PVA-Modified Grout Strengthens a Fragile Sandstone Aquifer Interface
Weakly cemented sandstone in coal mine aquifers fails for a familiar reason: water and mining disturbance strip away cohesion faster than the rock can respond. The result is particle softening, a widening of water-conducting channels, and a roof or aquifer system that loses integrity under load. In that setting, grouting is not simply a filling operation. It has to restore continuity, carry stress, and seal pathways before seepage spreads through the pore network.
This study tested whether polyvinyl alcohol could solve a practical problem in coal gangue-based grouts: high gangue content weakens early hydration activity, while the resulting slurry often struggles to bond consistently with weakly cemented sandstone. Cement and coal gangue formed the base system, and PVA was added at different contents to examine how the modifier changed slurry behavior, hydration products, and the interface between grout and rock. The work focused on a coal mine aquifer setting where the target is not an idealized crack in strong rock, but a porous, water-sensitive sandstone matrix that can fail at the interface before the grout itself fully hardens.
One result came through immediately. The grout did not behave the same way at every PVA dose.
Flow Stability, Setting Behavior, and Water Retention Shift with PVA Dose
High coal gangue content tends to blunt the early reactivity of cementitious slurry. Once PVA entered the mix, fluidity increased, the initial and final setting times lengthened further, and the bleeding rate declined. Those changes matter because a grout that separates water too quickly or stiffens too early cannot migrate into pores and fractures with enough continuity to create a sealed reinforcing zone. Better water retention also helps maintain particle suspension during placement, which is especially important when the target rock is soft and structurally irregular.
The dose response was not linear in a simple upward direction. Instead, the data showed a clear optimum inside the tested range. Peak stresses for the 0%, 0.3%, 0.6%, and 1.0% PVA groups were 4.87 ± 0.14 MPa, 5.35 ± 0.06 MPa, 6.04 ± 0.09 MPa, and 5.12 ± 0.09 MPa, respectively. The 0.6% group produced the highest bearing capacity of the grout-rock complex, while the 1.0% group did not continue the improvement. In a grouting system, more polymer is not automatically better; too much can begin to interfere with the balance between hydration, rigidity, and interface transfer.
Hydration Products Fill Pores While Polymer Film Softens the Interface
The microstructural findings explain why the 0.6% group performed best. XRD, FTIR, and SEM evidence showed that an appropriate PVA dose promoted the formation and deposition of C-S-H gel, AFt rod crystals, and C-H flake crystals on sandstone particle surfaces and within pores and fractures. Those hydration products provide the rigid skeleton needed to carry compressive load and lock the grout into the rock fabric. At the same time, PVA molecules with abundant hydroxyl groups can adsorb and coordinate with water molecules, hydration products, and active groups on coal gangue surfaces, making the polymer part of the interfacial architecture rather than a passive additive.
The interface becomes stronger because two different mechanical behaviors are working in tandem. Hydration products densify the structure and improve continuity, while the flexible PVA film buffers deformation and helps absorb stress transfer at the grout-rock boundary. In weakly cemented sandstone, where failure often begins at a poorly bonded surface, this combination is clinically important in engineering terms: the material has to bridge a mismatch between rigid mineral skeleton and deformable porous rock. A brittle grout alone can crack at the boundary. A polymer-only system would lack the mineral backbone to support sustained load. The modified mix links both functions.
Why the 0.6% Formulation Outperformed the Higher Polymer Dose
The most useful interpretation is not that PVA universally strengthens the system, but that it reorganizes the balance between hydration and flexibility. At the right level, the polymer supports slurry stability, delays premature water loss, and improves the contact conditions needed for hydration products to settle into pore spaces and fractures. At excessive content, the same modifier can begin to dilute or disrupt the mineral bonding network, limiting the final load-bearing response.
This is a recurring challenge in solid-waste-based grouting materials. Coal gangue helps with resource utilization and cost control, but its high proportion can suppress early-age cement activity. PVA can compensate, yet only within a narrow working window. The best-performing formulation in this study therefore reflects a compromise between rheology, setting behavior, water retention, microstructural densification, and mechanical capacity. In practical terms, a grout that is too fluid will escape the target zone; one that sets too fast will not penetrate; one that hardens without a strong interface will still fail when the surrounding rock deforms.
Coal Gangue Grouting Gains a More Realistic Design Framework
The work is especially relevant because weakly cemented sandstone aquifers do not behave like clean laboratory cracks. Their pore structure, water sensitivity, and poor natural cementation mean the grout must work at both the macroscopic and microscopic levels. This study ties those levels together by showing how PVA influences the slurry as a whole, the hydration products that form during hardening, and the interfacial microstructure that ultimately controls load transfer.
The engineering implication is direct: a 0.6% PVA content achieved the best balance of slurry performance, water retention stability, hydration cementation, and bearing capacity in the tested range. For grouting reinforcement of weakly cemented aquifers, thickening of aquicludes, and prevention of mining-induced water disasters, that balance is the point. The material has to be stable enough to place, reactive enough to harden, and compliant enough to survive the interface without tearing itself apart under stress.
The study does not claim that this formulation solves every aquifer reinforcement problem. It does, however, define a workable material window for coal gangue-rich grouts in soft, water-sensitive sandstone, where interface failure often decides whether sealing succeeds or collapses.