CO2-induced changes in rheology, structural evolution, and particle characteristics of cement paste

Abstract

The injection of carbon dioxide (CO2) into cementitious systems has emerged as a promising approach to enhancing sustainability in concrete production. This study explores the impact of CO2 injection on the rheological behavior, structural build-up, and particle size evolution of cement paste by systematically varying the water-to-cement ratio (w/c) (0.35, 0.4, and 0.5) and CO2 concentration (0.06, 0.28, and 0.84 mol). Rheological analysis revealed a significant increase in initial yield stress with CO2 injection. Using the Modified Bingham model, yield stress increased by 72 % and 150 % for 0.28- and 0.84-mol CO2, respectively, while the Herschel-Bulkley model indicated corresponding increases of 31 % and 76 %. Structural build-up rates also exhibited substantial enhancement, reaching 23 Pa/min for 0.28-mol CO2 and 106 Pa/min for 0.84-mol CO2, whereas negligible changes were observed in CO2-free samples. Note that lower w/c (e.g., 0.35) demonstrated superior rheological improvements, with a 310 % increase in yield stress, highlighting the role of free water in facilitating carbonation reactions. Shear-thinning behavior intensified with CO2 injection, particularly at low shear rates (10−2 to 10−1 s−1), while viscosity changes diminished at higher shear rates (101 to 103 s−1) due to particle network disruption. Slump flow tests further confirmed alterations in flow characteristics following CO2 injection. Analytical techniques, including multiple light scattering, X-ray diffraction (XRD), and thermogravimetric analysis (TG), indicated the formation of CaCO3 in the form of aragonite and calcite, contributing to particle agglomeration and enhanced stability.