Evaluation of Mechanical and Microstructural Properties of Engineered Geopolymer Composites with Construction and Demolition Waste-Based Matrices

The main purpose of this work was to combine the advantages of increased material greenness, waste upcycling, reduced raw material demand, and the superior characteristics of traditional engineered cementitious composites (ECCs). To this end, engineered geopolymer composites (EGCs) with matrices based entirely on components from construction and demolition waste (CDW) as precursors and aggregates were developed. The CDW-based precursors included roof tiles, red clay bricks, hollow bricks, glass, and concrete. Different combinations of sodium hydroxide, sodium silicate, and calcium hydroxide were used as alkaline activators. Hybridized polyethylene and nylon fibers were used as fibers. To investigate the influences of the additional calcium source, slag-substituted versions of the same mixtures were produced. At the fresh state, Marsh cone and mini-slump tests were performed. At the hardened state, mechanical property tests (compressive strength and four-point bending) and microstructural characterization tests (X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy) were conducted. The findings revealed that, regardless of the mixture composition, all EGCs exhibited a deflection-hardening response coupled with multiple microcracking behavior. The 28-day average ranges for compressive strength, flexural strength, and midspan deflection results were 25.2-42.1 MPa, 6.2-9.5 MPa, and 14.1-28.3 mm, respectively. Slag substitution mostly improved the mechanical performance of EGCs. The main geopolymerization products were sodium aluminosilicate hydrate (NASH), calcium aluminosilicate hydrate (CASH), and C-(N)-ASH gels, the formation of which varied depending on the type of precursor and activator.