Keywords

1 Introduction

With the rapid development and accelerated urbanization process in our country, the volume of urban sewage treatment and collection has continued to increase. As of 2022, the total length of urban drainage pipelines has exceeded 914,000 kilometers [1]. However, approximately 30% of these pipelines were built before the year 2000, mainly serving as urban sewage mains, and are now nearing the end of their service life. These pipelines suffer from defects in materials and structures, compounded by the erosion of sewage, geological factors, and environmental influences, leading to aging and leakage issues. Consequently, the actual sewage treatment rate is only between 40% to 60%, with approximately 40% of sewage being discharged untreated directly into natural water bodies [2]. This underscores the urgency of pipeline repair, necessitating breakthrough research.

Traditional methods of pipeline network repair mainly involve excavation and replacement, which not only affect traffic and the environment but also incur high costs and construction difficulties. In contrast, non-excavation repair techniques address defects through inspection well entrances, reducing their impact on urban traffic and the environment, while being cost-effective and safe [3]. Currently, non-excavation repairs primarily use cement-based materials, but their high energy consumption and susceptibility to corrosion limit their application scope. With the increasing awareness of environmental protection, there is an urgent need to seek environmentally friendly and low-carbon alternative materials [4]. Polymer-based composite mortar, due to its excellent performance, abundant raw materials, and potential for utilizing solid waste, has become a research hotspot.

Due to the harsh working environment of sewage pipelines, higher standards are demanded for the mechanical performance and durability of the repair materials used on them. Additionally, it is difficult to achieve basic maintenance conditions within sewage pipelines, which necessitates repair materials with characteristics of high early strength and low shrinkage rate. Research has shown that modifying polymer mortar with fibers and cellulose ethers can effectively enhance its strength and durability while reducing shrinkage rate [5].

2 Experiment

2.1 Raw Materials

Kaolin (MK), obtained from Shanxi Chaopai Calcined Kaolin Co., Ltd., carries the model number K1100 and possesses a fineness of 800 mesh. The cementitious crystalline waterproofing material (CCCW), a gray powdered Xypex admixture, is supplied by Beijing Chengrong Waterproofing Materials Co., Ltd. As shown in Fig. 1 and Table 1, X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses indicate kaolin mainly comprises amorphous Al2O3 and SiO2, plus minor TiO2. CCCW contains calcium and silicon predominantly as tricalcium silicate, along with MgO and traces of Mg(OH)2 due to moisture absorption.

Fig. 1.
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XRD spectra of MK and CCCW

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Table 1. Main chemical composition of Metakaolin and CCCW
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To ensure the stability and reproducibility of multiple experiments, standard sand from Xiamen ISO Standard Sand Co., Ltd. Was selected as the aggregate. The particle size range is 0.08 mm to 2 mm, with a silica content of over 98%, and a mud content of less than or equal to 0.18%.

The alkali activator used is a laboratory-prepared water glass solution, combining previous laboratory research findings, utilizing commercial liquid sodium silicate solution, adjusted with deionized water and NaOH to a modulus of 1.2, and a concentration of 43.5%.

Hydroxypropyl methylcellulose (HPMC), a common water-retaining agent in building mortar, can slow down the rate of water loss in mortar and reduce the likelihood of drying cracks. It is purchased from Hebei Weiteng Building Materials Co., Ltd., with a molecular weight (degree of polymerization) of 200,000, a fineness of 80 to 100 mesh, and a density of 1.26 to 1.31g/cm3.

For use in non-excavation repair spraying operations of pipelines, the goal was to find a flexible and finer diameter fiber. Polypropylene fiber (PPF) meets these requirements. It is produced by Shanghai Chenqi Chemical Technology Co., Ltd., with an industrial grade polypropylene fiber (PP), length of 6mm, fiber diameter of 31μm, and a Young's modulus of ≥ 3.5 Gpa.Experimental design.

2.2 Response Surface Method

The experimental design adopted in this study is Response Surface Methodology (RSM), which is implemented using the Design-Expert software as a platform, employing the Box-Behnken design method (abbreviated as BBD) within the software for experimental design.

The HPMC dosage, CCCW dosage, and PFF dosage are selected as independent variables, represented respectively as A1, A2, and A3. The compressive strength at 28 days and 7-day shrinkage of the mortar are the response values, denoted as Y1 and Y2, respectively. A three-factor three-level experiment is conducted, with each factor corresponding to high, medium, and low levels encoded as + 1, 0, and -1. Specifically, -1 represents the low level, 0 represents the center point, and + 1 represents the high level. The specific values are shown in Table 2.

Table 2. Coding and level of independent variables
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3 Result

3.1 Experimental Plan and Results

According to the design of Box-Behnken experiment scheme, there are 17 groups of experiment points, each group contains 3 shrinkage tests and 6 compressive tests. The average value of experimental data is taken as the experimental result. After the experiment, the measured data of compressive and compressive values of specimens are obtained, as shown in Table 3.

