A mixture of sewage sludge and red gypsum as an alternative material for temporary landfill cover

This study proposed the recycling of sewage sludge (SS) and red gypsum (RG) as potential temporary landfill cover materials. Mixtures with different SS and RG compositions were prepared and tested in determining the most suitable design mix based on the resulting physical, mechanical, and geotechnical properties, namely the hydraulic conductivity, compressive strength, and plasticity. A maximum compressive strength of 524 kPa was achieved for the optimum SS:RG composition of 1:1, corresponding to Ca:Si composition of 2.5:1, which was appropriate to form the calcium silicate hydrate (CSH) gel. The SS and RG compositions did not affect the hydraulic conductivity, which was in the order 10-5 cm/s for all mixtures. Mixtures with RG greater than SS in composition exhibited plastic behaviour due to the Fe content in the RG, which helped minimize the risk of cracking. The optimum mixture had compressive strength greater than the specified minimum of 345 kPa, medium hydraulic conductivity, and moderate plasticity, thus appropriate for application as an alternative material for the temporary landfill cover in the tropics.