Heat preservation propertiesThe thermal conductivity of grouting materials exhibits different insulation performances due to the influence of varying component ratios. Pore size is a crucial factor impacting material insulation, but direct measurement of small pore diameters can result in significant errors. The numerical density of a material can more intuitively reflect its compactness, offering a rough estimation of the extent of pore size.From Fig. 3, it is evident that when the LY-4110 content is 35%, and MDI content is 55%, the thermal conductivity is minimized, and the density is 42.48 \(\text{kg}/{\text{m}}^{3}\). The significant increase in the fifth set of data is attributed to incomplete reaction between LY-4110 and MDI, preventing the full utilization of their respective advantages. Excluding the fifth set of data, a 10% change in PU ratio results in an approximately 8% variation in thermal conductivity. A noticeable inflection point appears in the second set of data, and at this point, the density is minimized. We attribute this phenomenon to the complete reaction between LY-4110 and MDI, forming a uniformly dense PU foam, thereby endowing it with excellent insulation properties. This result aligns with the research on PU formulation ratios conducted by Yin35.Figure 3Thermal conductivity of PU with different proportions of components.\(\text{Mg}{(\text{OH})}_{2}\) is a finely powdered material that can effectively fill the pore structure of PU, improving the insulation performance of the grouting material. From Fig. 4, it is evident that the thermal conductivity of PU with added E-51 and PU with added E-51, PMMA both show an increasing trend. With the increase in E-51 content, the thermal conductivity of the grouting material gradually decreases. An inflection point occurs when the E-51 content is 11%, at which point the thermal conductivity of the grouting material is 0.0353 W/(m K). Without considering the first set of data, adding 11% E-51 reduces the thermal conductivity of the grouting material by about 10%, compared to a 5% reduction in thermal conductivity of PU before adding the flame retardant.Figure 4Effect of different proportions of additives on the thermal conductivity of PU.When E-51 and PMMA are added to PU, the overall thermal conductivity of the grouting material decreases. Disregarding the first set of data, when the PU contains 3% E-51 and 15% PMMA, it exhibits better insulation properties. At this point, the thermal conductivity decreases by about 1% compared to 0.0353 W/(m K). We consider that E-51, being an adhesive material, disrupts some of the pore structures of PU when reacting with it, causing an enlargement of certain pore diameters. This leads to an increase in the thermal conductivity of the grouting material, consequently a decline in its insulation performance. When E-51 is added separately to PU, the initial thermal conductivity decreases. This circumstance occurs because the E-51 content is not high enough to offset its advantage in preventing temperature leakage into or out of the material. The overall decrease in thermal conductivity of the grouting material is attributed to PMMA entering the pore structure and filling some voids. This action effectively reduces the pore diameter of PU, enhancing the insulation performance of the material.ImpermeabilityTo address issues such as lining damage caused by the infiltration of water into rock pores, we aim for the new grouting material to possess excellent anti-permeability. For materials with small and interconnected pores, their mass water absorption rate is often substantial, sometimes exceeding 100%. In these cases, the volumetric water absorption rate is used to indicate the water absorption situation. However, materials with predominantly closed pores, where water is less likely to penetrate large and interconnected pores, use the mass water absorption rate to express their water absorption characteristics. The mass water absorption rate represents the ratio of the mass of water contained in the material to the mass of the material in its dry state, also known as the material’s moisture content36,37. From the above description, a lower mass water absorption rate indicates a lower moisture content in the material, implying a higher closed-cell ratio. PU, as observed under a microscope, predominantly exhibits closed-cell pores. Therefore, the mass water absorption rate is used to express its water absorption characteristics, reflecting its anti-permeability. The pore structure of PU under a microscope is depicted in Fig. 5.Figure 5Microscopic structure of PU pores.LY-4110 has hydrophilic properties that enhance the moisture sensitivity of PU, making it more prone to water absorption. The impact of varying LY-4110 concentrations on the water absorption rate is illustrated in Table 8.Table 8 Data of water absorption rate changes with LY-4110 content.From Table 8, it is evident that with an increase in LY-4110 content, the water absorption rate gradually rises. Disregarding the last set of data, for every 10% increase in LY-4110 content, the water absorption rate increases by approximately 30%. The water absorption rate of PU with 65% LY-4110 content is about 68% higher compared to PU with 25% LY-4110 content, underlining the significant impact of LY-4110 concentration on water absorption.From Fig. 6, it can be observed that additives are effective in reducing the water absorption rate of the grouting material, thereby significantly improving its anti-permeability. As the E-51 content increases, the water absorption rate of the grouting material decreases. A turning point is evident at the fourth set of data, which still aligns with the previously explained mechanism for the trend in thermal conductivity changes. When the E-51 content is low, it effectively prevents moisture from entering the pores. With an increase in the proportion of E-51, its ability to disrupt the PU pore structure enhances, transforming the originally closed-cell structure of PU into an open-cell structure, leading to an increase in the water absorption rate. When the E-51 content is 11%, the water absorption rate of the grouting material is 1.37%, representing a reduction of approximately 72% compared to the water absorption rate of the grouting material