Physico-chemical properties and durability of a fly-ash-based geopolymer

Due to the environmental problems linked to the production of portland cement, the search for new more economic and non-polluting construction materials has become a current issue of interest. Geopolymers represent new types of “polymeric” materials of mineral nature. The aim of this work is to study the influence of the Na2SiO3/NaOH mass ratio and curing time on the mechanical and microstructural properties of fly-ash-based geopolymers GP-Fs. The samples were synthesized with different Na2SiO3/NaOH mass ratios (2.5, 4, and 6) and curing times (2, 12, and 24 h) at 60°C. The GP-F pastes elaborated were analyzed by scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD). The sample with the highest compressive strength and density and the lowest water absorption was produced with a Na2SiO3/NaOH mass ratio of 2.5 and a curing time of 24 h, which is GP1, as confirmed by FTIR and XRD analyses; in addition, it had a compact structure attributed to a higher pozzolanic reactivity. These optimized geopolymer pastes were studied to assess the durability test, evaluating the resistance to fire and acid attack. Fire resistance was assessed by heating the GP-F pastes to 800°C for 2 h, and acid resistance was studied by immersing specimens in a solution of 6% acetic acid (GP-F-CH3COOH) and 6% hydrochloric acid (GP-F-HCl) for 2 months. Physico-chemical and microstructural changes before and after heat and acid exposure were studied using several analyses. Samples exposed to an acidic environment display a completely porous shape with some micro-cracks, inducing a decrease of the residual compressive energy. FTIR analysis showed that the geopolymer gel deformed after heating to 800°C, and pores were formed in their structure due to evaporation of water.