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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]

One of the key issues associated to mineral carbonation is the slow kinetic rate of leaching process. Hence it is highly important to study the kinetics of the leaching process in detail. This paper deals with the detailed analytical and statistical studies of calcium leaching from waste concrete sample. For comparison, another potential waste i.e. CKD was also selected. Different analytical approaches such as ICP, EDS, XRD, and SEM were used. Based on the preliminary results, at the experimental conditions of 2 M solution of each ligand for 2 h at 70 °C, HNO 3 was found to be more proficient as compared to the rest of ligands (propionic acid, NH 4 Cl, and NaOH) used. In addition, detailed kinetic study was also performed for different temperatures and HNO 3 concentrations. Different heterogeneous (M1-M9), homogeneous (M10, M11), and nucleation i.e. JMAEK (M12) models were implemented to represent the experimental data. Statistical tools such as SSE, SEE, R 2 , F-value, and t-values of the regression parameters were used to discriminate various kinetic models and among them, nucleation model (M12) was found best to describe the experimental data. A low activation energy (25 kJ/mol) indicates that diffusion could be the rate controlling step for the dissolution of waste concrete sample. [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]

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

This paper investigates the effect of magnesium oxide (MgO) on the carbonation resistance of alkali-activated slag-fly ash blend containing 75% ground granulated blast furnace slag (GGBS) and 25% low-calcium fly ash. Two types of GGBS were used with different magnesium oxide content. Phenolphthalein indicator and pH profiles showed that the GGBS with higher levels of magnesium oxide offered no significant improvement in resistance against natural and 1% accelerated carbonation. X-ray diffraction confirmed no hydrotalcite formation, although the magnesium oxide content was 9·1%. A very small amount of free magnesium ions (Mg2+) was available in the pore solution, which was deemed insufficient to form hydrotalcite. Lack of its formation was the major reason for the lower carbonation resistance. Excessive silicate in the system reduces the calcium oxide/silicon dioxide ratio, which leads to the incorporation of magnesium ions in the calcium silicate hydrate structure. Hydrotalcite was observed when the activator concentration was reduced. The results suggest that in addition to magnesium and aluminium ion (Al3+) availability, silicate concentration also plays a strong role in deciding the hydrotalcite formation in alkali-activated GGBS concrete. [ABSTRACT FROM AUTHOR]

The paper addresses the carbonation of CSA based binders from the mechanical, chemical and mineralogical point of view. Three different binders have been investigated, revealing that many hydrated phases can protect ettringite against carbonation. In particular, the presence of AFm phases gives a significant contribution to buffer the pH in alkaline range and to preserve the protective film of steel reinforcement bars. In a carbonated environment AFm phases act as ionic exchangers: carbonate ions replace sulfates in the interlayer and in these conditions the released sulfate ions contribute to the formation of new ettringite, with a beneficial effect on compressive strength performances. Experimental data are compared with models and carbonation mechanisms are described. [ABSTRACT FROM AUTHOR]

This paper deals with the construction, analysis and computation of a numerical method to solve a moving boundary coupled nonlinear system of parabolic reaction–diffusion equations, arising in concrete carbonation problems. By means of a front-fixing transformation, the domain of the problem becomes fixed, and the position of the moving carbonation front has to be determined together with the mass concentrations of the involved chemical species. Qualitative properties like positivity and stability of the numerical solution are established. Spatial monotone behaviour of the solution is also proved. Numerical examples illustrate these results. [ABSTRACT FROM AUTHOR]

Vaterite-type calcium carbonate particles have some unique properties such as high hydrophilicity, large surface areas, and hierarchical structures consisting of primary vaterite particles in comparison with calcite or aragonite-type polymorphs. In this paper, gypsum (CaSO 4 ·2H 2 O) suspension is used to synthesize micro-sized vaterite CaCO 3 through magnetic stirring (MS) and ultrasonic probe vibration (UPV) methods. The effects of ammonia concentration, CO 2 flow rate, solid-liquid ratio on the gypsum carbonation process, mineral phase composition, morphology and particle size distribution of CaCO 3 are investigated. The results show that the carbonation process is significantly influenced by ammonia concentration, CO 2 flow rate and ultrasound. Comparing with magnetic stirring, ultrasonic probe vibration take less time to reach the complete carbonate reaction. Gypsum is transformed to vaterite with the conversion rate about ∼95% when the mole ratio of NH 4 + /Ca 2+ is 2.4 otherwise the carbonation reaction was uncompleted with gypsum residues left. Comparing with MS method, the UPV method resulted in smaller size and narrower size distribution of as-prepared microparticles and approximately 80% reduction of the particle size was achieved. It is established that increasing the solid-liquid ratio resulted in larger particle size in MS system and smaller particle size in UPV system. Increasing CO 2 flow rate caused the particle size decreased in MS system and increased in UPV system. [ABSTRACT FROM AUTHOR]

