Alkali activation of basalt

To further reduce the emission of greenhouse gasses, raw materials that can be directly alkali activated are looked for. In this study the reaction of basalt with concentrated NaOH or KOH solutions to form geopolymer like materials is investigated. A comparison with alkali activated kaolinite is made. The reactivity and the mechanical properties are investigated and a preliminary study into the structure of the material is made. The workability of the mixture basalt/NaOH solution is better, but the reactivity is much lower (DSC study). A wet compressive strength of about 20MPa can be reached, depending on the amount of NaOH added. SEM-EDX and XRD show that not all crystals in the initial basalt react with the alkaline solution. This hampers a deeper study of the reactions going on during polymerization. With XRD it was possible to show that the main reactive phases in basalt are nepheline (NaAlSiO4) and anorthite (CaAl2Si2O8) whereas the main reaction products are hydrated hydroxysodalite and gehlenite (Ca2Al2SiO7). According to the TGA measurements the hydroxysodalite is probably the dihydrate Na8[AlSiO4]6(OH)2* 2(H2O). The results show that basalt can indeed be alkali activated, but it needs a cure temperature of about 80°C (24h).