The use of ladle furnace slag in soil stabilization

Abstract

Natural clayey soils have often to be stabilized by mixing them with certain materials, such as cement, lime, or similar products, so that they acquire the necessary properties for civil works. Some industrial by-products can also be used for this purpose, among which steelmaking slags. Our research studies the properties of Ladle Furnace Slag (LFS) and the characteristics of several clayey soils susceptible to improvement with additions of this by-product. The behaviour of the different soil and slag mixes was similar to the behaviour of the soil and lime mixtures reported in the literature. The results of a series of test report improvements in various geotechnical properties, such as the plasticity index, expansiveness, bearing capacity and durability.

Introduction

Natural soil stabilization is a classic problem in civil engineering, when it is necessary to prepare a subbase or foundation bed with natural soils upon which to construct light foundations or transportation infrastructure such as roadbeds, railroads, or airports. The function of the grading roadbed is to provide geometric regularity, lineal continuity and adequate transmission of loads to the subjacent ground. Grading roadbeds may be prepared in different ways; this study examines the construction of a surface layer, in which the natural soil, if of poor quality, may be efficiently improved.

An outstanding problem in geotechnical engineering has involved improvements to the bearing capacity of clayey soils and the achievement of volumetric stability when they show expansive behaviour in the presence of water [1]. For decades, various proven industrial products (lime, Portland cement) have been used, in order to achieve levelled areas of acceptable surface quality for civil works, although they are currently considered to incur high economic and environmental costs [2], [3], [4], [5].

Nevertheless, when mixed with a clayey soil, various industrial by-products are able to transform its properties. In this way, cheap materials or compounds capable of activating light pozzolanic activity, ionic exchanges and the flocculation of clay can be used for the improvement of soft natural soils. The most prominent are Ground Granulated Blast Furnace Slag (GGBFS) [6], fly ashes from coal burning [7] and cement kiln dust [8], [9], [10], [11], [12], [13], [14]. Bottom ashes from coal burning [15], municipal solid waste incinerator ashes [16], [17] and small proportions of chemical activators that improve the binding capacity of the soil [18] are also used, but to a lesser extent. Most of the aforementioned by-products may be sourced in a dusty form, which makes their application in large proportions mixed with natural soils considerably easier and more economically viable.

Slags from iron and steel manufacture have long been regarded as useful materials in building and civil works. Relevant studies published in the 1990s [19] and at the beginning of this millennium [20], [21] have demonstrated the suitability and even the excellence of some of these materials, when used in acceptable and viable applications, from both a technical and an economic perspective [22], [23], [24], [25], [26], [27], [28].

In the early 1990s [29], pioneering attempts to use ground steel slag from converter furnaces (LD) in soil stabilization for rural roads with low-traffic met with limited success and the milling process of the slags came at a high economic cost. Subsequent results obtained [30] for the stabilization of soils in road bases using LD slags were not encouraging; despite the use of activators, it was necessary to use excessively high proportions (15–20%) of finely milled slag and long curing periods, all of which were negative factors, in order to achieve acceptable performances. Similarly, the works of several other authors [6], [19], [20] considered LD slag and Electric Arc Furnace Slag (EAFS) inappropriate for soil stabilization. Nevertheless, high proportions of slag have recently been used for the stabilization of dredged marine clay embankments [31], and the use of EAF and LD slag mixed with fly ash has also provided successfully results [32], [33].

A promising type of slag for such applications is Ladle Furnace Slag (LFS), which is also known as basic slag, reducing slag, white slag and secondary refining slag. Its production in Spain stood at half-a-million tons in 2010; partially, it is reused in Portland cement manufacture (Oficemen – Unesid), being the rest allocated to new uses at sites close to the centres of production.

LFS is a dusty material with limited hydraulic reactivity [34], [35], [36], [37], [38], due to the majority presence of both calcium and magnesium oxides, silicates and aluminates. The foremost question hanging over the use of LFS in engineering concerns its volumetric instability [39], [40], [41], [42] as a result of weathering and consequent exposure to atmospheric agents, air and water. However, this short and medium-term expansiveness is irrelevant when used in low proportions (less than 10%) for flexible matrices, such as soil mixtures.

LFS from any origin shares common chemical features [43], [44], such as the presence of dicalcium silicate (in any of its crystalline varieties), calcium aluminates, free lime (lime, portlandite) and free magnesium oxide (periclase). These and other substances found in LFS are useful when considering their applications in the improvement of soils [45]. The replacement of quicklime by LFS is advantageous from the economic and environmental point of view; the results may be excellent for certain combinations of soil and LFS and poor in other cases, due to the great variety of clayey soils [46] and the varied chemical and mineralogical composition of LFS [47].

Two European Standards – EN 14227-13 and ENV 13282 [48] – specify the use of industrial products and by-products for natural soil stabilization, when the soils show sufficient capacity for improvement. Encouraging results in the work of Kanagawa and Kuwayama [49] reported successful improvements to soft clayey soils through the use of LFS. The present paper develops this same line of research, by mixing clayey soils of low bearing capacity with LFS in various proportions, in order to analyse the results in absolute and comparative terms with conventional lime-based stabilization.