Self-healing cementitious materials are a promising means for ensuring sustainable concrete infrastructure and promoting long-term service lives. To obtain microcapsules that are versatile in varying environments, in this study, absorbing microcapsules with calcium alginate as the shell and epoxy resin as the core were prepared. The absorbing microcapsules exhibit self-healing and can reduce the shrinkage of cementitious materials. Volume changes of the microcapsules in the hardened paste with increasing hydration age were observed using three-dimensional X-ray computed tomography. In the hardened cement paste with a water-cement ratio of 0.29, the absorption of the microcapsules lasted for several days, and the release of water lasted for 28 days. The absorption of microcapsules affected the fluidity of cement paste, and it was significantly weakened and delayed due to the lower absorption rate. The addition of absorbing microcapsules significantly reduced the autogenous and drying shrinkage of mortars. For microcapsules with a core content of 55% added at 3.5% of cement weight, autogenous shrinkage was almost eliminated. Most importantly, the addition of absorbing microcapsules could achieve a certain degree of recovery of compressive strength as well as satisfactory recovery of impermeability in dry and wet environments.
The purpose of this study was to investigate how the addition of shrinkage-reducing polycarboxylate superplasticiser (SRPC) affected the shrinkage of cement-based materials (CBMs). SRPCs can significantly reduce the surface tension of cement solutions. In this work, a new molecular structure of SRPC was synthesised; its surface tension was reduced to 34·1 mN/m, which is lower than that of ordinary polycarboxylate superplasticisers (PCEs). Compared with mortars without additives, the shrinkage-reducing ratios of mortars containing SRPC at 7 and 49 d were about 33 and 22%, respectively, for an SRPC dosage of 0·2% by mass of cement. The results showed that the SRPC reduced the shrinkage of CBMs more effectively than an ordinary PCE. Furthermore, the shrinkage mechanism of CBMs containing SRPC was assessed regarding three aspects: the change in the pore size distribution of the cement pastes with pore size below 50 nm, the reduction of the capillary pore pressure in the hardened cement pastes after the addition of a low-surface-tension SRPC and the reduction in the evaporation rate of moisture through SRPC aggregation on the gas–liquid interface in cement pastes.
Shrinkage-reducing polycarboxylate superplasticizer (SRPC) has a good effect on reducing shrinkage of cement-based materials because of its low surface tension. However, the effect of SRPC on the properties of cement-based materials needs to be further explored. A comparative study of SRPC and polycarboxylate superplasticizer (PCE) was carried out with the aim of correlating the adsorption in fresh cement pastes with their dispersing capability and the impacts on cement hydration. The results indicate that the adsorption amounts of SRPC are less than PCE. Both of them exhibit a multilayer adsorption; the fluidity of cement pastes with SRPC and PCE are related to the first adsorbed layer, and the secondary adsorbed layer does not contribute to the enhancement of fluidity. The retarding cement hydration of SRPC is weaker than PCE because of weak adsorption capacity of SRPC.
Concrete cracking has a negative impact on the durability of the structure. Pre-implanting microcapsules containing healing agents into the concrete are expected to induce the cracks to self-heal. However, the self-healing effect can potentially be influenced by several environmental conditions, thus limiting its applications. To address these challenges, we developed a new type of water-absorbing microcapsules, using calcium alginate hydrogel as the wall material and an adhesive epoxy polymer as the core material, to improve the self-healing adaptability in complex and changing environments. We explored the healing properties and mechanism of cementitious materials containing microcapsules under various environmental conditions. The experimental results showed that the water-absorbent microcapsules exhibit multiple self-healing effects under different external conditions: (1) in an anhydrous environment, fissures prompted the activation of microcapsules, and the epoxy polymer flowed out to seal the cracks. (2) When exposed to water, the microcapsules inflated to form a seal around the fissures. (3) The microcapsules facilitated the autogenous healing of cracks in the cementitious material when wet and dry conditions were alternated. The three self-healing mechanisms worked synergistically and contributed to the effective restoration of the impermeability and strength of concrete under different environments. Particularly, the recovery of compressive strength and impermeability exceeded 100% when the microcapsule content was 4% and the pre-pressure was 40% of fmax.
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