Carbonation-Activated Microstructural Refinement in GUL-GGBFS Blended Mortars: Shrinkage Mitigation and Strength Enhancement

Abstract

Carbonation curing, a potential method for achieving carbon-neutral concrete, enables cement-based materials to react with CO₂ and form stable carbonates. The objective of this study is to investigate the volume stability and microstructural changes of mortars exposed to drying (0.04% ± 0.001%) and accelerated carbonation (3% ± 0.5%). Five mixtures with general use limestone cement (GUL) and up to 80% ground granulated blast furnace slag (GGBFS) replacement were analyzed. Macroscopic properties, including compressive strength and shrinkage, were assessed up to 174 days. Mineralogical composition was analyzed via X-ray diffraction (XRD) and thermogravimetric analysis (TG). Pore structures were investigated using X-ray computed tomography (XCT) and dynamic vapor sorption (DVS). Results indicate that 80% GGBFS reduced 28-day compressive strength by 65.6% compared to 0% GGBFS under drying, while accelerated carbonation compensates for this reduction, increasing 46.9% compressive strength as GGBFS rises from 0% to 40% due to pore refinement from calcite and dolomite formation. Accelerated carbonation increases shrinkage by 49.2% in specimens with 0% GGBFS, whereas incorporating over 40% GGBFS reduced shrinkage by 20.3%. Although carbonation densified the pore structure and limited CO₂ ingress, specimens with GGBFS showed higher carbonation rates attributed to the lower Ca(OH)₂ from cement dilution and pozzolanic reactions. XCT further revealed crack in high-GGBFS mixes (60% and 80%) after carbonation, which critically compromised their strength and durability. This study demonstrates that moderate GGBFS replacement combined with carbonation curing can improve strength and shrinkage resistance while advancing carbon-neutral construction.