# Effect of Different Fine Aggregate Characteristics on Fracture Toughness and Microstructure of Sand Concrete

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Experimental Programs

#### 2.1. Experimental Materials

#### 2.1.1. Fine Aggregate

#### 2.1.2. Cement

^{3}and a specific surface area: 446.5 m

^{2}/kg.

#### 2.1.3. Admixtures

#### 2.2. Mix Proportion and Specimen Preparation

#### 2.3. Test Methods

#### 2.3.1. Physical Properties Test of Fine Aggregates

#### 2.3.2. XRD Test

#### 2.3.3. Compressive Strength and Flexural Strength Test

#### 2.3.4. Box-Counting Fractal Dimensions Calculation

#### 2.3.5. SEM Test

## 3. Results and Discussion

#### 3.1. The Characteristics of Fine Aggregate

_{2}, Al

_{2}O

_{3}and Fe

_{2}O

_{3}. The river sand is constantly washed in water, and its mineral composition is relatively single; while tailings sand exists in the field environment, its mineral composition is more complex. Therefore, the quartz content of river sand is higher than that of quartz in tailings sand.

_{min}is the minimum particle size (mm), D

_{max}is the maximum particle size (mm) and q is the distribution modulus. The value of q is between 0.23 and 0.37. The q value of the target curve in the close packing model is fixed at 0.23 based on the properties of fine aggregates [29]. The gradation quality of the fine aggregate is assessed by the determination coefficient of the target curve (R

^{2}, as shown in Equation (2)).

_{mix}is mixed fine aggregates, the P

_{tar}is the target grading calculated from Equation (1), n is the number of points used to calculate the deviation and $\overline{{P}_{mix}}$ represents the mean of the entire distribution. The target curve under this model and the particle size distribution curve of different fine aggregates are shown in Figure 6. The results showed that the correlation coefficient of ITS (R

^{2}= 0.86) was highest compared to the target curve, while the correlation coefficient of GTS(R

^{2}= 0.64) and RS (R

^{2}= 0.60) were lower compared to the target curve. As a result, ITS showed the most reasonable aggregate gradation, while RS showed the most undesirable aggregate gradation. This suggests that the gradation is closely related to the particle size distribution of fine aggregate. Finally, it should be noted that the basic difference between these aggregates lies in physical characteristics, grain shape, granularity and particle size distributions.

#### 3.2. The Brittleness Coefficient of Sand Concrete with Different Fine Aggregates

#### 3.3. Box-Counting Fractal Dimensions of Fracture Surface

#### 3.4. Microstructural Analysis

## 4. Conclusions

- (1)
- The fracture toughness of sand concrete is related to the characteristics of the aggregate. However, the brittleness coefficients of sand concrete with different aggregates are between 5:1 and 6.5:1 at different strength grades, and the brittleness coefficient of sand concrete is much lower than that of ordinary concrete. Sand concrete exhibits good fracture toughness than ordinary concrete.
- (2)
- The effect of particle fineness of fine aggregates on the fracture toughness of sand concrete is not significant.
- (3)
- The fracture toughness of sand concrete is strongly affected by the FAA. As the FAA value increases from 32 s to 44 s, the fractal dimension of the fracture surface of sand concrete increases from 2.25 to 2.69, and the average flexural strength increases by 13.65% under the same compressive strength.
- (4)
- The fracture toughness, microstructure and hydration products of sand concrete are also related to the gradation of fine aggregates. A good grading distribution can improve the performance of ITZ. When the correlation coefficient of the gradation compared to the target curve increased from 0.60 to 0.86, the microcrack width of ITZ decreased by about 38.55%, and there was also a difference in the hydration products produced. Therefore, the better the gradation of fine aggregates, the better the crack resistance of sand concrete.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**Crack propagation of sand concrete. (

