1. Introduction
The most commonly used sand in building mortar is natural river sand [
1]. River sand has good mechanical properties and low mud content, and it is easy to obtain [
2]. However, the overexploitation of natural river sand will lead to a series of environmental problems [
3,
4]. Therefore, the research on fine aggregate substitutes in building mortar is very diverse [
5]. Since mortar is made by mixing cementitious materials and fine aggregate, fine aggregate not only has excellent mechanical properties, but also has good compatibility with cementitious materials [
6].
Among the artificial fine aggregates, pottery sand is considered to be an energy-efficient and environmentally friendly building material [
7]. Pottery sand is a small-sized ceramsite with a particle size of 0.15~4.75 mm, which is calcined in a high temperature environment by a variety of silicate and aluminum metal salt minerals and foaming agents. It has the advantages of light weight, high strength, stable chemical properties, and the artificial regulation of physical and mechanical properties [
8]. In addition, pottery sand is widely used as water treatment filter material and petroleum fracturing proppant [
9].
The production mode of pottery sand has the characteristics of regionality and diversity. The use of low-cost local mineral or industrial solid waste results in using rotary kiln high-temperature firing [
10,
11]. For some solid wastes of clay-like mineral composition, firing them into pottery sand is not only environmentally friendly, but also economically beneficial [
12]. There are many solid wastes that can be used to burn ceramsite and pottery sand, such as coal gangue, heavy metal tailings, and municipal sludge [
8,
13]. Zhang et al. [
14] used a mixture of biochar and sewage sludge incineration ash as a lightweight building material added to concrete. Yu et al. [
15] made super sewage sludge light environmental ceramsite and analyzed the synthesis mechanism of it.
Among the abovementioned solid wastes, municipal sludge is one of the pollutions that every city and community has to face together [
16]. Therefore, the effective utilization of this kind of waste produced by the water supply and drainage plants to make sludge pottery sand (SPS) plays an important role in the water safety of urban residents and the prevention and control of local water and soil resource pollution [
17]. At present, the common process is to mix it with a foaming agent and binder, and at a high temperature of 1100~1400 °C, it presents a molten glass state and produces a large number of expansion pores [
18,
19]. After cooling, a stable sludge pottery sand can be obtained. Although it consumes a certain amount of fuel in the process of thermal processing, in fact, this turns the sludge from a harmful solid waste into a usable lightweight building material, and the heat consumed in the production process is not more than other types of artificial pottery sand. Therefore, SPS reduces the actual energy consumption from the perspective of saving waste treatment costs [
20].
As a new type of building material, alkali-activated slag mortar has a good phase with the SPS while effectively replacing ordinary Portland cement mortar. Chang et al. [
21] studied the potential volcanic ash activity of sludge. Therefore, in the process of the setting and hardening of alkali-activated slag paste, the substances in the SPS will not have an antagonistic effect on the hydration products, and they have a positive effect on the performance of the interface transition zone [
22]. The comprehensive performance of sludge ceramsite alkali-activated slag mortar (PSAM) composed of the SPS and alkali-activated slag paste was studied, which has a positive role in promoting the application of SPS as an environmentally friendly building material [
23,
24].
In the present study, Jiao et al. [
25] explored the effect of an alkaline activator on the performance of ceramic sand to alkali-activated slag paste. Xie et al. [
26] used a sludge lightweight aggregate and alkali-activated slag materials to partially replace sand and Portland cement to make non-bearing structural concrete with an application value; however, for the SPS, a fine aggregate with large pores inside, the performance of its porous structure under dynamic loading is worth exploring. As a kind of material with a short setting time and fast strength growth [
27], the static and dynamic mechanical properties of alkali-activated slag mortar using ordinary river sand have been widely studied [
28]; however, the research on the static and dynamic mechanical properties of PSAM is relatively lacking. In view of the large difference between the strength and apparent density of different kinds of ceramic sand, it is valuable to study the static and dynamic mechanical properties of PSAM for the subsequent modification and enhancement of the SPS aggregate.
