Increasing the early age strength of High Volume Fly Ash (HVFA) concrete

The construction industry is heavily reliant on production of Portland cement and, in the UK alone, 12 MT of cement is produced per annum. Depending on the specific production processes used, manufacture of 1 kg of Portland cement produces 0.7 kg - 1.0 kg of CO2. Many sources suggest that cement manufacture accounts for up to 5% of the world's CO2 emissions. There is an urgent need for a step change in technology to achieve the radical reductions in carbon emissions necessary to stabilise climate change.
Many different approaches can be used to mitigate the effects of cement production. Considerable improvements have been made with kiln efficiency and waste fuels are now commonly used. However, during the production process, calcium carbonate decomposes into calcium oxide and carbon dioxide. This calcination reaction causes over half the CO2 emissions from the production process so there is limited scope for improvement. A number of initiatives have examined cement alternatives or replacement materials to reduce the Portland cement requirement of concrete. "Novacem" is an innovation from Imperial College London that made significant progress on a radical alternative to calcium-silica based cements based on magnesium oxide produced from magnesium silicates. Although these developments are encouraging, this process would require entirely new plant and with world production of cement at 2.5 billion tonnes per annum, this technology will take a long time to make a significant impact. Other initiatives such as "Ecocem" in Australia are based on cement replacement materials. A considerable amount of research and development of cement replacement materials has been carried out but replacement levels have specified maximum limits in current standards to ensure concrete behaviour does not differ significantly from Portland cement concrete.
Fly ash is a by-product from coal-fired power stations which can be used as a partial cement replacement in concrete. It reacts with calcium hydroxide (produced during hydration of Portland cement) to form stable calcium silicate and aluminate hydrates - the pozzolanic reaction. Fly ash typically replaces 20% - 35% of the cement content within a concrete mix but there are obvious environmental benefits for incorporating higher proportions of cement replacement. However, the pozzolanic reaction between the fly ash and calcium hydroxide occurs quite slowly, which increases setting times and reduces the rate of strength gain of the concrete. This can cause problems associated with surface finishing, delayed removal of formwork etc. which can increase the cost and duration of a construction project. Researchers have consistently found that the higher the proportion of fly ash, the lower the early age strength of the concrete. Therefore, improvement of early age strength of fly ash concrete, particularly when incorporating high volumes of fly ash, warrants investigation.
This project has been developed by Coventry University after detailed discussions with key industry figures representing cement, fly ash and admixture suppliers and concrete users. A comprehensive experimental programme will investigate the use of mineral activators to reduce setting times and enhance early age strengths of HVFA concretes. Cement kiln dust is a by-product of the cement manufacture process and its high alkalinity makes it a suitable activator of fly ash. Waste gypsum is also available in abundance and has been shown to increase the rate of strength gain of fly ash concrete. The aim of this study is to incorporate these by-products into HVFA concrete mixes to give comparable early age performance to equivalent Portland cement concretes. The effect of intergrinding the cementitious materials and activators will also be assessed. Also, a range of fly ash sources will be investigated to account for variations in chemical composition of the fly ash, which have been shown to affect concrete strength.

Environmental impact
The "Climate Change Act 2008" has set a target of 80% reduction in CO2 emissions in the UK (relative to the 1990 emissions) to be achieved by 2050. Production of Portland cement is a major contributor of CO2 emissions, with an industry accepted embodied CO2 content of 930 kg(CO2) / tonne of cement produced compared to 4 kg(CO2) / tonne for fly ash. Therefore, promotion of cement replacement materials such as fly ash in concrete has obvious environmental benefits and increasing the proportion of cement replacement commonly used would help to achieve the 2050 target.
Consider the environmental impact of a construction project per m3 of concrete. If Portland cement concrete is used with say, 300 kg/m3 of cement, the associated carbon emissions would be 280 kg(CO2) / m3 of concrete. Replacing 35% of the cement will give a mix with 247 kg of cement and 133 kg of fly ash (assuming an "efficiency" of 0.4 in accordance with BS EN 206-1), saving just 50 kg(CO2) / m3 of concrete. If the replacement proportion is increased to 70% to give a mix with 155 kg of cement and 362 kg of fly ash, a further 84 kg(CO2) / m3 is saved, giving a total reduction in CO2 of almost half that for the Portland cement concrete. This type of radical reduction would have an enormous impact in the UK and worldwide.

Social impact
The aim of the research is to balance society's needs for cement products for the provision of safe homes and workplaces with conservation of energy and natural resources and minimising environmental impact. High quality research of sustainable construction materials is a huge step towards reaching the government set targets for reductions in CO2 emissions. Sustainable design and construction can facilitate the reduction in CO2 emissions associated with the public and private sectors and domestic use, which impacts directly on society at large.

Economic impact
Energy companies associated with coal fired power stations are likely to benefit from use of high volumes of fly ash. Power stations themselves would benefit from higher ash use as disposal of fly ash is reduced - at present, almost half the fly ash produced is disposed of in landfill. If power stations demonstrate that the majority of this by-product can be reused with minimal energy input, it promotes a greener public image for the related energy companies.
Cement manufacturers who interact with research on cement replacement materials benefit from having a more environmentally friendly public image. Even though reducing the Portland cement content of concrete reduces the throughput requirement of cement plants, most cement manufacturers also produce blended cement products. Coordination between the cement manufacturers and the coal power industry (processing of the ash and blending with Portland cement) allows these blended cement products to be brought to market. Recent improvements in kiln design have attempted to reduce kiln and bypass dust arising but even the most modern kilns still produce significant quantities resulting in disposal problems. Therefore, cement manufacturers also stand to benefit from maximising the reuse of cement kiln dust.
Sustainability is now a major factor at the tendering stage of a project and contractors benefit from being seen to use environmentally friendly materials to lower their carbon footprint. This is particularly true of contractors on high profile projects where fly ash concrete has been used (e.g. Heathrow-Terminal 5 and the Canary Wharf development). Use of sustainable materials allows contractors to earn lower BREAM and CEEQUAL ecopoint scores on construction projects. Also, construction clients place huge emphasis on the carbon footprint of a project and this must be carefully considered by bidding contractors so they are competitive.