The existing Portland cement concrete cannot ensure the long-term service life of tunnel engineering in high geothermal and high salinity environments. This work was to adopt a calcined clay limestone cementitious (LC³) system as an alternative material and the microstructural evolution mechanism of LC³ mortar under high geothermal conditions was investigated. A chloride ion transport model for thermal damage accumulation and adsorption-fixation effects was established, and the accelerating effect of thermal damage degree on chloride ion penetration was revealed.

Raw powders included cement, limestone powder, calcined clay, gypsum, and fly ash. The cement was P·Ⅱ 52.5 grade Portland cement. The limestone powder consisted of ≥90% CaCO₃. The calcined clay was produced from a clay with a kaolinite content ≥40%. The gypsum was dihydrate gypsum with a purity ≥90%. The fly ash was Class F fly ash. The aggregate was machine-made sand from tunnel muck. Mortar specimens were prepared according to the standard GB/T 17671—1999, China. The LC³ system contained 55% cement, 30% limestone powder, and 15% calcined clay. The specimens were cured under different conditions (i.e., 20 ℃–95%RH, 65 ℃–10%RH, and 105 ℃–10%RH). The microstructure was characterized by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The total and free chloride ion contents of the specimens after immersion in 3% NaCl solution were determined. A chloride transport model incorporating temperature gradient and the Langmuir nonlinear adsorption was proposed, and the related results were simulated by a software named COMSOL Multiphysics.

The results show that a high geothermal exposure leads to significant microstructural deterioration of the LC³ mortar. The porosity increases to 45.14% (i.e., an increase of 40.36%) and the average pore diameter increases to 115.25 nm (i.e., an increase of 356.25%) at 105 ℃, mpared to those at 20 ℃. The XRD patterns and the NMR analyses indicate a decomposition of hydration products like portlandite (CH) and ettringite (AFt), and changes in aluminum coordination, reducing the structural stability and chloride binding capacity. The LC³ system demonstrates a superior resistance to chloride ion penetration, compared to plain cement and binary systems (i.e., limestone, calcined clay, or fly ash). At 105 ℃, the total chloride content in LC³ is only 83.9% and the free chloride content is only 73.6% of those in the plain cement system. Based on the prediction by the proposed chloride transport model, an increase of 92% in the total chloride content at 2.5 mm depth at 105 ℃, compared to that at 20 ℃, confirming the promoting effect of high geothermal conditions.

High geothermal conditions could deteriorate the pore structure of LC³ mortar and reduce its resistance to chloride penetration. The LC³ system exhibited the superior performance against chloride ion transport, compared to traditional cement and binary systems under high geothermal conditions, due to the synergistic effect of limestone and calcined clay. The chloride concentration decreased non-linearly with depth, and elevated temperature significantly accelerated the chloride diffusion due to the pore structure coarsening and reduced binding capacity.