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Article

Mechanical Properties, Durability and Leaching Toxicity of Cement-Stabilized Macadam Incorporating Reclaimed Clay Bricks as Fine Aggregate

by
Ermao Zhang
1,†,
Xirui Wang
2,†,
Wensheng Wang
3,* and
Haoyun Wang
3
1
Nanning Expressway Construction & Development Co., Ltd., Nanning 530029, China
2
Bridge Engineering Research Institute, Guangxi Transportation Science and Technology Group Co., Ltd., Nanning 530007, China
3
College of Transportation, Jilin University, Changchun 130025, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2022, 14(14), 8432; https://doi.org/10.3390/su14148432
Submission received: 15 May 2022 / Revised: 8 July 2022 / Accepted: 8 July 2022 / Published: 10 July 2022
Browse Figures
Versions Notes

Abstract

:
The utilization of reclaimed clay brick (RCB) from construction and demolition (C&D) waste is an extremely troublesome problem, which is beneficial and necessary for environmental protection and resource conservation. The objective of this study is to evaluate the mechanical properties, durability and environmental impact of cement-stabilized macadam (CSM) incorporating RCB. The physical and chemical properties of RCB were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) technologies. RCB exhibited a porous surface micro-morphology, high water absorption and pozzolanic activity. The higher RCB substitution ratio resulted in a lower unconfined compressive strength of CSM. Meanwhile, the higher the RCB substitution ratio was, the larger the 90 d indirect tensile strength of CSM at the late curing period. The RCB substitution ratio within 50% was beneficial for the freeze-thaw resistance of CSM. Additionally, RCB had a smaller aggregate size, causing a negative influence on the anti-scouring property of CSM. CSM incorporating RCB had an overall increasing accumulative water loss rate, and average coefficients of dry shrinkage and temperature shrinkage, except that 20% RCB substitution ratio resulted in an excellent dry shrinkage property. Based on the chemical analysis of EDTA-2Na, the pozzolanic RCB reacted mainly at later curing to form the crystal structure, enhancing the interfacial transition zone. Additionally, the leaching solutions could meet the identification requirements for extraction toxicity, surface water and groundwater referring to Chinese standards. Utilizing RCB in road engineering as the substitute for natural aggregate would be a promising step forward to sustainable development and green construction.

1. Introduction

The continuous progress of economic society is promoting rapid urbanization and industrialization, and meanwhile, the generation of construction and demolition (C&D) waste, e.g., clay brick, masonry, concrete, etc., is being accelerated by the construction of new and old urban areas and renovation of the urban-rural fringe [1,2]. The landfill or stacking disposal of C&D waste not only covers massive and rare land resources, but also pollutes the soil and water, resulting in an excessive waste of resources and ecological environmental burden [3,4,5,6]. Bricks, as a widely used construction material, are inevitably damaged during the reconstruction process, such as demolition and construction activities. In the past 50 years, approximately 20–30 billion clay bricks were produced in China, which would be eventually turned into a huge amount of C&D waste [7]. Therefore, the exploration of the utilization of reclaimed clay bricks (RCB) from C&D waste is an extremely troublesome problem of great significance.
Road engineering construction in China is still booming with goals of 47,000 km of expressways and 100,000 km of national highways by 2030 [8]. However, based on the consideration of environmental protection and resource conservation including carbon emissions reduction, the mining of natural aggregates has been banned in many areas of China. Given the imbalance between the supply and demand of aggregates as well as the increasingly prominent contradiction between natural aggregate consumption and C&D waste disposal, the feasibility of reusing RCB from C&D waste in road engineering construction as the substitute for natural aggregates has attracted a lot of attention, which will be a promising step forward to sustainable development and green construction. Several new scientific and engineering approaches have been proposed to explore new applications for composites [9,10,11,12]. It has been observed that it is feasible to reuse RCB as a partial substitute in cementitious construction materials [13,14,15]. Although it may slightly reduce the mechanical and durability properties, the experimental results showed that the addition of RCB can still meet the application requirements [16].
Semi-rigid base has been widely used for road engineering due to its higher strength, better loading distribution and excellent wholeness for decades, the main materials of which generally include cement-stabilized macadam (CSM), lime-fly ash stabilized macadam, cement-fly ash stabilized macadam and lime stabilized macadam, etc. [17,18]. The semi-rigid base is becoming the predominant type of base and subbase for highways and urban roads, especially in China [19]. CSM, as one of the main semi-rigid base materials, is a kind of composite construction material composed of proper aggregate gradation, 3–8% cement of aggregate by weight as well as optimum content of water [20,21]. The potential of CSM containing RCB is worth studying to supply an opportunity to use RCB from C&D waste, which is widely accepted as a cost-effective utilization means [22].
Some studies have attempted to explore the application of RCB as aggregate for cement-based material. The comparative study of Arulrajah et al. reported that about 25% RCB could be satisfactorily added to aggregates in the application of pavement subbase, and the RCB content has marginal effects on the mechanical properties and relatively obvious effects on dry density and moisture content of CSM [23]. Generally, there will be a reduction in the mechanical properties of cement-based materials by replacing coarse aggregate with RCB. The lower inherent strength of RCB may be considered as one of the major factors for the strength reduction in cement-based materials due to the main skeleton component of coarse aggregate [24]. Nevertheless, several studies have reported that the replacement rate of coarse aggregate by RCB could be up to 50% in cement-based material, which still meets the application requirements, and the RCB replacement rate within 20% would have no prominent negative influence on cement-based material [25,26,27].
Unlike coarse aggregate replacement, the possibility of applying RCB instead of fine aggregate in cement-based materials has been investigated, and the clear evidence from experimental observations fully supported the utilization of RCB and the benefits of RCB fine aggregate for the machinal and shrinkage properties [28]. It has been demonstrated that a lower drying shrinkage was measured for cement-based material with 20% RCB replacing fine aggregate, which may be due to the restraining effect of RCB and the internal curing effect [29]. In addition, the findings of this research confirmed that the incorporation of RCB with pozzolanic activity into cement-based material can inhibit alkali-silica reactions [30,31,32]. Consequently, the active pozzolanic materials in RCB aggregates would react with the hydration product (calcium hydroxide, Ca(OH)2), generating the hydrated calcium silicate (C-S-H) [33]. The benefits of RCB aggregate, therefore, may be driven by pozzolanic action provided of fine RCB aggregate. Through the above literature research, it can be found that the replacement of RCB coarse aggregate often leads to a large decline in mechanical properties, and the replacement of RCB fine aggregate has little risk on the performance of cement-based materials. Therefore, this study proposed the cement-stabilized macadam incorporating RCB as fine aggregate.

