Coprecipitation of Crystalline Calcium Silicates and Carbonates from the Hydrothermal Reaction of Pseudowollastonite

Abstract

Calcium silicates are abundant but sparingly soluble feedstocks of interest for making low-carbon alternative cements. Under hydrothermal and alkaline conditions, they can form crystalline calcium silicate hydrate (CCSH) products, which are abundant in Roman concrete, or they can form carbonates when CO 2 is present. To understand when coprecipitation of CCSH and carbonate phases is possible, we studied the hydrothermal carbonation of a model calcium silicate, pseudowollastonite (α-CaSiO 3 ), at 150 °C and high pH as a function of CO 2 source [CO 2 (g) or Na 2 CO 3 ] and different concentrations of sodium, alumina, and silica. Our experiments produced a range of CCSH phases including tobermorite−13 Å, rhodesite, and pectolite, as early as 1 day after the start of our experiments. After 7 days of curing in a 2 M NaOH solution, over 10% of the samples had been converted to these CCSH phases. We also observed the formation of CaCO 3 as both aragonite and calcite when carbon was introduced to our experimental system. The carbon source impacted the ratio of the CaCO 3 to CCSH phases in the reaction products. The availability of Na 2 CO 3 produced a balance between the CaCO 3 and CCSH phases, whereas CO 2 (g) produced more CaCO 3 , with samples that were over one-third carbonate by mass. Higher concentrations of Na + increased the precipitation of both the CaCO 3 and CCSH phases. The presence of excess silica, in the form of dissolved borosilicate glass from our reaction vessels under alkaline reaction conditions, also enhanced the formation of CCSH phases formed in some experiments. Supplemental Al 2 O 3 , a common constituent in many silicate feedstocks, also enhanced CCSH formation, likely by forming aluminum-substituted phases under the conditions tested here. These chemical insights can enable the design of formulation and curing guidelines for novel cementitious materials.