Jens Mose and John Terembula, Product Line Management, FLSmidth A/S, introduce the type and availability of supplementary cementitious materials (SCMs) in this first part of a three-part article about the potential to increase the use of SCMs to reduce carbon emissions from cement manufacture. You will find parts two and three in the September and October issues of Indian Cement Review.

While much has been done to reduce emissions from cement production, the calcination of limestone is still a major contributor of harmful pollutants and must be addressed if cement manufacturers are to meet their sustainability targets. Fortunately, a number of supplementary cementitious materials (SCM) are available, which enable cement manufacturers to replace clinker and thus reduce CO2 emissions. However, adoption of these SCMs varies widely on a regional basis. In India and Brazil, for example, it is common to use fly ash and slag to reduce the clinker factor to as little as 65 per cent. In the US, however, the clinker factor remains high at around 95 per cent.
Worldwide, according to the Climate Technology Centre and Network, the ‘average clinker/cement ratio is about 0.81, with the balance comprising gypsum and additives such as blast furnace slag, fly ash, and natural pozzolana.’ The UNEP-sponsored white paper ‘Eco-efficient cements: Potential economically viable solutions for a low-CO2 cement-based materials industry’ suggests a reasonable worldwide average of 0.60 is achievable by 2050.
But how do we get to this figure? With many decades’ experience grinding a wide range of different materials in vertical roller mills, ball mills and hydraulic roller presses, FLSmidth is in a position to share our knowhow to help the industry make strides in reducing its carbon footprint.

Types of SCMs
SCMs include both naturally occurring materials and byproducts of other industrial processes. The Global Cement and Concrete Association (GCCA) groups SCMs according to how they harden : hydraulic SCMs harden in the presence of water (like Portland cement) and include granulated blast furnace slag (GBFS) and burnt shale oil. Pozzolanic materials require the additional presence of dissolved calcium hydroxide (Ca(OH)2) – a byproduct of the hydration of Portland cement – in order to harden. These include fly ash, silica fume, calcined clays, burnt rice husk and natural pozzolans. Hydraulic cements have a higher early age strength, while pozzolans continue to gain strength for a longer period, giving a higher long-term concrete strength. Both have been proven in construction applications. Limestone is not classified as either hydraulic or pozzolanic but also contributes to the hardening of concrete, putting it in an SCM category all of its own.

Table 1: Common types of SCMs

Find parts two and three of this article in the September and October issues of Indian Cement Review:
Part 2: Equipment selection for SCM grinding, Indian Cement Review, September 2023
Part 3: Expanding SCM use in the future, Indian Cement Review, October 2023

1 https://www.ctc-n.org/technologies/clinker-replacement
2 https://wedocs.unep.org/bitstream/handle/20.500.11822/25281/eco_efficient_cements.pdf
3 https://gccassociation.org/sustainability-innovation/health-safety-cement-innovation/clinker-substitutes/