Effect of Ph on Compressibility Behaviour of Cement Treated Soil

B. Bhavana * T. Vamsi Krishna **

*-** PG Student, Department of Civil Engineering, VNR Vignana Jyothi Institute of Engineering and Technology, Hyderabad, India.

Abstract

To enrich the performance of structures, the stability of underlying soils must be checked. The stability of soil may change due to the discharge of pollutants into water bodies without adequate treatment, which causes momentous changes in the behavior of water, further leading to changes in the pH of water. In the present study, the black cotton soil has been treated with Ordinary Portland Cement (OPC) of 53 Grade and Portland Slag Cement (PSC) with different percentages (3%, 6%, and 9%), and laboratory tests such as Atterberg limits, Compaction, Unconfined Compressive Strength (UCS),California Bearing Ratio (CBR) and Consolidation have been performed. This paper attempts to assess the effect of pH (pH=5, pH=7, pH=9) levels on curing time (7, 14 and 28days) and behavior of cement treated soil. Tests results have shown that there is an increase in Optimum Water Content (OMC) which leads to decrease in Maximum Dry Density (MDD). The UCS and CBR strengths of soil got increased due to the addition of cement, besides that there is a reduction in the compressibility with the addition of cement content which were cured for ages (7, 14 and 28 days).

Keywords :

pH,
Compressibility,
Strength Characteristics,
Curing Time.

Introduction

Generally, industries are the main source for human life and developing countries. Due to the increase in the industrial activities, the waste which has been released from these industries causes enormous changes. In most of the developed countries, among 70% of solid and liquid wastes are dumped into water bodies without appropriate treatment, due to which the ground water is getting polluted. The contaminated water contains heavy metals which creates problems in environment, due to which the pH of water may vary. If pH level of water is < 7, it comes under acidic water and if it is > 7, it comes under alkaline water. Changes in the pH of water may affect the soil which leads to geotechnical problems. The reduction in strength properties may be found because of the mineralogical changes in the behavior of soil. Mostly, expansive soils are the most problematic soils that exert pressure on the foundation or basement, which results in lateral moment of the structure. These soils become sticky when they absorb water, and become hard and brittle when they are in dry state. This can be avoided by improving the properties of soil which further increases its strength, stiffness, etc. To accomplish higher strength, stabilization can be used by adding different additives. In the present study, two different types of cements namely Ordinary Portland Cement and Portland Slag Cement have been used. The technical information about cement treated soil mixtures has been carried out from various research works in the past. Teja et al., (2015) revealed that the maximum dry density decreased with an increase in Optimum Water Content (OMC) with the addition of cement. From the results obtained by using lime and cement, Bekki et al. (2015) concluded that cement treatment is more efficient than lime treatment in improving the values of California Bearing Ratio (CBR). IS 1498 (1970) shows that the unconfined compressive strength increases with increase in the amount of cement content, and for mixtures with 0% cement, the CBR for unsoaked specimens are greater than those of soaked specimens. However, the CBR for soaked specimens are higher than those of the unsoaked specimens. Bahar et al., (2004) shows the investigation of stabilized soil by mechanical means such as chemical stabilization or vibration, and compaction, by using cement. Properties of stabilized sand were evaluated and curing periods with variations in cement content were quantified by Al-Aghbari and Dutta (2005). The compression index (C_c ) for cement treated soils was found to be decreased when compared to the untreated soil samples with an increase in the curing ages, as mentioned by Ho and Chan (2011). In this investigation the effect of acids and alkalis were noted separately by varying pH levels (pH=5, pH=7 and pH=9) and curing time behavior of Black cotton soil, treated with different proportions of OPC and PSC. The behaviour and strength characteristics of soils can be found by performing a series of UCS, CBR and Consolidation tests.

1. Aim of the Study

The aim of this research is to establish the effect of acidic and alkaline nature on curing time behavior of cement treated soil.

