Assessement of the impact of sawdust ash on the sulphate resistance of concrete during early strength development

Abstract

Sulfates are detrimental to the structural integrity of concrete throughout its service life. In the project for the construction of the overhead transmission line grid for power evacuation from the Karuma Hydro Power plant, aggressive levels of sulfates were encountered along Karuma – Lira 132kV Transmission Line. This necessitated use of Sulfate Resisting Cement (SRC) in foundations to counter sulfate attack. Sulfate Resisting Cement is twice as expensive as Ordinary Portland Cement (OPC) in Uganda and may only be available on a minimum special order of 200 tons. Based on the above the study was therefore aimed at evaluating the impact of sawdust ash on the sulphate resistance of concrete during early strength development. A total of 384 concrete cubes of 4 designed mixes were used in two different sulphuric acid solutions of differing concentrations to simulate sulphate attack on concrete. Cubes were exposed to sulfate attack for 28 days in sulphuric acid solutions in the early stages of hardening (i.e., at the age of 6, 24 and 72 hours) and in the later stage of hardening (i.e., aged 28 days). SDA dosages used to replace part of the OPC were 0%, 5%, 10% and 15%. It was observed that concrete in the early stages of hardening (i.e., age of 6, 24 and 72 hours) exhibited improved resistance to sulfate attack compared to that in the later stage of hardening (i.e., aged 28 days). Moderate strength concrete grades (i.e., C16/20, C20/25) with design cement content less than 18.1% performed better than the higher strength concrete (i.e., C25/30 and C30/37). The ingress of SO4 2- ions into C20/25-5% SDA concrete followed a decreasing linear function for concrete cured for 72 hours before exposure to sulfate attack. A maximum cement replacement of 10% with SDA as well as partial back filling of foundation concrete after 72 hours of casting is