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

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 degreesC and 65 degreesC), and fly ash/ slag ratios (100/0, 70/30, 50/50, 30/70, and 0/100). The equations of the m odels describing the mechanical behaviour of these pastes have been establi shed as a function of the factors and levels considered. The ratio of fly a sh/slag and the activator concentration always result to be significative f actors. The influence of curing temperature in the development of the stren gth of the pastes is lower than the contribution due to other factors. At 2 8 days of reaction, the mixture 50% fly ash/50% slag activated with 10 M Na OH and cured at 25 degreesC, develop compressive mechanical strengths of ab out 50 MPa. The nature of the reaction products in these pastes has been st udied 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 th at the fly ash is partially dissolved and participates in the reactive proc ess, even in pastes activated at ambient temperature. The main reaction pro duct in these pastes is a hydrated calcium silicate, like CSH gel, with hig h amounts of tetracoordinated Al in its structure, as well as Na ions in th e interlayer spaces. No hydrated alkaline aluminosilicates with three-dimen sional structure characteristics of the alkaline activation of fly ashes we re formed. (C) 2000 Elsevier Science Ltd. All rights reserved.