Hydraulic behaviour of calcium sulfoaluminate cement alone and in mixture with Portland cement

The basic component of calcium sulfoaluminate (CSA) cement is C4A3$, able to produce ettringite when hydrated with lime and/or calcium sulfate. Commercial CSA cements generally contain calcium sulfate both included in CSA clinker (as anhydrite) and added to it (as anhydrite or gypsum); lime, if required, can derive from CSA clinker (as free CaO) or hydration of Portland cement blended with CSA cement (as calcium hydroxide). Hydration features and technical properties of a CSA cement, arising from a clinker without free lime, cured from 90 minutes to 180 days, both alone (a) and blended with Portland cement (b), were investigated in this paper. Drying-shrinkage and compressive strength tests were performed and the results were interpreted on the basis of the microstructural data obtained through XRD and DTA-TGA analyses as well as mercury intrusion porosimetry. The phase (a) of the investigation looked at the use of CSA cement as a high-performance binder characterized by rapid hardening and dimensional stability as well as high chemical resistance and reduced shrinkage. The hydration of C4A3$ in the presence of calcium sulfate alone quickly generates, besides aluminium hydroxide, non-expansive ettringite particles developing in the shape of large prismatic crystals responsible for an elevated mechanical strength. The most relevant features of the hydration process are: 1) a high reaction rate and phase stability; 2) a considerable free water consumption and a reduced capillary porosity; 3) a peculiar pore size distribution shifted towards low-porosity regions. The phase (b) of the investigation mainly aimed at floor concrete applications, where both high compressive strength and low drying-shrinkage are needed at early curing times. Blending Portland cement with CSA cement enables to exploit not only the intrinsic contribution of the latter in terms of early strength and reduced shrinkage, but also the shrinkage-compensating effect exerted by microcrystalline ettringite particles, responsible for an expansive behaviour and formed by the C4A3$ reaction with water, calcium sulfate and calcium hydroxide generated by Portland cement hydration. The hydration process of a suitable CSA-Portland cement blend is mainly characterized by: 1) the quick formation of ettringite and its dominant role among the hydration products; 2) the reduced hydration rate of C3S and C3A at early ages; 3) the absence of calcium hydroxide; 4) the more concentrated porosity distribution (within the submicronic region) compared to that of Portland cement.