Features of Processes for Preparation and Performance of Foamed Lightweight Soil with Steel Slag Micronized Powder and Granulated Blast Furnace Slag

Abstract

Steel slag micronized powder, granulated blast furnace slag, and cement were used as cementitious materials to prepare a foamed lightweight soil for roadbed filling to reduce the settlement and additional stress of the foundation and to solve the environmental problems caused by the storage of large amounts of steel slag. However, the instability of steel slag and the multi-angular nature of its surface limit the resource utilization of steel slag. Currently, concrete technology is unable to achieve a large amount of steel slag. Therefore, it is necessary to deeply explore the influence of steel slag content and the specific surface area of steel slag on the working performance, compressive strength, durability, and micro-mechanism of foam light soil. Through the modification of steel slag and the improvement of the production process, the preparation of foam light soil with a large amount of steel slag can be realized. In this study, the foamed lightweight soil with 1.0 Mpa was prepared by cementitious materials composed of 40% cement and 60% multi-mixture of steel slag micronized powder and granulated blast furnace slag. The study of SEM images and BET demonstrated that the larger specific surface area of steel slag powder was more conducive to improving the durability of the foamed lightweight soil. Meanwhile, XRD analyses confirmed that the reactions of f-CaO and f-MgO in steel slag were slowly released in the porous foamed lightweight soil system, which compensated for the shrinkage properties of porous materials. When the SSMP content was 0%, the shrinkage rate was 2.34 × 10⁻³, while when the SSMP content was 60%, the shrinkage rate was only 0.54 × 10⁻³. Furthermore, our study of the hydration process of samples indicated that the strong alkalinity of steel slag micronized powder hydration was helpful to stimulate the potential activity of the slag powder, which was beneficial to the improvement of the compressive strength of foamed lightweight soil. Thus, this study provides a valuable idea for reducing the settlement and additional stress of the original foundation and for solving the environmental problems caused by a large amount of steel slag storage.

1. Introduction

The prolonged use of roads leads to engineering challenges, such as differential settlement and roadbed cracking. Traditional methods for settlement control in such cases often prove ineffective, primarily relying on conventional soft ground treatment approaches [1,2,3,4]. The conventional soft ground foundation, characterized by high compressibility, elevated water content, and low strength, tends to result in uneven settlements and cracks during road improvement and expansion projects. This significantly impacts road capacity and safety, prompting a shift towards using innovative road foundation filling materials instead of traditional soils. Foamed lightweight soil, categorized as a type of lightweight concrete, is artificially created by introducing a suitable foaming agent to mortar [5]. When replacing conventional soil for roadbed filling and reinforcement, this material is commonly referred to as foamed lightweight soil. This novel construction material offers advantages such as reduced weight, adjustable density and strength, and convenient construction [6,7,8,9,10]. Consequently, foamed lightweight soil, as a filling material, effectively alleviates the self-weight of the roadbed, minimizing settlement and presenting considerable prospects for widespread application [11,12,13,14].

As a byproduct of steel production, the massive stacking of steel slag would not only occupy land resources, but it would also cause environmental problems, such as pollution of groundwater and dust. Therefore, it is urgent to study the resource utilization of steel slag micronized powder [15,16]. The composition of steel slag is similar to the mineral composition of Portland clinker, which contains 2CaO·SiO₂ (C₂S), 3CaO·SiO₂ (C3S) and other active components [17]. This means that the steel slag has good hydrogel properties. However, due to the small amount of f-CaO and f-MgO in steel slag, cement or concrete added with steel slag is prone to expansion, leading to problems such as decreased stability, which further limits the large-scale application of steel slag [18,19,20,21,22]. Targeting the characteristic of steel slag being prone to expansion, researchers ingeniously applied the expansion property of steel slag in foam concrete to compensate for the large shrinkage of foam concrete. For example, Tiong et al. [23] partially replaced the density of 2.65 g/cm³ of manufactured sand with the density of 3.8 g/cm³ of the steel slag. In this study, when the replacement amount was 25%, the compressive strength could meet the structural strength requirements of ASTMC 330-89. Foamed lightweight soil has internal pores of different sizes, and when mixed with steel slag, it would largely alleviate the expansion brought about by the hydration process of steel slag, which has better volume stability and could inhibit the self-shrinkage of concrete to a certain extent [24,25].

