Concrete is already a low carbon product compared to many other common construction materials. The vast majority of cement is used as a binder in concrete; it is mixed with water and aggregates to make concrete. Various options to reduce the carbon footprint of cement are covered in the first three parallel routes presented here, but to further reduce its impact, it is important to look at the whole life cycle, including how cement is used in concrete. Indeed, there are several ways to reduce the environmental footprint of concrete:
Aggregates and recycled materials from construction and demolition waste are relatively low-cost products and average delivery distance is less than 40 km. In environmental and economic terms, local sites serve local markets. Moreover, the price tends to double beyond a delivery radius of about 40 km. In order to keep transport emissions low, aggregates have to be sourced locally.
A theoretical possibility for reduction of CO2 in concrete buildings is use of high-performance cements instead of conventional cements. Use of high-strength cements could lower to some extent the amount of cement needed to make the same quantity of concrete, in spite of the concrete having a higher clinker content.
Extensive research is being conducted to test new combinations of aggregates for high-strength cements, but the cement content itself has a strong influence on the workability and durability of the concrete. The cement paste component causes a dense microstructure and guarantees the alkalinity of the concrete, which prevents corrosion of reinforcing steel. For these reasons, minimum cement contents are defined in regulations and standards for concrete constructions.
In addition to cement, gravel, sand and air, modern concrete contains one or more admixtures. Admixtures are chemicals added in very small amounts to concrete to modify the properties of the mix in its plastic and/or hardened state. Today, approximately 80% of ready-mixed and precast concrete production is modified with a concrete admixture. The quantity of admixture included is usually based on the cement content and for most admixtures are in the range of 0.2-2.0% by weight.
The main sustainability benefits of admixture use are:
By using admixtures to optimise mix constituents, the net improvement in water use and reduction in global warming potential of the concrete can be around 10-20%. In addition, some admixtures are derived from renewable raw material sources, such as corn or wood. In the latter case, the chemicals are produced from a by-product of paper pulp manufacturing, which in the past was considered a waste material sent for disposal.
Stringent testing has shown that admixtures are bound to concrete and do not leach out into the environment during the lifetime of concrete. Testing for admixtures in end-of-life scenarios has shown that even when old concrete is crushed, the admixture leaching rate is so slow that the admixtures biodegrade fast before they can reach significant concentrations in the natural environment.
Finely divided material can be used in concrete in order to improve or achieve certain properties. These materials include virtually inert additions (type I) and pozzolanic or latent hydraulic additions (type II). Additions to concrete are a class of materials that can be considered in the manufacture of concrete although this is very much dependent on the cement type used. The materials that can be used are sometimes the same as those used as constituents in cement manufacturing. The most frequently used additions to concrete are fly ash, a by-product of coal-fired power plants, ground granulated blastfurnace slag (GBFS), a residue of steel production, and silica fume, also known as microsilica. Less commonly used additions to concrete include natural pozzolans (or volcanic ash), metallurgical slag (known as GSCem), and rice husk ash.