Dry-wet cycles durability of solid waste based cementing materials solidifying different characteristic soils

A comparative study of the durability of multi-source solid waste-based soil solidification materials in solidifying different soil types has not yet been conducted. Therefore, the properties of multi-source solid waste-based solidification materials (SBM) solidifying clay soil (CS), sandy soil (SS) and organic soil (OS) subjected to dry-wet cycles of damage were studied in this work. The unconfined compressive strength (UCS) of the SBM solidified soil was tested to evaluate the mechanical properties of the solidified soil. Scanning electron microscopy (SEM) and mercury injection porosimetry (MIP) tests were conducted in order to study the micro-action mechanism. The results demonstrated that the SBM showed wide applicability and good long-term performance. The rate of strength increase of the SBM solidified soil during the long-term curing period was found to be dependent on soil characteristics. All the types of SBM solidified soils exhibited increased UCS during the first 10 cycles of the D-W. As the number of D-W cycles increased from 10 to 50, the UCS loss rate for CS reached 78%, with OS experiencing the least at 58%. The structure of SBM solidified soil exhibited softening and weakened resistance to deformation with each additional D-W cycle. The types of hydration products were consistent across all three soil types. The quantity of hydration products was influenced by the characteristics of the soil, which also contributed to the deterioration of damage resistance in D-W cycles. The number of pores within the SBM solidified soil increased with the number of D-W cycles (>10 cycles), resulting in a deterioration of the compact structure.


Introduction
As the global economy develops, there is a concomitant expansion in the scale of infrastructure construction.This results in the generation of a significant quantity of abandoned soil [1].Currently, cement is the principal solidification material employed for the stabilisation and modification of the abandoned soil [2][3][4].However, the production of cement represents an industrial process with high carbon emissions.It is estimated that one tonne of cement results in the emission of 0.8 tonnes of carbon dioxide [5,6].Consequently, the extensive utilisation of cement in the context of soil solidification is likely to result in significant environmental degradation.Consequently, research is being conducted into alternative low-carbon technologies, including biocement [7][8][9][10] and solid waste-based cementing materials.
A significant area of research is the use of industrial solid waste as an alternative to cement in the field of soil solidification [11][12][13].The most commonly studied solid waste-based solidification materials include ground granulated blast furnace slag, steel slag, fly ash and coal slag [14][15][16][17][18][19], which are being investigated as potential replacements for cement.Chemical activation is typically employed to expedite the reaction process in solid waste, thereby improving the mechanical properties of solid waste-based solidified soil [20,21].The activators sodium hydroxide, sodium carbonate and sodium silicate are employed to activate solid wastes [21,22].It is demonstrated that the dissolution and re-polymerisation rate of the glass phase in solid waste is enhanced by the use of alkaline materials, which subsequently results in a notable improvement in the binding ability of solid waste [23,24].
Wu et al [25] proposed a unified design method for utilising various solid wastes to create a composite solidification binder, which was employed to replace cement for the stabilisation of soft clay.The strength of the multi-solid waste based soil solidification binder is greater than that of cement.Liu et al [26] investigated the influence of architectural residue soil, solid waste desulfurization gypsum, slag and construction soil on the mechanical properties of solidified dredged silt.Slag demonstrated the most significant effect in solidifying the silt.C-S-H, N-A-S-H gel and C-A-S-H formed within the silt.Shang et al [27] proposed a novel solid waste solidification material composed of slag, ceramic powder and phosphogypsum.After a 7days cure period, the compressive strength of the solidified soil reached 2.382 MPa.Additionally, the formation of C-S-H and C-A-H in the solidified soil led to notable improvements in soil compactness.Jia et al [28] developed a solidification material composed of steel slag and desulfurization ash.This solidification material exhibited a compressive strength of 30 MPa after 28 days of curing.
The composition of solid waste-based solidification materials, their mechanical properties and microstructures have undergone extensive investigation.However, there has been relatively little research conducted on the durability of solid waste-based solidification materials.Wang et al [29] found that cement solidified soil with recycled fine aggregate exhibited enhanced freeze-thaw resistance.After undergoing 20 freeze-thaw cycles, the mass loss rate of the solidified soil was 18.8%.The microstructure is essentially unaltered.Xu et al [30] developed a solid waste binder comprising ground granulated blast furnace slag and calcium carbide residue in a ratio of 8:2.The UCS of the solid waste solidified construction mud decreased with dry-wet cycles.The number of pores increased and hydration products decreased under the influence of dry-wet cycles.
To date, there have been few studies examining the effects of solidified materials composed of multiple solid wastes (at least three) on solidified soil.Moreover, there is still a lack of understanding regarding the durability differences of solidified materials when applied to various soil types.In light of this, a comparative study was conducted to examine the impact of the dry-wet cycle on the performance differences of solidification materials comprising three distinct waste types (steel slag powder, fly ash and desulfurization gypsum).The effectiveness of these materials in solidifying clay, sandy and organic soils were investigated, with particular emphasis on the visual and mechanical properties exhibited by each material.Additionally, a detailed microscopic analysis was conducted to assess the morphology and pore structure of each material, with a view to elucidating the underlying influencing mechanisms.