1.1 Objectives of the Study

To analyze the properties of black cotton soil such as its consistency limits, grain size distribution and compaction properties, using standard proctor test before and after stabilization with two different types of cements (OPC and PSC).
To assess the behavioral changes and strength characteristics of treated soils which were cured under different pH levels (pH=5, pH=7 and pH=9).
To evaluate the effect of pH (pH=5, pH=7 and pH=9) and curing time on various parameters such as coefficient of consolidation, compressibility, coefficient of volume compressibility and permeability of untreated and treated soil.
A comparative study is made between the soil treated with Ordinary Portland Cement and Portland Slag Cement, which were cured under varied pH conditions. The soil treated with cement (OPC, PSC), which yields more strength is considered to be best suited for construction purposes.

2. Materials Used

2.1 Black Cotton Soil

The black cotton soil which was collected from Medchal Road, Ranga Reddy district, Telengana at the depth of 0.3 m was blackish grey in color. Before conducting the Laboratory experiments, the soil was oven dried and later, the index and engineering properties of soil was determined as per Indian Standards. The properties of black cotton soil are given in Table 1.

Table 1. Properties of Black Cotton (BC) Soil

2.2 Cement

In the present study, the Ordinary Portland Cement and Portland Slag Cement were purchased from a Cement Company in Hyderabad. The Ordinary Portland Cement (OPC) of 53 Grade is a type of hydraulic cement, which is being used worldwide and Portland Slag Cement (PSC) is an intimate mixture of OPC and granulated slag cement.

3. Laboratory Mix Design

In the present study, the chosen black cotton soil was mixed in dry condition with Ordinary Portland Cement (53 Grade) and Portland Slag Cement individually with various percentages (3%, 6%, and 9%).

4. Methodology

4.1 Laboratory Experiments

Various tests were carried out in the laboratory, the index and engineering properties of the soil samples have been determined as per Indian Standard (IS) specifications.

The specific gravity of the chosen soil was determined by using density bottle method, based on IS 2720 (1980a). The Grain size distribution of the soils has been strong-minded as per IS 2720 (1985a) and the soil classification is based on IS 1498 (1970). The casagrande liquid limit apparatus was used to determine the liquid limit of untreated and various soil-cement mixes and plastic limit test has been conducted using rolling thread method as outlined in IS 2720 (1985b). The standard proctor test was conducted to determine the direction of OMC and MDD of the soil according to IS 2720 (1980b). The Organic matter of the chosen soil can be determined using IS 2720 (2010) and pH of the soil can be found using IS 2720 (1997).

4.2 Unconfined Compressive Strength

4.2.1 Preparation of Specimen

The soil sample which was sieved through 425 μ sieve was mixed with various percentages (3%, 6% and 9%) of cement at required OMC. Later, the design mix was compacted in the compaction mould. Further, the sample was ejected from the mould and checked for required dimensions (38 mm diameter and 76 mm height) as per standards. For each cement content, 9 set of specimens have been prepared for chosen soil with Ordinary Portland Cement and Portland Slag Cement, separately. The specimens were wrapped in a polythene cover for one day, and later the specimens were directly immersed in water at 3 different pH levels (pH=5, pH=7, and pH=9) for curing ages of 7, 14, and 28 days.

4.2.2 Test Procedure

The Unconfined Compressive Strength (UCS) was determined as per IS 2720 (1991) by placing the uncured and cured cylindrical shaped specimens on the base plate. Without any stress application on the sample, the load frame was fixed. Later, by adjusting the dial gauge and proving ring readings are set to zero at a strain rate of 1.5 mm/min; the proving ring readings are noted for regular intervals of dial gauge readings. The UCS specimens under testing is shown in Figure 1.

Figure 1. UCS Specimens Under Testing (a) Before Curing, (b) After Curing

4.3 California Bearing Ratio Test

California Bearing Ratio (CBR) test is mostly important for pavements, because the thickness of pavement depends upon CBR value of soil as per Indian Roads Congress (IRC).

