REN Keqi¹ ，WANG Yong¹ ，LEI Bingbing^2a ，ZHANG Jiangpeng^2b ，LIU Jie^1，2ab

1. College of Water Conservancy & Architectural Engineering， Shihezi University， Shihezi 832003， China；

2. Xinjiang Engineering Research Center for Disaster Prevention and Control Technology of Mountain Transportation Infrastructure；

a. Xinjiang Key Laboratory for Safety and Health of Transportation Infrastructure in Alpine and High-altitude Mountainous Areas，

b.Xinjiang Transportation Planning Survey and Design Institute Co. ， Ltd. ， Urumqi 830000， China.

Abstract

Objective To promote waste utilization and rational use of resources， aeolian sand and pyrolysis residues of oil sludge are selected as raw materials to produce a sustainable and economical foamed lightweight soil. The study analyzes the fluidity and unconfined compressive strength of foamed lightweight soil of aeolian sand and oil sludge pyrolysis residues （SOFS） to prepare an environmentally friendly subgrade filling material with high compressive strength.

Methods Foamed lightweight soil was prepared using aeolian sand， pyrolysis residues of oil sludge， cement， a composite foaming agent， and water as raw materials. 25 sets of orthogonal experiments with six factors and five levels were designed. The fluidity and 28-day unconfined compressive strength were evaluated to determine the influences of various factors and obtain the optimal mix ratio. X-ray diffraction （XRD） analysis was conducted to identify the ion components contained in various samples and assess the involvement of aeolian sand and oil sludge pyrolysis residues in the hydration reaction of cement. Electron microscope tests were used to examine the internal microstructures of test specimens， and the structural characteristics contributing to higher compressive strength were summarized.

Results and Discussion The content of aeolian sand， pyrolysis residues of oil sludge， and the water-solid ratio were found to have a greater impact on both the fluidity and unconfined compressive strength of the SOFS. The amount of aeolian sand affected fluidity due to the lack of cohesion in aeolian sand particles. Under the action of water flow and gravity， the slurry of the foamed lightweight soil tended to spread more easily. In contrast， the amount of oil sludge pyrolysis residues affected fluidity by reducing it because the sticky nature of these particles inhibited slurry spreading. The water-solid ratio also impacted fluidity. A higher water-solid ratio led to more water， which enhanced the driving force of the water， and consequently increased fluidity. Factors such as mixing time， dilution factor， and foam-to-slurry volume ratio had less impact on fluidity but significantly impacted compressive strength. The dilution factor of the foaming agent affected the foam size and its distribution in the slurry， thus affecting the compressive strength. The foam-to-slurry volume ratio mainly affected the porosity of the specimens， as higher porosity reduced the compressive strength. Moreover， water-solid ratio also influenced the impact of the admixture’s water absorption on the foam. A higher water-solid ratio minimized the effect of water absorption on the foam， allowing for a complete hydration reaction， which improved the compressive strength of the specimens. Moderate amounts of aeolian sand and oil sludge pyrolysis residues contributed to a denser structure， further improving the compressive strength of the specimens. Microscopic analysis showed that both aeolian sand and pyrolysis residues of oil sludge participated in the hydration reaction of cement， forming uniform C-S-H floccules with consistent pore sizes that improved the strength of the material. Based on these findings， the optimal mix ratio for the SOFS was determined to be as follows： a foam-to-slurry volume ratio of 0. 8∶1， a foaming agent dilution factor of 45 times， a water-solid ratio of 0. 33∶1， a mixing time of 100 s，120 g of pyrolysis residues， and 90 g of aeolian sand.

Conclusion The SOFS prepared with the optimal mix ratio achieves a wet density of 928 kg/m³， i. e. ， wet unit weight of 9. 3 kN/m³， and a 28-day compressive strength of 3. 4 MPa， which exceeds the requirements for CF2. 5 but falls below CF5. 0. The SOFS meets the specification requirements of a minimum compressive strength of 0. 8 MPa and a minimum unit weight of 5 kN/m³ for subgrade filling within 0-0. 8 m beneath urban expressways， highways， and first-class roads outlined in the Technical Specification for Foamed Mixture Lightweight Soil Filling Engineering（CJJ/T 177—2012）.

Keywords：aeolian sand； oil sludge pyrolysis residue； foamed lightweight soil； unconfined compressive strength； fluidity； microscopic analysis