1. Introduction
As a result of the process of global urbanization, more than 50% of the world’s population now live in cities [
1]. Cities are often built near water sources such as rivers, which bring drinking water and transportation to people. However, with the expansion of the city, the total impervious area of natural water sources gradually increases in the form of hardened ground, roof, lawn, etc. This will lead to the reduction of water source filterability and the change of sediment and solute export in the global river basin, which will seriously affect the natural environment [
2]. Furthermore, many urban diseases have emerged during urban expansion, for example: air pollution caused by the concentration of transportation and industry; eutrophication and change of hydrological system caused by point source pollution into waters; changes in urban biodiversity [
3]. Among such urban diseases, the typical ones are “urban pluvial flooding” caused by urban hydrological change and “heat island effect” caused by urban atmospheric change.
“Urban pluvial flooding” refers to the phenomenon of waterlogging disaster in the city caused by heavy precipitation or continuous precipitation exceeding the urban drainage capacity. There are pluvial flooding and surface runoffs in many cities around the world [
4,
5,
6,
7], and this reflects the hydrological and environmental impact of urban expansion that increase the runoff rate and flow, reduce the soil water that usually supports infiltration, reduce groundwater supply and base flow, and reduce evapotranspiration [
8].
The heat island effect refers to the phenomenon of urban microclimate change with the increase of urban heat waves. This phenomenon is due to man-made reasons, which change the local temperature, humidity, air convection and other factors of the urban surface and buildings. It results in reduced sensible heat convection and urban land evaporative cooling during the daytime, the use of energy and the heat released by solar energy stored in buildings make the city warm during the night [
9,
10,
11,
12].
Urban diseases affect the natural ecological environment, they also threaten the safety of people living in cities, and they cause immeasurable economic losses. It is urgent to solve these problems. Since the 1970s, lots of developed countries (e.g., USA, Germany, United Kingdom, Japan, Singapore and Australia) have put forward targeted programs to guide urban planning and construction, e.g., best management practices (BMP), low impact development (LID), green infrastructure (GI), sustainable urban drainage system (SUDS), and water sensitive urban design (WSUD), low impact urban design and development (LIUDD), active beautiful and clean waters program (ABC) [
13,
14,
15,
16,
17,
18,
19], and these plans combine municipal engineering, urban hydrology, environmental science, social science, etc., and are undertaken at the planning level to the specific construction level. In previous studies [
20,
21], it can be seen that in the process of urbanization, the hardened road surface gradually replaced the ecological road surface, which aggravated the urban pluvial flooding and heat island effects, and the impervious surface replaced the previous surface. An ecological pavement is capable of absorbing, accumulating and slow-releasing natural water, it can supplement soil water and groundwater, enhance convection and evaporation, and reduce urban heat waves. By contrast, the surface-hardened pavement is an aggregate of asphalt pavement, cement concrete pavement, etc. It usually does not have the functions of ecological pavement, so they have a negative correlation with the treatment of urban diseases. Urbanization is an irreversible process for a region. To solve the aforementioned urban diseases, urban roads must have both mechanical and ecological functions. According to this idea, researchers [
22] proposed a porous permeable pavement system (PPS), which typically includes porous concrete, permeable asphalt, clay permeable brick and cement-based permeable brick [
23,
24,
25,
26,
27,
28], and their corresponding scenarios are light load urban roadway and pedestrian footpath that can not only bear the load of an urban pavement, but also have the function of water permeability, drainage and water storage, which can solve the contradiction between hardened pavement and ecological pavement. This kind of material also plays a role of filtration in promoting hydrological recovery in urban environment. But it also has corresponding disadvantages, e.g., it has a large surface-exposed pores, it is easy to block, has a short life cycle, has a high cost in terms of use and maintenance, and has an unsightly appearance, etc. [
29], and these defects hinder the promotion and use of porous permeable materials.
