Tea polyphenol as a bio-admixture: A strategy for performance enhancement in alkali-activated slag

Abstract

The fast hydration rate resulting in the rapid hardening and workability loss is one of the critical challenges to the large-scale engineering application of alkali-activated slag (AAS). In this regard, this study innovatively introduces tea polyphenol (TP) as a bio-admixture to regulate the hydration process and mechanical performance of AAS. The work mechanisms of TP on AAS were confirmed through a series of experimental methods, including total organic carbon (TOC), low-field ¹H nuclear magnetic resonance (LF NMR), X-ray diffraction (XRD), thermal gravimetric analyzer (TGA), inductively coupled plasma photoemission spectrometry (ICP-OES) and nanoindentation (NI). The results reveal that TP effectively retards the early hydration process of AAS by decreasing pH in the pore solution and inhibiting the precipitation of hydration products. The initial and final setting times of AAS are extended from 1.25 h to 12.53 h and from 3.88 h to 20.00 h, respectively. Additionally, TP optimizes the rheological properties and increases the workability retention of AAS, which is attributed to limiting the consumption of capillary water and the formation of surface water. TP exhibits an accelerating effect on the formation of Hydrotalcite (Ht) at 3 d. The large capillary pore of AAS was restricted. Such a result leads to a 4.8 %-27.9 % increase in compressive strength at 3 d. Moreover, TP comprehensively enhances the hydration process and refines the pore structure of AAS at later ages. Consequently, the compressive strength of AAS is improved by 39.8 % and the average elastic modulus of the paste is strengthened by 12.22 % at 28 d. Moreover, A blending ratio below 0.30 % TP can suppress the later-stage shrinkage of AAS. These findings highlight the capacity of TP to enhance the performance of AAS, providing a theoretical foundation for the development of novel bio-based concrete admixture.