Improving Concrete Sustainability and Performance with Use of Portland-Limestone Cement Synergies

The increased use of portland-limestone cements (PLCs) in the United States is anticipated in response to a new provision that has been added to blended cement specifications for PLCs containing up to 15% limestone. Published research has documented the performance synergies of cementitious mixtures with finely ground limestone (which has particle sizes generally smaller than cement particle sizes), especially in combination with certain supplementary cementitious materials (SCMs). Time of setting and strength development benefits are reported, generally in proportion to limestone fineness. It appears possible to fully develop the potential for these performance synergies in mill-ground PLCs, in which limestone comprises the majority of the finest particles. This study further investigated performance trends observed in concrete with PLC using separately proportioned, commercially ground limestone and ordinary portland cement, as well as cement mill ground PLC samples. The influences of variables such as SCM type and limestone fineness were evaluated with laboratory paste mixtures. Set acceleration increased with limestone fineness for all combinations, including mixtures without SCMs. Strength improvements were clearly evident with all SCMs, more significantly with Class C ash and slag cement than with Class F ash. All strength trends improved as limestone fineness was increased. Consistently enhanced setting and strength performance appear achievable with PLCs. Optimizing particle fineness will be a key factor in achieving these benefits. As performance contributions of SCMs in combination with PLCs may exceed those of similar mixtures with traditional ordinary portland cements, SCM use can be maximized and related sustainability impacts further extended.