Influence of composition of curing agent and sand ratio of engineering excavated soil on mechanical properties of fluidized solidified soil

The three kinds of excavated soil studied here are all from the project construction site. Type A soil is plastic clay with a sand ratio of about 2%; Type B soil is mainly composed of plastic clay, containing a small amount of building slag, with a sand ratio of about 15%; Type C soil is mainly composed of completely weathered mudstone, containing a small amount of clay, with a sand ratio of about 68%. Physical characteristics and appearance of different soils were shown in Table 1 and Figure 1, respectively.

Fig. 1.

The cement is ordinary Portland cement produced by Sichuan Jinding (group) Emei Cement Plant Co., Ltd. Both fly ash and quicklime are commercially available materials. The performance parameters of cement and fly ash are shown in Tables 2 and 3.

The amount of curing agent in this paper was 15% of the dry soil mass. By changing the type of soil and the composition ratio of the curing agent, one can control the water and solids ratio or slump, respectively, to mix the premixed fluidized solidified soil. Before mixing, all raw materials shall be dried, and the excavated soil be crushed into particles smaller than 10mm. The detailed sample preparation steps were as follows.

The solidifying effect of the curing agent on the soil was closely related to the composition of the curing agent. Figure 2 showed the influence of different composition ratios of curing agents on the strength of fluidized solidified soil under the same dosage of curing agent. The results showed that the 28 day unconfined compressive strength of the fluidized solidified soil increased first and then decreased with the increase of the ratio of quicklime in the curing agent. This was mainly because the plastic clay had a poor engineering property. Quicklime is a calcium-containing stabilizer, which could exchange calcium ions with clay to produce flocculation, which reduced the thickness of bound water film in clay minerals, thus reducing adsorption and plasticity of clay, improving the engineering properties [27]. In addition, quicklime reacted with water to generate Ca(OH)₂, increasing the alkalinity of the mixture, which was conducive to stimulating the activity of fly ash, generating pozzolanic reaction to generate hydrated calcium silicate C-S-H gel, and further improving the strength of fluidized solidified soil. However, if the proportion of quicklime was too high, the excess Ca(OH)2 could absorb carbon dioxide to produce calcium carbonate, which has no direct contribution to strength. If the fly ash is too low, the growth rate of the late strength and the negative effects of early strength could be weakened, resulting in a decrease in the 28 day strength and a slowed growth rate in the 7 day strength. For example, in the A-4 and A-5 samples, the content of fly ash was lower, the late strength increased slowly, and the overall strength was low due to the reduction of the secondary hydration reaction and the micro-aggregate reaction of fly ash. Therefore, after a comprehensive comparison, the ratio of cement: fly ash: quicklime in the curing agent was 2:1:1, which was more appropriate.

Fig. 2.

In order to study the influence of sand ratio of excavated soil on the strength of fluidized solidified soil, the above-preferred composition ratio of curing agent and its dosage was selected to prepare fluidized solidified soil with the same slump. Its unconfined compressive strength was tested, and the results are shown in Figure 3. The results show that the strength of the prepared fluidized solidified soil increased in turn with the increase of sand ratio in the excavated soil. This was because the increase of sand ratio reduced the content of clay particles, so that the engineering plasticity of the soil mixed with quicklime was enhanced. At the same time, the amount of water used to mix the fluidized solidified soil to the same fluidity was reduced, which indirectly increased the amount of the curing agent in the fluidized solidified soil, so that the final strength was significantly improved. In general, the excavated soil contains sand or hard materials, which can play the role of fine aggregate in the fluidized solidified soil and help to improve its strength.

Fig. 3.

To study the micro mechanism of the strength change of fluidized solidified soil, the XRD of fluidized solidified soil with different sand ratio after 28 days of curing was tested, and its spectrum is shown in Figure 4. Cement will generate calcium hydroxide (Ca(OH)2) and calcium silicate (CSH) during hydration, but there was almost no diffraction peak of Ca(OH)2 in Figure 4. This was because it has a secondary hydration reaction with silicon dioxide (SiO2), and continuously generates hydrated calcium silicate (CSH). In addition, with the increase of sand ratio in excavated soil, the diffraction peaks of ettringite (AFt), hydrated calcium silicate (CSH), and calcium silicoaluminate hydrate (CASH) were enhanced. This showed that the reaction products with strengthening effect in the fluidized solidified soil could increase with the increase of sand ratio. This showed that the reaction products with strengthening effect in the fluidized solidified soil could increase with the increase of sand ratio.

Fig. 4.

