Textural properties of waste silica sludges
Industrial waste silica sludges, which their main chemical compositions are shown in Table S1, are the key raw materials for the IHC-s. As observed, the dominant chemical element of waste silica sludges is silicon (ca. 98.8%) which is originated from the manufacture of precipitated silica. Additionally, a small amount of sulfur elements (ca. 0.98%) observed in the waste silica sludges is attributed to the usage of sulfuric acid during the process. Some heavy metals such as calcium, alumina, iron, magnesium, copper and arsenic are found in the waste silica sludges. As shown in Fig. 2. the waste silica sludges have a broad XRD diffraction peak at 2θ = 22o, indicating the existence of amorphous silica [32]. As observed in Fig. 3a, the particle size of waste silica sludges is mostly located between 10 and 50 µm. The averaged particle size of waste silica sludges is calculated to be ca. 26.8 µm. The SEM image (Fig. 3b) of waste silica sludges show the formation of aggregated silica particles with amorphous structure. The porous structure of waste silica sludges (i.e., specific SBET and Vtotal) were determined by N2 adsorption at 77 K and the results are summarized in Table S2. The result shows that waste silica sludges have a SBET of 58.7 m2 g-1 and Vtotal of 0.64 cm3 g-1. In addition, waste silica sludges exhibit a type-IV isotherm (Fig. 3c) with pore size distribution of 10–100 nm (Fig. 3d). The presence of meso-macropore in the waste silica sludges is responsible for the moisture adsorption and desorption performance as discussed in the following section.
Physicochemical properties of IHC-s
As reported earlier [33], transformation of CaSO4·0.5H2O (bassanite) into CaSO4·2H2O (gypsum) via self-assembly process (as indicated in the Eq. (7)) and the formed gypsum can serve as the skeleton structure which provides mechanical properties such as hardness, compressive strength and porosity for the coating.
CaSO4·0.5H2O + nH2O → CaSO4·2H2O + (n-1.5) H2O (7)
As shown in Fig. 4a and b, the bassanite with the brick-like morphology is hydrated into gypsum with rod-like particles during casting process. The gypsum with rod-like particles is also observed for IHC-0 (without waste silica sludges), as can be seen in Fig. 4c. This indicates that the crystallization of gypsum can occur even in the existence of the acrylic resin, which also can be confirmed by XRD pattern (see Fig. 2). The characteristic peaks of gypsum are located at 11.6°, 20.7°, 23.3°, 26.6°, 29°, 31°, 33.4°, 35.9° and 40.6°, suggesting the formation of gypsum in the IHC-s. The waste silica sludges is an amorphous SiO2 with micro-sized particles which can suspend completely in the water. The dissolution of SiO2 (0.01–0.012% by weight in water at 25oC) produces monomeric form, i.e., Si(OH)4 and the solid phase [34]. The dispersed waste silica sludges in the solvent can be adhered by the resin and then consolidate between the cross-linking structure of rod-like particles of gypsum, as shown in Fig. 4d. The mesoporous and microporous structure can be developed by the dispersed waste silica sludges and gypsum. Therefore, the higher ratios of waste silica sludges can supply more mesopore for the IHC-s coatings. However, the excessive waste silica sludges (> 60 wt%) can make coating surface crack, as displayed in Fig. 5. In addition, the surface cracking also may be due to the insufficiency of gypsum, which make the skeleton weak and thus excessive agglomeration of waste silica sludges by resins. In this study, different ratios (0–58 wt%) of waste silica sludges were used and fabricated as IHC-s.
The porous structure of IHC-s and commercial coatings is investigated by N2 adsorption-desorption isotherms. All the samples exhibit Type-IV isotherms due to the presence of mesoporous structure, as can be seen in Fig. 6a. Also, the Type-H3 hysteresis loops can be observed for IHC-s with different amounts of waste silica sludges because slit-shaped pores are formed in the presence of gypsum and waste silica sludges particles. Accordingly, specific SBET and the Vtotal of IHC-s with different ratios of waste silica sludges and commercial coatings are summarized in Table 1. As a result, the SBET and Vtotal of coatings are increased as the ratios of waste silica sludges are increased. The SBET values f IHC-s (s = 38, 48 and 58 wt%) are measured to be 10.0, 13.1 and 13.5 m2 g-1, respectively, which are larger than original IHC-0 (3.9 m2 g-1) and commercial coatings (2.0 m2 g-1). The Vtotal values are also increased as amounts of waste silica sludges are increased (from 0.049 to 0.206 cm3 g-1). The above result suggests that the addition of waste silica sludges can increase the Vtotal and SBET. It should be noted that the total volume of commercial coatings is greater than those of IHC-s. However, pore volumes of commercial coatings are mostly attributed to the contribution of macropore (> 50 nm). Moreover, the mesopores (i.e., 2–50 nm) in the IHC-s are the most effective pores for moisture adsorption. Therefore, the presence of waste silica sludges can increase the volumes of mesopores, as evidenced in the pore size distributions of IHC-s (see Fig. 6b).
