Recycling of clay brick powder to improve the microstructure and mechanical properties of oil well cement pastes at high temperature and high pressure conditions

In oil well cementing involving high temperature and high pressure (HTHP), silica sources are usually incorporated into oil well cement to prevent its strength retrogression. This paper aims to use recycled clay brick powder (CBP), a silica-enriched material, as an alternative to the use of silica flour (SF) to inhibit the strength retrogression of the cement pastes after thermal cycling curing at 300 degrees C/13.0 MPa. Pure pastes (G), blended pastes with 40 wt% SF (40SF) and blended pastes with 40 wt% CBP (40CBP) were prepared to comparative analysis. The results showed that 40CBP and 40SF exhibited lower compressive strength and poorer pore structure than the G samples at 50 degrees C. However, the incorporation of CBP could effectively improve the compressive strength of the cement at 300 degrees C/13.0 MPa compared to that of the G paste. Mineralogical analysis indicated that xonotlite phases were the main crystalline phases in 40CBP and 40SF after thermal cycling curing. Compared to reinhardbraunsite in G samples, xonotlite could improve the pore structure and make the microstructure denser. In addition, there are some hibschite phases in 40CBP, which may make its compressive strength slightly lower than 40SF. Nevertheless, the environmental impact and cost analysis showed that the energy intensity and CO2 emissions of 40CBP were significantly lower than those of G and 40SF, and it had great cost advantages. This study is expected to provide some help in the rational recycling of CBP and reducing the CO2 emissions in cement production.