## 1 Introduction  
Fiber-reinforced polymer (FRP) composites such as Carbon FRP (CFRP), Glass FRP (GFRP) and Aramid FRP (AFRP) have been widely used in infrastructure construction and other fields [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. However, large-scale applications of FRP composites in infrastructure construction are still limited for some reasons, e.g., high cost of CFRP and AFRP composites, poor chemical stability of GFRP composites [12; 13; 14; 15; 16]. Over the past few years, basalt fiber has gradually received more attention as a new inorganic green fiber for its environment-friendly features. The basalt fiber is made of basalt stone after melting at 1450-1500 \({}^{\circ}\)C. There is no pollution during its production process [17; 18]. The basalt fiber has a greater elongation at break than carbon fiber, higher elastic modulus, and greater chemical stability than glass fiber, and lower cost than carbon fiber and aramid fiber [18; 19; 20; 21]. Moreover, the basalt fiber is also a good flame retardant [21; 22]. Therefore, basalt FRP (BFRP) composites are increasingly used in civil engineering, such as externally strengthening sheets of reinforced concrete (RC) structure. The literature [23; 24] revealed that BFRP laminates would be degraded due to the strong alkaline environment inside concrete (pH = 12-13), and such degradation would be further accelerated when the BFRP laminates were exposed to hygothermal environments. The reason is that the resin matrix of BFRP laminates is highly sensitive to the change of the temperature and the moisture [25; 26; 27], which is seriously harmful to BFRP laminates. Therefore, the hygothermal ageing properties of BFRP laminates in alkaline solution need to be studied. At present, some researches [28; 29; 30; 31; 32] have been done on the hygothermal ageing properties of BFRP laminates subjected to alkaline solution. [PERSON] et al. [28; 29] investigated the long-term mechanical properties of pultruded BFRP laminates after a long-term immersion. The test results showed that as the increase of immersion time, the water absorption increased, and the tensile strength and interlaminar shear strength decreased dramatically due to the severe interfacial debonding between the basalt fiber and resin matrix after ageing. [PERSON] et al. [30] tested the tensile strength of BFRP laminates fabricated by the vacuum assistant resin infusion (VARI) method. The tensile strength of BFRP specimens decreased by 37% and 34% at 20 \({}^{\circ}\)C and 40 \({}^{\circ}\)C for distilled water and decreased by 67% and 90% for alkaline solution after 180 ageing days, respectively. [PERSON] et al. [31] studied the tensile properties of wet-layup BFRP laminates. After immersion of 180 ageing days, it was observed that a higher temperature (e.g., 60 or 80 \({}^{\circ}\)C) would cause a greater reduction of tensile strength during the same ageing time. [PERSON] et al. [32] studied the degradation of basalt fiber and BFRP laminates and concluded that the tensile properties of BFRP laminates were superior to that of basalt fiber in alkaline solution. The tensile properties reduction of BFRP laminates might be attributed to the severe degradation of interfacial adhesion between the basalt fiber and epoxy resin, which was observed by scanning electron microscopy (SEM) images, rather than the ageing of basalt fibers. It can be concluded from the existing studies that the long-term mechanical properties of BFRP laminates were greatly affected by the hygrothermal environment. A higher temperature and alkaline solution would aggravate the ageing process, leading to a severe interfacial debonding between the basalt fiber and resin matrix. Although the above researches showed that the hygothermal ageing properties of BFRP laminates have been studied by many test methods, the long-term durability of BFRP laminates still cannot be evaluated comprehensively. One reason is that different specimen thicknesses (ranging from 0.45 to 4.5 mm) were adopted for test specimens in the existing researches [21; 28; 30; 31; 32; 33].