Introduction During the long-term service of the tunnel in high-geothermal environment, one side of the lining structure is in contact with the high-temperature rock wall, and another side is the normal temperature opening area of the tunnel. The temperature field is formed inside the lining concrete. The durability deterioration of lining concrete is the result of the coupling effect of temperature field and sulfate attack. The existence of temperature field under the coupling effect accelerates the diffusion of SO42- from the outside to the inside of concrete as well as the chemical reaction and damage degree of attack. The deterioration mechanism of sulfate attack in high-geothermal environment is different from that in normal temperature environment. However, an indoor simulation test in high-geothermal environment adopts a hot sulfate solution soaking method. Although this method can achieve the acceleration of high temperature, it cannot simulate the effect of temperature field. The concrete corner is subjected to two-way high temperature under the method of hot sulfate solution immersion, resulting in more serious corner deterioration. Therefore, the scientific basis and rationality of the hot sulfate solution soaking method are still debatable. In addition, the pore structure characteristics of concrete are a key to affecting sulfate attack. The existing research on the pore structure of sulfate attack concrete mostly adopts the index of pore size distribution and its corresponding pore content, which cannot fully reflect the mechanism of sulfate attack. Fractal theory is proved to quantitatively characterize the complexity and irregularity of the pore structure of cement-based materials, and can also link the pore structure characteristics of cement-based materials with macroscopic properties. Therefore, this paper proposed a durability simulation system for the coupling of one-dimensional temperature field and sulfate attack. A fractal dimension method was used to investigate the pore structure characteristics of lining concrete under the coupling of temperature field and sulfate attack. Based on the layered damage theory, the relationship between the pore structure characteristics and the average compressive strength of the damaged layer, and the overall concrete strength under the coupling effect were analyzed.Methods A Portland cement 42.5 (GB175—007 standard) and a fly ash with grade II (GB/T 1596—2017 standard) were used. The fineness modulus of the river sand was 2.7, and the coarse aggregate was gravel with a gradation of 5-25 mm. A water reducing agent was polycarboxylate as a superplasticizer, and the water reducing rate was 25%. According to the actual engineering test, a mix ratio is cement of 320 kg/m3, fly ash of 80 kg/m3, water-binder ratio of 0.38, as well as sand and gravel of 740 kg/m3 and 1 108 kg/m3. After de-moulding, the specimens were placed and cured in a curing box for 28 d at different curing temperatures (i.e., 40, 60 ℃ and 80 ℃, respectively). After reaching the curing age, the durability test of specimens was carried out in a self-designed high geothermal environment attack test device. The wet sand containing SO42- solution was used to simulate the attack environment. The wet sand and heating system were placed on the top of the test block. The coupling effect of single-sided attack and one-dimensional temperature field from top to bottom was realized. Water was added every 12 h to keep the humidity of the wet sand constant. The wet sand changed every 7 d to keep the salt concentration constant. The upper part of the wet sand was sealed with a plastic film to prevent water evaporation.Results and discussion The change of compressive strength of concrete under the coupling effect of temperature field at 40 ℃ and 60 ℃ and sulfate attack are divided into the early growth and the subsequent decline. The coupling effect is 30.0% and 23.6% lower than that of single factor. However, the compressive strength of the coupling effect of temperature field at 80 ℃ decreases with a maximum decrease of 22.5%. The temperature field at 40-80 ℃ accelerates the attack chemical reaction, but a greater initial damage of concrete under the temperature field at 80 ℃ occurs. The most probable pore size and pore size distribution of concrete increase with the increase of temperature field. The large pores and transition pores in concrete are dominant, which are 40.38%-46.72 % and 31.09%-38.51%. Under the coupling effect of temperature field at 60 ℃, the content of gel pores and capillary pores decreases, while the content of macropores increases with the increase of attack depth. The surface of transition pores and capillary pores of concrete under coupling action has fractal characteristics, while the surface of gel pores and macropores does not have fractal characteristics. The pore tortuosity also has fractal characteristics in different pore size ranges. The Dt of gel pores increases with the increase of temperature field. The Dt of macropores decreases with the increase of temperature field, but the Dt of transition pores, capillary pores and macropores does not change with the depth. The influence of the surface roughness of the transition pore and the large pore on the average compressive strength of the damaged layer concrete is dominant. The surface roughness of the gel pore has a less influence. On the contrary, the complexity of gel pore structure has a dominant effect on the average compressive strength of damaged layer concrete. The influence of fractal dimension of pore structure on the strength of integral concrete is not consistent. There is a negative linear correlation between Ds of macropores and the compressive strength of integral concrete. However, the influence of Ds of capillary pores, transition pores and gel pores on the compressive strength of integral concrete is dominant, which is different from the results of previous studies. However, the Dv of macropores, capillary pores and transition pores is negatively linearly correlated with the compressive strength of the whole concrete.Conclusions The overall compressive strength of concrete under the coupling effect of temperature field at 40-80 ℃ was 22.5%-30.0% lower than that of single factor. The proportion of gel pores in concrete was decreased by 43.17%, and the capillary pores were increased by 573.10% as the temperature field was increased from 40 ℃ to 80 ℃. The surface of transition pore became more rougher, and the spatial structure of gel pore and transition pore was more complex than that of capillary pore and macropore under the coupling action. The tortuosity of gel pore, transition pore, and capillary pore increased but the macropore decreased. The surface roughness of transition pores and macropores, and the structure complexity of gel pore had a dominant influence on the concrete damaged layer average compressive strength. There was a negative linear correlation between the fractal dimension of pore volume of macropores/capillary pores/transition pores and the overall concrete compressive strength.