boundary controlled stage and product layer diffusion controlled stage. Theoretical solid-state reaction approach, including contracting volume model and Jander's equations were used to determine the carbonation rate constants for the calcium silicate phases. Phase boundary controlled stage was found to be dominantly dependent on the type of the starting calcium silicate phases. On the other hand, during the diffusion controlled stage the reaction rate constants were found to depend on the type of carbonation products (in this case Ca-modified silica gel and calcium carbonate). The mechanical properties of the individual microscopic phases were evaluated using nanoindentation test whereas the overall strength of the carbonated paste was evaluated using macroscale three-point bending test. Correlations between the mechanical performances and microstructural characteristics revealed the performance controlling factors of the carbonation activated binders. The higher bound water contents of the carbonated matrix tend to increase the short-term (up to 3 h) creep deformation of the matrix when subjected to constant stress. The presence of a higher proportion of poorly-crystalline forms of calcium carbonates (i.e., vaterite and amorphous calcium carbonate) were observed to increase the flexural strength but decrease the elastic modulus of the carbonated matrix. [ABSTRACT FROM AUTHOR]

Abstract This paper first introduced a numerical carbonation model (NCM) for fatigue-damaged concrete. Then, based on the NCM, a simplified carbonation model (SCM) for fatigue-damaged concrete was established, comprised of a non-damaged concrete contribution term and a fatigue damage contribution term, through Monte Carlo simulations. Both the NCM and SCM were verified by experimental results reported in the literature. Subsequently, an incremental method was proposed to consider the time-variant fatigue damage and exposure conditions. Finally, a case study was conducted, which determined that cumulating fatigue damage could have a big influence on carbonation depth evolution. Meanwhile, differences in the cumulating fatigue damage could lead to noticeable differences in carbonation depth evolution among different points in the same concrete bridge. Moreover, drastic increases of CO 2 concentration and temperature could induce relationships of carbonation depths with square roots of service times to remarkably deviate from the proportional laws widely accepted under a time-invariant environment. [ABSTRACT FROM AUTHOR]

The paper describes a study carried out to explore the potential for recovery of fly ash from wet-storage areas for use as an addition in concrete, and involved drying and processing material to achieve finer fractions/reduce particle size. Three relatively coarse fly ashes with medium/high carbon contents (loss-on-ignition; LOI) were obtained from two UK lagoons and a stockpile. Initially, drying (105 °C) and screening (600 μm) were carried out, giving acceptable handling properties. Thereafter, processing using (63 μm) sieving, air classifying or grinding was investigated. The methods gave increased fineness levels (greatest with grinding), with some reductions in LOI also found after sieving and air classifying. Assessment of fly ash reactivity indicated improvements with processing, and most noticeable effects by grinding. Tests on concrete found that both consistence (slump) and compressive (cube) strength increased with processing and tended to follow fly ash fineness. Further analysis showed that strength could be related to the sub 10 μm particle level in fly ash. This appeared to be irrespective of processing method, with progressive increases (in strength) up to sub 10 μm contents of about 60% (maximum in the study). Concrete tests for water absorption and intrinsic (air) permeability, carbonation and chloride diffusion showed that these were also influenced by fly ash sub 10 μm content. The factors affecting behaviour are discussed and practical implications of the research considered. [ABSTRACT FROM AUTHOR]

Recycling of waste concrete has become an important issue worldwide due to the continued increase of construction wastes. Also, the growing global construction activities urge to find sustainable resources to replace natural materials for the production of concrete. In the past few decades, many researches have been carried out on the use of recycled aggregate (RA) derived from construction and demolition wastes to produce concrete products. This paper reviews the previous findings on the effects of use of RA on durability of concrete. In general, the amount of adhered mortar and the quality of the original concrete have a significant effect on the properties of resulting concrete. The increase of RA content and w/c ratio results in poorer durability of concrete. In comparison, the negative effect of recycled fine aggregate is more obvious than that of recycled coarse aggregate. The use of pozzolanic materials either for surface coating of RA or intermixed within the concrete are effective and feasible to improve the overall durability of concrete. Recent researches on CO 2 treatment indicate that it can enhance the properties of RA and durability of concrete significantly. [ABSTRACT FROM AUTHOR]

