Utilization of water treatment sludge as a partial replacement for fine aggregates in making of solid concrete blocks

Abstract

Sludge production at the Ggaba III water treatment plant was approximated at 113,000 metric tons/year where by the means of its disposal was damping in nearby wetlands posing a significant risk to native ecosystems. This, coupled with an increase in demand for sand brought about due to the continued growth of Uganda’s construction sector, continues to pose significant environmental impacts to local ecosystems. Finding sustainable and effective ways to manage this waste material and decrease the rate of sand mining was crucial to alleviate these detrimental impacts. There have been studies carried out on the use of waste materials such as silica fume, blast furnace slag, and fly ash as partial replacements for sand as a fine aggregate in the concrete industry have yielded commendable results. The aim of this research was to determine whether Water Treatment Sludge (WTS) can be utilized in a similar way. This report presents results from the utilization of water treatment sludge as a partial replacement for fine aggregates in the production of concrete blocks. The evaluation of the physical and chemical characteristics of the WTS was essential to determine whether the sludge could be viable as a partial replacement for fine aggregates in the manufacture of concrete blocks. This was done by determining the particle size distribution (PSD), fineness modulus, specific gravity, bulk density and elemental composition of the sludge by XRF spectrometry. In addition, the effect of the sludge on cement hydration was determined by evaluating the chemical shrinkage using the dilatometry method in accordance to ASTM C1608. Compressive strength tests were carried out on mortar cubes made with 0%, 5%, 10%, 15%, 20%, and 30% sand replacement with WTS in order to obtain the ideal sand replacement with WTS before determining that of the concrete blocks. A comparative study on the difference in GHG emissions released during the production of both the WTS and the mining of sand was done to assess whether the use of the sludge had a possible environmental benefit as opposed to the use of sand. The results obtained indicated that the WTS had a lower bulk density than the sand which resulted in lighter mortar cubes and concrete blocks. The WTS contained less silica (3.9%) than sand (98%) failing to conform to US_EAS 148-1 2017. The WTS retarded the rate of cement hydration with each increase in the percentage of WTS resulting in lower values of chemical shrinkage. Compressive strength tests on the mortar cubes showed that 5% sand replacement had delivered the best results out of the other replacements. The compressive strengths of the concrete blocks made with the optimum 5% replacement as determined from analysis of the mortar cubes were below the 10 MPa limit prescribed in US 2032-2019 for non-loadbearing concrete blocks, signifying that the WTS did not have adequate physical and chemical properties to result in adequate strength. The determination of the Greenhouse Gas (GHG) emissions from sludge production and sand mining in Nsonga sand mines showed that the emissions from sludge production exceeded that of sand mining for that particular mine.