2. Goal and Scope

The primary goal of this study is to explore the feasibility analysis and environmental assessment of CSM containing RCB as partial fine aggregate through laboratory tests. In this investigation, RCB fine aggregate crushed by a jaw crusher was selected to analyze the physical and chemical properties by SEM, EDS and XRD technologies. The unconfined compressive strength, indirect tensile strength, freeze-thaw resistance, anti-scouring and dry shrinkage as well as temperature shrinkage were carried out to evaluate the mechanical and durability properties of CSM varying with RCB substitution ratio. And the strength mechanism of CSM was analyzed using a chemical detection method for further understanding. To evaluate the environmental impact of CSM, the leaching toxicity detection of heavy metal elements was performed. The whole experimental procedure is shown in Figure 1.

3. Experimental Program

3.1. Materials

In this study, the rapid hardening ordinary silicate cement (P.O 42.5R) conforming to Chinese standards (GB175-2007, JTG/T F20-2015) was used for CSM, of which the initial and final setting time are 202 min and 372 min, 3 d and 28 d flexural strength are 5.2 MPa and 8.5 MPa and 3 d and 28 d compressive strength are 25.0 MPa and 53.0 MPa, respectively. The locally available natural basalt aggregate (BA) with the apparent specific density of 2.735, water absorption of 1.24%, needle and flake content of 9.57% and crushed stone value of 21.63% was used as coarse aggregates. RCB collected from the construction waste in a shantytown demolition site near Changchun and natural BA were used as fine aggregates. The grain size distributions of aggregates are shown in Figure 2. The physical properties of BA and RCB fine aggregates were tested referring to the Chinese standard (JTG E42-2005) and the technical results are summarized in Table 1.

3.2. Mixture Design

The performances of cement-stabilized macadam are mainly influenced by cement, aggregate, gradation, water and other compositions, in which cement as a kind of cementitious material plays an important role in a denser gap structure in CSM. Considering the performance and economy of cement-stabilized macadam, the cement dosage should not be too low or too high, and 5% of cement dosage in mixtures was used in this study. On the other hand, the difference in aggregate gradation is also the main reason for the diversity of mechanical and shrinkage properties of CSM. Referring to the Chinese standard (JTG/T F20-2015), the aggregate gradation range and gradation curve of CSM are illustrated in our previous study [34]. Referring to the Class C heavy compaction test method T0804-1994 in Chinese standard (JTG E51-2009), the optimum water content (OWC) and maximum dry density (MDD) of CSM with different substitution ratios of RCB (abbreviated as CSM-BA, CSM-RCB20, CSM-RCB40, CSM-RCB50, CSM-RCB60 and CSM-RCB80) could be determined by quadratic polynomial fit method, which are summarized in Table 2. Compared to BA fine aggregate, RCB fine aggregate has higher water absorption and lower density than natural limestone aggregate. Therefore, there is a decreasing trend of MDD and an increasing trend of OWC with the RCB substitution ratio increasing in CSM.

3.3. Specimen Preparation

In this investigation, CSM specimens with 98% compactness incorporating various RCB substitution ratios were prepared by the static compaction in Chinese standard (JTG E51-2009). A series of cylindrical specimens with the dimension of Φ 150 mm × 150 mm and cuboid beam specimens with the dimension of 100 mm × 100 mm × 400 mm were fabricated. The mixture including cement, aggregates and water was poured into the cylindrical or cuboid beam mould three times, and then placed onto the compactor with a loading rate of 1 mm/min. The cylindrical CSM specimens were kept by steady pressure for 2 min and then de-moulded after 4–6 h, and the cuboid CSM beams were kept by steady pressure for 5 min and then de-moulded after 24 h. The specimens were covered with sealed plastic bags and stored in a standard curing condition with temperature-controlled at (20 ± 2) °C and relative humidity above 95%. CSM specimens were then stored till the day before the curing period (7 d, 28 d, 90 d, 180 d), and soaked in water with a temperature of (20 ± 2) °C and a water surface of 25 mm higher than specimens for one day.