Table 1
Porous properties of IHC-s samples
Samples
|
IHC-0
|
IHC-38
|
IHC-48
|
IHC-58
|
Commercial coatings
|
SBET (m2 g-1)
|
3.9
|
10.0
|
13.1
|
13.5
|
2.0
|
Vtotal (cm3 g-1)
|
0.049
|
0.147
|
0.193
|
0.206
|
0.505
|
Surface contact angles of IHC-s samples are shown in Fig. 7. The adsorption time relates to the porous properties of materials, i.e., the faster rate of water drop adsorption the higher porosity of samples. The pristine IHC-0 shows lower adsorption rate that the water drop cannot be adsorbed completely over 130 s. Upon adding waste silica sludges, the initial contact angles become smaller and the adsorption rates for water drop are increased. Therefore, the more ratios of waste silica sludges result in the increased pore volumes that promote the ability for liquid water adsorption.
Moisture adsorption-desorption capacity tests
The moisture buffering performance (in the range of 50–90% RH) of the IHC-s is shown in Table 2. The moisture buffering capacities and content values of IHC-0 are 216.9 g m-2 and 16.3%, respectively. The moisture buffering capacities of IHC-s gradually increase as the amounts of waste silica sludges increase, i.e., the performance of samples with waste silica sludges is better than that of IHC-0. As a result, the moisture buffering capacities of IHC-38, IHC-48 and IHC-58 can reach 270.3, 303.7 and 316.1 g m-2, respectively. Also, moisture contents of IHC-38, IHC-48 and IHC-58 are 23.6, 25.6 and 26.7%, respectively. The moisture buffering capacities and contents of IHC-s are remarkably superior to those of commercially available coatings (58.7 g m-2 and 11.4%), which are possibly due to the unique porous properties. Moisture buffering ability has the positive correlation with SBET and Vtotal of coatings.
Table 2
Moisture buffering capacities and contents of IHC-s and commercial coating
Sample
|
IHC-0
|
IHC-38
|
IHC-48
|
IHC-58
|
Commercial coatings
|
Wa (g m-2)
|
216.9
|
270.3
|
303.7
|
316.1
|
58.7
|
u (%)
|
16.3
|
23.6
|
25.6
|
26.7
|
11.4
|
Humidity buffering performance (in the range of 50–75% RH) of IHC-s and their corresponding results are shown in Fig. 8a. Except to IHC-0 coating, IHC-s (s = 38, 48 and 58%) have the higher moisture buffering capacities in the range of 50–75% RH, indicating the waste silica sludges possess the positive effect on moisture adsorption. In addition, the benchmarks of adsorbed capacities for HBMs issued by Japanese Industrial Standards (JIS, see Table S3) are 29 g m-2 (Level 1) and 50 g m-2 (Level 2) for the adsorption time of 12 h. The IHC-38, IHC-48 and IHC-58 samples possess moisture buffering capacities of 48.6, 48.4 and 46.1 g m-2, respectively for the adsorption time of 12 h, which reach the requirement of Level 1 and close to Level 2. Also, adsorption-desorption gradients of various coatings are shown in Fig. 8b. The IHC-48 coatings have the highest adsorption rate (20.1 g m-2 h-1) and the optimal desorption rate can be observed for IHC-38 (-17.3 g m-2 h-1). The results show that the adsorption and desorption rates of IHC-s (s = 38–58%) are higher than that of IHC-0 and commercial coatings. Hygroscopic curves of the IHC-s coatings are shown in Fig. 8c. The IHC-48 coatings start to adsorb moisture at the RH of 40%. It can be seen that no significant difference between IHC-48 and commercial coatings can be observed in the humidity range of < 50%. While relative humidity are higher than 75%, the adsorbed capacities are increased sharply upon the addition of waste silica sludges. Note that curves of adsorption and desorption are not overlapped due to the hysteresis effect. Durability of adsorption-desorption process for IHC-48 was performed by cyclic tests, as presented in Fig. 8d. The moisture buffering capacities of IHC-48 coatings keep stable for four cyclic runs (i.e., the entire run time = 96 h). In other words, the moisture can be adsorbed by IHC-48 spontaneously and then desorbed from pores entirely.
As shown in Table S4, leaching contents of heavy metals (Cu, Cr, Cd, Ni, Ba, Co and Pb) for IHC-48 were investigated by the TCLP tests. No detectable heavy metals are observed in leaching solutions of IHC-48 coatings. Moreover, the antibacterial property of IHC-48 is evaluated via the standard method from JIS (JIS Z 2801). As observed in Table S5, the bacteria concentration of 99 CFU mL-1 observed for IHC-48 after the contact time of 24 h is significantly lower 6.0 ⅹ 106 CFU mL-1 for blank. The R factor of antimicrobial activity is ca. 4.8 which is greater than the threshold value (i.e., 2) of Japanese Industrial Standards (JIS Z 2801). It is worthy to note that the IHC-48 recycled from industrial wastes possesses an excellent moisture buffering ability which can reach the benchmark of Japanese Industrial Standards and is also superior to the commercial coatings. More importantly, the simple and energy-saving sol-gel method to prepare IHC-48 coatings with an excellent antimicrobial efficacy (> 99.99%) and environmental friendliness (non-detectable heavy metal leaching) recycled from inorganic wastes under room temperature may be a promising candidate for practical applications in the indoor coatings.