Development Of Low PH And High-performance Grouting Materials And Their Engineering Performance Studies

In the construction of a deep geological disposal repository for highly radioactive nuclear waste,the combined effects of excavation disturbance,groundwater movement,buffer/backfill material swelling,and adverse geological factors caused by long-term crustal movement can cause certain damage to the surrounding geological rock mass of the repository.Therefore,grouting is necessary to improve the internal structure of the surrounding geological rock mass.The most commonly used silicate cement-based grouting material currently produces alkaline hydration products with p H≥11 during its hydration process,and this alkaline substance will continuously migrate with the movement of water,causing the main minerals in the buffer/backfill material to dissolve and phase change,greatly weakening the buffering and sealing performance of the buffer/backfill material,and thus unable to ensure the stability and safety of the disposal warehouse construction and service period.Magnesium phosphate cement has many excellent properties that ordinary Portland cement does not have,and is an inorganic cementitious material with p H<11.The magnesium phosphate cement slurry under the common mixing ratio is viscous,the setting time is short,the workability is poor,the compressive strength is too high,and the working performance and mechanical properties are extremely unbalanced,and it cannot be directly used as a surrounding rock grouting material.Therefore,this paper proposes to take magnesium phosphate cement as the research object,study the influence of different factors on the working properties,mechanical properties,rheological properties and durability of magnesium phosphate cement,and develop high-performance magnesium phosphate cement-based grouting materials suitable for the construction of high-level waste storage warehouses.（1）Design and performance optimization of basic mix ratio for magnesium phosphate cement based grouting materialsBy adjusting and optimizing the material mix ratio,combined with macroscopic properties and microscopic analysis,the effects of water-cement ratio,magnesium-phosphorus ratio,borax content and ACID salts content on the setting time,fluidity and 28-day compressive strength of hydrated stone body of magnesium phosphate cement slurry were studied.By testing the hydration temperature,hydration ion concentration and hydration p H value of magnesium phosphate cement hydration system under different ACID salts content,and using microscopic testing techniques such as SEM and XRD,the retardation mechanism of ACID salts was further improved.The results show that the addition of ACID salts can significantly improve the setting time of magnesium phosphate cement slurry without significant effect on its fluidity.With the increase of ACID salts content,the compressive strength of magnesium phosphate cement stone body first increased and then decreased.The addition of ACID salts inhibited the dissolution of Mg²⁺in the hydration system,and also inhibited the conversion of HPO₄^2-to PO₄^3-.（2）Study on rheological properties of freshly mixed magnesium phosphate cement pasteBy changing the material mix ratio,the effects of magnesium-phosphorus ratio,water-cement ratio and ACID salts content on the rheological properties of newly mixed magnesium phosphate cement slurry were studied.Different rheological models were used to fit the rheological curves of the slurry,among which the Herschel-Bulkley model had the highest fit,and the rheological parameters of the fitted slurry were analyzed.The results show that when the magnesium-phosphorus ratio is controlled within3,the rheological properties of the freshly mixed magnesium phosphate cement slurry are not much different.With the further increase of the magnesium-phosphorus ratio,the yield stress and rheological index of the slurry gradually increased,and the rheological properties deteriorated significantly.When the content of ACID salts is controlled within 5%,ACID salts has little effect on the rheological properties of the slurry.With the further increase of ACID salts content,the yield stress and rheological index of the slurry gradually increased,and the rheological properties deteriorated significantly.With the increase of the water-cement ratio,the yield stress and rheological index of the slurry gradually decrease,and the rheological properties of the slurry can be improved by increasing the water-cement ratio.（3）Study on the modification of magnesium phosphate cement based grouting material by water resistant modifierBased on the benchmark mix ratio of the best magnesium phosphate cement-based grout material,the effects of water resistant modifier content on the hydration characteristics,physical properties,mechanical properties,water resistance and p H value of stone leaching liquid of magnesium phosphate cement were studied by the mixture of water resistant modifier in magnesium phosphate cement-based grout materials.At the same time,XRD,SEM and other microscopic test methods were used to study the microstructure and phase composition of magnesium phosphate cement stone body with different immersion times under different water resistant modifier content.Considering the properties of the material,the optimal content of water resistant modifier was finally determined to be 2%.water resistant modifier participated in the hydration reaction of magnesium phosphate cement slurry,generated new hydration products,and improved the water resistance of magnesium phosphate cement-based grout materials.There are 46 figures,23 tables and 80 referrences in this dissertation.