Characteristics of Concrete Corroded by Landfill Leachate in Extremely Cold Regions

The landfill leachate used in this experiment was selected from a representative sample collected on-site at a landfill in the Xiangfang District, Harbin. The landfill has been in place for over 5 years and mainly handles household waste. The samples required for the experiment were obtained from the conditioning tank of the plant. Its physical parameters are presented in Table 1. Once sealed in barrels, the landfill leachate was transported to a low-temperature laboratory for use. Experiments were conducted to test and analyze the water quality of aged landfill leachate (landfilled for more than 5 years) and fresh landfill leachate (landfilled for less than 5 years). Testing revealed that the ion content in the aged landfill leachate was much higher than that in the fresh landfill leachate. Therefore, the aged landfill leachate was selected as the corrosion solution for this experiment.

The cementitious material used in this test was Swan PO 42.5 grade composite silicate cement, and the fine and coarse aggregates were natural medium-coarse river sand and 5–15-mm continuous graded stone, respectively. Laboratory tap water was also used. The cement:water:fine aggregate:coarse aggregate ratio of the concrete was 1.00:0.48:1.71:3.12. Owing to the special nature of the test, the concrete test blocks used were 40 × 40 × 160 mm³ prisms and 40 × 40 × 40 mm³ cubes, prepared according to the standard process of production. The standard curing was completed after 28 days. The preparation process is shown in Fig. 1.

The single-factor tests consisted of four groups: waste landfill leachate full immersion (Group S), full immersion corrosion (Groups Y1 and Y2), and freeze–thaw cycle (Group D) (Table 2). The coupling tests consisted of three groups: waste landfill leachate coupled with freeze–thaw cycle groups (SD and DS) and compound salt solution coupled with freeze–thaw cycles (Group YD) (Table 3).

This study involved a single-factor experiment that was divided into four groups: the Full immersion group S, which was soaked in the landfill leachate (with the addition of 5% sodium chloride and 5% sodium sulfate); the Full immersion group Y1, which was soaked in a 5% sodium chloride solution; the Full immersion group Y2, which was soaked in a 5% sodium sulfate solution; and the Freeze–thaw cycle group D, which underwent a daily cycle of freezing for 12 h and thawing for 12 h. The corrosion periods for each single-factor experimental group were 30, 60, 90, and 120 days. The results of the compressive strength test of the concrete after being soaked in different single-factor corrosion solutions are shown in Fig. 2, and the results of the flexural strength test are shown in Fig. 3.

The compressive strength of the Freeze–thaw cycle test group D decreased in a single downward trend; after 120 freeze–thaw cycles, it decreased by 17.19%. The trend of the compressive strength variation in Test groups Y1, Y2, and S subjected to three different types of soaking corrosion was divided into two stages. The first stage showed an upward trend, with the smallest increase in compressive strength observed in the S group samples corroded by landfill leachate and the largest increase observed in the Y2 group samples corroded by the sodium sulfate solution. The compressive strengths of the three sample groups increased by 10.61%, 18.51%, and 5.73%, respectively. The second stage showed a downward trend, and the decreases in the compressive strengths of the S group samples were much greater than those of the Y1 and Y2 groups. After 120 days of corrosion, the compressive strengths of the S group samples decreased by 6.88%, whereas those of the Y1 and Y2 group samples increased by 3.78% and 14.39%, respectively. Therefore, the effect of landfill leachate on the compressive strength of concrete was greater than that of sodium sulfate and sodium chloride solutions, consistent with the conclusion drawn by Feng (2022).

The flexural strength of concrete under the influence of a single factor was studied, and the experimental results showed that the trend of flexural strength with age was similar to that of compressive strength; however, the magnitude of the variation was larger. In the first stage, the flexural strengths of the Y1, Y2, and S groups increased by 23.07%, 29.36%, and 17.77%, respectively. After 120 days of corrosion, the flexural strengths of the Y1 and Y2 groups increased by 10.37% and 23.73%, respectively, whereas those of the D and S groups decreased by 32.12% and 6.40%, respectively. Therefore, within a certain period of time, the corrosion of concrete by landfill leachate was less than that of freeze–thaw cycles; however, compared with the corrosion of concrete by the sodium sulfate and sodium chloride solutions, the corrosion of concrete by landfill leachate was greater. This may be because the salt ions contained in the landfill leachate reacted to generate calcium aluminate, which filled the pores and rendered the internal structure of the concrete dense, increasing the strength of the concrete in the early stages (Xia et al. 2023). With the extension of the corrosion age, other substances in the landfill leachate severely corroded the concrete, accelerating the decline in compressive and flexural strengths. We found that freeze–thaw cycles had the greatest impact on concrete, although landfill leachate and salt solutions also had significant impacts. Based on this phenomenon, we investigated the corrosion characteristics of concrete by conducting coupled tests to compare the effects of landfill leachate in cold regions.

