Toward Carbon-Negative Construction Materials: CO2-Storing Alkali-Activated Waste-Based Binder

This study examines the carbonation behavior and CO2 storage potential of a Ca-rich alkali-activated binder produced entirely from industrial residues-ladle furnace slag (LFS), coal ash (CA), and cement kiln dust (CKD). The system was designed as a one-part alkali-activated material (AAM), with CKD acting as an internal activator, and subjected to ambient curing, water curing, and accelerated CO2 curing at ambient pressure. Phase evolution, microstructural development, and pore-structure characteristics were investigated using X-ray diffraction, FTIR spectroscopy, DSC–TG analysis, scanning electron microscopy, and X-ray micro-computed tomography, together with measurements of density, water absorption, and compressive strength. Loss-on-ignition measurements combined with chemical analysis were further used to quantify CO2 uptake and evaluate the degree of carbonation of the binder system. CO2 curing fundamentally altered the reaction pathway of the binder, shifting it from hydration-dominated to carbonation-controlled phase evolution, leading to the decomposition of calcium-bearing hydrates and complete carbonation of non-hydraulic γ-belite with the formation of vaterite, aragonite, and calcite. These transformations induced pronounced microstructural densification, reflected in a near-doubling of compressive strength (>48 MPa), increased apparent density, reduced water absorption, and simplified pore-network topology. A preliminary carbon footprint assessment indicates that the production of 1 m3 of the developed LFS–CA–CKD concrete generates about 14.36 kg CO2-eq, while the carbonation process enables significant CO2 sequestration, resulting in a net negative carbon balance. The results demonstrate that controlled carbonation is an effective post-treatment strategy for waste-derived alkali-activated binders, enabling simultaneous performance enhancement and permanent CO2 sequestration.