Producing cement uses a great deal of energy, so finding a waste product that can substitute for cement makes good environmental sense.
To varying degrees, cement substitutes work in two ways:
• First, they hydrate and cure like portland cement.
• Second they are "pozzolans," providing silica that reacts with hydrated lime, an unwanted by-product of concrete curing.
Pulverised Fuel Ash (PFA), aka ‘Fly ash’ as a cement substitute.
PFA is a by-product of coal-burning power stations. As part of the combustion process, coal is pulverised into a powder before being burned. About 18% of the fuel forms fine glass spheres, about 75% of which rise with the flue gases from the combustion. The ‘ash’ is recovered from the gases and used, amongst other functions, as a cement substitute.
PFA is always used in conjunction with Portland Cement. It is employed in ratios ranging from 80% Pc and 20% PFA – 60% Pc and 40% PFA according to the ultimate function of the cement. PFA can’t be used completely as a substitute for cement, because it relies on the water and lime from the cement to hydrate as part of the overall chemical reaction.
In the future, as coal-burning power stations are phased out, PFA will gradually become unavailable.
Ground Granulated Blast-furnace Slag (GGBS) as a cement substitute
GGBS is a by-product of the iron and steel industry. In the blast furnace, slag floats to the top of the iron and removed. GGBS is produced through quenching the molten slag in water and then grinding it into a fine powder.
Chemically it is similar to, but less reactive than, Portland cement (Pc).
When mixed with water it will hydrate in a similar way to Portland cement. It is always used in combination with Portland cement, typically in the range 60% Pc and 40% GGBS - 30% Pc and 70% GGBS, according to the ultimate function of the cement. Very occasionally, it can be found up to a ratio of 90% GGBS and 10% Pc.
Concrete made with GGBS cement sets more slowly than concrete made with ordinary Portland cement, depending on the amount of GGBS in the cement mix, but also continues to gain strength over a longer period leading to improved overall durability and life expectancy.
Silica fume is a by-product from the manufacture of silicon. It is an extremely fine powder (as fine as smoke) and therefore it is used in concrete production in either a densified or slurry form. Due to economic considerations, the use of silica fume is generally limited to high strength concretes or concretes in aggressive environmental conditions. The most commonly used proportion of silica fume in UK - produced combinations is 10% by mass of total cementitious content.
Limestone fines can be used as a constituent of cement to produce Portland limestone cement. BS 7979  provides additional information on the specification of limestone fines for use with Portland cement. The most commonly used proportions of limestone fines in UK-produced combinations is 6-10% by mass of total cementitious content.
The use of alternative fuels not only diverts waste from landfill and saves on the need for fossil fuels, but can reduce the need for raw materials; for example, the use of waste tyres provides a fuel and minimises the need to add iron-oxide to cement due to wire content.
It is important to note that ECO2 of concrete should not be considered or specified in isolation of other sustainability factors such as strength gain.