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

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

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]

Energy storage capacity and heat transfer ability are two important indexes for shape-stabilized phase change materials (ss-PCMs). In this paper, a core-sheath CNT@PC was prepared via carbonation of CNT@ZIF-8, simultaneously 3D structural supports were obtained due to the porous carbon (PC) sheath stabilized the CNT@PC network structure. Porous carbon (PC), derived from carbonized metal organic frameworks (MOFs), exhibited high porosity and large specific surface area. PCMs, absorbed by capillary force of porous structure, was stabilized in the pores of PC sheath. Further, the interaction between PCMs and CNTs reduced the interfacial thermal resistance greatly. Carbon nanotubes (CNTs), acting as heat transfer pathways, provided continuous channels for phonons transfer and realized rapid heat transformation between ss-PCMs and external environment. The obtained SA/CNT@PC ss-PCMs exhibited excellent thermal conductivity (1.023 W/mK), large phase change enthepy (155.7 J g −1 ) and high thermal storage capabilities (99.9%). The thermal conductivity of SA/CNT@PC was improved 222.6% and phase change enthalpy was increased 92.6% over SA/PC ss-PCM. SA/CNT@PC with large energy storage density, flexible designation, simple operation and near-constant temperature properties during phase change process shows great potential in waste heat utilization. [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]

Abstract: An alternative of wellbore cement system, formed through alkali‐activation of fly ashes also knows as fly ash geopolymers, provides attractive properties due to its high compressive strength and durability against acidic conditions. However, changes in microstructure and mineralogical during accelerated carbonation affected by CO2 alteration under elevated temperature and pressure have not been fully understood. This paper investigates and characterizes the alkali‐activated binders fly ash based in the acidic environment under elevated temperature and pressure. The results found that scales such a layer was formed in the gel phase at the spherical surface of fly ash after carbonation process. The effect is that the surface of the gel phase became rough and porous. Carbonate and/or bicarbonate salts were identified as major phase in the sample after immersion test. In the activated fly ash cement, both calcium aluminate silicate hydrate (C–A–S–H) and sodium aluminosilicate hydrate (N–A–S–H) gels were formed; C–A–S–H gel was decalcified, forming calcium carbonate phase in the form of calcite, aragonite or vaterite. These findings were confirmed by the analysis and identification of Fourier‐transformed infrared spectroscopy, X‐ray diffraction and scanning electron microscopy tests on the changes in the cement system. [ABSTRACT FROM AUTHOR]

Nanolime is a promising consolidant for the conservation of most historic structures thanks to its high compatibility with carbonate-based substrates. Nanolime can recover the superficial cohesion of deteriorated surfaces thanks to its potential to complete the carbonation process, recreating a thin network of new cementing calcium carbonate. In this paper, the nanolime was produced by an innovative, time and energy-saving and scalable method, and its efficacy was tested preliminary on biocalcarenite stones from Agrigento. The stones characterization as well as the treatment effectiveness, in terms of protection against water and superficial consolidation, was investigated by several techniques such as X-ray fluorescence, X-ray diffraction, scotch tape test, water absorption by capillarity, mercury intrusion porosimetry, drilling resistance measurement system and colorimeter. Investigations showed that nanolime could guarantee a complete transformation in pure calcite together with a superficial consolidation and a reduction in water absorption. [ABSTRACT FROM AUTHOR]