3.4. Testing Procedure

As shown in Figure 1, several tests were performed to investigate the mechanical, durability and environmental behaviors of CSM specimens incorporating various RCB substitution ratios in the presented study. The experimental methodologies are described in the following.
The effect of the RCB substitution ratio on the mechanical properties of CSM specimens has been evaluated by an unconfined compressive test and indirect tensile test. In line with the standard methods of T0805-1994 and T0806-1994 in JTG E51-2009, unconfined compressive test (curing age of 7 d, 28 d, 90 d, 180 d) and indirect tensile test (curing age of 90 d) were carried out for cylindrical CSM specimens by using a hydraulic pressure universal testing machine with a measuring range of 2000 KN. The cylindrical CSM specimens were placed at the center of the vertical load and loaded at a constant rate of 1mm/min until the specimen was damaged, and the maximum load was recorded.
The durability behavior of the CSM specimen was determined based on shrinkage properties at the curing age of 7 d, freeze-thaw resistance (curing age of 28 d and 180 d), and an anti-scouring test (curing age of 28 d) following JTG E51-2009. Dry shrinkage and temperature properties of cuboid beam CSM specimens with different substitution ratios of RCB at the curing age of 7 d were performed. Parts of areas on the cuboid beam specimens were polished with sandpaper or cement mortar. Two dial indicators were fixed at both ends of the shrinkage instrument to contact the two ends of the cuboid beam CSM specimen, which were then put into a dry shrinkage chamber with a constant temperature of 20 ± 1 °C and relative humidity of 60% ± 5%, and dry shrinkage coefficient could be calculated to evaluate the volume shrinkage degree of specimens after a water loss. After the cuboid beam CSM specimens were in an oven at 105 °C till a constant weight, a temperature shrinkage test was performed on the CSM specimens pasted with foil-type resistance strain gauges through a programmable high and low-temperature alternating test chamber, and the temperature range selected was set as −20 °C–30 °C. The freeze-thaw cycles were set as 5 or 10 for the cylindrical specimens after curing for 28 d or 180 d, respectively, to evaluate the freeze-thaw resistance of the cylindrical CSM specimens. The anti-scouring test was performed by using a novel improved test method based on a vibration platform for concrete, in which water is 5 mm higher than the specimen’s upper surface and the test vibration lasted for 360 s.
The leaching test of CSM specimens was performed in line with the horizontal vibration extraction method of the Chinese standard (HJ557-2009). 100 g of fragmentized CSM specimens with a liquid-solid mass ratio of 10:1 was weighted and oscillated at a frequency of 110 times/min for 8 h and then left to stand for 16 h to prepare the leaching solution. Then, the toxicity metal elements in the filtered leaching solution were detected by an inductively coupled plasma mass spectrometer (ICP-MS) and compared with the Chinese standard (GB 5085.3-2007). Meanwhile, the environmental impact of CSM specimens dominated by water quality was conducted concerning Chinese standards regarding surface water and groundwater (GB 3838-2002 and GB/T 14848-2017).

4. Results and Discussion

4.1. Physical and Chemical Properties of RCB

The microstructure and element analyses of RCB were performed by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) technologies, which have the advantages of large magnification, depth of field, view and three-dimensional imaging. RCB specimens were scanned by SEM-EDS point by point with a focused narrow electron beam. The interaction between the electron beam and the RCB surface would produce physical signals such as characteristic X-ray, backscattered electron and secondary electron signals. The microstructure and element analyses were realized by projection imaging after collection, amplification and processing of physical signals, in which the morphological characterization mainly depends on secondary electron signals and chemical elements could be determined by characteristic X-ray. The microstructure and microscopic surface morphology of RCB aggregate are observed by SEM technology and presented in Figure 3a. SEM image shows that the RCB aggregate has irregular shapes with rough edges, and some micropores and agglomerations on its surface. It also can be found from EDS spectrum analysis in Figure 3b that RCB contains a lot of oxygen, silicon, aluminum, calcium, carbon, iron, magnesium, and other elements, although the EDS spectrum results in different detection areas fluctuate slightly. There are many oxide forms in RCB due to the high oxygen element content, which would provide some potential activity. Thus, the porous surface micro-morphology and high oxides are responsible for the higher water absorption and crushed value [35,36].
The mineralogical composition of RCB was analyzed by the X-ray diffraction (XRD) technique. The composition, internal atomic or molecular structure, and morphology of RCB can be obtained from the XRD pattern, which was formed by the XRD phenomenon of the transition of the inner electron of the atom under the bombardment of the high-speed moving electron. RCB powders of 10 g with particle size less than 0.015 mm after screening and drying were ground to powders with particle size less than 300 mesh for XRD scanning. Figure 4 shows the mineral compositions of RCB aggregate by the XRD pattern. There are some strong peaks representing the inorganic crystalline phase of quartz, a small number of weak peaks representing the crystalline phase of feldspar, and hematite in the XRD pattern, in which the quartz is the major crystalline phase. The mineral and element compositions of RCB aggregate shown in Figure 2 could provide the compositions that react with the hydration products. RCB has the potential for pozzolanic reaction and cementitious activity according to its physical and chemical properties [31,33,37].

