1. Introduction
The indoor environment plays a crucial role in determining individuals’ physical and mental well-being, a relationship that is primarily affected by indoor environmental parameters [
1,
2,
3]. Relative humidity is one such parameter that significantly influences thermal comfort, building load, indoor air quality, and occupant work efficiency [
4,
5]. To create a comfortable indoor environment, it is pivotal to regulate relative humidity based on active or passive methods. The former involves using heat, ventilation, and air conditioning systems to either humidify or dehumidify indoor air, thereby keeping relative humidity at cozy levels for occupants. However, this method consumes energy and causes environmental pollution, undermining China’s strategy for renewable energy development. Reports indicate an average annual growth rate of 5.39% in China’s energy consumption during building operations from 2005 to 2018 [
6]. In contrast, the latter method utilizes renewable energy or materials with moisture absorption and desorption capabilities. This approach aims to achieve the same goal of enhancing comfort while reducing building energy consumption [
7,
8,
9]. This ecological approach holds great significance for improving indoor living space and building energy conservation.
As a passive energy-saving technology for regulating indoor humidity, humidity-control materials stand out among numerous passive technologies. The concept of humidity-control materials was initially proposed by Nishito and Miyano in Japan in the 1940s [
10]. It refers to the method of automatically regulating indoor air humidity without relying on energy-consuming equipment, based on the moisture absorption and desorption characteristics of the material. It is acknowledged as an environmentally benign passive control method. It can manage indoor relative humidity fluctuations and maintain indoor relative humidity within a comfortable range, thereby avoiding negative impacts on human health caused by high or low relative humidity environments [
11,
12].
In recent years, the utilization of humidity-control materials for regulating indoor relative humidity has yielded promising results [
13,
14,
15]. Zhang’s experimentation with humidity-control materials placed in an artificial climate chamber indicated their effectiveness in regulating indoor humidity levels [
16]. Ge’s inquiry focused on the moisture buffering performance of common humidity-control materials. The results revealed significant variations in the moisture absorption and desorption capabilities across similar materials [
17]. Simonson’s numerical simulation study demonstrated that incorporating humidity-control materials in indoor spaces led to a reduction in required ventilation rates. Importantly, this reduction was achieved without compromising comfort or air quality [
18]. Zhang’s application of numerical simulation techniques revealed the significant impact of humidity-control materials on building energy consumption in different climatic conditions. This study highlights their potential for up to 25% energy savings in temperate and semi-arid climates [
19].
Currently, various types of humidity-control materials have been developed and classified as biomass, organic, inorganic, and composite materials [
20]. Biomass materials have relatively large moisture capacity, but they exhibit smaller vapor permeability and slower moisture transfer rate [
21]. Organic materials can absorb moisture hundreds of times their own weight, but they have weaker desorption ability [
22]. Inorganic materials possess open porous structures and strong adsorption capabilities. Among them, gypsum is a widely used lightweight construction material known for its cost-effectiveness and eco-friendly production process. Its high porosity and uniform pore size distribution enable favorable permeability, making it suitable for humidity-control applications. Roel investigated the moisture absorption and desorption performance of gypsum boards coated with latex paint compared to bare gypsum boards. The findings indicated that applying latex paint on the surface of gypsum boards significantly diminishes their moisture buffering capacity [
23]. Zhang evaluated the humidity-control capabilities of magnesite board, diatomaceous earth, and gypsum board. The results revealed that they all possess varying degrees of humidity-control capabilities, with magnesite board performing the best, followed by diatomaceous earth, and gypsum board performing poorly [
24]. Shahrzad found that placing gypsum inside concrete walls under higher indoor ventilation rates can maintain indoor relative humidity at approximately 60% [
25]. Although gypsum possesses certain moisture control capabilities, it presents two limitations in terms of its moisture absorption and desorption performance. Firstly, within the relative humidity range of 40–70%, gypsum exhibits relatively low equilibrium moisture content, which fails to meet the demand for substantial moisture absorption within the standard humidity range. Secondly, gypsum displays a slow rate of moisture absorption and desorption, making it challenging to promptly respond to dynamic humidity changes.
To enhance the humidity-control performance of gypsum, numerous researchers have chosen gypsum as the matrix and incorporated other materials to develop gypsum-based composite materials [
26,
27]. Jiang employed sepiolite powder activated by calcium chloride as an additive and incorporated it into gypsum to prepare a composite material. The adsorption and desorption performance were tested, revealing that the adsorption and desorption capacity of the samples increased with the increase in the dosage of activated sepiolite powder. However, with a continuous increase in the content of activated sepiolite powder, wetting phenomena were observed on the sample surface. Consequently, the optimal additive content was found to be 20% of activated sepiolite powder [
28]. Shang introduced lithium chloride into gypsum and developed a novel material. The research indicated that this material exhibited good humidity absorption and desorption performance, with a maximum moisture absorption capacity of 0.410 g/g [
29]. Lee added activated clay into gypsum to fabricate a new gypsum-based composite material. The experimental results showed that the humidity absorption and desorption performance of the material improved with an increasing amount of clay addition, reaching its peak when the clay content reached 70% [
30]. Shang successfully developed a gypsum-based humidity-control material by mixing it with adsorbent materials such as plant fiber, kaolin, and activated carbon [
31].
In conclusion, gypsum is a traditional building material with humidity-control properties, but it has limitations in terms of moisture absorption and desorption performance. The current focus of research lies on composite humidity-control materials [
32,
33], but there has been no study on the preparation of a novel composite material by mixing gypsum with silica gel. Therefore, in previous studies, the author employed silica gel as a functional material to modify gypsum and prepare a gypsum–silica gel composite [
34]. Through experimental measurements, the composite material exhibited markedly enhanced moisture absorption and desorption capacity as well as rate compared to pure gypsum. To further enhance the humidity-control capabilities of this composite material, we developed a novel gypsum-based humidity-control material. This was achieved by adding sepiolite powder activated by calcium chloride at a mass ratio of 20%, based on existing research [
28,
35]. The humidity-control performance of the material was then evaluated through experiments and simulated tests. The study aims to provide a new idea for continuously seeking low-cost and practical humidity-control building materials suitable for the construction field.
