Intelligent Carbon Reactive Cement Systems Using Dynamic CO2 Absorption During Concrete Hydration

Abstract

Chinenye Elizabeth Onumadu

Cement production accounts for ≈8% of anthropogenic CO2 emissions, yet the material itself could become a carbon sink if CO2 absorption is engineered into its hydration sequence. Here we introduce an intelligent carbon-reactive cement system that dynamically absorbs CO2 during defined curing stages rather than post-hardening. We blended ordinary Portland cement with 20% reactive MgO and exposed specimens to a three-stage CO2 regime: ambient air (0–30 min), 5% CO2 (30 min–4 hr, coinciding with the acceleration period of hydration), followed by air (4–24 hr). Compared to constant CO2 exposure or no CO2, the dynamic protocol achieved 22% CO2 uptake by mass of MgO (vs. <5% in constant exposure) within 24 hours, with no surface carbonation crust or microcracking. X-ray diffraction revealed nesquehonite and calcite formed exclusively during the dynamic absorption window, filling capillary pores (median diameter reduced from 45 nm to 12 nm). Compressive strength at 28 days increased by 12% relative to control, contrasting with constant CO2 exposure which reduced strength by 17% due to diffusion-limited carbonation and crack formation. Pore solution pH remained above 11.5, ensuring steel passivation. The mechanism is attributed to dissolved CO2 reacting with Ca2+ and Mg2+ ions during the percolated but still water-filled capillary network (between initial and final set), generating nanocrystalline carbonates that act as both nucleation sites and pore fillers. This work challenges the paradigm that CO2 is only a durability threat to concrete and demonstrates that dynamic, phase-targeted carbonation can turn cement hydration into a CO2 valorization process. The system is compatible with dilute CO2 streams (e.g., flue gas) and opens pathways for carbon-negative precast concrete and potentially in-situ carbonation tents.