The yield stress and viscosity development influenced according to CO₂-CPB time with and without 1.0‰ SDS are shown in Figures 3, 4, respectively. A sample without SDS was designated as the control sample.

According to the results shown in Figure 3, the yield stress for all specimens exhibits a gradual increase between 0 and 2 h. This gradually increasing yield stress is correlated with the cement hydration degree in the slurry, which increased over time. Figure 5 shows the TG/DTG curves of specimens without SDS after curing for 15 min and 2 h. As can be seen, all samples have three distinct weight loss peaks between 60 and 180, 380–480, and 600°C–800°C. The weight loss occurring at 60°C–180°C is mainly due to the dehydration reactions of the hydrates, such as C-S-H and ettringite (Rocha et al., 2015; Zhou et al., 2022). The apparent peaks in the second stage (380°C–480°C) are attributed to the de-hydroxylation of the calcium hydroxide (CH) (Gabrovšek et al., 2006; Yilmaz and Fall 2017), as shown in Equation 1. The weight loss in the last stage (600°C–800°C) is due to the decomposition reaction of calcium carbonate (Gaviria et al., 2018; Wang et al., 2024), as shown in Equation 2. As can be seen from Figure 5, all three peaks of the 2-h curve are higher than those of the 15-min curve, indicating that more hydration products (C-S-H, ettringite, and CH) and calcium carbonate are produced at 2 h. In other words, from 0 to 2 h, the products in the sample gradually increased, which led to an increase in the friction resistance between particles, in turn increasing the yield stress (Haiqiang et al., 2016; Malkin et al., 2017). C a O H 2 → C a O + H 2 O (1) C a C O 3 → C a O + C O 2 (2)

Additionally, as shown in Figure 3, SDS addition reduced the yield stress of the CO₂-CPB sample for 0–1 h. The lower yield stress of the SDS sample compared to that of the control specimen can be demonstrated by the gas content parameter. The gas content development of the CO₂-CPB samples with and without SDS during 0–2 h is shown in Figure 6. As can be seen, the gas content of the CO₂-CPB sample without SDS remains at approximately 2.2% from 0 to 2 h, whereas SDS addition increases the initial gas content to 4.6%. This increase in gas content is due to the enhanced effect of SDS on the surface activity of the slurry; that is, SDS addition causes the slurry to introduce more CO₂ small bubbles during mixing, resulting in an increase in the gas content of the slurry (Li et al., 2023; Pham and Cramer, 2021; Tunstall et al., 2021). Moreover, as shown in Figure 6, the gas content of CO₂-CPB with SDS gradually decreases from 0 to 1 h because of the reaction of CO₂ gas with the CH generated by cement hydration to produce calcium carbonate (Chen et al., 2022), as shown in Equations 3, 4. In other words, the CO₂ gas in the sample was consumed and converted into solid calcium carbonate particles, resulting in a lower gas content in the slurry. Although the CO₂ gas in the SDS sample was gradually consumed, the gas content of the SDS specimen remained higher than that of the control specimen from 0 to 1 h. However, after 2 h, the gas content of the SDS sample decreased to 2.21% (almost the same as that of the control sample), indicating that the CO₂ gas in the SDS sample was completely consumed at 2 h. C O 2 + H 2 O → H 2 C O 3 (3) H 2 C O 3 + C a O H 2 → C a C O 3 + 2 H 2 O (4)

Therefore, according to Figure 6, from 0 to 1 h, the SDS sample exhibits a higher gas content than the control sample, that is, there are more small CO₂ bubbles in the SDS sample, which act as ball lubrication, resulting in a lower yield stress in the SDS specimen (Samson et al., 2017; Yang et al., 2022; Zuo et al., 2023). However, the yield stress of the SDS specimen at 2 h (Figure 3) was higher than that of the control specimen, which does not correspond to a nearly identical gas content at 2 h, as shown in Figure 6. From the zeta potential results at 2 h (Figure 7), the absolute zeta potential value of the SDS specimen was higher than that of the control specimen, indicating greater repulsion between particles in the SDS sample (Elakneswaran et al., 2009; Xiapeng et al., 2019). Previous studies have shown that a greater repulsive force between particles reduces the frictional resistance during flow, thereby reducing the yield stress of the slurry (Simon and Grabinsky, 2013; Xiapeng et al., 2019; Zhou and Fall 2022). Therefore, neither the gas content nor the zeta potential at 2 h could be responsible for the higher yield stress of the SDS sample compared with that of the control sample.

