Enhancement of heat-cured cement paste with tannic acid

The improvement of cement-based materials' performance by natural organic compounds can greatly promote the green and sustainable development of the construction industry. However, such compounds are not widely used yet because of their retarding effect on cement. In this study, the retardation effect of tannic acid (TA, a well-known retarding compound) is overcome and the enhancing effect is achieved by adding less than 0.1% content and curing samples in thermal regime. Then the mechanism of TA enhancing heat-cured cement pastes is studied systematically. Mechanical properties results suggest that addition of 0.025% TA can reduce the compressive and flexural strengths of cement pastes by up to 3.4% and 17.1% under normal curing regime at 3 days, but enhance these two strengths by more than 11.4% and 34.6% after thermal curing, respectively. XRD patterns and TGA analysis indicate that, under thermal curing regime, 0.025% TA can improve the hydration degree of cement where the bound water content is increased by 21.4%. SEM observations and MIP results show that TA can compact the microstructure and the porosity is decreased by more than 7.0%. Furthermore, FTIR spectrums prove that TA can bond with hydration products. Molecular dynamics simulation demonstrates that TA cross-links with calcium silicate hydrates (C-S-H) through ionic and hydrogen bonds, which could increase the tensile strength by 12.5% and the ultimate strain by 100%.