**a**) sand concrete with round and coarse aggregate, (

**b**) sand concrete with angular and fine aggregate.

**Figure 6.**Size distribution curves of fine aggregate and target curves under Andreasen close packing model.

**Figure 9.**Binarization of image and fractal dimension calculation (

**a**) ITS sand concrete, (

**b**) GTS sand concrete, (

**c**) RS sand concrete, (

**d**) Ordinary concrete.

**Figure 10.**Propagation paths of microcracks in concrete (

**a**) ITS sand concrete, (

**b**) GTS sand concrete, (

**c**) RS sand concrete, (

**d**) Ordinary concrete.

**Figure 11.**ITZ of concrete (

**a**) ITS sand concrete, (

**b**) GTS sand concrete, (

**c**) RS sand concrete, (

**d**) Ordinary concrete.

**Figure 12.**Hydration products of concrete (

**a**) ITS sand concrete, (

**b**) GTS sand concrete, (

**c**) RS sand concrete, (

**d**) Ordinary concrete.

Specimens | Ratio of Cement to Sand | Cement /kg·m ^{−3} | GTS /kg·m ^{−3} | ITS /kg·m ^{−3} | RS /kg·m ^{−3} | Water /kg·m ^{−3} |
---|---|---|---|---|---|---|

SC1 | 585.9 | — | 1757.1 | — | 310.5 | |

1:3 | 585.9 | 1757.1 | — | — | 310.5 | |

585.9 | — | — | 1757.1 | 310.5 | ||

SC2 | 669.3 | — | 1673.7 | — | 274.4 | |

1:2.5 | 669.3 | 1673.7 | — | — | 274.4 | |

669.3 | — | — | 1673.7 | 274.4 | ||

SC3 | 781.2 | — | 1561.8 | — | 234.4 | |

1:2 | 781.2 | 1561.8 | — | — | 234.4 | |

781.2 | — | — | 1561.8 | 234.4 | ||

SC4 | 937.5 | — | 1405.5 | — | 206.3 | |

1:1.5 | 937.5 | 1405.5 | — | — | 206.3 | |

937.5 | — | — | 1405.5 | 206.3 | ||

SC5 | 1171.5 | — | 1171.5 | — | 175.7 | |

1:1 | 1171.5 | 1171.5 | — | — | 175.7 | |

1171.5 | — | — | 1171.5 | 175.7 |

Type | Chemical Compositions of Raw Materials (wt. %) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

SiO_{2} | Al_{2}O_{3} | Fe_{2}O_{3} | CaO | MgO | Na_{2}O | K_{2}O | P_{2}O_{5} | TiO_{2} | MnO | LOI | TOTAL | |

GTS | 73.34 | 7.06 | 3.82 | 3.22 | 1.26 | 1.58 | 2.08 | 0.25 | 0.34 | 0.10 | 3.66 | 96.71 |

ITS | 75.85 | 7.54 | 6.64 | 4.19 | 2.12 | 0.60 | 0.48 | 0.13 | 0.30 | 0.15 | 0.86 | 98.86 |

RS | 81.32 | 4.75 | 2.83 | 2.50 | 0.48 | 0.23 | 1.57 | 0.09 | 0.18 | 0.05 | 2.16 | 96.16 |

Type | Fineness Modulus | Apparent Density (g/cm ^{3}) | Bulk Density (g/cm ^{3}) | Void Ratio (%) | Average Size (µm) | Specific Area (m ^{2}/kg) | Powder Content (<0.075 mm) |
---|---|---|---|---|---|---|---|

GTS | 0.62 | 2.67 | 1.40 | 47.62 | 113 | 323 | 32.91% |

ITS | 1.29 | 2.78 | 1.52 | 45.33 | 225 | 282 | 16.93% |

RS | 1.92 | 2.64 | 1.48 | 43.57 | 450 | 256 | 6.92% |

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## Share and Cite

**MDPI and ACS Style**

Sun, Z.; Xiong, J.; Cao, S.; Zhu, J.; Jia, X.; Hu, Z.; Liu, K.
Effect of Different Fine Aggregate Characteristics on Fracture Toughness and Microstructure of Sand Concrete. *Materials* **2023**, *16*, 2080.
https://doi.org/10.3390/ma16052080

**AMA Style**

Sun Z, Xiong J, Cao S, Zhu J, Jia X, Hu Z, Liu K.
Effect of Different Fine Aggregate Characteristics on Fracture Toughness and Microstructure of Sand Concrete. *Materials*. 2023; 16(5):2080.
https://doi.org/10.3390/ma16052080

**Chicago/Turabian Style**

Sun, Zhihua, Jin Xiong, Shubo Cao, Jianxiong Zhu, Xuzhi Jia, Zhigang Hu, and Kaiping Liu.
2023. "Effect of Different Fine Aggregate Characteristics on Fracture Toughness and Microstructure of Sand Concrete" *Materials* 16, no. 5: 2080.
https://doi.org/10.3390/ma16052080