Individual and combined effects of modified starch, bentonite and their composite powder with HPMC on the performance of cement mortars

Abstract

Abstract A series of acetate starch was synthesized and mixed with bentonite to prepare acetate starch/bentonite composite as a novel additive in order to improve the thixotropic property of cement mortar. The thixotropic index and water retention value of the fresh cement mortar containing acetate starch, bentonite, and acetate starch/bentonite composite, respectively, were measured with and without hydroxypropyl methyl cellulose ether (HPMC). Acetate starch with the degree of substitution (DS) of about 0.5 provided the largest increase in the thixotropic index of the cement mortar. Application of HPMC gave high water retention ability and viscosity to the mortar, but the thixotropic property was not improved. Adding acetate starch or starch/bentonite composite to the mortar can increase its thixotropic index, whereas no significant improvements on the water retention ability and viscosity of mortars were observed. By the combined use of acetate starch/bentonite composite with HPMC, the water retention value (WRV) of the mortar was as high as 98%, and its thixotropic index increased from 4.06 to 5.30, with a high value of viscosity.