About Supplementary Cementing Materials

Supplementary Cementing Materials are materials that when used with Portland cement contribute to the properties of the hardened concrete through hydraulic or pozzolanic activity or both. Typical examples are fly ash, ground granulated blast furnace slag (GGBFS) and silica fume. The benefits derived from the use of SCMs in the cement and concrete industries can be divided into three categories: engineering, economic and ecological benefits.

Engineering benefits

First, the incorporation of finely divided particles into a concrete mixture tends to improve the workability, and reduce the water requirement at a given consistency (except for materials with a very high surface area, such as silica fume). Secondly, in general, there is an enhancement of ultimate strength, impermeability, and durability to chemical attack. Thirdly, an improved resistance to thermal cracking is obtained due to the lower heat of hydration of blended cements and increased tensile strain capacity of concrete incorporating SCMs.

Economic benefits

Typically, Portland cement represents the most expensive component of a concrete mixture, as it is a highly energy-intensive material. Most of the materials that can displace cement in concrete are industrial byproducts and, as such, require relatively little or no expenditure of energy to be made useful as SCMs and cost significantly less than Portland cement. As distance between the source of SCMs and the end user increases, however, high transportation costs may outweigh the potential economic benefits of SCM use. Lack of affordable storage for SCMs can also be a barrier to increased use in some markets.

Future pricing is a difficult prediction. SCM prices may not remain less than cement if the use of concrete continues to grow, in a CO2 constrained world. The value of these commodities will be driven by the market's demand for concrete with a limited CO2 footprint.

Environmental benefits (Greenhouse gas emissions)

The production of every tonne of Portland cement results in the release of a nearly equivalent amount of carbon dioxide to the atmosphere thus whenever a quantity of Portland cement is replaced by SCMs, CO2 emissions are reduced by a similar quantity. Fly ash and silica fume generally do not require energy-intensive processing to be used as SCMs. Slag, however, must be ground; a process that releases about 0.07 tonnes of CO2 per every tonne of GGBFS (ground granulated blast furnace slag) produced. The transportation of these materials to the job site also increases the CO2 emissions related to their use. Studies indicate that transportation (truck and rail) of 1 tonne of SCMs over a distance of 1000 km releases around 0.022 tonnes of CO2. It is necessary to consider a wide range of parameters when determining if the use of SCMs provides environmental benefits over cement in terms of reduced emissions of CO2 to the atmosphere.