Informing the Design of Hydrogel-Based Internal Curing Agents for Mortar: Effects of Hydrogel Chemistry on Mortar Microstructure and Mechanics
Despite the widespread usage of concrete and significant increases in concrete technology, including the use of new admixtures, supplementary materials, and improved batch design, two consistent problems for the concrete industry are carbon dioxide emissions and reduced service life due to cracking. High performance concrete with a water-to-cement ratio of below 0.42 is utilized to reduce greenhouse gas emissions and create a concrete with a dense microstructure and high compressive strength. Unfortunately, low water content increases the probability of early-age cracking due to volumetric shrinkage. Such cracking will decrease compressive strength, durability, and thus shorten service life. Internal curing has been shown to be a viable method of reducing or eliminating early-age cracking; however it is only in recent research that structure-property relationships between polymeric internal curing agents and bulk concrete performance are investigated. Superabsorbent polymer hydrogels of poly(sodium acrylate-co-acrylamide) are explored as a solution to early-age cracking in high performance concrete. Different weight fractions of monomers are used to formulate several chemically distinct types of hydrogels. Hydrogels are characterized with swelling tests in pure water, various salt solutions (sodium, calcium), and cement pore solution. Mortar with and without hydrogels is examined with autogenous shrinkage measurements, backscattered electron microscopy, and compression experiments. Model cement-hydrogel systems are also investigated with quasielastic neutron scattering. The chemical structure of the hydrogels is directly linked to swelling performance and is affected by the presence of cations, which were found to severely limit hydrogel swelling and, at times, cause deswelling. Mortar containing hydrogels displayed dramatically reduced shrinkage. Microscopy showed that while void space remained in mortar as the hydrogels deswelled, majority-acrylamide hydrogels allowed for the formation of calcium hydroxide inside the hydrogel void. Thus it is demonstrated that hydrogel composition can be tuned to change the microstructure of mortar depending on the desired application, which has wide-reaching implications for the admixture industry and advancement of cementitious materials.
Erk, Purdue University.
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