{Question} My cement supplier is adding limestone to their cement. Is there anything to worry about? Will I have to reduce the fly ash content I normally use?

{Answer} Recently, ASTM C150 was harmonized to meet the AASHTO M85 to allow up to 5% limestone in the cement. The history of limestone in cement is a little checkered, and as a result, there is some controversy in this decision, along with some confusion and concern.

Here in the U.S. it is long past when finely ground limestone was added to cement to make “adulterated” cement. Although this remains a problem, mainly in the third world where cement is used in bags or sold by unscrupulous middle men, this is not the addition of limestone that is occurring in some markets now.

The added limestone in the U.S. is modern portland cement and consists of ground clinker, a source of readily soluble sulfates and functional additives. The change is that the specification adds up to 5% limestone. The biggest difference in production is that the limestone is added to the clinker blend before grinding. Because the limestone is softer than the clinker it will grind preferentially, resulting in a cement with a better particle size distribution with less energy. The limestone also requires less processing and does not undergo calcination, so it releases less waste in its production.

The resulting cement will perform at least as well as cements that do not contain limestone. The limestone acts as a seed crystal for the cement, better distributing the reaction products and increasing the reactivity of the cement.

As there is always unhydrated cement in the concrete, this change will have no real measurable effect on the use of fly ash. The cement might have a reduced water demand, and the coarser cement gradation will likely improve the efficiency of water reducers. In general, air entrainment will not be affected, nor will viscosity-modifying admixtures.

The change is not all on the positive side though. We have always used the “traditional” specific gravity of cement as 3.15, the specific gravity of the clinker. That is no longer the case, and it might be necessary to reduce this value slightly or errors in yield or unit weight determinations may result. The mill test results that rely on X–Ray Florescence, will need to be adjusted to account for the calcium in the limestone or the theoretical composition will favor C3S and discount C2S.

Still, with the production of portland cement clinker representing a sizeable fraction of the carbon dioxide emissions worldwide and the focus of many politicians and environmental groups, limestone addition is one very good way of reducing the environmental impact of concrete production.

So ask your cement salesman what specific gravity to use, make sure the mill test reports are accurate, and keep making quality concrete.

Kevin MacDonald is president of Beton Consulting Engineers. Contact him at kmacdonald@betonconsultingeng.com or visit www.betonconsultingeng.com.