The use of new binders for structural concrete raises questions about the durability of reinforced concrete structures. Calcium sulfoaluminate cement (CSA) has become in recent years an environmentally-friendly alternative to ordinary Portland cement (OPC), but the protection capacity of CSA concrete in relation to the corrosion of embedded steel reinforcement needs to be studied. This paper describes a study on the corrosion behaviour of carbon steel embedded in concrete made with CSA-based binders. It is shown that the pore solution of sulfoaluminate concrete was sufficiently alkaline to passivate embedded carbon steel rebars. Compared to traditional Portland and Portland-limestone cements, CSA cement led to higher carbonation rate of concrete, but to lower corrosion rate of steel in carbonated concrete, likely due to the higher electrical resistivity. Corrosion rate was negligible up to 95% RH at 20 °C. Blending of CSA and OPC cements improved steel passivity and concrete resistance to carbonation. [ABSTRACT FROM AUTHOR]

The prediction of carbonation depth for recycled aggregate concrete (RAC) is investigated in this paper. The existing prediction models were evaluated, and it showed that the coefficient of variation (COV) of model error for the existing models is high. By introducing the weighed water absorption of aggregates, the COV of model error can be effectively decreased. Compared with the existing models, the proposed model can predict more accurate carbonation depths. For RAC specimens, compared with the fib model and Xiao and Lei's model-a, the COV of model error of the proposed model is 0.36 which is decreased by 33.3%, and when compared with Xiao and Lei's model-b and Silva et al.’s model, the corresponding decreases are 55.2% and 16.2%. Finally, the proposed model is validated by a 10-year-old carbonation experiment, which indicates that the proposed model is reasonable and can be applied to predict the carbonation depth of RAC. [ABSTRACT FROM AUTHOR]

A model is proposed for prediction of natural carbonation in reinforced concrete (RC) structures, and is potentially applicable to existing and new RC structures. The major components of the model comprise mathematical functions applied to predict the influence of concrete composition, and environmental factors on natural carbonation. This paper introduces the model concept and explains its structure including derivation, optimization and calibration. Over 163 data sets taken from a 10-year carbonation study were used in the model development and calibration. Only the experimental data that were based on outdoor natural exposure environment were employed in this research. Also in this study, the proposed model is compared with fib-Model Code 2010 using carbonation predictions generated from 346 data sets involving real world, highway structures. It is shown that the proposed model is comparably accurate and involves mainly basic tests with no major anticipated costs. [ABSTRACT FROM AUTHOR]

An alternative CO 2 curing method for precast concrete products has been proposed in order to achieve rapid strength development at early age, as well as to capture and store greenhouse gas (CO 2 ). In this paper, an experimental study for the development of a maturity approach is presented to estimate the strength development of carbonated concrete blocks. In order to promote the use of industrial flue gas containing CO 2 , a flow-through CO 2 curing regime at ambient pressure and temperature was employed using different atmospheric conditions, such as various CO 2 concentrations, RH values and gas flow rates. The experimental results showed that the compressive strength or maturity of the carbonated concrete blocks was affected by two factors: accelerated cement hydration and carbonation extent. A high CO 2 concentration, a fast gas flow rate and a moderate relative humidity were essential for enhancing the maturity and the strength development. The developed model based on the maturity approach may accurately predict the strength development of the carbonated concrete blocks. [ABSTRACT FROM AUTHOR]

This paper develops a kind of molded disc samples to investigate the carbonation and related behaviors of hardened cement pastes under different previous hydration degrees. Weight and length changes of cement pastes over time are monitored during a multistep process including carbonation, drying, rewetting, and redrying. The combination of X-ray diffraction (XRD) and thermogravimetric analysis (TGA) is used to identify and quantify the mineral compositions of carbonated cement pastes. An exponential function between CO 2 uptake capacity and hydration time of cement pastes is established, which shows that the CO 2 uptake capacity of cement pastes decreases dramatically at the very beginning days of hydration and then remaining relatively stable as hydration time is prolonged. Two reasons for this finding are revealed: i) the equilibrium between the carbonation and the post-carbonation reaction of carbonation product, i.e., silica-alumina gel; ii) refining of pore structures by hydration products which hinders carbonation. A clearer zonation of carbonation areas is proposed, and the spatial distribution equations of CO 2 absorption are initially established. By monitoring carbonation and drying behavior of cement pastes with different hydration ages, it is revealed that carbonation reduces drying shrinkage of cement pastes especially for early-age samples, whereas drying increases carbonation shrinkage. By investigating the water changes during the multistep process, it is found that water is little released during the carbonation of C–S–H gels. New insight into mechanism of carbonation shrinkage is provided by a newly proposed model. • Molded disc and multistep aided by self-made device are used for all-round study. • Exponential function between CO 2 uptake capacity and hydration time is established. • A clearer zonation of carbonation areas is proposed. • Carbonation reduces drying shrinkage for early-age cement pastes. [ABSTRACT FROM AUTHOR]