Three sets of coupling tests were conducted, divided into SD and DS groups under the coupling of landfill leachate and freeze–thaw cycles, to simulate the different conditions of concrete corrosion and freeze–thaw cycles. The SD group was first immersed in landfill leachate and then subjected to freeze–thaw cycles, while the DS group was the opposite. The YD group was coupled with a composite salt solution (5% sodium chloride + 5% sodium sulfate solution) and subjected to freeze–thaw cycles. Each group was evaluated for four cycles, with one period consisting of 30 days of soaking and 25 freeze–thaw cycles. The compressive strength test results of concrete after the coupling factor action are shown in Fig. 4, and the flexural strength test results are shown in Fig. 5.

The compressive strength results of the coupling test groups showed that in the first cycle, the compressive strength of group YD was the highest, and the compressive strength of group DS was lower than that of group SD. In the second cycle, the compressive strengths of the DS and YD groups reached their peaks, increasing by 2.00% and 5.09%, respectively, whereas that of the SD group showed a slow downward trend, decreasing by 2.19%. In the third cycle, the compressive strength of group SD reached its peak, whereas those of groups DS and YD decreased significantly. After four corrosion cycles, the compressive strengths of the three coupled test groups, SD, DS, and YD, decreased significantly. Among them, the freeze–thaw cycle and landfill leachate coupling test group DS was the most affected, with a reduction of 18.74%, and the freeze–thaw cycle and salt solution coupling test group YD was the least affected, with a reduction of 10.94%. Therefore, concrete subjected to freeze–thaw cycles and then corroded by landfill leachate was the most severely damaged.

The results of the flexural strength tests for the coupled test groups showed that the flexural strengths of the DS and YD groups reached peak values in the second cycle, increasing by 1.88% and 8.17%, respectively. The flexural strength of the SD group reached its peak value in the third cycle, increasing by 1.55%. In the fourth cycle, the flexural strengths of the three coupled test groups, SD, DS, and YD, decreased significantly, with reductions of 30.35%, 37.97%, and 26.7%, respectively. This may be attributable to the fact that the freeze–thaw cycle first damaged the concrete structure, making it easier for inorganic salt ions to enter the specimen and react with the concrete matrix to generate calcium aluminate and other hydration products with expansive properties, filling the cracks in the specimen and continuously increasing the compressive and flexural strengths of the concrete until they reached their peak values (Xiao et al. 2019). As the freeze–thaw cycle and immersion corrosion continued, the expansive materials and frozen water pressure generated expansion stress inside the concrete, exacerbating the damage inside the concrete and easily allowing corrosion solutions to enter the concrete, creating a cycle that ultimately leads to a rapid decrease in concrete strength (Jiang et al. 2015).

Based on the aforementioned experimental results, the damage to concrete caused by the coupling effect of landfill leachate and freeze–thaw cycles was determined to be more severe than that caused by the coupling effect of the salt solution and freeze–thaw cycles. Additionally, the concrete that was first subjected to freeze–thaw cycles was more affected than that subjected to landfill leachate corrosion, indicating that microorganisms and other acidic substances in the landfill leachate, in addition to inorganic salt ions, could also damage the internal structure and significantly impact the strength of concrete. Furthermore, compared with concrete that was first corroded by landfill leachate, concrete that was first subjected to freeze–thaw cycles reached its peak and entered a damaged state earlier.

In the coupling test group, the apparent damage characteristics of the concrete specimens under different corrosion conditions and action cycles are shown in Fig. 6.

