The differential regulator was set on the upside of the syringe to fine-tune the syringe ejection water volume. The droplet stand was set on the differential lifting platform, the syringe was on the upside of the droplet stand, the diameter of the stand rod was 3 mm. The water samples were dropped from the syringe needle, which was very slow to the droplet stand. To increase the water droplet’s surface area, the water from the syringe should be added very slowly to the droplet on the stand. The larger the droplet’s surface area, the contact chance between droplet and dust sample can be reinforced. Figure 1A1 and Figure 2 show the stand, water droplet, and experiment apparatus set up. To maintain the water droplet staying in a good and stable shape, the stand was rounded into an arc-shaped suppression on the top.

In this experiment, the water droplet was set under a stationary condition and dust particles were under a motive condition. Dust particles were collected in a tiny tube, and it was pushed to the droplet without any initial velocity. Dust samples were sprayed onto the droplet by gravity (Figure 1A2), where H represents the distance between dust and droplet on the stand. The velocity of dust particles can be calculated according to the H value because of the free-fall motion of dust particles. It was easy to find out the microscopic typical characteristics of single dust and droplet collision at a specific speed.

A high-speed camera with a 12X microscopic lens and a point light source was used to record the shape of the droplet and the dust-droplet collision phenomenon. The droplet on the stand was in the middle between spot light and high-speed camera. The experiments were carried out under the condition of room temperature of 20°C and relative humidity of 60–80%.

Unlike the experiment in 2.3.2, in this experiment, the dust samples were spread on a flat glass plate to form a dust layer, water droplet was dropped into the dust particles layer. The dust particles were set under a stationary condition and water droplet is under a motive condition in this experiment. In Figure 1B, water samples were dropped from the syringe needle very slowly into the dust, to record the contact image between the moving droplet and static dust particles. In Figure 1C, water droplet was dropped into the dust from a water pipette, and a polarizing microscope was used to record the impacting results of the motive water droplet and stationary dust particles.

Particle size distribution obtained by using a laser particle size analyzer (LPSA) for the cement sample and BDD is shown in the square of a logarithmic graph, Figure 4. The BDD samples were collected from Figure 13A. The most abundant cement powder particles are around 25 μm, and BDD particles are around 90 μm. These values indicated that the average dust particle diameter of BDD was much larger than the cement powder. The larger size of BDD particles may be due to the detonation force on the hydrated and hardened formed cement blocks. In other words, after the cement is hydrated and hardened to form a cement block, and the blasting dust formed by the detonation force acting on the cement block, this may become the main cause of the characteristics difference between blasting dust and cement powder particles.

Table.1 shows the characteristics of two samples. The median size of D50 is 23.666 μm for the cement powder sample and 65.890 μm for the BDD sample, 90% cement powder diameter is less than 29.25 μm, and 90% BDD is less than 176.278 μm. Most BDD is a kind of falling dust. The diameter of the cement powder is smaller than that of BDD. The specific surface area S/V values of the cement powder sample are much larger than the BDD sample. A similar trend can be observed in the optics concentration values; this is because aerosols with smaller dust particle sizes will restrict the penetration ability of the laser beam in LPSA operation. These values indicate that there is an essential difference between BDD and cement powder. Obviously, in terms of dust suppression methods, it is not appropriate to simply transplant cement powder suppression methods into BDD. Cement powder is a kind of continuous source of pollution during the cement producing process, and the suppression method is continuous technology. However, the BDD is a kind of instant source of pollution, to suppress BDD, it is a key to create a large amount of mist in a very short time. “Explosion water mist” is created according to this requirement. Figure 3 to acquire the typical dust and droplet collision experiment results, the BDD was sieved by a 200-mesh (75 μm) screen for subsequent dust and droplet collision experiments because most of the BDD diameter is below 200 mesh.

The surface micromorphology of the cement sample and BDD are observed in the SEM images (Figure 5). The dust samples were amplified at 1,700 times by SEM, and the images are all in the same contrast scales (50 μm). Figure 5A shows that cement powder has a sharp edge and no crack structures and exists in each dust particle. Figure 5B shows that the BDD samples have a smooth edge, and many cracks existed in dust particles. The typical characteristics of BDD are the main causes of larger porosity than cement powder. The particle diameter information of the two samples has similar characteristics as Figure 4.

The structure of blasting demolition particles became looser than cement powder, as seen in Figure 5B. The cracks and fluffy construction was may be caused by the detonation force in the blasting operation. The micromorphology of cement powder is more stable than that of BDD. When the BDD are contacted with each other or impacted with other materials, they may disintegrate into small pieces because of the fluffy structure, which will increase the concentration of dust in the air.