4.3.1 Preparation of Specimen

4.3.1.1 Unsoaked Specimen

The soil-cement mix at optimum moisture content for various percentages (3%, 6% and 9%) of cement was compacted by placing a spacer disc on the bottom of the CBR mould in five equal layers. The excess soil is trimmed off by removing the collar. Later, the spacer disc was removed by reversing the mould by adding of surcharge loads on the top of the soil specimen.

4.3.1.2 Soaked Specimen

The soil sample was prepared by compacting the design mix in CBR mould. Before performing the test, the porous plate was placed on the of the soil sample along with a filter paper, in addition to surcharge loads. Later, the mould containing soil is engrossed in water at three different pH levels (pH=5, pH=7, and pH=9) for curing period of 7, 14 and 28 days.

4.3.2 Test Procedure

The CBR mould is placed on the testing machine and the plunger is brought in contact with the soil sample. The proving ring and dial gauge readings are set to zero. The CBR value can be found as per IS 2720 (2002) at a penetration rate of 1.25 mm/min. The specimen under testing is shown in Figure 2.

Figure 2. CBR Specimen under Testing

4.4 Consolidation Test

4.4.1 Preparation of Specimen

The test specimens were prepared by compacting the soilcement mixture with different cement percentages (3%, 6% and 9%) in standard proctor compaction mould at obtained optimum moisture content on non-treated and cement treated soils. After completion of compaction, the specimens were extracted from the compaction mould as per the required dimensions (2 cm height and 10 cm diameter) and excess soil was trimmed off. The consolidation ring containing soil sample was placed in the consolidation cell, by providing porous stones on top and bottom of the specimen along with a filter paper, which was placed between the porous stones and specimen. Now, the consolidation cell is adjusted in a suitable position in the loading device.

4.4.2 Test Procedure

The consolidation test was carried out according to IS 2720 (1965) at a pressure increment of 0.25, 0.5,1.0 and 2.0 kg/cm² by varying pH levels (pH=5, pH=7 and pH=9). For each load increment, the corresponding dial gauge readings are taken using time (minutes) sequence of 0.00, 0.25, 1.00, 4.00, 9, 16, 25, 36, 49, 64, 81, 100 and 1440.

4.5 Analysis of Test Results

4.5.1 Effect of Cement on Engineering Properties

4.5.1.1 Atterberg Limits< /h3>

The Liquid Limit (LL) and Plastic Limit (PL) of black cotton soil increases with increasing cement contents, and a decrease in the Plasticity Index (PI) was found, with the increase in the cement contents. Figures 3 and 4 show the variation of Liquid Limit and Plastic Limit of black cotton soils respectively, when treated individually with OPC and PSC. Figure 5 shows the variation of Plasticity Index of black cotton soils, when treated with OPC and PSC individually.

Figure 3. Variation of Liquid Limit of Black Cotton Soil when treated with Ordinary Portland Cement (OPC) and Portland Slag Cement (PSC) Individually

Figure 4. Variation of Plastic Limit of Black Cotton Soil when treated with Ordinary Portland Cement (OPC) and Portland Slag Cement (PSC) Individually

Figure 5. Variation of Plasticity Index of Black Cotton Soil when treated with Ordinary Portland Cement (OPC) and Portland Slag Cement (PSC) Individually

4.5.1.2 Compaction Characteristics

The increase in the addition of cement decreases the maximum dry density and increases the corresponding values of optimum moisture content. Figures 6 and 7 show the variation of Optimum Moisture Content and Maximum Dry Density of black cotton soils respectively, when treated individually with OPC and PSC.

Figure 6. Variation of OMC of Black Cotton Soil treated with OPC and PSC Separately

Figure 7. Variation of MDD of Black Cotton soil treated with OPC and PSC Separately

4.5.1.3 Strength Characteristics

The Unconfined compressive strength and California bearing ratio values increase when black cotton soil is treated with Ordinary Portland Cement and Portland Slag Cement individually, with increasing cement content. Table 2 shows the UCS and CBR values of Black Cotton (BC) soil treated with OPC and PSC.