4. Discussion
Through the analysis of experimental results, it was found that intermittent dehumidification and humidification have shown more beneficial long-term performance for the material compared to continuous dehumidification and humidification. Moreover, although high temperature can promote the desorption process of gypsum-based humidity-control material, its desorption capacity is still limited under continuously high-humidity conditions. In contrast to temperature, relative humidity has a more significant impact on the material’s humidity-control performance.
The simulation results also confirmed the findings. In climates characterized by sustained high temperature and humidity, such as Xiamen, outdoor high temperature increases the accumulated heat load on the material. Additionally, outdoor high humidity hinders the desorption process, resulting in suboptimal discharge of internal moisture. Conversely, in regions with greater fluctuations in temperature and relative humidity throughout the year, such as Beijing, Paris, and Atlanta, the material exhibits excellent energy-saving performance.
To better elucidate this finding, Xiamen was taken as an example to categorize the impact of gypsum-based humidity-control material on annual building energy consumption according to the seasons: spring, summer, autumn, and winter. The results are presented in
Figure 13. During spring, it can be observed that there is a large temperature and relative humidity difference between daytime and nighttime. The temperature is higher during the day, and the relative humidity is lower. Under this circumstance, the material can absorb the heat from the surrounding environment by releasing the moisture stored in the internal capillary pores, thereby effectively reducing the indoor temperature. Conversely, at night, the temperature drops, and the relative humidity increases, allowing the material to absorb moisture from the air that condenses inside the material and releases heat, leading to an increase in indoor temperature. Consequently, this material exhibits excellent energy-saving performance during spring. During summer, due to the continuous high temperature and humidity in Xiamen, the internal waste heat and moisture are not fully discharged and released. As a result, this limits the energy-saving capacity of the material. Similarly, during autumn, although outdoor temperature and humidity decrease relative to summer, they remain relatively high overall. This indicates that the material cannot efficiently discharge excess internal heat and moisture, thereby resulting in weak energy-saving performance. During winter, it exhibits similar characteristics to spring, with significant diurnal temperature variation and relative humidity. However, the average temperature and average relative humidity are the lowest throughout the year, with an average temperature of 17.43 °C and an average relative humidity of 58.3%. During the daytime, the temperature is relatively high, and the humidity is lower. This creates an environment where the material can release moisture from its pores, effectively reducing the indoor temperature and decreasing the cooling load on air conditioning. However, as the nighttime temperature drops, the remaining moisture may further lower the indoor temperature, increasing the heating load on air conditioning. Overall, the material still demonstrates good energy-saving effects in winter.
5. Conclusions
To improve the humidity-control performance of traditional building materials and augment their efficacy in regulating indoor relative humidity, this study incorporated sepiolite powder activated by calcium chloride into gypsum–silica gel humidity-control material to prepare a new type of gypsum-based humidity-control material. Through experimental and simulation studies, the following main conclusions were drawn:
(1) Gypsum-based humidity-control material exhibits the ability to absorb moisture in high-humidity environments and desorb moisture in low-humidity environments. However, the material’s humidity-control capacity decreases over time during continuous dehumidification or humidification. Therefore, intermittent dehumidification and humidification are more conducive to maintaining relative humidity stability for extended periods of time.
(2) The moisture absorption content of the gypsum-based humidity-control material is minimally influenced by changes in environmental temperatures, while its moisture desorption content is noticeably affected. On the other hand, variations in relative humidity impact both the moisture adsorption and desorption content of the material. The larger the discrepancy between environmental relative humidity and the internal moisture content of the material, the greater the effectiveness of the material’s humidity-control performance.
(3) Addition of calcium-chloride-activated sepiolite powder significantly enhances the adsorption and desorption abilities of the gypsum–silica gel humidity-control material. At a relative humidity of 97.4%, the equilibrium moisture content of the gypsum-based humidity-control material can reach a maximum of 0.225 g/g. It is 1.4 times higher than that of gypsum–silica gel material, and 4.5 times than that of pure gypsum materials.
(4) Gypsum-based humidity-control material can mitigate the influence of exterior and interior humidity loads on indoor relative humidity. The material can reduce fluctuations in indoor relative humidity and maintain it within a narrow range, providing a more stable indoor environment.
(5) Gypsum-based humidity-control material has the potential to reduce building energy consumption. Simulation results show that compared with regions with high temperatures and high humidity throughout the year, this material is more suitable for areas with large diurnal temperature differences and differences in relative humidity.
The uniqueness of this study lies in the successful preparation of a novel gypsum-based humidity-control material. It provides a new approach for low-cost and practical humidity-control building materials in the field of architecture. However, there are still several limitations to be considered. This study explores the practical application of gypsum-based humidity-control materials through simulation research methods. Further work requires long-term monitoring and analysis in actual architectural environments to evaluate their actual moisture regulation performance. Additionally, during the preparation process of gypsum-based composite materials, the silica gel and activated sepiolite powder as additives significantly enhance the moisture absorption and desorption properties of the materials. However, considering that silica gel itself belongs to a “strong absorption and weak desorption” material, it leads to relatively poor desorption performance of gypsum-based materials. The next focus of research will concentrate on exploring modification methods to improve the moisture desorption performance of the materials. Simultaneously, it aims to achieve cost reduction objectives and ensure meet energy-saving requirements in continuous high-temperature and high-humidity environments.