New Insights into Potential Capacity of Olivine in Ground Improvement Mohammad Hamed Fasihnikoutalab Department of Civil Engineering, University Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia Corresponded author: [email protected]

Paul Westgate Department of Architecture and Civil Engineering, University of Bath, Bath, BA2 7AY, UK

Huat B.B.K Department of Civil Engineering, University Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia

Afshin Asadi HRC, Faculty of Engineering, University Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia

Richard J Ball BRE, Center for Innovative Structural Materials, Department of Architecture and Civil Engineering, University of Bath, Bath, BA2 7AY, UK

Haslinda Nahazanan Department of Civil Engineering, University Putra Malaysia, 43400 UPM, Serdang, Selangor,Malaysia

Parminder Singh Independent Civil Engineering Professional, 154 LorongMaarof Taman Bukit Bandaraya, 59000, Kuala Lumpur

ABSTRACT Today, cement is one of the most popular materials used in geotechnical engineering projects because of its excellent soil stabilizing properties. The big concern, however, is that its production results in the emission of a large amount of carbon dioxide (CO2) which contributes to the cumulative CO2released worldwide which is recognised as one of the main contributing factors to climate change and the greenhouse gas effect, which results in global warming. Olivine with chemical formula (Fe,Mg)2SiO4 is a suitable mineral for CO2 sequestration; additionally due to its high Magnesium oxide (MgO) content, it may be a sustainable material for soil stabilization. The chemical combination of Olivine categorizes this mineral in the class of pozzolans. Magnesium oxide has a high potential for CO2 capturing and carbonated magnesia provides stability in soil and is thus an active layer in works on stabilizing slopes against sliding.

KEYWORDS:

Olivine, Magnesium oxide, climate change, CO2 sequestration, Pozzolanic material, soil stabilization

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INTRODUCTION Unfortunately the use of cement comes with several significant disadvantages, the most important of which is the resultant release of carbon dioxide (CO2) into the atmosphere. Carbon dioxide is the leading greenhouse gas which is an outcome from human activity and causes climate change and global warming. In the 1950s, CO2 doping grew about 0.7 ppm per year. In the last decade, the CO2 level significantly increased by2.1 ppm per year (WAYMAN, 2013). Olivine is a sustainable material that has the potential for use in treating soils as it can capture CO2. Cement is today widely used as the main binder in ground improvement due to its many properties. As an additive in soil, it minimizes soil settlement. Today cement is one the main binders in ground improvement. Using cement as an additive in soil reduces the settlement, inhibits shear deformation, helps excavation, blocks sliding failure and averts liquefaction. This article discusses the properties and potential application of olivine in geotechnical engineering.

CHEMICAL AND PHYSICAL PROPERTIES OF OLIVINE Olivine with the formula (Mg,Fe)2SiO4 is a magnesium iron silicate. It is a prevalent mineral within the Earth's subsurface and is usually found in mafic to ultramafic igneous rocks. It is found less commonly in marbles and some alternative metamorphic rock types (JESSA, 2011). The ratio of magnesium to iron can vary in any proportion from pure Mg2SiO4 (fosterite) through to pure Fe2SiO4 (fayalite).Olivine can exist with colours ranging from yellowish green, olive green, greenish black and reddish brown with densities from 3.27 to 3.37 and averaging 3.32 g/cm3 (Barthelmy, 2010). Table 1&2 show the nominal chemistry and physical properties of olivine, which, according to chemical composition, consists of approximately 45% to 49% of MgO. Table 3 shows the physical properties of olivine; with pH slightly basic and the bulk density of it is between 1.45 and 1.75 g/cm3.

