Circular construction: Brick fine fractions from construction and demolition waste as processed hydraulic additives in cementitious materials

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The potential of industrially processed fine brick fractions from construction and demolition waste (CDW) as partial cement replacements in standardized mortar systems was systematically evaluated. Two brick fractions – a dry-processed sieved fraction and a wet-processed fraction – were ground to Blaine fineness values of approximately 4500 cm²/g and 7500 cm²/g and tested as type II hydraulic additives according to ÖNORM B 3309–1:2010. Mortars were prepared with 5–25 wt% cement replacement, and compressive strengths were measured at 7, 14, and 28 days. Microstructural analyses were conducted using mercury intrusion porosimetry (MIP) and energy-dispersive X-ray spectroscopy (EDS). At low replacement levels (5–10 wt%), both brick fractions achieved 28-day activity indices ≥ 90%, and mortars with 5 wt% replacement exhibited compressive strengths comparable to the control specimen. Higher replacement levels (15–25 wt%) caused significant strength reductions due to clinker dilution, while increasing the fineness from 4500 cm²/g to 7500 cm²/g had no measurable effect at high substitution rates (25%). MIP revealed a slight increase in total porosity of 3.4% for 5 wt% replacement with dry-processed materials, and more pronounced increases of 11.5–11.6% for 10–15 wt% replacement. Long-term measurements at 56 and 90 days under constant consistency conditions, achieved by adapting the water amount, showed that 10 wt% dry-processed materials contributed to strength development up to 56 days, indicating participation in long-term hardening processes. These findings demonstrate that industrially processed brick CDW materials can act as reactive type II additives at low replacement levels, achieving compressive strengths compliant with the Austrian standard ÖNORM B 3309–1:2010.