Alkali-activated materials from different aluminosilicate sources: Effect of aluminum and calcium availability

The present study aims to determine the influence of aluminum and calcium availability from two different aluminosilicate sources (COx argillite and sediment), thermally treated at 750 °C, on the formation, the structure and the working properties of the resulting alkali-activated materials. Despite their similar chemical composition, some differences in terms of aluminum coordination, amorphous phase and calcium availability were detected between the two studied aluminosilicate sources. Indeed, sediment sample exhibits higher reactive aluminum and lower reactive calcium compared to argillite sample. Moreover, FTIR spectroscopy has revealed that available calcium interacts with free silicon and aluminum and alkali cation in excess (sodium or potassium) leading to the formation of several networks (SiOAl and SiOCa bonds). For both aluminosilicates, the calcium content (0.33 < Ca/Si < 0.36) seems to be not sufficient to produce C-S-H hydrated phases. It was also demonstrated that the low availability of aluminum in addition to the high availability of calcium favor the precipitation of secondary reaction products in the detriment of the geopolymer network. These products fill the voids inducing a decrease of the porosity and the pore size but also lead to higher heterogeneity that decreases the mechanical properties of the final material. However, the higher availability of aluminum and the lower availability of calcium in presence of high alkaline conditions favor the polycondensation reaction.