**1. Introduction**

Cemented sand and gravel (CSG) is a new material for dam construction works, produced by adding cementitious materials and water to easily accessible rock based material, including sand and gravel at the river bed or excavation muck near the dam site, and mixing them with simple equipment and process. The cemented sand gravel dam (CSGD) built on CSG material has the advantages of both a roller compacted concrete dam (RCC dam) and a rockfill dam. Compared with an RCC dam, it demands less cement, with simplified aggregate preparation and mixing facilities. Moreover, temperature control measures are not necessary in the construction. As a result, it can effectively speed up the construction and lower the project cost. Compared with rockfill dams, a CSG dam requires a significantly lower amount of engineering works, but has batter capacity to withstand seepage deformation and scouring. Through the utilization of waste materials, CSG technology lowers the demand for artificial materials and high-quality aggregate, and thus promotes the efficient use of resources, less destruction of land vegetation, and a lower impact on the natural environment. Therefore, CSGD is a new kind of easily constructed dam that is economic, safe, low carbon, and eco-friendly.

**Citation:** Chai, Q.; Wan, F.; Xiao, L.; Wu, F. The Influence of Fly Ash Content on the Compressive Strength of Cemented Sand and Gravel Material. *Crystals* **2021**, *11*, 1426. https://doi.org/10.3390/ cryst11111426

Academic Editors: Peter Taylor, Yifeng Ling, Chuanqing Fu, Peng Zhang and Tomasz Sadowski

Received: 23 October 2021 Accepted: 18 November 2021 Published: 21 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In recent years, the relationship between dams and the natural environment has attracted increasingly public attention. It has become a trend of dam technology development to strike a balance between the low-cost and efficient construction of modern dams as well as a lower impact on the natural environment. As the basis and carrier of water resources and hydropower development, reservoir dams play a major role in the comprehensive utilization of water and hydropower resources. Their position will be further strengthened for the sustainable social and economic development of China, to which the CSGD development is the key technology.

According to statistics, dozens of CSGDs have been built around the world since the 1980s. Japan, Greece, Dominica, the Philippines, Pakistan, and Turkey are among countries that have already carried out engineering explorations and applied the technology [1–7]. China began the study of CSGD at the end of last century. By the beginning of this century, many colleges and research institutes have made extensive studies and discussions on the property, constitutive model, calculation and analysis methods, design principles, and standards of CSG material. The representative researchers include Jia Jinsheng, Zheng Cuiying et al. of China Institute of Water Resources and Hydropower Research [8–11], Sun Mingquan et al. of North China University of Water Resources and Electric Power [12–16], He Yunlong et al. of Wuhan University [17–20], and Cai et al. [21,22] of Hohai University. The previous studies believe that as CSG material only uses a small amount of cement, fly ash can be added in large quantities to improve its strength. However, the specific increased value of material strength and the optimal content of fly ash are not fully discussed. Therefore, the primary purpose of this study is to examine the influence of fly ash content on the compressive strength of CSG material. This quantitative examination provides various crucial insights for building dams of this type in the future.

## **2. Test Design**

### *2.1. Test Materials*

CSG material is composed of water, sand (fine aggregate), stone (coarse aggregate), and cementitious materials (cement and fly ash). The proportions of them can vary. In this test, (1) water: Zhengzhou tap water; (2) Sand: Natural river sand in Ruzhou County section of Ruhe River was adopted, the fineness modulus FM = 2.57, in accordance with the requirements of the Technical Guidelines for Damming with Cemented Granular Materials (SL 678-2014) that " . . . in natural materials, the fineness modulus of sand should range from 2.0 to 3.3". (3) Stone: to study the effect of aggregate gradation on the strength of CSG material, natural graded aggregates in Ruzhou County section of Ruhe River were used. After artificial screening, aggregates were divided into 5–20 mm, 20–40 mm, 40–80 mm, 80–150 mm and distributed in silos, as illustrated in Table 1. (4) Cementitious materials: the 425# ordinary Portland cement (P.O.42.5) was produced by Henan Duoyangda Cement Co., Ltd., Zhengzhou, China, which met the requirements for cement in the CSG material in the Technical Guidelines for Damming with Cemented Granular Materials (SL 678-2014) that "all Portland cement series conforming to GB175 and GB200 can be used to build dams with cemented granular materials. When mineral admixtures such as fly ash are added into cementitious materials, Portland cement, ordinary Portland cement, medium or low heat Portland cement should be preferentially selected." The dry discharged F Class II fly ash from Zhengzhou Thermal Power Plant was used in the test. The performance index is shown in Table 2.




