Gypsum-based Silica Gel Humidity-controlling Composite Materials: Preparation,Characterization,and Performance

LI Xi ,RAN Maoyu＊

(1.School of Architecture,Huaqiao University,Xiamen 361021,China;2.Xiamen Key Laboratory of Ecological Building Construction,Xiamen 361021,China)

Abstract: Gypsum was used as substrate,and silica gel was mixed into substrate at a certain mass ratio to prepare humidity-controlling composites;moreover,the moisture absorption and desorption properties of gypsum-based composites were compared with adding different silica gel particle size and proportion.The morphological characteristics,the isothermal equilibrium moisture content curve,moisture absorption and desorption rate,moisture absorption and desorption stability,and humidity-conditioning performance were tested and analyzed.The experimental results show that,compared with pure-gypsum,the surface structure of the gypsum-based composites is relatively loose,the quantity,density and aperture of the pores in the structure increase.The absorption and desorption capacity increase along with the increase of silica gel particle size and silica gel proportion.When 3 mm silica gel particle size is added with a mass ratio of 40%,the maximum equilibrium moisture content of humidity-controlling composites is 0.161 g/g at 98% relative humidity (RH),3.22 times that of pure-gypsum.The moisture absorption and desorption rates are increased,the equilibrium moisture absorption and desorption rates are 2.68 times and 1.61 times that of pure-gypsum at 58.5% RH,respectively.The gypsum-based composites have a good stability,which has better timely response to dynamic humidity changes and can effectively regulate indoor humidity under natural conditions.

Key words: humidity controlling;composite materials;gypsum;silica gel

1 Introduction

The building is the main place for people daily production,living and learning.More than 80% of human time is spent in the building interior.The quality of the building environment directly affects people's physical and mental health,and the health and comfort of the indoor environment are closely related to the air humidity[1].Relative humidity has a significant impact on thermal comfort,building load,and indoor air quality[2].Although heating,ventilation,and air conditioning systems can maintain relative humidity at a comfortable level,providing better comfort environment,it will cause lots of energy consumption.On the other hand,since the use of equipment is often accompanied by the increase of the air-tightness of the building,it is also easy to cause sick building syndrome[3].Humidity-controlling materials were first proposed by Japan's Nishito and Miyano in the 1952[4],refers to a method of automatically adjusting indoor air relative humidity by virtue of the material's own moisture absorption and desorption characteristics without relying on energy-consuming equipment.When the relative humidity of the indoor air increases,the humidity-controlling materials will absorb the moisture in the air and prevent the indoor relative humidity from rising;and when the relative humidity of the indoor air drops,the humidity-controlling materials will release moisture to inhibit the decline of the indoor relative humidity[5].From the point of view of building materials,using humidity-controlling materials to passively adjust the room air humidity can improve indoor comfort and reduce building energy consumption.

At present,people have developed a variety of humidity-controlling materials,which are divided into biomass class,organic class,inorganic class and composites[6].Biomass class has a relatively high moisture storage capacity,but the steam permeability coefficient is low[7].The absorption capacity of organic class can reach hundreds of times of its own mass,but the desorption capacity is weak[8].Inorganic class has an open porous structure and strong absorption capacity.Among them,gypsum is a widely used lightweight building material due to its low-cost and environmentally friendly production process.It has high porosity and homogeneous pore size distribution,which creates favorable conditions for its humidity controlling application.Zhang[9]evaluated the humidity controlling ability of glass magnesium board,diatom mud and gypsum board,and showed that all three have different degrees of humidity controlling ability,glass magnesium board is the best,diatom mud is second,and gypsum board is poor.Shahrzad[10]found that indoor relative humidity can be kept to 60% by placing gypsum inside concrete walls under conditions of high indoor ventilation rates.The above studies have shown that gypsum has a humidity controlling ability.However,in terms of the moisture absorption and desorption performance of gypsum,there are two deficiencies: one is that its equilibrium moisture content is relatively low under the relative humidity of 40%-70%,which is difficult to meet the requirements of a large amount of moisture absorption in the conventional required humidity range;the other is the moisture absorption and desorption rate of gypsum are slow,and it is difficult to meet the requirements of timely response to dynamic humidity changes.

