Study on freeze-thaw damage of antique-style bricks based on grey relational analysis

To comprehensively evaluate the damage evolution of six types of antique-style bricks under freeze–thaw cycling and to identify rapid indicators for assessing their frost resistance, 60 freeze–thaw cycles were applied to the bricks, and the mass loss, compressive strength loss, and pore-structure evolution were measured. The entropy weight method (EWM) was employed to integrate the mass loss rate and the compressive strength loss rate into a comprehensive freeze–thaw damage index, and grey relational analysis (GRA) was used to establish the relational degrees between this index and the mechanical properties, water absorption, total porosity, and the porosity growth rates of four pore-size intervals. The results show that the six brick types differ significantly in mechanical properties, water absorption, and post-freeze–thaw mass and strength losses, and that a high initial compressive strength does not necessarily correspond to high frost resistance. Among them, the large city-wall brick 03CZ from Shanxi exhibited the best frost resistance, with a mass loss rate of 0.37% and a compressive strength loss rate of 2.15%, whereas 04NT showed the poorest performance, with a mass loss rate of 3.67% and a compressive strength loss rate of 25.18%. After freeze–thaw cycling, micropores smaller than 20 nm decreased while mesopores of 20–50 nm and 50–100 nm increased in all six bricks. The grey relational analysis results show that, among the pre-freeze–thaw indicators, the flexural-to-compressive strength ratio (0.783) had the highest relational degree with the comprehensive freeze–thaw damage index, indicating that this ratio should be given primary attention in discriminating the frost resistance of antique-style bricks; among the post-freeze–thaw indicators, the growth rate of 50–100 nm pores (0.655) was the highest. The severity of brick damage depends more on the migration of micropores to mesopores than on the absolute increase in the total pore volume.