## 1 Introduction  
Lightweight concrete (LWC) and lightweight cement composites (LWCC) have been widely regarded as promising materials for the building and construction industry for their superior advantages in engineering properties such as low thermal expansion, high fire resistance, and outstanding heat and sound insulation [1]. The decreased weight contributes to convenient fabrication, shipping, transportation, and installation, and leads to a significant reduction in material consumption, energy requirements, and construction costs [2, 3]. Based on unit weight, concrete materials can be classified into three categories: (a) heavyweight concrete, with a density of 3200 to 4000 kg/m\({}^{3}\); (b) normal concrete, with a unit weight from 2400 to 2600 kg/m\({}^{3}\); and (c) LWC, with a density less than 1920 kg/m\({}^{3}\)[4]. Traditionally, LWC and LWCC are produced via the addition of lightweight fillers as aggregates into the cement [4]. These fillers alter the mechanical properties and durability of the material, and their effects depend on the characteristics of the fillers as well as the bonding effect between aggregates and cement matrix [5]. Lightweight aggregates can be classified into (a) natural aggregates such as pumice, (b) organic aggregates like palm oil shells and rubber seeds, and (c) synthetic aggregates such as waste glass and expanded polystyrene beads [6, 7, 8, 9, 10]. Fly ash cenosphere (FAC) is also a widely incorporated artificial lightweight aggregate in the fabrication of LWC and LWCC due to its special physicochemical characteristics and outstanding engineering performance, such as low density and superior thermal insulation [11]. The properties of FAC rely on a variety of parameters such as the category of the coal, grinding approaches, combustion procedures, and withdrawal process [12]. As with fly ash, FAC is an industrial waste yielded from thermal power plants as a by-product of the combustion of coal. According to [PERSON] et al. [13], the production of concrete results in a tremendous consumption of aggregates, with a demand in the building sector of approximately 49% raw stones and sand, 25% wood, and 16% water. This contributes to great concerns about their future availability and the environmental effects of their manufacture. Under these circumstances, the introduction of recycled aggregates, especially the reuse of industrial wastes, is regarded as a solution to the raw material shortage and meanwhile can improve the utilization rate of materials by converting wastes into resources [14]. In addition, the fabrication of cement is associated with considerable environmental burdens, such as climate change and global warming, and requires intensive energy. Around 7% of global anthropogenic emissions of carbon dioxide, as well as 14% of the world's industrial energy consumption, are attributed to the production of cement [15]. Thus, the incorporation of FAC in the manufacture of lightweight building and construction materials is a sustainable movement for environmental conservation as well as the preservation of natural resources. The presence of FAC can remarkably reduce the density of cement-based materials, which is ascribed to the incorporation of a great number of small air voids, for FAC is difficult to pack densely due to its spherical shape [11]. However, along with reduced density, a decrease in compressive, flexural, and tensile strength is observed in lightweight cement materials with FAC, similar to other lightweight aggregates. Based on the analysis by [PERSON] et al. [11], a strong linear evolution was witnessed between the compressive strength of LWC and the density of FAC. Along with the growth of the percentage of FAC, the compressive strength decreased in a linear pattern. Efforts have been made in order to ameliorate these mechanical behaviors and develop high-strength LWCC, by introducing supplementary cementitious materials. In this regard, for instance, [PERSON] et al.