without E-51.Figure 6Variation trend of water absorption rate of PU with different substances added.The water absorption rate of the grouting material, with the addition of both E-51 and PMMA, gradually decreases without a distinct turning point. The initial decrease in the graph is rapid, followed by a gradual stabilization. This is primarily attributed to the role of E-51 in preventing moisture from entering the pore structure, effectively leveraging its advantages. Simultaneously, PMMA efficiently fills the open-cell structure, further impeding water penetration into the pores and enhancing the impermeability of the grouting material. Observing the two curves, the performance of the grouting material with the addition of E-51 and PMMA exhibits a relatively stable change without abrupt or drastic variations. This indicates a complementary synergy between the two materials, showcasing their mutual advantages and contributing to the improvement of the grouting material’s performance indicators. When the E-51 content is 15% and PMMA content is 3%, the water absorption rate of the grouting material is 1.29%. Compared to PU with only a flame retardant, the water absorption rate is reduced by approximately 74%. Additionally, compared to the grouting material with both a flame retardant and E-51, the water absorption rate decreases by around 6%. Excluding the first set of data, for every 4% increase in E-51 content and every 4% decrease in PMMA content, the water absorption rate of the grouting material decreases by approximately 22%.The lower the numerical value of the water absorption rate, the lower the moisture content in the material. This indicates that moisture is less likely to penetrate the material, or if it does, it doesn’t linger. This indirectly implies that a lower moisture content corresponds to a higher closed-cell content in the material. Consequently, the material’s anti-permeability performance is more outstanding when the moisture content is lower.Compressive strength characteristicsIn the study of thermal insulation and anti-permeability of PU, the addition of E-51 and PMMA has been found to enhance the thermal insulation and anti-permeability properties of PU. Additionally, the additive significantly influences the compressive strength of PU. The standard specification requires a compressive strength of thermal insulation boards to be ≥ 100 kPa38. However, the grouting material investigated in this study is intended for use between rocks in the surrounding rock of tunnels, where it is not subjected to substantial compression. Therefore, the compressive strength requirements can be appropriately reduced. We conducted tests for the 10% deformation compressive strength of materials with different compositions, adjusting the universal testing machine speed to 0.3 mm/min, with a maximum compression deformation of 4.5 mm. This testing method assesses the material’s compressive resistance under a certain level of deformation, providing useful insights into its performance in practical applications.The composition of PU typically comprises two segments: the hard segment and the soft segment. The soft segment refers to the long-chain alcohol groups within the PU molecular chain, often composed of alcohol-based compounds like polyether polyols. The soft segment primarily influences the flexibility and elongation of PU. On the other hand, the hard segment refers to the diisocyanate groups within the isocyanate molecular chain, predominantly impacting the hardness, brittleness, and tightly linear structure of PU39. The variation in compressive strength of the grouting material with changes in MDI content is illustrated in Table 9.Table 9 Compressive strength data as a function of MDI content.From the table, it is evident that with a decrease in MDI content, the compressive strength of the grouting material also decreases. When the MDI content is 25%, the compressive strength of PU is only 2.3 kPa. This is because the insufficient MDI content prevents the formation of a complete chain structure through reaction with LY-4110. As the MDI content decreases to 35%, the compressive strength of PU significantly drops by around 70%. For every 10% decrease in MDI content, there is an approximately 1.5% reduction in the compressive strength of PU.The addition of flame retardants makes it difficult for the PU interface to form a strong bond, disrupting the cellular structure of PU, and resulting in a 63% reduction in compressive strength.From the Fig. 7, it can be observed that, with the addition of E-51 and PMMA, the compressive strength of the grouting material shows an overall increasing trend with a distinct turning point. After adding E-51, there is initially a significant decrease in compressive strength, but as the E-51 content gradually increases, the compressive strength begins to rise. This phenomenon is primarily due to E-51 disrupting the cellular structure of PU, making its connections fragile and causing a substantial decrease in compressive strength. As the E-51 content increases, the excellent compressive strength advantages of E-51 become increasingly evident. With an 11% addition of E-51, the compressive strength of the grouting material increases by approximately 60%.Figure 7Effect of different additives on the compressive strength of materials.PMMA is a resin material with a certain compressive strength. From the graph, it can be seen that the compressive strength of the grouting material with the addition of E-51 and PMMA remains relatively stable. The overall compressive strength of the grouting material with the addition of both E-51 and PMMA is superior to that of the grouting material with only E-51. When the E-51 content is 11% and the PMMA content is 7%, the compressive strength of the grouting material is 89.57 kPa. This represents an increase of approximately 26% compared to the compressive strength of the grouting material with only E-51. Furthermore, compared to the compressive strength of PU with only flame retardant, there is an increase of around 25%. The descending segments in both curves are considered to be due to an excessively high E-51 content, causing more damage to the cellular structure, and PMMA being unable to fill all the voids, resulting in a subsequent decrease in compressive strength.
