Cement is changing its recipe: what LC3 means for limestone

Limestone is the main raw material for clinker, the core ingredient of cement, and in some cements it is ground in as a powder as well. The industry is now changing the recipe, and that affects how much limestone each tonne of cement needs.

Where the CO2 comes from

A 2019 global inventory in Earth System Science Data put process emissions from cement at 1.50 billion tonnes of CO2 in 2018, about 4% of CO2 from fossil fuels. Process emissions are the CO2 released when limestone is heated and breaks down. Burning fuel to heat the kiln, the paper notes, citing the IEA, could add a further 60% on top.

The process part comes from the chemistry, so a cleaner fuel does not remove it. One of the main ways to cut it is to make less clinker.

Less clinker

Clinker is the hard material made by burning limestone and other raw materials in the kiln. The International Energy Agency’s net zero pathway for cement has the clinker-to-cement ratio falling from 0.71 in 2022 to 0.57 in 2050.

What LC3 is

LC3 is limestone calcined clay cement. A typical recipe is 50% clinker, 30% calcined clay, 15% limestone powder and 5% gypsum. Compared with the 2022 global average of 0.71, that is about 30% less clinker for each tonne of cement.

A Nigerian clay study

A 2023 paper in Materials collected four natural clays from around the Ashaka cement plant in Gombe State and calcined them in a laboratory. All four met the ASTM C618 requirements for natural pozzolans, which are materials that can replace part of the cement. The authors conclude they can replace up to 45% of Portland cement when combined with limestone powder to form LC3, with at least a “Level 1” cut in greenhouse gas emissions. These were laboratory results.

What it means for limestone

LC3 still needs limestone. About half of the mix is clinker made from it, and 15% is ground limestone. Less limestone is burned per tonne, but some is used as powder instead. What this adds up to for limestone demand depends on how fast producers change their recipes, and we have not found a published estimate for Nigeria.