Selective CO₂ sequestration in cement using HMDA: preserving structural integrity while enhancing carbonation

This study explores a novel method for enhancing CO₂ uptake in cement systems using 1,6-hexamethylenediamine (HMDA) as a water-soluble additive. Conventional CO₂ curing methods are limited by low CO₂ solubility in cement and adverse effects on cement alkalinity. In this work, HMDA was introduced into CO₂-saturated water to promote chemical absorption of CO₂ through carbamate formation while maintaining favorable pH conditions for cement hydration. Cement pastes were prepared using four mixing solutions—tap water (W1), carbonated water (W2), and carbonated water with 0.1 % and 0.6 % HMDA (W3, W4). A comprehensive suite of analyses was conducted, including total inorganic carbon (TIC), pH, X-ray diffraction (XRD), micro-computed tomography (μCT), Fourier-transform infrared spectroscopy (FTIR), mechanical testing, and pore structure characterization. Results showed that HMDA significantly enhanced CO₂ uptake, from 0.74 g/L in W2 to 8.2 g/L in W4 (measured at w/c = 0.5). Despite lower early-age strength, HMDA-treated samples exhibited superior 28-day compressive strength, with W3 achieving the highest value of 113.8 MPa at a w/c ratio of 0.3. XRD and FTIR confirmed increased calcite formation and reduced portlandite content, indicating deeper carbonation. μCT and PSD analyses revealed reduced porosity and refined pore structures, especially in W3 and W4. Mechanical testing showed that despite lower early-age strength, HMDA-treated samples exhibited superior 28-day compressive strength, stiffness, and elastic modulus. W3 achieved the highest strength (113.8 MPa), confirming the long-term benefits of controlled internal carbonation. This HMDA-based approach offers a dual advantage: enhanced CO₂ capture and improved cement performance. It provides a scalable and cost-effective pathway for carbon utilization in cement production, supporting the development of high-performance, low-carbon construction materials.