Valorization of industrial siliceous wastes in the production of geopolymers binders

To develop a practical alternative to classic cement materials, alkali-activated materials were manufactured from industrial wastes (fly and bottom ash). Microstructural behavior of these blends was investigated through X-ray diffraction, scanning electron microscope, thermal analysis, Fourier-transform infrared spectroscopy and surface response methodology. Natrolite, phillipsite, and calcium aluminum silicate hydrate and sodium carbonate newly formed after alkali activation. Empirical ternary diagrams as well as modeling equations were united to predict the physico-mechanical evolution of the prepared specimens. The used model described well the evolution of the studied properties against the processing factors; temperature, NaOH concentration and curing time. The weight of the effect of each parameter was estimated and factor to factor interaction was considered. NaOH concentrations and ageing temperature had a considerable positive effect on density and mechanical strength, respectively. Water absorption and electrical conductivity were influenced differently by the factors, the electrical conductivity increases as the NaOH concentrations increase, and time and temperature decrease. The response surfaces and their results were correlated with the microstructure of the hardened samples. Overall, alkali activated binders with good engineering properties could be synthesized in the considered conditions. The optimal alkali activation conditions were ∼12 M and the corresponding final properties were as follows: 50 MPa for the compressive strength and 20% for water absorption while density was recorded at a 1.75 g/cm3 value.