Variation in hybrid cements over time. Alkaline activation of fly ash–portland cement blends

The primary aim of the present paper was to determine the variations over time (one year) in the main cementitious gels forming during the alkaline activation of hybrid cements (70% FA/30% OPC). The 28 and 365 day hydration products were characterised by different techniques. The findings showed that the C-S-H/N-A-S-H mix of gels precipitating did not precipitate in a pure state, but rather that their composition was affected by the presence of dissolved species. In the presence of aluminium C-S-H gel development was: C-S-H → C–(A)–S–H → C-A-S-H, whilst in the presence of calcium, N-A-S-H gel evolved as follows: N-A-S-H → (N,C)-A-S-H → C-A-S-H. This last conversion is not complete in these systems because the amount of calcium present is thought to be insufficient. On the grounds of the findings, a microstructural model is proposed to describe the development of the reaction products forming in these hybrid cements.