Efficiency of a polycarboxylate superplasticizer and low water content in improving the performance and reducing the carbon footprint and cost of cement-wood biochar mortar

Although biochar is a promising admixture, its use can alter the workability and mechanical strength at certain low cement substitution rates. Using a superplasticizer (SP) is an effective solution, but more is required as the biochar content increases, which can increase costs and the carbon footprint. This study evaluated the efficiency of adjusting the SP dosage while maintaining or reducing the water-to-binder ratio (w/b) in order to optimize the key properties of mortars containing cement blended with biochar, compared to standard mortar (100% cement, standard sand and 0.5 w/b). A cost and environmental impact assessment was conducted to determine the potential reduction in CO 2 emissions and cost of optimized mortar mixtures. The results showed that, at a w/b ratio of 0.5, to maintain the same price as the reference mortar, the cost of the biochar, SP dosage and biochar rate should not exceed 53 €/ton, 0.2 wt% and 5 wt%, respectively. Although mortars containing 10 wt% biochar had a similar workability to the reference mortar, they exhibited increased porosity, resulting in a 13% reduction in compressive strength. However, at a w/b ratio of 0.44, the addition of 10 wt% biochar and 0.7 wt% SP, enhanced hydration, and reduced porosity, thereby increasing compressive strength relative to cement content in addition to reducing CO₂ emissions by 15% (per m 3 of mortar). This resulted in a greater strength-to-cost-to-carbon performance ratio than that of the reference mortar (1.45 vs. 1.24 MPa.m 3 /k€.kg CO 2 eq.). The improvement in this performance ratio was observed even in worst-case scenarios, where the price of biochar was assumed to equal that of cement and its global warming potential was considered to be zero rather than negative (1.26 vs. 1.24 MPa.m 3 /k€.kg CO 2 eq.), thanks to gains in environmental impact and compressive strength.