Alkali-activated fly ash/slag cements. Strength behaviour and hydration products

Abstract

The activation of fly ash/slag pastes with NaOH solutions have been studied. The parameters of the process studied are: activator concentration (NaOH 2 and 10 M), curing temperature (25 °C and 65 °C), and fly ash/slag ratios (100/0, 70/30, 50/50, 30/70, and 0/100). The equations of the models describing the mechanical behaviour of these pastes have been established as a function of the factors and levels considered. The ratio of fly ash/slag and the activator concentration always result to be significative factors. The influence of curing temperature in the development of the strength of the pastes is lower than the contribution due to other factors. At 28 days of reaction, the mixture 50% fly ash/50% slag activated with 10 M NaOH and cured at 25 °C, develop compressive mechanical strengths of about 50 MPa. The nature of the reaction products in these pastes has been studied by insoluble residue in HCl acid, XRD, FTIR and MAS NMR. It has been verified that slag reacts almost completely. It has also been determined that the fly ash is partially dissolved and participates in the reactive process, even in pastes activated at ambient temperature. The main reaction product in these pastes is a hydrated calcium silicate, like CSH gel, with high amounts of tetracoordinated Al in its structure, as well as Na ions in the interlayer spaces. No hydrated alkaline alumino-silicates with three-dimensional structure characteristics of the alkaline activation of fly ashes were formed.