This paper innovatively uses the SPS and slag to prepare low-density alkali-activated slag mortar, and quantitatively analyzes the static and dynamic mechanical properties and energy absorption characteristics of the lightweight aggregate mortar completely composed of pottery sand. The static strength of the SPS was quantified by a cylinder pressure test, and the relationship between the apparent density of the SPS and the static and dynamic strength of PSAM specimens was analyzed. The split Hopkinson pressure bar (SHPB) test system can collect and process the peak compressive strength and energy absorption characteristics of PSAM under the dynamic load, which can systematically reflect the dynamic mechanical properties and energy absorption characteristics of PSAM [
29]. The images collected by the ultra-high-speed camera can accurately determine the crack evolution process and specimen breakage of PSAM under the dynamic load [
30].
In the experiment, the size of the SPS was taken as the main variable, and the static and dynamic mechanical properties of PSAM with different aggregate size ratios were tested. The energy dissipation law and crushing morphology of the specimens in the dynamic compression experiment were analyzed [
31,
32]. The images collected by the electron microscope and the hardened pore-measuring instrument were comprehensively evaluated. The influence of the pore structure of the SPS on the strength and energy absorption characteristics of PSAM specimens was analyzed from the micro level, and the characteristics of the interface transition zone between the SPS and alkali-activated slag paste were analyzed.
3. Experimental Results and Discussion
3.1. Static Mechanical Properties
The static compressive strength of the PSAM test group and the reference group is shown in
Figure 4. The static compressive strength of the X
1Y
0 group is 33.74 MPa, which is the highest value in the experimental group, and the lowest value appears in the X
0Y
1 group, which is only 52.4% of the X
1Y
0 group. Therefore, the PSAM specimen with 0.15 mm~2.36 mm SPS has the best static compressive performance, which is consistent with the static compressive strength of the SPS. It can be seen from the data of the six reference groups that the static compressive strength of the specimens with coarse ceramsite instead of the SPS is 73.8% lower than that of X
1Y
0 group, indicating that large-sized ceramsite is the weak link of the overall structure in PSAM. Therefore, considering the static mechanical properties, the use of PSAM to prepare lightweight concrete should not use large-sized sludge ceramsite coarse aggregate. In
Figure 4, the aggregate ratio of the X group and Y group is regarded as the independent variable, and the static compressive strength (σ
s) is regarded as the dependent variable. The influence of the change of the aggregate volume ratio on the static compressive strength can be fitted by a quadratic equation. The fitting result and R2 value are given in the diagram, so the quadratic equation can be used to fit the data law. The reason for this data pattern is that the static compressive strength of PSAM decreases with the increase in SPS size, but the rate of strength reduction decreases with the increase in SPS size. The decrease in aggregate strength and the increase in the uneven mortar system are the main causes of this phenomenon.
Because the total volume is the same, the volume ratio of the SPS and coarse ceramsite of the X-type and Y-type in SSAM can be set as three coordinate axes, and a three-term data space can be obtained. The height in the space is the strength value. The non-linear surface fitting of the data points in the three-phase space of the SPS volume ratio and static compressive strength is carried out. The basic unit of the model is quadratic, and the fitting results are shown in
Figure 5. The main factors of the model are analyzed. The R
2 value is 0.989, approaching 1. The non-linear regression model can better predict the data trend of static compressive strength caused by aggregate size variables in the SPS.
Due to the difference in size, the traits of the X-type and Y-type SPS are not completely consistent. Although the gap of the SPS cylinder compressive strength data is one of the factors of the change of static compressive strength, the static strength ratio of X1Y0 and X0Y1 can not be simply explained by the difference in aggregate strength; otherwise, the fitting law of the above data would be approximately linear. The difference in pore structure is a potential factor in the influence of these two SPSs on strength, which can be analyzed from two aspects. One is the difference in pore size. In the case of a certain porosity of concrete, the larger the average size of all independent bubbles, the less the total number of bubbles, and the negative impact on the strength is obvious. The average pore size of the X-type SPS is much smaller than that of the Y-type SPS, and the static compressive strength of the SPS is lower than that of the alkali-activated slag paste. Therefore, the Y-type SPS is more likely to become a weak point in the compression test than the X-type SPS in PSAM. However, the macropores still have a positive effect, that is, the saturated SPS has a slow-release effect of water. Because the SPS can be well-dispersed in PSAM and the water storage is moderate, the SPS can play a good internal curing role in PSAM, reduce the internal reaction exothermic temperature peak in the middle and late stages of the alkali-activated slag hydration reaction, supplement in a timely manner the evaporated water, and reduce the generation of internal microcracks. Therefore, from the perspective of the internal curing effect, the Y-type SPS has a more positive effect on the hydration reaction of PSAM.
