The experiment used pure cement grout and cement-clay grout to create grouted stone samples, with mix proportions corresponding to standard practices for grouting curtains in karst areas. The cement utilized was again 42.5R Ordinary Portland Cement, adhering to the national standard of normal Portland cement in China. The clay component consisted of specialized calcium-based clay from Lingshou County, Hebei Province, China. The water-cement ratio for the pure cement slurry was 1:2, while the mixing ratio for the cement-clay slurry was 2:1, with the clay content making up 50% of the cement weight. The grouting process parameters, such as pressure and duration, play a critical role in ensuring uniform penetration and minimizing head losses during injection, as highlighted in prior fluid dynamics studies (Haroon et al., 2017). During the preparation process of grouting stone bodies, a grouting pressure of 1 MPa was applied for a duration of 6 s. The samples were cured under standard conditions of 20°C ± 1°C and ≥95% humidity. Figures 1–3 illustrate the test preparation process, test equipment and grout stone samples.

The critical challenge in conducting laboratory accelerated degradation tests lies in constructing an accelerated erosion environment that effectively replicates the long-term service conditions of grout curtains. In this work, the degradation process of grouted specimens was accelerated by controlling the concentrations of aggressive ions in simulated karst groundwater and adjusting the curing temperature. The transport mechanisms governing the ingress of aggressive ions into grouted matrices typically include adsorption, diffusion, pressure-driven permeation, and dispersion. Among these, the diffusion of aggressive ions is generally considered to be the most important migration mechanism (Yu and Sun, 2006). When neglecting the binding capacity of aggressive ions within grouted matrices and the influence of time-dependent structural defects on diffusion coefficients, the theoretical model for ion diffusion can be derived from Fick’s Second Law (Yu and Sun, 2006; Wang, 2016), as shown in Equation 1 and Equation 2. c =   c 0 + c s − c 0 1 − erf x 2 D t (1) e r f u = 2 π ∫ 0 u e − t 2 d t (2) where c ₀ and c _s are the initial erosive ion concentration in the grouted stone and the erosive ion concentration on the exposed surface, respectively. And erf is the error function.

According to the test design, it can be considered that c ₀ = 0, then Equation 1 can be simplified to Equation 3: c =   c s 1 − erf x 2 D t (3)

Therefore, the diffusion of aggressive ions in the experiment exhibits direct proportionality to the surface concentration c _s. When the erosive ion concentration at the exposed surface of grouted matrices increases by a factor of 20, the corresponding acceleration factor s ₁ for the test duration also increases by 20 times.

The fundamental mechanism of aggressive ion diffusion in grouted matrices involves random thermal motion of ions through the pore solution within the matrix. The diffusion coefficient of aggressive ions demonstrates a positive temperature dependence (Lindvall, 2007). As described by the Arrhenius equation (Zuo, 2021), the influence of ambient temperature on the diffusion of aggressive ions within grouted matrices is quantified by Equation 4: D t T = D 0 e E R 1 T 0 − 1 T (4) where: D ₀ is the diffusion coefficient of aggressive ions at temperature T ₀, m²/s; D _T is the diffusion coefficient of aggressive ions at temperature T, m²/s; T ₁ is the reference temperature, generally 293 K; E/R is the activation energy, which is taken as 14,242 in reference (Wang, 2019).

According to literature (Jiang et al., 2014; Pan et al., 2023), the annual average water temperature of rivers in Changsha area is about 18°C (290.9 K), and constant temperature 30°C (303 K) was used in this experiment to maintain the grouted stone test. The acceleration coefficient s ₂ is calculated in Equation 5. S 2 = D T 303 D T 290.9 = D 0 e 14242 1 293 − 1 303 D 0 e 14242 1 293 − 1 291 = 6.95 (5)

Therefore, the total acceleration coefficient S of this degradation test is equal to the product of external erosive ion concentration and temperature acceleration coefficient, that is, S = 138.94.

Based on the pre-determined 20 times accelerated erosion factor, the karst water curing solution was prepared at 20 times the concentration of natural karst water. According to investigations, chloride (Cl⁻), bicarbonate (HCO₃ ⁻), and sulfate (SO₄ ²⁻) are the primary aggressive ions in karst water affecting grouted matrices. Referencing groundwater monitoring data from a karst-area dam in Hunan Province, Table 1 lists the normal and accelerated concentrations of these three ions.

The curing solution was prepared using sodium chloride, sodium bicarbonate, and potassium sulfate, with an analytical purity of ≥ 99%. Based on calculations of ion concentration, each liter of the prepared karst water curing solution contained 1.52 g of sodium chloride, 8.91 g of sodium bicarbonate, and 21.61 g of potassium sulfate. A self-made accelerated curing box was utilized to accelerate the erosion and deterioration of the stone test block. A small heater was placed at the bottom of the curing box to control the temperature of the karst water, which was set to 30°C within a control range of 20°C–35°C. The accelerated karst water erosion test is shown in Figure 4.

To investigate the degradation of physical and mechanical performance in CGS (Group A) and CCGS (Group B) specimens under karst water erosion, we conducted analyses of compressive strength, porosity, mass loss, and scanning electron microscopy (SEM) on pre-prepared specimens. The tests adhered to the technical code for testing in China (Ministry of Transport of the People’s Republic of China, 2019; Ministry of Construction of the People’s Republic of China, 2019). A summary of the experimental design, including test conditions, is presented in Table 2. The procedures were as follows.