Table 2. UCS and CBR values of BC Soil treated with OPC and PSC

4.5.2 Influence of Curing Time

The effect of curing time for 7, 14, and 28 days with varying cement contents (3%, 6%, and 9%) were studied. The strength of soil-cement mixes has been increased with the curing ages. This gain in strength may continue for years which further increase the UCS and CBR values. The UCS and CBR specimens at the time of curing are shown in Figures 8 and 9.

Figure 8. UCS Samples at the Time of Curing

Figure 9. CBR Samples at the time of Curing

Figures 8 and 9 shows the Curing of UCS and CBR specimens when BC soil is treated with OPC and PSC under different pH levels (pH=5 (Acid), pH=7 (Neutral) and pH=9 (Base)).

4.5.2.1 Effect of pH and Curing Time on Strength Characteristics

The UCS and CBR strengths of black cotton soil increases with an increase in curing periods/days. The black cotton soil when treated with Ordinary Portland Cement and Portland Slag Cement separately in acidic solution (pH=5) for 7, 14, and 28 days gives best results for Ordinary Portland Cement when compared to Portland Slag Cement. The black cotton soil when treated with ordinary Portland Cement and Portland Slag Cement separately in (pH=9) base for 7, 14, and 28 days gives best results for Portland slag cement when compared to ordinary Portland cement. The black cotton soil when treated with Ordinary Portland Cement and Portland Slag Cement separately in neutral solution (pH=7) for 7, 14, and 28 days gives best results for Ordinary Portland Cement when compared to Portland Slag Cement. Figures 10-12 show the variations in the Unconfined Compressive Strength of black cotton soil, treated with cement (OPC, PSC) for curing ages of 7,14 and 28 days in acidic, base and neutral solutions respectively. Figures 13-15 show the variations in the California Bearing Ratio of black cotton soil, treated with cement (OPC, PSC) for curing ages of 7,14 and 28days in acidic, base and neutral solutions respectively.

Figure 10. The Variations in the Unconfined compressive Strength of Black Cotton Soil Treated with Cement (OPC, PSC) which were Cured under Different Acid (pH=5) for Curing Ages of 7,14 and 28 Days

Figure 11. The Variations in the Unconfined Compressive Strength of Black Cotton soil Treated with Cement (OPC, PSC) which were Cured under Different Base (pH=9) for Curing Ages of 7,14 and 28 Days

Figure 12. The Variations in the Unconfined compressive Strength of Black Cotton Soil Treated with Cement (OPC, PSC) which were Cured under Different Neutral (pH=7) for Curing Ages of 7,14 and 28 Days

Figure 13. The Variations in the CBR Values of Black Cotton Soil Treated with Cement (OPC, PSC) which were Cured under Different Acid (pH=5) for Curing Ages of 7, 14 and 28 Days

Figure 14. The Variations in the CBR Values of Black Cotton soil Treated with Cement (OPC, PSC) which were Cured under Different Base (pH=9) for Curing Ages of 7,14 and 28 Days

Figure 15. The Variations in the CBR Values of Black Cotton Soil Treated with Cement (OPC, PSC) which were Cured under Different Neutral (pH=7) for Curing Ages of 7, 14 and 28 Days

4.5.2.2 Effect of pH and Curing Time on Consolidation Parameters

When black cotton soil is treated with ordinary Portland Cement and Portland Slag Cement separately under different pH levels (pH=5, pH=7 and pH=9), the coefficient of consolidation and compressibility decreases with the increase in the cement contents and curing ages. Whereas, on the other side, the coefficient of volume compressibility and permeability increases with curing ages and decreases with the addition of cement content. The permeability of black cotton soil decreases with the increase in cement content and increases with the increase in curing ages. This is due to the decrease in the voids present in the soil particles because of the increase in the cement content, which improves the bonding between the cement and soil particles.

Table 3 shows the variation in consolidation parameters such as coefficient of consolidation (C_v ), coefficient of volume compressibility (m_v ), Compression Index (C_c ), coefficient of permeability (k) with the effect of Acid (pH=5) and curing time when Black cotton soil treated with OPC.

