Formation Mechanism of Carbonation Profiles in Cement Pastes by Pre-Conditioning

Early-age carbon dioxide (CO₂) curing of concrete is an innovative carbon utilization technology, enabling resource-oriented utilization of carbon dioxide that allows carbon neutrality. Thus, it is a promising technology for the concrete industry. Enhancing carbonation reactivity contributes to the promotion of CO₂ curing technology. Among all the influencing factors, previous studies showed that water loss during pretreatment could be one of the most important parameters affecting carbonation degree and mechanical properties of concrete.

In a previous work, water loss was primarily evaluated based on the overall water loss of specimens. Note that water loss varies significantly at different depths due to the complex pore structure and heterogeneity of cement-based materials. This causes depth-dependent carbonation reaction, forming gradients in both water content and carbonation degree. However, the existing studies neglect water loss gradient and its impact on carbonation. This is mainly due to a lack of direct visualization and continuous tracking of moisture distribution and carbonation products inside concrete by conventional testing methods, making it difficult to accurately characterize water loss gradients and carbonation gradients. A more precise and efficient water loss and carbonation characterization method is required.

This study was to apply X-ray imaging technology to non-destructively and quantitatively track the formation of water loss gradients and carbonation gradients in cement-based materials at different pre-drying durations.

This study cast cube specimens with the size of 40 mm for compressive strength tests and specimens with the sizes of 10 mm × 20 mm × 40 mm for X-ray imaging tests. The raw materials consisted of water and P.O.42.5 cement at a water-to-binder ratio of 0.4. X-ray imaging technology was employed to capture projection images of the 20 mm × 40 mm face of the specimens before and after pre-drying and accelerated carbonation.

Image processing for water loss distribution and carbonation distribution included field non-uniformity correction, normalization, image registration, grayscale difference calculation, water loss distribution characterization, attenuation coefficient variation calculation, carbonation-water signal decoupling and carbonation distribution characterization.

Furthermore, a phenolphthalein alcohol solution was used to validate carbonation depths determined by X-ray imaging. The mechanism of different water loss gradients on carbonation behavior was analyzed by thermogravimetric analysis (TGA). The porosity changes of carbonated specimens under different water loss gradients was characterized by low-field nuclear magnetic resonance, thereby elucidating the formation mechanisms of water loss gradients and carbonation gradients. The 3-d compressive strength of the specimens was determined using a universal testing machine.

Based on the X-ray projection images, water loss mapping and water loss-depth curves are obtained, revealing a decrease of water loss with increasing depth. Besides, the overall water loss increases with prolonging pre-drying duration. A substantial water loss difference appears between the surface and internal sample core, forming a distinct water loss gradient. The peak water loss and the water loss gradient both increase with extended drying.

In addition, 2D mapping of attenuation coefficient variation caused by carbonation and the corresponding variation along the depth can be acquired, introducing carbonation gradient and total attenuation coefficient change as indicators for carbonation behavior. Among all the samples, the sample CC18-0 exhibits the shallowest carbonation depth, with the maximum carbonation degree appearing at the surface. Carbonation depths of sample CC18-1.5/3/6 show a slight increase with drying time, while the decline rate within the partially carbonated zone remains consistent. Finally, the sample CC18-24 achieves the maximum carbonation depth of approximately 6.6 mm, while its peak carbonation degree is the lowest. Moreover, prolonged drying time leads to a significant decrease in the carbonation gradient, dropping progressively from 0.0088 mm^-1 for the sample CC18-0 to 0.0061 mm^-1 for the sample CC18-24 (a maximum reduction of 30.7%). In addition, the total attenuation coefficient change within the carbonated zone also increases with drying time (from 0.093 to 0.138).

A comparison of carbonation depths determined by X-ray imaging, thermogravimetric analysis, and the phenolphthalein test indicates that X-ray imaging is a more precise carbonation testing method. Furthermore, the porosity of carbonated specimens decreases as water loss during pre-treatment increases. The sample CC18-24 has the lowest porosity of approximately 25.29%. Eventually, the early-age strength of carbonated cement pastes slightly improve from hydrated samples.

The design of water loss gradients in cement-based materials prior to carbonation can be achieved via controlling the pre-drying duration. In general, short pre-drying period results in a gentle water loss gradient due to the larger pores and lower resistance to moisture migration. The high surface water content impedes CO₂ diffusion, causing the carbonation reaction to be dominated by the transport rate of CO₂. As a result, carbonation mainly occurs near the surface, leading to a high carbonation degree that is limited to a shallow depth. In contrast, a prolonged pre-drying promotes a higher hydration degree and pore refinement, which hinders outward moisture migration. This leads to a sharp increase in water loss at the surface, forming a steep water loss gradient. The low surface water content broadens the pathways for CO₂ diffusion, facilitating a deeper carbonation penetration. Meanwhile, the presence of continuous water films in the internal regions enhances the dissolution efficiency of calcium ions, resulting in a gradual carbonation gradient. The steep water loss gradient drives CO₂ to diffuse inward, thereby increasing the overall carbonation degree, which is evidenced by higher CO₂ uptake and calcium carbonate content. The increased calcium carbonate content further refines the pore structure, significantly reducing the total porosity of the matrix, which in turn enhances the compressive strength.

The samples subjected to shorter drying durations exhibited less and more uniform moisture losses from surface to interior. The relatively high residual moisture at the surface favored in-situ carbonation but limited CO₂ transport inwards, resulting in a higher surface carbonation degree with a shallower carbonation depth. Conversely, the samples dried longer experienced a greater moisture loss and an increased surface drying. The lower surface moisture facilitated CO₂ diffusion and slowed down calcium ion dissolution, leading to a relatively lower surface carbonation degree yet a greater carbonation depth. It was indicated that a steep moisture loss gradient promoted CO₂ diffusion, enhancing the overall carbonation degree of cement pastes. This significantly reduced total porosity, which consequently improved the compressive strength.