Effect of alkali on the synthesis of single phase gyrolite in the system CaO-quartz-H2O

The influence of ammonium and Li, K, Rb, Cs hydroxides on the synthesis of gyrolite in the system CaO-quartz-H2O system were investigated. Mixtures of CaO and quartz (CaO/SiO2 =0.66) with different amount of alkali (0.50%, 0.75%, 1.00%, 1.25%, 2.00%, 5.00% as molar Na2Oeq corresponding to the dry substrates mass) were treated hydrothermally at 200 degrees C for 24 h. Phase composition of prepared samples was characterized by XRD and was confirmed by DTA and IR analyses. It has been found that the content of hydroxides is crucial for gyrolite crystallization. The results indicate that the optimum amount of hydroxides is in a narrow range between 0.75% and 1.25% of Na2Oeq for RbO and CsOH and from 0.50% to 2.00% for KOH. Significant changes in phase composition were observed in samples in which hydroxides were added in the amount of 5.00%. The maximum dosage level of hydroxides promotes the formation of poorly crystalline C-S-H and prevents the formation of gyrolite. The paper provides some important insights into the role of alkali hydroxides in the synthesis of gyrolite and the route of hydrothermal reactions in the CaO-SiO2-H2O system. (C) 2019 Elsevier Ltd. All rights reserved.