Table 3. Response Surface Experimental Design Plan and Results
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3.2 Analysis of Variance

Utilizing DesignExpert software, regression model variance analyses were conducted for the 28-day compressive strength, 28-day flexural strength, and 7-day shrinkage values, with the analysis results shown in Table 4.

Table 4. Analysis of variance table
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In response surface methodology, the significance testing of parameters is a crucial step to evaluate their impact on the response variable. The p-value serves as a key metric in this assessment, where values greater than 0.05 are considered not significant. On the other hand, the Lack of Fit test assesses whether the model adequately accounts for the random errors in the data. A larger p-value in this test indicates a lack of significance, suggesting a lower probability of errors in the model.

From Table 4, it's observed that for the 28-day compressive strength and shrinkage values model, all p-values are below 0.0001, signifying a highly significant model with excellent fitting accuracy. Moreover, the Lack of Fit test's p-value exceeds 0.05, indicating that the lack of fit is not significant, hence the model is less likely to contain errors. According to Table 4, within the compressive strength model, the factors HPMC, CCCW, and PPF all show p-values less than 0.05, suggesting a significant influence on compressive strength. The order of impact on compressive strength from these factors is CCCW > HPMC > PPF. In the model for 7-day shrinkage values, the p-values for HPMC and PPF are below 0.0001, demonstrating their substantial effect on the mortar's 7-day shrinkage. Meanwhile, CCCW's p-value is 0.0483, closely approaching the threshold of non-significance at 0.05, indicating a minimal impact on 7-day shrinkage of mortar. The ranking of these factors’ effects on 7-day shrinkage of mortar is HPMC > PPF > CCCW.

28d Compressive strength.

Based on the variance analysis of the 28-day compressive strength, only the P-value of the interaction term AB, with 0.0303 < 0.05, indicates that the interaction between HPMC and CCCW has a significant effect on the strength of the mortar. As shown in Fig. 2(a), at a mid-range dosage of PPF (0.6%), the compressive strength of the mortar initially increases and then decreases with the increase in CCCW dosage. This phenomenon is due to the unique action mechanism of the penetrating crystalline material: in the alkaline environment within the polymer mortar, the active substances in CCCW react in the mortar's micropores and cracks to form C-S-H gel and generate a large number of ettringite (Aft) crystals. These crystals interconnect to form a three-dimensional network structure, reinforcing and filling the voids6. With the increase in CCCW dosage, the excessive generation of Aft will instead cause damage to the N-(C)-S-H gel, leading to microcracks. When the damage effect outweighs the filling reinforcement effect, it results in an increase in internal defects and a decrease in compressive strength [6]. Meanwhile, with the increase in HPMC dosage, the compressive strength shows a gradual decrease trend. This is because HPMC reduces the surface tension of the solution, has a strong air-entraining effect, increases the porosity of the mortar dramatically, reduces compactness, and consequently decreases compressive strength [7].

Fig. 2.
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The Effect of HPMC, CCCW, and PPF on 3d and 7d Strength

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Figure 2 (b) and (c) reveal that with the increase in PPF, the strength of the mortar also shows an initial increase followed by a decrease trend. This is because PPF acts as a bridging agent during hydration, restricting the occurrence and propagation of cracks in the interfacial transition zone during loading. Moreover, the dense PPF fibers form a grid structure, creating a skeleton. After filling with N-(C)-S-H gel, they enhance the continuity of the material, reduce internal defects, and thus increase compressive strength [8]. However, excessive addition of polypropylene fibers leads to noticeable aggregation during the mixing stage, resulting in a large number of macropores and interconnected pores during the hardening stage, increasing the number of defects in the mortar and reducing compressive strength.

7d Shrinkage Value

According to the analysis of variance in Table 4, the P-value of the AB interaction term is less than 0.0001, the P-value of the AC interaction term is 0.0006 < 0.05, indicating significance, while the P-value of the BC term is 0.1351 > 0.05, indicating insignificance. This proves that the AB and AC interactions have a significant impact on the 7-day shrinkage of the mortar. Analyzing the response surface plot of the AB interaction, Fig. 3 it can be observed that when the PPF dosage is at the middle level (0.6%), the response surface plot exhibits a cloak-shaped pattern with lower values in the middle and higher values around the edges. As the HPMC dosage increases, the shrinkage initially decreases and then increases. This is because the appropriate amount of HPMC forms a three-dimensional network structure and membrane structure inside the mortar, acting as both a skeleton and a filler, reducing the shrinkage of the mortar [9]. With increasing HPMC dosage, the internal pores of the mortar increase, leading to decreased structural stability and increased drying shrinkage. Similarly, with an increase in CCCW dosage, there is a trend of initially decreasing and then increasing 7-day shrinkage values. This is because the active substances in CCCW, such as C3S and MgO, generate expansive forces outwardly when forming ettringite and magnesium aluminate hydrate in the pores [10], providing compensation during the contraction period of the mortar and thus reducing shrinkage values.

