Zeolite formation in the presence of cement hydrates and albite

Zeolite formation caused by interactions between cement hydrates and rock forming minerals was investigated by two sets of batch experiments and supported by thermodynamic modelling. The first set of batch experiments investigated the interaction between calcium silicate hydrates (C-S-H) (Ca0.8SiO2.8 center dot 32H(2)O) and ettringite (Ca6Al2(SO4)(3)(OH)(12)(H2O)(26)) as cement hydrate minerals and albite (NaAlSi3O8) as a rock forming mineral at 20, 50 and 80 degrees C. The dissolution of C-S-H, ettringite and albite led to relatively high calcium and low silicon and sodium concentrations and to the formation of zeolite P(Ca) (Ca2Al2Si2O8 center dot 4.5H(2)O) and natrolite (Na2Al2Si3O10 center dot 2H(2)O). The second set of experiments used ettringite and silica fume as cement phases and NaAlO2 to represent a rock forming mineral. High initial sodium, hydroxide and aluminium concentrations were observed leading to the precipitation of zeolite X (Na2Al2Si2.5O9 center dot 6.2H(2)O) and C-S-H gel at 20 and 50 degrees C where only 40-60% of the silica had reacted after 3 years. At 80 degrees C where more silica fume had reacted, the formation of SiO2-rich zeolite Y (Na2Al2Si4O12 center dot 8H(2)O) and chabazite (CaAl2Si4O12 center dot 6H(2)O) was observed. Solubility products for the zeolite P(Ca), natrolite, chabazite, zeolite X and zeolite Y were obtained from the measured concentrations. Comparison with values published in the literature shows a high variability due to the flexibility of the Si to Al ratio in zeolite structures and underlines the need for systematic experimental determination of the solubility of different zeolites. (C) 2017 Elsevier Ltd. All rights reserved.