Recoverable hydrogel with high stretchability and toughness achieved by low-temperature hydration of Portland cement†
A novel calcium hydroxide nanospherulite (CNS) strengthened super elastic hydrogel with excellent mechanical properties has been successfully invented and investigated. The CNSs are one major hydration product of cement hydrated at low temperature. They are incorporated into the hydrogel polymer network through the Ca2+ ions diffusing from the cement particles into the hydrogel matrix first and then forming calcium hydroxide nanospherulites with diameters <4 nm uniformly in the matrix. This develops an innovative method to form such nanometer-sized calcium hydroxide. The uniformly distributed 4 nm-sized spherulites formed by such a method can act as a crosslinker and hence enhance the properties of the hydrogel remarkably. By incorporating about 40 ppm of 4 nm-sized calcium hydroxide spherulites, the modified hydrogel sample can be stretched to 112 times of its initial length without breaking and withstand a maximum stress of 400 kPa. The strain recovery rate Rr, which is defined as the ratio of recovered strain to the maximum strain, increases from 18.0% for the original hydrogel to 96.7% for the hydrogel with incorporation of around 200 ppm 4 nm-sized calcium hydroxide spherulites. This research contributes to the field with a unique formulation method of uniformly distributed 4 nm or smaller nanoparticles acting as crosslinkers of a hydrogel, achieving enhancement of the excellent overall mechanical properties for the hydrogel. It opens a new direction for nanoparticle strengthened material development by controlling the nanoparticle size from hydrolyzing the matrix.