Effect of calcium-rich additions on the mechanical and microstructural properties of metakaolin-based geopolymer concrete cured in ambient sub-Saharan climate

One of the remaining challenges to extend the field of applications of geopolymers is their relatively weak performance under ambient curing conditions, which could be resolved by the addition of calcium-rich products. However, depending on the nature of these calcium-rich products, their effects may differ from a physicomechanical or microstructural point of view. This study assesses the effect of different calcium-rich additions on the mechanical properties and microstructure of metakaolin (MK) based geopolymer concrete activated by NaOH solution 12 M and cured in ambient conditions of sub-Saharan climate. The MK was partially substituted by different types of calcium-rich additions, namely calcium carbide residue (CCR), quick lime (QLM), slaked lime (SLM), and ordinary Portland cement (OPC). After curing at ambient temperature (30 degrees C) for 7 to 90 days, concretes were tested to evaluate their compressive strength. The microstructural properties were also investigated. The results showed that the substitution of MK by calcium-rich products leads to an improvement in mechanical strength of geopolymer concrete compared to plain MK-based geopolymer concrete. However, earlyage performances differ according to the nature of the calcium product: QLM and SLM have an overall accelerating effect on early-age strength development, while CCR and OPC generally develop strength slowly. Moreover, microcalorimetry analysis reveals that the substitution of MK by calcium products leads to a global decrease in total heat released, regardless of the type of calcium product. Furthermore, it was observed that the exothermic peak was delayed and generally spread out, resulting in more prolonged curing of the pastes and consequently higher mechanical strength. The microstructural analysis shows that the reaction products mainly consist of 2 types of zeolites (Zeolite A and Zeolite X) and low-Ca C-(N)-A-S-H gel. The more heat the calcium-rich product tends to release, the faster the geopolymerization reactions and vice versa. This study demonstrates the feasibility to design metakaolin-based geopolymer concretes at room temperature to reach up to a compressive strength of 30 MPa for engineering applications.