Enhancing fluidity and mechanical properties in Limestone Calcined Clay cements with one-third Portland clinker content

Abstract

Limestone Calcined Clay Cements (LC3) are attracting considerable attention due to their low CO2 footprints and relatively fast hydration kinetics. LC3-50 formulations (approximately 50% Portland clinker, 30% kaolinitic calcined clay, 15% limestone and 5% gypsum) are being extensively investigated and the hydration chemistry, rheology, mechanical strength developments and durability performances are starting to be well established. LC3 binders with lower clinker factors are possible, offering an opportunity for further CO2 footprint reduction. However, these blends have been much less studied. This work contributes to fill this gap. Here, a LC3-35 binder (i.e. 35% Portland clinker content) is investigated and its performances are compared to those of the original Portland cement (PC) and a standard LC3-50 binder. The effects of two superplasticizers and a strength-enhancing admixture are thoroughly investigated in mortars and pastes. Larger calcined clay contents result in an exacerbation of the slump retention problem, but this issue is solved by the use of a new superplasticizer. Moreover, the low mechanical strengths at early ages can be partly mitigated with a C-S-H nucleation seeding admixture. Employing the right admixtures, LC3-35 binders can develop strength values similar to those of neat PC and LC3-50 at 7 and 28 days. The laboratory X-ray powder diffraction results for LC3-35 pastes, when using the strength-enhancing admixture, show that C4AF exhibits an accelerated reaction rate, leading to the formation of larger amounts of AFt and AFm. Approximate mass balance calculations are carried out to estimate the metakaolin reaction degree and an environmental performance indicator is used to place the footprints of the reported binders within the LC3 landscape.