Figure 8 shows the TG/DTG results for the 2-h specimens with and without SDS. As can be seen, the first weight loss peak (60°C–180°C) of the SDS specimen (6.65% weight loss) is higher than that of the control specimen (3.81% weight loss), and the calcium carbonate weight loss peak (600°C–800°C) is also higher for the former (6.35% weight loss) than for the latter (1.70% weight loss). A greater weight loss indicates that more products (such as C-S-H and calcium carbonate) were generated in the SDS sample. The formation of more C-S-H in the SDS sample is attributed to the presence of CO₂, which consumed the hydration product CH, causing the hydration reaction of the cement (Equations 5, 6) to proceed. On the other hand, as mentioned earlier, the presence of CO₂ bubbles in the SDS sample led to the formation of more calcium carbonate. 2 3 C a O ∙ S i O 2   C 3 S + 6 H 2 O → 3 C a O ∙ 2 S i O 2 ∙ 3 H 2 O   C − S − H + 3 C a O H 2 (5) 2 2 C a O ∙ S i O 2   C 2 S + 4 H 2 O → 3 C a O ∙ 2 S i O 2 ∙ 3 H 2 O   C − S − H + C a O H 2 (6)

Therefore, the formation of more products in the SDS specimen after 2 h resulted in a higher yield stress than in the control specimen (Figure 3). In summary, compared with the control specimen, there were two competitive factors in the yield stress of the SDS specimen at 2 h: the yield stress reduction effect caused by the increase in the repulsion force between particles and the increase effect caused by the increase in products. According to these results, it can be inferred that the influence of the former is weaker than that of the latter; therefore, the yield stress of the SDS specimen at 2 h is higher than that of the control specimen.

Furthermore, as can be seen from the viscosity results in Figure 4, the viscosity increases over time for both the control and SDS samples. This viscosity increase over time is due to the hydration reaction progression, that is, the increase in hydration products over time, resulting in greater sample density and frictional resistance between particles, which in turn leads to a tendency for viscosity to increase (Ouattara et al., 2017; Xiao et al., 2021). Additionally, as shown in Figure 4, from 0 to 1 h, the viscosity of the SDS specimen is lower than that of the control specimen. This lower viscosity is due to the larger gas content value shown in Figure 6, that is, the higher the gas content, the lower the slurry density and thus the lower the viscosity (Cao et al., 2022; Şahin et al., 2023). However, the higher viscosity of the SDS sample compared with the control sample at 2 h is due to the formation of more products (such as C-S-H and calcium carbonate) in the SDS sample, as mentioned earlier. In other words, more products lead to a higher sample density, which leads to higher viscosity.

It is worth noting that from 0 to 2 h, both the yield stress and viscosity of the SDS specimen increased at a higher rate compared with the control specimen, as shown in Figures 9, 10. For the SDS samples, this higher yield stress or viscosity growth rate was due to the coincidence of these two factors. First, as previously mentioned, the presence of CO₂ in the SDS sample carbonates CH and promotes the hydration reaction of the cement, leading to the formation of more products and a higher rheological parameter growth rate. Second, CO₂ consumption in the SDS samples leads to a decrease in the internal gas content, resulting in an increase in slurry density, which in turn leads to a larger slurry rheological parameter growth rate than in the control samples.

Figures 11, 12 show the effects of different SDS contents (0.0‰, 0.5‰, 1.0‰, and 1.5‰) on the CO₂-CPB yield stress and viscosity, respectively. As can be seen, both the yield stress and viscosity increase from 0 to 2 h. The reason for rheological parameter increase with time has been mentioned previously and will not be discussed here for brevity.

