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This paper presents the results of a study on the evaluation of resistance of pastes from carbonated, low-lime calcium silica cements to external sulfate attack. The extent of chemical interaction between sulfate solutions and paste powders was assessed by quantifying the amount of species that leached out from carbonated pastes using ICP-OES and IC techniques. In addition, the loss of carbonates from the carbonated pastes exposed to sulfate solutions and the corresponding amounts of gypsum formed were also monitored by using the TGA and QXRD techniques. The changes in the structure of silica gels were evaluated using FTIR analysis. The results of this study revealed that the level of resistance of carbonated, low-lime calcium silicates to external sulfate attack was affected by the degree of crystallinity of calcium carbonate, the type of calcium silicate, and the type of cation present in the sulfate solution. [ABSTRACT FROM AUTHOR]

Highlights • CSC can store up to 18% of CO 2 (by wt) in the form of CaCO 3 during carbonation. • Reaction rate of CSC was highest for water to cement ratio of 0.4. • Free moisture is essential for the carbonation reaction to progress. • The activation energy for carbonation of CSC was in the range of 44 to 57 kJ/mol. Abstract CO 2 cured calcium silicate cement (CSC) can substantially improve the sustainability of concrete-like materials when used as an alternative to ordinary portland cement (OPC). CSC is produced from the same raw materials as the OPC but at kiln temperatures around 250 °C lower than those required for the production of the OPC. This paper presents a summary of the results from a comprehensive study on the evolution of microscopic phases, reaction kinetics, and strength development in CSC paste and mortar samples during carbonation. The primary carbonation products of CSC are calcium carbonate and Ca-modified silica gel. All three polymorphs of crystalline calcium carbonate (i.e. calcite, aragonite, and vaterite) were found to be present in carbonated CSC paste, calcite being the most abundant polymorph. Influences of water to cement ratio (w/c, by weight), temperature, relative humidity, and CO 2 concentration on the carbonation rate constants of CSC have been studied here. The carbonation rate constants of CSC were found to be the highest for w/c of 0.4 in a 99.9% CO 2 environment. The carbonation activation energies of these systems varied from about 44 kJ/mol to about 57 kJ/mol, depending on the carbonation curing conditions (i.e., w/c ratio, CO 2 concentration, etc.). The CSC mortar samples achieved compressive strength as high as 40 MPa after only 3 days of carbonation. [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]

This article features an investigation of the accelerated carbonation products of four calcium silicate phases, namely monoclinic-tricalcium silicate (3CaO·SiO 2 or C 3 S), γ-dicalcium silicate (2CaO·SiO 2 or γ-C 2 S), tricalcium disilicate (3CaO·2SiO 2 or C 3 S 2 , rankinite), and monocalcium silicate (CaO·SiO 2 or CS, wollastonite). During the carbonation reaction, the calcium silicate minerals form calcium carbonate and Ca-modified silica gel. These reaction products were examined using thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), 29 Si magic angle spining (MAS) nuclear magnetic resonance (NMR), 13 C { 1 H} cross polarized (CP)/MAS NMR, and dynamic vapor sorption (DVS) techniques. Along with the crystalline forms of CaCO 3 (i.e., calcite, vaterite, and aragonite), amorphous calcium carbonate (ACC) was also found to be present in the carbonated calcium silicate systems. Based on the experimental results, it is proposed that the ACC particles were stabilized by the deposition of silica layers on their surfaces. Presence of ACC also affected the pore size distribution of the matrixes, which eventually influenced the diffusion based carbonation rate of the matrixes. Moreover, the degree of carbonation of CS was found to be most effective compared to other calcium silicates in terms of the percentage of the phase reacted for the experimental conditions used in this study. [ABSTRACT FROM AUTHOR]

Non-hydraulic calcium silicate-based cement (CSC) is a newly developed binder that hardens upon carbonation reaction. The primary components of CSC are low-lime calcium silicates, including rankinite, wollastonite and pseudo-wollastonite. During the carbonation, CSC binder forms Ca-modified silica gel and CaCO 3 . Microscopic evaluation of the carbonated CSC paste using the 29 Si { 1 H} CP-MAS NMR revealed that the Ca-modified silica gel phase consists primarily of Q 3 and Q 4 species, indicating a substantially higher degree of polymerization in comparison to that of the C-S-H. The elastic modulus and hardness of this gel phase were found to be close to those of the high density C-S-H. A composite phase, formed by the intermixing of Ca-modified silica gel and CaCO 3 , was also identified using SEM X-ray microanalysis and nanoindentations. The distribution of the microscopic phases in ITZ was similar to that observed in the bulk paste region for this system. [ABSTRACT FROM AUTHOR]

This paper presents a study on the carbonation behaviors of hydraulic and non-hydraulic calcium silicate phases, including tricalcium silicate (3CaO·SiO or CS), γ-dicalcium silicate (γ-2CaO·SiO or γ-CS), β-dicalcium silicate (β-2CaO·SiO or β-CS), rankinite (3CaO·2SiO or CS), and wollastonite (CaO·SiO or CS). These calcium silicate phases were subjected to carbonation reaction at different CO concentration and temperatures. Thermogravimetric analysis (TGA) tests were performed to quantify the amounts of carbonates formed during the carbonation reactions, which were eventually used to monitor the degree of reactions of the calcium silicate phases. Both hydraulic and non-hydraulic calcium silicates demonstrated higher reaction rate in case of carbonation reaction than that of the hydration reaction. Under specific carbonation scenario, non-hydraulic low-lime calcium silicates such as γ-CS, CS and CS were found to achieve a reaction rate close to that of CS. Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscope (SEM) tests were performed to characterize the carbonation reaction products of the calcium silicate phases. The FTIR spectra during the early stage of carbonation reaction showed formation of calcium silicate hydrate (C-S-H) from CS, γ-CS, β-CS, and CS phases with a similar degree of polymerization as that of the C-S-H that forms during the hydration reaction of CS. However, upon further exposure to CO, these C-S-H phases decompose and eventually converted to calcium-modified (Ca-modified) silica gel phase with higher degree of silicate polymerization. Contradictorily, CS phase started forming Ca-modified silica gel phase even at the early stage of carbonation reaction. This paper also revealed the stoichiometry of the Ca-modified silica gel that formed during the carbonation reaction of the calcium silicate phases using the SEM/energy dispersive spectroscopy (EDS) and TGA results. [ABSTRACT FROM AUTHOR]