In addition, the alkalinity of steel slag and cement could stimulate the slag alkali and improve the overall strength of foamed lightweight soil when the composite system of steel slag micro-powder and slag is used [26,27,28]. Therefore, the application of steel slag in foamed lightweight soil as road filling material is a practical technical approach and an ideal development direction.

However, during the preparation and mixing of foam light soil, part of the foam is broken because of the many edges and corners on the surface of the prepared steel slag [9]. Baalamurugan thought that steel slag morphology could be changed by grinding steel slag to reduce foam loss [10], but the grindability of steel slag is poor [14]. Hence, the balance between the fineness of steel slag and the loss of foam is worth exploring. The related research can provide a practical scheme for the application of steel slag foam lightweight soil.

To address the scientific issues mentioned above, the purpose of this paper is to study the preparation process of slag-blended foamed lightweight soil, to explore the performance of the foamed lightweight soil prepared by blending different proportions of steel slag micronized powder and mineral powder with different specific surface areas, and to discuss the effect of steel slag micronized powder on the self-shrinkage of the foamed lightweight soil and the hydration of the foamed lightweight soil under the blending of steel slag micronized powder and mineral powder. This work provides theoretical and technical support for the better use of steel slag resources in foamed lightweight soil practical engineering.

2. Materials and Methods

The cementitious system studied in this article is Cement (PC) + Granulated blast furnace slag (GBFS) + Steel slag micronized powder (SSMP). Many researchers have achieved rich research results in the research and application of cement and concrete, and the potential activity of slag powder has also been widely recognized in the industry. However, the research on steel slag micro-powder is not yet sufficient. In dense cement or concrete applications, the amount of steel slag micro-powder is very small, usually not exceeding 5%. Due to the presence of small amounts of f-CaO and MgO in steel slag micro-powder, there is a volume stability issue. Cement or concrete can generate internal stress, causing expansion, resulting in product cracking and even a decrease in strength. However, in the foam light soil system, because there are many pores in the system, the expansion of steel slag does not cause internal stress but it compensates for the contraction of foam light soil, and the strong alkalinity of steel slag stimulates the potential activity of slag. Therefore, the expansion performance of steel slag has also been reasonably and scientifically applied in the foam lightweight soil system.

According to the above analyses, we built the flowchart of the study in Figure S1. At first, we used cement (PC), steel slag micronized powder (SSMP), granulated blast furnace slag (GBFS), foaming agent, and water as the main materials for making the specimens. The PC was P.O 42.5 cement with an apparent density of 3360 kg/m³ produced by Anhui Conch Group Co., Ltd. (Wuhu, China). After grinding SSMP for 0, 2, 4, 6, and 8 min, the corresponding specific surface areas were 410, 520, 620, 710, and 790 m²/kg, respectively. Additionally, the phase composition of SSMP was investigated by X-ray diffraction (XRD) and is shown in Figure 1.

The preparation of the foamed lightweight soil in this experiment was prepared by the pre-foaming method, i.e., the slurry and foam were prepared separately, and then they were mixed thoroughly, as shown in Figure S2a [29,30]. The foam was prepared by diluting an appropriate amount of the foaming agent stock solution, with water to 100 times, and physically foaming it through an intelligent micro foaming machine. The foaming principle of the intelligent micro-foaming machine is the same as that of the large compressor foaming used in engineering, and both use compressed air to produce more stable foam. The foam is fine, is smooth, has no large bubbles, and is uniform in size. By adjusting the knob to change the size of the pressure of the compressed air, the resulting foam density also changed. The density of the foam used in this experiment was controlled at about 50 g/L. According to the technical requirements of this work, the water solid ratio of foamed lightweight soil was 0.6, and the wet density was 600 ± 30 kg/m³. According to the mix design theory of foamed lightweight soil, the material composition and mix proportion of each sample are shown in Table S1. The cementitious material of foamed lightweight soil was composed of 40% PC and 60% SSMP and GBFS. The PC, SSMP, and GBFS were weighed and mixed according to the ratio designed in Table S1. The mixture was premixed for 1 min with a mixer, and then water was added in a certain water/solid ratio to obtain a uniformly mixed slurry. The prepared foam was added to the slurry in a certain proportion and continued to be stirred until there was no obvious bubble burst on the slurry surface. Finally, the prepared foamed lightweight soil was poured into a 100 mm × 100 mm × 100 mm mold (as shown in Figure S2b, the size of the mold for shrinkage measurement was 100 mm × 100 mm × 515 mm). Due to the low strength of the foamed lightweight soil, it was de-molded after 48 h of curing. After de-molding, the test pieces were sealed in plastic bags and then placed in the cement curing room for curing (temperature 20~25 °C, humidity 50~70%) for 48 h.

