Influence of metakaolin and calcined montmorillonite on the hydration of calcium sulphoaluminate cement

Calcium sulfoaluminate (CSA) cement-calcined clay blends have shown great potential as new lower CO2 footprint cementitious materials. The influences of metakaolin and calcined montmorillonite on the hydration of CSA cement were studied from aspects of setting time, physical strength and drying shrinkage, while fly ash was chosen for comparison. Meanwhile, hydration heat evolution and the microstructure of hydrates of modified CSA cement paste at early age were determined by means of calorimetry, XRD, SEM and DTA-TG. Results show that both metakaolin and calcined montmorillonite do not contribute to the formation of new kinds of hydrates. They shorten the setting time and reduce the strength, while effectively reduce the cumulative hydration heat within 24 h and inhibit the drying shrinkage of CSA cement mortars. By contrast, calcined montmorillonite has a better shrinkage compensation than metakaolin, owing to that it contributes to the formation of ettringite. It is notable that low dosage of calcined montmorillonite can effectively decline the drying shrinkage of CSA cement mortar, whereas higher content is detrimental to the strength development. It lies in that calcined montmorillonite dramatically accelerates the initial heat evolution rate of CSA cement paste in 0.4 h and leads to a rapid setting. On the contrary, increasing amount of metakaolin prominently reduces the drying shrinkage of CSA cement mortar, with a slight shortening on the setting time.