Evaluation of alkali surface modification on pyrolysis chars for use as supplementary cementitious materials in mortars

With rising global demand for Portland cement, the construction sector faces pressure to reduce CO 2 emissions and meet net-zero targets by 2050. Traditional supplementary cementitious materials (SCMs) like ground granulated blast-furnace slag (GGBS) and pulverised fuel ash (PFA) are declining due to shifts in steelmaking and energy production. This study explores pyrolysed chars from municipal plastic waste and sewage sludge as alternative SCMs, replacing 2–5% Portland cement in mortars. Compressive strength development was assessed in relation to char mineralogy, physico-chemical properties, KOH treatment, and curing conditions. Pyrolysed plastic chars (PPC) outperformed sludge biochars, achieving strengths comparable or superior to control mortars and supporting hydration of C-S-H, C-A-S-H, and ettringite. Statistical analyses identified pozzolanic reactivity, Ca/pozzolan ratio, and effective w/b ratio as key predictors of strength. While KOH treatment improved some char properties, its effectiveness varied. Further optimisation of feedstock composition and pyrolysis conditions is needed to enhance char performance as SCMs. • Pyrolysed sewage treatment sludge and municipal solid wastes are pozzolanic. • Using up to 5% municipal solid waste char increased strength. • Sewage treatment sludge char addition up to 5% caused strength reduction. • Key variables controlling strength were char reactivity, Ca/pozzolan and effective w/b ratios. • KOH treatment of residue chars increased their porosity and hydrophobicity.