Utilization of water treatment sludge as a partial replacement for fine aggregates in making of solid concrete blocks
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Date
2023-06Author
Muhabuzi, Asiimwe Muhoozi
Giipakrwot, Paul
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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.