MICRO-STRUCTURAL CHARACTERIZATION OF CEMENT STABILIZED TROPICAL lATERITE CLAY SOIL CONTAMINATED BY HEAVY METALS

The presence of contaminated soils due to industrials and mining activities is a major concern in today’s heavily industrialized world. The contaminants lead to poor engineering properties for these soils. In this study 10% cement is used to stabilize laterite clay soil contaminated by heavy metals of Cu and Zn. However, the effect of these contaminants on the geotechnical properties of clayey soils can be altered through chemical stabilization using traditional stabilizers like cement, which result in achieving suitable material for construction purposes. The micro-structural characterization of the cement-treated Laterite clay is presented in this research. The changes in the mineralogy and morphology structure due to the effect of stabilizer of cement and heavy metals were discussed based on X-ray Diffractometry (XRD), and Field Emission Scanning Electron Microscopy (FESEM). According to the micro-structural characterization of the stabilized soils, cement was effective stabilizer in terms of improving the strength of contaminated treated soils. However, the heavy metals have retarded effect on the cement treated samples. This was due to precipitation of the metals onto the surface of calcium and aluminium silicates as insoluble hydroxides or sulphates. Thus, these compounds form an impermeable coating that acts as a barrier to inhibit cement hydration by impending transport of water into cement grain. Finally, it can be concluded that the mechanical and the physico-chemical behaviours of the compacted specimens, as determined during testing, formed the basis for evaluating both the degree of immobilization of the heavy metal in the soil matrix, and the potential for rehabilitation of contaminated sites.


1.Introduction
The global environmental issue today focused on the problem of contamination of land, water and air as a result of industrial development in the world.Thus, thousands of industrial around the world are contaminated by metallic pollutants, mainly due to waste deposits, chemical leakages, or fall-out from atmospheric emission.These sites can represent important source of heavy metals for the soils.However, at the same time it was noticed an increasing the presence of weak contaminated soils at various sites [1].
For that, various in situ and ex situ techniques have been used to reduce the impact of metals in the soil, including excavation, solidification, stabilization, soil washing, electroremediation and phytoremedition.
The chemical stabilization of clays by cement is a common methods that can be used to improve the properties of soil to provide a workable platform to construction projects [2].Cement is often used as an additive to improve the strength and stiffness of soft clayey soils [3][4][5][6][7][8].The researchers indicate that the injecting of cement into soil leads to the formation of a new structure within the soil grain accumulation.The term structure in this paper refers to the cementing materials resulting from pozzolanic reactions.
According to European standard EN197-1,"Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO 2 and 2CaO.SiO 2 ), the remainder consisting of aluminium-and ironcontaining clinker phases and other compounds.The chemical composition is expressed in terms of oxides, the substances are actually present in the form of compounds known as clinker minerals.The most important clinker minerals in the Portland cement are alite, belite, aluminate, and alumnate-ferrite [9].These four main constituents (as listed in Table 2) are the major strength producing components.The fundamental mechanism of soil cement stabilization has been outlined by [10][11][12] and [5].In general, mixing cement with water initiates chemical reaction named hydration, which forms a hard cement paste.When the pore water of the soil encounters the cement, hydration reaction of cement compounds (C 3 S) and (C 2 S) are occur rapidly.Calcium ions are rapidly released into solution and hydroxide ions form, causing the pH to raise.The reaction rate, and hence the strength gain are mainly controlled by; the ratio of C3S to C2S, which C3S gives a rapid-hardening cement, while C2S gives a cement that react slowly, the fineness of the grind, and the temperature.The hydration that may take place can be illustrated as shown in Figure (1) [12] Fig. 1 Steps of Cement Hydration [12] In this study, in order to accomplish and clarify the physicochemical changes prompted by stabilizers on the surface and interlayer of clay particles after contaminated by heavy metals, it was fundamental to describe the stabilization of the soil prior to and after being added contaminants.It was also important to refer to the engineering properties of the soils, heavy metals and the stabilizers that were used in this study, since they determined the effectiveness of the stabilization technique.

Materials
The soil used in this study was a natural soil deposits.The deposits were collected from a hillside (Balai Cerap) located in the campus of Universiti Teknologi Malaysia, Skudai [1].Typically, the natural laterite soil deposits have clay minerals constituents.In addition, they may also contain some sort of impurities such as salts and organic matters.In this study, contaminants were added to the deposits.As a result and in order to diminish the intervention of these materials and additives on soil-stabilizer reactions, more pure soil samples were required.It should be mentioned (as seen in Figure 2), the reddish brown colour for the Laterite clay soil is due to the presence high quantities of free iron oxides within the soil compositions [13].Furthermore, the tropical laterite clay can be characterized by the following properties [13] a) This type of soil is a little acidic in nature.b) It has high specific surface area value.d) This soil contains kaolinite, the prevailing clay mineral.e) The proportions of silicon and aluminium oxides are almost comparable in this soil.Ordinary Portland cement has been used as a stabilizer in this study.Due to its cost effectiveness, availability and compatibility with a variety of wastes, it considers the most widely used binders in stabilization technique [11].
The reagents that were added to emulate inorganic metal contamination ,were Copper nitrate tri-hydrateate (Cu(NO 3 ) 2 * 3 H 2 O) to provide the copper (II) ion (Cu 2+ ) ,and the zinc in the form of zinc nitrate tetra-hydrated (Zn(NO 3 ) 2 * 4 H 2 O) to provide zinc (II) ion (Zn +2 ).Both of them were from Merch KGaA-Germany.The two reagents were chosen due to their prevalence at many contaminated sites.

