Effectiveness of nano-SiO2 on the mechanical, durability, and microstructural behavior of geopolymer concrete at different curing ages

The invention and development of new binding construction materials to replace conventional Portland cement are now essential from the perspective of environmental concerns. Geopolymers are a potential solution to this problem. Geopolymers are innovative cementitious materials with the potential to replace Portland cement in manufacturing concrete composites. Nanomaterials offer novel features and performances to geopolymer composites by enhancing the composite's microstructural characteristics by forming extra calcium-silicate-hydrate (C–S–H), sodium-alumino-silicate-hydrate (N–A–S–H), and calcium-alumino-silicate-hydrate (C–A–S–H) gels, as well as the filling nano-pores in the matrix. In this study, extensive experimental laboratory works have been conducted on around 250 geopolymer concrete (GPC) specimens to investigate the effects of adding different dosages (1, 2, 3, and 4%) of nano-silica (NS) on the fresh, compressive strength, splitting tensile strength, flexural strength, stress–strain behaviors, modulus of elasticity, water absorption, rapid chloride permeability, resistance to an acidic environment, and microstructural properties like scanning electron microscopy (SEM) and X-ray diffraction (XRD) of geopolymer concrete composites. As a result of the addition of NS, it was found that the largest improvement in compressive strength was occurred at 3% NS, which was 6.3, 13.4, 20.5, 21, and 21.9% at 3, 7, 28, 90, and 180 days, respectively, compared to the control GPC mixture. Also, the maximum improvement in water absorption was nearly similar for 2 and 3% of NS content, which was 32.2 and 38% at 28 and 90 days, respectively, compared to the control GPC mixture. Furthermore, according to SEM observations, the addition of NS improved the microstructural characteristics of the GPC specimens due to the formation of additional geopolymerization products, as revealed by XRD analyses. However, the fresh characteristics of the geopolymer concrete mixtures are reduced due to the addition of NS to the GPC mixtures.