Characterization of Superabsorbent Poly(Sodium-Acrylate Acrylamide) Hydrogels and Infuence of Chemical Structure on Internally Cured Mortar

Internal curing of mortar through superabsorbent polymer hydrogels is explored as a solution to self-desiccation. Four different hydrogels of poly(sodium-acrylate acry- lamide) are synthesized and the impact of chemical composition on mortar is assessed with relative humidity and autogenous shrinkage testing. The hydrogels are characterized with swelling tests in different salt solutions and compression tests. Chemical composition af- fected both swelling kinetics and gel network size. Mortar containing these hydrogels had increased relative humidity and markedly reduced autogenous shrinkage. Additionally, the chemical structure of the hydrogels was found to signifcantly impact the mortar’s shrink- age. Hydrogels that quickly released most of their absorbed fuid were able to better reduce autogenous shrinkage compared to hydrogels that retained fuid for longer periods (\u3e 4 hours), although this performance was highly sensitive to total water content. The release of absorbed water in hydrogels is most likely a function of both Laplace pressure of emptying voids and chemically-linked osmotic pressure developing from an ion concentration gradient between the hydrogels and cement pore solution. If the osmotic pressure is strong enough, the hydrogels can disperse most of the absorbed water before the depercolation of capillary porosity occurs, allowing the water to permeate the bulk of the mortar microstructure and most effectively reduce self-desiccation and autogenous shrinkage

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

The objective of this research is to define the fundamental structure-property relationships of water-swollen polymer hydrogel particles that are employed as internal curing agents in cementitious mixtures, in addition to reporting a novel synthesis procedure for combining pozzolanic materials with hydrogel particles. Solution polymerization was performed to incorporate amorphous nanosilica particles within acrylic-based polymer hydrogel particles of varying chemical compositions (i.e., monomer ratio of acrylic acid (AA) to acrylamide (AM)). Experiments were designed to measure the absorption capacity and kinetics of hydrogel particles immersed in pure water and cementitious pore solution, as well as determine the impact of particles on cement paste microstructure. While majority-AM hydrogel particles displayed relatively stable absorption values during immersion in pore solution, majority-AA hydrogel particles desorbed fluid over time, most likely due to the interactions of multivalent cations in the absorbed solution with the anionic polymer network. Interestingly, the addition of negatively charged nanosilica particles accelerated and enhanced this desorption response. When incorporated into cement paste, majority-AM hydrogel particles encouraged the formation of calcium hydroxide and calcium silicate hydrate within the void space previously occupied by the swollen particles. When nanosilica was added to the hydrogel particles, a 53 % increase in the number of hydrogel voids containing hydrated product was observed for the 17 % AA hydrogel particles, and a 140 % increase was observed for the 83 % AA hydrogel particles. These results suggest that the combination of nanosilica with polymeric hydrogel particles provides a favorable environment for the pozzolanic reaction to proceed and that nanosilica aids in the replenishment of hydrogel void space with hydrated cement phases

Characterization of Superabsorbent Polymers in Aluminum Solutions

Over the past few decades, super absorbent polymers (SAPs) have been the topic of research projects all around the world due to their incredible ability to absorb water. They have applications in everything from disposable diapers to high performance concrete. In concrete, aqueous cations permeate the polymer network, reducing swelling and altering properties. One of these ions, aluminum, alters SAP properties by creating a stiff outer shell and greatly reducing absorbency, but these effects have not been well characterized. One method of characterizing the effects of aluminum on SAP hydrogels was performing gravimetric swelling tests to determine equilibrium water capacity at different aluminum ion concentrations. Compressive strength was also determined for swollen particles using a rheometer to perform compression tests. Results from this testing show that low concentration solutions take several hours to permeate the polymer network and reduce swelling capacity, while high concentration solutions are able to limit swelling immediately. The compressive strength of the gel was increased greatly in polymers containing mostly poly(acrylic acid), while SAPs containing more poly(acrylamide) did not have their strength as greatly influenced by the aluminum ions. These results help elucidate the negative effects that may be caused by multivalent cations in concrete. Further research will include studying the interactions of aluminum ions with polymer strands using polymer brushes on a quartz crystal microbalance. This will hopefully reveal the mechanism and kinetics of salt absorption in polymer networks

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

Improved Concrete Materials with Hydrogel-Based Internal Curing Agents

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