Workability and fresh properties of a low CO2 footprint concrete

Blends of calcined clay - ordinary Portland cement (CC-OPC) provide low CO2 footprint cementitious materials. The effects of supplementing OPC with calcined clay on the fresh properties of concrete, e.g. slump, rheology etc., and the impact that these fresh properties exert on hardened characteristics have not been studied in detail. We present herein a thorough investigation of the impacts that calcined clay addition may have on freshly mixed concrete’s workability and resulting mechanical performance. The research program included size and shape analysis of the OPC and CC particles, determining the slump and measuring rheological parameters such as the static and dynamic yield stress, plastic viscosity and thixotropy of the mixes. We also trialed a simple method for measuring initial water uptake of various cementitious materials (OPC vs CC-OPC blend) to determine its role on reducing workability. Due to particle shape (e.g. sphericity) and the quick initial water uptake of the CC particles, partial replacement of OPC with CC results in greatly reduced workability of fresh concrete. We also show that workability issues can be largely ameliorated with the use of an appropriate rheology modifier during concrete mixing. Furthermore, our findings indicate either a similar or greater (depending on the hydration time) mechanical strength of the CC-OPC concrete compared to that of the pure OPC concrete.