Sample Characterization
The characterization studies conducted in this study can be classified by the microstructural characteristics.The changes induced in soil stabilizer-metals matrix along curing time, were detected by using X-ray diffractometry (XRD), and Field Emission Scanning Electron Microscopy (FESEM).

XRD results
To assess effects from cement on soil mineralogical transformations, XRD examinations were applied mainly to samples with higher cement content (10% wt of soil).This limit content (10%) was used for all mineralogy and molecular tests in order to include the cementitious phase as stated by [14].As shown in Figure 3, major minerals present in natural Laterite clay soil were kaolinite, quartz, and gibbsite.These results were in agreement with findings of [15].Other reflections observed corresponded to Potassium propanoate at 2θ (24.9º and 33.38º).
X-ray diffraction patterns for post-cured (7, 100 and 200 days), untreated and chemically treated cement-Laterite clay soils are illustrated in Figure 4.As indicated, and in the context of comparison of effects from stabilizers on the soil, a number of new peaks indicative of the formation of crystalline reaction products were observed.Calcite minerals induced by the carbonation of calcium hydroxide or calcium oxide after exposure to CO 2 gas, were detected in cement stabilized lateritic clay for all curing periods.Furthermore, calcium hydroxide (portlandite) was identified in cement-lateritic clay; a primary product of cement hydration attributed to the Brucite group of minerals.
Tri-calcium aluminate (calcium aluminium benzene-sulfonate) was also detected in cement-lateritic clay at 30 and 90 days, which contributed slightly to early strength development.Some of these results were consistent with the results of [16,17].As the pozzolanic reaction developed in time, the formation of crystalline reaction products were observed by x-ray diffraction of cement lateritic clay.A poorly-crystallized calcium aluminum silicate hydrate (CASH) was detected at 200 days curing for both cement treated samples as shown in Figure (4).It is a Gismondine compound, which is attributed to the zeolite group.This compound is responsible for long-term durability as with age, as it forms a permanent compound that binds soil particles, thus, leading to increased strength in stabilized soils.With the progressing in time, this compound developed and slowly converted to a full crystalline phase as stated by [18].
While, fully crystalline compound (calcium silicate hydrate, CSH) was detected at the same age in cement treated laterite samples.It was a Tobermorite gel, which is produced as a result to hydration of tri-calcium silicate and di-calcium silicate.It plays a dominant role in the setting and hardening of cement paste and then the strength of soil-mixtures.Thus, this compound can be represented the heart of the concrete [19].Furthermore, iron rich member of the smectite groups (Nontronite) were observed in cement treated Laterite soil.This compound has variable amounts of adsorbed water associated with interlayer surfaces and the exchange of cations.The mineralogical changes induced due to contamination by adding heavy metals were also investigated by XRD analysis.Figures (5a,b) illustrate the X-ray diffraction patterns for cement stabilized Lateritic clay contaminated by Copper and Zinc.When comparing these results with uncontaminated cement-Laterite clay (Figure 4), products of cement hydration such as calcium hydroxide and other silica compounds were not visible during early curing.
However, over time up to 200 days, the presence of pozzolanic compounds such as CSH and CASH was detected, but in low quantities.This perhaps occurred due to the stabilized conversion of metals over time into cementitious compounds by adsorption on surfaces of pozzolanic products.This phenomenon agreed with statements made by [20].They each reported this despite the retardant effect from metals during early curing stages from their adsorption, precipitation, ion exchange, and chemical incorporation into cement compounds to form more complex compounds coated with C 3 S grains that prevent the transport of materials necessary to complete cement hydration.
However, hydration reactions continued over the long term due to the rising pH of pore water as a result of cement hydration and the dissolution of calcium ions.This indicated that heavy metals probably promoted cement hydration at a later period, thus, enhancing strength development despite their having initially delayed the hydration process.Another new mineral detected in copper-cement treated clay samples is Clinozoisite (Epidote group); which is a crystallized monoclinic system scientifically classified as an individual mineral species [21].

