Improved Concrete Materials with Hydrogel-Based Internal Curing Agents

Abstract

This research article will describe the design and use of polyelectrolyte hydrogel particles as internal curing agents in concrete and present new results on relevant hydrogel-ion interactions. When incorporated into concrete, hydrogel particles release their stored water to fuel the curing reaction, resulting in reduced volumetric shrinkage and cracking and thus increasing concrete service life. The hydrogel's swelling performance and mechanical properties are strongly sensitive to multivalent cations that are naturally present in concrete mixtures, including calcium and aluminum. Model poly(acrylic acid(AA)-acrylamide(AM))-based hydrogel particles with different chemical compositions (AA:AM monomer ratio) were synthesized and immersed in sodium, calcium, and aluminum salt solutions. The presence of multivalent cations resulted in decreased swelling capacity and altered swelling kinetics to the point where some hydrogel compositions displayed rapid deswelling behavior and the formation of a mechanically stiff shell. Interestingly, when incorporated into mortar, hydrogel particles reduced mixture shrinkage while encouraging the formation of specific inorganic phases (calcium hydroxide and calcium silicate hydrate) within the void space previously occupied by the swollen particle.

Synthesis and Characterization of Polymer-Silica Composite Hydrogel Particles and Influence of Hydrogel Composition on Cement Paste Microstructure

“…This portion was immediately covered to prevent carbonation and used for gravimetric swelling tests. The gravimetric "teabag" method was used to determine the transient absorption response of the hydrogel samples used in this study, following past protocols [57,63]. For both solutions, 200 ml of liquid was added to a beaker, and a teabag was fully immersed into the solution for 30 seconds.…”

“…This gradient creates an osmotic pressure which drives the diffusion of water into the hydrogel particle along with other ions in solution, and the particle will continue to swell until the net osmotic pressure is reduced to zero. Cations present in the aqueous fluid (including sodium, calcium, and aluminum ions) will be electrostatically attracted to the COO − moieties and form ionic complexes that effectively act as crosslinks within the polymer network, the formation of which decreases the equilibrium absorption capacity of the hydrogel [56,61] and can ultimately lead to the collapse of the polymer network, as we recently demonstrated [57]. In the concrete materials community, there is a growing body of research to accurately quantify the absorption and desorption behavior of hydrogel particles in cementitious pore solutions, as certain hydrogel compositions (including commercially available products) have been found to display strong sensitivity to the mono-and multivalent cations that are naturally present in pore solutions depending on mixture age [17,57,[62][63][64][65][66].…”

“…In contrast to most LWA, however, hydrogel particles are not chemically inert within cementitious mixtures. In the case of hydrogel particles containing AA, the carboxylic acid functional groups undergo deprotonation in alkaline conditions (as the pKa of AA is approximately 4.5 [55]), forming anionic carboxylate (COO − ) moieties in the polymer network and allowing significant amounts of water to be absorbed by virtue of ion-dipole interactions [56][57][58][59]. For these charged polymer networks, the primary mechanism of hydrogel swelling can be attributed to the creation of a chemical potential gradient across the hydrogel-fluid interface, as the anionic polymer network results in a higher concentration of free counterions within the hydrogel particle compared to the surrounding fluid in order to preserve electroneutrality within the system [60].…”

“…In addition to changes in swelling performance, hydrogel particles used as internal curing agents have also been found to impact the inorganic chemistry of cement, specifically the development of calcium hydroxide (CH) and calcium-silicate-hydrate (CSH) phases [24]. Recent findings by our research group [57] suggested that hydrogel particles composed mainly of polyacrylamide were able to form significant amounts of CH phases within the hydrogel void space. Thus, the incorporation of a pozzolanic material within the hydrogel network may form an additional CSH phase, perhaps leading to a further increase in the mechanical properties and durability of internally cured concrete.…”

“…Our recent research [57] on hydrogel-ion interactions and influence of hydrogel chemistry on cement paste microstructure indicated that the chemical structure of hydrogel particles could be tuned to increase the amount of CH phases that forms within the hydrogel void. It is thus anticipated that the incorporation of nanosilica in these hydrogel particles will facilitate the formation of additional CSH within the void space, by taking advantage of the combination of a pozzolan and absorbed water within the hydrogel network.…”