Hydration and microstructural development of portland limestone cement-based materials

Abstract

The manufacture of portland cement contributes an estimated 5-8% of all anthropogenic carbon dioxide (CO2) emissions each year. One strategy to curb these cement-induced CO2 emissions is to reduce the amount of clinker in portland cement by replacing a portion with a widely available mineral filler, such as limestone. This research examines the relative influences of the limestone’s composition, blending rate with portland cement, and particle size distribution on the hydration, microstructural development, and long-term durability of limestone-blended cement-based materials. Investigation into the early-age hydration kinetics of portland limestone cements (PLCs) revealed that hydration rate is primarily dependent upon the fineness of the cement, with finely ground PLCs exhibiting increased hydration rates compared to ordinary portland cements (OPCs) as a result of dominant heterogeneous nucleation effects, and more coarsely ground PLCs exhibiting similar or decreased hydration rates compared to OPCs as a result of dominant dilution effects. Experimental and computational studies further revealed that the early-age chemical and autogenous shrinkage of finely ground PLC pastes are increased relative to OPC pastes through a combination of physical and chemical effects, suggesting that concrete produced from finely ground PLCs may be more susceptible to cracking at early ages. Assessment of microstructural development revealed that dilution of the clinker by limestone increases the total porosity of the cement paste matrix, while improvements to particle packing by filler effects reduces the average size of the pores. The net effect is that neat PLC concretes have similar permeabilities to neat OPC concretes. When combined with supplementary cementitious materials (SCMs), chemical interactions between the limestone and the SCMs further reduce the porosity of the PLC concretes, leading to additional reductions in permeability compared to OPC-SCM blended concretes. Overall, the findings suggest that while hydration mechanics are altered in PLC-based materials, their long-term durability is comparable to that of OPC-based materials, provided that the increases to chemical shrinkage do not cause increased cracking at very early ages.