Preparation of multi-functional sepiolite-based foam concrete and its adsorption and thermal insulation properties

Foam concrete is one of the most valuable porous materials used in civil and industrial buildings. Sepiolite, a fibrous hydrous magnesium silicate, possesses exceptional qualities such as a low thermal conductivity, non-polluting nature, and eco-friendliness. In this paper, a multi-functional sepiolite-based foam concrete has been fabricated using a physical foaming technology. The high sepiolite content and unique hierarchical structure endow the foam concrete with excellent adsorption and thermal insulation properties. The foam concrete exhibits a high adsorption capacity towards methanal and ammonia gases. Furthermore, the foam concrete, with its high porosity and good forming ability, shows great potential for application in thermal-insulation materials and wastewater treatment.


Introduction
Sepiolite is an abundant, natural and nontoxic phyllosilicate with a compositional formula of Mg 8 Si 12 O 30 (OH 2 ) 4 •8H 2 O [1,2].The structure of sepiolite contains extended silicon-oxygen sheets, making it a layered silicate mineral.These sheets are formed by tetrahedral SiO 4 groups, which give rise to the mineral's fibrous characteristics [3].As one of the non-swelling and lightweight porous clays, sepiolite possesses a large specific surface area and a fiber-like morphology.The high surface area and unique porous structure of sepiolite account for its impressive adsorption capacity and colloidal properties [4][5][6].Compared with the current mainstream adsorbents, sepiolite has the advantages of relatively low price and wide sources.Adsorption mechanism at sepiolite surface is strictly related to the dual nature of the surface charge [7][8][9][10].Sepiolite materials exhibit flexible surface properties including chemical and interfacial modification.The cation exchange sites at the edges of sepiolite offer a route for accommodating charged entities with specific functions [8].The silanol groups (Si-OH) located along the edges of the channels give rise to interact with a wide variety of ion, molecular or polymeric species.Therefore, sepiolite has demonstrated efficacy in the elimination of diverse cations and cationic dyes.The adsorption capacity of sepiolite, however, is not satisfactory [11].The lumpy nature of sepiolite makes it difficult to separate from the aqueous phases and hinders subsequent processes for recovery and reuse [12].Organic modification [13], pillared treatment [14], and acid activation [15] are some methods used to improve natural clay's adsorption performance.
In numerous studies, the utilization of clays in their unbound state presents a challenge, thus necessitating their incorporation into a support medium, such as a polymer, to facilitate ease of handling [16,17].Thus, sepiolite has been widely used to fabricate composites, which inherits the advantages of both sepiolite and matrix.D. Saraydın et al prepared a polyacrylamide/sepiolite hydrogel composite, which shows high removal efficiency towards monovalent cationic dyes [18].Tang et al designed a γ-Fe 2 O 3 /sepolite nanocomposite modified with 3-aminopropyltriethoxysilane by chemical coprecipitation method [19].The adsorption capacity of the composite is four times of that of raw sepiolite.Therefore, the design and preparation of sepiolite composites have emerged to be among the hottest topics in the study of clay materials.Based on the issues mentioned above, the present work attempted to fabricate sepiolite-based foam concretes by a physical foaming technology.The unique structure endows the foam concrete with high adsorption capacity and low thermal conductivity coefficient.

Materials and chemicals
Sepiolite powders were provided by Xiangtan Sepiolite Technology Co. (Xiangtan, China).Ordinary Portland cement (OPC, 32.5 N) was used to prepare foam concrete.Sodium dodecyl sulfate, dodecanol and anhydrous calcium chloride were purchased from Sinopharm Chemical Reagent Co. Ltd.

Preparation of foam concretes
In a typical procedure, a certain proportion of sepiolite, OPC powders, calcium chloride and water were mixed to prepare the cement mortar.The foam required for this experiment was obtained by physical foaming method.4.0 g sodium dodecyl sulfate (foaming agent) and 2.0 g dodecanol (foam stabilizer) were added into 1000 ml water under stirring to prefabricate foaming solution.100 ml of as-prepared solution was treated by an agitator with different stirring speed, from low to high.Then the foam with uniform size was added into the cement mortar with strong stirring.Finally, the mixture was poured into the molds.After 72 h of maintenance, the mold was removed and continued to be maintained under standard curing conditions for 7 days.