Table 1: Chemical properties MgO 45% to 49%

SiO2 39% to 42%

FeO 6% to 8%

CaO 0.2% to 0.3%

Al2O3 0.2% to 0.8%

Table 2: Physical properties of olivine Melting

Nominal

Specific

Fusion

Loss on

Mohs

point

Bulk

Gravity

Point (°C)

Ignition

Hardness at

(°C)

Density

Free Silica Content (%)

20°C

3

(g/cm ) 1800

1.45-1.75

3.2-3.4

2800-3200

<1.5%

6.5-7

<0.1

Figure 1shows the perfect olivine structure contains of a hexagonal close packed array of oxygen atoms in which one half of the available octahedral voids are occupied by magnesium atoms and one eighth of the available tetrahedral voids by Si atoms (Birle, Gibbs, Moore, & Smith, 1968). Olivine crystals in the groundmass are subhedral to anhedral, and have grain sizes from 10μmup to 500 μm in length (Gross et al., 2012).

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Figure 1: Crystal structure of olivine

OLIVINE WORLDWIDE DISTRIBUTION Olivine is found all over the world: India, Egypt, Myanmar, South Africa, Russia, Pakistan, Norway, Sweden, France, Brazil, Germany, Mexico, Ethiopia, Australia, China and the US. As early as 1500 BC Egyptian pharaohs mined olivine on Zabargad Island in the Red Sea. Olivine can be detected in the green beaches in Hawaii as well as in meteorites, on Mars and on the Moon (Olivine, 2013). In Malaysia, according to its geology there is a possibility of olivine being mined. According to a study of the Tawau geological heritage area located in the eastern part of Semporna Peninsula, in Sabah, West Malaysia, in this area, volcanic rocks of the andesite-dacite- association form the major mountainous backbone. The rocks contain plagioclase, olivine, clinopyroxene, hornblende phenocrysts and magnetite microcrystals (Tahir, MUSTA, & Rahim, 2010). There is every possibility of surface mining the substantial amount of olivine that exists here. Alternatively in tropical areas, the great advantage of this is the fast weathering in tropical areas (Schuiling & Praagman, 2011).

OLIVINE WEATHERING Dissolution and hydrolysis is the result of reactions with acids. In hydrolysis occurs as the transformation of silicate and carbonate minerals into new minerals. In this process, there is total dissolution of the original rock leaving no solid residue (Kayar, 2011). It can be generally explained that olivine dissolution occurs when carbonate saturation is achieved in the fluid stage, resulting in the precipitation of magnesite, and summarized as (Dufaud, Martinez, & Shilobreeva, 2009):

Mg2SiO4 + 3H2O + 2CO2 → Mg2+ + Mg3Si2O5(OH)4 + 2HCO−

(1)

The study has proven that surface reaction determines the effects of olivine dissolution kinetics, at low temperature, far from equilibrium, and in aqueous solution. This proton driven mechanism involves an ion exchange reaction preceding an adsorption reaction (Prigiobbe & Mazzotti, 2011). In an important investigation of olivine dissolution at 25 °C and the effect of pH, CO2 and organic acid, the following was shown: in basic solutions, when the partial pressure of CO2 equals atmospheric levels (PCO = 10 -3.5atm), olivine dissolution rates are nearly pH independent throughout the pH range 6 to 12 and are about equal to the minimum rate of dissolution under CO2 purged conditions. However, at pH 11, the presence of atmospheric levels of CO2 decreases the dissolution rate by over an order of magnitude to 10-14mol cm-2 s-1. Additionally, experiments undertaken in the acidic and near neutral pH ranges show that organic ligands chelate surface Mg increasing the olivine

