*Article* **Effect of Ultrafine Metakaolin on the Properties of Mortar and Concrete**

**Shengli Zhang <sup>1</sup> , Yuqi Zhou <sup>2</sup> , Jianwei Sun 3,\* and Fanghui Han <sup>4</sup>**


**Abstract:** This study investigated the influence of ultrafine metakaolin replacing cement as a cementitious material on the properties of concrete and mortar. Two substitution levels of ultrafine metakaolin (9% and 15% by mass) were chosen. The reference samples were plain cement concrete sample and silica fume concrete sample with the same metakaolin substitution rates and superplasticizer contents. The results indicate that simultaneously adding ultrafine metakaolin and a certain amount of polycarboxylate superplasticizer can effectively ensure the workability of concrete. Additionally, the effect of adding ultrafine metakaolin on the workability is better than that of adding silica fume. Adding ultrafine metakaolin or silica fume can effectively increase the compressive strength, splitting tensile strength, resistance to chloride ion penetration and freeze–thaw properties of concrete due to improved pore structure. The sulphate attack resistance of mortar can be improved more obviously by simultaneously adding ultrafine metakaolin and prolonging the initial moisture curing time.

**Keywords:** ultrafine metakaolin; silica fume; strength; durability

### **1. Introduction**

Concrete made with Portland cement has been available for nearly two hundred years. In the past two centuries, with the development of science and technology, the composition and performance of Portland cement have undergone great changes, and construction technology using concrete has also undergone earth-shaking changes [1,2]. The two changes above have led to complete changes in the composition, preparation methods and performance of modern concrete. Mineral admixtures such as supplementary cementitious materials are an indispensable part of modern concrete [3,4]. The use of highactivity admixtures such as slag and low-activity or inert admixtures such as limestone powder effectively reduces energy and resource consumption by decreasing the amount of cement used in the concrete production process and ensures the green and sustainable development of the concrete industry [3,4]. More importantly, the use of admixtures reduces the dependence of modern concrete strength on cement strength, improves the rheological properties of the mixture, and increases the durability of concrete structures.

At present, the mineral admixtures used in the concrete preparation process worldwide are mainly fly ash, slag, and limestone powder [5–8]. However, with the development of the concrete industry, the stock of high-quality admixtures has decreased, leading to gradually rising prices. Therefore, the available high-quality and economical mineral admixtures will become increasingly scarce and unable to meet the needs of the national construction market. On the premise that the annual production of admixtures cannot be changed, the main technical means to offset the scarcity of high-quality admixtures

**Citation:** Zhang, S.; Zhou, Y.; Sun, J.; Han, F. Effect of Ultrafine Metakaolin on the Properties of Mortar and Concrete. *Crystals* **2021**, *11*, 665. https://doi.org/10.3390/cryst11060 665

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

Received: 21 May 2021 Accepted: 8 June 2021 Published: 10 June 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/).

is to increase their reactivity to reduce the dosage. Mechanical grinding is the most direct method used to improve the reactivity of powders. Ultrafine grinding involves the application of strong mechanical forces that distort the crystal structure of the mineral, rapidly resulting in lattice dislocations, defects, and recrystallization, which improves the reaction activity [9–12]. On the other hand, ultrafine grinding increases the specific surface area of the admixture, thereby enhancing micro aggregation and nucleation effects in the cement matrix [13–15]. In addition, ultra-high-performance concrete (UHPC) has attracted increasing attention due to it meeting the requirements for special concrete structures such as super high-rise buildings and long-span bridges [16–18]. Ultra-high-activity mineral admixtures, such as ultrafine powders, are indispensable for the preparation of UHPC [19–21]. Therefore, it is necessary to study the action mechanism and application effects of ultrafine admixtures in concrete, whether from the perspective of improving the overall quality of admixtures, solving the problem of the scarcity of high-quality admixtures, or meeting the needs of special engineering and ensuring the supply of UHPC.

