Evaluating long-term durability of nanosilica-enhanced alkali-activated concrete in sulfate environments towards sustainable concrete development

The primary concern for the durability of ordinary Portland cement concrete is the breakdown of calcium silicate hydrate (C-S-H) formed during hydration when exposed to sulfate environments. The disintegration mechanisms of the binder structure in alkali-activated concrete (AAC) derived from aluminosiliceous precursors may vary, owing to the distinct reaction products formed during the alkali activation process. This paper presents the results of a study conducted to evaluate the long-term performance of alkali activated pulverized volcanic pumice (PVP) incorporating nanosilica (nSi) exposed to sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4). In order to enhance the properties, the PVP was partially replaced with nSi up to 7.50 %. The degeneration of polymeric compounds was examined by conducting scanning electron microscope (SEM) and X-ray diffraction (XRD) on the alkali activated paste (AAP) together with visual examination, loss of weight and strength of concrete over a period of 5-years of exposure. There was about 45.2 % and 27.1 % decrease in strength of OPC concrete exposed to sodium and magnesium sulfate solutions after 5 years, respectively. 100 % PVP activated concrete performed better compared to conventional concrete. Exceptionally, nSi integration from 2.50 % to 7.50 % wt. enhanced the resistance sulfate attack. There was about 5 % decrease in strength was noted in the 5.00 % and 7.50 % nSi modified AAC. The greater decrease in strength of OPC concrete resulted due to decalcification of concrete making C-S-H gel unstable leading to the formation of expansive products needle-like ettringite (Ca6Al2(SO4)3(OH)12·26 H2O) and plate-like (CaSO4·2 H2O) gypsum. The substitution of PVP with nSi improved microstructure thereby resisting sulfate ion penetration. A consistent higher Ca/Si ratio in the nSi modified binder indicated the preservation of the critical C-S-H phase under both sulfate environments. The results show that the PVP-based AAC may well be used towards sustainable concrete development in the aggressive environmental conditions.