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The flexural strength and compressive strength are the two most important parameters that characterize the mechanical properties of cement-based materials, but the two are usually mutually exclusive or negatively correlated. The flexural strength of cement-based materials cannot be effectively improved by a single nanomaterial without reducing their compressive properties. Therefore, compounding various micro–nano materials, fully utilizing their respective advantages, and developing composite materials with new properties and application characteristics are important techniques for enhancing the mechanical properties of cement-based materials. This study aimed to investigate the synergistic enhancement of the mechanical properties of concrete by using microcrystalline cellulose (MCC) and nano-silica (NS) composites. MCC and NS were mixed and modified to synthesize MCCNS materials, which were added to cement slurry for experiments. Results indicated that the flexural strength and compressive strength of the cement paste with MCCNS materials were significantly increased compared with those of the cement pastes without MCCNS or with only NS. The flexural strength of the cement paste with MCCNS-1 was increased by 47%, 20%, 6.3%, and 6.8% after curing for 1, 7, 28, and 56 days, respectively. Meanwhile, compressive strength was also increased by 14.4%, 6.3%, 6.8% and 6.5%, respectively. In addition, the reaction mechanism was speculated through a variety of test results. The addition of MCCNS material facilitated the formation of additional C-S-H, which also contributed to enhancing the mechanical properties of cement paste. • Novel nanocomposites were synthesized using MCC and NS through adsorption-phase reaction and sol–gel methods. • The mechanical properties of cement-based materials were notably enhanced by incorporating MCCNS materials. • The volcanic ash activity of NS led to additional C-S-H formation, improving the mechanical strength of the cement paste. [ABSTRACT FROM AUTHOR]

• Novel organic/inorganic nanocomposites LNTS-1 and CNTS-2 were synthesized from NS and alpha thioctic acid. • Compared with bulk CNTS-2, linear LNTS-1 can effectively improve the macro flexural strength of cement-based materials. • The results of adsorption experiments show that LNTS-1 has strong adsorption on calcium ions. Using organic/inorganic nanocomposites to optimize the microstructures of cement hydration products at the micro/nanometre scale is a promising method for improving the mechanical properties of cement-based materials. One type of novel nanohybrid material modified by the flexible autonomous healing polymer poly-TA (thioctic acid) was synthesized and used as a highly efficient mechanical improvement agent for cement paste. The nanohybrids linear LNTS-1 and bulk CNTS-2 were synthesized from silane-coupled nanosilica and alpha thioctic acid. Due to the existence of hydrogen bonds, coordination bonds and dynamic covalent S S bonds, this type of linear poly-TA with a certain amount of calcium content exhibited good self-healing properties. Linear LNTS-1 can effectively improve the macro flexural strength of cement-based materials without a negative effect on their compressive strength. The flexural strength of cement pastes with 1.0‰ LNTS-1 increased by 11.9%, 15.6%, and 16.9% after curing for 3, 7, and 28 d compared with the control cement paste. The results of adsorption experiments show that nanohybrid LNTS-1 has strong adsorption on calcium ions, which affects the formation of cement hydration products. [ABSTRACT FROM AUTHOR]