Differences in hydration kinetics and phase development of Australian bentonite and kaolinite based limestone calcined clay cements

Abstract

Given the current global shift towards a lower carbon economy, alternative supplementary cementitious materials, such as calcined clays, are expected to have huge role in reducing the embodied carbon of the concrete industry. The performance of limestone-calcined-clay cements (LC³) made with a unique, high purity Australian bentonite clay (CB) and with low-to-medium purity kaolin clays (MK) was evaluated. Reaction kinetics, compressive strength and phase development together with water absorption and sorptivity of pastes were examined after curing for up to 90 days. All LC³ mixes underwent faster hydration reactions than OPC, releasing less heat with distinct events for silicate and aluminate hydration. Compared to OPC, differences in the structure and composition of clay minerals e.g. 50–55% more silica and 2-times more alkali oxides in CB-based mixes and 60–90% more alumina in MK-based pastes resulted in distinctly different phase formation on hydration. For the mix design used herein, the early (3- and 7-day) strength of CB-based LC³ exceeded that of the MK-based LC³ and nearly equaled the control OPC; the 28-day compressive strength of CB- LC³ was also comparable to that of OPC. Greater portlandite consumption as well as ettringite formation was exhibited in the CB-based LC³, while greater amounts of carboaluminate and monosulphate were observed in the MK-based LC³. Water absorption and sorptivity was also lowest in CB-based LC³ and is interpreted to be due to lower connected porosity. The results demonstrate that alternative local clay minerals can be suitable for producing low-clinker cements.