Due to the high compressibility, high moisture content, and low strength of traditional fill soil foundations, uneven settlement often occurs during long-term road use, leading to uneven or cracked road surfaces. Compared to the filled subgrade, the volume weight of foam light soil is far less than that of the filled soil. The unit weight of foam light soil was 6.0 kN/m³, while the unit weight of general soil under the maximum density was about 18 kN/m³, which reduced the pressure on the subgrade by 2/3. Therefore, when the subgrade filler was foam light soil, the subgrade settlement value was the shrinkage value of foam light soil. In our study, the technical index requirements of foamed lightweight soil are as follows: wet unit weight (600 ± 30) kg/m³, foam density (50 ± 2) kg/m³, fluidity (180 ± 10) mm, compressive strength at 7 d ≥ 0.5 Mpa, and compressive strength at 28 d ≥ 1.0 Mpa. To measure relevant indicator data, three parallel samples were prepared for each group of experiments, and the average of the test results of the three samples was taken as the experimental results.

Working properties test method: The working properties of foamed lightweight soil, including wet density and flow factor, are shown in Figure S2c,d. The sample was placed in the center of the lower plate of the material testing machine. The testing machine continuously and uniformly applied pressure to the specimen at a loading speed of (20.5) kN/s and recorded the failure load at the time of specimen damage.

Shrinkage test method: First, 100 mm × 100 mm × 515 mm foam light soil specimens were prepared in a certain proportion and were cured in the steel mold for 2–3 days. Then, the demolded sample was placed in the cement curing room (20 ± 3 °C and measure its deformation reading at 90% humidity). The formula for calculating the shrinkage value is as follows:

ε_st = (L₀ − L_t)/515

Formula:

• ε_st—shrinkage ratio;
• L₀—Length of initial measurement (mm);
• L_t—Corresponding length of test piece at t day (mm).

3. Results and Analysis

3.1. Influence of SSMP on Flow Coefficient and Wet Density of Foamed Concrete

To investigate the effect law of SSMP on the flow factor and wet density of foamed lightweight soil, two groups of experiments were conducted as follows. The first group adjusted the ratio of SSMP and GBFS under the SSMP with the specific surface area (410 m²/kg). The second group adjusted the specific surface area of SSMP under the ratio of SSMP (40%) and GBFS (20%). The experimental data results are shown in Figure 2.

Firstly, the specific surface area of SSMP was kept at 410 m²/kg, and with the increase of the ratio of SSMP and GBFS, the flow factor and wet density of foamed lightweight soil gradually increased from 598 kg/m³ to 669 kg/m³ and 170 mm to 225 mm, respectively; their change tendencies were both first slow and then rapid. Here, the increase of wet density of foamed lightweight soil was because the SSMP density was higher than GBFS. In the production process, with the increase of the ratio of SSMP and GBFS, the theoretical wet density of foamed lightweight soil would increase. However, as shown in Figure 2a, the increase of the wet density of prepared foamed lightweight soil was higher than the theoretical value of wet density, which could be ascribed to the fact that SSMP has the characteristics of high density, multiple edges, and water absorption [9]. The above characteristics of SSMP would cause to the destruction of many foams in the slurry, further resulting in a substantial increase in wet density. In addition, the damaged foam would become water, which would result in an increase of light soil flow factor of foam. Secondly, when the proportions of SSMP and GBFS were 20% and 40%, respectively, the flow factor and wet density of prepared foamed lightweight soil were 181 mm and 615 kg/m³, respectively, which meet the technical index requirements of foamed lightweight soil construction in engineering construction. The flow value and wet density of prepared foamed lightweight soil were 192 mm and 629 kg/m³, respectively, under the ratio of SSMP and GBFS at 30%. Under the operating conditions, the corresponding flow value and wet density of foamed lightweight soil exceeded the corresponding technical requirement. Therefore, when the specific surface area of SSMP was 410 m²/kg and the content of SSMP and GBFS was 20 and 40%, respectively, then the prepared foamed lightweight soil could meet the working performance. The smaller the flow factor of foamed lightweight soil, the greater the viscosity of the slurry, the more difficult it is to transport and pour, and the foam is more easy to break; on the contrary, the larger the flow factor of foamed lightweight soil, the greater the chance that PC, SSMP, and GBFS solid particles in the system are prone to settlement, delamination, and collapse; thus, the flow factor needs to be controlled in a certain range.