4.2. Strength Mechanism Analysis Using EDTA-2Na

The cement used in this investigation is ordinary silicate cement, and its components are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium iron aluminate, etc., [34]. The components in cement exposed to water would produce a hydration reaction immediately inside the CSM specimen with different hydration rates accompanied by hydration heat, which would produce the hydration product (calcium hydroxide) with the prolonging of hydration time. In addition, the SEM-EDS results of RCB indicate that there is a higher content of calcium ions in RCB from the EDS spectrum shown in Figure 3. The divalent cation in the soaking solution comes from the above two sources of cement and RCB. The divalent cation from the hydration product could be considered the same for all CSM specimens due to the same cement dosage, while the divalent cation provided by RCB has a positive correlation with the RCB substitution ratio. In the meantime, the pozzolanic materials in RCB continuously react with the hydration product to produce the hydrated calcium silicate (C-S-H) insoluble in water, reducing the concentration of divalent cation. The ethylenediaminetetraacetic acid disodium (EDTA-2Na) is a chelating agent with six coordination atoms, which can combine with metal ions to produce complexes with a cyclic structure due to the chelation [38]. The EDTA titration method has been extensively applied as an effective means to check the cement content of CSM, which is a kind of chemical detection method to test the pozzolanic activity of RCB and excitation effects of cement on the pozzolanic activity. There would be interferences from divalent and trivalent cations, and a masking agent can be added to eliminate the impact. In line with T0809-2009 of Chinese standard (JTG E51-2009), the EDTA titration method was used for CSM specimens with different RCB substitution ratios, as shown in Figure 5a. The EDTA-2Na solution was added to capture divalent cation and the color of the solution changed from rosy to pure blue. The consumption of EDTA-2Na solution is positively correlated with divalent cation. In Figure 5b, modified CSM containing a pozzolanic RCB reacts slowly, and the consumption of divalent cation mainly occurs at later ages. These findings could prove the pozzolanic characteristic of RCB, and the pozzolanic reaction rate is slower than the hydration reaction rate. The continuous pozzolanic reaction will form the crystal structure, cementing into a whole and enhancing the interfacial transition zone, which has a positive effect explained by chemical reaction on the strength of CSM.

4.3. Mechanical Properties

4.3.1. Unconfined Compressive Strength

The unconfined compressive strength results of CSM specimens at curing ages of 7 d, 28 d, 90 d and 180 d are summarized in Figure 6. In general, the unconfined compressive strength (RC) of CSM specimens decreases with the RCB substitution ratio increasing, and RC value increases with curing age increasing. As shown in Figure 6, with the RCB substitution ratio, the unconfined compressive strength of all CSM groups at the same curing age generally shows a downward trend, which is consistent with the conclusions in the previous studies [8,39]. The unconfined compressive strength of CSM is related not only to the mechanical strength of aggregates, but also to the interfacial transition zone. As listed in Table 1, RCB has a larger crushed stone value compared with BA, indicating that the mechanical strength of RCB is lower than that of BA. In the unconfined compression process, the failure of CSM specimens generally starts from the interfacial transition zone, but the aggregate itself is rarely crushed. RCB used in this study was produced by the crushing of construction waste, and its angularity is not obvious compared with BA. Therefore, the bite force in the interfacial transition zone formed with mortar would become weaker, which is unfavorable to the unconfined compressive strength of CSM incorporating RCB. With the increase in the RCB substitution ratio, this kind of weak interfacial transition zone will also increase inside CSM, so the unconfined compressive strength of CSM incorporating RCB decreases with the RCB substitution ratio.
As illustrated in Figure 6, it is worth noting that the growth law of unconfined compressive strength of modified CSM is similar to that of the CSM control group without RCB. The unconfined compressive strength of all CSM increases sharply during the first 7 d curing period, and then its growth rate gradually slowed down from 28 d to 180 d. The growth rates of unconfined compressive strength are also different varying with the RCB substitution ratio. This may be because the hydration product by the reaction of cement and water would wrap the un-hydrated components, resulting in a slowdown tendency of unconfined compressive strength enhancement. In summary, most of the early unconfined compressive strength of CSM has been completed after curing for 7 d, and the 28 d unconfined compressive strength can reach about 70% of the 180 d unconfined compressive strength. Nevertheless, the 7 d unconfined compressive strength of all CSM can still meet the requirements of medium and light traffic (3–5 MPa). Referring to the Chinese specification (JTG D50-2017), CSM without RCB and with 20% RCB can be in accord with heavy and extra heavy traffic for expressways and first-class highways, and CSM with RCB of 20% to 60% could satisfy the standard of heavy traffic for expressways and first-class highways, while CSM with 80% RCB could fit the bill of medium and light traffic for expressways and first-class highways.
As listed in Table 3, the logarithmic regression function (i.e., strength = a + b × ln(x)) is used to fit the unconfined compressive strength results of CSM varying with curing age, in which strength is the unconfined compressive strength, and x represents curing time. The fitting models show that there has been a sharp drop in the fitting coefficient “a”, and the fitting coefficient “b” increases steadily as the RCB substitution ratio increases. These results are consistent with those of Yan’s findings indicating that RCB has an obvious effect on the early unconfined compressive strength, while the negative influence of RCB on the unconfined compressive strength of CSM would decrease gradually varying curing age [40]. The fitting models further support the idea that although the addition of RCB decreases the early unconfined compressive strength, the corresponding unconfined compressive strength in the later stage will be improved. This is because the pozzolanic reaction in CSM usually occurs at later stages after the hydration reaction of cement, and the pozzolanic reaction rate is slower than the hydration reaction rate, as discussed in Section 4.2.

