Abstract:

Leakage under flowing water is a challenge issue in underground engineering. Grouting materials must remain injectable before reaching the target zone and set rapidly after placement to resist washout. Magnesium phosphate cement （MPC） offers fast setting， high early strength and good bonding， making it promising for dynamic water sealing. However， MPC suffers from severe dilution and washout under the high water-to-solid ratio required for two-component grouting.

This study aims to improve the anti-washout performance of MPC grouting materials by incorporating two thickening components into the MgO slurry and the ammonium dihydrogen phosphate （ADP） solution， respectively. The combined effects of hydroxypropyl methylcellulose （HPMC） and sodium polyacrylate （PAAS） on the workability， rheology， mechanical properties， and water dispersion resistance of MPC were systematically investigated. The anti-washout mechanism was further explored via X-ray diffraction （XRD）， thermogravimetric analysis （TG-DTG）， scanning electron microscopy （SEM）， conductivity measurement， and pH analysis.

The results show that HPMC （0.2% by mass of MgO） can form a stable spatial network structure in the MgO slurry， effectively reducing bleeding to nearly 0% and improving suspension stability. However， when mixed with the phosphate solution， the HPMC network collapses due to the salting-out effect caused by the high ionic strength environment （Mg²⁺， NH{}_{\mathrm{4}}^{+}， H₂PO{}_{\mathrm{4}}^{-}）， leading to negligible improvement in anti-washout performance. In contrast， PAAS （0.2% to 1.1% by total mass of ADP and borax） can form a stable three-dimensional network in the presence of borax， which acts as a crosslinking agent. Even under high ionic strength， the PAAS network remains intact， significantly increasing the yield stress and consistency coefficient of the MPC slurry. The Herschel-Bulkley rheological model fitting results show that with increasing PAAS content， the yield stress （τ₀） increases from 36.18 MPa to 943.20 MPa， and the consistency coefficient （k） increases from 1.63×10^-3 to 60.81×10^-3， indicating a strong shear-thickening behavior （n>1）.

The optimal formulation is achieved with 0.2% HPMC in the MgO slurry and 0.8% PAAS in the phosphate solution （P-0.8 group）. Under a water flow velocity of 0.8 m/s， the grout retention ratio reaches 0.88， and the 3 d water-to-land strength ratio exceeds 0.9. Compared to the unmodified group （P-0）， the compressive strength at 28 d increases slightly from 3.60 MPa to 3.71 MPa. In contrast， the HPMC-modified group （H-0.8） shows significantly lower strength （2.64 MPa at 28 d） due to the introduction of micro-air bubbles and network collapse. XRD and TG-DTG analyses confirm that PAAS does not alter the hydration products （mainly MgO and struvite）， but promotes struvite formation slightly. SEM observations of freeze-dried hydrogels further reveal that borax crosslinking enables PAAS to maintain an intact porous network even in the presence of ADP， effectively resisting the salting-out effect.

In conclusion， PAAS is more suitable than HPMC for modifying MPC grouting materials in dynamic water environments due to its superior structural stability under high ionic strength. Borax plays a dual role as a retarder and a crosslinking agent， enhancing the network stability of PAAS. This study provides a new technical approach for applying MPC in dynamic water sealing of underground engineering， with tunable anti-washout performance by adjusting PAAS dosage.

Key words: magnesium phosphate cement, MgO, sodium polyacrylate, hydroxypropyl methylcellulose, water dispersion resistance, setting time