Materials
The solidifying binding materials (SBM) utilized in this research were independently developed by Shu et al [31][32][33].The primary constituents are steel slag powder, fly ash, and desulfurization gypsum.The properties of SBM fulfill the criteria for stabilizer for soft soil (CJ/T 526-2018, China), and the test results can be found in table 1.
The geotechnical characteristics of the three kinds of soils are shown in table 2.

Unconfined compressive strength test
A soil sample with a diameter of 50 mm and a height of 100 mm, which had been subjected to the requisite curing period, was subjected to testing on an electronic universal testing machine.The loading rate was maintained at 1 mm/min.Six samples were tested under each experimental condition, with the highest and lowest strength values being removed.The average of the remaining four strength values was determined as the unconfined compressive strength (UCS) of the group of samples.

Microscopic analysis
A SEM test was performed on the Quattro C field emission scanning electron microscopy apparatus.Prior to that, a solidified soil sample was allowed to dry for 24 h at 80 °C.The dried sample was then coated with gold.
During the test, the magnification was maintained at 5000 times.

MIP test
A mercury injection porosimetry test (MIP) was conducted on an AUTOPOREV 9620 machine.The testable aperture range is from 2 nm to 800 μm, and the testing pressure range is from 0.2 psi to 60000 psi.Prior to the commencement of the test, the solidified soil sample was prepared into a spherical shape with an approximate diameter of 2 mm, and subsequently dried at 80 °C over a 24-hour period.

Result and discussion
3.1.Strength changes in standard curing environment Figure 1 illustrates the UCS of OS, CS and SS with a standard curing time.The data shows that the strength of all types of solidified soil increases with curing time.This indicates that the SBM has wide applicability and stable long-term performance.Furthermore, SS exhibits the greatest UCS, followed by CS and finally OS with the lowest UCS.After 360 days of curing, the UCS of SS was 4.61 MPa, 26% higher than that of CS 3.65 MPa, and 275% higher than that of OS 1.23 MPa.
A graphical representation of the rate of increase in strength for three distinct categories of solidified soil, as a function of curing period, can be seen in figure 2. Figure 2(a) depicts the observed decrease in rate of strength increase with an increase in curing period.The comparative analysis of the three categories revealed that OS exhibited the most significant increase in strength, attaining an 88% increase from 7 to 28 days of curing.This value is notably higher than that observed in CS and SS.Over the 28d to 120d curing period, it is evident that the strength increase rate of CS and SS is significantly higher than that of OS.At the 120d to 360d curing period, the three kinds of solidified soil exhibited a strength increase rate of less than 20%.This may be attributed to the fact that after 28 days, the majority of solidification materials had undergone a hydration reaction.As the curing period is extended, the remaining solidification materials continue to react, resulting in a reduction in the strength growth observed in the long-term curing period compared to that observed in the recent curing period.Figure 2(b) demonstrates that, following the 28-day curing period, SBM solidified soil continues to exhibit an excellent strength increase rate.At 28 days and 360 days, the strength increase rates of CS, SS and OS are respectively 98%, 72% and 45%.These findings demonstrate the influence of soil characteristics on strength increase rates over extended curing periods.In contrast to the conventional 28-day assessment period, the 360-day assessment period is more reflective of the practical engineering application.

Appearance in D-W environment
Figure 3 illustrates the appearance of three types of solidified soil subjected to D-W cycles.It can be observed that the D-W cycles resulted in the generation of more cracks in SS.After 40 D-W cycles, a through crack was observed in SS, accompanied by local peeling and damage.In contrast, CS exhibited the least number of cracks, with the lowest number observed in OS at the same D-W cycles.This can be attributed to the presence of finer clay particles in CS and OS, which enhance the compactness of the structure.Additionally, montmorillonite clay particles demonstrate a hydrophilic and swollen nature, which mitigates to some extent the drying shrinkage induced by the D-W cycles.