4.3.2. Indirect Tensile Strength

Indirect tensile strength is a primary mechanical parameter in determining the suitability of CSM application, because there is usually some tensile stress to resist at the bottom of semi-rigid base course under the action of traffic load, and shrinkage stress will also occur while the ambient temperature and humidity changing [41,42]. Figure 7 graphically represents the indirect tensile strength results at the curing age of 90 d for CSM.
It can be inferred from the results that the indirect tensile strength overall presents an upward trend as the RCB substitution ratio increases, which is consistent with the analysis results in the previous studies [40,41,43]. Thus, RCB could obviously improve the indirect tensile strength of CSM specimens with a slow-growth trend. This is because the indirect tensile strength of CSM is mainly contributed by the cement mortar among the inside aggregates. The porous surface on RCB can play a positive effect in increasing the bite force with cement mortar, so as to enhance the bonding force between aggregates; hence, the indirect tensile strength of CSM is increased. The hydration products of Portland cement would set and harden to increase the inside adhesion again. Meanwhile, due to the hydration product of cement as an alkaline activator, the active components (silicon-oxygen and aluminum oxide microcrystal) in the pozzolanic materials in RCB continuously react with active silica and alumina to produce the hydrated calcium silicate (C-S-H) insoluble in water. This continuous pozzolanic reaction will form the crystal structure, cementing into a whole and enhancing the interfacial transition zone, which would play a positive role in indirect tensile strength for CSM.
In addition, it is clearly seen that the growth rate of indirect tensile strength of CSM gradually slows down. This result may be explained by the fact that the pozzolanic reaction usually occurs after cement hydration, and its reaction rate is relatively slow, which mostly occurs in the middle and late curing periods. Consequently, at the 90 d curing time of the late curing period, the indirect tensile strength of modified CSM incorporating RCB would be further enhanced, which could satisfy the requirements of the Chinese specification (JTG D50-2017).

4.4. Durability Properties

4.4.1. Dry Shrinkage Behavior

A dry shrinkage test is an effective tool to reflect a macroscopic volume change in CSM due to the change in inside water content. Results of accumulative water loss rate changing with time are demonstrated in Figure 8a. It can be seen that the accumulative water loss rate of CSM at the same curing time decreases first and then increases as the RCB substitution ratio increases and the turning point is the RCB substitution ratio of 20%. RCB has higher water absorption; herein substituting BA with RCB would increase the OWC of CSM specimens incorporating RCB, as presented in Table 2. The larger the water content in the early curing stage of CSM specimens, the more water will evaporate in the later curing stage of CSM, resulting in a greater dry shrinkage and water loss rate [44]. Moreover, it is clearly seen that as the curing time increases, the accumulative water loss rate curves of all CSM specimens are gradually slowing upward trends. The water loss rates change fast during the early curing period and gradually slow down during the later curing period.
The average coefficients of dry shrinkage are summarized in Figure 8b. It could be seen that the coefficient of dry shrinkage first decreases and then increases, and its minimum value appears at the RCB substitution ratio of 20%. And then due to the higher water absorption of RCB, the increase in RCB substitution ratio in aggregates can result in a larger coefficient of dry shrinkage. The more sensitive to water the base course material is, the greater its coefficient of dry shrinkage, indicating a worse crack resistance. This result may be explained by the fact that the water loss of macro voids inside CSM specimens has no obvious influence on dry shrinkage.
The above findings may be taken to indicate that adding an appropriate amount of RCB as a BA substitute can inhibit the dry shrinkage of CSM to a certain extent, as the lowest average coefficient of dry shrinkage of CSM-RCB20. A possible explanation for this might be that the water loss from dry shrinkage of CSM specimens mainly includes the dehydration of hydration products of cement such as C-S-H and the evaporation of pore water [45]. The reduction in C-S-H associated with the RCB substitution ratio from the source may cause a dehydration reduction in hydration products of cement. On the other hand, the active components in the pozzolanic materials of RCB aggregates react with the hydration product of cement, producing more cementitious substances to fill the inside pores and increase the number of capillary pores. It is probable therefore that the evaporation of pore water under the action of capillary tension would increase the strain of dry shrinkage. Along with the increase in RCB substitution ratio range from 20% to 80%, however, there is an increasing trend of accumulative water loss rate as well as average coefficient of dry shrinkage for CSM specimens, since the evaporation of capillary water is significant owing to its slow pozzolanic reaction rate. This view is supported by Bektas et al., who wrote that using RCB as the aggregate replacement will even increase the dry shrinkage of CSM [29].