(1) Specimen preparation and curing. CGS and CCGS specimens were cast, then placed in water-filled tanks for standard curing under controlled conditions (temperature: 20°C ± 1°C, humidity: ≥95%) for 28 days.

The compressive strength test results for grouted stones under accelerated karst water erosion are summarized in Figure 5. With prolonged curing time, the compressive strength of CGS (Group A) decreased from 32.89 MPa to 20.03 MPa, while CCGS (Group B) declined from 3.89 MPa to 2.03 MPa. Notably, Group A exhibited significantly higher strength than Group B. This difference stems from the charge interaction in cement-clay grout: positively charged cement hydration particles partially exchange charges with clay particles, inducing clay agglomeration to form a gel-structured skeleton. However, this skeleton demonstrates inferior mechanical performance compared to the robust particle skeleton created by cement hydration (Feng, 2015). Furthermore, both groups experienced progressive strength reduction over time due to chemical erosion from karst water. The dissolution of calcium hydroxide and degradation of hydration products (e.g., calcium silicate hydrate and calcium aluminate hydrate) into less cohesive compounds gradually compromised the structural integrity of the grouted stones, diminishing their load-bearing capacity (Zhou et al., 2022).

Figure 6 illustrates the reduction rates of compressive strength for Groups A and B grouted stones. After 120 days of curing in a karst water environment, Group A exhibited a strength reduction rate of 39.10%, while Group B showed a higher reduction rate of 47.67%, indicating that Group A experienced less strength degradation compared to Group B. This disparity arises because cement particles, with their coarser size and superior cementation properties, form a more stable granular skeleton structure in cement grout (Xie et al.,2025). This enhanced stability mitigates strength deterioration in cement grout stones relative to cement-clay grout, where the weaker clay-cement matrix is more susceptible to karst water erosion. These results further confirm that cement-clay materials exhibit inferior resistance to karst water erosion compared to pure cement materials (Bai, 2001).

Additionally, Figure 7 depicts the compressive failure morphology of grouted stone specimens cured in karst water for varying durations. Unconfined compressive strength tests revealed that failure occurred through a progressive crack propagation process: cracks initiated at the base and outer surfaces, extended upward and inward, and ultimately coalesced to cause specimen failure. The crack orientation aligned predominantly with the axial loading direction. Notably, no significant shear failure was observed during the entire unconfined compression process, demonstrating that structural disintegration was driven primarily by tensile splitting rather than shear mechanisms.

Figure 8 shows the porosity test results of grouted stone specimens under different accelerated karst water erosion conditions. With the increase in curing time, the porosity of Group A increased from 26.06% to 33.91%, while that of Group B increased from 58.04% to 77.01%. The porosity of Group B was significantly higher than that of Group A. This is because the cement-based grouting material used in Group A exhibited better fluidity and higher density compared to the clay-cement grouting material in Group B. The porosity of both Groups A and B increased with curing time. The primary reason is that in the karst water environment, calcium hydroxide (Ca(OH)₂) within the grouted stones dissolves and reacts with erosive ions, reducing its concentration in the surrounding water. This process accelerates the decomposition and dissolution of other hydration products, resulting in continuous loss of chemical substances and gradual enlargement of pores within the grouted stones (Tang, 2019).

As shown in Figure 9, after 120 days of curing in the karst water environment, the porosity increase magnitude of Group A reached 30.12%, while that of Group B was 32.68%. The smaller variation in Group A’s porosity aligns with its lower strength reduction compared to Group B, and the underlying mechanisms are similar. Additionally, it was observed that the porosity growth rates of both groups were relatively slow during the first 90 days of curing. However, after 90 days, the rates accelerated. This phenomenon can be attributed to calcium hydroxide being the primary dissolved component in the early stage. As pores enlarged and increased, the contact area between various hydration products and karst water increased, accelerating the dissolution and erosion rates of calcium hydroxide and other hydration products, thereby hastening porosity growth (He, 2019). Pore structure and thermal behavior of subsurface materials have been extensively modeled to assess effective thermal conductivity and degradation behavior (Rizvi et al., 2020b).

The experimental study on grouted stone body specimens cured in clear water and karst water environments revealed distinct mass variation trends over different periods (as shown in Figure 10). Prior to 60 days of curing, the mass change rates of Groups A and B showed minimal differences between the two environments. However, beyond 60 days, specimens in karst water exhibited significantly greater mass variations compared to those in clear water, with the divergence amplifying over time, indicating accelerated deterioration effects from karst water. At 150 days, Group A specimens demonstrated mass change rates of 3.00% (clear water) and 4.50% (karst water), while Group B reached 4.63% and 6.44%, respectively. Notably, Group B consistently displayed higher mass variation rates than Group A. Early-stage chemical reactions between erosive ions in karst water and hydration products (e.g., calcium hydroxide dissolution) initially reduced specimen mass. However, subsequent formation of expansive minerals like gypsum and ettringite partially filled pore structures, temporarily offsetting mass loss through physical expansion while retarding further dissolution. For Group A specimens, this expansive filling initially outweighed dissolution effects, causing a slight mass gain. Prolonged exposure ultimately weakened cementitious bonding, inducing surface softening and edge spalling in both groups, thereby gradually reversing the mass trends. Underground structural performance also varies with geometric and geomechanical parameters, as seen in tunnel stability studies (Alsabhan et al., 2021).