Cement substitutes

The production of cement is a significant contributor to global warming. In addition to optimising the energy efficiency of Portland cement production plants, the amount of cement used in concrete mixes can be reduced by using cement substitutes.

The problem with Portland cement

• Cement production is the third ranking producer of CO² in the world after transport and energy generation.

• Cement production is responsible for 7-10% of the world’s total CO² emission and 2% of that produced in the UK (according to the BCA).

• For every ton of cement produced, approx. 1 ton of CO² is produced from chemical reaction and the burning of fossil fuel.

• The UK produces around 12,000,000 tonnes of cement per annum

• Cement production is increasing worldwide by approx. 5% per annum.

Cement substitutes

All cement substitutes have the dual benefit of replacing energy-intensive Portland cement, and of using material that would otherwise be landfilled.

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 byproduct of concrete curing.

Pulverised Fuel Ash (PFA), aka ‘Fly ash’

PFA is a by-product of burning coal in power stations. The ash is removed from the flue gases using electrostatic precipitators.

PFA is routinely divided into 2 classes: ‘Type C’ and ‘Type F’. The main difference is the lime (calcium) content of the ash. Ash of high calcium content is classed as ‘Type ‘F. Both types are routinely used in concrete production.

For most purposes PFA can replace Portland cement at rates of 10-30%, though there have been examples of over 50% replacement.

Ground Granulated Blast-furnace Slag (GGBS)

Ground Granulated Blastfurnace Slag is a by-product of the Iron industry and is produced at the blastfurnace. The molten slag is rapidly quenched in water and then is ground into a fine cementitious powder.

The quality of the iron and the blastfurnace slag are interdependent. The composition of the iron particularly with regard to minor elements is monitored closely by measuring the chemistry of the blastfurnace slag. The process of recovering and producing one tonne of ground granulated blastfurnace slag powder produces approximately 0.1 tonne of CO².

GGBS can replace Portland cement at rates of up to 90%. More usually the maximum replacement level is around 70%. This reduces the CO² emissions of concrete by up to 50%. It is common practice in the UK for ready mixed concrete companies to produce concrete with a cementitious component of 50% GGBS and 50% Portland cement.

Strength

Blast-furnace slag is most like Portland cement and least like a pozzolan. Class F PFA is most like a pozzolan, with Class C PFA between . While stronger and more durable in the end, it takes more time for pozzolans to gain strength than it does portland cement. For most construction purposes, high early strength is very desirable because it allows quicker finishing of slabs and earlier removal of forms. Reducing the amount of water, in part, can compensate for slow strength gain.

Durability

There are two durability conditions that cement substitutes help alleviate:

• Alkali-Silica Reaction (ASR): High-silica aggregates and high-alkali cement (which is becoming more common) can create ASR, which causes internal expansion and crazing of concrete. Cement substitutes, especially slag, remove the alkalinity through pozzolanic action. ‘Class C’ PFA varies in this ability, while ‘Class F’ PFA is very effective.

• Sulfate Attack: Concrete made with 60% or more slag is very effective in mitigating attack by sulfates, found in some arid soils, seawater and wastewater. The pozzolanic action of fly ash also contributes to sulfate resistance.