The production of global solid waste is persistently accelerating with the advancement of industry and emerging technology applications, as well as the increase in the human population. In this regard, coal is considered as one of the most essential sources of energy throughout the world, the extraction and exploitation of which lead to the production of waste materials [
1,
2]. Coal is one of the most abundant resources used to produce energy. Coal production across the world is about 5.5 billion tons per year, and the volume produced in Iran reaches about 310 million tons per year. In general, there are three ways to dispose of such waste in nature: landfills, incineration, and recycling. The latter one has become a potential solution for the disposal management of such waste [
3]. In this respect, researchers have made great efforts in the field of waste recycling and its reuse in the civil engineering discipline [
4,
5,
6]. An efficient method for recycling solid waste is to use them in concrete; a practice that not only prevents the direct release of the solid wastes into the environment, but is also able to lower the consumption of quarried aggregates [
5,
7]. Over the past years, studies have been carried out on the recycling of various materials such as scrap tires, polyethylene terephthalate (PET), concrete, glass, etc. in various types of concrete. On the one hand, the use of coal waste as a partial replacement of aggregates in concrete reduces the depletion of natural resources and mitigates environmental hazards. On the other hand, coal waste can show similar properties to cement due to the presence of silica and alumina in its composition, and thus improve the mechanical and physical properties of concrete. Scholars have conducted extensive research on the use of coal waste as a volume replacement of natural aggregates in concrete. In a study, Cassiano et al. [
8] used coal waste as an alternative to fine aggregates in concrete. They observed that replacing 25 and 50% of fines with coal waste improved the mechanical properties of concrete after 28 days. Hesami et al. [
9] examined the effect of coal waste on the strength characteristics of roller concrete. The results indicated that replacing 5% of coal powder improved the strength properties of concrete for up to 90 days. However, adding 10 and 20% of coal powder reduced the compressive and tensile strengths of roller concrete at different ages. Karimaie et al. [
10] investigated the mechanical specifications of concrete with coal waste replacing aggregates. It was concluded that the effect of coal waste on concrete at 5% of volume addition improved the mechanical properties of concrete. The substitution of aggregates with coal waste increased the average compressive and flexural strengths by about 3–7 and 5–8%, respectively. In another study, Karimpour [
3] evaluated the effect of untreated coal waste as an alternative to coarse and fine aggregates on the mechanical properties of concrete. The experimental results showed that adding 5 vol.% of coal waste enhanced the mechanical properties of concrete, while a further increase of coal reduced such properties.
On account of the uncertainties present in the strength assessment of concrete specimens, as well as the localized damages induced by weathering, fire, and chemical attacks, civil engineers are increasingly demanding advanced and reliable methods for the evaluation and quality control of the concrete. In this respect, non-destructive test methods have attracted the attention of civil engineers due to the reduction in testing time, convenience of testing, and low cost compared to destructive approaches. Besides, these procedures have demonstrated promising results in inspecting and evaluating the quality of existing concrete structures [
11,
12,
13,
14,
15]. The ultrasonic pulse velocity (UPV) technique is a non-destructive test method of concrete based on the calculation of ultrasonic pulse transmission speed within concrete. It has been broadly implemented to estimate various specifications and integrity of concrete structures [
13,
14]. Basically, the characteristics of concrete, including strength, elastic modulus, porosity, depth of surface cracks, defects, and damages caused by chemical attacks and fire, can be evaluated using UPV [
5,
16]. The speed of the ultrasonic pulse is affected by many aspects, such as cement type and content, age of concrete, water-to-cement ratio, size and type of aggregates, curing method, temperature of the measuring medium, and the length of measuring distance [
15,
17,
18]. In recent years, several researchers have focused attention on the ultrasonic pulses in different fields of study. Washer et al. [
19], by examining the experimental results of longitudinal and shear pulse velocities of cylindrical and cubic concrete specimens, concluded that the velocity of the pulse is dependent on the presence of fibers, curing method, modulus of elasticity, and density of concrete. They also proposed that the propagation of ultrasonic pulses at high frequencies as high as 1 MHz or higher can be launched and received. It was also stated that increasing the test frequency slightly increases both the longitudinal and shear pulse velocities. Bogas et al. [
18] assessed the compressive strength of lightweight concrete using UPV. In their research, the pulse velocity of specimens with varying water-to-cement ratios, type and volume of aggregates, type of pozzolan, and different ages of concrete was investigated. They could provide pulse velocity ranges according to the type of variable, and then developed a relationship between compressive strength and UPV in the form of an exponential function. Thus, a simplified expression for estimating the compressive strength was recommended, regardless of the concrete type and composition. Despite the many studies on the estimation of concrete compressive strength using UPV [
18,
19,
20,
21,
22], there is little research in the literature on the relationship between compressive strength and UPV of concrete containing untreated coal waste. Therefore, this study aims to examine the performance of the UPV method in estimating the compressive strength and the quality of specimens containing untreated coal waste. Accordingly, the concrete specimens were made in 11 experimental groups, and the parameters of compressive strength and UPV at different ages, and with different percentages of coal waste substituting coarse and fine aggregates, were investigated. The UPV test was performed on specimens after 7, 14, and 28 days of fabrication.