As the action cycle progressed, the surface of the specimen became rough, and the peeling on the cement mortar protective layer on the surface became increasingly severe. This is because the stable structure of the alkaline cementitious material on the specimen surface was destroyed, resulting in a loss of its ability to protect the aggregates. After the third cycle, all three test groups exhibited intensified cement mortar peeling and exposure to coarse aggregates. The calculated mass loss rates showed that the DS test group had the most severe concrete specimen damage, with a mass loss rate of 0.85%, followed by the SD test group, with a mass loss rate of 0.73%, whereas the YD test group had the least damage, with a mass loss rate of 0.55%.

Based on the results of the mechanical tests, significant differences were observed between the first and third cycles. To further investigate the corrosion mechanism, concrete samples under different corrosion environments were analyzed using SEM (Fig. 7). The internal structure of Sample group S30 was dense and contained only a small number of micropores, whereas Sample group S90 produced needle-shaped and debris-like products with slight cracks. The blank group of concrete contained a large amount of C–S–H gel and plate-like Ca(OH)₂ crystals, and its structure was dense with no significant cracks. In the coupling test of the first cycle, Sample groups SDI and DSI had numerous plate- and fiber-like substances distributed internally, accompanied by slight cracks and a small amount of needle-like substances, resulting in slight damage to the concrete compared with Sample group S30. In the coupling test of the third cycle, compared with Sample groups SDI and DSI, both Sample groups SDIII and DSIII produced more new cracks and pores inside the concrete, filled with a large number of needle-shaped and debris-like products with higher porosity. Sample group DSIII had larger cracks and more debris-like products with a more severe level of damage than Sample group SDIII. Combined with the results of the macroscopic mechanical tests, concrete subjected to freeze–thaw cycles first suffered more severe damage than concrete subjected to landfill leachate corrosion; upon comparing Figs. 7(d and h), we found that numerous needle-shaped ettringite and debris-like gypsum were inside the concrete of the YDIII test group, with obvious pores and cracks. In contrast, the pores and cracks in the SDIII test group were much more numerous than those in the YDIII test group and were filled with many needle-shaped and debris-like products. Based on these phenomena, the coupling effect of freeze–thaw cycles and landfill leachate from waste landfill is greater than the coupling effect of freeze–thaw cycles and salt solution on concrete. In the early stages of the experiment, inorganic salt ions in the landfill leachate gradually penetrated the concrete specimens, forming crystalline substances that filled the pores of the concrete specimens and made the internal structure of the concrete denser. As the test cycle progressed, the external cement mortar began to peel off and an increasing number of water molecules and landfill leachate entered the specimens. The inorganic salt ions in the landfill leachate consumed the hydration products in the concrete specimens, such as C–S–H and calcium hydroxide, to generate expansible needle-shaped ettringite and gypsum. This, in turn, caused more micropores and cracks under the influence of freeze–thaw cycles (Zhao et al. 2016). As the test cycles increased, the degradation rates of the concrete specimens increased. These phenomena are consistent with the existing conclusions that inorganic salt ions fill cracks and pores in concrete during the early and middle stages of freeze–thaw cycles, and in the later stages, they have a significant mutual promotional effect on the damage caused by freeze–thaw cycles to concrete (Luo et al. 2018).

The XRD phase analysis diagram in Fig. 8 shows that the concrete corroded by landfill leachate contained not only C–S–H but also ettringite at a corrosion age of 30 days. At this time, the ettringite filled the pores and cracks inside the concrete specimens, improving its internal structure and making them denser. Concrete specimens corroded by landfill leachate and those corroded by salt in the third period in severely cold regions both contained ettringite and gypsum, which verifies the previous speculation, as shown in Fig. 9. As the corrosion age increased, the calcium hydroxide content inside the concrete specimens decreased continuously, and more needle-shaped ettringite and fragmentary gypsum appeared, generating outward expansive stress. When the expansive stress exceeded the tensile stress inside the concrete, numerous microcracks occurred, damaging its internal structure (Liu et al. 2020). Compared with single landfill leachate corrosion, the concrete specimens in the coupling test group had more cracks within owing to the freeze–thaw cycles, which increased the contact area between the corrosion solution and concrete and accelerated the generation of ettringite and gypsum. The combined action of freeze–thaw stress and expansive stress ultimately led to concrete damage, which explains the accelerated decline of the concrete's macroscopic mechanical properties in the later stage from a microscopic perspective.