Fibre and textile-reinforced mortars are increasingly being used for a variety of building applications, including the strengthening of masonry structures. Lime mortars reinforced with sustainable fibres (such as vegetable or cellulosic fibres) may provide an interesting solution. In this paper, a mixture of commercial lime with 20% metakaolin addition was used to produce composites reinforced with non-woven flax fabrics that were cured at different moisture contents (from 0 to 100%) for 7 or 14 days in a CO 2 incubator. The composites were characterised to determine their flexural behaviour, carbonation level and microstructure. According to the results, no differences exist in the flexural strength of the composites made in the moisture range of 33%–66%. At 7 days of curing, they attained Modulus of Rupture (MOR) values that exceeded 5.5. MPa. Moreover, it was observed that under high moisture conditions, the permeability of the fibres allows for CO 2 access, despite the saturation of the pores of the matrix – allowing a reaction in the vicinity of the fibres –, while under dry conditions, the fibre's moisture retention does not permit the carbonation of the matrix in their vicinity, even though complete carbonation takes place after 14 days. [ABSTRACT FROM AUTHOR]

Application of cement in high-pressure high-temperature (HPHT) carbonic acid containing environment poses serious well integrity issues. HPHT condition facilitates acid attack on well cement resulting in the loss of structural integrity. This paper presents the results of an experimental study conducted to investigate the effects of CO 2 concentration (mole fraction) and pressure on mechanical integrity of degraded Classes G and H cements after exposure to carbonic acid environment. Cement cores were prepared and aged in autoclave filled with 2% NaCl solution saturated with mixture of methane and carbon dioxide gases for durations up to 28 days. Experiments were performed by varying test pressure and CO 2 concentration. Compressive strength, porosity and permeability of unaged and aged samples were measured to assess the level of degradation. Besides, analysis of the mineralogy, the microstructure, and the morphology of specimens was conducted. After exposure, incomplete hydration of clinker materials, carbonation of cement hydrates, leaching of by-products of carbonation, and structural transformation of amorphous calcium silicate hydrate to its crystalline form were observed. Generally, compressive strength increased with CO 2 concentration due to carbonation of calcium hydroxide (CH) and calcium silicate hydrate (C-S-H). Correspondingly, porosity and permeability decreased. Furthermore, degree of degradation is greatly influenced by pressure. [ABSTRACT FROM AUTHOR]

This paper reports the carbonation characteristics of a cement-slag system exposed to accelerated carbonation testing, and its improved carbonation resistance with the increasing MgO content in blast furnace slag, in which hydrotalcite-like phase plays a key role. Our research showed that the hydrotalcite-like phase started to carbonate upon contacting with the carbonate ions and bound more than 15 wt% CO 2 −3 in the mildly carbonated and transition areas. This value was positively associated with the magnesia content of slag. Additionally, the proportion shared by hydrotalcite-like phase decreased in the fully carbonated area, and more CO 2 was fixed in the form of calcium carbonate. Consistent with the thermodynamic modelling, the ratio of CO 2 bound in carbonated hydrotalcite-like phase to the total CaCO 3 continued to decrease as the CO 2 ingress progressed. On the other hand, the reaction between hydrotalcite-like phase and CO 2 was found to be volumetrically stable due to binding CO 2 in the interlayer space, and Mg was still distributed within the original slag grain region. Mg/Al atomic ratio of hydrotalcite-like phase remained nearly the same before and after carbonation. Results of this study quantitatively emphasized the favorable effect of hydrotalcite-like phase to improve the carbonation resistance of slag-rich cementitious systems. [ABSTRACT FROM AUTHOR]

