Recycled wastewater treatment plant sludge as a construction material additive to ecological lightweight earth bricks

Sludge disposal has major drawbacks on the environment when damping wastewater treatment sludge in landfills and estuaries. In addition to financial drawbacks manifested by the poor sludge management due to the lack of valorization regulations or investments’ encouragement. This paper analyzes the mechanical, physicochemical, and thermal properties of earth bricks of unfired type thanks to their low energy demand and high energy savings’ potential. This enables promoting a cleaner production protocol in accordance to the Moroccan testing standards in the building sector. X-ray Diffraction and Fluorescence analysis of the earth and sludge deployed reflected a dominance in Quartz (SiO2) with a respective 59.6% and 28.37%. This affirms the high clayey composition in the used earth material. Various sludge additive percentages (0%, 1%, 3%, 7%, 15% and 20%) to earth material, by weight, are investigated. Higher sludge content in the brick samples’ matrix produced more porous specimens, up to 17%, compared to control samples, of 0% additive content, with 1.04% porosity level. This resulted in higher capillary water absorption coefficient 47.15 ​g/(cm2.min0.5) and lower compressive strength 3.95 ​MPa compared to reference values, of 0% additive, of 25.10 ​g/(cm2.min0.5) and 6.17 ​MPa, respectively. In addition, bulk density analysis classified produced brick samples as lightweight construction materials, following the Moroccan testing standards. This is due to specimens’ respective bulk density does not go beyond the 1.75 ​g/cm3 mark. Improvements in thermal performance were also recorded with 43% and 30% gains in thermal conductivity and specific heat capacity properties, respectively, compared to control samples. Moreover, the incorporation of sludge additive into the clayey earth matrix has shown a decrease in the mixtures’ pH level. This resulted in producing more porous brick samples with improved thermal properties and lower mechanical compressive strength due to deterioration. •Sludge additive to unfired bricks showed good potential as a clean building material.•Various proportions of sludge additives were added to unfired clay bricks.•Lightweight samples with highest porosity and capillarity levels with 20% additive.•Samples’ compressive strength is between 6.02 and 3.95 ​MPa, in the EB3 and EB4 class.•Thermal properties went from 0.51 to 0.29 ​W/m.K for the additive percentage used.