Application Progress of Aerogel / Inorganic Cementitious Composites in Building Thermal Insulation

: In recent years, the problem of energy saving and consumption reduction in the construction industry has attracted more and more attention. The state has also put forward higher requirements on the energy saving, environmental protection and fire safety of building exterior wall insulation in terms of policies. Aerogel materials are considered to be the most promising thermal insulation materials in the future due to their excellent thermal insulation properties and fire resistance. In this paper, the preparation process and application of aerogel inorganic cementitious composites were investigated, mainly aerogel cement composites and aerogel gypsum composites. We analyzed the recent research results, and focused on the preparation optimization and application prospects of aerogel inorganic cementitious composites. In the preparation of aerogel inorganic cementitious composites, many researchers have proposed different preparation methods to optimize the interface between aerogel and inorganic cementitious materials in order to avoid the strength degradation caused by doping aerogel. At present, preparing aerogel into aerogel slurry for compounding is considered to be a relatively optimal incorporation method. It is found that aerogel inorganic cementitious composites have great prospects for thermal insulation applications in the construction field. However, due to the high cost of aerogel preparation and the poor interfacial bonding between aerogel and inorganic cementitious materials, the application of aerogel in building thermal insulation is limited. This paper has reference significance for studying the application of aerogel in the construction industry.


INTRODUCTION
In recent years, the global community has reached a consensus on the imperative of low-carbon development. In line with this, China has set forth an ambitious target of reaching the peak of carbon dioxide emissions by 2030 and achieving carbon neutrality by 2060, as announced in late 2020. Notably, building operations contribute to approximately one-fifth of the nation's total carbon emissions, with the proportion of energy-efficient buildings in China's existing stock not surpassing 90%. In light of the imperative to achieve sustainable development, foster an aesthetically pleasing environment, and realize the dual objectives of carbon peak and carbon neutrality, reducing building energy consumption becomes an indispensable endeavor [1] .
Of the various approaches to curbing building energy consumption, the utilization of insulation materials exhibiting exceptional thermal insulation properties assumes a pivotal role. In this regard, aerogel has garnered increasing attention across diverse industries due to its remarkable thermal insulation performance. Nevertheless, aerogels possess certain limitations such as high cost, poor strength, and inadequate interfacial bonding, thereby presenting considerable research potential for their application. Given that inorganic cementitious materials (e.g., cement, gypsum, lime) dominate the construction industry, numerous researchers have dedicated significant efforts to effectively combine aerogels with such materials while ensuring the thermal insulation performance of the resulting composite materials.

AEROGEL AND ITS PROPERTIES
Aerogel is a gel material with a dispersed medium of gas. It was first discovered in the early 1930s and has found widespread applications in industries such as aerospace and chemistry, primarily due to its high thermal insulation performance [2] . Depending on the different precursors, aerogels can be classified into three main categories: organic aerogels (resin-based aerogels, cellulose aerogels, polysaccharide aerogels, starch aerogels, etc.), inorganic aerogels (silica aerogels, carbon aerogels, metal oxide aerogels, etc.), and organic-inorganic hybrid aerogels (cellulose/SiO 2 composite aerogels) [3] . Among them, the preparation and application of silica aerogels are the most mature. Hydrophobic silica aerogels (SiO 2 aerogels, SA) are considered superinsulating materials, composed of mesoporous (2-50 nm) and cross-linked SiO 2 nanoparticles [4] . They exhibit ultra-low density (approximately 3 kgꞏm −3 ) and an extremely low thermal conductivity (approximately 0.02 W/m/K). Consequently, they have significant applications in building insulation, industrial pipelines, aerospace, and other fields, making them a hot topic in the research of insulation materials internationally. The utilization of aerogels in building insulation has demonstrated remarkable potential in reducing building energy consumption. With just 20 mm of aerogel, up to 90% of the heat loss from the interior to the exterior walls of a building can be minimized [5] .

