Synthesis and characterization of geopolymer foam based on construction and demolition waste

Construction and demolition waste (CDW) represents one of the largest waste streams worldwide, and its reuse in high-value applications is a key challenge in the transition toward circular economy. This work demonstrates for the first time that unsorted CDW can be used as the main aluminosilicate precursor (60 wt%) to fabricate geopolymer foams with 74% total porosity, filling a gap in CDW-based geopolymer research, which has so far focused on dense matrices or separated waste fractions. A direct foaming approach was employed using hydrogen peroxide (H2O2) as a blowing agent to generate a macroporous structure. Macro-microstructural, mechanical, gas permeation, and thermal properties of the resulting geopolymer foam were analyzed. The bulk density was about 0.62 g/cm3 with a bimodal distribution in the mesopore to ultra-macropore range, and the compressive strength exceeded 2 MPa. Gas permeability measurements indicated comparable performance to gel-cast foams and honeycomb filters. The foam showed a thermal conductivity of 0.166 W/(m·K) and good thermal stability up to 650 °C, above which sintering occurred leading to a shrinkage of about 15%. These findings demonstrate the feasibility of valorizing unsorted CDW into lightweight geopolymer foams with high porosity, providing a novel and cost-effective route for thermal insulation materials in the construction sector.