Hydrogel-based internal curing agents can be used in Portland cement systems to produce microstructural refinement of the cement matrix, leading to more durable cement with longer service life. Crosslinked polyacrylamide-based hydrogel particles serve as reservoirs of water to promote the cement hydration reaction responsible for hardening and strength development. Cementitious systems are highly basic (pH ~13) with many ionic species in solution, including K+, Na+, Ca2+, H2SiO42- (ionic strength ~0.3 molal). Variation of crosslinking density and of co-polymer ionic fraction have been shown to affect hydrogel properties in cementitious systems with respect to both absorption behavior and hydration product formation within the void left in the cement matrix as the hydrogel desorbs. Using methods inspired by biomineralization research, this study explores nucleation and growth of calcium hydroxide and other cement hydration products within polyacrylamide hydrogels with the goal of improving understanding of the role of hydrogel properties in mediating cement hydration product formation. Cross-linking densities and co-polymers were varied to control localized degree of supersaturation and ion-polymer interactions. Optical microscopy and image analysis were used to quantify crystallization kinetics within bulk hydrogels immersed in supersaturated solutions including calcium hydroxide solution and simulated cement pore solution. Hydrogels and crystals were further characterized using small angle x-ray scattering and x-ray diffraction to determine the hydrogel’s influence on crystal morphology as well as crystallization’s influence on hydrogel morphology. Extracted crystals and dried crystal-containing hydrogels were characterized with scanning electron microscopy for morphology and thermogravimetric analysis. Results from these experiments will be presented along with implications for informed selection or design of hydrogels for internal curing applications.

Hydrogel-mediated crystallization of Portland cement hydration products

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