**Table 2.** Performance index of fly ash.

## *2.2. Mix Proportion in the Test*

In this test, "weight method" was used in a mixed proportion design. According to the Technical Guidelines for Damming with Cemented Granular Materials (SL 678-2014) and previous research [23–30], the apparent density of CSG material was selected as 2350 kg/m<sup>3</sup> (the apparent density was checked after the specimen test. The samples were all about 2350 kg/m<sup>3</sup> , and the maximum fluctuation was within ±2%). According to the research results, factors such as water–binder ratio, sand ratio, and aggregate gradation have great influence on the material strength, and all factors have optimal values. This paper mainly focused on the influence of fly ash content on the strength of CSG material, so the proportion of other materials followed the reference of previous studies with the water–binder ratio at 1.0, sand ratio at 20%, coarse aggregate ranging from 20–40 mm, and 5–20 mm, mass ratio at 6:4, and cement content of 50 kg/m<sup>3</sup> and 60 kg/m<sup>3</sup> . The fly ash content was designed to be 0 kg/m<sup>3</sup> , 20 kg/m<sup>3</sup> , 30 kg/m<sup>3</sup> , 40 kg/m<sup>3</sup> , 50 kg/m<sup>3</sup> , 60 kg/m<sup>3</sup> , 70 kg/m<sup>3</sup> , 80 kg/m<sup>3</sup> , 90 kg/m<sup>3</sup> , and 100 kg/m<sup>3</sup> , respectively as the admixture for cementitious materials. The designed mix proportion is shown in Table 3.



Note: Volume of material per unit volume/(kg/m<sup>3</sup> ); C, cement; F, fly ash; W, water demand; S, sand; NA1, coarse aggregate (20–40 mm); and NA2, coarse aggregate (5–20 mm).

### *2.3. Specimen Preparation*

Considering that no standard CSG material test exists, and the composition of CSG is similar to concrete, this test followed the Test Code for Hydraulic Concrete (SL352-2006). As the construction and dam filling of CSGDs home and abroad have all adopted roller compaction, the CSG specimens in this paper were also formed through rolling compacted concrete, with the upper part of the specimens compacted and the lower part vibrated. After the vibration, the surface of specimens was smoothed and covered with a film (to prevent water evaporation). Before demolding, they were placed in a room with the

temperature of 20 ◦C ± 5 ◦C for 48 h [31–34]. After demolding, the specimens were sent to the standard maintenance room for maintenance lasting 28 d and 90 d. Eighteen groups were formed, with eight cubic specimens of 150 mm × 150 mm × 150 mm in each group. The preparation process is shown in Figure 1. the temperature of 20 °C ± 5 °C for 48 h [31–34]. After demolding, the specimens were sent to the standard maintenance room for maintenance lasting 28 d and 90 d. Eighteen groups were formed, with eight cubic specimens of 150 mm × 150 mm × 150 mm in each group. The preparation process is shown in Figure 1.

roller compaction, the CSG specimens in this paper were also formed through rolling compacted concrete, with the upper part of the specimens compacted and the lower part vibrated. After the vibration, the surface of specimens was smoothed and covered with a film (to prevent water evaporation). Before demolding, they were placed in a room with

*Crystals* **2021**, *11*, x FOR PEER REVIEW 4 of 10

**Figure 1.** Flow chart of CSG sample preparation. **Figure 1.** Flow chart of CSG sample preparation.

Preparation process of cemented sand and gravel material: Premixed stone and sand weighed according to test mix ratio, adding cement and fly ash, mixing evenly, and then adding water to mix, and finally mixing evenly and charging and vibrating. After the vibration is completed, the surface of the specimen is smoothed, and the film is covered (to prevent water evaporation). After standing at 20 °C ± 5 °C for 48 h, the demolded specimen is sent to the standard maintenance room for maintenance until the test age. Preparation process of cemented sand and gravel material: Premixed stone and sand weighed according to test mix ratio, adding cement and fly ash, mixing evenly, and then adding water to mix, and finally mixing evenly and charging and vibrating. After the vibration is completed, the surface of the specimen is smoothed, and the film is covered (to prevent water evaporation). After standing at 20 ◦C ± 5 ◦C for 48 h, the demolded specimen is sent to the standard maintenance room for maintenance until the test age.