Strength change in D-W environment
Figure 4 illustrates the UCS of the three types of solidified soils with D-W cycles: SS, CS and OS.The results demonstrate that SS exhibited the highest UCS value throughout the D-W cycles, followed by CS, and finally OS, which displayed the lowest UCS.The UCS values for SS, CS and OS are 4.6 MPa, 3.2 MPa and 1.5 MPa, respectively.It is observed that the UCS of all three types of solidified soil exhibited an upward trend with an increase in D-W cycles, reaching a maximum at 10 D-W cycles.This phenomenon can be attributed to an acceleration of the reaction process caused by the rise in temperature.
During the initial 10 D-W cycle period, the formation of hydration products led to an enhanced cementation solidifying effect, which offset the destructive impact of D-W cycles during this phase.The destructive impact of drying shrinkage became increasingly pronounced with each additional D-W cycle, resulting in a gradual decline in the strength of the three types of solidified soil.
The effect of D-W cycles on the UCS of the three types of solidified soil is illustrated in figure 5.The first 10 D-W cycles in figure 5(a) demonstrate that OS exhibited the greatest strength increase rate, followed by CS and SS, which exhibited the lowest strength increase rate.There is a negative correlation between the strength increase rate and the strength of the solidified soil at the first 10 D-W cycles.This suggests that unreacted binder is enclosed within the solidified soil matrix, and its content is inversely proportional to the UCS.From figure 5(b), it can be discerned that CS demonstrated the greatest UCS loss rate, SS was second, and OS showed the lowest UCS.From the D-W cycles of 10 to 50, the UCS loss rate of CS, SS and OS are 78%, 60% and 58% respectively.The results indicate that OS exhibited the most robust D-W cycles damage resistance, followed by SS, while CS demonstrated the least resilience to D-W cycles damage.
The stress-strain curve of the three types of solidified soil subjected to D-W cycles is depicted in figure 6.As illustrated in figure 6(a), the first 10 D-W cycles resulted in a noticeable increase in the stiffness of SS.As the number of D-W cycles increased, the failure strain typically increased, and the stress-strain curve gradually flattened, indicating that the D-W cycle damage resulted in a softening of the solidified soil structure and a reduction in its deformation resistance.Similar patterns were also observed in CS and OS (figures 6(b) and (c)).soil.After 60 D-W cycles, the degree of damage to the three types of solidified soil structures is as follows: OS>SS>CS.The CS type demonstrated the lowest number and size of cracks, while SS exhibited a greater number of cracks with a longer duration and a larger area.The OS type displayed an increased crack size and the presence of numerous large-sized holes.It can be concluded from the results that the characteristics of soil have   no effect on the nature of the hydration products formed by SBM.However, the characteristics do affect the quantity produced and the resulting differences in resistance to damage from D-W cycles.

Pore analysis
Figure 8 illustrates the pore size distribution of the three types of solidified soil subjected to D-W cycles.Figure 8(a) depicts the most probable aperture of CS after 10 D-W cycles, which is 28 nm.Following the 60 D-W cycles, the most probable aperture increased to 38 nm, accompanied by a notable expansion in the incremental pore area of pore size diameter ranging from 30 nm to 80 nm.Similar patterns can also be observed in figures 8(b) and (c).The most probable aperture of SS increased from 27 nm to 32 nm, while that of OS increased from 8 nm to 12 nm.Furthermore, SS and OS exhibited a larger cumulative pore area at this pore size.The results demonstrate that the D-W cycle damage increased with the increase in D-W cycles from 10 to 60.This was accompanied by an increase in the internal pore amounts and pore size, and a corresponding decrease in the compactness of the solidified soil structure.These findings are consistent with those of other researchers [34][35][36].Additionally, the results indicate that the most probable aperture was affected by soil characteristics.

Conclusion
This paper presents a study of the durability of SBM solidified CS, SBM solidified SS and SBM solidified OS in dry-wet cycles.The UCS of the solidified soil subjected to D-W cycles was tested, and the microscopic action mechanism was revealed.The following conclusions can be drawn: (1) SBM solidified soil demonstrated stable long-term performance.The strength increase rate of solidified soil at the long-term curing period (>28d) is dependent on soil characteristic.In the process of engineering application, it is recommended that the evaluation time of SBM solidified soil be extended beyond 28d, in contrast to the standard evaluation period for cement.
(2) In the initial 0-10 D-W cycles period, the detrimental impact of D-W cycles on the soil structure was mitigated by an increase in the formation of hydration products, resulting in an elevated UCS.With the progression of D-W cycles, the UCS of the three types of solidified soil decreased.Clay exhibited robust resilience to dry-wet cyclic damage due to its smaller particle size.Furthermore, the compactness of the structure was augmented.Consequently, CS exhibited the most effective resistance to D-W damage, followed by OS, and SS exhibited the least effective resistance.D-W cycle damage softened the SBM solidified soil structure and weakened deformation resistance.
(3) Pore size and the amount of SBM solidified soil increased with D-W cycles.Following 60 D-W cycles, the most probable apertures for CS, SS and OS were 38 nm, 32 nm and 12 nm, respectively.

Figure 2 .
Figure 2. UCS increase rate of OS, CS and SS at (a) different curing period and (b) long-term curing time versus 28d curing time.

3. 4 .
Microscopic morphology The microscopic morphology of different types of solidified soil with D-W cycles is shown in figure 7.After 10 D-W cycles (as shown in figures 7(a), (b) and c), all types of solidified soil exhibited a dense microscopic morphology, with no obvious cracks or holes.The soil characteristics did not change the hydration product types, but did affect the amount of hydration product.C-H-S and AFt were generated in all types of solidified

Figure 3 .
Figure 3. Appearance of OS, CS and SS with D-W cycles.

Figure 4 .
Figure 4. UCS of OS, CS and SS with D-W cycles.

Figure 5 .
Figure 5. UCS change of solidified soil with D-W cycles: (a) from 0 to 10 D-W cycles, (b) from 0 to 50 cycles and from 10 to 50 cycles.

Table 2 .
Geotechnical characteristics of the three kinds of soils.Figure 1. UCS of OS, CS and SS with curing time.