Abstract
Geopolymers are synthesized by mixing powdery solids, rich in amorphous silicon and aluminum species, with an alkaline solution, which leads to the formation of an inorganic alumosilicate network. Their acid resistance is affected by the composition, the porosity, and pore size distribution of the hardened binder as well as the type and concentration of the acidic solution. In the present study, two geopolymer mixtures with varying liquid-to-solid ratios and Si/Al ratios were exposed to a sulfuric acid solution (pH = 1) and analyzed after different durations of exposure (7, 14, 28, 56, and 70 days) by using a light microscope and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). SEM-EDX elemental mapping was used to evaluate the degradation from depth profiles of silicon (Si), aluminum (Al), and potassium (K) leaching. The results clearly show the leaching kinetics of potassium and the dealumination of the network. The separate consideration of specific reaction steps in the course of degradation, namely the depth of erosion (DE), the depth of deterioration (DD), and the depth of reaction for certain elements (DR(e)), indicate a combination of chemical and diffusion controlled degradation mechanisms.
Highlights
Geopolymers are synthesized by mixing alumosilicate powders with alkaline solutions
The subsequent polycondensation reaction leads to the hardening of the newly formed alumosilicate network [2], which consists of Si and
Even though in some publications it is claimed that potassium silicate solutions reduced the acid resistance of geopolymers [29,40], compared to metakaolin activated with sodium silicate solution, the low viscosity of the alkaline solution used in this study enabled significantly lower l/s ratios with good processability at the same scale
Summary
Geopolymers are synthesized by mixing alumosilicate powders with alkaline solutions. In the process of geopolymerisation, hydroxide (OH− from the alkaline solution hydrolyses soluble silicon (Si) and aluminum (Al) species from the alumosilicate powder [1]. The subsequent polycondensation reaction leads to the hardening of the newly formed alumosilicate network [2], which consists of Si and. The alkali metals (Na+ , K+ ) from the alkaline solution are integrated into the network to charge balance the negatively charged aluminum tetrahedrons [4]. Smaller proportions of alkalis that are not integrated into the network remain mobile in the geopolymer pore solution [5]. The alumosilicate network is generally characterized by the Si/Al molar ratio, which according to the Loewenstein-rule [6], has a minimum value of 1.0 [7]
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