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dissolution rate (Roy a. Wogelius & Walther, 1992). In another study, investigating the kinetic dissolution of olivine near the surface, the results of the batch experiments at 65°C on forsteritic olivine indicated the strong dependence of the olivine dissolution reaction temperature (RA Wogelius & Walther, 1991). In2000, the investigation of the dissolution of forsteritic olivine at 65°C and 2< pH < 5, at various increased temperatures indicated that the dissolution rate of forsteritic olivine was more pH dependent (Chen & Brantley, 2000). Earlier research findings on the characterization of the dissolution rate of olivine at low temperature can be extended to higher temperature, which could include its behaviour as a function of pH, in the presence of CO2, and at pH ≤ 5 (at 120 °C).Dissolution rates have been found to be two times greater than those without CO2 at the same pH. Citric acid, another ligand previously studied in the literature, also exhibits a dissolution enhancement effect (Hänchen, Prigiobbe, & Storti, 2006). As a result, certain scholars have pointed out that sorption of inorganic carbon species to surfaces can affect dissolution behaviour for Fe oxides and Ca or Mg silicates. Even under alkaline states; silicate dissolution rates demonstrate only a negligible or at best weak dependence on PCO2 when pH is held constant. For instance, the liberation of Ca and Mg from diopside during dissolution decreased slightly or not at all for PCO2 > atmospheric (Brantley, 2008). Mineral carbonation is dependent on how CO2 reacts to metal oxide bearing materials in the creation of insoluble carbonates, with Ca and Mg being the leading metals. In the natural weathering of silicates this is referred to and occurs in the context of a geological time scale. It involves naturally occurring silicates because the source of alkaline and alkaline-earth metals and expends atmospheric CO2 (Mazzotti, 2005). Olivine could be a suitable candidate for CO2 sequestration as a result of: first, it is present in basalt; second, it is a neosilicate, the silicate cluster with the lowest ratio of Si cations, therefore the best rate of dissolution and also the highest capability for carbon capture; third, olivine does not contain aluminium, that tends to create clays, so removing a number of the cations that could form carbonate minerals; and fourth, mafic and ultramafic rocks are basic, contributing neutralizing capacity for the protons created when CO2 reacts with H2O to make H2CO3, thus permitting additional mineral carbonation to proceed to what would occur throughout weathering of felsic rocks, like granite or andesite (Olsson et al., 2012). There are so many factors that effect of dissolution rate of olivine, some most important factors are: the grain size, temperature, solution chemistry (pH, concentration of carbonate, magnesium, silica, organic acids, ionic strength) and the formation of the coating on the grains (Veld, Roskam, & Enk, 2008). Mineral carbonation, that involves the reaction of Mg-rich minerals with CO2 to create geologically stable mineral carbonates, has been proposed as a promising CO2capturing technology. Suitable feed stocks embody olivine (Mg2SiO4) and serpentine (Mg3Si2O5(OH)4) minerals, and the general reaction is shown as follow (Maroto-Valer, Fauth, Kuchta, Zhang, & Andrésen, 2005):

(Mg,Ca)xSiyOx+2y + xCO2 → x(Mg,Ca)CO3 + ySiO2

(2)

The carbonation reaction of Mgrich olivine will be described in three steps. Firstly CO2from the atmosphere dissolvesin offered water formingcarbonic acid with a pH around 5.6. The second reaction is the dissolution of Olivine incarbonated water. The third step is the precipitation of carbonates and silica (Haug, Kleiv, & Munz, 2010).

Mg2SiO4(S) + 2H2O + 2CO2 → 2Mg2+ + 2CO32- + H4SiO2

(3)

The mineral carbonation can take place in carbonic acid or through the solution of CO2 in water. This was seen in a study which set out to determine carbon dioxide sequestration by direct mineral carbonation with carbonic acid. The study explained that the CO2 is dissolved in water to form carbonic acid (H2CO3), which dissociates to H+ and HCO3-; the H+ reacts with the mineral, liberating

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Mg2+cations which react with the bicarbonate to form the solid carbonate. In another major study it was explained that direct mineral carbonation is an operation to convert gaseous CO2 into a geologically stable, solid final form. This process utilizes a solution of sodium bicarbonate (NaHCO3), sodium chloride (NaCl), and water, mixed with a mineral reactant, such as olivine (Mg2SiO4) or serpentine [Mg3Si2O5(OH)4].The direct mineral carbonation method utilizes a slurry of fine particlesized mineral in the water, at solids concentrations from 15-30%. Decomposition of the mineral and succeeding carbonation occur in an exceedingly single unit function. The theorized reactions (4), (5) and (6) follow (O’Connor, Dahlin, & Nilsen, 2000, O’Connor, Dahlin, & Nilsen, 2001):

CO2 + H2O → H2CO3 → H+ + HCO3Mg2SiO4 + 4H+ →2Mg2+ + H4SiO4 or SiO2 + 2H2O Mg+2+HCO3-→ MgCO3+ H+

