# Experimental Study on the Effects of Freeze–Thaw Cycles on Strength and Microstructure of Xining Region Loess in China

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

#### 2.1. Sample Preparation

#### 2.2. Experimental Design

#### 2.3. Basic Physical Properties of Loess

## 3. Analysis of Micro-Test Results

#### 3.1. SEM Test

#### 3.2. XRD and XRF Test

## 4. Analysis of Mechanical Test Results

#### 4.1. Unconfined Compressive Strength

#### 4.2. Stress–Strain Curve

#### 4.3. Elastic Modulus

^{2}= 0.80172), which indicates that the elastic modulus of undisturbed loess shows an exponentially decreasing trend with the increase in freeze–thaw cycles. With the void increases, the compressive strength of the undisturbed loess decreases, and its strength tends to be stable with the increase in freeze–thaw cycles. For the remolded loess, with the increase in the number of freeze–thaw cycles, its elastic modulus changes gradually, and the fitting degree between the values is low. This may be due to the low strength of the remolded loess itself and the high compressibility between soil particles. The effects of freeze–thaw cycles on soil elastic modulus are roughly the same, so the fitting curve is relatively smooth, and the fitting degree is poor.

## 5. Conclusions

- With an increasing number of freeze–thaw cycles, the unconfined compressive strength of undisturbed loess and remolded loess first increases and then decreases as the strain increases, and the stress–strain curve exhibits strain-softening characteristics. During the freeze–thaw cycle, through the mutual conversion of free water to ice water inside the sample, the structure of the loess is destroyed. At the same time, due to the influence of the primary structure, the unconfined compressive strength of the undisturbed loess is higher than remolded loess.
- The unconfined compressive strength of undisturbed loess and remolded loess decreases after 6 freeze–thaw cycles, and the strength increases after 8 to 20 freeze–thaw cycles.
- It can be seen from the SEM images that with increasing freeze–thaw cycles, the large particles inside the sample gradually decompose into small particles, increasing the fine particles inside the sample. At the same time, the larger pores inside the sample gradually reduce in size. With the increase in the number of freeze–thaw cycles, the particles inside the soil become denser, and the strength increases.
- The mineral composition of loess after freeze–thaw cycles are studied, and it is found that the internal composition of loess did not change significantly, and the mineral content is stable. Therefore, the effect of freeze–thaw cycles on the mineral composition of loess can be ignored in engineering.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**Sample preparation flow chart. (

**a**) Wrapped in plastic; (

**b**) marked sample; (

**c**) sealed sample; (

**d**) freeze–thaw cycle test box.

**Figure 3.**Temperature curve. (

**a**) Winter temperature change curve in Xining (in recent years); (

**b**) sine curve set during the test.

**Figure 4.**Physical property curve of Xining loess. (

**a**) Grain size distribution of Xining loess; (

**b**) compaction curve of Xining loess.

**Figure 7.**SEM images of undisturbed loess at different freeze–thaw cycles (500 times). (

**a**) The number of freeze–thaw cycles is 0. (

**b**) The number of freeze–thaw cycles is 2. (

**c**) The number of freeze–thaw cycles is 6. (

**d**) The number of freeze–thaw cycles is 10. (

**e**) The number of freeze–thaw cycles is 15. (

**f**) The number of freeze–thaw cycles is 20.

**Figure 8.**SEM images of remolded loess at different freeze–thaw cycles (500 times). (

**a**) The number of freeze–thaw cycles is 0. (

**b**) The number of freeze–thaw cycles is 2. (

**c**) The number of freeze–thaw cycles is 6. (

**d**) The number of freeze–thaw cycles is 10. (

**e**) The number of freeze–thaw cycles is 15. (

**f**) The number of freeze–thaw cycles is 20.

**Figure 9.**XRD patterns of loess under different freeze–thaw cycles. (

**a**) The number of freeze–thaw cycles is 0. (

**b**) The number of freeze–thaw cycles is 2. (

**c**) The number of freeze–thaw cycles is 6. (

**d**) The number of freeze–thaw cycles is 20.

**Figure 10.**Relationship between the compressive strength of loess and the number of freeze–thaw cycles at different freeze–thaw cycles.

**Figure 11.**Stress–strain curves of loess with different freeze–thaw cycles. (

**a**) Undisturbed loess. (

**b**) Remolded loess.

**Figure 12.**Relationship between the initial elastic modulus of loess and the number of freeze–thaw cycles.

Type of Soil | Freeze–Thaw Cycle Test | UCS Test | SEM Test | XRD and XRF Test |
---|---|---|---|---|

Undisturbed loess | (1) Freeze–thaw cycles were 0, 2, 4, 6, 8, 10, 15, 20 | Each group has 3 samples, a totalof 48 samples | Each group has 2 samples, a totalof 32 samples | Each group has 2 samples, a totalof 32 samples |

Remolded loess | (2) The freezing and thawing temperature was set as ±15 °C | |||

(3) 12 h for freezing and 12 h for thawing as a cycle |

Specific Gravity of Soil Solids (Gs) | Natural Water Content (%) | Dry Density (g/cm ^{3}) | Liquid Limit (%) | Plastic Limit (%) |
---|---|---|---|---|

2.71 | 13.40 | 1.72 | 25.30 | 13.95 |

Chemical Constituent (Mass%) Freeze–Thaw Cycles | SiO_{2} | Al_{2}O_{3} | CaCO_{3} | MgO | K_{2}O | Na_{2}O | Fe_{2}O_{3} | Others |
---|---|---|---|---|---|---|---|---|

0 times | 48.0 | 13.5 | 25.9 | 3.15 | 2.74 | 1.26 | 4.17 | 1.28 |

2 times | 48.8 | 14.0 | 24.4 | 3.25 | 2.84 | 1.32 | 4.28 | 1.11 |

6 times | 49.4 | 13.7 | 24.1 | 3.27 | 2.82 | 1.35 | 4.28 | 1.08 |

10 times | 49.1 | 13.5 | 24.8 | 3.13 | 2.70 | 1.30 | 4.28 | 1.19 |

20 times | 48.8 | 13.5 | 24.4 | 3.06 | 2.85 | 1.29 | 4.80 | 1.30 |

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**MDPI and ACS Style**

Xie, B.; Zhang, W.; Sun, X.; Huang, Y.; Liu, L.
Experimental Study on the Effects of Freeze–Thaw Cycles on Strength and Microstructure of Xining Region Loess in China. *Buildings* **2022**, *12*, 795.
https://doi.org/10.3390/buildings12060795

**AMA Style**

Xie B, Zhang W, Sun X, Huang Y, Liu L.
Experimental Study on the Effects of Freeze–Thaw Cycles on Strength and Microstructure of Xining Region Loess in China. *Buildings*. 2022; 12(6):795.
https://doi.org/10.3390/buildings12060795

**Chicago/Turabian Style**

Xie, Banglong, Wuyu Zhang, Xianglong Sun, Yuling Huang, and Leqing Liu.
2022. "Experimental Study on the Effects of Freeze–Thaw Cycles on Strength and Microstructure of Xining Region Loess in China" *Buildings* 12, no. 6: 795.
https://doi.org/10.3390/buildings12060795