Evolution of Unconfined Compressive Strength and Microstructural Properties of Geopolymers From Blended Waste Concrete Powder and Metakaolin

The use of waste concrete powder (WCP) in geopolymer synthesis offers a sustainable method for recycling construction and demolition waste (CDW). However, its low reactivity and high calcium content frequently result in geopolymers with inadequate mechanical strength. To overcome this challenge, this study proposes a novel method that involves the synergistic addition of metakaolin (MK) to improve the performance of WCP‐based geopolymers. The study systematically examines the effects of NaOH concentration (6, 10, 14 M) and MK content (0%–100%) on the unconfined compressive strength (UCS), microstructure, and reaction products. The findings indicate that the inclusion of MK significantly improves the UCS. An optimal blend of 75% MK and 25% WCP, activated by a 14 M NaOH solution, achieves a peak strength of 46.84 MPa, which is considerably higher than that of geopolymers made from either precursor independently. Microstructural analyses (X‐ray diffraction [XRD], scanning electron microscopy/energy‐dispersive X‐ray spectroscopy [SEM/EDS], and Fourier‐transform infrared spectroscopy [FTIR]) confirm that the strength improvement is due to the formation of a denser and more uniform matrix. This is characterized by the cohesive coexistence of sodium aluminosilicate hydrate (N–A–S–H) gel from MK and calcium (aluminate) silicate hydrate (C–A–S–H) gel from WCP. The innovation of this study lies in demonstrating the effective synergy between a highly reactive aluminosilicate source (MK) and a low‐reactivity waste material (WCP), transforming a potential performance inhibitor (calcium) into a strength‐enhancing element. This study presents a practical and efficient strategy for converting waste concrete into valuable geopolymer materials, offering significant environmental advantages and fostering sustainable development within the construction sector.