Results of element determinations of cement powder samples and BDD samples are presented in Table 2. The abundances of Si in cement powder are two times higher than those in the BDD, the widely used cement in China is Portland cement, and the high content of Si is evident. In addition, this may be also evidenced that the dust of demolition blasting is not complete from cement particle dust. Element Ca is nine times and S is 12 times in the BDD than those in cement powder. The abundances of heavy metals, such as Mn, Ni, Zn, and As are also higher in the BDD. However, there is no obvious relationship between the contents of Na, Mg, Ti, and Fe determined by the XRF technique.

The mass percentage of elements Ca and S in the BDD is much higher than that of the cement powder, which may be due to the large number of gray bricks used in the construction of the demolition building, these bricks were produced from flue gas desulfurization (FGD) ash in nearby coal fired power plants (Telesca et al., 2013; Li and Jiang, 2021); in other words, the content of elements Ca and S in the FGD ash (Fu et al., 2018; Fu et al., 2019) is much higher than that of the cement used in the structural column, which leads to this result. Moreover, element Cl has the same characteristics, which is also because of the high concentration in FGD ash (Fu et al., 2018). These values of Na, Mg, Ti, and Fe are rather ambiguous, and it is hard to determine some relationships and draw appropriate conclusions due to the dust sample sources. The widely used coal fire power plants waste increases the migration risk of heavy metals during the blasting demolition process.

Wetting of BDD is a complicated process involving the interaction among solid, liquid, and gas (three phases). BDD has different porous structures (Figure 5B), which may influence the wetting ability of the dust. This experiment was carried out according to Figure 1B. Figure 6 shows the contact angle image of a municipal water droplet and BDD layer on a glass plate, which was a record by high-speed camera.

In Figure 6, the contact angle of cement powder is 89^o, while BDD is 84^o, and the hydrophilicity of cement powder and BDD is similar. Cement powder is a kind of hydrophobic particle because of the smaller particle diameter, and BDD’s hydrophobic characteristic may be due to the microporous and fissure structures on the particle surface, which increases the specific surface area of BDD. In addition, there is a layer of air film on the surface of dust particles and water mist, only when there is a high relative movement speed between dust particles and water droplets, do the water droplets break through this layer of air film to capture dust particles, the collision speed may become the dominated factor for dust suppression (Wang et al., 2015).

To compare the difference between pendant liquid droplet and droplet on the stand, municipal water was used to form the two kinds of droplets. The image was recorded by the high-speed camera, in Figure 7. To catch the biggest pendant liquid droplet, this image was a snapshot in the high-speed camera films during the pendant liquid droplet almost dropped to the stand. The volume and surface area of the droplet on the stand is much larger than the pendant liquid droplet. The droplet on the stand has much better stability as well. To investigate the collision phenomenon between dust and droplet, the droplet on the stand was chosen to fulfill the following experiments.

To find out the relationship between fluid–air surface tension and droplet shape, four kinds of typical fluids with different surface tension coefficients were chosen to form droplets on the stand. It includes 1) 15% NaCl solution, 2) municipal water, 3) surfactant solution, and 4) hydraulic oil HM46#. The surface tension of NaCl solution is 94 ± 27 mN/m, water is 74 ± 22 mN/m (Bahadur et al., 2007), and surfactant solution is 49 mN/m (Hajibagheri et al., 2018), and surface tension coefficient of different kinds oil is between 17 mN/m–34 mN/m (Sett et al., 2017). The shapes of these fluid droplets on the stand are shown in Figure 8.

The images in Figure 8 were snapshots in the high-speed camera films, during different fluid droplets almost overflow to the outside of the stand rod. The fluid droplet surface tension decreases from Figure 8A to Figure 8D, and the size, volume, and surface area of these droplets decreased from the biggest to the smallest. According to the concept of “surface tension,” the liquid surface produces a force that shrinks the surface as much as possible. The radius and radian of each droplet from 1) to 4) decreased accordingly; this is also the evidence of surface tension decreased from 1 to 4 in Figure 8.

Municipal water and 0.005% surfactant solution were chosen as the sample fluid for the further dust and fluid collision experiments. Because municipal water is widely used as the material to control BDD during the demolition process, Sodium dodecyl benzene sulfonate is widely used as a surfactant agent in many industrial fields to suppress dust pollution (Salehi et al., 2008; Shi et al., 2019).

