Reaction mechanism, kinetics and high temperature transformations of geopolymers

The reaction kinetics and mechanism of geopolymers are studied. The dissolved silicate concentration decreases from the beginning of the reaction. A characteristic time ‘t _0,vit’ for the setting of the reaction mixture is derived from isothermal Dynamic Mechanical Analysis experiments. ‘t _0,vit’ increases with SiO₂/R₂O but goes through a minimum for increasing water content. The reaction is slower for K compared to Na-silicate based systems. ²⁹Si and ²⁷Al solution NMR are used to probe the molecular changes. ²⁷Al NMR and FTIR reveal that an ‘intermediate aluminosilicate species’ (IAS) is formed from the start of the reaction. The concentration decrease of OH⁻ during low-temperature reaction is related to the formation of IAS. The rate law of this process seems to be obeyed by a total reaction order of 5/3, with a partial order of 1 for OH⁻ and 0 for Na⁺ in the silicate solution. During first heating after polymerization water is lost leading to a distortion of the Al environment. According to XRD, no crystallization occurs below 900 °C. However, between 950 and 1100 °C a crystallization exotherm of nepheline is observed with DSC for a geopolymer with SiO₂/Na₂O = 1.4. Neither T _g of the amorphous geopolymer, nor the shrinkage and expansion around T _g during first heating, cause a measurable heat effect.