Types of aerogel/cement composites and preparation process
Aerogel is primarily incorporated into cement as a lightweight aggregate, leading to the preparation of aerogel insulation mortar, aerogel lightweight concrete, and aerogel foam concrete, depending on the specific manufacturing processes and procedures. However, due to the low strength and density of aerogels, as well as their hydrophobic nature, achieving uniform dispersion with cement and other inorganic binder materials during mixing presents challenges. Issues such as scattering and floating on the surface may occur. Furthermore, as shown in Fig.1, at the microscopic level, there exists an interface transition zone of approximately 10-50 μm between the hydrophobic aerogel and the hydrophilic cement matrix [6,7] . Therefore, during the investigation of aerogel-cement composites, researchers have identified several issues. For instance, the aerogel particles tend to break during the mixing process, resulting in an uneven mixture. Additionally, the aerogel may not withstand sufficient load, thereby affecting the mechanical properties of the composite material. In order to address these challenges, researchers have proposed different solutions.
Some authors have employed traditional mixing methods, where the dry components, including cement, aerogel, aggregates (fine/coarse), and additives, are first mixed. Then, water and water-reducing agents are gradually added. However, this method can lead to the fragmentation of aerogel particles during mixing [7] . On the other hand, other scholars have adopted a method where cement, aggregates, additives, and water are mixed first, followed by the addition of aerogel particles. This approach ensures that the aerogel particles do not undergo excessive breakage during mixing and results in a homogeneous mixture [8] .
Furthermore, some researchers have proposed surface modification of aerogel using methanol, surfactants, silane coupling agents, and other substances to improve compatibility between aerogel and inorganic binder materials. For example, Kim et al. [9] attempted to "wet" the SiO2 aerogel by treating it with methanol, enhancing its compatibility with the cement matrix during the mixing process. The volatility of methanol ensures that it does not negatively affect the cement matrix or the aerogel itself during the subsequent curing process. Gao et al. [10] found that introducing water-reducing agents with an excess of surfactants as the main component can achieve good workability without the need for modifying hydrophobic aerogel.
Although treating aerogel can greatly improve its compatibility with cement, research indicates that the mechanical properties of aerogel-cement composites are still compromised due to the inability of aerogel to withstand sufficient load. To overcome this limitation and avoid compromising the strength of the aerogel-cement composites, some researchers, such as Yoon et al. [11] , have employed a technique involving the preparation of foam concrete followed by the introduction of sol through negative pressure. This method promotes gel formation by adjusting the pH value and ultimately produces aerogel foam concrete. This approach preserves the strength of the foam concrete while introducing aerogel to enhance its hydrophobic and thermal insulation properties. Hence, improving the compatibility between aerogel and cement and ensuring the strength of aerogel-cement composites are essential areas for future research.

Properties of Aerogel/Cement Composite Materials
In the preparation of aerogel-cement composites, researchers often focus on their thermal insulation and mechanical properties. As shown in Table 1, the introduction of aerogel can greatly enhance the thermal insulation performance of the material but may lead to a decrease in its strength. Zeng et al. [12] observed that when SA replaced 50 vol.% of cement, the compressive strength decreased from 26 MPa to 5 MPa, and the flexural strength decreased by approximately 80%. The thermal conductivity decreased from 0.36 W/m/K to 0.08 W/m/K, representing an 80% reduction. Studies investigating the use of SA as a substitute for fine aggregates in cement mortar also demonstrated a significant decrease in compressive and flexural strength. For example, when SA was substituted for sand in cement composite (CC) up to 80 vol.%, the compressive and flexural strengths decreased by up to 95%, and the thermal conductivity decreased by a maximum of 75% [13] . When SA completely replaced sand, the compressive strength loss was as high as 98%, the flexural strength loss was as high as 95%, and the thermal conductivity decreased by a maximum of 92%. Similarly, when SA was used as a substitute for lightweight aggregates in lightweight cement-based materials, the compressive strength and thermal conductivity of the cement-based materials also decreased significantly. Yiren et al. [14] used lightweight vitrified microspheres as fine aggregates and observed a reduction of approximately 35% in compressive and flexural strength when 45 vol.% of lightweight fine aggregates were replaced by SA, accompanied by a 40% reduction in thermal conductivity. Likewise, Adhikary et al. [15] replaced 60 vol.% of lightweight expanded glass aggregates with SA in lightweight cement-based materials, resulting in a reduction of approximately 60% in compressive strength, approximately 45% in flexural strength, and approximately 10% in thermal conductivity.
The aforementioned research results indicate that the introduction of aerogel in cement-based materials can significantly decrease their strength, with a maximum reduction of 98%. However, due to its extremely low thermal conductivity (~0.020 W/m/K), the inclusion of aerogel enhances the thermal insulation performance of cement-based materials.

