Engineering aluminosilicates with high surface area extracted from waste concrete to form a viable supplementary cementitious material

Aluminosilicate residues can be produced by acid leaching waste concrete. These are pozzolanic and have the potential to be used as a supplementary cementitious material (SCM). However, the workability of cement mixes is seriously compromised when aluminosilicate particles are used, due to their high surface area and excessive water absorption. This negatively influences the compaction and performance of mixes. In this work, aluminosilicates extracted from waste concrete have been heat-treated, and the effects of temperature on their specific surface area, pozzolanic reactivity, and consequently the workability and strength of mortars are reported. Thermal treatment at 900℃ for 1 hour reduces the specific surface area of aluminosilicate particles to values comparable to commercial SCMs such as coal fly ash (FA) and ground granulated blast furnace slag (GGBFS), while retaining significant pozzolanic activity. Mortars containing CEM I replaced by 20 wt% of optimally heat-treated aluminosilicate residue at a 0.56 w/b ratio show improved flow properties compared to untreated residues, and similar compressive strength to mortars containing the same amount of silica fume (SF) or GGBFS. The research shows that aluminosilicates extracted from cement paste can be heat treated to form a viable SCM, and this contributes to the future development of circular concrete. • Aluminosilicate residue from indirect carbonation of waste concrete has high surface area and causes workability loss. • Heat treatment causes sintering of the aluminosilicate and reduces the surface area. • Optimal heat treatment at 900 °C maintains the pozzolanic reactivity of aluminosilicate residue. • 900°C treated aluminosilicate residue gives good mortar flow and comparable strength to mortars with conventional SCMs.