Preparation and properties of novel superabsorbent polymer (SAP) composites for cementitious materials based on modified metakaolin

Superabsorbent polymer (SAP) is becoming the most promising internal curing material for cementitious materials. However, commercial SAP products, most of which are designed for hygiene products, controlled drug release, and agriculture, usually impair the strength of cementitious materials. In view of this, a novel type of poly(acrylic acid-co-acrylamide) SAP composites based on modified metakaolin (MK) was prepared for cementitious materials in this work. Modified MK, acrylic acid (AA) and acrylamide (AM) were polymerized by initiating of ammonium persulfate (APS). The kinetics characteristics of the fabricated SAP composites were studied by absorption and desorption in different solutions. The microstructures and phase compositions of SAP composites, as well as the mechanical properties of SAP-modified cement mortar were characterized by a suite of complementary techniques, including Xray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and compressive strength test. Results show that the modification of silane coupling agent (SCA) and xanthan gum (XG) can increase the recombination degree between MK and copolymers. The water retentions of self-developed SAP composites, SAP-N-1 and SAP-N-2, are much better than those of two commercial SAPs in alkaline solution and cement filtrate, respectively. The maximum absorbency of SAP-N-2 in sodium hydroxide solution is even greater than that in tap water. When SAP saturated with absorbed tap water is placed in saturated lime water, SAP-N-2 can release only 0.49% water at a 4 h soaking period. Furthermore, either of the commercial SAPs has negative effects on the strength of cement mortar at 7 d and 28 d curing ages, while compared with the reference mortars, the incorporation of the SAP-N-2 increases the 28 d compressive strength of the mortars by about 8% to 16%. (C) 2020 Published by Elsevier Ltd.