Improving cracking resistance of cement mortar by thermo-sensitive poly N-isopropyl acrylamide (PNIPAM) gels

Abstract

Cracking problems are threats to durability and sustainability of high performance cement-based composites. Due to the complex behaviors of cement materials under various environmental conditions, accurate prediction of the cracks is very difficult. In this work, effects of noval thermo-sensitive polymer N-isopropylacrylamide (PNIPAM) on the cracking resistance of cement mortar were investigated. The micro structures of cement mortar and PNIPAM were characterized by environmental scanning electron microscopy (ESEM) and Fourier Transform Infrared Spectoscopy (FT-IR). The pulse velocity, water absorption and water content of cement mortar were tested to observe the inner structure changes of cement mortar with PNIPAM. Artificial neural network (ANN) technology was used to predict the cracking resistance of cement mortar with PNIPAM. The results show that PNIPAM is cross-linked macromolecule polymer with special thermo-sensitive characters of shrinkage at high temperature and expansion at low temperature. When the mixing temperature is lower than LCST of PNIPAM, it can expand and is soluble in water. However, PNIPAM can shrink and release water to cure the hardened mortar when temperature is higher than LCST due to the cement hydration heat accumulation. The proposed model built by ANN can be used to predict the cracking resistance of cement mortar. The model was further applied to evaluate the effects of different PNIPAM contents on the cracking performance of cement mortar. PNIPAM with suitable contents can decrease the internal deflects of cement mortar. The content of PNIPAM can be used below 1.2% of cement mass for the consideration of cracking resistance improvement of cement mortar.