Effect of calcium formate on the compressive strength, and hydration process of cement composite containing fly ash and slag

The replacement of Portland cement (PC) by fly ash (FA) or slag will cause low early age strength development of concrete. However, the addition of chemical additives can eliminate the well-known drawbacks above. The effect of calcium formate (CF) on the compressive strength development of ternary cement (containing FA and slag) was studied based on simplex centroid design in this paper. Results show that CF can be safely used to increase the strength of ternary cement composite. In addition, the influence of CF on the hydration process and microstructural features of PC and PC-FA/slag blends was investigated using isothermal calorimetry, X-ray diffraction, thermal analysis, and scanning electron microscopy. These results show that the addition of CF can enhance the strength of PC and PC-FA/slag blends at early curing ages, which is mainly because of a kind of calcium aluminate silicate hydrate called gismondine (CaAl2Si2O8·4H2O) and more formation of ettringite (AFt). Furthermore, at later ages of hydration, CF can promote the silicate reaction of PC and PC-FA/slag blends and accelerate the pozzolanic reaction of FA in order to improve the strength. •Simplex centroid design introduced to interpret the strength development of ternary cement composite with or without CF.•With the addition of CF, a kind of calcium aluminate silicate hydrate called gismondine was formed at early curing ages.•CF would improve the formation of AFt and make the crystal size of AFt smaller.•CF could promote the silicate reaction and accelerate the pozzolanic reaction of FA at later curing ages.