Phase compositions and pore structure of phosphate modified calcium aluminate cement hardened pastes with varied dosages of sodium polyphosphate

Phosphate modified calcium aluminate cement (CAPC) has been demonstrated to be superior to Portland cement under harsh conditions including high temperature and severe corrosiveness. A systematic and quantitative investigation into phase evolution of hardened CAPC pastes cured under 60 degrees C is conducted to lay a technical basis for cementing CO2 storage wells. It is found that increasing phosphate dosage results in decreasing contents of C3AH6 and AH3, the typical hydration products of calcium aluminate cement (CAC), and increasing content of C-A-P-H gel in hardened CAPC pastes. The critical phosphate dosage for complete depletion of C3AH6 lies in 20%-30% by weight of CAC. Furthermore, C-A-P-H gel, known as the characteristic reaction product of CAPC, is proposed to be constituted of Na-substituted nano-hydroxyapatite and pseudo boehmite clusters. Hardened CAPC pastes exhibit remarkably smaller pore sizes due to C-A-P-H gel, explaining their higher compressive strength compared with hardened CAC pastes.