Improving the Performance of Superabsorbent Polymers as Internal Curing Agents in Concrete: Effects of Novel Composite Hydrogels on Microstructure and Hydration of Cementitious Systems
Superabsorbent polymer (SAP) hydrogel particles have been used as internal curing agents in concrete mixes as they are capable of absorbing and subsequently releasing large amounts of water. This reduces autogenous shrinkage during early stages of hydration. The size, shape, and composition of the hydrogel particles can be controlled during the synthesis, hence providing the opportunity to custom synthesize these internal curing agents to elicit desired structure-property relationships. Utilization of optimized dosage and formulation of SAP has the potential to improve the microstructure, durability, and strength of internally cured concrete.
The first study focuses on the synthesis and application of novel composite hydrogel particles as internal curing agents in cementitious mixes. Composite polyacrylamide hydrogel particles containing two different amorphous silica–either nanosilica or silica fume–were used to investigate whether the internal curing performance of hydrogel particles could be enhanced. The dosage and type of silica, crosslinker amount were varied to identify the composite polyacrylamide hydrogel particle composition that provides optimum benefits to internally cured cementitious systems. The synthesized hydrogels were characterized by means of absorption capacity tests, compositional and size analysis. The beneficial impacts of the addition of composite hydrogels on cement paste microstructure are highlighted, including the preferential formation of cement hydration products (such as portlandite) within the hydrogel-induced voids that appeared to be influenced by the composition of the hydrogel particles. The interrelationship between extent of hydration, size of hydrogel voids, and void-filling with hydration products was found to strongly influence mechanical strength and is thus an important structure-property relationship to consider when selecting hydrogels for internal curing purposes. This study informs the design of composite hydrogel particles to optimize performance in cementitious mixes. Additionally, it provides a novel means of incorporating other commonly used admixtures in concrete without facing common challenges related to dispersion and health hazards.
The second study focuses on the utilization of two retarding admixture-citric acid and sucrose-to custom synthesize composite polyacrylamides to investigate whether the composite hydrogels could delay hydration of cement paste. Isothermal calorimetry analysis results showed that composite sucrose-containing polyacrylamide hydrogel particles were successfully able to retard main hydration peak of cement paste, beyond the retardation capabilities of the pure polyacrylamide hydrogels. Thus, this study provides avenues of exploring the utilization of common admixtures to formulate novel composite hydrogels that imparts specific properties to cementitious systems.
In another study, SAP formulated by admixture industries were used to investigate the feasibility of internal curing of bridge decks and pavement patches with SAP particles. The microstructure and early age hydration properties of SAP-cured cementitious systems were studied. Mitigation of microcracks in the matrix, along with portlandite growth in SAP voids, were observed in SAP-cured mortars. Presence of SAP also mitigated autogenous shrinkage and improved early age hydration as observed by isothermal calorimetry analysis. This thesis highlights some of the beneficial impacts of SAP-cured cementitious systems, and the potential to harness those benefits in large-scale applications of SAP-cured concrete.