1. Introduction
Predictions show that a large amount of sand consumption in the future will not only create economic pressure on the construction industry but also cause the erosion and degradation of the world’s major ecosystems. The global construction waste crisis will get worse; global waste production will increase by 70% by 2050 in comparison to 2016, and the global waste crisis will spur innovation in waste management. Due to the boom of the construction industry, the price of sand will increase significantly [
1]. However, the large amount of construction waste produced by the process of demolition and reconstruction is one of the inevitable additional products in the process of urbanization. Among the construction waste includes a large number of waste concrete blocks, waste clay bricks, waste steel bars, waste wires and cables, waste plastic products, and so on. Among them, clay bricks and concrete blocks account for about 80% of the total [
2]. Marzouk and Azabbuilt a system dynamics methodology of the construction and demolition waste management sector by developing a dynamic model that is capable of studying the behavior of landfill process on both the short and long run and its impacts on the environment and economy. In Egypt, this system dynamics methodology results show that the recycling of construction and demolition waste would reduce the costs required to mitigate air pollution by
$16,161.35 billion over 20 years [
3].
In order to reduce the environmental and economic pressure of construction waste, the reuse of it is an effective method. As for the reuse of construction waste in road engineering, it is usually used for filling roadbeds without secondary crushing, while after secondary crushing, it can be used as aggregate in asphalt pavement and as a cement stabilized base layer [
4,
5]. Waste concrete blocks are widely used in road engineering. The superiority of concrete materials for housing construction and considerable secondary hydration are the reasons why waste concrete blocks are widely used among several types of construction wastes. However, due to its high strength, the crushing process is complicated, the requirements for the crushing machine are higher, and the crushing process is difficult. There are also uncontrollable interference factors, such as cement that did not fully hydrate, aggregates from different sources with different strengths, and some waste bricks that cannot be completely removed [
6]. In comparison, waste clay bricksare easy to be picked out and crushed and have a lower transportation burden. A stone-crushing machine can be used to crush waste clay bricks, which can save more expenditure.
In response to the management of construction waste, many developed countries began to take action to solve the problem in the 1970s and 1980s. Examples include the Solid Waste Treatment Act of 1965 in the United States and the Waste Treatment Act of 1970 in Japan [
7]. Many other developed countries, such as Germany, Korea, Denmark, and Singapore also made initial efforts to solve this problem and had momentous success [
8,
9]. In China, regulations related to construction waste have been carried out since 1992. The Regulations on the Management of City Appearance and Environmental Hygiene have stipulated that the responsible persons for the disposal of construction waste must take timely clearance and transportation. A series of plans also have been implemented [
10]. These actions above are the first step that has been taken in encouraging the recycling of construction waste and governing the unreasonable disposal of construction waste, which laid a foundation for improving the relevant laws and regulations for the recycling and reuse of construction waste.
In terms of construction waste recycling, most research studies have focused on cement concrete and asphalt concrete. Xue et al. considered that the optimum amount of construction waste composite powder in cement concrete is 30%. This amount of construction waste composite powder putsthe concrete in a more compact and low-alkali condition, thus enhancing the macroscopic frost resistance [
11]. Peng et al. combine silicon powder, slag powder, and recycled construction waste regenerated powder in concrete; they comprehensively study the performance test of using construction waste regenerated powders to replace silicon powder and cement in concrete, obtaining the optimal water–binder ratio range of 0.18–0.20. The results show that the particle size of regenerated powders is mostly larger than that of silicon powder particles, and some of them are smaller than that of cement particles. Furthermore, the pozzolanic reaction of regenerated powders as active admixtures is weak, reflecting the macroscopical performance in which the resistance to chloride ion permeability and the resistance to shrinkage are enhanced with the increase of the amount of the regenerated powders, while simultaneously the unconfined compressive strength and flexural strength are weakened [
12]. Cheng studied the application of fine aggregate partially replaced by fly ash and coarse aggregate partially replaced by waste brick in concrete. The results showed that when the replacement rate of both fly ash and waste brick was 30%, the compressive strength of recycled concrete reached the highest value [
