Hydrothermal synthesis of α-C2SH and kilchoanite mixture and its application in CO2 hardening mortar

Abstract

In this work, the conditions for the hydrothermal synthesis of kilchoanite–α-C2SH binder from a lime-tripoli mixture were studied, and the effects of the amount of water and binder in the mixture as well as the carbonation temperature and duration on the hardening process of mortar in CO2 environment and its microstructure and properties were investigated. X-ray diffraction analysis, simultaneous thermal analysis, scanning electron microscopy and Fourier transform infrared spectroscopy showed the optimal hydrothermal synthesis conditions for obtaining a high degree of crystallinity C2SH and kilchoanite, while also considering the economic and technological advantages, as follows: molar ratio of CaO/SiO2 mixture = 1.5, temperature is 200 °C, isothermal curing duration is 8 h without stirring. In order to obtain mortars of sufficient strength, it is recommended to use mixtures with a water/binder ratio of 0.3, to press the samples at a pressure of 12.5 MPa and to cure them in a CO2 environment at 45 °C for 24 h. The compressive strength of the samples exceeded 20 MPa. Hence, this binder should be suitable for the production of medium strength mortars. Most probably, when using calcium silicate hydrate as a binder, the diffusion of CO2 from the surface of the particles to their center occurs more quickly than with anhydrous calcium silicate. This binder can be very attractive for several reasons: high temperature synthesis is no longer required; the mixture contains significantly less calcium carbonate needed to prepare the initial CaO–SiO2 mixture; this reduces CO2 emissions from raw materials by a factor of almost 2; during hardening, greenhouse gases are bound into inert CaCO3.