Acoustic emission and microstructural insights into a ternary OPC-SAC-UFA cementitious grout

To address the issues of poor injectability, delayed setting, and low early strength of cement-based grouting materials, a ternary composite grout was developed by partially replacing ordinary Portland cement (OPC) with calcium sulfoaluminate cement (SAC) and ultra-fine fly ash (UFA), combined with a polycarboxylate superplasticizer (PCE). The effects of the SAC-to-UFA mass ratio (mSAC:mUFA), PCE dosage, and water-to-binder ratio on the workability, mechanical properties, and acoustic emission (AE) damage characteristics were systematically investigated. The intrinsic relationship between microstructural evolution and macroscopic performance was elucidated through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results demonstrate that: (1) As the mSAC:mUFA ratio increased from 0.1:1 to 2.0:1, the initial setting time was shortened from 41 min to 16 min and the 3 d compressive strength increased from 2.8 MPa to 7.5 MPa; however, the 28 d strength decreased by 51.2% and the fluidity was reduced by 40.7%. At an optimal PCE dosage of 0.3%, the fluidity reached 29.3 cm and the compressive strength attained its maximum value at all curing ages. When the water-to-binder ratio was raised from 0.4 to 2.0, the apparent viscosity declined from 97.3 mPa s to 5.1 mPa s, whereas the bleeding rate increased from 0.8% to 48.8% and the 28 d strength was substantially degraded.(2) The mSAC:mUFA ratio, PCE dosage, and water-to-binder ratio jointly governed the crack failure mode and AE characteristics of the hardened grout. A low mSAC:mUFA ratio resulted in stable AE activity, whereas a high ratio induced high-energy brittle failure. At a PCE dosage of 0.3%, tensile cracks dominated the entire loading process, with their proportion increasing from 51.5% at the compaction and crack initiation stage to 82.3% at the post-peak failure stage; under insufficient or excessive PCE dosages, the shear crack proportion increased markedly. Specimens with a low water-to-binder ratio displayed fewer AE events with higher energy, accompanied by a stepwise rise in the cumulative ring-down count, whereas those with a high ratio exhibited sustained low-energy AE emissions.(3) SAC provided early-age strength through the rapid formation of an ettringite skeleton, UFA improved fluidity via the physical ball-bearing effect of its spherical particles and densified the hardened matrix through subsequent pozzolanic reactions, and PCE optimized workability and regulated the hydration rate through the synergistic dispersion effect arising from side-chain steric hindrance and electrostatic repulsion. These three components acted sequentially during the early-age skeleton formation stage, the intermediate dispersion and regulation stage, and the later-age densification stage, achieving a synergistic balance among fluidity, setting time, and strength development that is difficult to attain with conventional single- or binary-component grouting materials.