13]. The physical and mechanical properties of four kinds of coarse aggregate with different particle sizes in fresh concrete and hardened concrete were also studied. The results show that harder original brick directs the stronger compressive strength of the concrete, and the higher density directs the higher concrete strength. The flexural strength of concrete produced with a crushed brick aggregate was approximately 8% less than that of concrete made with granite aggregate [
14]. Kristina et al. replaced natural macadam fine aggregate with recycled brick fine aggregate of 0%, 25%, and 50% in concrete, obtaining 25, 50, 75, and 100 routine freeze-thaw cycles at 28 and 60 days of curing age, respectively. Combining the subsequent compressive strength and flexural strength tests, it was concluded that the addition of recycled brick fine aggregate could improve the frost resistance and long-term strength of concrete to a certain extent [
15]. Zhao et al. mix coarse and fine aggregates of construction waste into concrete, respectively. The hardening density, drying shrinkage coefficient, and water absorption rate were taken as reference, the optimum replacement ratio of coarse aggregate and fine aggregate is less than 25% and within the scope of 50%–75%, respectively [
16]. Hu et al. mix recycled clay brick aggregate with 20%, 40%, 60%, 80%, and 100% of coarse aggregate and fine aggregate in cement stabilized macadam, respectively. The compressive strength, splitting strength, compressive modulus of resilience, shrinkage strain, and freeze resistance index were used as references in the study, and the maximum mixing ratio of coarse aggregate and fine aggregate obtained was 70% and 90%, respectively [
17]. Turanli et al. used clay bricks as pozzolanic materials after fine grinding. An ASTM accelerated mortar bar test and scanning electron microscopy (SEM) showed that fine grinding clay bricks could effectively inhibit the expansion caused by the alkali–aggregate reaction [
18]. Peng replaced recycled aggregate for coarse aggregate in asphalt concrete and obtained the optimum replacement ratio of 30%–40% with the high-temperature performance taken as a reference; their results also showed an optimum replacement ratio range of 60%–70%, using the residual stability and freeze–thaw splitting strength as references. When the replacement ratio of recycled aggregate is less than 60%, the adverse effects on low-temperature performance and fatigue resistance are controllable [
19].
It can be seen that the research of recycled construction waste clay brick aggregate (RBA) in concrete and asphalt concrete is more indepth, and some referential conclusions are also drawn in terms of the replacement ratio and pozzolanic activity. In this study, the application of clay brick in cement stabilized macadam subbase will be studied, and the influence of the potential activity of RBA using it instead of fine aggregate partly on the unconfined compressive strength of this structure layer will be discussed. To study the microscopic mechanism of the tendency of strength, we also used the physical and chemical methods to verify the practicability and feasibility of RBA application in road subbase.
4. Test Results and Analysis
4.1. Unconfined Compressive Strength Test
The unconfined compressive strength of 7 d is the only hard and fast rule regarding the design of cement stabilized macadam(JTG/T F20—2015). Specimens were used with curing ages of 7 days, 28 days, and 90 days, respectively, and the values of specimens with replacement amounts of RBA of 0%, 20%, 40%, 50%, 60%, and 80% are shown in
Table 5.
(1) Replacing part of CBA with RBA in cement stabilized macadam fine aggregate will have a great effect on its strength, and the change in the law of strength at various agesis slightly different from the change of RBA content.The strength of 7d and 28d decreases at first, andthen increases and decreases again with the increasing of RBA content. The 7d strength is slightly higher than that of the standard specimen (RBA 0%) when the RBA content is 50%, and the strength of other RBA contents is lower than that of the standard specimen. The 28d strength of 40%, 50%, and 60% RBA content is close to or slightly larger than the standard specimen, and the strength of other RBA contents is less than the standard specimen. However, the 90d strength of the specimens mixed with RBA is higher than that of the standard specimens.
(2) The property of aggregate has a non-negligible influence on strength at an early stage. Strengths of 7d and 28d show that 20% is smaller than 0% because the pozzolanic reaction is weak; as the replacement ratio increases, the pozzolanic reaction becomes stronger and counteracts the weakening caused by the RBA aggregate, so the strength increases gradually. At the same time, the crushed stone value and the plastic index also increase, exceeding the reference values until the 60% RBA replacement ratio directly caused either the strength to decrease again or be maintained at a relatively stable level.
