Experimental analysis of decalcification and recalcification of cement paste

Decalcification — calcium leaching and carbonation, poses a threat to the longevity of concrete structures in civil engineering. Traditionally, the resulting damages are treated through patching, grouting, or complete demolition and rebuilding. However, such techniques are challenging in certain applications, particularly those in hard-to-reach locations or in environments where the surrounding conditions are hazardous. A notable example is geological disposal facilities, where concrete is used to encapsulate radioactive waste and construct gallery lining. The service life of concrete in this specific application is compromised by the contact with groundwater, which induces leaching and carbonation. Given that decalcification results in the removal of calcium from hydration products, the resupply of calcium offers a promising solution. Calcium ions can react with SiO2 to produce calcium-silicate-hydrate. They are also able to restore calcium-silicate-hydrate in decalcified cementitious materials, as reported in a previous study by Chen et al. However, little is known about the impacts of this restoration process on a bulk cementitious structure. The mentioned pioneer work only utilized small, crushed samples, which limited the understanding of the effects of recalcification on microstructure. The small specimens also undermined the diffusion-controlled nature that is more relevant to larger structures. Concerning the number of studies on recalcification in the literature, there is clearly a lack of attention paid to this topic. This project aims at investigating the effects of resupplying calcium, referred to as recalcification, on decalcified cement paste. The study focuses on changes in chemistry — specifically the restoration of pH, mineralogy, silicate environment, and microstructure to elucidate the mechanisms and influences of recalcification. Additionally, the transport properties, particularly permeability, of leached cement paste are assessed both before and after recalcification, given the substantial alterations to the microstructure induced by calcium leaching. Lastly, the comparison between recalcified calcium-silicate-hydrate and the freshly synthesized product from CaO and SiO2 provides deeper insights into the recalcification mechanism. The results demonstrate that recalcification effectively restores decalcified cement paste. The pH of the pore solution, which exists in equilibrium with hydration products, exceeds 10, as indicated by phenolphthalein, confirming that the solid phases are restored and can function as pH buffers. The calcium-to-silicon ratio of the solid phases increases post-recalcification. Portlandite does not reform, indicating that recalcification by immersion in saturated Ca(OH)2 solution is not the exact reverse of decalcification. However, alterations to the silicate environment, as monitored by 29Si NMR and FTIR, confirm the restoration of calcium-silicate-hydrate. The additional calcium not only consumes silica gel to form new calcium-silicate-hydrate, but also breaks down the polymerized silicate species to form shorter silicate units. This restoration densifies the microstructure, with recalcified paste exhibiting a significant reduction in porosity (an ≈ 50% drop) and permeability (≈ 50% drop) compared to leached paste. Such a recalcification-induced drop in porosity is not as evident in carbonated paste, where the formation of CaCO3 already densifies the pore structure. Recalcification takes longer than the direct reaction of CaO and SiO2 to increase the Ca/Si ratio of calcium-silicate-hydrate by the same amount, possibly due to the need to destabilize and reorganize the quasi-stable structures of this phase. The restored calcium-silicate-hydrate differs from the original when formed from silica gel in severely decalcified samples, especially when pore space is unrestricted. In contrast, calcium-silicate-hydrate restored from the decalcified counterpart, and/or in confined spaces closely resembles the original gel. Overall, the process of recalcification produces calcium-silicate-hydrate similar to that formed by the reaction of CaO and SiO2, with variations in structure determined by factors such as space availability, Ca2+ concentration, and pre-existing calcium-silicate-hydrate structure. Recalcification can be considered a special form of autogenous healing. The findings of this project contribute to the existing knowledge on recalcification and suggest the potential for integrating recalcification with self-healing strategies to provide an additional barrier against concrete degradation, thereby enhancing durability and prolonging the service life of concrete structures.