The above phenomenon is often encountered in the use of PPS, and the problems behind it are that the composition of such materials, aggregates and binder materials, are limited to common materials, and performance design and material selection cannot be based on the required pore diameter, water permeability, strength, etc. Research has been undertaken to explore the relationship between pore size and permeability [
30], Because the Reynolds number of liquid flowing in this kind of porous media is generally less than 10, all of them are in laminar flow state, so the permeability of permeable materials can be calculated by using the Kozeny–Carman equation. In all of the permeable materials with laminar flow state, it is satisfied that with the increase of pore diameter, the effective porosity will increase, and the permeability will also be improved and, therefore, permeable concrete and permeable asphalt with large pore size have higher water permeability. However, the pore size of the exposed surface is too large, which leads to pore blocking, stone falling and other problems. The permeable pavement material with a pore size of millimeters or even smaller can overcome the aforementioned defects, and has a good consideration of water permeability, water storage, bearing, long-term performance, stability and other properties.
In order to reduce the pore size, fine aggregates can be used to prepare water-permeable materials. The fine aggregate permeable brick mainly includes cement-based permeable brick and sintered permeable brick [
31,
32,
33], researchers have prepared small pore size cement-based permeable bricks using aggregates with a particle size of 3–5 mm [
24], and sintered permeable bricks with small pore diameters were prepared from fine aggregates with a particle size of 0.85–2 mm [
31]. The sintering process of clay permeable brick has high energy consumption and is not environmentally friendly, there will also be secondary pollution even if waste slag and waste glass are used to make bricks. At the same time, quartz, which is the main component of sintered permeable bricks, will expand in volume during the sintering process, so a large number of microcracks will be produced, which exist in the crystal interior and the contact part between particles and glass phase. The existence of microcracks reduces the strength of the crystal under stress, while the brick has a large volume, and the accumulated deformation and cracks will also reduce the strength and service life of the brick. For cement-based permeable brick, it is difficult to balance the strength and content of cementitious materials, at the same time, the cement-based permeable brick has a long curing time due to the limitation of the cementitious material, which is not conducive to turnover and storage in industrialization. Therefore, materials that are environmentally friendly and have balanced strength and dosage need to be studied urgently.
To regulate the properties of permeable bricks, the constituent materials such as binder and aggregate must be studied. Digital image processing (DIP) is more convenient and accurate to obtain the particle group parameters of aggregate when the fine aggregate is used to prepare permeable brick. The particle group parameters of aggregate are the key to coupling the permeable rate and mechanical properties of permeable brick, such as the median diameter, distribution, roundness, etc. [
34,
35], but there is no such research on permeable bricks with micron-sized pores. In the use of binder, the conventional thinking is to use inorganic cementitious materials such as cement. Undoubtedly, this kind of material is widely used and mature in large-pore permeable materials. However, in order to meet the performance requirements of small-pore permeable bricks, a new binder is required, which should have the properties of short curing time and small amount of admixture (small volume proportion) but can provide enough strength and strong pressure-bearing capacity. In fact, it is not difficult to think of resin binders with excellent bonding and pressure-bearing capabilities in commonly used binder materials.
The innovation of this study is as follows:
A kind of permeable brick was developed, which uses organic material as binder and fine sand with particle size in the range of 0.08–0.6 mm as aggregate. It can be compatible with the performance of hardened pavements and ecological pavements. Its performance in all aspects exceeds the national standard GB/T 25993-2010 [
36], and the parameters stipulated in national standard are shown in
Table 1. In order to distinguish it from cement-based permeable brick and sintered permeable brick, this kind of brick is named resin-based permeable brick (RBPB).
In this study, the performance of resin binder material was tested, and the particle group parameters of aggregate were obtained. On this basis, the parameters and the related performance of permeable materials were analyzed, and the preparation and aggregate optimization method of micron porous permeable brick were obtained. Scanning electron microscopy (SEM) was used to observe the micro pore characteristics, binder material characteristics and aggregate defects. It has reference value for the research and design of permeable pavement materials.