3.2. Effect of SSMP on Shrinkage of Foamed Concrete

To improve the SSMP utilization rate, so that the foamed lightweight soil can be prepared to meet the technical index requirements even at higher SSMP doping. The technical route of increasing the specific surface area of SSMP was adopted. As can be seen from the dashed line in Figure 2b, when the ratio of 40% SSMP to 20% GBFS was kept constant, the flow factor and wet density of the foamed lightweight soil gradually decreased with the increase of SSMP specific surface area, although the decrease was small, but at the SSMP specific surface area of 710 m²/kg, the flow factor and wet density were 187 mm and 630 kg/m³, respectively, which just meet the requirements of technical indexes. The larger the SSMP specific surface area is, the smoother the surface grinding is, and the easier it is to mix evenly, and the foam is not easy to break. Therefore, to improve the utilization rate of SSMP, the specific surface area of SSMP needed to reach 700 m²/kg or more.

According to the above results, the shrinkage variation of sample 1# was similar to that of ordinary concrete, while the shrinkage of other samples added to SSMP at 28 days was significantly smaller than that of sample 1#, similar to a parabola. The above results indicate that a small amount of f-CaO and f-MgO in SSMP underwent hydration reactions to a certain extent, compensating for the shrinkage of foamed lightweight soil. With the increase of SSMP content, the change in shrinkage rate showed a downward trend. When the SSMP content was 0%, the maximum shrinkage rate was 2.34 × 10⁻³, and when the SSMP concentration was 60%, the minimum shrinkage rate was 0.54 × 10⁻³. This is because the volume of foamed lightweight soil decreased after pouring, and the foamed lightweight soil shrunk. Based on XRD analysis, we believe that SSMP contains free calcium oxide and magnesium oxide, which react with water to form calcium hydroxide and magnesium hydroxide and expand. On the one hand, it can counteract the natural shrinkage of foamed lightweight soil itself, and on the other hand, the porosity of foamed lightweight soil itself can provide some space for the expansion of SSMP, solving the instability problem of SSMP.

3.3. Influence of SSMP on Compressive Strength of Foamed Concrete

In the foamed lightweight soil of the compound SSMP–GBFS system, both PC and SSMP were strongly alkaline after hydration, which had an excitation effect on the potential activity of GBFS. SSMP–GBFS–PC hydration products interact and promote each other to contribute to compressive strength. The change patterns of compressive strength for different SSMP and GBFS ratios and different SSMP specific surface areas are shown in Figure 3a,b, respectively. As shown in Figure 3a, the compressive strength of the foamed lightweight soil at 7 d and 28 d generally showed a decreasing trend with the increase of SSMP admixture in the SSMP–GBFS system. At 20% and 30% of SS admixture, the decrease of compressive strength was small, but when the SSMP exceeded 30%, the compressive strength of the foamed lightweight soil decreased greatly. At 20% of SSMP admixture (40% of GBFS admixture), the compressive strength of the foamed lightweight soil at 28 d could reach a maximum of 1.52 Mpa. Thus, the results show that the proportional relationship between SSMP and GBFS is important in the SSMP–GBFS–PC system (PC was fixed at 40%).