Characterization of the foam concretes
The mechanical properties of the foam concrete were characterized by microcomputer-controlled electronic universal testing machine (HD-B615-S).Each piece of data is the mean value of five experiments performed.The structural features of the foam concrete were studied by scanning electron microscopy (SEM, JSM-6610LV).The crystal structure of the samples was studied by x-ray diffraction (XRD) technology (Bruker D8 Advance).The adsorption capacities of the foam concrete towards methanal and ammonia were tested based on the JC/T1074-2008 and GT18582-2008 standards in China.In a typical procedure [20], the adsorption test was conducted in a transparent and airtight container.A gas-generating device consisting of 100 ml of methanal solution at a typical concentration was placed at the bottom of the container.To expedite the volatilization of methanal, a fan and heating plate were prearranged.The foam concrete specimen was then introduced into the container, which was subsequently sealed.The remaining concentration of methanal gas in the device was determined through a detection port.

Results and discussion
The mechanical performance is one of the crucial parameters of composites for their practical application.Therefore, the effects of the water-cement ratio, sepiolite content, and heat-treatment temperature of the sepiolite powders on the mechanical properties of the composites have been investigated.Firstly, the sepiolite powders were heat-treated at 100 °C, 200 °C, 300 °C and 400 °C.Previously reported studies have shown that sepiolite contains four different water molecules in its structure: hydroscopic water, zeolitic water, bound water (coordinated water) and structural water (hydroxyl groups).These water molecules can be removed by means of thermal treatments at different temperatures.As an example, when temperature comes to 220 °C, hydroscopic water from the outer surface and zeolitic water from the voids of the structure leave the mineral.However, excessive temperature leads to blockage of the channels and deformation of the crystalline structure, which further causes a decrease in the surface area of the sepiolite.The XRD patterns of the heat-treated sepiolite are presented in figure 1. Sepiolite powders have been employed to improve the physical properties of concrete previously.However, the effect of heat-treatment on the performance of the sepiolite did not cause attention.In this case, the intent of the introduction of the heat-treatment is to enhance the adsorption capability of sepiolitebased foam concrete.Moreover, the heat-treatment is in favor of improvement of the activation of the sepiolite.In the XRD patterns of the sepiolite, a series of diffraction peaks can be observed, which can be ascribed to the feature peaks of sepiolite.The increase in the heat-treated temperature did not bring out a significant change in the XRD patterns.Generally, the diffraction peak at 2θ = 7.26°corresponds to the (110) facet of sepiolite [21].In the range of 100 °C-400 °C, the crystal structure of the sepiolite can be maintained after the heat-treatment.The morphological features of the sepiolite treated at different temperatures are shown in figure 2. The fibrous structure of the sepiolite powders can be identified in all images.The sepiolite fibers are uniform in size and have an average diameter of 30 nm.Due to the complex chemical and crystalline composition, some nanoparticles can also be found in the sepiolite samples.
The effect of heat-treatment temperature on the mechanical properties of the foam concrete has been studied.The concrete mix design was listed in table 1.The compressive strength of the foam concrete is shown in figure 3(a).With increasing heat-treatment temperature, the compressive strength of the foam concrete gradually increases except for the case of 400 °C.Purification of the sepiolite can improve the mechanical properties of the sepiolite fibers, however, high treatment temperature would destroy the channel structures of sepiolite [22].Thus, the 7-days compressive strength of the foam concrete firstly increased from 0.55 MPa to 0.68 MPa, and then decreased to 0.57 MPa.Besides, the effect of sepiolite content on the mechanical properties of the foam concrete has been studied.The mix design of the foam concrete was listed in table 2.
