Experimental study on the physical and mechanical properties of magnesium phosphate cement-based composite grouting material and its performance in steel rebar connections

To meet the requirements of prefabricated building steel rebar connection joints for early strength, high fluidity, minimal expansion, and excellent durability of grouting materials, a novel Magnesium phosphate cement-based composite grouting material (MPCCGM) is proposed. Through systematic experiments, the effects of the magnesium-phosphate ratio (M/P), water-binder ratio (W/B), borax (Na 2 B 4 O 7 ·10H 2 O) content, glucose (C 6 H 12 O 6 ) content, and steel fiber content on the physical and mechanical properties and microstructure of MPCCGM were investigated. The optimal mix proportion was determined, and the vertical expansion rate, autogenous drying shrinkage, chloride ion (Cl - ) content, and mechanical performance of steel rebar sleeve grouting connection joints were evaluated. The results indicate that MPCCGM exhibits optimal engineering performance when M/P = 3.2, W/B = 0.16, a combined addition of 7% Na 2 B 4 O 7 10H 2 O and 1% C 6 H 12 O 6 is used as retarders, and the steel fiber content is 2%. Under these conditions, the fluidity reaches 304 mm, the setting time is 16.9 min, and the 28d compressive strength attains 85.1 MPa. The 3 h vertical expansion rate is 1.1%, the 28d autogenous drying shrinkage is 0.02%, and the Cl - content is 0%, all of which satisfy the relevant standard requirements. The hydration products of MPCCGM are mainly struvite (MgKPO 4 ·6H 2 O) crystals with a dense microstructure, and good bonding is observed between the steel fibers and the matrix. In addition, when the anchorage length of Φ 14 steel rebar is 8D (eight times the steel rebar diameter), the tensile strength of the connection joints reaches 626 MPa at 14d and 634 MPa at 28d, both exceeding the standard tensile strength of the steel rebar. The residual deformation ranges from 0.03 to 0.04 mm, and the failure mode is steel rebar fracture, with firm bonding observed between the grouting material and the steel rebar as well as the sleeve. MPCCGM exhibits advantages including rapid hardening and early strength development, good fluidity, slight expansion, low shrinkage, and no risk of chloride-induced corrosion, providing a high-performance and construction-friendly grouting material for prefabricated building steel rebar connections with broad application prospects.