An amphoteric hydrogel: Synthesis and application as an internal curing agent of concrete

A new crosslinked amphoteric hydrogel (PAC) was synthesized and evaluated as an internal curing agent of concrete. PAC was prepared from acrylamide (AM) and 4-(2-((carboxylatomethyl)dimethylammonio)ethoxy)-4-oxobut-2-enoate (CMD) through free-radical polymerization by using ammonium persulfate (APS) as an initiator and methylene-bisacrylamide (MBA) as a crosslinker. The structure of the prepared polymer was verified by the IR spectra. The effects of the polymerization variables on the swelling capacity of PAC were investigated. The water absorbency of PAC was found to increase with increasing CMD/AM ratio, reach a maximum value, and decrease afterword. Increase of either APS or MBA content decreased the water absorbency. The highest water absorbency of PAC hydrogel (P7) achieved in deionized water, 0.1M NaCl solutions and 0.1M CaCl₂ solutions, was 306 g/g, 32 g/g, and 22 g/g, respectively. Compared with a commercial acrylate-based hydrogel 283HA, P7 was less sensitive to the existence of Ca²⁺ ions in cement pore solutions, and more effective in reducing the cracking tendency of cement pastes. Finally, mortars incorporated with proper amounts of P7 showed smaller drying shrinkage and higher compressive strength than that without hydrogel present.