By comparing the difference between 28 d and 7 d compressive strengths of foamed lightweight soils corresponding to different ratios of SSMP and GBFS, it was found that when SSMP: GBFS was 0: 60%, the 7 d and 28 d compressive strengths of foamed lightweight soils were 1.17 Mpa and 1.5 Mpa, respectively, with a difference of 0.33 Mpa at different ages; when SSMP: GBFS was =20%: 40%, the 7 d and 28 d compressive strengths of the foamed lightweight soil were 0.08 Mpa and 1.52 Mpa, respectively, with the difference of 0.64 Mpa at different ages; when SSMP: GBFS was =60%: 0, the 7 d and 28 d compressive strengths of the foamed lightweight soil were 0.29 Mpa and 0.51 Mpa, respectively, with the difference of 0.22 Mpa at different ages, indicating that SSMP and GBFS have certain hydration synergistic effects. Due to the poor hydration of SSMP, the synergistic effect was not obvious when the amount of SSMP was more than 30%. When SSMP and GBFS were dosed at 20% and 40%, respectively, the later hydration effect was good, and the strength increase was more obvious. The dashed line in Figure 3c is the technical index requirement of the compressive strength of foamed lightweight soil (7 d ≥ 0.5 Mpa, 28 d ≥ 1 Mpa). Figure 3b shows the variation law of the compressive strength of foamed lightweight soil mixed with different specific surface areas of SS. It can be seen in Figure 3b that, with the increase of SSMP specific surface area, the 7 d and 28 d compressive strengths show an overall increasing trend, and the trend is from sharp to slow. As mentioned above, when the specific surface area of SSMP was 710 m²/kg, the workability of the lightweight soil with 40% SSMP 20% GBFS foam was to meet the design technical index requirements, and the corresponding 7 d and 28 d compressive strengths were 1.12 Mpa and 1.55 Mpa, respectively, which also meet the technical index requirements. However, it is not economical to use SSMP with high specific surface area due to the high energy consumption of SSMP milling.

3.4. Microscopic Analysis of Foamed Lightweight Soil

The shrinkage variation laws of specimens 1#, 3#, 5#, 7#, and 10# at the age of 56 d are displayed in Figure 3c. Only specimen 1# (0% SSMP) showed an increasing trend of shrinkage with the increase of age, while the rest of the specimens showed a trend of increasing and then decreasing. The shrinkage rates of the other SSMP-doped specimens were significantly smaller than that of specimen 1# at the age of 28 d, like a parabola, indicating that the small amount of f-CaO and f-MgO in SSMP produced a certain expansion effect after hydration during this period, which compensated for the shrinkage of the foamed lightweight soil. Thus, more SSMP admixture would produce a greater expansion effect and a smaller shrinkage rate, in which their shrinkage rate decreases sharply after 21 d when the specimen contains high SSMP doping, while the change of the shrinkage rate tends to decrease slowly with the increase of SSMP doping during the age of 28~56 d. The maximum shrinkage rate was 2.34 × 10⁻³ when the SSMP doping was 0%, and the minimum shrinkage rate was 0.54 × 10⁻³ when the doping was 60%. Comparing specimens 5# and 10#, it was found that the larger the SSMP specific surface area, the smaller the shrinkage of the samples.

The microscopic morphology and mineral composition of samples 1#, 5#, 7#, and 10# at 28 d of age were studied by using SEM and XRD, respectively. The SEM images of the four samples at 30× and 2000× magnification are shown in Figure 4, respectively. It can be seen in Figure 4a–d that all four samples had a spherical pore shape with different pore sizes, and the maximum pore size reached about 0.7 mm. Compared with samples 5# and 7#, when the SSMP admixture was 0%, 40%, and 60%, respectively, 1# sample had the most uniform pore size of about 0.2 mm, and the pore wall was smooth with fewer connected pores and mostly individual pores. Samples 5# and 7# had more large pores, and some of them were through and broken pores, with thin pore walls. The admixture of SSMP would cause some damage to the foam and affect the formed foamed lightweight soil pore structure. Compared with sample 5# with a specific surface area of 410 m²/kg, when the ratios were 40% of SSMP and 20% of GBFS, sample 10# with a specific surface area of 710 m²/kg had a smooth pore wall, few through holes, and mostly small individual holes. Additionally, the increase of SSMP specific surface area made the angles smooth, which made it easier to mix uniformly and reduced the damage to the foam. The morphology of the hydration products of the foam walls were characterized by SME technology, and their corresponding results are shown in Figure 4e–h. It can be seen that sample 1# was the most compact, followed by samples 10#, 5#, and 7#. The four samples had low crystallinity, and their hydration products were amorphous gel (C-S-H) and flake calcium hydroxide (CH). Samples 5# and 7# had obvious spatial mesh structures, which could be ascribed to the fact that the increase of the C-S-H gel system would make the surface of dehydrated SSMP particles wrapped in hydration products. Compared to sample 5#, sample 10# had fewer reticular structures, more C-S-H, finer SSMP, higher hydration, and higher compressive strength [31,32].