FESEM results
To study the effects of cement on the microstructure and morphology of the soil clay, it was employed a Field Emission Scanning Electron Microscope (FESEM) to visually examine hydration products and micro-structural changes in the matrix of cement/lime stabilized samples.
A micrograph of natural Laterite clay soil is presented in Figure 6.As expected in Laterite soil, free oxides within its microstructure coated and bonded with clay particles.This result was consistent with study of [22].The morphology of untreated soil reveals an insignificant, yet open type of microstructure with platy clay particles in a scattered composition.Over time and due to the effects of chemical treatment, a noticeable change in the morphologic texture of cement treated Laterite soil at both 100 and 200 days was observed.Soil particle clusters incorporated large openings in a modified flocculated texture with a flaky appearance at 100 days curing, with evidence of white lumps compounds.
Moreover, and as the pozzolanic reactions continued to 200 days, newly formed lamellar and reticular phases became very clear.The reticular texture observed in (Figure 7c) was attributed to the formation of CSH, which provided superior cementitious strength to that of CASH.Both phases had been recognized as tobermorite and laumontite by this X-ray diffraction studies, respectively.Thus, it appears that long term curing developed strength in cement stabilized clay soils and was markedly dependent on the continuous formation of new cementitious materials.
Field Emission Scanning Electron Microscope (FESEM) also has been used to investigate the morphology of the treated clay soils contaminated by heavy metals.Figure 8 explains the microstructure of cement-stabilized soils contaminated by heavy metals (copper and zinc).The micrographs of copper and zinc contaminate samples at 7 days reveal different textures from that of cement treated uncontaminated soil at 7 days.This was attributed to the coating of soil particles by large clusters, which are produced by the metals compounds reactions.Furthermore, mixing heavy metals with soil samples led to the production of new compounds as previously observed by X-ray diffraction.These compounds coated soil particles and prevented further hydration of cement as stated previously by [20,21].This may explains the reduction in compression strength when compared to uncontaminated soil as a result of the retardation of cement hydration.
After 100 days, the microstructure of contaminated samples modified towards flocculation with signs of reticulation, particularly in samples contaminated by copper.These alterations in texture may be attributed to the formation of aluminum oxide hydrate compounds as proved by the XRD.With age, hydration reactions were continuous and the crystalline phases progressed further for both copper and zinc contaminated soils, as observed at 200 days.This growth in texture was related to the formation of pozzolanic compounds (CSH and CASH).Note also that comparison of microstructures for contaminated and uncontaminated samples demonstrated different alterations in the crystalline structure of uncontaminated soils treated with cement, This, may give an evidence of dissimilar strength developments.

Conclusions
An experimental investigation on clay soils with various amounts of heavy metals, which were then stabilized by cement has been carried out in this project.The main objective was to study the physicochemical behaviour of cement treated tropical clay Mineralogy studies by XRD on cement treated Laterite samples revealed the presence of calcite during early stages of curing due to the carbonation of calcium hydroxide.However, over advancing curing time, the XRD analysis of cement treated Laterite samples showed the presence of the cementitious compound, calcium silicate hydrate (CSH), and the poorly-crystallized compound, calcium alumina silica hydrate (CASH), in addition to an iron rich member of the smectites groups (Nontronite).Compared to Laterite mineralogy.
Morphology analyses by FESEM detected free oxides within the microstructure of Laterite clay soil that coated and then bonded soil particles together.As appeared in early stages, the fabric of Laterite soil developed a flocculated texture with signs of reticulation due to the presence of Portland.With the progression of time, obvious changes in the morphologic texture of cement treated Laterite was noticed through decreasing in pore spaces between particles due to the formation of cementitious pozzolanic compounds (CSH and CASH) and the flocculated morphology of Kaolin's structure that modified the edges of its clay particles.
The reasons for retardant effects of heavy metals on cement hydration can be attributed to several mechanisms such as adsorption of metal compounds by the cement surface.This is only one significant mechanical hindrance which produces an impervious coating on the surface of CSH that then prevents water movement inside the cement grains; consequently retarding cement hydration.Precipitation of metals as hydroxides, sulphates and oxides (detected by XRD) has further retarded cement hydration and strength development due to precipitates on cement particles and the forming impermeable barriers.In addition, the presence of zinc nitrate in higher percentages (5% and 10%), led to the production of an amorphous layers of zinc calcium, zinc hydrate, and zinc nitrate hydrate as shown by XRD.These layers may also retard cement hydration and reduced the overall strength of treated soils.

Fig. 2 :
Fig. 2: The image of natural Laterite Clay soil from its excavated site

Figure 7
Figure 7 represents a micrograph of cement treated Laterite clay soil after (7, 100, and 200 days) and showing the transformation of soil fabric towards a more flocculated texture with signs of reticulation during the early stages of curing (7 days).Over time and due to the effects of chemical treatment, a noticeable change in the morphologic texture of cement treated Laterite soil at both 100 and 200 days was observed.Soil particle clusters incorporated large openings in a modified flocculated texture with a flaky appearance at 100 days curing, with evidence of white lumps compounds.Moreover, and as the pozzolanic reactions continued to 200 days, newly formed lamellar and reticular phases became very clear.The reticular texture observed in (Figure7c) was attributed to the formation of CSH, which provided superior cementitious strength to that of CASH.Both phases had been recognized as tobermorite and laumontite by this X-ray diffraction studies, respectively.Thus, it appears that long term curing developed strength in cement stabilized clay soils and was markedly dependent on the continuous formation of new cementitious materials.

Table ( 1
(2)resents a typical chemical composition of Portland cement while Table(2)presents components of Portland cement.

Table 1 .
Typical Composition of Portland cement

Table 2 .
The Components of Portland cement