Orimulsion Fly Ash In Clay Bricks: Prospects And Limitations
Free (open access)
M. Dondi, G. Ercolani, G. Guarini, M. Raimondo & A. Ruffini
A bitumen-in-water emulsion (Orimulsion) is currently used as a fuel in several thermal power plants worldwide, producing a fly ash rich in S, Mg, V and Ni. In order to assess the feasibility of a recycling in clay brick production, a physico- chemical characterization of ash was performed together with a laboratory simulation of the brickmaking process on two clays, with waste additions up to 6%. Plasticity, extrusion and drying behaviour, shrinkage, water absorption, bulk density, modulus of rupture, pore size distribution, microstructure and phase composition were measured on fired and unfired bricks as well as efflorescence, amount of water soluble salts and their elution, fraction of metals immobilized in the ceramic matrix, and volatile elements released during firing. Orimulsion ash resulted to be fine-grained, highly hygroscopic, consisting mainly of magnesium sulphate, vanadyl sulphates, magnesium and nickel oxides, thus thermally instable in the usual brick firing conditions. It caused some detrimental changes of technological properties of bricks, particularly plasticity, drying sensitivity, porosity and colour. The stabilising mechanism acts through the capture of metals into the crystalline structure of silicates formed at high temperature (pyroxene and melilite). However, a complete reaction of sulphates is not achieved in brick firing and considerable efflorescence and soluble salts are formed, producing a risk of sulphate attack to the mortars. On the other hand, the decomposition of sulphates during firing could bring about remarkable SO, emissions, especially in carbonate-free clay bodies. In conclusion, the disposal of Orimulsion fly ash in clay bricks must be practiced with caution and amounts below 1% weight are strongly recommanded.