Dust and droplet collision experiments were carried out according to Figure 1A2, dust particles were dropped to the droplet on the stand at the height of 1 cm, and the collision speed between a single BDD particle and droplet was 0.44 m/s. Ignoring the influence of dust particle size, air resistance, and electrostatic force, only the effect of gravity was considered on dust particles during each experiment. The influence of the surface tension coefficient and particle size was calculated by the criteria and the following results were found (Wang et al., 2015). As the surface tension coefficient increased from 0.034 N/m to 0.074 N/m, the rebound velocity range changed from 3.96– 4.67 m/s to 5.96–7.06 m/s. As the particle size increased from 2 μm to 20 μm, the rebound velocity range changed from 9.69 m/s– 11.79 m/s to 2.73 m/s– 3.78 m/s. The collision speed of BDD particle and droplet was 0.44 m/s, which was much less than the rebound velocity, the rebound phenomenon would not occur, and all the BDD particles would submerge into the droplets. However, the submergence time would differ according to the surface tension coefficient difference of the droplets.

Figure 9 shows the collision images of dust and municipal droplets on the stand, and Figure 10 shows the collision images of dust and surfactant solution on the stand, which can verify the calculation results of Wang et al. (2015).

When the BDD particle first collision with the droplet is recorded as the starting time point, which is 0.000 ms. In Figure 9, when the time was 0.600 ms, half of the particle body submerges into the municipal droplet. However, it took another 1.400 ms for the whole particle to submerge into the droplet completely. In Figure 10, when the time was 0.475 ms, most of the particle body submerges into the surfactant solution droplet. Moreover, it only took another 0.650 ms for the whole particle to submerge into the droplet completely. The submerge time of BDD and surfactant solution droplets collision was only half of the municipal water droplet. It is obvious that there is a significant positive correlation between the contacting time of dust particle droplet and the surface tension; this is also evidenced by the better efficiency of the surfactant in the process of dust particle suppression.

However, the collision and submergence of a dust particle with a droplet as a free-fall mode is only one of the collision types, many other collision phenomena cannot be recorded by this experiment. If the dust particles can be ejected from a different angle with different speeds to collide with the droplet, other contacting images would be recorded, such as oscillation and rebound (Lee and Kim, 2008; Wang et al., 2015). Due to the lack of precision dust aerosol generator instruments, further studies are recommended to identify the contacting angle and velocity affection on the collision results.

To compare the dust suppression efficiency of municipal water and surfactant, the agglomeration effect should be investigated (Yuan et al., 2021). These experiments were carried on according to Figure 1B,C.

Agglomeration effect of municipal water Figure 11A1–A5 and surfactant solution in Figure 11B1–B5 drop to the BDD layer. Figure 10A1 and Figure 11B1 show the original images right before the droplet contacted the dust.

Figure 11A2 and Figure 11B2 show the start time of the contact, in Figure 11A3, after 5 ms, municipal water droplets stir up a large number of dust particles on the dust layer, few dust particles adhered to the surface of the droplet. In Figure 11A4, after 10 ms, the wetting condition did not improve, and the dust particles scattered around the droplet irregularity. Finally, in Figure 11A5, the droplets stay in a stable shape, which indicates the hydrophobic characteristic of BDD. In Figure 11B3, no dust particles were stirred up by the surfactant solution droplet. Unlike the municipal water droplet in Figure 11A4, in Figure 11B4 the surfactant solution droplet became flat and dust participles are on the top of the flat surfactant solution, it adhered to the solution surface accordingly. Finally, in Figure 11B5, the dust particles were completely submerged in the surfactant solution. It indicates that surfactant solution with lower surface tension has stronger agglomeration ability than municipal water for blasting dust.

Figure 12 shows the BDD condensed by municipal water and surfactant solution. Figure 12A1 and Figure 12B1 show the initial dust distribution of the two experiments, which shows that the BDDs were scattered in a state of irregularity.

Figure 12A2 and Figure 12B2 show the condition of 20 s after municipal water and surfactant solution dropped into the dust layer. Figure 12A2 shows the effect of spraying municipal water over the BDD, which indicates that the municipal water sprayed over the BDD can hardly wet the dust. Due to the hydrophobic characteristic of BDD, water tends to flow away or evaporated instead of condensing to the BDD surface. However, in Figure 12B2, the effect of spraying surfactant solution over the BDD, clearly indicates that the BDD is completely wet by surfactant solution, being surrounded by lots of conglomerated liquid particles containing BDD. The BDD condensed together into clumps under the function of surfactant solution, the BDD can be immediately wet and captured by surfactant solution. Figure 12A3 and Figure 12B3 show the final agglomeration condition of BDD with municipal water and surfactant solution. After the BDD and municipal water droplet contact, the agglomeration of dust can be neglected. This result is consistent with the wetting result of BDD with municipal water in 3.1.4, which also indicates that BDD is hydrophobic particles to municipal water. Moreover, the agglomeration of dust in Figure 12B3 is obvious, and the density of the dust heap is larger. It indicates that the BDD hydrophilic improved with a surfactant solution.