Effects of tannic acid on the late-age strength of autoclave curing cement paste

Abstract

The long-term performance regression of thermal curing concrete threatens the durability and safety of structure. With the popularization of prefabricated components, the problem needs to be solved urgently. Tannic acid (TA), a natural polyhydroxyl compound, has retarding and enhancing effect on cement. The former effect can partially offset the rapid hydration of thermal curing initially and the latter can resist drying shrinkage in subsequent air curing. Thus TA is proposed to mitigate the strength retrogression of autoclave curing cement paste in this research. As a result, the flexural strength of autoclave curing cement paste mixed with 0.025% TA is improved by 44.1% than samples without TA at 28 days. And the flexural strength loss rates because of total damage and subsequent air curing are decreased to −8.9% and 12.5% from 24.4% and 26.9%, respectively. XRD and TGA results show that the hydration of autoclave curing cement paste can be improved by 0.025% TA where the bound water is increased by 6.6%. SEM observations and MIP analysis prove that 0.025% TA can make the matrix dense and decrease the porosity by 14.4% under thermal curing. Besides, TA can promote the generation of nano network structure and bridge the hydration products, which is beneficial to disperse the tensile load. Thus TA can improve the long-term properties (especially flexural strength) of autoclave curing cement paste greatly.