Credit: Holcim (US)
PLC and OPC mixtures without SCMs have basically equivalent initial set and strength gain; with 40% SCMs, PLC mixtures gain strength somewhat more quickly.
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.