**1. Introduction**

Owing to the development and increased utilisation of the ocean, a significant amount of marine infrastructure is being constructed, which necessitates more building materials. Corrosion-resistant structural steels have been developed, such as fibre-reinforced polymer bars [1–3], stainless-clad bimetallic steel [4–8], and titanium-clad bimetallic steel [9–11]. The application of reinforced concrete structures in marine engineering is becoming increasingly extensive. Concrete, as one of the main raw materials for reinforced concrete structures, is in high demand. Aggregates constitute 70% to 80% of concrete volume and contribute significantly to concrete [12,13]. For construction on islands and reefs distant from the mainland, the transportation of sand aggregates significantly increases the construction cost [14]. Therefore, alternative materials that satisfy engineering requirements must be identified. Sea sand has been used as a fine aggregate to prepare concrete [15–17]. Coral is a relatively easy building material for marine engineering. In the 1830s, corals were first used as aggregates in concrete. After investigating coral aggregate concrete buildings in Guam, Howdyshell [18] concluded that coral aggregates can be used in concrete structures. In recent years, scholars have extensively investigated coral aggregate concrete. Wu et al. [19] investigated the physical and mechanical properties of different coral aggregates, revealed the pore morphology and distribution of coral aggregates. The results indicated that the mechanical properties of coral aggregate are poor due to its high porosity. Ma et al. [20] prepared high-strength coral aggregate seawater concrete and investigated its impact resistance, which illustrated that the high porosity of coral aggregate led to a large dynamic increase factor of concrete. Chen et al. [21] revealed

**Citation:** Wang, F.; Hua, J.; Xue, X.; Wang, N.; Yan, F.; Feng, D. Effect of Superfine Cement Modification on Properties of Coral Aggregate Concrete. *Materials* **2023**, *16*, 1103. https://doi.org/10.3390/ ma16031103

Academic Editor: Wei Wang

Received: 3 January 2023 Revised: 14 January 2023 Accepted: 17 January 2023 Published: 27 January 2023

**Copyright:** © 2023 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/).

the internal damage and fracture mechanism of coral aggregate concrete via numerical simulations and discovered that microcracks initiated from concrete pores expanded into aggregates. Cai et al. [22] performed a comparison between coral aggregate concrete and ordinary Portland cement concrete, which revealed the low strength of coral aggregate under impact load. Furthermore, the energy required for coral aggregate concrete to initiate the main crack is much lower than that of ordinary Portland cement concrete. Zhou et al. [23] discovered that coral aggregates had a lower strength but higher porosity than sand gravel, which resulted in the strength of coral aggregate concrete being lower than that of ordinary concrete. The above research indicates that although coral aggregate can be used to prepare concrete, the workability and mechanical property of coral aggregate concrete are relatively poor.

To effectively improve the workability and mechanical properties of coral aggregate concrete, the coral aggregates to be used must be pretreated. Coral aggregates are characterised by low density, high crushing index, high porosity, and high water absorption, which are typical of lightweight aggregates; therefore, coral aggregates can be regarded as a lightweight aggregate. To modify lightweight aggregate concrete, researchers typically use inorganic cementitious materials to treat the aggregates, reduce the aggregate porosity, and improve the aggregate strength and durability. He et al. [24] formed a slurry comprising 80% cement, 10% fly ash, and 10% silica fume to modify recycled aggregates of broken brick concrete. The results showed that the slurry increased the apparent density and strength of recycled brick concrete aggregates. Yang et al. [25] investigated the strengthening effect of cement paste on the mechanical properties of broken brick aggregate concrete. It was found that the broken brick aggregate concrete demonstrated better mechanical properties and higher resistance to chloride ion migration after being treated with a cement-coal fly ash slurry via soaking for 4 h. Compared with before modification, the compressive strength of the brick aggregate concrete increased by 22–34% after modification. Hossein et al. [26] improved the durability of recycled concrete aggregate concrete by soaking it in silica fume paste. The results showed that the water absorption of recycled aggregates decreased by 14–22%, and the resistivity increased significantly after modification with silica fume paste. The above research shows that the inorganic cementitious materials modification method can improve the mechanical properties and durability of lightweight aggregate concrete. However, the above modification methods still cannot satisfy the requirements of coral aggregate concrete, due to the high porosity and low strength of coral aggregate. At present, the research on modification methods of coral aggregate is relatively limited, and the influence of modification on aggregate properties and concrete performance is still unclear.

Considering the high porosity of coral aggregates and the typical methods used to modify porous lightweight aggregates, superfine cement is used innovatively in this study to modify coral aggregates. Changes in the water absorption and strength of coral aggregates before and after modification are investigated. Modified and unmodified coral aggregates are used to prepare coral aggregate concrete. Subsequently, the effects of using modified superfine cement on the workability, strength, and failure mode of coral aggregate concrete are revealed. The aim of this study is to provide a new method for modifying coral aggregates, which can promote the use of coral aggregate in the construction of islands and reefs. The experimental results are conducive to the design and construction of modified coral aggregate concrete structures.

#### **2. Materials and Methods**


The coral aggregates used in this study were obtained from the South China Sea. As shown in Figure 1, corals have different shapes, which may be elongated, forked or irregular. In addition, there are many holes in the coral, showing a honeycomb-like surface. The aggregate particle size distribution indicates the proportion of aggregates with different

particle size ranges relative to the total aggregate quality. The particle size distribution affects the slump, expansion, consistency, bleeding rate, and other performances of concrete mixtures [27,28], as well as the mechanical properties [29] and durability [30] of concrete. In accordance with GB/T 17431.1-2010 [31], the coarse aggregates used in this experiment were screened and analysed to obtain the gradation of the coral aggregates, as shown in Figure 2. The experimental coral aggregates satisfied the requirements of GB/T 17431.1- 2010 [31] in terms of the upper and lower limits of gradation for lightweight aggregates measuring 5–20 mm. Table 1 lists the physical properties of the coral aggregates used in the experiment.

**Figure 1.** Coral.

**Figure 2.** Coral aggregate gradation.

**Table 1.** Bulk density, apparent density, and porosity of coral aggregate samples.


## 2.1.2. Cement and Superfine Cement

Owing to the high porosity of coral aggregates, the coral aggregates were modified by soaking them in a high-activity micro-inorganic cementitious material. The internal pores and microcracks of the coral aggregates were filled with slurry to improve the internal structure of the aggregates as well as the workability and mechanical properties of the concrete. Ordinary Portland cement, which is typically used for modification, features a large particle size; thus, the particles do not permeate easily into microcracks. Generally, particles of ordinary Portland cement can only penetrate into pores larger than 0.2 mm; therefore, the modification effect on porous aggregates is unsatisfactory, and improvements to fine pores require cement with a smaller particle size. Superfine cement is a highperformance cement-based grouting material with a much higher fineness than ordinary cement [32]. It has higher strength and better groutability. Because of the superfine grain size and larger surface area of cements, superfine cement slurry has good fluidity and particle filling [33]. Therefore, to fill the fine pores of coral aggregates and reduce the water absorption and crushing index of aggregates, superfine cement was adopted to strengthen and modify the coral aggregates. The superfine cement used in the current experiment was 1250 mesh superfine cement produced by China Resources, and its performance index is shown in Table 2. The cement used to prepare concrete was P.O 42.5 ordinary Portland cement produced by Sichuan Esheng Cement Plant, which satisfies the requirements of GB175-2020 [34], and its performance index is shown in Table 3.



**Table 3.** Performance index of cement.