This paper investigates the feasibility of mechanical activation as a pre-treatment method as part of an integrated mineral carbonation process for a nickel mining operation in British Columbia. The physical, structural and chemical characteristics of forsterite and lizardite in mine waste rock after mechanical activation were monitored to determine their CO 2 sequestration efficiency for a direct aqueous carbonation process. Economic analysis on the process was developed through considering the energy requirements, cost modeling and carbon balance. Mechanical activation on mining residue reduced the particle size, enlarged the surface area, distorted the crystal structure of forsterite, and enhanced the CO 2 sequestration efficiency. However, it didn't create new phase or induce phase transfer between forsterite and lizardite, and distort the crystal lattice of lizardite. The optimum condition for 60% CO 2 sequestration efficiency was found at mechanical activation for 210 kWh t −1 ore and carbonation for 260 kWh t −1 ore, which had an operating cost of 104.1–107.1 $ t −1 CO 2 avoided. Using mechanical activation, the integrated mineral carbonation plant in the mine can potentially sequester 14.62 Mt y −1 CO 2 using mine waste rock and tailings during the 28-year life of mine. [ABSTRACT FROM AUTHOR]

This paper studies hydration stages and phase transformation mechanism of the Sr2+-doped calcium zirconium aluminate cement at room temperature. Features of different hydration stages of this cement paste are identified by X-ray diffraction, scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, evolved gas analysis and heat evolution test. It was found that the partial isovalent substitution of Ca2+ for Sr2+ such as Ca7−xSrxZrAl6O18, with x = 0.3, 0.6, 1.0, reduced the hydraulic reactivity of Ca7ZrAl6O18 phase. The course of hydration of mixed oxides of the type 6CaO·SrO·3Al2O3·ZrO2 documented using microcalorimetry was supported by investigations of the solid hydration products. Research showed that the hexagonal hydrates were stable at early and middle curing ages, an indication that including Sr in solid solution could effectively inhibit the conversion from both C4AH13–19 and C2AH8 to C3AH6 and AH3. Calcium monocarboaluminate C4AC¯H11 phases were also found in this system due to the carbonation process of C4AH13 phase. Substitution of Ca2+ ions by Sr2+ ions in hexagonal calcium aluminate hydrates causes structural disorder and then contributes to the broadening of lines in the powder X-ray diffraction patterns of layered structures of metastable hydrates. At later curing ages, the formation of two hydrogarnet phases, one Sr-rich (C,Sr)3AH6 and the other Ca-rich (C,Sr)3AH6, was proved. [ABSTRACT FROM AUTHOR]

In this paper we define and study a finite volume scheme for a concrete carbonation model proposed by Aiki and Muntean in [Adv. Math. Sci. Appl.19 (2009) 109–129]. The model consists in a system of two weakly coupled parabolic equations in a varying domain whose length is governed by an ordinary differential equation. The numerical sheme is obtained by a Euler discretisation in time and a Scharfetter-Gummel discretisation in space. We establish the convergence of the scheme. As a by-product, we obtain existence of a solution to the model. Finally, some numerical experiments show the efficiency of the scheme. [ABSTRACT FROM AUTHOR]

Micro-Raman spectroscopy is a powerful technique for qualitative and quantitative analysis of different mineral mixtures. In this paper, micro-Raman spectroscopy was used for quantification in local regions (180 × 180 μm area) of ternary mixtures of the synthetic calcium carbonate (CaCO 3 ) polymorphs (vaterite, aragonite, calcite) as well as CaCO 3 formed during the carbonation of nanolime suspension. The obtained results of localized quantification were in agreement with the detected concentrations obtained from bulk quantitative phase analysis of X-ray powder diffraction patterns. The detection limits were found to be below 0.5 wt.% for each CaCO 3 polymorphs. Through the use of 2D mapping, localized quantification of CaCO 3 polymorphs can be achieved. This information could be potentially useful for conservation of valuable Cultural Heritage objects, as it might influence the consolidation treatment chosen. [ABSTRACT FROM AUTHOR]

This paper deals with the construction and computation of numerical solutions of a coupled mixed partial differential equation system arising in concrete carbonation problems. The moving boundary problem under study is firstly transformed in a fixed boundary one, allowing the computation of the propagation front as a new unknown that can be computed together with the mass concentrations of CO 2 in air and water. Apart from the stability and the consistency of the numerical solution, constructed by a finite difference scheme, qualitative properties of the numerical solution are established. In fact, positivity of the concentrations, increasing properties of the propagation front and monotone behavior of the solution are proved. We also confirm numerically the t -law of propagation. Results are illustrated with numerical examples. [ABSTRACT FROM AUTHOR]

