Strength mineralogy and microstructure of a lime stabilized expansive soil amended with waste materials

CHAPTER one/ INTRODUCTION

 BACKGROUND

Soils exhibit a wide range of characteristics, so much so that it led to the development of a whole branch of study in order to understand it better. In his endeavour to understand soil better, man has encountered several different soils posing problems to his developmental activities. However, all soils are not problematic from engineering point of view. Different soils exhibit different levels of difficulty in handling them in actual field conditions. Expansive soil is one such problematic soil, found all over the world. They are considered hazardous, which can cause severe damage, if proper mitigation methods are not adopted.

Expansive soil is any soil composed predominantly of clay, which undergoes significant volume change in response to changes in soil moisture content. They swell when they come into to contact with water and shrink when they lose water due to drying. The volume change behaviour of expansive soils are influenced by factors like type and the amount of clay minerals and cations, moisture content, dry density, soil structure and loading conditions (Al-Rawas et al. 2002). Seco et al. (2011) classified the factors that influence the swelling of such soils into three types: geology, engineering factors of the soil and local environmental conditions.

Expansive soils result in extensive damage to structures and infrastructure built on them. Buildings constructed on expansive soils are often subjected to severe movement due to non-uniform soil moisture changes, with resultant cracking and damage related to structural distortion. Especially, lightly loaded structures are easily prone to damage in such situations resulting in cracks in walls, beams, columns, door and window openings and subgrade beams. These moisture variations can be caused due to evaporation and precipitation, ruptured water pipelines, irrigating gardens through hoses or sprinklers or roots of vegetation and trees.

There are several methods available to mitigate or eliminate the effects of expansive nature of such soils. These include stabilization, soil replacement with compaction control, pre-wetting, moisture control, surcharge loading and use of geosynthetics (Al-Rawas et al. 2002). Soil stabilization using chemicals has been a simple but effective method in modifying the properties of expansive soils. Soil stabilization is addition of an external binder to improve the chemical and mechanical properties of the soil (Castro-Fresno et al. 2011). Chemical stabilization involves addition of one or more external chemical agents, which results in a chemical interaction between them, leading to modification in engineering properties of the soil. Thus, stabilized soil can be considered as a composite material that is obtained by combining and optimizing the properties of the individual components.

Several chemicals have been adopted in chemical stabilization of soils. However, lime and cement have been standout performers in soil stabilization over the years with extensive research carried out on these two. There have been several instances of successful remediation of poor soils using lime and cement stabilization. However, there have also been instances whence they have resulted in poor performances in soil stabilization

especially in sulphate rich environments wherein the formation of the minerals ettringite and thaumasite result in the swelling of the stabilized soils even more than the virgin soil (Rajasekaran 2005; Ouhadi & Yong 2008). However, there are studies in which the formation of ettringite has been cited as reasons for the improvement in performance of the stabilized soil. Thus, several factors like type of soil, type and quantity of binder, water-binder ratio, type of curing, duration of curing and curing temperature influence the development of strength of stabilized soil.

Industrial revolution was a major milestone in the history of human civilization. Since the dawn of machines and industrialization of various manufacturing processes, there has been a rapid boom in development and urbanization surrounding industrial centres. The standard of living of the society started to rise but the standard of the living environment started to decline. It was not noticed until it started affecting humans directly. Today, industrial waste management is an area of concern with tons of waste being generated each day.

The number and quantity of industrial wastes produced around the world is huge. Globally, cities generate about 1.3 billion tonnes of solid waste per year. This volume is expected to increase to 2.2 billion tonnes by 2025 (Hoornweg & Bhada-Tata 2012). Merely cataloguing the various types of wastes and their quantity produced around the world itself is a huge task. However, a general idea of the proportion of the problem can be drawn by analysing the waste production statistics of some of the most widely  generated wastes. According to 2010 data, the worldwide generation of coal combustion products including fly ash (FA), bottom ash, cenospheres, conditioned ash and flue gas desulphurization gypsum, was approximately 780 million tonnes (Heidrich et al. 2013). The global annual production of blast furnace slag is approximately 400 million tonnes whereas the production of steel slag is around 350 million tonnes (Motz et al. 2013). Red mud production, another waste product generated during Bayer process for manufacture of aluminium, is estimated to be between 70 -120 million tonnes globally (Rai et al. 2013; Mišík et al. 2014; Sutar et al. 2014). The generation of cement kiln dust (CKD), a by-product of cement manufacturing process is approximately in the order of 510-680 million tonnes all over the world (Kunal et al. 2014). It can be seen that solid wastes generated around the world is mounting to huge proportions and needs different strategies for their effective management. Coming to the status of waste production in India, being a developing nation, the waste produced in India is no small amount. Table 1.2 shows the major industrial wastes and their quantity generated annually in India.

Table 1.1 Major Industrial Wastes Generated in India

(Source: Central Pollution Control Board 2006, 2012; Department of Industrial Policy and Promotion 2011; Parlikar et al. 2011; Central Electricity Authority 2015; FICCI 2014)

Generation of waste materials in such huge quantities results in massive problems of pollution, disposal and management. A lot of efforts have been put into effective waste management practices. One avenue for the management of solid wastes is to find suitable uses for it in various sectors of engineering and manufacturing. Usage of solid wastes in manufacture of materials has been one of the most effective avenues in the field of Civil Engineering. In recent times, such utilization of wastes in Civil Engineering has increased in order to achieve sustainable waste management practices. Industrial wastes have been recycled in manufacture of bricks, blocks and pavers, as aggregates in concrete and mortars, as raw materials for manufacture of cement and building lime, plaster boards, floor and wall tiles to name a few. In the same line, utilization of solid wastes in soil engineering is being researched heavily in recent times, especially in soil stabilization. Waste utilization in soil stabilization has only recently gained widespread acceptance with more and more solid wastes being researched for their efficacy in modifying soil properties and serving as mechanical and chemical stabilizers. Some solid wastes have also been used in geotechnical fill applications. Solid waste reuse has gained rapid momentum for achieving sustainable waste management and hence, they have been adopted in soil stabilization as standalone stabilizers as well as additives to augment the performance of conventional stabilizers like lime and cement. Research has shown that the use of solid wastes as additives with and replacement for conventional stabilizers has resulted in better results than the performance of either individually.