The micromorphology of fluidized solidified soil with different sand ratio was tested by SEM, and the results are shown in Figure 5. The gaps between soil particles in Figure 5a were obvious, while there were also gaps that basically filled with needle-like ettringite (AFt) in Figure 5b and c. Ettringite is a crystal formed by the combination of cement hydration product C-A-H (calcium aluminate hydrate) and sulfate ions. The higher content of the curing agent can strengthen the cation exchange on the surface of soil particles and generate more hydration products (AFt: ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O); CSH: calcium silicate hydrate (Ca5Si₆O₁₆(OH)*4H₂O); CASH: calcium silicoaluminate hydrate (Ca₂Al₂SiO₇(OH)*4nH₂O)) which can compact soil particles, and improve the strength of fluidized solidified soil. In addition, it can be seen in Figure 5b and c that the generated hydration products that could fill gaps were mainly needle-like AFt. Comparing Figure 5b with 5c, it can be found that the AFt crystals in Figure 5b were loosely packed, while the AFt crystals in Figure 5c were larger and more tightly packed, which was consistent with the high-intensity AFt diffraction peak of the liquid solidified soil prepared by Type C soil in the XRD analysis.

Fig. 5.

As a foundation backfill material, the fluidized solidified soil will inevitably be eroded by rainwater or groundwater during its service. The change of its performance after soaking in water is one of the key factors affecting the foundation settlement. In this paper, the water stability of fluidized solidified soil with different sand ratio was tested, and the results are shown in Figure 6. It could be found that the softening coefficient of fluidized solidified soil with the same sand ratio increased with the increase of curing age before soaking under the same soaking time. This was related to the curing age before soaking, that is, with the extension of the curing age, the more hydration products of the curing agent, the better the soil solidification effect. When soaking, it is difficult for water to soak into the solidified soil, so that the softening coefficient is higher. For the fluidized solidified soil with different sand ratio, under the same curing age before soaking, the internal voids caused by more hydration products were decreased and the compaction density between soil particles was increased with the increase of the sand ratio, making it more difficult for water to be absorbed during soaking, resulting in less impact on the performance of the fluidized solidified soil with higher sand ratio. In a word, according to the requirements of construction standards, it is required to cure for not less than 7 days after pouring and covering a membrane. Even if the membrane is removed and soaking occurs in the rainstorm weather, the softening coefficient of the fluidized solidified soil prepared in this paper can reach more than 80%, which is applicable to the backfilling of most projects.

Fig. 6.

Green building is an important way to achieve energy conservation and emission reduction, and the research and application of green low-carbon building materials is one of the methods of green building. As the main component of fluidized solidified soil is excavated soil, it can reduce the emission of construction waste and environmental pollution after use and is a green low-carbon building material. In order to better understand the low carbon characteristics of fluidized solidified soil, carbon emissions of it and traditional backfill materials were calculated on raw materials. The composition, amount, and carbon emission factor of each backfill material were shown in Table 5.

The calculation formula for total carbon emissions generated by each raw material used in this paper was shown in Formula (1): 1CSC=∑k=1nMk×Fk$${\rm{CSC}} = \mathop \sum \limits_{k = 1}^n {M_k} \times {F_k}$$ Where, CSC is the total carbon emission of each raw material used in the production stage of building materials (kg CO₂e/m³); M_k is the consumption of the kth main raw material; Fk is the carbon emission factor of k main raw materials (kgCO₂e). The calculation results are shown in Figure 7. Cementitious materials in different backfill materials had an important impact on carbon emissions, that was, their carbon emissions account for more than 90%. For example, in lime-soil and foamed concrete, the carbon emissions of cementitious materials reached 99.9%, which was mainly due to the high consumption of cementitious materials and the amplification of carbon emissions. Through comparison, it was found that the amount of cementitious material used for fluidized solidified soil was the lowest, so that it had the lowest carbon emission, which was only 111.29 kgCO₂e/m³. At the same time, due to the high fluidity and selfcompactness of the fluidized solidified soil, mechanical compaction or vibration was not required during the backfilling construction, which could further reduce the construction carbon emissions. Therefore, the fluidized solidified soil is an excellent green low-carbon building material, which can be widely used in the green construction process.

Fig. 7.

The fluidized solidified soil prepared in this paper was used for engineering demonstration, and the demonstration effect is shown in Figure 8. By using the mix ratio of fluid solidified soil described in this article, and using excavated soil on the construction site, a self-compacting backfill material with high flow and controllable strength was prepared. This material was mainly used for backfilling projects such as foundation trenches, cavities, and roadbed. During the application process, it was also necessary to pay attention to curing the poured fluidized solidified soil for no less than seven days, otherwise, the surface of the solidified soil was prone to cracking. The results of curing and noncuring are shown in Figure 9. The application of fluidized solidified soil can save costs, reduce environmental damage and pollution, greatly alleviate the shortage of mineral resources, improve the utilization rate of solid waste resources, and reduce carbon emissions.

Fig. 8.

Fig. 9.