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

Mineral sequestration stabilizes carbon dioxide in carbonate form, its thermodynamic ground state. In the case of waste concrete, it produces precipitated calcium carbonate, a commodity. We propose an indirect, aqueous process consisting of six steps: (i) recovery of waste cement (primarily calcium silicate hydrate) from waste concrete, (ii) leaching of calcium from waste cement using hydrochloric acid, (iii) purification of the leachate through alkalinity addition, (iv) precipitation of calcium carbonate via aqueous reaction with sodium carbonate, four of which were investigated here. The omissions include regeneration of hydrochloric acid and sodium hydroxide solutions from sodium chloride solution through bipolar membrane electrodialysis and absorption of CO 2 by sodium hydroxide solution. Hydrochloric acid was capable of leaching all the calcium ions present in the fines when the stoichiometric amount was added. Results showed up to 70% of the total calcium ions recovered were associated with waste cement with the remainder from limestone in the aggregates. A bright, high-purity PCC was produced after purification to pH ∼11. The high-purity PCC particles were larger than what is typically used in paper manufacturing (∼2 μm) with metastable morphology. The particle size was reduced to ∼2 μm when the purification stopped at pH 9.0 and a magnesium containing PCC was produced; the morphology remained stable in de-ionized water for 24 h. [ABSTRACT FROM AUTHOR]

Alkaline anion exchange membrane fuel cell (AEMFC) is a promising technology to replace precious metals used today as fuel cell catalysts. However, AEMFC does not yet demonstrate high performance when running on ambient air where they are exposed to CO 2 . The resulting carbonation reaction reduces membrane conductivity. This paper analyses and quantifies the effect of CO 2 from ambient air on the concentration profiles in the membrane and the anode and, thus, assesses the CO 2 impact on fuel cell performance. The physico-chemical model contains chemical and electrochemical reactions, liquid-gas phase equilibria as well as the transport processes in the cell. Results imply that a significant part of fed CO 2 is absorbed in the cathode and is transported as carbonate ions to the anode. Concentration profiles in the membrane reveal an enrichment zone of CO 2 in the membrane close to the anode, negligible HC O 3 − and a wide distribution of C O 3 2 − across the membrane. The carbonate distribution affects overall anion exchange membrane conductivity. For practical relevant current densities of i > 500 mA cm − 2 and typical excess ratios of 1.5 for the hydrogen feed, less than 10% of the anions in the membrane are C O 3 2 − . We show that while increasing cell temperature has an ambiguous effect on the carbonation process and on the total effect of CO 2 on the cell, current density has a significant effect. The impact of CO 2 on AEMFC performance can be significantly decreased when operating the cell at high current densities above 1000 mA cm − 2 . [ABSTRACT FROM AUTHOR]

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

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]

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

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

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

This paper presents a numerical study of the gas-liquid flow inside a novel Venturi carbonation reactor for red mud processing. The model is based on the Eulerian-Eulerian model, with population balance method being used to describe bubble size distribution. The validity of the model is confirmed through different applications. It is then used to analyze the flow and gas dispersion inside the reactor at different liquid flow rates. The numerical results reveal that the new reactor can produce periodic variations of fluid pressure and gas holdup with time. Correspondingly, the ascending liquid flow presents a vortex column, swinging periodically in the radial direction. This flow oscillation becomes more obvious at an increased liquid flow rate, promoting bubble refining and uniform distribution. The results of this study should be useful not only for better understanding the gas-liquid flow but also for designing and controlling this new reactor. [ABSTRACT FROM AUTHOR]

Steel fibre reinforced concrete (SFRC) is increasingly being used in the construction of civil infrastructure. However, there are inconsistencies among international standards and guidelines regarding the consideration of carbon-steel fibres for the structural verification of SFRC exposed to corrosive environments. This paper presents a review of the published research regarding carbonation- and chloride-induced corrosion of SFRC, and proposes a deterioration theory for cracked SFRC exposed to chlorides and carbonation, based on the damage at the fibre-matrix interface. The review confirms an overall agreement among academics and regulators regarding the durability of uncracked SFRC exposed to chlorides and carbonation. Contrariwise, the durability of cracked SFRC is under discussion at the technical and scientific level, as there is a large dispersion on the experimental results and some of the mechanisms governing the corrosion of carbon-steel fibres in cracks and its effects on the fracture behaviour of SFRC are not fully understood. [ABSTRACT FROM AUTHOR]