(4) (5) (6)

In a study which sets out to determine the direct dry carbonation for CO2 emission reduction in Finland the researchers pointed out that for mineral carbonation the utilization of magnesium primarily based silicates, xMgO•ySiO2 •zH2O is favoured as they are available in large amounts worldwide. Magnesium silicates can be divided into several subgroups and the largest quantities are olivine, (Mg,Fe)SiO4, and serpentine (Mg3Si2O5(OH)4; while other appropriate minerals exist but in smaller quantities. The chemistry of CO2 stabilization can be summarized in reactions (7) and (8) as below (Zevenhoven, 2002):

xMgO.ySiO2.zH2O (s) → xMgO (s) + y SiO2 (s) + zH2O MgO (s) + CO2 →MgCO3 (s)

(7) (8)

Ca and Mg-silicate minerals can be obtained from igneous rocks, which are suitable for CO2 fixation due to the fact that they are basically free from carbonates. The main candidates are magnesium-rich ultramafic rocks such asdunites, peridotites, and serpentinites. Table 3 shows the composition of some candidate rocks and minerals and their particular CO2 capturing capacity, RCO2 (Huijgen & Comans, 2007).

Table 3: Composition of some selected rocks and pure minerals and their potential CO2 sequestration capacity ROCK Mineral Serpentinite Serpentine, Mg3Si2O5(OH)4 Dunite Olivine, Mg2SiO4 Basalt

MgO

CaO

RCO2

[wt%]

[wt%]

[kg/kg]

~40

~0

~2.3

48.6 49.5 57.3 6.2

1.9 0.3 9.4

1.8 1.6 7.1

Note: RCO2 = mass ratio of rock to CO2 required for CO2 sequestration

Studies by (W. K. O’Connor, Dahlin, Rush, Gerdemann, & Penner, 2004), (Munz et al., 2009), (Olsson et al., 2012) describe the advantages of mineral carbonation, the biggest of which is the long term safety of the repository. Energy assessment of a one-step method shows a net sink for CO2 and on average 76 % of the CO2 can be removed, when olivine is used. A further advantage is that, while

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stable, the solidified CO2 is immobilized on a geological time scale. With the vast capability worldwide; there is abundant sequestration material accessible to bind the CO2. Carbonate mineral formation is energetically favourable; the thermodynamic driving force promotes these reactions. Preliminary work on olivine shows it can react fully with CO2, trapping up to (57 ± 2) % (equation of initial CO2) as magnesite with some amorphous silica additionally formed. Olivine grain diameter and solution mineral ratios were identified as important factors in controlling the reaction with salinity acting as a second order parameter. During the experiments, fluid analyses might not be performed with the approach adopted but, geochemical modelling was attempted to relinquish data regarding the preferred decomposition. This showed an interesting mineral matrix evolution. Under the experimental conditions, olivine appeared to be a sensible candidate for CO2 trapping into a geologically stable carbonate, magnesite. The potential use of mafic and ultramafic rocks for CO2 sequestration is mentioned (Garcia et al., 2010).The study of dissolution of mechanically activated olivine for carbonation shows the dissolution rate dependency on crystallinity, specific surface area and particle size distributions in pH drifting experiments (Haug et al., 2010). In another major study about the factors that affect the direct mineralization of CO2 with olivine, laboratory scale CO2 carbonation with olivine was carried out as a demonstration for CO2 capture directly out of exhaust streams without the need of olivine pre-treatment. Different factors including carbonation temperature, inlet CO2 concentration, residence time, and water vapour concentration affect the olivine based CO2 mineralization to various degrees. The thermodynamic calculations for the CO2 carbonation process show that substantial heat energy is dropped during the reaction between CO2 and olivine. The heat can be recycled as a part of the energy resource required for heating olivine to the specified temperature prior to its reaction with CO2 (Kwon, Fan, DaCosta, & Russell, 2011). As a result when olivine comes into contact with water it consumes the CO2 and separates into sand, magnesium and bi-carbonate, where this reaction can be described as:

