Effect and mechanism of nano-Ca10(PO4)6(OH)2 additive on compressive strength of calcium aluminate cement at high temperature

Abstract

Calcium aluminate cement (CAC) is widely used as a binder for refractory materials, and thus the improvement in compressive strength is of vital importance for CAC applied at high temperature. For this purpose, nano-Ca10(PO4)6(OH)2 additive with a ratio of 0.5–1.5 mass% was added with the water-cement ratio to be 0.4. X-ray diffraction and isothermal calorimetry analysis demonstrate that nano-Ca10(PO4)6(OH)2 additive can shorten the hydration process and promote the formation of main hydrates of CaAl2O4·10H2O (CAH10) and Ca2Al2O5·8H2O (C2AH8). In addition, scanning electron microscopy results suggest that nano-Ca10(PO4)6(OH)2 can protect CAH10 and C2AH8 from being destroyed during the calcination, guaranteeing that these thin lamellar crystals are intertwined to form the denser microstructure. Benefited from above effects, nano-Ca10(PO4)6(OH)2 can obviously improve the compressive strength of the CAC mortar samples cured for 7 d after calcination at 1100 °C, while the improving effect is dependent upon its contents. Especially, compared with the one without the additive, the compressive strength of the sample with 1.0% nano-Ca10(PO4)6(OH)2 is increased by 14%.