For the purpose of improving the humidity controlling performance of gypsum,many scholars use gypsum as the substrate and add other materials to it to make gypsum-based composites.Jiang[11]prepared a humidity-controlling material by adding activated sepiolite powder to gypsum,and tested its absorption and desorption performance.It was found that the absorption and desorption capacity of sample increased with the increase of activated sepiolite powder dosage.Shang[12]added lithium chloride in gypsum to prepare a new type of humidity-controlling material.The research showed that it has a good moisture absorption and desorption property,and the maximum moisture absorption can reach 0.410 g/g.Recently,Lee[13]prepared a new gypsum-based composites by adding active clay.The test showed that its absorption and desorption properties increased with the increase of clay content,and the moisture absorption was the highest when the clay content was 70%.Shang[14]developed a gypsum-based humidity-controlling materials by mixing plant fibers,kaolin,activated carbon and other absorbent materials with gypsum.

In summary,the research on humidity-controlling composites has achieved certain academic achievements,but the continuous search for low-cost humidity-controlling building materials suitable for practical applications in the construction field is still a hot research issue.For gypsum,it has always been a highly competitive traditional building material,but it has the problem of poor moisture absorption and desorption properties.Silica gel is a common humidity-controlling material with a significant absorption performance.It is widely used as a desiccant due to its non-toxicity,high porosity,strong absorption capacity,and recyclability[15,16].There have been many studies using silica gel as an additive to prepare humidity-controlling composites.Fang[17]used silica sol as a dispersant and a binder for zeolite molecular sieves to develop composites.Zheng[18]prepared new humidity-controlling composites with silica gel as matrix and loaded with nanoparticles using sol mixing technology.Zhang[19]prepared silica gel and calcium chloride composites,while Jiang[20]impregnated silica gel with lithium chloride solution to make the composites.However,there is no research on mixing gypsum and silica gel to prepare a new type of humidity-controlling composites so far.

In this work,gypsum is used as the substrate,and silica gel with different particle sizes,is mixed into gypsum in different mass ratios to prepare gypsum-based silica gel humidity-controlling composites.The morphological characteristics,the isothermal equilibrium moisture content curve,moisture absorption and desorption rate,moisture absorption and desorption stability,and humidity-conditioning performance was tested and analyzed to reveal the influence of the particle size and proportion of silica gel on various properties.

2 Experimental

2.1 Materials

In these experiments,the gypsum material was purchased from Sinopharm Chemical Reagent Co.Ltd;the silica gel used to be from Henan Pubang Environmental Protection Materials Co.Ltd.After the gypsum and the silica gel were mixed,distilled water was added to the composites to make gypsum-based silica gel humidity-controlling composites.

2.2 Preparation of humidity-controlling composites

Table 1 Experimental scheme of preparing humidity-controlling composites

In the process of sample preparation,firstly,the silica gel was crushed with crusher (MT-10S,made in China),and the silica gel with different particle sizes of 0.2,0.5,1,2,and 3 mm were screened by sieve.Then,use an electronic balance (TP-213,accuracy: 0.01 g)to respectively weigh the materials.According to the material mass ratio in Table 1,and mixed them evenly in the beaker,and then added the required amount of distilled water to the beaker for full stirring.Finally,the mixed composite slurry was solidified in a prefabricated mold of 50 mm×50 mm×5 mm and cured for 24 h under the conditions of maintaining a temperature of 50±2 ℃ and a relative humidity of 40±5%.

2.3 Testing methods

The characteristics of the samples were determined by a variety of techniques.

Scanning electron microscopy (SEM): The morphological characteristics of each sample were analyzed using an SEM (Hitachi,Japan).