SiO2 HCO32Mg2+

Mg2SiO4 + 4CO2 + 2 H2O 2Mg2+ + SiO2 + 4HCO3(Silicon oxide, also known as quartz) (Hydrogenbicarbonate) (Magnesium ions)

(9)

Effect of grain size on Olivine carbonation Diminution of grain size causes an increase of the surface area and has an effect on the degree of carbonation (Veld et al., 2008). A study described that particle size was recognized as a significant factor of reaction, as most mineral dissolution reactions are surface controlled, as a measure of the stoichiometric conversion of the silicate to the carbonate, increased dramatically with decreasing particle size, to over 90% for the test conducted on 37 micron olivine (W. O’Connor et al., 2001). In order to attain an adequate reaction rate the minerals should be ground. The reaction rate will augment the surface area. Among others, O’Connor et al. inquired the influence of the particle size of the conversion. These researchers found that a reduction from between 106 and150µm to less than 37µm increased the conversion in their experiments from 10% to 90% (Huijgen & Comans, 2007). Smaller particle sizes are weathered by chemical means more rapidly compared to larger particles due to an increase of surface area. Figure 2 shows that as the particles get smaller, the total surface area accessible for chemical weathering increases (Stimac, 2004).

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Figure 2: Illustration of the increasing surface area obtained as particle size is decreased for a constant volume of material

Effect of pH, temperature and CO2 pressure on olivine dissolution and carbonation The impact of temperature is interrelated with many alternative factors. At higher temperatures the dissolution of forsterite is increasingly pH dependent and plant degradation that leads to organic acids is quicker. In keeping with previous studies, the dissolution of olivine can proceed 10 times faster at a temperature of 298°K compared to 273°K (Olsson et al., 2012). The previous study involved the effect of pressure on the carbonation of olivine. They compared the influence of pressure and temperature on the result of olivine carbonation, increasing the temperature to 185°C whereas holding PCO2 constant resulted in over 65% extent of reaction in six hours. The degree of reaction was improved further, to almost 85%, by increasing the PCO2 to 115 atm while holding the temperature constant at 185°C (W. O’Connor et al., 2001).

POTENTIAL APPLICATION OF OLIVINE IN GROUND IMPROVEMENT Regarding the chemical properties of olivine as shown in Table 1, olivine is the major source of magnesium oxide (MgO) which makes up between45% to 49% by weight. Prior research has demonstrated that magnesium oxide can be successfully utilized for soil stabilization, for example, the use of Magnesium hydroxide to stabilize swelling clay (Xeidakis, 1996). Another investigation studied how low grade MgO affected the stability of contaminated soil, and the results showed that the contaminated soil was successfully stabilized with low grade MgO (García et al., 2004). A significant study conducted in 2010 addressed that use of sustainable materials for soil stabilization. The study investigated the effect of mixing industrial by-products with innovative materials such as reactive magnesia and zeolite. These were demonstrated to clearly show a variety of sustainability benefits over PC in terms of reduced environmental impacts and enhanced technical and durability performance (Jegandan, Al-Tabbaa, Liska, & Osman, 2010). Another investigation considered the properties of two soil types, each with different blenders and proportions of ground granulated blast slag (GGBS), lime, MgO and PC. The investigation focused on the effect of mixed MgO and GGBS and compared the results with the use of PC and a mix of GGBS-lime on the stability of soil (Y. Yi,

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Liska, Unluer, & A, 2013). A recent study in 2013 investigated the effects of carbonating magnesia on soil stabilization by comparing it with PC blended in soil. The results showed that soil treated with reactive carbonated magnesia after a few hours had the same stability of soil stabilized by PC after 28 days. The main reaction products of carbonated magnesia responsible for the increased stability were nesquehointe and hydromagnesite-dypingite (Y. Yi & Liska, 2013). The formulations below show the main products of reactive carbonated magnesium:

MgO + H2O → Mg(OH)2 Mg(OH)2 + CO2 +2H2O → MgCO3 . 3H2O 5Mg(OH)2 + 4CO2 + H2O → (Mg)5(CO3)4(OH)2 . 5H2O 5Mg(OH)2 + 4CO2 → (Mg)5(CO3)4(OH)2 . 4H2O