On the other hand, the performance of the interfacial transition zone between the SPS and paste is an important part of the overall strength of PSAM. The concentration of alkali metal ions and hydroxide ions in the two-component activator is higher, so it has higher ion osmotic pressure, which is also the most important way for the water stored in the SPS to be replaced. Therefore, in the early stage of hydration reaction, ITZ exists in the paste with low solid solubility and high fluidity. This will cause its static compressive strength to be slightly lower than that of the paste away from the SPS, but its high fluidity will also help it penetrate into the pores and cracks on the SPS surface and avoid bubbles adhering to the SPS surface. Since the X-type SPS has a finer particle size and a larger contact area with the paste at the same volume, although the total area of the ITZ increases, tight contact between the aggregate and the paste reduces this negative effect.
The cross-section of the PSAM specimen in the static compression test is shown in
Figure 6. It can be observed that although ITZ is the contact zone between the aggregate and paste, the breakage of the specimen must be induced by the failure of SPS strength, and ITZ does not have an obvious peeling phenomenon with SPS on the fracture surface. In the X group and the Y group, there is often a relatively thick skeleton structure inside, which makes it have better force transmission ability when subjected to load. The coarse ceramsite introduces a large-size cavity in the PSAM, which will lead to internal stress concentration when subjected to load, further accelerating the destruction of the specimen.
3.2. Dynamic Compressive Strength
The SHPB impact compression test is carried out on PSAM specimens, and the peak compressive strength can be collected as shown in
Figure 7. The dynamic compressive strength of the X
1Y
0 group is 27.89 MPa, and the lowest dynamic compressive strength of the X
0Y
1 group is 16.29 MPa. The dynamic compressive strength of PSAM specimens with the X-type SPS is 171.2% of that with the Y-type SPS, which is consistent with the data trend of the static compressive strength of the PSAM. The data of the six reference groups also show further decline, and the dynamic mechanical properties of coarse ceramsite in the alkali-activated slag paste are still the lowest. In addition, this negative effect is deeper than the static compressive strength. In
Figure 6, the aggregate ratio of X and Y is regarded as the independent variable, and the dynamic compressive strength (σ
d) is regarded as the dependent variable. The influence of the change of the aggregate volume ratio on the dynamic compressive strength can be fitted by the quadratic equation. The fitting result and R
2 value are given in the diagram; therefore, the quadratic fit is accurate in predicting data trends.
The volume ratio of the X-type and Y-type SPS and coarse ceramsite in SSAM is set as three coordinate axes, and the height in space is the dynamic compressive strength value. Thus, a three-phase space scatter diagram can be drawn, and non-linear surface fitting can be performed on the data points of the SPS volume ratio and dynamic compressive strength in the coordinate space. The basic unit of the model is a quadratic form, and the fitting results are shown in
Figure 8. The main factor significance analysis of the model reveals that the R
2 value is 0.982, approaching 1. The non-linear regression model can better predict the data trend of static compressive strength caused by aggregate size variables in the SPS.
3.3. Failure Morphology and Dynamic Absorption Energy
The dynamic impact test will cause the specimen to break and produce a large number of fragments. This process starts from axial compression and ends at the same time as the radial tensile failure and axial compression failure. The impact process of the PSAM specimen is collected and recorded in real-time by the ultra-high-speed camera. The failure process of the specimen is shown in
Figure 9.
It can be seen from
Figure 9 that the X
1Y
0 group with the highest dynamic compressive strength is cracked by axial cracks. After the first crack is born, multiple axial cracks are generated, and the crack width continues to expand. With the further compression of the incident bar on the specimen in the impact test system, the axial crack of the PSAM specimen will develop into a circumferential crack, and the specimen will be further broken until the end of the impact process. The impact process of the X
0Y
1 group with the lowest strength is relatively simple. When the impact load is applied to the specimen by the incident rod, the PSAM specimen undergoes brittle failure, resulting in penetrating the axial main cracks and some oblique fine cracks. Then, the specimen is completely destroyed, and the impact test is completed. These are the two extreme cases in the captured image, corresponding to the maximum and minimum values of the dynamic peak strength when the specimen is destroyed. In summary, the higher the degree of fragmentation of the specimen, the higher the dynamic compressive strength.