4.4.2. Temperature Shrinkage Behavior

Semi-rigid base course is a solid-liquid-gas three-phase material with different temperature shrinkage properties. The temperature stress stemming from the temperature variety of the environment could lead to the shrinkage of materials, and if repeated, the internal structure will fatigue and gradually induce cracks. The temperature shrinkage results of CSM specimens are shown in Figure 9. It is noted that the coefficient of temperature shrinkage varying with temperature ranges first decreases and then increases with the decreasing of test temperature, presenting a similar variation trend, as shown in Figure 9a. Although the temperature shrinkage behaviour of CSM-RCB50 fluctuates greatly, the overall change trend conforms to the overall law, which may be caused by the test error at a certain stage. When the temperature drops from 30 °C to (0~−10) °C, the coefficient of temperature shrinkage declines gradually, afterwards, the coefficient value increases as the temperature sequentially drops to −20 °C. Additionally, it can be seen from Figure 9a that for any temperature range, the higher the substitution ratio, the larger the coefficient of temperature shrinkage, which implies a higher probability of cracks while cooling. The reason for this phenomenon is that the capillary water and structural water inside CSM specimens will freeze and frost heave below 0 °C, increasing the coefficient of temperature shrinkage. In the temperature range of 10–20 °C, the coefficient of temperature shrinkage has a relatively gentle variation.
The average coefficient results of temperature shrinkage varying with the RCB substitution ratio are shown in Figure 9b, and the average coefficient of temperature shrinkage increases with the RCB substitution ratio increasing. The addition of RCB has an effect not only on the dry shrinkage characteristics of CSM specimens, but also on the temperature shrinkage characteristics. It seems possible that these results are because RCB has many pores, large pore size, and a rough surface in contrast to BA. With the RCB substitution ratio increasing, the internal porosity of CSM also increases, providing a relatively large space for the deformation of temperature shrinkage, thus, the average coefficient of temperature shrinkage presents a clear trend of growing with the RCB substitution ratio.

4.4.3. Freeze-Thaw Resistance Behavior

As known, there are inevitable voids in the interior of pavement materials, which often contain more or less water [46,47]. When the ambient temperature changes, the water within the materials will expand with low temperatures and contract with high temperatures. The residual strength ratio (BDR), i.e., the ratio of unconfined compressive strength of saturated cylindrical CSM specimens after the freeze-thaw cycle to that before the freeze-thaw cycle, is employed to assess freeze-thaw resistance. The freeze-thaw resistance results of CSM with different RCB substitution ratios at curing ages of 28 d or 180 d are represented in Figure 10. It is apparent that unconfined compressive strength results of CSM specimens after curing for 28 d and 180 d are in overall downward trends before and after freeze-thaw action varying with RCB substitution ratio increasing. On the other hand, the larger the BDR value, the better the freeze-thaw resistance of CSM. By comparison, with the RCB substitution ratio increasing, BDR values of CSM increase slightly in a lower RCB substitution ratio range and then decreased rapidly, in which CSM-RCB50 achieves the BDR peak value. All BDR values can meet the freeze-thaw resistance requirements of the semi-rigid base course for heavy frozen areas, that is, the BDR value should not be less than 50% after curing for 28 d [40].
There would be sufficient pozzolanic reaction between more active components in RCB and hydration products under the promoting effect of alkaline activator, in which more cementitious materials were produced to fill internal pores to make the internal structure denser and thus reducing the damage of frost heave to CSM specimens. In the meantime, it has previously been observed that the BDR value of CSM-RCB50 decreases slowly and increases slightly [41]. In fact, at an excessive RCB substitution ratio, thanks to higher porosity and water absorption of RCB, OWC increases with the increase in RCB substitution ratio. During the freeze-thaw period, the repeated action of frozen and frost heaving inside CSM will cause damage to some extent, and the induced damage is more serious [48]. Moreover, as a result of the mechanical strength of RCB lower than that of BA, considering the porous structure and weak interface transition zone, CSM incorporating RCB are more likely to be damaged by the expansion of water at a higher RCB substitution ratio.

4.4.4. Anti-Scouring Behavior

Given the rainfall or surface water seeping into pavement inevitably, some aggregates inside pavement will be continuously washed out under the action of hydrodynamic pressure, hence, the cementation ability among aggregates decreases. In this case, the looseness and strength reduction in the semi-rigid base course is also accompanied. Scouring mass loss rate (SML) results representing the anti-scouring property of CSM with RCB at a curing time of 28 d are displayed in Figure 11. It is obvious from the figure that SML values of CSM present an upward tendency with the RCB substitution ratio, as concluded by Miao et al., [49]. The degradation of the anti-scouring property can be mainly attributed to the poor angularity of RCB. Considering RCB was produced by the crushing of construction waste, the angularity of RCB is not obvious and poorer compared to BA, resulting in the weaker bite force of the interfacial transition zone formed with cement mortar. On the other hand, the most obvious finding to emerge from the gradation distribution in Figure 2 is that the aggregate size of RCB is relatively small in contrast to BA. Due to the pumping effect inside pavement, substituting BA with RCB would increase the proportion of fine aggregate washed away and then reduce the anti-scouring property of CSM. Therefore, the mass loss of CSM specimens incorporating RCB is large under the same scouring condition, which means that RCB aggregates have a negative influence on the anti-scouring property of CSM.

