Microstructural evolution and mechanical behaviour of alkali activated fly ash binder treated clay

This work focuses on the use of alkali activated fly ash-based binder to enhance engineering characteristics of soft clay-rich soils and as a substitute to standard stabilisers (e.g., lime or cement). Especially, it examines the microstructural evolution of a calcium-rich fly ash from coal combustion-based binder activated by a sodium-based alkaline solution. To this end, the processes generating the microstructure and the evolution of the pore network over time are investigated. A second point addressed by this study is how the presence of kaolin particles affects the microstructural features of the binder. The microstructure has therefore been investigated by considering the binder alone and the binder mixed with kaolin. The effects of microstructural evolution have been observed at macroscopic level by means of one-dimensional compression tests. The combination of completing techniques has been used including Optical microscopy, Scanning Electron Microscopy and Mercury Intrusion Porosimetry in order to gain an overview of the complex pore structure. Microstructural changes occur around calcium-containing phases derived from fly ash which are the reactive phases of the system. Namely, the dissolution of calcium-rich grains leads to the formation of new compounds that first cover the grain surfaces and then further grow into the available space. Furthermore, the evolution of the pore network over time is characterized by a progressive filling of capillary pores by new compounds while small nanometric pores are being formed and associated with the newly formed silicate-calcium chains. Similar tendencies are observed when the binder is mixed with the soil although the general porosity is lesser due to the filling of pores by small-sized kaolinite platelets. Experimental evidences at microscale level have been linked to the macroscopic behaviour of treated soil

DOI: 10.1016/j.enggeo.2010.12.002 Performance of lime-treated silty soil under long-term hydraulic conditions

The durability of the mechanical improvements brought by lime once the involved earth structures are exposed to long-term hydraulic conditions (long-term water contact and water circulation) is still under discussion. This paper aims at assessing i) the impact of such hydraulic conditions on the mechanical shear strength of a lime-treated silty soil, and ii) the effect of initial conditions (lime content and compaction conditions) on the lime treatment durability. The results showed that water contact leads to a significant decrease in shear strength. The results also indicated that the mechanical performance of soils exposed to water circulation is highly related to the quantity of water passing through the soil specimens and hence to the permeability of the soils: the lower the permeability, the better the durability. Futhermore, it has been observed that a lime content higher than the lime modification optimum (LMO) enables a better homogeneity of the treatment, giving rise to a longer resistance to water circulation

Chemo-mineralogical evolution and microstructural modifications of a lime treated pyroclastic soil

An extensive experimental multi-scale analysis was developed on a pyroclastic soil, coming from Central Italy, to investigate the link between the mineralogical and microstructural evolution of the system and the mechanical improvements induced by lime treatment. Chemo-mineralogical and microstructural evolution of the hydrated lime-soil system was monitored by means of Thermo-Gravimetric Analysis, 29Si Nuclear Magnetic Resonance, X-Ray Diffraction, Mercury Intrusion Porosimetry and Scanning Electron Microscopy. Rietvield analysis of X-Ray Diffraction results allowed a quantitative evaluation of amorphous phase and mineralogical components over time. Compressibility properties were measured by means of oedometer tests on raw and treated specimens. Amorphous phase plays a fundamental role in the development of pozzolanic reactions and in the formation of new cementitious compounds, whereas the contribution of zeolites, abundant in the raw soil, and other minerals is negligible. Formation of new phases induces microstructural modifications of the system, mainly detected within 28 days of curing. Chemo-mineralogical evolution of the system and the microstructural modifications affect the macroscopic behaviour of lime treated samples towards a structured behaviour, with a decrease of compressibility and an increase of yield stress

A multidisciplinary approach for the assessment of the environmental behavior of Basix Oxygen Furnace slag used in road construction

6th European Slag Conference, MADRID, ESPAGNE, 20-/10/2010 - 22/10/2010The production of steel slags in France was about 1.6 Mio.t in 2009, among them 0.735 Mio.t of Basic Oxygen Furnace (BOF) slags. The use of BOF slags has been restrined due to insufficient volume stability and lack of environmental regulation. The ECLIR research program has been launched in 2006 in order to implement a multidisciplinary approach for the assessment of the environmental behaviour of a BOF slag used in a civil engineering scenario. The main results of the study indicated that BOF slag contained relatively large amounts of chromium and vanadium which were mostly associated with stable phases consisting of iron oxides, and to a lesser extent, to more reactive calcium silicates. The monitoring of the quality of seepage water through a civil engineering structure built with a BOF slag, showed that these elements were present at very low concentrations and that the waters did not reveal real toxic effect. The atmospheric conditions were responsible for the carbonation of the surface layer of slag with a consequent decrease in permeability and increased leaching potential of vanadium

Environmental assessment of a BOF steel slag used in road construction : The ECLAIR research program

International audienceSteel production generates great amounts of by-products as steel slag. Unlike blast furnace slag, the use of Basic Oxygen Furnace slag (BOF slag) has been restrained due to insufficient volume stability and to the lack of environmental regulations. This study aimed at investigating the potential release and impact of pollutants, especially Cr and V that are present in rather high concentrations in slag, from a BOF slag used in a civil engineering structure (an industrial platform), using a multi-scale approach. The oneyear follow up of the experimental platform showed that concentrations of Cr and V were generally low in seepage waters, and in leachates from leaching test. Microanalyses carried out on slag allowed us to confirm the location of these metals in rather stable ferrous mineral phases, but V was also bound to more reactive silicates. No real toxicity effect of seepage waters has been revealed from eco-toxicological tests carried out with earthworms.La fabrication de l’acier s’accompagne d’une production d’importantes quantités de co-produits, les laitiers d’aciérie. Contrairement aux laitiers de haut-fourneau, l’utilisation des laitiers d’aciérie de conversion (laitiers LD) a été limitée en raison de leur instabilité volumique et de l’absence de réglementation environnementale. L’objectif de cette étude est d’étudier le relargage potentiel et l’impact des polluants, plus particulièrement de Cr et V qui sont présents à des concentrations assez élevées dans les laitiers, par un laitier LD utilisé dans une structure de génie civil (plateforme industrielle) à l’aide d’une approche multi-échelles