Conventional cement-based materials exhibit excellent compressive performance, but are inherently brittle, with inadequate tensile, flexural and deformation capacities, which readily induce cracking and impair the durability and long-term service performance of structures. At present, incorporating organic polymers or inorganic nanomaterials is proven as one of the effective strategies to enhance the mechanical properties of cement-based materials. Organic polymers are widely adopted in cement-based materials for their superior plasticity and toughness. They can adsorb onto the surface of cement particles via electrostatic interaction or hydrogen bonding, changing the hydration process of cement and optimizing the distribution and microstructure of hydration products. In addition, inorganic nanomaterials (e.g., nano-silica, carbon nanotubes and graphene oxide) can also improve the mechanical properties of cement-based materials due to their high specific surface area and reinforcement effect. However, these modification methods have some limitations. For most polymers, their effect on the flexural and compressive strengths of cement-based materials has a negative correlation, and improving the flexural strength can result in a significant reduction in the compressive strength. Furthermore, inorganic nanomaterials suffer from a poor dispersion in cement matrix, which easily causes agglomeration and uneven distribution. In addition, inorganic nanomaterials mainly improve the early compressive performance of cement-based materials, while their enhancement effect on flexural strength is relatively limited. Using organic/inorganic nanocomposites to optimize the microstructures of cement hydration products at the micro/nanometre scale is thus a promising method for improving the mechanical properties of cement-based materials. In this work, α-lipoic acid (TA), a natural bioactive molecule with a five-membered ring disulfide bond, was innovatively introduced and self-polymerized at a high temperature to prepare a dynamic polylipoic acid (PTA) network. Selective esterification modification of reactive hydroxyl groups on microcrystalline cellulose (MCC) molecular chains with PTA could improve MCC dispersibility and endow it with organic polymer characteristics. The prepared PTA-MCC hybrid material exhibited excellent mechanical enhancement in the cement matrix, which can significantly improve the flexural strength of cement-based materials at a low dosage without compromising compressive strength, thus effectively breaking through the technical bottleneck of synergistically enhancing strength and toughness of cement-based materials.

For the synthesis of polylipoic acid fiber hybrid material (MCPTS), TA powder was weighed according to a selected ratio, reacted in an oil bath at 150 ℃ for 1 h and cooled naturally to obtain linear PTA. The activated MCC and linear PTA were added to DMF solvent in a proportion, fully stirred to form a uniform system, and reacted in an oil bath at 120 ℃ for 6 h. After the reaction, the product was centrifuged and dried by rotary evaporation to obtain the final product of MCPTS. The products were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM), confirming that the activated MCC and linear PTA could be fully combined, and the target hybrid material was prepared.

The results of mechanical properties of cement paste show that the MCPTS synthesized can effectively improve the comprehensive mechanical properties of cement-based materials, especially a small increase in its compressive strength, which greatly improves the flexural strength of cement-based materials. Under the premise of ensuring that its compressive strength does not decrease, the flexural strength of cement-based materials is synergistically improved. The XRD patterns of cement-based materials at 3-d and 28-d curing ages reveal that the MCPTS-added group promotes cement hydration to a certain extent. Some cement hydration products like calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) gel are formed in the system as the hydration age increases. The SEM images demonstrate that the cement paste has a much denser microstructure with a large number of clustered C-S-H hydration products after the incorporation of MCPTS. The results of cement hydration exothermic tests indicate that cement pastes with MCPTS show a significant hydration acceleration effect, with the occurrence time of their main hydration exothermic peaks advanced to varying degrees and the peak hydration rate increases remarkably.

In this paper, a type of MCPTS hybrid material with dynamic hybrid of PTA and MCC was prepared for a synergistic enhancement of organic-inorganic hybrid. It has reversible covalent bond and non-covalent bond dynamic network structure. The optimal performance was achieved at a dosage of 1.0‰ MCPTS-1. The flexural strength at 3, 7 d and 28 d was increased by 8.56%, 25.91% and 22.68%, respectively, and the compressive strength at 7 d and 28 d was enhanced by 11.80% and 6.45%, respectively. The XRD patterns, SEM images and hydration exothermic analysis indicated that MCPTS could accelerate cement hydration, increase heat release, and promote the formation of a denser hydration product structure, thereby improving mechanical properties. The underlying mechanism was mainly as follows, i.e., the carboxylic acid ion network formed by PTA hydrolysis could chelate Ca²⁺ and optimize the structures of CH crystals and C-S-H gels. MCC dispersed stress concentration through the fiber bridging effect. The synergistic effect of the two components realized the performance enhancement of cement-based materials.