4.5. Environmental Impact Based on the Leaching Toxicity

RCB is a kind of solid waste, which may contain harmful trace elements such as Zn, Cu, Cr, Cd, Ni, Pb, As, etc., in addition to the main elements such as C, O, Si, Fe, Al, Ca, Mg and Na. These harmful trace heavy metal elements may leak from the semi-rigid base course, causing a potential threat to the surroundings. Whether RCB pollutes the environment is the key premise for the application of RCB on road bases. There is no violent chemical reaction in CSM incorporating RCB. In view of raw materials including RCB from C&D waste for CSM products, it is necessary to perform the leaching toxicity detection of heavy metal elements. Results of trace toxicity of heavy metal elements in the filtered leaching solution of CSM specimens after curing for 7 d are summarized in Table 4.
As shown in Table 4, despite that the concentrations of leaching toxicity heavy metal elements significantly exhibit an increasing tendency as the RCB substitution ratio increases, the leaching concentrations of several heavy metal elements are far lower than the threshold values of identification for extraction toxicity referring to Chinese standard (GB 5085.3-2007). Meanwhile, in contrast to the quality standards of surface water and groundwater in Chinese standards (GB 3838-2002 and GB/T 14848-2017), the leaching concentrations of heavy metal elements for CSM specimens with RCB substitution ratio from 0% to 20% could meet the Class I standard requirements of surface water and groundwater. On the whole, the standard for groundwater quality is stricter than the quality standards for surface water, in which the groundwater satisfying Class I and Class II has a wide range of applications, and the Class III groundwater can be as centralized drinking water and industrial and agricultural production water [50]. The leaching concentrations of heavy metal elements for all CSM would conform to the Class III groundwater standard.
Several factors could explain this observation. Firstly, the hydration product (C-S-H) of cement can reach the nanoscale and some heavy metal ions could be fixed inside CSM due to its physical wrapping and solidification [51]. And the heavy metal ions could be absorbed owing to the large specific surface area of the hydration product (C-S-H) [52]. In addition, some heavy metal ions will also be fixed in CSM to further reduce their concentrations on account of chemical reactions and ion replacement [53]. In reality, the leaching of toxicity heavy metal elements generally is a slow process, and it is evident that the toxicity heavy metal elements immobilized in CSM are more reliable. Therefore, the modified CSM products incorporating RCB from construction waste will have no secondary pollution potential to the environment, which can be directly put into engineering application.

5. Conclusions

The primary objective of the study was to evaluate the feasibility and environmental impact of CSM incorporating various RCB fine aggregate substitution ratios. A comprehensive evaluation concerning the mechanical and durability properties of modified CSM has been carried out based on laboratory tests. Simultaneously, an in-depth analysis of CSM included both environmental and strength mechanism aspects. The utilization of RCB in CSM is proved as a promising solution for C&D waste disposal. The experimental results and discussions lead to the following conclusions:
(1)
A higher substitution ratio results in lower unconfined compressive strength as for the lower inherent strength and inconspicuous angularity of RCB compared with natural aggregate. The higher the RCB substitution ratio is, the larger the indirect tensile strength of the late curing period is.
(2)
RCB negatively impacts upon the freeze-thaw resistance of CSM, and the increase in CSM curing time can promote its freeze-thaw resistance, since the pozzolanic reaction would produce cementitious materials to fill internal pores. RCB has a smaller aggregate size and poorer angularity compared with natural aggregate, causing a negative influence on the anti-scouring property of CSM.
(3)
Substituting RCB for natural aggregate resulted in an overall increasing accumulative water loss rate, the average coefficient of dry shrinkage and temperature shrinkage. Moreover, the coefficient of temperature shrinkage has a relatively gentle variation in the temperature range of 10–20 °C.
(4)
CSM incorporating RCB with pozzolanic activity reacts very slowly to form the crystal structure, which has a positive effect on the strength of modified CSM mainly at later stages. The leaching concentrations of several heavy metal elements were far lower than the threshold values of identification for extraction toxicity, which also meet the standard requirements of surface water and groundwater referring to the Chinese standard.
The limitation of this study lies in the lack of practical engineering. The future research plans to explore the performance prediction model of modified cement-stabilized macadam incorporating RCB fine aggregate, and verify the prediction model in combination with actual projects. Incorporating RCB from C&D waste as fine aggregate into CSM of pavement base would be a sustainable means for environmental protection and resource conservation, and even reduce the carbon footprint of raw material mining, for which more research is needed for a deeper investigation into the practical applications.

