Limestone filler

Adding fine limestone to cement was originally used as a way of reducing costs and raiding production. But it also benefits concrete workability and early strength, while reducing the risk of segregation

Fine limestone has been added to Portland cement for decades, usually by intergrinding crushed limestone with Portland cement clinker in the cement mill. Generally regarded as an inert filler in early literature, it is now understood to make a significant contribution to the way concrete hardens when co-ground with clinker in small amounts. As a result, it can arguably be classed as a supplementary cementitious material (SCM) in its own right.

The first significant use of limestone in Portland cement took place during World War Two, when it was used by several European countries to reduce the cost and increase the quantity of production. Its widespread use today can be traced back to the 1973 oil crisis and to France, which had low production of other SCMs, such as fly ash and granulated blastfurnace slag. In current standards, many countries now allow clinker replacement by limestone in cement.

The benefits of using limestone include improved concrete workability, reduced bleeding, and lower water demand. At levels below 5%, it also improves early strength. Limestone cement is also lighter in colour than ordinary Portland cement.

In cement manufacture, the addition of limestone reduces costs (limestone being a cheaper material than clinker), while its good grindability allows producers to easily increase production capacity without detrimental impact on existing equipment or costly investment in new equipment. Limestone also offer positive synergies with other SCMs, enabling higher clinker replacement rates: for example, in European CEM II M cements, mixtures of limestone and other SCMs may replace up to 46% of the clinker in cement.

Challenges posed by the addition of limestone in cement include potential contamination by clay or organic materials, which have a negative impact of the frost and freeze-thaw resistance of concrete, as well as concrete’s resistance to chemically-aggressive environments. Addition rates above 5% can also lead to lower long-term strengths and raise the risk of steel reinforcement corrosion.