4. Conclusions
This report on a pre-industrial hybrid cement with a low clinker content manufactured with an alkaline activator shows that the production of cement with a very low clinker factor and high mechanical strength is feasible. The material in question set and hardened at ambient temperature, giving rise to a binder with higher mechanical strength than required for general purpose cement (UG) under Colombian standard NTC 121, and for class 32.5 cement defined in European standard EN 196-1.Hybrid cement owes its good mechanical strength to the main hydration product, a mix of cementitious gels C-(A)-S-H þ C-A-S-H. These gels take up a larger percentage of aluminium and are more intensely polymerised than traditional C-S-H gel. In hybrid cement a high pre-curing temperature expedites hydration reactions in much the same way as in OPC and alkaline cements. Raising the temperature from 25 to 85 C accelerated the initial reaction in the cement studied, which exhibited 3-day compressive strength values of up to 30 MPa. Nonetheless, the subsequent development of mechanical strength was slower than when the cement was pre-cured at ambient temperature. Initial curing at 85 C for 20 h induced the formation of more polymerised gels (lower ratio) and inhibited ettringite formation (or favoured its swift decomposition), at least during the first 90 days, the maximum age of the materials analysed here. The possibility of delayed ettringite formation (DEF) at later ages in these thermally pre-cured hybrid cements cannot therefore be ruled out.
