Partial Replacement of Reactive Magnesium with GGBS has an Impact on the Geotechnical Properties of Soil

This study is an attempt to examine proper usage of GGBS in the field of Civil Engineering to reduce existing dumping problems and escalate the geotechnical properties of inadequate soil. According to laboratory findings, certain soil qualities that were treated with MgO and GGBS at specific concentrations exhibited remarkably beneficial changes when compared to natural soil. The compressive strength value of modified soil improved, showing increase in the bearing capacity of treated soil The increased CBR value of modified soil both in soaked (4 days) and unsoaked condition indicates the economical designing of flexible pavement by reducing the thickness of base and subbase layers.


Introduction
Ground granulated blast furnace slag (GGBS) when activated with an alkali activator, acts as a sustainable binder to Portland cement (PC) pertaining to its low energy consumption and energy utilization, low initial capital cost leading to low amount of carbon dioxide (CO2) emission.Type of activator and its content, GGBS composition and curing condition also effects the properties of activated slag.Alkali activator dissolves in water to start the slag hydration mechanism, which is followed by covalent bond breakdown (Si-O-Si, Al-O-Al and Ca-O).Within minutes of covalent bonds breaking, an impenetrable coating forms on the slag grains followed by the creation of hydration products such hydrotalcite and calcium-silicate-hydrates (C-S-H) (Ht).One of the regulating factors for starting the hydration mechanism of alkali activated GGBS has been discovered to be pH.GGBS has previously been activated using a variety of alkali activators, including lime, NaOH, KOH, cement, and Na2SO4 etc (Choudary Y et al, 2022).1327 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1327/1/012008 2 Many researchers have recently employed reactive magnesia (MgO) as a novel activator for stabilising GGBS blended soil.The strength of GGBS activated by Ca(OH)2 and MgO activated GGBS at two distinct activator contents of 5% and 10% were evaluated by (Yi, Y. et al. 2012) who came to the conclusion that the latter one had a 30% higher strength than the former.According to Benhaha (Fruhwirth et al. 1985), the main hydration products created during the hydration of MgO-GGBS are C-S-H and Ht similar phases, which give the soil matrix less porosity and more strength.
The current study seeks to investigate the appropriate application of GGBS in the field of civil engineering in order to lessen its problem of dumping and enhance the geotechnical qualities of deficiency soil.

Materials Used
The natural soil sample was collected from Sitarganj region of Udham Singh Nagar district, Uttarakhand, India at 0.60 m depth from the top soil, which was removed to avoid vegetation and organic matter.Before conducting any test, the soil was mixed thoroughly.The magnesium oxide, purchased from the depot of Almora Magnesite Limited (A Joint Undertaking of U.P.S.I.D.C. Tata Refractories Ltd. and S.A.I.L.) in Haldwani, was used an activator for GGBS obtained from Ecogen Industries Private Limited, Dehradun.The GGBS was purchased from Ecogen Industries Private Limited, Dehradun, Uttarakhand, India and was kept in an airtight container so as to avoid lump formation.The following table shows the chemical composition of GGBS:   x Third phase: MgO was added to GGBS in fixed proportion i.e. 1:4 so as to activate its hydration process with varying dosage binder content (5%, 10%, 15%, 20%, 25%).Thereafter, the modified soil was subjected to a range of tests such as consistency limit, standard proctor test, UCS and CBR so as to examine its enhanced behaviour and modified geotechnical properties.The specific gravity of GGBS, PC (Portland cement) and virgin soil was determined using Pycnometer method in accordance with IS: 2720(Part III section I/II)1980.The soil was passed through 4.75 mm IS sieve and was then oven GULHG DW Û& WR Û& WHPSHUDWXUH IRU IXUWKHU determination of the specific gravity.A sample of soil, GGBS and PC, was passed through 1 mm and 425 μ IS sieve, kept in an oven for drying and then the usual procedure was followed.Kerosene was used as a standard fluid for GGBS and PC because of their high affinity for water.

Standard proctor test (SPT) (IS: 2720 Part VII-1980)
The testing apparatus comprises of a 1000 cc cylindrical mould of 127.30 mm height and 100 mm diameter with a detachable base plate and collar of 50 mm height.The soil was filled in the mould in three layers and each layer was compacted by 25 number of blows by standard hammer (weight 2.60 kg) falling through 310 mm height.Compaction curve has been plotted between moisture content and dry unit weight.The peak of compaction curve corresponds to the maximum dry unit weight is denoted by Ȗd(max) and the moisture content corresponding to Ȗd(max) is known as optimum moisture content (OMC) at a given compactive effort.