Brucite Nesquehonite Dypingite Hydromagnesite

(10) (11) (12) (13)

Additionally, according to the literature, there have been many studies undertaken on the use of MgO in a variety of cements and as additives in concrete. The results showed that magnesium oxide is a good additive for combination with cement in concrete (Ali & Mullick, 1998, Altun & Yılmaz, 2002, Liu, Li, & Zhang, 2002, Harrison, 2003, Liu & Li, 2005, Li & Chau, 2008, Vandeperre, Liska, & Al-Tabbaa, 2008, Liska & Al-Tabbaa, 2008, Cwirzen & Habermehl-Cwirzen, 2012, Unluer & AlTabbaa, 2013). Furthermore, in accordance with ASTM D5370, and Table 1, olivine can be considered as a pozzolanic material because of the substantial amount of SiO2, Fe2O3, and Al2O3, which have the potential to contribute to soil improvement. The Fe2O3, Al2O3 and SiO2 amount to almost 50% by weight. A pozzolan is a siliceous and aluminous or siliceous material that on its own has no cementitious properties but, in finely divided form and in the presence of moisture, will chemically react with calcium hydroxide (lime) at ordinary temperatures to form cementitious compounds(Dunstan, 2011) . The pozzolanic reactions associated with SiO2, Al2O3 and Fe2O3are shown below(Beeghly & Schrock, 2010):

Silicates: (calcium silicate) SiO2 + Ca(OH)2 + H2O → CaO-SiO2- H2O Aluminates: Al2O3 + Ca(OH)2 + H2O → CaO-Al2O3-H2O (calcium aluminate) Ferro Aluminates: Fe2O3 + Al2O3 + Ca(OH)2 → CaO-Al2O3-Fe2O3-H2O (calciumferroaluminate)

(14) (15) (16)

The effects of pozzolanic materials on the properties of building composites were studied and specific surface, particle size, and amorphousness of the materials were found to be affective factors. It was observed that an increase of specific area and/or decrease in particle size would result in increased chemical reaction. There was also evidence of higher levels of reactivity in amorphous structures, compared to crystalline forms, attributed to the higher energy level of their atoms. It was also found that there was clear relationship between compressive strength and the pozzolanic reactivity of the resultant composite. Yet another study noted that the strength is chiefly determined by the amorphousness and particle size of the pozzolan, indicating a strong relationship between increased amorphousness and improved strength for all pozzolans(WALKER & PAVIA, 2010) . In light of various recent investigations, it is clear that there is great interest in the use of MgO for soil stability in cement and for CO2 capture. While it is advantageous to use MgO for binding purposes in cement and soil because it is environmentally friendly, the downside is that MgO cannot be found in nature. MgO for industrial use is formed by heating magnesium carbonate (MgCO3) to release CO2 (decarboxylation). The associated release of CO2, into the atmosphere has raised some environmental

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concerns due to its activity as a greenhouse gas. However, olivine is a mineral with a high source of natural MgO which has the potential to capture CO2.It shows great promise as an additive for ground improvement in both the uncarbonated and carbonated states. The potential of olivine for applications in geotechnical engineering is significant and more studies should be carried out to further investigate its properties and its potential for industrial applications.

CONCLUSIONS This paper introduces the concept of using olivine as a sustainable material capable of contributing to climate change control, soil stabilization and slope stability. Olivine (Fe,Mg)2SiO4) is a major source of magnesium oxide which can capture CO2 and also act as an efficient binder for the stabilization of soil. Composition wise, it is classified as a pozzolanic material comparable to class C flies ash according to ASTM. The use of olivine for soil stabilization has two benefits: natural CO2sequestration, and contributing to soil stability by acting as an active layer for slope stability against sliding. The origin of olivine is from mafic and ultramafic rocks with high source of MgO, which is generally an excellent substitute for cement in making concrete and also in soil stabilization. The high potential of olivine as a material for soil stabilization and climate change requires further and extensive research into new methods of using this mineral in geotechnical engineering.

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