This paper studied in situ , by Micro-Raman spectroscopy, the very early hydration of C 3 A in the presence and absence of sulphates and with sucrose as an additive. For C 3 A hydration in the absence of gypsum, when carbonation is not avoided, carbonate-AFm phases are formed, but in the presence of gypsum, hydroxi-AFm are the main phases. Ettringite is the AFm stable phase developed initially at 70 min of hydration with gypsum and no monosulphate is formed. In the presence of sucrose, this salt, instead of sulphate, is adsorbed over the surface of the C 3 A, avoiding its reaction with sulphates until sucrose desorption. Three hours are necessary to lead to ettringite formation. A nucleation poisoning/adsorption surface mechanism is proposed for added sucrose systems. [ABSTRACT FROM AUTHOR]

This paper describes the multiscale characterization of the carbonated wollastonite paste using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), and statistical nanoindentation (SNI, also known as ‘grid indentation’) methods as well as micromechanical homogenization models. Wollastonite (CaSiO 3 ) fibers are commonly used as filler in ceramics or plastics. However, wollastonite can also be regarded as non-hydraulic binder material since upon carbonation it forms a heterogeneous matrix with mechanical properties similar to those of the conventional hydrated cement pastes. Carbonation reaction of wollastonite results in the formation of two main products: calcium carbonate (CaCO 3 ) and amorphous silica gel (SiO 2 ). The SEM/EDS microanalysis performed on this system revealed that the average calcium to silica (Ca/Si) atomic ratio of the silica gel phase was around 0.40. Three individual carbonated wollastonite paste samples, each representing a different degree of carbonation were selected for nanoindentation tests. The obtained elastic moduli for silica gel, calcium carbonate, and unreacted wollastonite grains were, respectively, 41.7 GPa, 67.3 GPa, and 134.7 GPa. The micromechanical homogenization models were then utilized to predict the effective (also referred to as ‘homogenized’) elastic moduli of the carbonated wollastonite paste. The predicted values of the effective elastic moduli of carbonated wollastonite pastes were found to be in the range of corresponding values for hydrated high to ultra-high performance cement pastes. Additionally, the values of the effective elastic moduli of the carbonated wollastonite pastes were observed to increase with the increase in the degree of carbonation. [ABSTRACT FROM AUTHOR]

Concrete carbonation is one of the main triggers of the corrosion process and hence of the premature deterioration of concrete reinforcements. In order to monitor and control the carbonation process, a potentiometric sensor using thick-film technology was developed. This paper includes the results of the research carried out to characterise the pH sensor, including the materials, response time, sensitivity, detection limit, reproducibility, reversibility and the study of chloride interference. The sensors have been studied in pore solutions and concrete specimens. The study shows that the thick-film sensor has a potentiometric response to variations in pH and is capable of providing continuous, real-time information on the progress of the carbonation front. [ABSTRACT FROM AUTHOR]

This paper identified carbonation products in the slag-rich cementitious systems upon three different exposure conditions, namely, long term exposure in the field, indoor natural exposure, and accelerated carbonation testing. Overall, mineralogy of the carbonation products was found to be fundamentally similar under different exposure environments. In the fully carbonated areas, no monosulfate and calcium hydroxide was observed, and calcium carbonate, carbonated hydrotalcite-like phase and Ca–Al AFm phases were identified as the main carbonate phases. In the mildly carbonated areas, monosulfate and calcium hydroxide clusters were detected again. With the continuous supply of CO 2 , monosulfate appeared to be consumed at first. Despite different exposure environments, carbonated Ca–Al AFm phases bound around 5% of CO 2 penetrated into the matrix. Hydrotalcite-like phase was able to absorb more than 15% CO 2 initially. However, this value decreased to around 10% in the fully carbonated areas. Therefore, more than 20% CO 2 entered into hydrotalcite-like phase as well as Ca–Al AFm phases at first, and the involved reactions were harmless without any detrimental effect on cement matrix. Meanwhile, hydrotalcite-like phase was able to keep intact and its Mg/Al atomic ratio did not vary significantly during carbonation. [ABSTRACT FROM AUTHOR]

