**1. Introduction**

Aggregates are widely used in the field of construction, specifically in the manufacture of concrete. The increasing demand for concrete will result in an increase in the requirements of all concrete ingredients, including aggregates, which represent 60–70% of the total volume of the concrete [1]. Most of the aggregates used are obtained from natural resources, including rocks. The concrete industry has an impact on the global environmental problem due to the utilization of large amounts of natural resources. Natural resources are

**Citation:** Hao, D.L.C.; Razak, R.A.; Kheimi, M.; Yahya, Z.; Abdullah, M.M.A.B.; Burduhos Nergis, D.D.; Fansuri, H.; Ediati, R.; Mohamed, R.; Abdullah, A. Artificial Lightweight Aggregates Made from Pozzolanic Material: A Review on the Method, Physical and Mechanical Properties, Thermal and Microstructure. *Materials* **2022**, *15*, 3929. https:// doi.org/10.3390/ma15113929

Academic Editor: Alessandro P. Fantilli

Received: 9 May 2022 Accepted: 21 May 2022 Published: 31 May 2022

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**Copyright:** © 2022 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/).

decreasing at a quicker rate because of the high demand for usage in the manufacturing of concrete. The development of lightweight materials, such as lightweight aggregate, will help to minimize the use of natural resources.

In the construction field, lightweight aggregate that is used in concrete production is a type of material that is very environmentally friendly. Lightweight aggregate is dramatically different from conventional aggregate. The modifications bring advantages for the designers for a number of reasons other than weight reduction, such as decreased early cracking, decreased permeability, and longer lifespan [2]. Lightweight aggregates can be grouped into the following categories [3]:


Sintered fly ash aggregate, also known as Lytag, is the most widely used artificial aggregate in the construction field [4]. According to Nadesan and Dinakar [5], sintered fly ash aggregate is capable of producing concrete with high strength performance. In addition, the application of lightweight aggregate in concrete has increased in popularity due to its low density, good thermal conductivity, being environmentally friendly, and many economic advantages [6]. Concrete with lightweight aggregate also has low thermal conductivity [6]. The concrete with lightweight aggregate had lower thermal conductivity [2]. The objective of this paper is to review the manufacturing method of lightweight aggregate for the production of lightweight concrete, as well as the properties of lightweight aggregate. The properties include physical properties, such as specific gravity and water absorption, and mechanical properties, such as crushing strength and aggregate impact value. The mechanical and thermal properties of concrete consisting of lightweight aggregate that have been reported previously are also reported in this review.

#### **2. Aggregate**

The main component of concrete is aggregates, which occupy around 70% to 80% of the total volume, with fine aggregate accounting for 25% to 30% and coarse aggregate accounting for 40% to 50% [7]. The coarse aggregate that is usually utilized in construction work comes from various types of resources, such as rock, crushed stone, and gravel [8]. Crushed rocks are commonly used as coarse aggregate and river sand as fine aggregate, both of which can be found naturally. A large amount of natural aggregate, such as sand, gravel, or crushed rock, is mined for concrete manufacturing, and the world's aggregate usage is estimated to be in excess of 40 billion tonnes per year, with concrete accounting for 64% to 75% of all mined aggregate [9]. The massive use of raw materials for aggregates is expected to reach 62.9 billion metric tonnes per year by 2024 with global construction aggregates consumption, and this can deplete natural resources, as it creates an immediate risk to the environment [10]. The construction projects need a considerable amount of natural aggregate for the production of concrete, which increases the depletion of natural aggregate resources and makes the sustainability of the construction projects more challenging [11]. Due to the increasing expansion of building construction, natural aggregate supplies are rapidly decreasing, resulting in a shortage of resources. This shortage of resources requires proper utilization for sustainable growth [7]. The natural sand can be replaced by using by-products of coarse aggregate, such as copper mine waste rocks, as it reduces the manufacturing cost, reduces CO2 emissions from the industrial process of producing natural sand, and water consumption for sand washing would be minimized [12]. Manufactured sands are essentially a waste product from the production of coarse aggregate, which are generally available, have a cheaper cost, and would reduce natural sand mining [13]. In addition, manufactured sand had become a popular choice to be used to replace fine aggregate, as it is generally mined from stream beds, and harvesting

sand is thought to be environmentally detrimental [14]. In addition, manufactured sand which is made from hard granite rocks can be produced locally by lowering the expense of shipping from a distant river sand bank, and it is dust-free, which is readily regulated to suit the needed grading for the construction [15]. Furthermore, the application of lightweight aggregate in concrete has been developed in order to resolve the depletion of natural aggregate. Due to its advantages in decreasing load bearing and also improving performance in thermal insulation, the development of lightweight concrete by including lightweight aggregates has caught researchers' attention [16].
