Effect of waste type and activator content on alkali-activated pumice-slag foam concrete: Evaluation of physical, mechanical, and embodied energy properties

This study investigated the effects of brick dust (BD), rice husk ash (RHA), and hazelnut shell ash (HSA) on the physical, mechanical, and environmental performance of alkali-activated pumice – slag foam concrete. Ground granulated blast-furnace slag was replaced by each waste at 10% by mass, and mixtures were produced at two activator levels while maintaining a constant 10 M sodium hydroxide concentration. Oven-dry unit weight, water absorption, apparent porosity, 7- and 28-day compressive strength, embodied energy, and embodied carbon were evaluated. The results showed that activator content affected each waste differently. Increasing the activator level reduced water absorption and porosity and markedly improved 28-day strength in BD- and RHA-containing mixtures, whereas it reduced unit weight and compressive strength and increased water absorption and porosity in HSA mixtures. HSA at the low activator level achieved the best overall physical and mechanical performance, with a 28-day compressive strength of 25.21 MPa, water absorption of 16.50%, and apparent porosity of 22.96%. Total embodied energy and embodied carbon ranged from 1822.01 to 3691.41 MJ/m³ and from 379.92 to 574.78 kgCO₂e/m³, respectively. The additional sodium hydroxide used for waste pre-activation outweighed the environmental benefit obtained from slag replacement, resulting in higher environmental loads for waste-containing mixtures than for the corresponding waste-free mixtures. Overall, the findings demonstrate that a single activator level cannot be universally applied to different waste-derived precursors and that mixture design should jointly consider pore structure, strength development, embodied energy, and embodied carbon.