Coupled carbonation-hydration mechanisms in high-volume slag-cement binders: Role of carbonation intervention timing

High-volume slag binders and carbonation curing are two promising strategies for reducing the carbon footprint of concrete while maintaining its structural performance. This study presents a laboratory study on carbonation curing of a high-volume slag (70 wt.%) cement-slag binary binder, with particular emphasis on how carbonation intervention timing influences slag reactivity and performance evolution. Carbonation was applied at 10 min, 6 h, and 18 h after mixing, corresponding to different hydration stages. The pozzolanic reactivity of slag was quantified using inductively coupled plasma-optical emission spectrometry, while phase assemblage and carbonation products were characterized using X-ray diffraction and thermogravimetric analysis. Complementary analyses, including Fourier-transform infrared spectroscopy, mercury intrusion porosimetry, backscattered electron imaging, and nanoindentation, were performed to reveal the coupling mechanism between carbonation and hydration. The results show that carbonation markedly inhibits the pozzolanic reactivity of slag, and this inhibitory effect becomes increasingly pronounced with delayed intervention. Compared with the non-carbonated control, the pozzolanic reaction degree of slag decreases by 39.3%, 49.1%, and 76.0% when carbonation is introduced at 10 min, 6 h, and 18 h, respectively. Early carbonation produces mainly amorphous CaCO 3 , whereas delayed carbonation generates highly crystalline calcite that restricts ion migration and slag activation. Although early-age strength is enhanced due to the micro-filling effect of CaCO 3 , long-term strength development is hindered by the suppressed hydration and pozzolanic reactions. This study provides mechanistic insights and practical guidance for selecting appropriate carbonation curing schedules in high-volume slag systems to balance CO 2 uptake, early-age strength, and long-term structural performance.