The use of china clay waste as a construction material using alkali-activated cement technology

Every 1 t of china clay produced in the UK generates 9 t of waste material. A limited quantity of the coarser waste has beneficial use as a building stone or secondary aggregate in concrete and asphalt, but there are currently limited uses for the finest waste fraction. ‘Mica’ waste is a mixture of fine minerals and is one of the forms of waste with little beneficial use other than the restoration of old quarries. Looking for innovative solutions for the needs of a new Eco-town in the UK and with an aim to find new commercially viable and low environmental impact uses in construction, this PhD introduces the idea of using the china clay waste in alkali-activated binders. First, it was investigated whether the contained mica mineral could be used as an alkali-activated binder. This was done using different MAS NMR and XRD analysis, and mechanical strength tests based on European standards. The material was shown to have low reactivity and some direct trials of alkali-activation of thermally treated mica mineral produced in average weak binders. Later, three series of alkali-activated binders were studied, one based on Ground Granulated Blastfurnace Slag (GGBS), one based on Fly Ash (FA) and one based on a 50%GGBS-50%FA blend (50/50). The china clay waste was incorporated in selected optimum binders from the three series as aggregate to make mortars and concrete. Compared to control specimens, the test specimens using the china clay waste always showed lower strength. It is suggested that the high water demand of the waste is the main problem. When the waste was used in mortars, the impact of the water declined over time, with sand waste from china clay extraction showing approximately equivalent strength to the control silica based mortar after 6 months of curing for the GGBS and 50/50 series. For the same series, mica-waste specimens gave about 50% of the strength of the control mortar on the 7th day of curing but increased to 70-80% of the control compressive strength at 6 months. In the FA series, the impact of additional water resulted in very low strengths and that series did not proceed in concrete making. The concrete design was accompanied by an environmental analysis to ensure environmentally beneficial outcomes were obtained. Testing in compression shows a similar decreasing strength for the Portland and the alkali-activated series for increasing amounts of waste used. However, the test specimens of the GGBS series showed potential for replacing Portland control specimens. The potential for making blocks and tiles using the new concrete would be greater if the mix design is optimized. Durability testing will be required on an optimized design and final product, but initial analysis of test results and the literature indicate this is unlikely to be a concern. Not using the final product to run durability tests would lead to arbitrary conclusions. The environmental analysis shows that although the carbon emissions would be reduced using alkali-activated concrete, most of the other environmental impact categories would be affected negatively which has to be considered when making the final decision on whether to use this new material in the Eco-town or elsewhere.