Composition-microstructure-property relationships of cement slurries incorporating aluminosilicates under elevated temperatures and pressures

• HPHT curing alters cement phase chemistry compared to hydrothermal model systems. • Tobermorite intermixed with Al-rich amorphous hydration products with temperature. • Mixed Al-based products & mineral-like phases reveal design rules for phase stability. Model systems, such as synthesized calcium (alumino) silicate hydrate (C-(A)-S-H) prepared at target Ca/Si and Al/Si ratios and saturated steam pressure, are ideal for thermodynamic stability predictions. However, they overlook the complexity stemming from the concurrent formation of multiple hydration products as they compete for available sites in cement slurries while setting at high pressure and high temperature (HPHT). Here, we developed realistic formulations incorporating different aluminosilicate sources to capture the heterogeneity of the microstructure as a function of the starting oxide compositions. We performed a comprehensive microstructural analysis to link phase development to the bulk-scale performance. We found that crystalline and amorphous C-(A)-S-H, at a wide range of Al uptake, co-formed and intermixed with mineral-like structures in cement slurries with varying Ca/(Si + Al) ratios after 48 h. These findings highlight the importance of identifying and monitoring early-age hydration products in realistic HPHT cement slurries for understanding the long-term durability of subsurface infrastructure.