INCORPORATING WASTE PAPER SLUDGE ASH AS PARTIAL CEMENT REPLACEMENT IN CONCRETE

The disposal or potential recycling of paper mill sludge (a by-product of the de-inking and repulping of paper) constitutes a major economic and environmental problem for the paper and board industry. In the UK approximately 1 million tonnes of paper mill sludge are produced annually. Whilst the main alternatives to landfilling are the use of the sludge as a fuel and its land-spreading as agricultural fertiliser, further outlets for this waste (or by-product) material are needed. The presented research investigated the use of waste ash produced by the incineration of the paper mill sludge, as a partial Ordinary Portland cement (OPC) replacement for concrete production. OPC production is responsible for approximately 10 % of the global carbon dioxide emission caused by anthropogenic activity. Therefore the need to use alternative, less energy intensive cementitious materials for concrete is imminent. In the present study a series of tests were performed on concrete mixes in which OEM-I type cement was partially replaced by Paper Sludge Ash (PSA), in order to identify the best mixes that could also allow for the highest OPC replacements. Mixes were also made in which in addition to PSA other constituent binders such as Ground Granulated Blast Furnace Slag (GGBS), or metakaolin were used to prevent potential deleterious reactions and the production of expansive products by consumption of CaO, leading to concrete unsoundness. The tests investigated a number of fresh and hardened concrete properties including workability, density, compressive and tensile strength as well as the soundness of the resulting cement. The results showed that the ash can partially replace OPC and that when used at modest OPC replacement levels it can actually improve the concrete strength compared to regular OEM-I concrete. It can therefore be concluded that the ash can be used to partially replace OPC with overall environmental advantages (discussed in the paper) and at the same time have a beneficial effect on properties of the resulting concrete.