Application of Aerogel/Cement Composite Materials in Building Insulation
Due to the extremely low thermal conductivity of aerogels, aerogel/cement composite materials can effectively reduce the energy demand of buildings. Cementitious materials based on aerogels can be used as insulation blocks, panels, and plastering materials. As shown in Fig. 2, Zhang et al. [16] prepared aerogel foamed concrete using aerogel powder as a raw material and compared its thermal insulation performance with expanded polystyrene (EPS) and ordinary concrete. The research results demonstrated that aerogel foamed concrete exhibited superior thermal insulation capabilities compared to ordinary concrete and EPS. The heat loss of aerogel foamed concrete was one-third of that of ordinary concrete, and it demonstrated better thermal insulation performance under solar radiation or extreme hot/cold weather conditions. Particularly for low-energy and zeroenergy buildings, aerogel/cement composite materials have significant potential for improving the thermal performance and energy efficiency of buildings. Researchers abroad [17] developed low-emissivity coatings coupled with aerogel-based plastering materials for application on wall surfaces to study their thermal insulation performance. The research results indicated energy-saving potentials of 7% to 13% for these coatings. In another experiment depicted in Fig. 3 [18] , aerogelcement was brushed onto external walls of different buildings. By measuring the relative temperature and humidity, it was found that the thermal loss of all insulated walls coated with the composite material was reduced by 80% to 90% compared to uninsulated walls. Moreover, due to the hydrophobic properties of aerogels, the walls did not accumulate moisture over extended periods.
Multiple experiments and studies have demonstrated the significant application potential of aerogel/cement composite materials in building energy-efficient insulation.

Types and Preparation Processes of Aerogel/Gypsum Composite Materials
Gypsum and its products possess excellent properties such as thermal insulation, sound insulation, moisture regulation, and fire resistance. Additionally, they are lightweight and easy to process, making gypsum an inorganic environmentally friendly building material. Scholars from both domestic and international backgrounds have conducted extensive research on gypsum-based insulation materials. Dong [1] prepared plastering gypsum using SiO 2 aerogel and expanded perlite as aggregates. The study investigated the effects of SiO 2 aerogel dosage and aggregate dosage on the mechanical properties and thermal conductivity of the plastering gypsum. An optimized proportion of SiO2 aerogel-modified plastering gypsum was obtained, addressing the issue of poor water resistance in conventional gypsum. Quan [19] utilized aerogel slurry and semihydrated gypsum as raw materials to develop a novel aerogel gypsum building material. As the surface of the aerogel slurry is hydrophilic, the need for surface treatment of the aerogel was avoided. The incorporation of aerogel significantly improved the hydrophobicity of gypsum (contact angle ~122°) and its thermal insulation ability (thermal conductivity ~0.10 W/m/K). As shown in Fig. 4, Wan et al. [20] dispersed aerogel in water to form a 15% aerogel slurry, which was then mixed with gypsum, cement, and fly ash. By varying the dosage of the aerogel slurry, the study explored the mechanism of how aerogel affects the properties of inorganic cementitious materials. The approach of mixing aerogel slurry with inorganic cementitious materials greatly improved the issues of floating and fragmentation during the mixing process of aerogel. In general, the preparation method of aerogel/gypsum composite materials is similar to the method of incorporating aerogel into cementitious materials.  The introduction of aerogel into gypsum materials results in similar effects to its incorporation in cementitious materials, namely a decrease in strength and thermal conductivity. As shown in Table 2, some researchers have conducted composite studies by using aerogel slurry with gypsum materials [19,20] . They found that with the addition of 40 wt.% aerogel, the compressive strength decreased by 86%, the flexural strength decreased by up to 95%, and the thermal conductivity decreased by 72%. D. Sanz-Pont et al. [21] discovered that the compressive strength of samples practically disappeared when doped with 57.1 vol.% aerogel. This is because gypsum itself has lower strength compared to cement, and the addition of aerogel results in a rapid decrease in strength, while achieving a minimum thermal conductivity of 0.028 W/m/K and demonstrating excellent thermal insulation performance. However, due to insufficient strength, it is not suitable for practical use. Other researchers have prepared aerogel/gypsum composite materials as coatings for internal and external wall insulation. Since the application of coatings does not require high strength from aerogel gypsum composites, as long as the aerogel coating exhibits excellent thermal insulation performance, it can meet the usage requirements. As the amount of aerogel increases, the thermal conductivity of plastering coatings can be reduced by up to 97%, reaching around 0.015 W/m/K, demonstrating excellent thermal insulation performance.