Fig. 3.
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Effects of HPMC, CCCW, and PPF on 7d shrinkage values

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When CCCW is held constant at the middle level (0.8%), the response surface plot shows a noticeable downward slope with an increase in PPF dosage. This is because the addition of fibers forms a three-dimensional randomly distributed network support system in the mortar, improving the homogeneity and integrity of the mortar and reducing the occurrence of inherent cracks [11].

3.3 Best Mix Proportion

Using Design Expert software, optimization of the response surface model can be conducted. After optimization, it is determined that the optimal ratio exists for the effects of HPMC, CCCW, and PPF on the compressive strength and drying shrinkage of geopolymer repair mortar. The optimal ratios are when the HPMC content is 0.306%, CCCW content is 0.764%, and PPF content is 0.423%. Predicted values for compressive strength at 28 days and shrinkage at 7 days are 73.6 MPa and 133.98 μm, respectively. Experimental verification yielded measured values of 71.9 MPa for compressive strength at 28 days and 128 μm for shrinkage at 7 days. The errors were 2.3% and 4.5%, respectively, indicating that the use of response surface methodology results in higher accuracy in both optimization of the mix proportion and prediction.

4 Micro Analysis

4.1 Scanning Electron Microscopy (SEM) Analysis

Through scanning electron microscopy (SEM), it can be observed in Fig. 4(a) that the N(C)-A-S-H gel tightly wraps around the fibers, forming a skeleton-filling structure. This structure enhances the cohesion of the mortar, increases its overall integrity, and reduces the probability of defect formation. In Fig. 4(b), cracks can be seen in the mortar under stress or drying conditions. The addition of PPF effectively relieves some of the stress and restricts the formation and propagation of cracks.

However, the excessive addition of fibers can lead to some adverse effects. As shown in Fig. 4(c), the aggregation of fibers within the mortar is evident, with long and slender fibers intertwining, resulting in voids and defects. Additionally, the mixing process can reduce the flowability of the mortar, and the bubbles within the mortar are less likely to be broken during vibration, leading to an increase in porosity and a decrease in strength. The decrease in structural stability also affects the overall shrinkage of the mortar.

Fig. 4.
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SEM images of fiber doped samples

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4.2 X-Ray Diffraction (XRD) Analysis

Figure 5 presents the XRD spectra of polymer mortar prepared with varying proportions of HPMC and CCCW compared to the blank polymer mortar. From the graph, a significant broad peak is observed in the diffraction angle range of 25° to 30°, characteristic of the amorphous phase of N(C)-A-S-H gel12. It is clearly observed that as the HPMC content increases, the diffraction peak around 22° originating from the amorphous SiO2 characteristic peak of the original kaolinite gradually disappears, while the broad peak of N(C)-A-S-H gel becomes more prominent. This indicates that the addition of HPMC promotes the formation of more N(C)-A-S-H gel in the polymer mortar. This is because water in the polymer does not directly react but rather provides a medium and space for the reaction of the polymer [13]. The addition of HPMC enhances the water retention of the mortar, delaying the rate of water loss, allowing more sufficient reaction time between the polymer precursors and activators. However, this does not lead to an increase in mechanical strength of the polymer mortar, which may be related to the introduction of more pores internally after adding HPMC.

Fig. 5.
figure 5

XRD patterns of samples and blank groups with added HPMC and CCCW

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Comparing the XRD spectra after adding CCCW, a slight enhancement is observed in the broad peak at the diffraction angle range of 25° to 30°. This is attributed to the formation of C-S-H gel and M-A-S-H gel from the tricalcium silicate and active magnesium in CCCW under alkaline conditions.

5 Conclusion

(1) By incorporating HPMC, CCCW, and PPF, modifications can be made to geopolymer-based repair mortar, and an optimal mix ratio exists. The Box-Behnken experimental design in response surface methodology can be used to optimize the mix ratio for different objectives accurately. The established model can predict experimental results and optimal mix ratios precisely.

(2) PPF and CCCW enhance the mechanical properties of geopolymer mortar and reduce shrinkage by forming a three-dimensional support system and generating substances such as ettringite (AFt) and C(M)A-S-H gel. HPMC improves the water retention of the mortar, allowing for the early formation of more N-A-S-H gel, reducing shrinkage. However, the special air-entraining effect of HPMC can introduce a large number of harmful pores to the mortar, adversely affecting its mechanical strength.

(3) After optimization through response surface methodology, the optimal dosages of HPMC, CCCW, and PPF for achieving the dual objectives of compressive strength and shrinkage are determined to be 0.306%, 0.764%, and 0.423%, respectively. Predicted values for compressive strength at 28 days and shrinkage at 7 days are 73.6 MPa and 133.98 μm. Experimental verification yields errors of 2.3% and 4.5%.

This study highlights the effectiveness of response surface methodology in developing high-performance, low-shrinkage repair materials and provides theoretical and practical insights into material formulation optimization. However, the study acknowledges some limitations, such as insufficient consideration of environmental impacts on material performance and the lack of discussion on long-term durability, which are crucial for practical applications. Future research should delve deeper into the environmental adaptability and long-term durability of materials to provide a more solid foundation for their application reliability.