Isothermal moisture absorption/desorption curve:Following the Chinese standard GB/T 20312-2006[21],each sample was dried in an air drying oven (10HS,made in China),and the temperature was kept at 80 ℃.When the weight change was less than 0.1% every 24 h for three consecutive times,it was regarded as constant.The dried samples were put into a constant temperature-humidity chamber (LY-2150B,accuracy: +2℃;+3% RH,made in China),in which the temperature was maintained at 23 ℃.Moisture absorption studies were carried out at 32.9%,43.2%,53.5%,64.9%,75.4%,84.6%,and 97.4% RH,and moisture desorption studies were carried out at 97.4%,84.6%,75.4%,64.9%,53.5%,43.2%,and 32.9% RH.

Moisture absorption/desorption rate: To test the moisture absorption rate of the humidity-controlling composites,the dry sample was placed in a container at 58.5% RH to absorb moisture,monitor the weight of the sample in real time,and use the Origin software to fit the data to obtain the equation of the moisture absorption rate of the sample.To test the moisture desorption rate of the humidity-controlling composites,put the sample that has reached equilibrium moisture absorption at 97.4% RH,and put it into the 58.5% RH for desorption.The weight of the sample was monitored in real time,and the equation of the sample moisture desorption rate was obtained by fitting the data with Origin software.

Moisture absorption/desorption stability: To test the moisture absorption and desorption stability of the humidity-controlling composites,32.9% RH and 97.4% RH were selected.The dried sample was placed at 97.4% RH to absorb moisture.After the moisture absorption was balanced,it was placed under 32.9% RH for desorption.Dry the sample again to constant weight and repeat the above test.This cycle was repeated three times to ensure experimental accuracy.

Humidity-conditioning performance: Using the closed box thermal action method[22],prepare two stainless steel boxes of the same specification,the length,width and height of the box are 500 mm×500 mm×500 mm,which are marked as box A and box B.Then,the sample B4 and the sample C of the same size were placed in box A and box B respectively,and the sample size was 300 mm×300 mm×1 mm.The two boxes were arranged on the roof of the 4th floor of the ADEB(Architecture Discipline Experimental Building) in Xiamen Park of Huaqiao University,so that the outside of the two boxes can experience the same periodic thermal effect.Use an automatic temperature-humidity recorder (AZ8829,made in China) to automatically monitor the temperature and humidity of the air in the box every 1min.Through the relative humidity and air temperature changes in the box,the curve of the air moisture content and absolute humidity in the box with time can be obtained,and the humidity-conditioning performance of the gypsum-based silica gel humidity-controlling composites and pure gypsum under natural conditions can be compared.

3 Results and discussion

3.1 Morphology characterization

SEM images of the respective samples are shown in Fig.1.The SEM image of sample C at 40 times magnification shows that its surface is relatively dense.When the structure of sample C is magnified 800 times,it can be seen that the sample surface has multiple dense crystallites,forming open pores.Comparing the SEM images of sample C and group A shows that the gypsum structure became loose after adding silica gel,the quantity,density and aperture of pores in the structure increased at the same time.Further comparing sample A1 and sample A5,it can be seen that compared with the addition of silica gel with 0.2 mm particle size,the surface structure of the sample added with silica gel with 3 mm particle size is looser.This indicates that the larger the particle size of silica gel added,the more conducive to the loosening of the sample structure.Comparing sample B1 and sample B4,it can be seen that when the silica gel with the same particle size is added,the more silica gel is added,the more pores of the sample are increased,the porosity of the sample is higher,and the structure of the sample is looser,which provides space for the sample to absorb moisture.

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Fig.1 SEM images of humidity-controlling composites

