Sulfate Resistance Study of Carbonated Low-Calcium Silicate Systems

This paper summarizes the results of sulfate resistance study of carbonated mortar specimens made with Solidia CementÔ (SC) and tested for expansion according to ASTM C1012 specification while exposed to three types of soak solutions: sodium sulfate, magnesium sulfate and deionized water. A control set of ordinary portland cement (OPC) mortars was also evaluated. Besides the length change measurements, visual observations of changes in the appearance of specimens were conducted after various lengths of exposure. In addition, microstructural characterization of the specimens was conducted using scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermo-gravimetric analysis (TGA) techniques. Finally, changes in the concentration of the chemical species present in the soak solutions in contact with the SC specimens were evaluated using both, the ion chromatography (IC) and the inductively coupled plasma optical emission spectrometry (ICP-OES). As expected, the OPC mortar specimens started deteriorating early and reached the critical (i.e.0.1%) level of expansion in about 4 months in case of sodium sulfate solution and in about 6 months in case of magnesium sulfate solution. With respect to the SC mortar specimens, those exposed to magnesium sulfate solution showed higher expansion than those exposed to sodium sulfate solution. However, after 18 months of exposure to both types of sulfate solutions the maximum expansion levels of specimens were still only about 33% of the critical (value. The SEM examination of SC mortar bars indicated that the matrix of the specimens exposed to magnesium sulfate solution showed evidence of formation of gypsum and magnesium-silica compounds. Magnesium and sulfate ions seem to have altered the morphology of the carbonation-generated silica phase and produced gypsum deposits in the air-voids, within the matrix and at the paste – aggregate interfaces. The formation of gypsum in those specimens was confirmed by the results of thermal and XRD analyses. Finally, the ionic analysis of the magnesium sulfate soak solution indicated consumption of sulfate ions whereas the concentration of the sulfates in sodium sulfate soak solution didn’t change during the exposure period

Characterizing the Pore Structure of Carbonated Natural Wollastonite

This paper focuses on examining the pore structure of a cementitious paste made with a calcium silicate (wollastonite) that reacts with carbon dioxide and water to form a hardened solid. The pore structure of the hardened solid has been characterized using vapor sorption and desorption, low-temperature differential scanning calorimetry (LT-DSC), and scanning electron microscopy (SEM). The total porosity was also measured using mass measurement in oven-dry and vacuum-saturated conditions. Evidence exists that support the hypothesis that the solid has two main pore sizes: large macropores (\u3e10 nm) appear to form between the initial calcium silicate particles and small micropores (\u3c10 \u3enm) were found in the reacted silica gel. The bimodal nature of the pore structure was evident from the desorption and LT-DSC responses. The extent of reaction was also investigated and was found to be the result of the function of the raw material particle size: only particles with radius \u3c10 \u3eμm were found to have entirely reacted even in highly reacted systems. Moreover, the degree of reaction influenced the uniformity of reaction across the sample. Only the highly reacted system showed a uniform microstructure with continuous reaction products path and low porosity

Study of Sulfate Attack Resistance of Carbonated Low-Lime Calcium Silicate Systems

The increasing awareness of the impact of cement production on the greenhouse gasses emissions (directly, in the form of carbon dioxide released during decomposition of calcium carbonate in the cement kiln as well as indirectly, through the combustion of fossil fuels) stimulates innovations in development of materials with reduced carbon footprint. One of such new materials, Solidia Cemen™, is a low-lime calcium silica binder that can be produced from the same raw ingredients, and using the same kiln, as ordinary portland cement but at lower temperature (thus requiring less fuel) and at reduced calcium:silica ratio (thus requiring less calcium carbonate). While this low-lime binder is non-hydraulic (and thus it will not harden as a result of chemical reaction with water) it solidifies by the process of carbonation, therefore further reducing carbon footprint. However, in order to determine to what extent such material can serve as a replacement for concrete based on the ordinary portland cement (OPC), a comparative study of durability of these carbonated low-lime calcium silicate systems (CCS) is needed. One of the durability issues facing OPC concrete exposed to sulfate-rich environment (e.g., certain types of soils, sea water, drainage or ground water, etc.) is the potential for an external sulfate attack which can lead to leaching of components from the hydration products, softening of the CSH gel, formation of new reaction products, precipitation and growth of expansive crystallo-hydrates of sulfate salts in free space of the matrix. When continuing over prolonged periods of time, all of these processes ultimately contribute to disintegration of the hydrated cement paste. Despite sizeable amount of previous work on the carbonation of calcium silicates, little data can be found in the literature regarding the potential performance issues associated with the CCS based cementitious systems. Therefore, a specific motivation for the work presented in this thesis was to contribute to the body of knowledge on the sulfate resistance of the CCS materials. The specific topics explored as a part of the work leading to this dissertation included: chemical interactions and kinetics of reactions between CCS and sulfates, the role of chemical and mineralogical composition of calcium silicates, response of the CCS system with respect to the type of the sulfates, verification of the possibility of thaumasite sulfate attack (TSA) in CCS systems, and compositional alterations and damage processes in the CCS matrix resulting from the sulfate attack. The scope of the study included evaluation of four different types of CCS materials, three different types of sulfate solutions and two different exposure temperatures. The main findings from the study indicate that CCS binder type systems are much more resistant to sulfate attack than the based system. However, some matrix alterations were, nevertheless, observed in the CCS-based systems, with the degree of these changes strongly depending on the type of sulfate solution. Specifically, while sodium sulfate did not cause any observable changes, magnesium and aluminum sulfates cased formation of gypsum as a result of decalcifications of calcium carbonates. It was also found that CCS materials that formed stable, crystalline phases were more resistant to sulfate attack

Resistance of Pastes from Carbonated, Low-Lime Calcium Silica Cements to External Sulfate Attack

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