Table 3. Variation in Consolidation Parameters (C_v , M_v , C_c , k) with the Effect of Acid (pH=5) and Curing Time when Black Cotton Soil Treated with OPC

Table 4 shows the variation in consolidation parameters (C_v , M_v , C_c , k) with the effect of Base (pH=9) and curing time, when black cotton soil is treated with OPC.

Table 4. Variation in Consolidation Parameters (C_c , M_v , C_c, k) with the Effect of Base (pH=9) and Curing Time when Black Cotton Soil Treated with OPC

Table 5 shows the variation in consolidation parameters (C_c , M_v , C_c , k) with the effect of Neutral (pH=7) and curing time when black cotton soil is treated with OPC.

Table 5. Variation in Consolidation Parameters (C_c , M_v , C_c, k) with the Effect of Neutral (pH=7) and Curing Time when Black Cotton Soil Treated with OPC

Table 6 shows the variation in consolidation parameters (C_c , M_v , C_c , k) with the effect of Acid (pH=5) and curing time when black cotton soil is treated with PSC.

Table 6. Variation in Consolidation Parameters (C_c , M_v , C_c, k) with the Effect of Acid (pH=5) and Curing Time when Black Cotton Soil Treated with PSC

Table 7 shows the variation in consolidation parameters (C_c , M_v , C_c, k) with the effect of Base (pH=9) and curing time when black cotton soil is treated with PSC.

Table 7. Variation in Consolidation Parameters (C_c , M_v , C_c, k) with the Effect of Base (pH=9) and Curing Time when Black Cotton Soil Treated with PSC

Table 8 shows the variation in consolidation parameters (C_c , M_v , C_c, k)with the effect of Neutral (pH=7) and curing time when black cotton soil is treated with PSC.

Table 8. Variation in Consolidation Parameters (C_c , M_v , C_c, k) with the Effect of Neutral (pH=7) and Curing Time when Black Cotton Soil Treated with PSC

5. Discussions

The liquid limit and plastic limit values of black cotton soil increases after addition of various percentage (3%, 6% and 9%) of cements. Besides that, the drop in the plasticity index values was observed with the addition of cement contents. The reduction in the plasticity index was due to cementitious links between the soil particles and, calcium silicates and aluminate hydration products as per the previous study (Bekki et al., 2015). However, the decrease in the plasticity index values increases the workability of soils and also improves the volume change characteristics.
As per Figure 6, maximum OMC is required to stabilize the black cotton soil with Ordinary Portland Cement compared to Portland Slag Cement and, Figure 7 illustrates that maximum MDD is attained for BC+OPC when compared to BC+PSC. The increase in the optimum moisture content was because of cement which consisted of fine particles in it. With the increase in the fine content of soil, the surface area is going to be increased which further needs a supplementary quantity of water, and the decrease in the maximum dry density with the addition of cement was due to the phenomenon of flocculation and agglomeration of fine grained soil. Similar results have been observed by Sarkar et al. (2012).
Figures 10 to 15 show that the maximum UCS and CBR values are accomplished for black cotton soil when treated with OPC compared to PSC. This indicates that, increase in the CBR value increases the bearing capacity of the soil and there is a decrease in the total thickness of the pavement. The increase in the strength with increase in the percentages of cement was due to earlier hydration reactions and formation of secondary cementitious products as studied by Bushra and Robinson (2010). The UCS and CBR strengths increased when the black cotton soil was treated with Ordinary Portland Cement under pH=7 (Neutral), and the same soil when treated with Portland Slag Cement, the UCS and CBR strengths increased in pH=9 (Base) compared with pH=7 (Neutral) and pH=5 (Acid). This is because of slag cement, which acts as an excellent resistance to alkali and sulphate attacks.
From Tables 3 to 8, it is clear that the coefficient of consolidation decreases with increase in the percentages of cement and curing ages. Similarly the compression index decreased with the increase in the cement contents and curing ages. This is because of the effect of cement content, which affects the structuration of the soil and makes the stabilized soil to become stiffer as studied by Ho and Chan (2011). Besides that, the coefficient of volume compressibility increases with the increase in the curing ages and decreases with the addition of cement content, and Similarly the permeability of black cotton soil decreases with the increase in the cement content and increases with the increase in the curing ages. This is due to the decrease in the voids present in the soil particles with increasing cement content, which improves the bonding between the cement and soilparticles.
The black cotton soil when treated with Ordinary Portland Cement under pH=7 (Neutral) gives best results than the soil-cement (OPC) mixes which were cured under pH=5 (Acid) and pH=9 (Base) compared to Portland Slag Cement and the same soil when treated with Portland Slag Cement under pH=9 (Base) gives best results than the soil cement (PSC) mixes which were cured under pH=5 (Acid) and pH=7 (Neutral) compared to Ordinary Portland Cement. This is because of slag cement, which acts as an excellent resistance to alkali and sulphate attacks.

