The concrete industry has been getting beat up for the past decade over the large carbon footprint of cement, and it’s true that cement manufacturing releases a significant amount of carbon dioxide into the environment. So if cement manufacturers come up with a way to improve concrete sustainability with no negative impacts on constructibility, performance, or durability, why would we resist using it? We in the construction industry are justifiably cautious with new materials, but the evidence is in and there is no reason not to adopt portland-limestone cements (PLC) with open arms.

One of the strongest advocates for PLC over the past few years has been Tim Cost, senior technical service engineer with Holcim (US). We caught up with Cost recently to learn more about PLC.

So what is portland-limestone cement?

PLC is a slightly modified version of portland cement that improves both the environmental footprint and potentially the basic performance of concrete. It is now described in ASTM and AASHTO specifications and is used just like traditional portland cement in mix designs. It can be made at any portland cement manufacturing plant. While ordinary portland cement (OPC) may contain up to 5% limestone, PLC contains between 5% and 15% limestone.

How is it made, and what’s different about it?

A metered proportion of crushed, dried limestone is fed to the finish grinding mill, along with clinker and gypsum. In almost all cases, the limestone used in PLC is the same limestone used as a raw material for cementmaking at that plant. This makes sense not only because the supply is relatively unlimited and low in cost, but because most cement plants use a limestone raw material that is more conducive to grinding than, say, dolomitic limestone or related mineralogies that are very hard and would be more expensive to grind. The limestone is more easily ground than the clinker, which is harder, and becomes concentrated in the finest particles.

Overall fineness must be higher (for equivalent performance) for fineness of the clinker fraction to be similar to OPC. This means the production rate is slowed and some additional grinding energy is required. This is more than offset by the lower clinker content and related kiln fuel savings. Hydration is enhanced by both physical and chemical interaction and greater overall cementitious efficiency is possible. Finally, the sustainability benefits are significant via reduced associated carbon emissions and embodied energy (less clinker).

How is increased hydration efficiency possible?

The physical mechanisms include enhanced particle packing and paste density due to the enhanced overall cement particle size distribution and the “nucleation site” phenomenon, where small limestone particles are suspended in paste between clinker grains and become intermediate sites for calcium silicate hydrate (CSH) crystal growth which improves efficiency.

The chemical mechanisms include that limestone contributes calcium compounds that go into solution and become available for hydration interaction, and that the calcium carbonate reacts with aluminate compounds to produce durable mono- and hemi-carboaluminate hydrate crystals.

How does PLC affect concrete properties?

Fresh concrete effects are all favorable (though slight). There is no difference in water demand or slump loss. Even though PLC is finer than Type I or II OPC, all of the extra fineness is due to fine limestone particles which are relatively inert in fresh concrete.

What is the impact on durability?

Extensive durability testing has been done by Holcim (US) and many others with generally favorable results. PLC performance is essentially equivalent to that of non-limestone cements from the same plants in terms of shrinkage, permeability, freeze-thaw resistance, and resistance to salt scaling, with even slight enhancements in some cases.


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?

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.

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