How do contractors like working with PLC?

Concrete made with PLC has excellent placing and finishing properties. Some contractors report that PLC concrete finishes and pumps slightly better than OPC concrete, not surprising considering the resulting difference in cementitious particle size distribution. Shrinkage and heat of hydration attributes are similar or even slightly improved.

How does PLC work with higher replacement mixes using SCMs?

The setting time of PLC concrete is generally the same as for concrete made with OPC or slightly shorter. But for mixes with supplementary cementitious materials (SCMs, such as fly ash or slag cement) the retardation effects of the SCMs are reduced. Recent research by Dr. Isaac Howard and PhD student Jay Shannon at Mississippi State University has found that PLC mixtures with high replacement levels of SCMs (both 1- and 2-SCM combinations up to about 50% replacement) produced higher strengths at all ages and generally more favorable setting performance than similar OPC mixtures.

What specifications cover PLC?

  • Set times of portland-limestone cement are generally the same as with concrete made with ordinary portland cement.
    Set times of portland-limestone cement are generally the same as with concrete made with ordinary portland cement.
For several years, some U.S. cement manufacturers have supplied PLC containing of up to 15% limestone under ASTM C1157, Performance Specification for Hydraulic Cement. PLC containing from 5% to 15% limestone is now included in the current blended cement specifications ASTM C595-12 and AASHTO M 240-12, Type IL.

Can PLC be used in the same mix designs as OPC?

Yes, and the efficiency of fly ash and slag cement may even be improved. No special admixtures or dosage changes are needed and there is no difference with entrained-air management. There are simply no operational distinctions.

How does PLC improve the sustainability of concrete?

PLC substitution for OPC is the most significant improvement to concrete sustainability with current technology. When OPCs with up to 5% limestone are replaced with PLCs containing 10% to 15% limestone, the resulting impact per million tons of cement produced equates to 443,000 to 664,000 million BTU less clinkering energy used and 189,000 to 283,000 tons reduction of CO2 emissions.

How does PLC affect the economics of a concrete mix design?

There would certainly seem to be the opportunity to reduce total cementitious content in some cases, considering the interaction benefits with certain SCMs as documented in research, or possibly even to SCM replacement of cement. Both of these cases would lower mix cost as compared with OPC mixes designed for the same criteria, given the same cement costs (OPC and PLC).

What is your conclusion of the benefits of PLC?

PLC has the potential to significantly improve concrete sustainability with performance equal to or better than OPC. It can be used seamlessly as a direct substitution for OPC in mix designs, and PLC hydrates with synergies contributed by limestone that enable enhanced setting and strength performance, especially in combination with SCMs.