Conclusions

The liquid limit and plastic limit of two different types of soils increases with increase in cement content and the plasticity index of BC soil decreases with increase in percentage of OPC and PSC cement separately, which is favorable, since it increases the workability of soils.
The optimum moisture content values of treated soil samples were increased continuously for all percentages of cement than that of untreated soil samples. Besides that, there is a slight decrease in MDD with the increase in the percentages of cement.
The UCS and CBR strength was more pronounced in Ordinary Portland Cement of 53 Grade than Portland Slag Cement. The Unconfined compressive strength has increased from 1.7 kg/cm² to 8.3 kg/cm² for black cotton soil when treated with OPC. The CBR values when treated with OPC (53 Grade) got increased from 3.74% to 21.7% for black cotton soil.
The maximum improvement in the strength can be observed with the addition of cement under curing period of 28 days. Based on UCS and CBR results, it has been concluded that the black cotton soil when treated with Portland Slag Cement was effective than Ordinary Portland Cement when cured under pH=5 (Acid) and pH=9 (Base), and the same soil when cured under pH=7 (Neutral), the Ordinary Portland Cement was effective than Portland Slag Cement.
The reduction in the permeability, compression index, coefficient of consolidation and coefficient of volume compressibility was observed with increase in the cement content. The BC+OPC when cured under pH=7 (Neutral) improve the properties of soil compared to pH=5 (Acid) and pH=9 (Base). BC+PSC when cured under Base improve the parameters of consolidation than that of BC+OPC.

Recommendations

The leakage of effluent in to subsoil directly affects the use and stability of the supported structure. Disposal of wastes from industries and accidental spillage of chemicals during the course of industrial operations are the major sources which affects the pH of water. In this investigation, an attempt has been made to find out the behavioral changes and strength characteristics of stabilized soils, when cured under different pH levels (pH=5, pH=7 and pH=9).
The Ordinary Portland Cement which has been used in the present study is highly recommended where fast pace of construction is required, as it requires minimum curing period of 7 days because of its fast increase in strength in the earlier days. When exposed to hot and dry weather conditions, the minimum curing period of 10 days is required. Hence, Ordinary Portland Cement can be preferred, where faster removal of framework or faster work rate is required. Also, OPC can resist against sulphates, alkalies, chlorides and chemicals. But compared to Portland Slag Cement, the Ordinary Portland Cement has lower resistance to attacks against chemicals.
The Portland Slag Cement which has been used in the present study is used virtually in all concrete applications such as concrete pavements, waste water treatment and marine applications, structures and foundations. It is also used in non-concrete applications such as soil-cement and hazardous waste solidification. Also, the use of slag cement reduces the emission of green house gasses. Slag cement improves compressive and flexural strength, reduces permeability and provides resistance to corrosion and chloride intrusion. It has the ability to mitigate, moderate to severe sulphate attack and higher- alkali cements.

Acknowledgements

The authors would like to thank the Principal of VNR VJIET and Dr. A. Mallika, Professor and Head of the Department of Civil Engineering for providing all facilities required for the completion of this research successfully and also they express their sincere gratitude to the management authorities for their continuous support, patience, motivation, enthusiasm and immense knowledge towards completion of this research.

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