As shown in Figure 5, the main hydration products of the four samples were calcium hydroxide (portlandite) and gel (calcium-silicate-hydrate). C-S-H gel peaks were not obvious. The characteristic peaks of calcium hydroxide were samples 7#, 10#, 5#, and 1# from large to small, which were consistent with the shrinkage rate change pattern. SSMP contains f-CaO, which would be hydrated to generate calcium hydroxide and cause swelling in the later stage, corroborating the trend of dry shrinkage of foamed lightweight soil in Figure 3c. There were also faint C3S peaks in Figure 5, and the C3S peaks of sample 10# were relatively low because the SSMP itself contained C2S, C3S, and other RO phases, which are not fully hydrated. SSMP doped with a high specific surface area was easily hydrated; thus, the C3S characteristic peaks were correspondingly small.

Since SSMP mineral composition contains a certain amount of C3S and C2S, it is like the mineral composition of PC clinker, except that the C3S and C2S in SSMP have dense crystals and a large size, and the hydration rate was slow, but the hydration products were still C-S-H and CH, which can also react with the hydration product CH produced during the hydration of GBFS in a secondary way. In addition, the C3S and C2S in SSMP are also directly related to its alkalinity level [33,34], and the strong alkalinity of SSMP and PC plays a good role in stimulating the potential activity of minerals. Thus, the synergistic hydration of GBFS-SSMP and GBFS would contribute to the improvement of the strength of foamed lightweight soil.

4. Conclusions

In this study, foamed lightweight soil used for subgrade filling was prepared with steel slag powder, blast furnace slag particles, and cement as cementitious materials. The mechanical properties of the foamed lightweight soil were improved by adjusting the doping amount and specific surface area of steel slag powder and by enhancing the mechanism of the microstructure evolution of foamed lightweight soil on the macroscopic mechanical properties. The above study drew the following conclusions:

(1)

In the steel slag micronized powder-granulated blast furnace slag-cement system, the foamed lightweight soil prepared displayed the best compressive performance by adding 20% steel slag micronized powder and 40% granulated blast furnace slag when steel slag micronized powder with a specific surface area of 410 m²/kg was used. The flow factor and wet density of the corresponding foamed lightweight soil were 181 mm and 615 kg/m³, respectively, and its compressive strength (28 d) reached up to 1.52 Mpa.

(2)

The compressive performance of the prepared foamed lightweight soil could meet the design technical requirements (7 d ≥ 0.5 Mpa; 28 d ≥ 1 Mpa) by adding the 40% steel slag micronized powder and 20% granulated blast furnace slag, when steel slag micronized powder with specific surface area of 710 m²/kg was used. The flow factor and wet density of prepared foamed lightweight soil were 187 mm and 630 kg/m³, respectively, and its compressive strength (28 d) reached up to 1.55 Mpa.

(3)

The steel slag micronized powder with the stronger hydration expansion effect enhanced the shrinkage compensation effect of foamed lightweight soil and boosted the stability of foamed lightweight soil. When the SSMP content was 0%, the shrinkage rate was 2.34 × 10⁻³, while when the SSMP content was 60%, the shrinkage rate was only 0.54 × 10⁻³.

(4)

The specific surface area of steel slag micronized powder was more conducive to the integrity of the foam pore structure. The main hydration products of the steel slag micronized powder-granulated blast furnace slag-cement system were low crystalline or amorphous gel (C-S-H) and tabular calcium hydroxide (CH). The strong alkalinity of steel slag hydration products could help to stimulate the intrinsic activity of mineral powder, further improving the compressive strength of foamed lightweight soil.