Effect of the crystalline state of SiO2 on the compressive strength of cement paste at HTHP

Abstract

Silica flour is often added to oil well cement to prevent the strength retrogression of cement paste at high temperature and high pressure (HTHP). Fused silica, as a non-crystalline form of silicon dioxide (SiO2), has a higher solubility than silica flour during cement hydration at HTHP and has a different effect on the strength stability of cement paste. This paper investigated the compressive strength and permeability of cement pastes incorporating SiO2 with different crystalline state (silica flour and fused silica) cured at 260 °C and 21 MPa for 1, 7 and 28 d. The phase evolution and microstructure of hardened cement pastes (HCPs) were characterized by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS). The result showed that HCPs incorporating 30 % silica flour had the highest compressive strength. 35 % silica flour was excess and α-SiO2 was observed from XRD pattern. Fused silica was more beneficial to the compressive strength than silica flour when their replacement rates were both 25 %. XRD showed that xonotlite was the main crystal phase in HCPs incorporating silica flour or fused silica. In addition, truscottite was identified in HCPs incorporating 35 % fused silica. The transformation of truscottite was not conducive to the pore structure and strength of HCPs. The properties and microstructure of HCPs were influenced by the different phase evolution. Solubility of SiO2 with different crystalline state affected the equilibrium of CaO-SiO2-H2O system of during the early age cement hydration at HTHP.