In our previous work, a method of stabilizing high-temperature polymorphs (α-CS) by rapid cooling was reported to improve the carbonation activity of ultra-low calcium CO 2 sequestration binder (LC-CSB). This paper further investigates the effect of sodium (Na) doping on the properties of α-CS minerals, such as burnability, mineral composition and microstructure. In addition, the carbonation reactivity, carbonation hardening properties, carbonation products and microstructure of α-CS with different sodium doping were also characterized by pH and conductivity, XRD, TGA, FT-IR, SEM-EDS. The mechanism of the effect of sodium doping on the structure and carbonation activity of α-CS minerals was revealed. The results show that the incorporation of sodium significantly improved the burnability of α-CS, lowered the sintering temperature by 180 °C and formed a new phase of Na 2 O·2CaO·3SiO 2 (NC 2 S 3). As sodium doping increased, the early carbonation hardening properties were slightly lower than those of pure α-CS, but there was a significant increase in compressive strength and carbonation degree in the later stages. This can be attributed to the fact that sodium doping increases the alkalinity of the paste and delays the dissolution of calcium ions from the α-CS. Furthermore, in the presence of sodium ions, metastable vaterite crystals can be stabilized in addition to calcite. The above results provide potential applications for the low-temperature sintering of alkali-containing solid waste into high carbonation activity LC-CSB. • The incorporation of sodium significantly improves the burnability of α-CS. • A new phase of Na 2 O·2CaO·3SiO 2 is formed by sodium doping. • The alkali-containing solid waste can be upcycled into high carbonation activity LC-CSB. [ABSTRACT FROM AUTHOR]

When structural concrete is exposed to a fire, various changes in chemical and mechanical properties proceed. The concentration of carbon dioxide CO 2 rises to about 15% depending on the situation of fire accidents accompanying combustion of carbide. The self-healing of damaged concrete is expected by absorbing CO 2 after fire. Thus, consideration of high temperature heating for carbonation is indispensable for performance assessment of structural concrete. This paper proposes a multi-scale thermo-chemo-physics model for carbonation, de-carbonation and re-carbonation processes during and after high-temperature heating. The proposed integrated model is experimentally validated by using the thermo-gravimetry experiments of cement paste and the strength of mortar composites immediately after high-temperature heating and after post-fire-curing. The CO 2 concentration and the humidity are experimentally changed as the thermodynamic boundary conditions for wide-range verification and validation. The compressive strength is treated not as the material property but the computed structural capacity of a cylindrical solid in which the temperature, hydration degree and carbonation develop non-uniformly. The proposed model can capture the tendency of experimental results and allows practically reasonable assessment of fire-damaged and moist-cured concrete as a multi-scale composite. [ABSTRACT FROM AUTHOR]

Uncertainties related to durability of concrete arise when Portland Cement (PC) is combined with supplementary cementitious materials, urging a more thorough understanding of the deterioration mechanisms. This paper focusses on carbonation, specifically the effect of replacement level of PC by ground granulated blast-furnace slag (GGBFS), curing duration and CO 2 concentration on the reaction products and CO 2 buffer capacity. The experimental test program included the determination of the hydration degree of PC, the reaction degree of GGBFS and an analysis of the reaction products by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectra of cement paste powders showed a delay in calcium silicate hydrate (C-S-H) decalcification upon carbonation in case of 40wt% GGBFS by increasing the curing period from 3 to 7 and 28 days. However for 70wt% GGBFS, C-S-H decalcification was quick and irrespective of the curing period whereas in neat PC, C-S-H decalcification was minimal. When comparing elevated and natural CO 2 concentrations, portlandite contents and the formed CaCO 3 polymorphs were comparable while the carbonate content and C-S-H decalcification lack resemblance suggesting different carbonation stages. ATR-FTIR spectra from the exposed surface towards the centre showed a systematic agreement between the peak area of carbonates and the colour change boundary by phenolphthalein. The spectra also suggested the transformation of aragonite to calcite during ongoing carbonation of mixes with 70wt% GGBFS and 40wt% GGBFS cured for 3 days. The use of phenolphthalein to distinguish carbonation does not reflect the heterogeneous composition of the microstructure of (partially) carbonated areas. [ABSTRACT FROM AUTHOR]

