Durability of geo-polymer mortar with respect to alkali silica reaction

Abstract

Despite been a highly consumable material, OPC is labelled as a less sustainable and less eco-friendly material. Geopolymer cement is considered a possible alternative to OPC. But, geopolymer possesses a high risk of ASR due to its high initial alkali content. However, though fly ash based geopolymers have performed satisfactorily even with reactive aggregates use of GGBFS seems to increase ASR risk in geopolymer binders. Since GGBFS governs early age properties of geopolymer concrete, optimisation of GGBFS is essential in the commercial adaptation of geopolymer concrete. This research is focused on identifying the ASR mechanism in geopolymer and the role of GGBFS in it. In this study, three main geopolymer mixes (fly ash/GGBFS ratio 9, 4 and 1) along with the standard OPC mix were used to assess four aggregates. Two subsidiary mixes (fly ash/GGBFS ratio 9, and 1) were used to analyse the effect of mechanical properties on the expansion. Mortar bars were exposed to 1M NaOH at 80 0C. In addition, three curing conditions: 0.03M NaOH, 1M NaOH saturated with Ca(OH)2 and water at 80 0C were adopted to assess the effect of curing solution on the ASR. SEM-EDS analysis was performed on each mix at 21 days and 150 days to identify the ASR gel formation. Si dissolution test was carried out on the reactive aggregates to assess their Si providing capability and factors affecting it. The experimental data articulate the low ASR potential of geopolymer compared to OPC. Final expansions of geopolymer mortar bars suggest that ASR increases with the GGBFS content. In addition, both expansion and microstructural analysis results emphasise the importance of Ca in deleterious ASR. Expansion results of mortar bars exposed to 1M NaOH saturated with Ca(OH)2 illustrate that even external Ca can induce ASR in geopolymer mortar. In addition, SEM-EDS analysis identified the formation of Al-rich ASR gel in geopolymer mortars which required further analysis. Experimental results suggest that short term tests might not be effective in identifying the ASR in geopolymers. It is recommended to revise existing parameters in the accelerated mortar bar test before adopted in geopolymer.