Abstract :
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.
