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 ‘t0,vit’ for the setting of the reaction mixture is derived from isothermal Dynamic Mechanical Analysis experiments. ‘t0,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 Tg of the amorphous geopolymer, nor the shrinkage and expansion around Tg during first heating, cause a measurable heat effect.