Abstract
Geopolymers are alternative binders made solely from industrial by-products and/or natural alumino-silicates, comprising no traditional cements. Reactive transport processes in geopolymer materials play a crucial role in both the degradation process of building materials as well as in the containment of hazardous wastes. A numerical model is presented for solving transport coupled to nonlinear ion-exchange equilibria between solid-liquid phases. Bound alkalies provide the geopolymer paste with a large reservoir of exchangeable (soluble) alkalies that allow for a more gradual drop in pH of the pore solution, as compared to a sudden drop when considering only diffusion transport mechanism. The model is robust enough to handle non-linearity of the ion-exchange equations, and presents a more reliable way to obtain long term durability predictions of geopolymer materials.
Highlights
Geopolymer is a sub-group of alkali-activated-alumino-silicate binders subcategorised by a criteria of low Ca content
Bound alkalies provide the geopolymer paste with a large reservoir of exchangeable alkalies that allow for a more gradual drop in pH of the pore solution, as compared to a sudden drop when considering only diffusion transport mechanism
Plain diffusion model is inappropriate to describe the leaching of alkalies from geopolymer materials
Summary
Geopolymer is a sub-group of alkali-activated (calcium)-alumino-silicate binders subcategorised by a criteria of low Ca content (below 5 - 10 mass%) They harden due to formation of AlO4− -SiO4 tetrahedral framework, where alkali or alkaline earth cations (Na+, K+, Ca2+), as well as H+ act as charge balancing species [1] [2]. Diffusive transport coupled with ion exchange reactions in geopolymer materials plays a crucial role in both the degradation process of building materials (e.g. mortar and concrete) [2] and the containing of hazardous wastes. Alkali ions such as sodium and potassium precursors readily leach out from the geopolymer material causing pH reduction and possible corrosion of reinforcing steel in concrete. Simulations were compared against literature measurement data [2]
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