3.2 Isothermal equilibrium moisture content curve

The isothermal equilibrium moisture content curve is one of the important indicators to measure the moisture absorption and desorption performance of the material.For a given material,the larger the slope of the isothermal equilibrium moisture content curve is,the higher its moisture storage capacity is.If the width of the lag ring between the moisture absorption and desorption curves is small enough,the material can stabilize the indoor relative humidity within a very narrow range.Fig.2 shows the isothermal equilibrium moisture content curves of different samples in group A.It can be seen from Fig.1 that the moisture absorption and desorption performance of A5 is obviously superior to other samples.Meanwhile,it shows that the moisture absorption and desorption capacity of the gypsum after adding silica gel is far greater than that of pure-gypsum.However,a reasonable amount to be added must be determined experimentally.As can be seen from Fig.3,with the increase of silica gel content,the moisture absorption and desorption performance of gypsum-based silica gel humidity-controlling composites are enhanced.When the content of silica gel is 40%,it has the best moisture absorption and desorption properties.At 97.4% RH,its equilibrium moisture content is 0.161 g/g,at 40%-60% RH,its absorption equilibrium moisture content is 0.072-0.105 g/g,and its desorption equilibrium moisture content is 0.138-0.150 g/g.The overall humidity controlling effect is ranked as follows: B4 >B3 >B2 >B1.In summary,the absorption and desorption capacity of gypsum is significantly improved after adding silica gel,and the humidity controlling performance of samples is closely related to silica gel.Silica gel can improve the structure of gypsum,while increasing the proportion of silica gel can increase the quantity,density and aperture of the pores,so as to improve the absorption and desorption capacity of gypsum-based silica gel humidity-controlling composites.

Fig.2 Isothermal equilibrium moisture content curve of group A and sample C

Fig.3 Isothermal equilibrium moisture content curve of group B and sample C

The ExpDec1 model in Origin software was used to fit the isothermal equilibrium moisture content curve of each sample,the formula is as follows:

Relevant parameters of the fitting curve are listed in Table 2.

Table 2 Parameters of sample isothermal equilibrium moisture content fitting curves

3.3 Moisture absorption/desorption rate

The moisture absorption and desorption rate of the material show the law of the amount of moisture absorption and desorption of materials changing with time,and it indicates the response ability of the material to the humidity change,which plays a decisive role in adjusting the indoor relative humidity[23].Fig.4 shows the absorption rates of sample B4 and sample C at 58.5% RH.It can be seen that the absorption rate of B4 is significantly better than that of sample C,the equilibrium absorption rate of B4 is 0.022g/h,and the equilibrium absorption rate of C is 0.0082 g/h.The equilibrium absorption rate of B4 was 2.68 times that of C.Fig.5 shows that under the condition of 58.5%RH,the equilibrium desorption rate of B4 and C is 0.014 and 0.0087 g/h,respectively.The desorption rate of B4 is faster,which is 1.61 times that of sample C.In summary,adding silica gel can not only improve the moisture absorption and desorption properties of gypsum-based silica gel humidity-controlling composites,but also increase its absorption and desorption rate.

Fig.4 Moisture absorption rate of sample B4 and sample C

Fig.5 Moisture desorption rate of sample B4 and sample C

Use the Origin software to fit the curve of moisture absorption and desorption rate of sample B4 under the condition of 58.5% RH[24].The fitting Eq.(2) is moisture absorption and Eq.(3) is moisture desorption.According to Eq.(2) and Eq.(3),formula (4) of the change law of moisture absorption and desorption rate can be obtained.

3.4 Moisture absorption/desorption stability

It is necessary to conduct experimental research on the attenuation of material cyclic moisture absorption and desorption to determine whether the material has stability.

Fig.6 and Fig.7 show the absorption and desorption cycle curve of sample B4.It can be seen that the absorption and desorption amount showed a certain but a limited change trend,with the absorption amount changed by 0.006 g/g,and the desorption amount changed by 0.002 g/g.According to the experimental data,the attenuation coefficient of the material can be obtained to determine whether the material has stable humidity-controlling ability.Eq.(5)[25]is used to calculate the decay coefficient:

Fig.6 Moisture absorption cycle curves of sample B4

Fig.7 Moisture desorption cycle curves of sample B4

where,Aiis the equilibrium absorption/desorption content of the initial materials andAi+nthe equilibrium absorption/desorption content of the th cycle.

Based on three cycles of moisture absorption and desorption,the result shows thatδabsorption=-0.012 andδdesorption=-0.005.The closer the attenuation coefficient is to 0,the better the moisture absorption and desorption stability of the material.At the same time,it can be seen that the material can be used repeatedly to show a continuous humidity-conditioning ability.