The cracks in the specimen during the impact process consume the energy that should be transmitted to the transmission rod. Therefore, the peak value of the impact energy absorbed by the specimen can be obtained by analyzing the waveforms collected by the transmission rod and the incident rod. The simplified three-wave method is used to calculate and count the peak absorption energy of the specimen during the dynamic load [
34]. The calculation results are shown in
Figure 10. The trend of the experimental data is positively correlated with the dynamic peak strength, which is also consistent with the diffusion of cracks during the impact process. The dynamic energy absorption of the six reference groups is also listed in
Figure 10, and their values are lower than the X
1Y
0 group. Therefore, the increase in SPS size will lead to a decrease in the dynamic energy absorption capacity of the specimen. The aggregate ratio X and Y is regarded as the independent variable, and the dynamic energy absorption (E) is regarded as the dependent variable. The influence of the change of the aggregate volume ratio on the dynamic energy absorption can be fitted by the quadratic equation. The fitting result and R
2 value are approaching 1, so the quadratic equation can be used to fit the data law.
Because the total volume is consistent, the volume ratio of the X-type SPS, Y-type SPS, and coarse ceramsite in PSAM can be set as three coordinate axes, and a three-term data space can be obtained. The height in the space is the dynamic energy absorption (E). The non-linear surface fitting of the data points in the three-phase space of SPS volume ratio and dynamic energy absorption is carried out. The basic unit of the model is quadratic, and the fitting results are shown in
Figure 11. The R
2 value is 0.981, so the non-linear regression model can better predict the data trend of dynamic energy absorption in the SPS.
6. Conclusions
In order to explore the feasibility of the SPS completely replacing ordinary river sand in alkali-activated slag mortar, static and dynamic uniaxial compressive experiments were carried out on PSAM with various fine aggregate volume ratios. The results show that the SPS with 0.15~2.36 mm has strong mechanical properties, and the PSAM completely using this SPS has the highest value in the transverse comparison of static and dynamic uniaxial compressive strength.
The static compressive strength of the Y-type SPS is lower than that of the X-type SPS. When it is mixed with alkali-activated slag mortar, the static compressive strength of the X1Y0 group is 386.4% of the X0Y1 group. The static compressive strength is negatively correlated with the size of the SPS, but they are not linearly correlated. This is because the Y-type aggregate has a strong water storage capacity and plays a positive role in the interfacial transition zone and hydration reaction.
The dynamic compressive strength and dynamic energy absorption peak of the experimental group show a similar change rule to the static compressive strength. The ultra-high-speed camera reveals the characteristics and sequence of crack formation when PSAM is damaged by the impact load. The specimens with higher strength include the PSAM specimens of the X1Y0 group. The dynamic compressive capacity and dynamic energy absorption capacity of the SPS with a larger size in the specimen are increasingly reduced, and the larger pores will lead to stress concentration, making the SPS’s interior a weak surface.
The pore morphology of the specimen further proves the influence of the pore structure inside the SPS on the strength of the PSAM specimen. The larger SPS is more likely to contain closed large pores, which introduce air into the PSAM and increase the air content of the specimen. The air content is closely related to the energy absorption characteristics and mechanical properties of the specimens. The air content of the X1Y0 group is 28.37% of the X0Y1 group. In the SEM image, it was found that the alkali-activated slag paste is filled in the outer pores of the SPS and produces hydration products, which indicates that the alkali-activated slag paste has excellent compatibility with the SPS fine aggregate.
In this paper, the mechanical properties and pore structure of PSAM were studied in detail, and there are still some shortcomings. In order to improve the application value of this kind of material, it is necessary to carry-out the research of the durability experiment in the follow-up work to explore the working performance of this kind of environmental-protection building material in a harsh environment. In addition, the improvement of the SPS is also the focus of improving the mechanical properties of mortar, and its pore structure should be further optimized in the manufacturing process.