Application progress of aerogel / gypsum composites in building insulation
Recently, many researchers and engineering professionals have gradually applied aerogel/gypsum composite materials to thermal insulation in both interior and exterior walls of buildings. Some researchers have prefabricated aerogel gypsum boards, which are made of gypsum or concrete and have lower thermal conductivity compared to traditional low-density fiberboards, although they have lower flexibility. Many researchers have developed aerogel gypsum plaster through various methods. Koebel et al. [22] introduced a high-performance insulation plaster and rendering system based on aerogel developed by the Swiss Federal Laboratories for Materials Science and Technology (EMPA). As shown in Fig. 5, the gypsum building plaster contains over 80% silica aerogel particles and can be spray-applied on walls using conventional industrial machinery. This material has more than twice the thermal insulation performance of traditional gypsum plaster. Buratti et al. [23] developed an aerogel plaster using aerogel and insulating gypsum for the renovation and restoration of old buildings. They compared its thermal performance with the commonly used traditional plaster system in buildings, as shown in Fig. 6. The study found that aerogel plaster exhibits excellent thermal and acoustic properties, as well as water repellency and water vapor permeability advantages. It significantly reduces the heat transmission rate of different walls and improves sound insulation. It also eliminates gaps that could be formed by moisture, reducing mold growth caused by condensation within the walls. The spray application method allows for the use of aerogel plaster on complex wall geometries. Fig.  7 shows the thermal benefits of applying gypsum-based aerogel plaster to building renovation through in-situ infrared thermography analysis. A thickness of approximately 5 mm of aerogel-based gypsum plaster (λ= 0.05 W/m/K) was applied to the interior walls of a threestory apartment. From the graph, it can be observed that the temperature values for M1, M2, and M3 (3rd floor) are around 9°C, while the temperature values for M4 to M9 (1st and 2nd floors) range between 10.5°C and 11.5°C. The approximately 2°C decrease is attributed to the application of aerogel-based gypsum plaster. However, the high cost of aerogel limits its application in the construction industry, making the price of this product five times higher than traditional plaster. Currently, aerogel gypsum-based plaster is primarily used for restoration and wall decoration in historical buildings. Due to its ability to reduce moisture risks and significantly improve thermal insulation in historical buildings, as well as its ease of installation, construction, and reversibility, aerogel gypsum plaster is suitable for irregular walls, typical curved areas in historical buildings, mural walls, stucco, or other historical wall decorations [24] . However, the high cost of aerogel gypsum building materials restricts their application in other construction sectors.

CONCLUSION
This literature review confirms that aerogel inorganic gelled composites are a very promising product for improving the energy efficiency of existing buildings, as has been confirmed by relevant applications. Relevant research findings indicate an inverse relationship between the thermal insulation properties and mechanical strength of aerogel-reinforced inorganic cementitious materials. As the aerogel content increases, the mechanical strength experiences a significant decline. However, it is crucial to strike a balance between thermal insulation and mechanical properties in the final product. For instance, achieving a compressive strength of 4 MPa while maintaining a thermal conductivity of 0.05 W/m/K demonstrates exceptional thermal insulation performance that meets the strength requirements. Nonetheless, the current cost of aerogel-related products remains high, necessitating future endeavors to optimize the costeffectiveness of aerogel-based materials for wider market acceptance. Additionally, limited studies exist on the weather resistance of aerogels when exposed in real building environments. Therefore, to ensure the long-term performance of aerogel-reinforced building products, it is imperative to prioritize research in this area, with future efforts focused on addressing these aspects.