Unified compression test (UCS) (IS: 2720 Part X-1991)
The test specimens were prepared by oven drying the soil passed through 425 μ IS sieve and then were mixed uniformly with water at OMC. Cylindrical soil specimens, prepared at OMC had initial diameter of 3.80 mm and 7.10 mm height.To examine the effect of different binder contents (GGBS, PC, and MgO activated GGBS) the samples of soil were subjected to 0, 7, 14 and 28 days of curing time.After curing, the specimens were subjected to the normal axial load at a strain rate of 1.25 mm per minute and the corresponding displacements were noted for strength determination through graphical representation of stress and strain.

California bearing ratio (CBR) (IS: 2720 Part XVI-1987)
For the preparation of the soil specimen, 5.00 kg of oven dried soil was mixed with water homogeneously at OMC and then filled up in the mould of 150.00 mm diameter and 175.00 mm height in three layers for light compaction.The mould is provided with a detachable extension collar of 50.00 mm height and a detachable perforated base plate 10 mm thick.Further, each layer was compacted by 55 blows with hammer (2.50 kg weight).After compacting the soil at OMC, the mould was set at the CBR testing machine at which the plunger of 50.00 mm diameter was allowed to penetrate into the soil for CBR value determination at 2.50 mm and 5.00 mm penetration.The dial gauge in the CBR test measures the load and the proving ring measures the corresponding penetration resistance of soil against the load.The test was conducted for both soaked and unsoaked conditions.For soaked condition, the mould was kept in water for 4 days and then thereafter, CBR value was noted at 2.5 mm and 5.00 mm penetration.
According to the Unified Soil Classification System (USCS), the soil was categorized as CL (clay with little plasticity) based on the results of its index properties and engineering properties of soil as shown in Table 2.

Binders' impact on consistency limits
The effect of inclusion of binders (MgO:GGBS=1:4) on the consistency limits of the expansive soil is shown in Table 3.When MgO activated GGBS was added to the soil to be tested, the liquid limit increased slightly at first, but as more binders were added, the liquid limit decreased, resulting in a decrease in plasticity index.The plastic limit (P.L) of soil increased from 19.81% at 0% binder

Binder's impact on the characteristics of compacted soil
Figure 2 illustrates how the maximum dry density (MDD) and optimum moisture content (OMC) of the test soil are affected by the addition of a binder.According to the plotted graphical representation in the preceding figures, the OMC increased as the binder content rises, but the corresponding MDD slightly decreased.Addition of binder content upto 20% reduced the dry unit weight of blended soil from 18.61 kN/m³ to 17.39 kN/m³.Since, the compactive effort was applied immediately after mixing, no chemical-reaction took place for GGBS alone blended soil, but for MgO activated GGBS mixed soil, MgO acts as an activator which accelerated the hydration process.The clay particles flocculate with the hydration products formed, thus, forming bigger sized particles and occupying and increasing the void spaces and thus reducing the dry weight.Formation of the cementitious products, causes resistance to compaction which can also lead to reduce in MDD.The rise in OMC is explained by, the water added was being used up immediately for the hydration process because of the accelerating impact of reactive magnesia (MgO) and thus, resulting in higher affinity for water.The reaction between MgO and Si present in GGBS and soil is mainly responsible for increase in OMC as the added MgO gets completely dissolved in the water rendering an alkaline environment which follows breaking of covalent bonds b/w Si-O and Al-O, resulting in the formation of hydrotalcite, a hydration product of GGBS-MgO mix soil.
Fig. 2. Variation in OMC and MDD

Binder addition's impact on strength characteristics
The strength of various soil-binder mixture combinations is evaluated using the UCS test.OMC and MDD prepared the UCS specimens.To measure the strength growth for each combination, the test was conducted for curing periods of 0, 7, and 28 days.With the inclusion of MgO-activated GGBS, the UCS strength of the modified soil increased dramatically.The strength of GGBS increases as a result of the MgO's faster hydration effect, which occurs right after mixing.The mechanism of MgO activating GGBS followed complete dissolution of Mg2+ ions and OH-ions directly.The MgO dissolves completely into water present in the pores to Mg2+ ions and OH-ions.These OH-ions activates the GGBS, forming calcium-silicate-hydrates (CSH) and hydrotalcite like phases.The dissolution rate of reactive magnesia is much faster due to its higher solubility, resulting in its higher activating efficiency.These hydration products, deposits in pore spaces available and bind the clay particles more efficiently thus, forming a compacted soil matrix.Hence, more is the MgO content, more is the formation of hydration product, hydrotalcite in the soil matrix resulting in increased early UCS of MgO activated GGBS mixed soil.Therefore, based on the UCS behaviour of soil (to be tested) with MgO-GGBS, 15% binder incorporation is suggested as an optimum content to effectively stabilize the expansive soil