Author Contributions

Conceptualization, E.Z. and X.W.; methodology, E.Z. and X.W.; validation, X.W. and H.W.; formal analysis, E.Z. and W.W.; investigation, E.Z., X.W., W.W. and H.W.; writing—original draft preparation, E.Z. and W.W.; writing—review and editing, X.W. and H.W.; project administration, W.W.; funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Scientific and Technological Development Plan Project of Jilin Province (grant number: 20210508028RQ), National Key R&D Program of China (Grant Number 2021YFB2600604), Scientific Research Project of Department of Education of Jilin Province (grant number: JJKH20221019KJ) and China Postdoctoral Science Foundation (grant number: 2021T140262).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic flowchart of the whole experimental program.
Figure 1. Schematic flowchart of the whole experimental program.
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Figure 2. Grain distributions: (a) BA, and (b) RCB.
Figure 2. Grain distributions: (a) BA, and (b) RCB.
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Figure 3. SEM-EDS results of RCB aggregate: (a) microstructure image, (b) EDS spectrum.
Figure 3. SEM-EDS results of RCB aggregate: (a) microstructure image, (b) EDS spectrum.
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Figure 4. XRD pattern of RCB aggregate.
Figure 4. XRD pattern of RCB aggregate.
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Figure 5. The EDTA titration test of CSM: (a) color change, and (b) EDTA consumption with curing.
Figure 5. The EDTA titration test of CSM: (a) color change, and (b) EDTA consumption with curing.
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Figure 6. The unconfined compressive strength results of CSM specimens.
Figure 6. The unconfined compressive strength results of CSM specimens.
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Figure 7. The indirect tensile strength results of CSM specimens.
Figure 7. The indirect tensile strength results of CSM specimens.
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Figure 8. The dry shrinkage results: (a) accumulative water loss rate, and (b) average coefficient of dry shrinkage.
Figure 8. The dry shrinkage results: (a) accumulative water loss rate, and (b) average coefficient of dry shrinkage.
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Figure 9. The temperature shrinkage results of CSM specimens: (a) coefficient of temperature shrinkage with temperature ranges, and (b) average coefficient of temperature shrinkage.
Figure 9. The temperature shrinkage results of CSM specimens: (a) coefficient of temperature shrinkage with temperature ranges, and (b) average coefficient of temperature shrinkage.
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Figure 10. The freeze-thaw resistance performance: (a) curing for 28 d, and (b) curing for 180 d.
Figure 10. The freeze-thaw resistance performance: (a) curing for 28 d, and (b) curing for 180 d.
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Figure 11. The anti-scouring properties of CSM specimens.
Figure 11. The anti-scouring properties of CSM specimens.
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Table
Table 1. Technical indexes of BA and RCB fine aggregates.
Table 1. Technical indexes of BA and RCB fine aggregates.
Fine AggregatesApparent Specific DensityWater Absorption (%)Liquid Limit (%)Plasticity Index
BA2.6891.7318.064.34
RCB2.11617.6037.918.50
Table
Table 2. MDD and OWC values for various CSM groups.
Table 2. MDD and OWC values for various CSM groups.
Group No.CSM-BACSM-RCB20CSM-RCB40CSM-RCB50CSM-RCB60CSM-RCB80
MDD (g/cm3)2.3622.2152.1672.1432.1222.152
OWC (%)5.116.877.047.358.178.31
Table
Table 3. The fitting model results of unconfined compressive strength for CSM.
Table 3. The fitting model results of unconfined compressive strength for CSM.
Fitting ModelsRCB Substitution Ratio (%)
02040506080
a3.33782.99412.59792.12511.50630.7652
b1.08731.13691.19181.25411.33331.4467
R20.99390.99190.99120.99260.99670.9899
Table
Table 4. Leaching toxicity test of CSM specimens incorporating RCB fine aggregates.
Table 4. Leaching toxicity test of CSM specimens incorporating RCB fine aggregates.
SpecimensConcentration of Leaching Toxicity Heavy Metal Elements (mg/L)
ZnCuCrCdNiPbAs
CSM-BA0.00330.00710.0087ND0.00550.0013ND
CSM-RCB200.00370.00840.0096ND0.00640.0021ND
CSM-RCB400.00390.00980.01050.00100.00690.0032ND
CSM-RCB500.00460.01110.01240.00130.00710.0039ND
CSM-RCB600.00520.01200.01400.00170.00710.0044ND
CSM-RCB800.00650.01320.01720.00210.00730.0057ND
Extraction toxicity
(GB 5085.3-2007)		10010051555
Surface water
(GB 3838-2002)		I0.050.010.010.001/0.010.05
II110.050.005/0.010.05
Groundwater
(GB/T 14848-2017)		I0.050.010.0050.0001/0.0050.001
II0.50.050.010.001/0.0050.001
III110.050.005/0.010.01
Note: Not detected (ND) (<0.001 mg/L).
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Zhang, E.; Wang, X.; Wang, W.; Wang, H. Mechanical Properties, Durability and Leaching Toxicity of Cement-Stabilized Macadam Incorporating Reclaimed Clay Bricks as Fine Aggregate. Sustainability 2022, 14, 8432. https://doi.org/10.3390/su14148432

AMA Style

Zhang E, Wang X, Wang W, Wang H. Mechanical Properties, Durability and Leaching Toxicity of Cement-Stabilized Macadam Incorporating Reclaimed Clay Bricks as Fine Aggregate. Sustainability. 2022; 14(14):8432. https://doi.org/10.3390/su14148432

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Zhang, Ermao, Xirui Wang, Wensheng Wang, and Haoyun Wang. 2022. "Mechanical Properties, Durability and Leaching Toxicity of Cement-Stabilized Macadam Incorporating Reclaimed Clay Bricks as Fine Aggregate" Sustainability 14, no. 14: 8432. https://doi.org/10.3390/su14148432

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