Accelerated carbonation of C2SH based dense concrete

Abstract In this work the possibility of producing a CO2 hardened concrete where the binder is a calcium silicate hydrate instead of anhydrous silicates was investigated and the technological parameters for obtaining calcite based products, combining Ca2(HSiO4)(OH) (α-C2SH) hydrothermal synthesis and accelerated carbonation curing were presented. α-C2SH was synthesized in a stirred suspension (water to solid ratio 10, 100 rpm) after 16 h at 200 °C from lime-opoka (silica-calcite sedimentary rock) mixture with molar ratio CaO/SiO2 = 2.0. Pressed cylindrical samples from synthesized binder and its mixtures with sand were carbonated in the CO2 environment at 1.5 MPa and at 45 °C from 8 to 48 h. It was shown that α-C2SH is, in fact, a binder that hardens in a carbon dioxide atmosphere. Compressive strength of the samples carbonated for 8 h was ~10 MPa and with extended curing, reached ~35 MPa and ~60 MPa after 24 and 48 h, respectively. By XRD and STA it was found that most of the α-C2SH sample carbonates within the first 8 h of curing. Amount of CaCO3 continues to grow gradually but steadily for up to 48 h. SEM and 29Si MAS NMR indicated that the compressive strength development is rather more reliant on the calcite crystal growth than silica gel polymerization. Concrete products from binder/sand mixture (1:3 by mass) with a compressive strength of ~25 MPa can be produced when carbonation is performed at 1.5 MPa CO2 pressure, 45 °C, and 24 h duration. α-C2SH binder, compared to Ordinary Portland cement, has a significantly lower negative impact on the environment due to lower amounts of calcareous raw materials and the elimination of high-temperature combustion.