Experimental assessment for compressive strength of sludge biochar added mortar

The cement industry is responsible for approximately 5–7% of global anthropogenic CO₂ emissions and significantly contributes to the depletion of natural resources, necessitating the exploration of more sustainable supplementary cementitious materials (SCMs). Biochar, a material gaining attention, is investigated for its potential as a partial cement substitute for mortar in this study. The biochar was derived from fecal sludge feedstock at a fecal sludge treatment plant (FSTP). The raw biochar was subsequently ground and sieved in the laboratory before incorporation into mortar. For the effect of biochar replacement on the curing mechanism of cement mortar cubes, specimens were prepared in accordance with ASTM C 109, with sludge biochar incorporated at 0% (control), 5%, 10%, 15%, and 20% by weight of cement. The specimens were subjected to three curing regimes—water curing, air curing, and sealed curing using polyethylene sheets—and tested for compressive strength at 7, 28, and 56 days. Additionally, the pozzolanic reactivity of the mixes was assessed through the Strength Activity Index (SAI) following ASTM C311. Wet curing is a conventional method to prevent moisture loss in mortar, but incorporating biochar provides a secondary curing mechanism by retaining and gradually releasing water to sustain the hydration process. Results of this study have shown that strengths were comparable to the control up to 10% replacement, with the highest value at 56 days under wet curing (41.66 MPa). Sealed curing achieved over 90% efficiency by achieving a compressive strength of 38.56 MPa, while air curing with 10% biochar (29.36 MPa) outperformed the control (23.40 MPa) by 120%. Pozzolanic activity peaked at 5–10% replacement, with sealed curing yielding the highest SAI (141.6% at 56 days). Beyond 15% replacement, dilution effects reduced performance. Overall, 10% biochar under sealed curing optimized strength and pozzolanic reactivity, confirming its feasibility as a sustainable cement substitute. Regression analysis is also carried out to explore the trend of biochar replacement on compressive strength, found more evident at 5% and 10% replacements across all curing regimes and weakened with higher percentages of biochar. The trendlines exhibited more consistent patterns and stronger correlations, with the highest R² value of 0.86 observed at 10% replacement in air-cured specimens, followed by 0.84 at 5% in the same curing condition. Overall, results indicate a positive influence of biochar on the strength properties of mortar.