Optimum utilisation of FGD waste in blended binders

The pollution caused by the wastes of semi-dry and dry flue gas desulphurisation (FGD) processes and the increasing cost of disposal make their utilisation imperative. The paper presents the first part of a comprehensive investigation to optimise the use of FGD wastes in concrete manufacture. Batches of FGD materials from actual industrial disposal sites and laboratory simulations based on fly ash and gypsum were investigated, representing the following categories: non-siliceous and deleteriously reactive; non-siliceous and non-reactive; siliceous and pozzolanic active. Generic relationships were derived that emphasise the importance of SO₃ content on concrete properties. There is an exponential relationship between strength and SO₃ content and also between strength and SiO₂ + Al₂O₃. The results show that SiO₂–Al₂O₃–CaSO₄-based FGD wastes with SO₃ < 15% retard early strength but increase long-term strength. Mortars containing fly ash–gypsum (FA–G) blend (25% replacement) with gypsum contents up to 35% (SO₃ content up to 18·37%) do not suffer long-term strength reduction. An optimum FA–G blend has an SO₃ content less than 10%.