**Mirjana Vuki´cevi´c, Miloš Marjanovi´c \*, Veljko Pujevi´c and Sanja Jockovi´c**

Faculty of Civil Engineering, University of Belgrade, Bulevar kralja Aleksandra 73, 11000 Belgrade, Serbia; mirav@grf.bg.ac.rs (M.V.); vpujevic@grf.bg.ac.rs (V.P.); borovina@grf.bg.ac.rs (S.J.)

**\*** Correspondence: mimarjanovic@grf.bg.ac.rs

Received: 26 July 2019; Accepted: 14 September 2019; Published: 18 September 2019

**Abstract:** Major infrastructure projects require significant amount of natural materials, often followed by the soft soil stabilization using hydraulic binders. This paper presents the results of a laboratory study of alternative waste materials (fly ash and slag) that can be used for earthworks. Results of high plasticity clay stabilization using fly ash from Serbian power plants are presented in the first part. In the second part of the paper, engineering properties of ash and ash-slag mixtures are discussed with the emphasis on the application in road subgrade and embankment construction. Physical and mechanical properties were determined via following laboratory tests: Specific gravity, grain size distribution, the moisture–density relationship (Proctor compaction test), unconfined compressive strength (UCS), oedometer and swell tests, direct shear and the California bearing ratio (CBR). The results indicate the positive effects of the clay stabilization using fly ash, in terms of increasing strength and stiffness and reducing expansivity. Fly ashes and ash-slag mixtures have also comparable mechanical properties with sands, which in combination with multiple other benefits (lower energy consumption and CO2 emission, saving of natural materials and smaller waste landfill areas), make them suitable fill materials for embankments, especially considering the necessity for sustainable development.

**Keywords:** fly ash; slag; soil stabilization; embankment; cement; lime

#### **1. Introduction**

The modern world is facing the consequences of the technological development followed by a huge environmental impact. This has stimulated recent scientific research in the field of identifying pollutants and the possibility of reducing the harmful effects of pollution. One of the major pollutants is fossil fuel, which produce huge amounts of CO2 in the combustion process. According to the World Bank data for 2015 [1], the total share of fossil fuels in energy production in the world is 65.2%. According to the same data in Serbia, this share is 73.1%, since the thermal power plants are the main producers of electricity. There are six thermal power plants within the Electric Power Industry of Serbia, which use lignite as the main fuel.

Thermal power plants have multiple harmful effects on the environment: They pollute air by emitting CO2, SO2, N2O and fly ash; landfills of ash and slag occupy large areas of mainly agriculture land; deposited ash can potentially pollute land and water due to the presence of microelements and radionuclides. The harmful effects of flue gases can be reduced by gas desulphurization, installation of efficient electro filters and application of methods for reducing the concentration of nitrogen oxides. The amount of deposited ash and slag can be significantly reduced by use in the construction industry.

According to the ECOBA (European Coal Combustion Products Association) data for 2016 [2], annual production of ash in the European Union (EU 15) was about 40 million tons, of which 64% was fly ash. About 50% of the produced amount was used in the construction industry, 41.5% was used

for land reclamation–restoration and only 6.7% was deposited. In the construction industry, it was mostly used for the production of cement and concrete (about 25%), while much less (about 6% today vs. 25% ten years ago) was used in the road construction. The data show that ash in developed EU countries is successfully used as raw material in the industry. In Serbia, the situation is completely different. About 7 million tons of ash is produced annually. A very small part of the ash is deposited in silos, while most of the total produced amount is deposited with the slag at the landfills. The landfills occupy an area of about 1600 ha, with about 300 million tons of ash and slag [3,4]. So far, only 3% of ash has been used for the production of cement [4].

In Serbia, the first major research related to the possibility of using ash in road construction began in the first decade of this century, with the aim of reducing the large amount of deposited ash. Since then, four extensive studies have been done [5–8]. Based on the results of these studies, in 2015 the Serbian Government has passed a regulation [9] on the use of ash from thermal power plants in Serbia, thus creating a legal framework for the use of ash.

During the research [7,8], about 1000 laboratory tests of physical and mechanical properties of fly ash, ash and slag, mixtures of ash and soil with or without hydraulic binders (cement and lime) were done in the Laboratory for Soil Mechanics of the Faculty of Civil Engineering in Belgrade. Additionally, chemical composition of fly ash was investigated. The main results and conclusions from these studies are presented in this paper.

The aforementioned studies included a very important ecological aspect of the use of ash, bearing in mind that ash contains harmful substances that can be a potential source of pollution of soil and water. Ash can be disposed of as waste material if the content of artificial and natural radionuclides is less than the values prescribed in the Rulebook on the Limits of Radioactive Contamination of the Environment [10]. Ash and slag from the Serbian thermal power plants meet the prescribed requirements [11]. Ash also contains trace elements that are hazardous to the environment, such as As, B, Cr, Mo and Se [12]. These elements could contaminate the soil, water and marine ecosystems in case of their leaching. The main factor in the control of leaching is the control of the mobility of the trace elements. There are appropriate procedures that can reduce or eliminate the leaching of toxic trace elements such as As, B, Cr, Sb and Se [11,12]. If it is proven that there is no risk of leaching, the use of ash for embankments provides economic and environmental benefits.

In the first part of the paper, the results of high plasticity clay stabilization using fly ash from Serbian power plants (with and without binders) were presented. In addition, the effects of ash application as a soil stabilizer were compared with the effects of chemical additives for the same purpose. In the second part, engineering properties of ash and ash-slag mixtures as embankment material in road construction were investigated. The mechanical properties important for fulfillment of the technical requirements for road subgrade were tested. The influence of common binders (activators) was also investigated.
