Nano Calcium carbonate reinforced with polypropylene fiber for alkali-activated recycled concrete: Mechanical properties and frost resistance study

Addressing the insufficient mechanical and frost resistance properties of alkali-activated recycled concrete, the synergistic modification effect of nano-calcium carbonate (NCC) and polypropylene fibers (PP) was investigated. Orthogonal experiments demonstrated that when 1% NCC and 0.3% PP were used in synergy, the 28-day compressive strength reached 28.3 MPa (an increase of 43.0% compared to the baseline group) and the flexural strength was 6.8 MPa (an increase of 65.9%). After 250 freeze-thaw cycles, the mass loss rate was 1.1% and the relative dynamic elastic modulus was 60%, indicating a significant improvement in frost resistance. Based on the Poly2D and Power2D models, a multi-factor prediction model for the influence of NCC and PP content, as well as age, on strength and frost resistance was established, with goodness-of-fit R² values exceeding 0.91. Microscopic analysis confirmed that NCC filled pores and accelerated hydration, while PP inhibited crack propagation. Together, they formed a "nano-filling - fiber bridging" reinforcement network, providing a basis for optimizing the performance of recycled concrete.