## 4 Discussion  
A general trend of increasing strength was observed when adding the industrial waste to a clayey soil as a ternary element together with lime or cement. For the cement samples, the UCS increased by 34% in 7 days when using limestone powder. These results agree with those obtained by [PERSON] et al. [48], who reported an increase of about 100% when 20% limestone powder was added to a marly soil with 2.5% ordinary Portland cement. In the study, the UCS obtained with 2.5% cement and 20% limestone powder was greater than that obtained with 7.5% cement but without limestone powder. Thus, this industrial by-product showed a very positive effect on soil stabilization as a partial cement replacement. Similar results were stated by [PERSON] et al. [49], as the UCS of a silty sand increased by about 18% when 2% of the industrial waste was added to samples with 6% cement. It was also concluded that the combined use of cement and limestone powder in a sulfate-bearing saline soil presented better behavior from a durability point of view. Although studies on using this industrial waste as a ternary element are very limited, considering the calcareous nature of the by-product, it could be said that similar results were obtained by [PERSON] et al. [28], who reported an increase of up to 22.8% when CaCO\({}_{3}\) nanoparticles were added to a clayey soil stabilized with 15% cement. Moreover, [PERSON] et al. [28] also observed a beneficial effect of the calcareous particles when the samples were affected by an aggressive environment, specifically, a marine environment. Nevertheless, in the present work, a general trend for UCS was not clearly observed when using the industrial waste in the cement samples made using the sulfate solution. Regarding the lime samples, an increase in the UCS was certainly observed when adding industrial by-product to the soil samples with lime. Greater values, up to 127%, were recorded compared to the samples without the by-product. These results agree with those obtained by [PERSON] et al. [26]. In that study, the soil strength was measured with the California Bearing Ratio (CBR) test, reporting an increase in the CBR value of 205% when 24% dolomitic limestone powder was added to soil samples with 3% lime. In addition, an increase in the UCS of mixed soils when using this by-product as a standalone stabilizer has been previously reported [20; 21; 50]. Regarding the microstructure of the mixed soil, the addition of the industrial waste resulted in a decrease in the total porosity of the samples compared to those without the by-product. This result could be expected from the UCS values, as higher results were observed for samples containing the by-product. Similar results were found in another study when using fly ash as a partial cement replacement, where a reduction in the total porosity was found from 0 to 28 days, remaining approximately constant from this age to 3 months [11]. Figure 11: SEM images of ettrinigite crystals at 6 months in (**a**) cement samples without the industrial waste and with the sulfate solution and (**b**) lime samples with the waste and sulfate solution. Similarly, [PERSON] et al. [28] concluded that the addition of CaCO\({}_{3}\) nanoparticles made the soil more compact by the nucleation effect and by filling voids, promoting the generation of cement hydration products. In that study, a beneficial effect on the durability was also stated when nanoparticles were added to the soil in a marine environment. This was also observed in the SEM images. [PERSON] et al. [50] also reported a decrease in the void ratio of a clayey soil improved using limestone powder. In the present study, the evolution of the total porosity showed a general decrease from 7 days to 3 months, remaining broadly constant from 3 to 6 months. The observed decrease was attributed to the reactions between the clay and the cement particles, similar to the pozzolanic reactions that, in the cement samples, resulted in the growth of cementitious products filling the pores.