Numerous researches have reported that there is a tendency chloride content first climbs to the maximum then declines with depth increasing in the surface layer of concrete under cyclic drying-wetting environments, which is temporally called ‘maximum phenomenon’ in this paper. This research focuses on the impact of different carbonation conditions on this phenomenon for both pastes and mortars. The distribution of chloride suggests that coupled effect of carbonation and capillary suction/moisture evaporation is more likely to lead to the formation of maximum phenomenon than the merely effect of capillary suction/moisture evaporation. Furthermore, analysis of pore structure and phase composition reveals that this particular phenomenon is directly related to the release of bound chloride fixed in Friedel's salt triggered by carbonation. In addition, the forming process of maximum phenomenon is proposed in the end based on Friedel's salt decomposition under cyclic drying and wetting condition. [ABSTRACT FROM AUTHOR]

Recycled concrete aggregate (RCA), obtained by crushing of original (old) concrete, consists of natural aggregate grains and a cement mortar matrix. The presence of old adhered cement mortar, which has higher porosity than natural aggregate, causes unfavourable properties of RCA. The research conducted in order to improve the quality of RCA and to enable its greater application in the construction industry is presented in this paper. Therefore, RCA was subjected to quality improvement treatments with hydrochloric acid and carbon dioxide (accelerated carbonation). The first procedure was aimed at partially removing the adhered cement mortar and the second at reinforcing the cement matrix. The physical, mechanical and chemical properties of all three types of RCA were tested. After the pre-soaking acid treatment (0.1 mol/dm3 HCl), RCA showed reduced water absorption (up to 3%); the process of accelerated carbonation also led to reduced water absorption (13-20%) as well as to improved mechanical properties (-10%). A scanning electron microscopy investigation revealed that the carbonation process, as expected, significantly reduces porosity of RCA. The overall results show that if RCA is obtained by crushing of compact, high- -quality concrete, the procedures of aggregate quality improvement are not necessary. [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]

We consider a new two-scale problem which is given as a mathematical model for moisture transport arising in a concrete carbonation process. In research for moisture transport, it is a crucial step how to describe the relationship between the relative humidity and the degree of saturation, mathematically. Here, we have proposed the two-scale model consisting of the diffusion equation for the relative humidity in a macro domain and the free boundary problems describing the relationship in infinitely micro domains. Accordingly, the structures of the micro domains are unknown in our model. This is a significant feature of our new model to emphasize. The aim of this paper is to establish local existence in time and uniqueness of a solution to the model. [ABSTRACT FROM AUTHOR]

Sorption Enhanced Steam Methane Reforming (SE-SMR) has been proposed as an efficient novel technology to increase hydrogen yield and reduce the environmental footprint in comparison to state of art H 2 production processes. Sorbent/catalyst materials characterized by stable behaviour over multiple reforming/calcination cycles may ensure to achieve almost stationary operating conditions utilizing a dual fluidized bed system (the reformer and the sorbent regenerator) with a solid circulation loop. Bifunctional, Combined Sorbent-Catalyst Materials (CSCM) are under development to integrate endothermic catalytic reforming and heterogeneous CO 2 sorption in one particle, decrease mass and heat transfer resistances and reduce the solid hold-up in the reactors. This paper deals with the numerical simulation of a pilot scale bubbling fluidized bed SE-SMR reactor by means of a Two-Dimensional Computational Fluid-Dynamic (2D CFD) approach. The hydrodynamic picture is supplemented with a comprehensive Particle Grain Model (PGM) previously developed to describe the kinetics of catalytic and sorption functions, and successfully validated with micro-reactor reactivity tests and multi-cycle thermo-gravimetric sorption tests. The effect of repeated carbonation-calcination steps (the “history” of the granular material) is included in the computation of the reactor performance by utilizing the appropriate size of the sorbent grains in the carbonation rate expression. The numerical results show quantitatively the positive influence of carbon dioxide sorption on the reforming process, at different operating conditions, specifically the enhancement of hydrogen yield and reduction of methane residual concentration in the reactor outlet stream. A preliminary validation of CFD simulations is also carried out utilizing experimental data obtained from a pilot scale bubbling fluidized bed SE-SMR reactor (total bed mass ≈ 14 kg). An estimate is provided for the inward heat flow that would be required to operate the reactor in stationary temperature conditions: it is substantially reduced by the exothermic sorption process and could be satisfied by means of the solid circulation loop connecting the SE-SMR reactor to the high temperature calciner in the whole dual fluidized bed system. [ABSTRACT FROM AUTHOR]

