Investigating the Evolving Crystallography and Chemistry of Sustainable Cements During Their Accelerated Carbonation

Cement production is responsible for 8 to 10% of global anthropogenic CO₂ emissions. As a solution to reduce net CO₂ emissions and valorize waste CO₂, using the reaction between CO₂ and cementitious materials for hardening, or carbonation curing, is a promising approach. However, cement carbonation involves multiple coupled physicochemical phenomena and depends on both the cement formulation and the curing environment. This makes the elaboration and validation of predictive models challenging, requiring extensive experimental data to optimize carbonation processes. With most carbonation monitoring methods requiring destructive sampling, the time resolution and the understanding of carbonation kinetics are limited and most experimental datasets are fragmented over multiple samples. This work addresses this gap by developing, validating and applying non-destructive operando characterization techniques capable of following a single sample throughout accelerated carbonation. The first part of this work focuses on the design and validation of a custom 3D-printed gas flow microreactor enabling operando monitoring of carbonating cement samples under controlled CO₂ concentration and relative humidity. Reducing the sample scale by two orders of magnitude compared to conventional samples shortens carbonation times from weeks to a few hours and allows the possibility of performing non-destructive transmission X-ray diffraction (XRD) to monitor the carbonating samples. The device was validated and applied to the carbonation of millimeter scale samples of hydrated lime and ordinary Portland cement under a wide range of CO₂ concentrations and relative humidities. The kinetics of carbonation were characterized by fitting the XRD Rietveld quantification of phase assemblage to a diffusion based model, revealing a strong influence of the environmental conditions on the rate of carbonation. The second part of this work focuses on another gap in conventional carbonation monitoring techniques, their limited spatial resolution. Operando line profile measurements by Raman spectroscopy and laboratory XRD together with operando 2D XRD mapping using synchrotron radiation were performed to track the evolution of phase composition as a function of position and time across cement samples. Working with spatial resolutions in the micrometer range highlighted local heterogeneities in the degree of carbonation that are usually averaged out, and the width of the carbonation front was measured, showing a diffuse front at the millimeter scale instead of the typically assumed sharp front. By coupling the microreactor with spatially resolved non-destructive operando characterization, this work establishes an experimental methodology for monitoring accelerated cement carbonation with unprecedented temporal and spatial resolution. The obtained datasets provide insight into the influence of environmental conditions on carbonation kinetics that can be used to refine and validate carbonation models, contributing to the development of low-carbon binders and CO₂ sequestration in the construction industry.