A first approach to the hydrothermal synthesis of zeolite–geopolymer lightweight aggregates (zeopolymers) from construction and demolition waste

A construction and demolition waste (CDW) was studied as a precursor of advanced lightweight aggregates rich in zeolite-geopolymer phases (zeopolymers). Spherical granules of 10 mm were prepared from mixtures of CDW with 0, 25, 50, 75 and 10 wt.% kaolin, which were then fired at 600 °C and finally subjected to hydrothermal treatment (HT) at 90 °C for 24 h with 2, 4, 6, 8 or 10 M NaOH solutions. After the HT, total porosity was reduced (from 38.6 – 44.2 % to 28.0 – 33.8 %), involving an increase in particle density (from 1.45 – 1.60 g/cm 3 to 1.72 – 2.12 g/cm 3 ) and mechanical strength (from 0.2 – 0.5 MPa to 1.9 – 5.4 MPa). Zeolitic phases crystallized mainly as zeolite A (1.3–21.7%), with lower amounts of sodalite (0.3–12.7%) and trace quantities of zeolites X and Y. Adding up to 65% CDW in the formulations permitted to produce zeolites content higher than those of materials made of kaolin only, with the maximum zeolite formation (22.7%) obtained for the mixture containing 37.9% CDW, using 4 M NaOH solution. Formulations containing 100% CDW formed up to 3.8% zeolite and presented enhanced crushing strength (up to 4.4. MPa) when treated with NaOH concentrations higher than 6M. This study provides a foundation for demonstrating the technical viability of valorizing mixed CDW through low-energy-demand processes, resulting in high value-added products for construction and environmental applications, thereby fostering circular economy as well as energy and material savings.