Metakaolin is an amorphous aluminum silicate formed by the calcination of kaolin clay at temperatures ranging from 500 ◦C to 800 ◦C [22,23]. In the process of heating, most octahedral alumina is converted into more active tetra-coordinated and penta-coordinated units [24,25]. When the crystal structure is completely or partially broken, or the bonds between the kaolinite layers are broken, kaolin undergoes a phase change and finally forms metakaolin with poor crystallinity [25,26]. As its molecular arrangement is irregular, metakaolin is in a metastable thermodynamic state and has gelling properties under appropriate excitation [23–26]. Metakaolin has high pozzolanic activity, which means that it can react with Ca(OH)<sup>2</sup> and produce C-S-H gel and alumina-containing phases, including C4AH13, C2ASH8, and C3AH6, at ambient temperature [23]. Therefore, metakaolin is mainly used as a mineral admixture in cement and concrete. Compared to silica fume and fly ash, metakaolin has a very high reactivity level [23]. Previous studies have shown that metakaolin can increase the mechanical strength of concrete to varying degrees, depending mainly on the replacement rate of metakaolin, the water/binder ratio, and the age at testing [22,23,27]. Remarkably, metakaolin has a positive effect on reducing drying shrinkage and improving durability [22,23,27–30].

In current practical engineering applications, 99.9% of metakaolin particles are less than 16 µm [23]. The mean particle size is generally 3 µm, which is significantly smaller than that of cement particles but not as fine as silica fume [23]. Ultrafine metakaolin with a specific surface area greater than 20,000 m2/kg can be obtained by grinding. The pozzolanic activity of metakaolin is related to the particle size. Theoretically, it is practical to grind metakaolin so that it can play a greater role in concrete. In this paper, ultrafine metakaolin obtained by grinding was used as a supplementary cementitious material in concrete and mortar. The macroscopic properties, including the mechanical strength and durability of concrete and mortar, were investigated. Plain cement concrete and silica fume concrete were employed as reference samples.

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

### *2.1. Raw Materials*

In this paper, Portland cement with a strength grade of 42.5 from Beijing Jinyu Group Co., Ltd. in China was used as the cementitious material. Ultrafine metakaolin and silica fume with similar specific surface areas were used as mineral admixtures. The chemical compositions of the raw materials are presented in Table 1. The total content of SiO<sup>2</sup> and Al2O<sup>3</sup> in ultrafine metakaolin is more than 99%. The particle size distribution of ultrafine metakaolin is shown in Figure 1a. The X-ray diffraction (XRD) patterns of ultrafine metakaolin are presented in Figure 1b. Figure 1b shows that the amorphous phase is the main constituent, and SiO<sup>2</sup> crystals are the main crystalline mineral phase in ultrafine metakaolin. The microstructure of ultrafine metakaolin is shown in Figure 1c. Ultrafine metakaolin is broken into small angular particles. Fine aggregates of mortar and concrete were ISO standard sand and river sand with a fineness modulus of 2.9, respectively.

Crushed limestone with a continuous size gradation from 4.75 mm to 20 mm was used as coarse aggregate. A polycarboxylate superplasticizer with a water reduction rate of 20% was used to adjust the flowability of fresh mortar and concrete. was used to adjust the flowability of fresh mortar and concrete. **Table 1.** Main chemical compositions of raw materials/%.

takaolin. The microstructure of ultrafine metakaolin is shown in Figure 1c. Ultrafine metakaolin is broken into small angular particles. Fine aggregates of mortar and concrete were ISO standard sand and river sand with a fineness modulus of 2.9, respectively. Crushed limestone with a continuous size gradation from 4.75 mm to 20 mm was used as coarse aggregate. A polycarboxylate superplasticizer with a water reduction rate of 20%

**Table 1.** Main chemical compositions of raw materials/%.  **CaO SiO2 Al2O3 Fe2O3 MgO SO3 Na2Oeq \* LOI** 

*Crystals* **2021**, *11*, x FOR PEER REVIEW 3 of 12


\* Na2Oeq = Na2O + 0.658K2O; LOI: loss on ignition.