Abstract: Concrete performance is traditionally based on assessing the effect of a single degradation mechanism. In the field, however, concrete is simultaneously affected by degradation mechanisms, possibly with a synergetic effect on deterioration. This paper presents the results of a Finnish research project assessing coupled deterioration mechanisms including frost attack, carbonation and chloride penetration. Research was composed of an extensive laboratory testing regime, in parallel to the exposure of several concrete specimens at field stations. Testing took into account the effects of ageing and repeated exposure cycles to different conditions. More than 60 concrete mixtures were evaluated with varying binder types and air contents. Testing results together with local weather data serve as a basis for modelling and development of service life assessment tools. The results show the synergetic effect on concrete deterioration of coupled deterioration and quantitatively support that a holistic approach should be adopted for predicting deterioration. [Copyright &y& Elsevier]

Abstract: Carbonation of cement-based materials is one area of concern for the durability of concrete structures. The calcium carbonate caused by carbonation is an important indicator of carbonation degrees. The present paper, using 3D tomography data, proposes a nondestructive method to characterize the 3D spatial distributions of calcium carbonate. It allows monitoring of 3D carbonation evolutions. The evolution of the calcium carbonate distributions in a specimen of cement paste with different carbonation degrees is given using the current method. The results are compared with the average quantity of calcium carbonate determined by thermal analysis. From the sharp edge of the calcium carbonate distribution, we conclude that the accelerated carbonation in this experimental condition is a diffusion controlling process. [Copyright &y& Elsevier]

In this paper, supercritical carbonation tests of concrete specimens with different water-to-cement ratios are carried out. In the test, the thickness of interfacial transition zone (ITZ) of the concrete is determined by the distribution of Ca/Si ratio across the interface between the coarse aggregate and cement paste. The microhardness distribution, microstructure and porosity of the ITZ before and after supercritical carbonation are analyzed. A geometrical and physical model considering the distribution of porosity, coarse aggregates, ITZ, and the supercritical carbonation of concrete is proposed, by which cracks, pores, calcium carbonates, and C-S-H gel at the interface of coarse aggregates and cement paste can be studied. The overall microstructures are relatively compacted after supercritical carbonation. The thickness of ITZ of concrete is reduced from 47-79 μm to 35–51 μm after supercritical carbonation. The average value and variance of carbonation depth of concrete increase with the increase of the thickness and porosity of ITZ. Comparing the carbonation results of concrete with different thicknesses and porosity of ITZ, it appears that porosity of ITZ has greater impact on the carbonation depth of concrete. • Ca/Si ratio distribution can be used to accurately determine the thickness of ITZ. • A method for generating the ITZ between the aggregates and cement paste was proposed. • A geometrical and physical model was proposed for the supercritical carbonation of concrete. • The carbonation depth of concrete is more sensitive to porosity than thickness of ITZ. [ABSTRACT FROM AUTHOR]

Supplementary cementitious materials (SCMs) can be used in concrete to enhance sustainability and reduce the concrete industry's carbon footprint. However, some negative perceptions about their long-term carbonation resistance are obstacles for large-scale implementation of such concretes. This study evaluated the carbonation resistance of 34 concretes (with Ordinary Portland Cement, fly ash, blast furnace slag, and limestone calcined clay) in natural tropical exposure conditions (Open and Sheltered) for 5 years and in accelerated exposure conditions (1 and 3% CO 2) for 112 days. Using these data and the square root of time function, the carbonation coefficients (K CO2, natl and K CO2, accl) of these concretes were estimated and a good correlation between them could not be observed. Hence, a more generic model (named as "A-to-N model") to estimate the K CO2, natl using the K CO2, accl , CO 2 concentration, and mixture proportion of concrete was developed, for which the mean absolute percent error is about 12% (reasonable accuracy). Using the A-to-N model, the carbonation depth at 50 years was estimated for various concretes. SCM concretes with low water-binder ratio and optimal binder content showed high resistance against carbonation at later ages; such information along with the target cover depth must be used while selecting materials for concrete design. Based on the model developed, a relatively simple 'service life design chart' was developed. This chart can be used by engineers to set the target K CO2, natl or K CO2, accl, and select the cover depth and binder type to provide the target service life (i.e., corrosion initiation time). This paper clearly shows that SCMs can be used to design concretes with comparable long-term carbonation depth as OPC concretes. • 5-year natural carbonation data from 34 concretes with various SCMs (blast furnace slag, fly ash and LC3). • Correlations between accelerated and natural carbonation coefficients for tropical climate. • Concretes with SCMs can be deisgned to exhibit long-term carbonation resistance. • Service-life design chart for reinforced concrete systems with SCMs subjected to carbonation-induced corrosion was developed. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the effect of curing on the susceptibility of cementitious composites to carbonation using supercritical carbon dioxide. Samples made using a compression moulding technique were cured in water before and/or after carbonation and the effect on porosity, microstructure, solid phase assemblage and flexural strength was determined. In terms of development of mechanical strength, no benefit was gained from any period of pre- or post-carbonation curing regime. Yet samples cured prior to carbonation underwent minimal chemical reaction between supercritical carbon dioxide and calcium hydroxide, unhydrated cement or C–S–H. Thus there was no correlation between chemical degree of reaction and strength development. The effects responsible for the marked strength gain in supercritically carbonated samples must involve subtle changes in the microstructure of the C–S–H gel, not simple pore filling by calcium carbonate as is often postulated. [Copyright &y& Elsevier]