CBR Impact of Binder Addition
The CBR is an index of the soil's mechanical strength.The test is based on the penetration resistance offered by soil to standard size piston.Fig. 4 shows a dramatic rise in the soaked CBR value on addition of MgO activated GGBS to the native soil.This increment in the soaked value was about 12 times the native soil.The unsoaked CBR increased significantly up to 20% addition of the binder content.The value of CBR in both soaked and unsoaked conditions gradually increased with the increase in binder concentration.In MgO-GGBS treated soil, the development of cementitious gel-like substances such hydrotalcite (Ht) and calcium-silicate-hydrates is responsible for the rise in CBR value (C-S-H).These gel like compounds have positive effect on filling the pore spaces and densifying the soil skeleton, resulting in higher CBR value.A denser structure is obtained on increasing the curing time of the specimen, which may be due to the chemical reactions and dissolution of MgO into the pore water, explaining the higher CBR value of soaked CBR than unsoaked condition.MgO-GGBS was added to test soil at a mixing ratio of 1:4 and was tested for its different combinations so as to access its effect on the engineering properties of soil.According to the results of extensive research conducted, by carrying out various experimental tests, following conclusions are made: x The liquid limit (WL) and Ip i.e. plasticity index (P.I) of modified soil decreases with rise in binder content while P.L increases.This decrease in P.I, is may be due to the non-plastic nature of slag and flocculation of clay particles.
x As the OMC increased, MDD decreased correspondingly it is due to the water added was being used up immediately for the hydration process because of the accelerating impact of reactive magnesia (MgO) and thus, resulting in higher affinity for water and formation of the cementitious products, causes resistance to compaction which can also lead to reduce in MDD.
x The UCS increased with binder content and curing period upto 15% MgO-GGBS addition.
The increase in UCS value was significant, suggesting the optimum content to effectively stabilize the expansive soil at 15% x The CBR of soil increased progressively as the percentage of binder content increased.It was also found that the value of Soaked CBR of soil is more than Unsoaked CBR is due to these gel like compounds have positive effect on filling the pore spaces and densifying the soil skeleton, resulting in higher CBR value also a denser structure is obtained on increasing the curing time of the specimen.x Cementitious gel like compounds such as hydrotalcite (Ht) and calcium-silicate-hydrates (C-S-H) are formed which have good effect on the strength property of soil and is primarily responsible for the high soaked CBR of the modified soil.

Limitation
There are still a lot more research areas to be covered in the future studies.The important points are summarized below: x More tests can be performed for examining the effect on durability, swell potential, shear strength and hydration properties of MgO-GGBS mixed soil.
x Analytical work such as X-ray diffraction (XRD), Scanning electron microscopy (SEM), Mercury intrusion porosimetry (MIP) and Thermogravimetric analysis (TGA) can be carried out for detailed microstructural and mineralogical studies.x As the subgrade of the pavement is subjected to continuous static and dynamic loading, further research can be carried out to study the effect of cyclic loading on the swelling behavior and strength of the treated soil

Fig. 3 .
Fig. 3. Variation in UCS with different % of Binder at Different Curing time

Table
. Chemical composition of GGBS (Sharma and Sivapullaiah 2016) Geotechnical properties of the collected soil were characterized by conducting various laboratory experiments in accordance with Bureau of Indian Standards (BIS).Laboratory tests such as grain size distribution, consistency limit test specific gravity test, standard Proctor test for MDD and OMC determination, Unified compressive strength (UCS) test, California Bearing Ratio (CBR) test were performed to determine the important properties of the soil.xSecond phase: The soil was modified with GGBS at five different mix proportions i.e. 5%, 10%, 15%, 20%, 25% (by dry wt. of soil).Subsequently, the soil was tested for MDD and OMC by standard Proctor test, unconfined compressive strength by UCS test, CBR value.

Table 2 .
Geotechnical properties of soil