# Performance Evaluation of Nano-Silica-Reinforced Mortar Containing Waste Tire Rubber and Recycled Fine Aggregate: Mechanical Properties, Frost Resistance, and Microstructure Assessment

## TL;DR

This study evaluates how adding nano-silica improves the strength and durability of mortar made with recycled materials and rubber.

## Contribution

The novel use of nano-silica to enhance the mechanical and freeze–thaw resistance of rubber-recycled cement mortar is investigated.

## Key findings

- Adding 1–3% nano-silica improved compressive and flexural strength of RRCM.
- Nano-silica reduced strength loss after freeze–thaw cycles and improved microstructure.
- 3% nano-silica provided the optimal improvement in RRCM properties.

## Abstract

In the preparation of rubber-recycled cement mortar (RRCM), recycled fine aggregates (RFA) were used to replace 95% of natural fine aggregates (NFA) by mass, with an additional 5% of NFA replaced by rubber particles (RP). Additionally, nano-silica (NS) was incorporated to replace ordinary Portland cement (OPC) by mass at a replacement of 0%, 1%, 2%, 3%, and 4%. The study aimed to investigate the effects of NS on the mechanical properties, freeze–thaw resistance, and microstructure of RRCM, using techniques such as X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), and scanning electron microscopy (SEM) to reveal the enhancement mechanisms. The results indicated that the compressive strength and flexural strength of RRCM at 28 days decreased by 10.3% and 10.1%, respectively, compared to NCM. After adding 1–3% NS, the mechanical properties of RRCM were improved, with the enhancements increasing as the NS content increased. Specifically, RRCM3 exhibited a 7.7% and 7.6% improvement in compressive and flexural strength, respectively, compared to RRCM0. After 30 freeze–thaw cycles, the strength loss rate of RCM was 27.51%, whereas the strength loss rate of RRCM3 was reduced to 20.13%, with better overall appearance integrity. Moreover, NS promoted the hydration of cement; reduced the contents of tricalcium silicate (C3S), and dicalcium silicate (C2S) and calcium hydroxide (CH); and facilitated the formation of additional hydration products that filled the interfacial transition zone (ITZ). The incorporation of 3% NS was found to provide the optimal improvement in RRCM.