1. Introduction
Road infrastructure in China has experienced a fast development in the past three decades. The total mileage of motorway is over 5 million kilometers at the end of 2021, including nearly 160 thousand kilometers of high-speed expressway [
1]. In the meantime, about 800 thousand kilometers of motorway needs rehabilitation or maintenance each year. A large number of aggregates are required to meet the needs of new constructions and maintenances, rendering a huge burden on natural resource and environment protection. There are urgent needs for sustainable and environmental substitute for natural aggregates in road engineering. At the same time, industrial wastes have increased as a result of rising population and technological development, posing increasingly serious social problems and environmental threats. Innovative approaches have been studied to decrease these wastes, or, as a clearer option, turn them into valuable reserves [
2,
3]. For several decades, various industry wastes have been extensively studied to replace aggregates or to enhance performance of construction materials [
4,
5,
6].
Steel slag is one of the by-products generated in steel making process. China’s crude steel production reaches 1.03 billion tons in 2021 [
1]. According to production rate of 60~150 kgt
−1 based on the current refining processes, the steel slag production is around 60~150 million tons yearly. A huge quantity of steel slag needs to be handled properly. Nevertheless, compared to 85~98% of utilization rate in industrialized countries such as USA, Europe and Japan, the utilization in China is only at 29.5% [
7]. Most of it is piled up or disposed as landfill, which not only occupies land resource, but also causes leaking of harmful component.
The physic properties of steel slag, i.e., hardness, crushing and grinding, makes it adequate for road engineering. If used appropriately, steel slag can partially or fully replace aggregates in mixtures; not only can the shortage of construction resources be alleviated, but also the damage to environment can be reduced. The utilization of steel slag for road construction in the United States is at nearly 50%; in Europe at 43%, and in Japan at 32.4%, whilst in China it is only at 7.6% [
7]. Much work has been done to apply steel slag in asphalt pavement in recent years. He et al. and Skaf et al. have argued that road construction is the most effective and lowest-cost way to utilize steel slag [
8,
9]. Studies have suggested that electric steel slag displayed almost the same mechanical and physical properties as traditional aggregates [
10,
11]. Scholars have also found that steel slag can improve high temperature performance [
12,
13], moisture stability [
14] and fatigue [
15] of asphalt mixture significantly. Moreover, the solutions to obtain functional pavement such as improving skid resistance [
16], deicing [
17] and self-healing material [
18] became more vital and diverse with the application of steel slag. In addition, the utilization of steel slag mixtures has sound economic benefit and potential environmental benefits compared to traditional mineral aggregate mixtures [
19,
20,
21]. Despite the many merits of steel slag over natural aggregates, the applications of steel slag in paving material are still conservative. Concerns include physical and morphologic property, replacement content, long-term volume stability and environmental impacts [
22,
23,
24].
Previous studies on steel slag in road engineering mainly focus on asphalt mixture and surface course, relatively few on applications of incorporating steel slag in base layer. Based on the reality that cement-stabilized base is the main base type, and the massive slag waste in China at this stage and beyond, the utilizing of steel slag in cement-stabilized base was investigated in this study. We designed four levels of replacement: 0%, 30%, 50% and 75% of steel slag, and the optimal moisture contents were determined through compaction test. Performances of samples with different steel slag contents were evaluated. Unconfined compressive strength, bending strength and resilient modulus were tested for mechanical properties. Volume expansion of steel slag and dry shrinkage of steel slag mixture were also checked. Environmental impacts were evaluated in view of heavy metal leaching and carbon footprint. Other means including SEM, X-ray and FAAS were also employed to check the surface morphology, element distribution and chemical composition to comprehend the strength development and environmental impacts of steel slag in cement-stabilized semi-rigid base.
5. Conclusions
Steel slag is a promising alternative for natural aggregates in pavement base course. In this study, different steel slag content (0%, 30%, 50%, 75%) was added to cement-treated aggregates. Throughout investigations including mechanical properties, volume characterization, economic benefits and environmental impacts, main conclusions are as follows:
- (1)
Compared to traditional mineral aggregate (i.e., limestone), steel slag aggregate has equal or superior engineering properties, including density, abrasion resistance and crushing resistance. Due to higher water absorption and larger gravity, the OMC and MDD increase proportionally as more natural aggregates are replaced with steel slag.
- (2)
Mechanical properties including strength and stiffness are significantly improved by steel slag within 50% replacement due to the increased hydration products and interlocked aggregate structure. However, the porosity and water absorption rate have greater negative effect on strength when steel slag replacement further increases.
- (3)
Appropriate treatment with CH3COOH or addition of silica fume can relieve the volume expansion of steel slag. Nevertheless, excessive silica fume may be detrimental to the aggregate structure and should be kept within a reasonable content: 3–4.5%.
- (4)
Smaller particle size shows higher risk of heavy metal precipitation, and the salty water environment in coastal area adds to it. Large particle size (>4.75 mm) and higher cement content are recommended to reduce heavy metal leaching threat.
- (5)
Steel slag mixture has sound economic gains and potential environmental benefits compared to limestone mixture. However, the transport emission of steel slag should be accounted for to determine an environmental haul distance of steel slag.
Further research would be focused on long-term performances of cement-treated aggregate base containing steel slag in real pavement projects in both the theoretic and engineering aspects.