Abstract: Recycled ground glass is an unconventional material for self-compacting concrete (SCC). Nevertheless, its use is becoming increasingly attractive representing a twofold contribution to economic and eco-efficient SCC. In fact, on one hand mixed-color glass culets that are not reusable for packaging purposes may be employed. On the other hand its supplementary cementing potential can be used to replace expensive materials such as silica fume, metakaolin and cement, reducing CO2 emissions. The present paper provides a comprehensive procedure for the design of SCC mortar mixtures incorporating fine glass powder. A central composite design was carried out to mathematically model the influence of mixture parameters and their coupled effects on deformability, viscosity, compressive strength, resistivity and resistance to carbonation. The derived models and a numerical optimization technique were used to select the best mixture, which maximizes durability and minimizes cost, while maintaining self-compactability. [Copyright &y& Elsevier]

Abstract: The fib-Model Code for Service Life Design, which is referenced within Model Code 2010, considers different deterioration mechanisms of concrete structures. In particular, it proposes a physical model for the assessment of the carbonation depth in time, which, for existing structures, requires data, such as the type of cement and the water-to-cement ratio of the concrete, that are often unavailable. In the paper the theoretical results obtained with the fib-model are discussed and compared with experimental data obtained during an extensive campaign carried out on cast-in-place uncracked concretes of in-field exposed existing r.c. structures from a highway infrastructure. This comparison has highlighted the key role played not only by the environment, but also by the quality of the concrete through the inverse effective carbonation resistance of concrete, , on the evolution of the carbonation depth in time. The direct measurements of the carbonation depth on existing r.c. structures allows the inverse effective carbonation resistance of concrete to be determined and correlated to the concrete mean compressive strength at 28days obtained from compressive tests on cores taken from the investigated structures. [Copyright &y& Elsevier]

Different models have been proposed to investigate and predict the carbonation process of cement-based materials. This paper firstly introduces the carbonation process occurred in cement-based materials and those influential factors. Then the proposed empirical, thermodynamic, kinetic and numerical models to characterize the carbonation process of cement-based materials are reviewed. The theoretical basis, input parameters and major applications of different models are summarized and discussed. The advantages and disadvantages of those models are comparatively reviewed. Finally, modelling of early CO 2 curing of fresh cement-based materials is also reviewed and compared with the carbonation of hardened cement-based materials. • Parameters and different forms of empirical carbonation models are summarized. • Thermodynamic carbonation modelling process and solving methods are discussed. • Kinetic carbonation models of diffusion, dissolution and reactions are compared. • Finite element and machine learning methods applied to carbonation are reviewed. [ABSTRACT FROM AUTHOR]

In this study, experimental investigations were conducted on enhancing the crack-healing potential of concrete using the internal carbonation process. The objective is to overcome one major limitation of autogenous healing (limited carbonation) by using Sodium Carbonate (Na 2 CO 3) solution as a healing agent. To replicate the different types of damages and internal cracking that could arise during the service life of concrete structures, the effects of drying shrinkage and mechanical stresses on the crack-healing proclivity of cement-based materials were explored. A novel technique to measure crack-healing under constantly applied stress is also introduced for better assessment of healing in real-world situations. Results show a significant improvement in healing by using 0.95 M Na 2 CO 3 solution over plain water. Moreover, permeability observations under stress revealed that the potential for healing by the proposed mechanism is strongly dependant on the stress level in samples. In the later part of the paper, Service Life Predictions were also undertaken by utilizing chloride ion diffusion coefficients extracted from the measured permeability coefficients and service life models available in the literature. Results from comparison of predicted service life indicate that at least in qualitative terms, healing with internal carbonation using the Na 2 CO 3 solution led to an increase in the service life of concrete composites. [ABSTRACT FROM AUTHOR]

