CEMENT EXPANSION ASSOCIATED WITH ETTRINGITE FORMATION

Interest in cements based on sulfoaluminate and sulfoaluminoferrite clinkers is growing. These clinkers are produced at lower temperatures than Portland cement clinker, which reduces their carbon footprint. Therefore, there is a need to study the hydration processes of cements based on them. Of particular interest is the behavior of these cements under various hardening conditions and the identification of their hydration products, in particular ettringite and iron-substituted ettringite. The aim of this study was to investigate the kinetics of ettringite formation from calcium sulfoaluminate and calcium sulfoaluminoferrite under different curing conditions such as water, humid and dry air. The amount of ettringite formed and the degree of expansion of the samples were also studied. It has been shown that samples hardening under wa-ter hydrate faster than samples hardening in humid air. However, the difference in the amount of ettringite formed is not too great. In contrast, the hydration process in testing samples cured in dry air virtually stopped within a few days. This is due to the loss of a significant portion of free water under these conditions. It was established that the kinetics of ettringite formation in the studied calcium sulfoaluminoferrite samples under conditions of hardening in humid air and water is noticeably slower than during the hydration of calcium sulfoaluminate. The reduced expansion of calcium sulfoaluminoferrite pastes is due to the slow formation of ettringite during hardening. A comparison of the expansion of samples occurring in humid air and in water showed that, with the same amounts of ettringite, the expansion is greater in samples hardened in water, which is due to increased water absorption under these conditions. Thus, it was established that the degree of expansion associated with ettringite depends on the curing conditions of the samples.