3.5 Humidity-conditioning performance

Based on the closed box thermal action method,the humidity-conditioning performance of sample B4 and sample C under natural conditions were tested.The author conducted a comparative experiment from December 18 to 20,2021.The changes of air temperature and relative humidity in the two boxes are shown in Fig.8.It can be seen from Fig.8 that the air temperature changes in the two boxes are similar,but the relative humidity changes are very different.The average relative humidity in box A was 48.9%,the maximum value was 58.4%,and the minimum value was 34.4%.The daily range changed 15.4% on the first day,9.3% on the second day,and the average was 12.4%.The average relative humidity in box B was 53.3%,the maximum is 77%,and the minimum is 22.3%.The daily range changed 48.6% on the first day,35.1% on the second day,and the average is 41.9%.The result shows that the relative humidity in box A is more stable than that in box B.However,the moisture absorption and desorption properties of the material cannot be seen from the change of relative humidity,so it must be described by moisture content and absolute humidity.Calculate the corresponding moisture content and absolute humidity using the air temperature and relative humidity in the test box,as shown in Fig.9.

Fig.8 Relative humidity and temperature in box A and box B

Fig.9 Moisture content and absolute humidity in box A and box B

As can be seen from Fig.9,humidity content and absolute humidity in box A have obvious changes,while in box B change relatively gently.The daily range can be obtained from the maximum and minimum values of air temperature and moisture content in the boxes,and the C value can be obtained by the ratio of the daily range of moisture content and the daily range of air temperature[22].The C value of box A is 0.407 on the first day and 0.345 on the second day.The C value of the B box is 0.155 on the first day and 0.136 on the second day.The higher the C value,the better the humidity-conditioning performance of the material.Therefore,it can be seen that the humidity-conditioning performance of sample B4 is better than that of sample C,and the timely response to dynamic humidity changes is also better.At the same time,it shows that the average relative humidity in both boxes decreases on the whole,and the change trend shows that the decline will decrease with the extension of the experimental period,and the average relative humidity will maintain a stable value lastly.The reason for this phenomenon is that after the material is placed in the box,it takes a certain time for the material and the wet air in the box to reach a new periodic heat and humidity balance under the action of periodic heat.This change is also reflected in the change of moisture content and absolute humidity.

4 Conclusions

In order to improve the moisture absorption and desorption properties of traditional building materials,so as to more effectively to adjust the indoor relative humidity,this study chose to modify gypsum to prepare gypsum-based silica gel humidity-controlling composites.According to the experimental results,the following conclusions are drawn:

a) After silica gel added to gypsum,the microstructure of gypsum changed.With the increase of silica gel particle size and proportion,the quantity,density and aperture of the pores in gypsum structure increased,providing space for the sample to absorb moisture.

b) The absorption and desorption capacity of gypsum-based silica gel humidity-controlling composites is significantly improved compared with pure-gypsum,and the absorption and desorption performance is closely related to silica gel.The absorption and desorption capacity increases with the increase of silica gel particle size and silica gel proportion.When adding 3 mm silica gel with a mass ratio of 40%,the material has a large moisture absorption and desorption capacity,and the maximum moisture absorption equilibrium moisture content is 0.161 g/g at 97.4% RH.

c) Under the condition of 58.5% RH,the equilibrium absorption rate of gypsum-based silica gel humidity-controlling composites is 2.68 times that of pure-gypsum,and the equilibrium desorption rate is 1.61 times that of pure-gypsum.

d) The moisture absorption and desorption cycle test shows that after three absorption and desorption cycles,the absorption amount changes by 0.006 g/g,the desorption amount changes by 0.002 g/g.The measured attenuation coefficients are all close to 0,indicating that gypsum-based silica gel humidity-controlling composites have a high degree of material stability.

e) Under natural conditions,based on the closed box thermal action method,the C value of gypsum-based silica gel humidity-controlling composites is higher than that of pure-gypsum,indicating that it has better timely response to dynamic humidity changes and is more conducive to regulating indoor humidity.

Conflict of interest

All authors declare that there are no competing interests.