**1. Introduction**

In the last few decades, demand for energy consumption in the residential building has risen and there is a high intention for reducing energy consumption in the buildings. The energy efficiency of the buildings has become increasingly critical with the rising costs of energy as well as increasing awareness on global warming effects [1,2]. Furthermore, waste managemen<sup>t</sup> has become a critical issue. In fact, non-degradable wastes such as glass are unable to break down naturally which is developing environmental problems [3]. In Australia, about 1.1 Mt of glass waste was generated in the year of 2016–2017 from that 43% was stockpiled. In New South Wales, companies accepting the landfill levy to dump their glass waste in landfill or arranging to relocate the waste to other states where the landfill levy does not apply. Thus, use of waste glass in the large scale is a sustainable solution in terms of reduction of carbon footprint and saving the costs and energy. The construction industry is a potential sector for utilization of waste glass. In this regard, use of solid wastes for manufacturing the building materials with high thermal insulation properties is an effective approach toward sustainable

development and decreasing the energy consumption in buildings [4–6]. It has been reported that the incorporation of insolation materials in the building can reduce the indoor temperature fluctuation up to 4 ◦C that would save 10–30% of energy usage [1,2].

Although many investigations have been carried out on utilizing waste glass in the form of glass powder [7–11] and glass bead [12–15] in concrete products, it has not found its position in the construction industry yet. Expanded glass aggregates (EGA), is a new commercial product, which are manufactured from waste and post-consumer glass. The EGA possess a relatively smooth surface with numerous encapsulated pores can be used as an insulating material [16]. The porous structure and low thermal conductivity of EGA can e ffectively reduce the heat transfer rate. The utilization of EGA in cementitious materials brings two-fold advantages, first, reducing the landfill cost and environment and secondly can reduce the energy consumption in buildings [16]. Recently, some studies [17–21] have been conducted to investigate the e ffect of EGA on mechanical and thermal properties of concrete and cement mortar, however, the utilization of EGA as an insulating material is at the initial stage. The grea<sup>t</sup> advantage of EGA is the possibility of production in a variety of size. Such a variety of particle sizes allows the improvement of the homogeneity of mixture and consequently reduces the possibility of segregation of the mixture [22].

Yu et al. [23] and Spiesz et al. [24] developed a cement-based lightweight composite using five di fferent size of EGA (range between 0.1 and 2.0 mm) and reported the density of 1280–1490 kg/m<sup>3</sup> and compressive strength of 23.3–30.2 MPa. Rumsys et al. [21] prepared cement mortar with two types of fine expanded aggregates (expanded glass and expanded clay) to compare their compressive strength and durability properties. In the mixes, they replaced the fine aggregates with expanded glass and expanded clay by the weight of the sand (8.5, 16.7, 33.3, 66.7, and 100 wt%). The obtained results revealed that in the mixtures with 100% EGA, the density decreased about 37% and the compressive strength after 28 days of curing dropped about 60%. The results also confirmed that EGA could be applied in the cementitious composites without limitation related to the alkali-silica reaction. In the experiment conducted by Namsone et al. [25], a foamed matrix was prepared using EGA and the mechanical, thermal and frost resistance properties were examined. They obtained the compressive of 4.7 and 5.7 MPa at the age of 7 and 28 days and the thermal conductivity of 0.152–0.108 W/m.K. Moreover, it was observed that reference samples had lower values of weight loss (g/m2) after the freeze–thaw test comparing to compositions with EGA. They also characterized the microstructure of the prepared foam matrix using optical microscopy and observed that EGA were distributed uniformly over the cross-section without any processes of segregation.

Abd Elrahman et al. [16] fabricated EGA-cement mortar and reported crushing resistance of 1.9–2.9 <sup>N</sup>/mm<sup>2</sup> and water absorption of 13.6–15.8 wt% depending on the particle size. The results showed a compressive strength of about 6 MPa and thermal conductivity less than 0.14 W/m.K. In the study conducted by [26], the influence of the grain size and percentage of EGA content on physical and mechanical properties of the cement composite were investigated. They reported an average porosity of 45–67% and bulk density of 903–1078 kg/m<sup>3</sup> in specimens containing 100% EGA with the size of 2–4 mm. Moreover, the compressive strengths of 6.68–12.49 MPa obtained for EGA cement mortar. In another attempt, [27] investigated the possibility of using artificial neural networks to design the composition of cement composite containing EGA with the desired properties. They established the relation between the quantity of EGA and the porosity, bulk density, and compressive strength of a cement composite. Moreover, previous studies revealed that high glass content (above 50%) could considerably increase the water absorption of cementitious composites [28,29]. It can be concluded that incorporation of EGA in cement mortar can significantly reduce the mechanical properties such as compressive strength and water resistance of cement matrix. Hence, in order to compensate the reduction in mechanical strength and water absorption of cementitious composites integrated with EGA, nanofillers such as TiO2 can be used. Previous researches have demonstrated that the addition of TiO2 nanoparticles e ffectively enhanced the compressive strength and reduce the water absorption of cementitious composites [30–35]. Indeed, nTiO2 fills the nanovoids in concrete, which leads to the

increment of compressive strength up to 40% [34,36]. Moreover, TiO2 accelerates the formation of C-S-H gel by increasing the amount of crystalline Ca(OH)2 at the early age of hydration [32,37]. Ma et al. [32] reported 37% and 44% increase in tensile and flexural strength respectively for the samples containing TiO2. In addition, the results indicated that the addition of TiO2 could significantly refine the pores and shift them to the harmless pores. In the research conducted by Behfarnia et al. [31], it was observed that TiO2 nanoparticles decreased the permeability of the cement matrix. In the research conducted by Khushwaha et al. [34] and Sorathiya et al. [33], the e ffect of various proportion of TiO2 was studied. It was concluded that addition of TiO2 up to 1% could significantly enhance the mechanical properties of concrete.

This research aims to develop a cement mortar with a lower heat transfer rate and insulating properties using substitution of a natural aggregate (NA) with EGA. In this study, the e ffect of incorporation of EGA and TiO2 nanoparticles on workability, bulk density, water penetration, compressive strength, and heat transfer rate of the cement mortar were investigated. Infrared thermography (IRT) was used to measure the thermal insulation property of EGA cement mortar. The IRT technique has been utilized to evaluate the thermal energy storage performance of building materials in previous studies [38], however it has not been used for measuring the thermal insulating property of the EGA cement mortar. The conducted research is an additional step toward development of insulating building material and sustainable application of EGA in the construction industry.

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