Additionally, from 0 to 0.25 h in Figures 11, 12, both the yield stress and viscosity exhibit a decreasing trend as the SDS content increases from 0.0‰ to 1.0‰, which is responsible for the gas content variation in the slurry as the SDS content changes, as shown in Figure 13. As shown in Figure 13, for both the 0- and 0.25-h samples, the gas content exhibits an increasing trend as the SDS content increases from 0.0‰ to 1.0‰. As previously mentioned, a higher gas content leads to a lower slurry density, resulting in a lower yield stress and viscosity. Furthermore, this yield stress and viscosity decrease with increasing SDS content (0.0‰–1.0‰) can also be supported by the zeta potential, as shown in Figure 14. As can be seen, the absolute zeta potential value increases as the SDS content increases from 0.0‰ to 1.0‰, which, as mentioned before, leads to an increase in inter-particle repulsion, resulting in the decrease in yield stress and viscosity.

However, according to Figures 11, 12, as the SDS content increases from 1.0‰ to 1.5‰, both the yield stress and viscosity of the 0- and 0.25-h samples show an increasing trend. This increasing trend is mainly attributed to the decrease in the gas content of the sample. As shown in Figure 13, as the SDS content increases from 1.0‰ to 1.5‰, the gas content of the 0- and 0.25-h samples shows a decreasing trend, which, as mentioned before, leads to an increase in sample density, resulting in an increase in yield stress and viscosity. Furthermore, as the SDS content increases, the gas content in the slurry exhibits a trend of first increasing and then decreasing, which is consistent with the findings of previous studies (Yang et al., 2022).

In contrast, as shown in Figures 11, 12, samples with 1.0‰ SDS exhibit the highest yield stress and viscosity at 2 h, followed by samples with 0.5‰ and 1.5‰ SDS (similar values), while samples with 0.0‰ SDS exhibit the lowest yield stress and viscosity. This difference in yield stress and viscosity depends mainly on the difference in the amount of products formed in the sample. According to Figure 15, the height of the first and third weight loss peaks is in the following order: 1.0‰ SDS >1.5‰ SDS >0.5‰ SDS >0.0‰ SDS. This is in line with the C-S-H and calcium carbonate amounts in the specimen. As previously mentioned, a greater number of products in the slurry resulted in higher yield stress and viscosity.

As previously mentioned, the calcium carbonate decomposition weight loss (600°C–800°C) can be measured via thermogravimetric experiments. Therefore, thermogravimetric analysis was adopted in this study to determine the CO₂ uptake ratio of CPB. Based on previous studies (Chen et al., 2022; Moon and Choi, 2019; Park and Choi, 2021) and on the TG curves shown in Figure 16, the CO₂ uptake ratio can be calculated as follows: C O 2   u p t a k e = ∆ w 600 − 800   °   c ∆ w 105   °   c × 100 % (7) where ΔW _600–800°C represents the calcium carbonate decomposition weight loss, and ΔW _{105 °C} represents the dry weight at 105°C.

The CO₂ uptake ratio of the samples with different SDS contents were calculated using Equation 7, as shown in Figure 17. As can be seen, the CO₂ uptake ratio of the samples with added SDS (0.5‰, 1.0‰, and 1.5‰) is significantly higher than that of the sample without SDS addition (0.0‰). This significant increase in the CO₂ uptake ratio is due to the increase in the CO₂ gas content in the slurry caused by SDS addition. As more CO₂ bubbles react with CH to form calcium carbonate, more CO₂ is absorbed by the CPB slurry. Additionally, based on Figure 17, by comparing the results for samples with 0.5‰, 1.0‰, and 1.5‰ SDS, it can be seen that the sample with 1.0‰ SDS has the highest CO₂ uptake ratio, which is confirmed by the test results for the largest gas content of the sample with 1.0‰ SDS, as shown in Figure 13.

From the above analysis, it can be concluded that the CO₂ uptake ratio increases significantly with SDS addition. This indicates that SDS can improve the carbon-sequestration efficiency of the mining backfill industry and has great application value and potential.