(3) Since the increased percentage of strength was composed of the continuous hydration of cement and the activation of the potential activity of RBA, as the curing time increased, the strength of specimens containing RBA increased as well. The strength of the 90d kept increasing because the pozzolanic reaction started during the hydration reaction and enough hydration product was formed, leading to the higher strength of the RBA specimens.
(4) According to the various strength characteristics at 7d, 28d, and 90d, the 50% RBA replacement ratio has the optimal strength, and it can be the recommended as the replacement percentage of RBA in cement stabilized macadam.
4.2. Modified EDTA titration experiment
In the comparison test, the solution with Cal-Red was purple at first and turned blue after 0.40 mL of EDTA-2Na standard solution was consumed. This change (in
Figure 7) proved that clay brick contains soluble calcium and can be dissolved in a solution environment after cement hydration. The titration results of specimens prepared in different replace proportions are shown in
Table 6.
Specimens with 50%, 60%, and 80% replacement ratios have the almost same content level of Ca2+, of which 50% has the lowest content of Ca2+ among them. This indicated that more Ca(OH)2 is consumed than other types of specimens and specimens with a 50% replacement proportion have the largest consumption of Ca(OH)2. The increase of the RBA proportion indicated that more soluble calcium in the ammonium chloride solution makes the content of Ca2+ in the specimens with 80% RBA larger than that of the 60% specimens. The standard specimen (0%) has the maximum ΔV and 50% has the minimal ΔV, referring to the corresponding Ca(OH)2 consumption.
4.3. Mechanism Analysis
From the results above, it can be seen that the unconfined compressive strength at 7d, 28d, and 90d have particular variation features as the proportion of RBA increases. The maximum value is obtained at 50% of the specimen at 7d. While at 28d it is kept in a stable range, after 50%, the proportion increases. At 90d, the strength increases as the proportion increases, and at 50%, it has the almost same value as that at 80%. It means that the potential activity of RBA activated and offset part of the weakness caused by the weak performances of it, which ultimately affected and enhanced the strength of the cement stabilized macadam layer. Combining with the plastic index and crushed stone value showed in
Table 1, 50% is the most appropriate proportion in this study. Comparing with the reference value at 7d stipulated for the first-class highway by specification (JTG/T F20—2015), 3–5 MPa is stipulated, and the strength of the 50% specimens exceeds the upper limit. The "modified EDTA titration experiment "shows the titration results after 28 days’ maintenance; the specimens with 50% of RBA have the minimum content of Ca
2+. Of this proportion, the potential activity of RBA was enormously stimulated and satisfied most of the specifications. When the replacement ratio increases to 80%, the CaO in clay brick increases because RBA in 80% consumes a similar Ca
2+ amount of 50%, and the strength results showed that the strength generated by potential activation can offset the strength weakened by the weak performances of the part of RBA and improve the strength level.
It can be proved that the activation of RBA activity is based on the hydration reaction of cement. The potential activation of RBA from the strength makes use of the product of the cement hydration reaction. The consumption of this product makes the activated part of RBA form strength at a rate less than that of the cement hydration reaction. xCaO·SiO
2·yH
2O (calcium silicate hydrate, short for C–S–H) and Ca(OH)
2 are major products of the hydration reaction. After a period of time, a certain amount of Ca(OH)
2 is produced. Then, Ca(OH)
2 and SiO
2 in RBA react as shown in Equation (1).
In Equation (1), Ca(OH)2 (form cement hydration), SiO2 (the main substances in clay brick), and H2O react and produce xCaO·SiO2·yH2O. This reaction to some extent increases the hydration product C–S–H of cement and increases the amount of the cement that participates in a hydration reaction. When the RBA content reaches a reasonable value, i.e., 50% in this paper, the SiO2 in clay brick consumes a large amount of the Ca(OH)2 produced by the cement hydration reaction, and produces xCaO·SiO3·yH2O with a cementitious effect and increases a part of the strength.
For 50%, the increased strength not only offsets the weakened strength due to the weak performances of RBA, but also achieves a higher strength level than the specimen without RBA replacing CBA, and satisfied most of the specifications.