Experimental Research On Water Erosion Modification Of Pisha Sandstone

Pisha sandstone（PS）is a special rock interlayer distributed in the bordering areas of Shanxi,Shanxi and Inner Mongolia in China,which is a harsh ecological environment,subject to multiple erosion coupling and serious soil erosion,and is a mainly source area of coarse sediment in the middle reaches of the Yellow River.The construction of silt dams is the primary means to prevent soil erosion in the PS area.However,the gullies and ravines in the PS area make traditional dam construction materials scarce,inconvenient transport and the excessive cost.It is economical and practical to use modified PS as dam building material in situ.In this paper,the precipitation patterns of each mineral ion in PS under different water erosion conditions and the effects of water erosion on the mineral chemical composition,mechanical properties and microstructure morphology of PS are investigated by water erosion modification of PS.The main conclusions are as follows:（1）The water erosion modification reduces the particle size of PS and decreases the distribution width of the particle size.When the leaching agent is water,the free swelling volume（FSV）in water of water erosion Pisha sandstone（WEPS）is lower than that of the untreated PS,and the FSV is mainly affected by the particle size and its distribution width.When the leaching agent is Na OH solution,the FSV of WEPS is even higher than that of PS,and the FSV is mainly affected by the hydration swelling of alkali-silicate reaction（ASR）gel.（2）The ASR gel will enhance the interparticle cementation and improve the strength of modified PS,and the unconfined compressive strength of the specimens reached up to 3.12MPa.When additional alkaline environment exists,the leaching agent alkali concentration and the increase of single leaching time both produce excessive ASR gels,resulting in the binding force outside the modified specimen being less than the swelling stress generated inside,thereby creating cracks on the surface of the specimen and adversely impacting the strength and water resistance of the specimen.（3）The blending of fly ash and the provision of additional alkaline environment play a decisive role in the enhancement of strength and water resistance of the modified specimens.The former provides abundant reactive calcium to the modified specimens,converts ASR gels into C-A-S-H gels,etc.,eliminates their hydration swelling effect and enhances interparticle cementation,while the latter leads to more adequate ion exchange.The strength of the modified specimens made based on the combined effect of fly ash and additional alkaline environment can reach 5.11～8.04 MPa,and the softening coefficient is between 0.40～0.98.（4）When the leaching agent is water,the water erosion modification has little effect on the XRD and SEM results of the PS.The increase in the number of stirring and the amount of single leaching agent causes the diffraction peak of montmorillonite to shift to the right,which decreases the intensity of the diffraction peak.ICP and XRF results show that the increase in the number of stirring and the amount of single leaching agent promotes the leaching of Si⁴⁺and reduces the amount of Si O₂ in the solid.The amount of Ca²⁺leached in the leachate was higher than that of Si⁴⁺and Al³⁺.Based on the concentration difference,the diffusion of cations adsorbed between the layers of calcium montmorillonite into the leachate occurred,and the concentration of Ca²⁺increased with the increase of single leaching agent dosage up to 0.62mmol/L.（5）When the leaching agent was Na OH solution,XRD and TG/DTG results showed that the calcium montmorillonite in PS was transformed to sodium montmorillonite by ion exchange,and the adsorbed water between its layers was decreasing with the increase of alkali concentration,and the crystal structure was destroyed and transformed to amorphous silicate.The ICP results showed that the concentrations of Si⁴⁺,Al³⁺and Ca²⁺increased with the increase of leaching agent alkali concentration up to 0.52,0.67 and 0.04 mmol/L,respectively,and increased and then decreased with the increase of single leaching time.SEM-EDS results show that a large number of ASR gels interconnected in a reticular form are present on the surface of the WEPS particles,and their number increases with increasing single leaching time,and dumbbell-shaped C-A-S-H gels are generated with Ca/（Al+Si）ranging from 1.0 to 1.7.