Passive radiative cooling includes spontaneously cooling a structure by reflecting and emitting light through atmospheric window (8 μm–13 μm) into the cold universe. This paper involves a systematical study on maximizing the radiative properties of cementitious composites by optimizing constituents and microstructures. Relatively high hemispherical solar reflectance (82 ± 2) % and middle infrared (MIR) emissivity (87 ± 2) % were achieved simultaneously, allowing for an inner temperature drop of 12 °C in field test. Whiter surface, micro-magnitude pore size and TiO 2 particle diameter all lead to higher reflectance in visible and near-infrared (VIS- NIR, 0.4 μm–2.5 μm) region. From the hydration products, complex infrared absorbance of O–H bonds yields low reflectance, while O–H, Si–O, Si–O–Si bonds stretching vibrations guarantee high emissivity. CaCO 3 , whether added as admixture or formed by carbonation, significantly increases the VIS reflectance. Both numerical simulation and field test demonstrate great potential of utilizing radiative cooling cementitious composites for building energy saving. [ABSTRACT FROM AUTHOR]

Degradation of Portland cement under supercritical CO 2 conditions (scCO 2) is one of the major concern in geological sequestration projects. This paper presents the research on two novel types of binders considered to display a CO 2 corrosion resistance potential: (1) Portland cement partially replaced with 20, 30 and 40 wt % of zeolite and blended with Styrene-Butadiene (SB) Latex and (2) geopolymer based on lime-slag and lime-pozzolan blends. Specimens were placed in an autoclave, covered with water, heated up to 100 °C and pressurized with 7 MPa CO 2 in order to simulate CO 2 environment in the Croatian wellbore. On behalf of tests on compressive strength, porosity, permeability and alkalinity, as well as thermogravimetry, X-ray diffraction and SEM microstructural analyses of samples at various penetration depths, carbonation mechanisms were discussed. Portland-Zeolite composites have been argued as potential CO 2 resistant cement systems, with an replacement rate below the minimally tested addition of 20 wt %, as excessive replacement rates resulted in an increase in porosity and permeability. Geopolymer composites based on lime activation unfortunately haven't exhibited required properties for application in well cementing of CO 2 injection wells. Calcium carbonate polymorphs precipitated throughout the whole specimen thickness of all samples, but preferably in mixtures based on Ca-richer raw materials. A re-precipitation of calcium carbonate filled in the porosity preferentially at the 0.5 mm surface layer of PC-based specimens, due to the outward diffusion of calcium at the early stage of carbonation. [ABSTRACT FROM AUTHOR]

This paper presents a Potentiostatic Step Voltammetry approach to corrosion rate measurement of reinforcements in concrete. We have termed this approach PSV-TE since it is based on the Tafel extrapolation method, but with the added advantage that long and slow potentiodynamic scans are no longer required to obtain the Tafel slopes. In this way, the irreversible polarization of rebars is prevented, so PSV-TE is considered to be a non-destructive method. The Tafel slopes are obtained by fitting the current-time response of the system to a theoretical model which we have outlined and validated previously. In this way, the concrete's electrical resistance and double layer capacity are also obtained. In this study, the optimal PSV-TE design has been established and validated on a range of reinforced concrete specimens. Results show minimal deviation between PSV-TE and reference methods. Therefore, PSV-TE is part of the corrosion monitoring system we have patented. • A Pulse Step Voltammetry approach is proposed to reliably assess rebar corrosion. • The corrosion rate measurement is based on the Tafel extrapolation method. • Potentiodynamic scans are not required, preventing irreversible steel polarization. • Concrete electrical resistance and double layer capacity are also determined. • It could be implemented as a non-destructive tool in corrosion monitoring routines. [ABSTRACT FROM AUTHOR]