With the ratio of sepiolite/cement was fixed at 2:3, the compressive strength reaches the maximum value.Besides, the effect of the water/cement ratio on the mechanical properties of the foam concrete has been investigated.The optimized water/cement ratio was set at 1:1.Due to the high water adsorption capacity of the sepiolite, the water/cement ratio of the foam concrete is much higher than that of the common concrete.All the results indicate that the foam concrete shows relatively low compressive strength (<1.0 MPa).Thus, the use of this kind of foam concrete alone is limited to non-load-bearing sites.The digital image of the foam concrete with sepiolite content at 40% is shown in figure 3(b).Plenty of small pores can be found on the surface of the foam concrete.The pores are homogeneously distributed.The inner structure of the foam concrete has been investigated by SEM analysis as shown in figures 3(c)-(e).The results indicate that the foam concrete possesses numerous internal pores and the diameter is in the range of 10-200 μm.The enlarged SEM image shows that a large number of pores with much smaller diameters are distributed in the wall of the micropores.The nanosized porous structure in the wall can greatly improve gas diffusion.Besides, sepiolite fibers can be observed in the wall.Adequate exposure of the sepiolite fibers is essential to the adsorption function of the foam concrete.
With the mass ratio of sepiolite/cement fixed at 1:1, the adsorption capacities towards methanal and ammonia are presented in figure 4. Short equilibrium time often reflects the nature of physical adsorption, whereas longer equilibrium times suggest either chemical adsorption or difficulties in accessing the micro pores [23].When compared to sepiolite powders, it takes a considerable amount of time for foam concrete to achieve adsorption equilibrium.This suggests that the adsorbed gas must traverse a longer path to access the pores.The underlying cause is the presence of sepiolite particles encased within cement, thereby lengthening the gas diffusion distance.The adsorption capacity increases gradually and then reaches the maximum value as the adsorption time increased.In the case of ammonia, the foam concrete exhibits similar adsorption behavior.After 25 h, the adsorption capacities of methanal and ammonia are 10.77 mg g −1 and 2.28 mg g −1 , respectively.The ammonia-adsorption capacity of the foam concrete was lower than in the case of methanal.There are attractive electrostatic and van der Waals forces between ammonia molecules, which make it easier for gas molecules to come into contact, collide, and combine together.The size of the NH 3 molecule aggregates is relatively large, which is the reason for the low adsorption capacity of sepiolite for ammonia gas.
It indicates that the sepiolite exhibits good selectivity towards methanal gas, which favors in-door air purification.The effect of sepiolite content on the thermal conductivity of the foam concrete was studied as shown in figure 5(a).The sepiolite has been employed to improve the thermal stability and thermal insulation performance of the composites [24,25].In this work, the coefficient of heat conductivity of the specimens was  measured three times.From the figure, the sepiolite-based foam concrete with a sepiolite content of 40% shows the lowest thermal conductivity coefficient of 0.072 W mK −1 .The foam concretes with 0% and 40% of sepiolite content were placed on the surface of a heating console where the temperature reaches 100 °C.The infrared thermal images of the foam concretes were measured as shown in figures 5(b) and (c).Due to the unique structure of the foam concretes, the thermal conduction in the foam concrete has been reduced significantly.
Adding sepiolite to concrete can effectively reduce the thermal conductivity of concrete.However, sepiolite also  has strong water absorption.When too much sepiolite is added to the concrete, excessive water will be consumed during the hydration process, resulting in the interconnection of pores of foam concrete and the increase of thermal conductivity.

Conclusions
In summary, sepiolite-based foam concrete has been successfully produced through a straightforward physical foaming technique.An SEM analysis reveals that the sepiolite fibers are evenly distributed within the foam concrete.It's worth noting that the sepiolite content significantly impacts the strength of the foam concrete, even though its overall strength remains relatively low.When the sepiolite content reaches up to 50%, these foam concretes demonstrate remarkable potential for indoor contaminant purification.Notably, they exhibit  adsorption capacities of 10.77 mg/g for methanal and 2.28 mg/g for ammonia.Furthermore, incorporating a specific quantity of sepiolite significantly enhances the thermal insulation properties of the foam concrete.Therefore, this type of sepiolite-based foam concrete can serve as a multifunctional material in the building and construction industry.

Figure 5 .
Figure 5. (a) Thermal conductivity of the foam concrete with different sepiolite content; (b), (c) infrared thermal images of the foam concretes.

Table 1 .
Concrete mix design of foam concrete with sepiolite heat-treated at different temperatures.

Table 2 .
Mix design of foam concrete with different mass ratio of cement/sepiolite.