Rheology and alkali-silica reaction of alkali-activated slag mortars modified by fly ash microsphere: a comparative analysis to OPC mortars

The poor workability and the risk of alkali-silica reaction (ASR) caused by alkaline activators restrict the wide range of applications of alkali-activated slag (AAS) cement. This paper presents a study on the effect of fly ash microsphere (FAM) on mitigating the rheological properties and ASR of AAS mortars for the first time. For comparison, the ordinary Portland cement (OPC) mortars blended with FAM are also prepared and investigated. The FAM addition is effective in decreasing the viscosity and meanwhile increasing the fluidity of the AAS pastes and mortars due to its 'ball-bearings' effect. According to the results obtained from the accelerated mortar bar test, the addition of more than 10% FAM reduces the ASR expansion effectively in both of the AAS and OPC mortars. Mineralogical studies demonstrate that for the AAS mortars, FAM promotes the formation of C-A-S-H gel phases with a better alkali binding capacity to mitigate ASR. The pozzolanic reaction of FAM consumes Ca(OH)(2) in the OPC mortars, which is essential to the ASR development. In addition, minor ASR products are observed in the AAS or OPC mortars modified with FAM, and meanwhile there is a more strong bonding between the interface of aggregate-paste.

Automated summary

This study investigated the effect of FAM on AAS mortars for the first time, showing that FAM addition can decrease viscosity, increase fluidity, and effectively reduce ASR expansion. FAM promotes better alkali binding capacity to mitigate ASR in AAS mortars, while consuming Ca(OH)2 in OPC mortars to influence ASR development. Minor ASR products were observed in both AAS and OPC mortars modified with FAM, with stronger bonding between aggregate-paste interfaces.