In the conservation of heritage materials, the effectiveness of diluted nanolime suspensions in consolidating surfaces is scarcely explored. This paper aims to examine the surface modification of stucco, adobe and stone by deposition of Ca(OH) 2 nanoparticles. The nanoparticles were applied in five consecutive coats and transformed into CaCO 3 by atmospheric CO 2 creating a compatible coating with the surface. The coatings were studied by erosion tests (pull-off tests) and examined by scanning electron microscopy (SEM) and optical microscopy. The tests confirmed the surface resistance was clearly improved due to formation of nanostructured cementing CaCO 3 coatings. In addition, the coatings did not practically alter the surface colour due to the diluted nature of the suspensions. To conclude, Ca(OH) 2 nanoparticles-based coatings are appropriate solutions to extend the durability of traditional heritage materials, such as stucco, adobe or stone. [ABSTRACT FROM AUTHOR]

This paper presents a method for the remote inspection of the structural integrity of engineering structures and materials, based on passive modulated scattering probes. In particular, a set of passive modulated scattering technique (MST) probes with diagnostic capabilities are embedded in structures, such as reinforced concrete slabs, with the objective of detecting water infiltrations, chemical deterioration, such as carbonation, and damages of the material structure. An external reader is used to provide the interrogating electromagnetic wave and to receive the signal generated by the MST probes. The material/structure integrity can be retrieved from the signal retransmitted by the probes. A set of preliminary experiments have been carried out to assess the potentialities of the method and to demonstrate how this system can be implemented for practical applications. The obtained results are quite promising. [ABSTRACT FROM PUBLISHER]

This paper investigates the mineral sequestration of carbon dioxide in circulating fluidized bed combustion (CFBC) boiler bottom ash. CFBC bottom ash, which originated from two sources, was prepared along with pulverized coal-fired (PC) boiler bottom ash as a control. These ashes were exposed to accelerated carbonation conditions at a relative humidity of 40% and 100%, in order to investigate the effects of humidity on the carbonation kinetics of the bottom ash. The obtained results showed that not only lime but other calcium-bearing phases (gehlenite, wollastonite, and brownmillerite) in CFBC bottom ash participated in the mineral carbonation reaction. In particular, these phases underwent hydration in a wet carbonation environment, whereby the carbon dioxide uptake and capacity of CFBC bottom ash are significantly enhanced. This study may have important implications, demonstrating the feasibility of carbon dioxide sequestration and recycling of CFBC boiler bottom ash. [ABSTRACT FROM AUTHOR]

Amine-based absorption is the primary contender for postcombustion CO2 capture from fossil fuel-fired power plants. However, significant concerns have arisen regarding the formation and emission of toxic nitrosamine and nitramine byproducts from amine-based systems. This paper reviews the current knowledge regarding these byproducts in CO2 capture systems. In the absorber, flue gas NOx drives nitrosamine and nitramine formation after its dissolution into the amine solvent. The reaction mechanisms are reviewed based on CO2 capture literature as well as biological and atmospheric chemistry studies. In the desorber, nitrosamines are formed under high temperatures by amines reacting with nitrite (a hydrolysis product of NOx), but they can also thermally decompose following pseudo-first order kinetics. The effects of amine structure, primarily amine order, on nitrosamine formation and the corresponding mechanisms are discussed. Washwater units, although intended to control emissions from the absorber, can contribute to additional nitrosamine formation when accumulated amines react with residual NOx. Nitramines are much less studied than nitrosamines in CO2 capture systems. Mitigation strategies based on the reaction mechanisms in each unit of the CO2 capture systems are reviewed. Lastly, we highlight research needs in clarifying reaction mechanisms, developing analytical methods for both liquid and gas phases, and integrating different units to quantitatively predict the accumulation and emission of nitrosamines and nitramines. [ABSTRACT FROM AUTHOR]

