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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.