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

Sorptivity is broadly used for characterising the pore connectivity of cementitious materials, with applications in design for durability. A water sorptivity coefficient (WSC) is typically obtained from the ratio between the amount of absorbed water and t 0.5 . This relationship is however not linear for cementitious materials, and conventions are needed for the computation. Variable criteria in the literature complicate the comparison of WSCs. This paper proposes a new approach for describing the entire absorption process. We substantiate the hygroscopicity of calcium silicate hydrates and the effect of swelling during the process as the main causes for the anomalous capillary absorption by cementitious materials. We present a theoretical model with a single descriptive coefficient of capillary absorption progressing linearly with t 0.25 . The model fits remarkably well to experimental data, and it solves the problem of lack of linearity with t 0.5 . A full description of the transport process is then offered. [ABSTRACT FROM AUTHOR]

The paper considers two deterioration mechanisms of concrete, frost-induced damage and carbonation. The objective of experimental study is to investigate how the microstructure degradation of cement-based materials caused by the cyclic water freezing affects the carbonation rate. Influence of water to cement ratio and addition of air-entraining admixture was analysed. The change of cement mortar pore structure due to frost damage was investigated by multi-cycle mercury intrusion porosimetry and capillary absorption test. The carbonation depth was examined using phenolphthalein indication and pH microelectrodes. The experimental results indicate that the ice-induced damage noticeably enhances the carbonation of concrete. Addition of AEA reduces considerably changes of the pore size distribution caused by the frost damage. [ABSTRACT FROM PUBLISHER]

The long-term durability of the wellbore system has become an important topic in recent decades. Specifically in a CO 2 underground storage system, both the ingress of chloride and the leaking of carbon dioxide (CO 2) can lead to severe degradation of the system, including corrosion of steel casing, and carbonation of the cement annulus. In this paper, a model was developed to consider the coupling effect of the carbonation reaction and chloride diffusion in well cement. The stoichiometric model was first applied to calculate the chemical compositions change of the well cement during the hydration process and the carbonation process. The chloride diffusion coefficient of carbonated well cement was calculated based on the general self-consistent (GSC) model and pore tortuosity. After that, the concentration of free chloride ions was predicted based on Fick's law, which takes into consideration released bonded chloride in the carbonated cement. The proposed model was validated based on available experimental data, and a comparison was made between the proposed model and other previously published models. The model has great significance for the safe and reliable geological storage of CO 2. As described in this paper, the model can be used to predict the durability of a CO 2 wellbore system and to inform the design of future systems. • The coupling effect of carbonation and chloride ingress on well cement was investigated. • The stoichiometric model was applied to simulate the phase transformations in the cement during the carbonation reaction. • The chloride diffusion coefficient of carbonated cement was analyzed by using the general self-consistent (GSC) model. • A prediction model of the free chloride ions concentration in carbonated cement was established based on modified Fick's law. [ABSTRACT FROM AUTHOR]

[Display omitted] • A mineral-carbonation-based process is used for the synthesis of nanosized amorphous CaCO 3. • The synthesis of alkanolamine-based DESs with improved CO 2 delivery properties is reported. • DES-H 2 O systems act as inhibitors in the crystallization pathway. • Size- and morphology-controlled synthesis of CaCO 3 nanoparticles is achieved via DES-H 2 O systems. This paper proposes a novel synthesis method for nanosized amorphous CaCO 3 based on advanced mineral carbonation using the macromolecular properties of deep eutectic solvents (DESs). DESs are generally synthesized by mixing hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs), and their properties vary depending on the HBDs and HBAs used for the synthesis. To synthesize DESs with CO 2 -philic properties, monoethanolamine (MEA) was selected as the HBD because it possesses a CO 2 -active site. However, alkanolamine-based DESs are difficult to use in practical applications because of their high viscosity following CO 2 absorption. To mitigate the onset of high viscosity, H 2 O was added to the synthesized DESs, with the molar ratio HBA:HBD:H 2 O as 1:5:10 and 1:5:15. In the proposed process, CO 2 was captured by the DESs-H 2 O system and converted into particle-size- and morphology-controlled CaCO 3. Nanosized amorphous CaCO 3 (ACC) with average particle sizes of 38.1 and 48.0 nm was formed by DESs based on the molar ratio of 1:5:10. The results demonstrated the need for an adequate ratio of H 2 O to DES to form nanosized ACC, implying that the strength of hydrogen bonding in DES-H 2 O systems is the primary factor affecting the performance of the solvent. The DES-H 2 O systems with inter- and intramolecular hydrogen bonding inhibited the crystallization of CaCO 3. The findings of this study are applicable to CO 2 utilization processes designed for the production of nanosized ACC. [ABSTRACT FROM AUTHOR]

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

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]