A lot of solid wastes have been investigated by researchers and have been found useful not only as a construction material but also an effective material for soil amendment. Sewage sludge ash, Silica fumes, Sugarcane bagasse ash (BA), Groundnut shell ash, Marble dust, Rice husk ash (RHA), Rice straw ash, Locust bean waste ash, Egg shell ash, CKD, Limen kiln dust, Sawdust ash, Waste paper sludge ash, Incineration ash, Limestone dust, Cement by-pass dust, Wood ash, Bottom ash, Calcined paper sludge, Palm oil fuel ash, Pumice waste, Lime sludge, Construction and demolition waste, Quarry dust and Crushed glass are some of the wastes that have been adopted successfully in soil improvement applications.

In this work, five different waste materials have been investigated to find their efficacy in enhancing the stabilization performance of lime in improving the geotechnical properties of an expansive soil. The materials include Phosphogypsum (PG), Ceramic dust (CD), Press mud (PM), BA and Coconut shell powder (CSP).

PG is an industrial by-product waste generated from fertilizer industries. PG was adopted as an additive because of the presence of gypsum in its composition. The worldwide PG production is estimated to be in the order of 100-280 million tonnes (Reijnders 2007; Tayibi et al. 2009). The annual generation of PG in India is 11 million tonnes as already mentioned in Table 1.1. Ceramic wastes are generated mainly from the construction industry. They are either produced in the form of rejects from the manufacturing plant or demolition waste from buildings. CD was adopted as an additive because of it being a known pozzolan with cement in concrete and lime in mortar, based on which it was adopted for investigation in soil stabilization. The global production of ceramic tiles is around 8500 million square meters (Tavakoli et al. 2013). The annual ceramics production in India is around 100 million tons worth 18,000 crores with an approximate production of 600 million square metres (Raval et al. 2013; Anwar et al. 2015). About 15 to 30% of waste is generated from the industry. PM is a waste generated during the manufacture of sugar from cane juice. PM has been adopted as a source for extraction of lime, which formed the basis behind its choice for use in this work. The worldwide generation of sugarcan

PM is estimated to be around 30 million tonnes (Tran 2015). About 7.5-12 million tonnes of PM is generated annually in India (Gupta et al. 2011; Bhardwaj 2013). It has been adopted as an organic additive to improve soil nutritive value but its use in soil stabilization has not been probed. BA is another waste generated as a by-product from the manufacture of sugar due to incineration of bagasse produced after extraction of cane juice. A lot of work has been done with BA as additive/replacement in concrete, however,  its work in soil stabilization has not been dealt with in the same level of detail which prompted its selection in this investigation. The worldwide annual production of sugarcane is around 1900 million tonnes out of which India produces 352 million tonnes (FAO 2015). Considering a yield of 0.6% ash (Souza et al. 2011), the bagasse ash generation is estimated to be 11.4 million tonnes worldwide and 2.1 million tonnes in India per annum. Coconut shell powder is obtained by fine grinding of shells of mature coconuts. The worldwide production of coconuts is 614 million tonnes (FAO 2015) with Indonesia being the largest producer in the world. Coconuts are also produced in huge quantities in India, the third largest producer, especially in the southern part of the country accounting for 90% of the country’s production, especially for extraction of oil. The annual production in India is around 11.1 million tonnes (FAO 2015). The shell comprises of around 15% by weight of the coconut (TNAU 2016), which works out to 1.67 million tonnes of coconut shell waste. This is bound to leave a huge quantity of shells as a waste remainder leading to disposal problems. There have been instances of usage of coconut shells as well as burnt ash of shells in concrete. But its usage in raw, powdered form has not been researched in much detail, especially in soil engineering where there is little evidence of its usage. Utilization of wastes in soil engineering has opened up a new avenue for solid waste reutilization. The benefit of environmental sustainability is also coupled with achieving engineering suitability of poor soils for use in developmental activities.

 NEED FOR SOLID WASTES IN SOIL STABILIZATION

Lime has long been used in the stabilization of poor soils. However, the use of solid wastes in recent times in soil stabilization has received a thrust with lots of research being carried out in the area. But in order to understand the need for solid wastes in soil stabilization, the following fundamental questions need to be answered. What is the need for lime to be mixed with solid wastes in soil stabilization? Is lime stabilization effective under all soil conditions? Can lime stabilization be made more cost effective? Are there any environmental concerns related to lime stabilization of soils and can they be used for remediation of contaminated soils? What benefits do we get by adopting solid wastes in soil stabilization? Answering the above questions will enable us to understand why there is a need for solid wastes in soil stabilization.

No single soil stabilizer is suitable for all soil conditions. What may be effective in one type of soil may not be as effective in another or may be even completely ineffective in stabilizing the soil. For example, in aggressive environments like sulphate rich soils, both cement and lime  stabilization result in poorer end products due to the formation of the mineral ettringite (Rajasekaran 2005; Ouhadi & Yong 2008). The result of this being excessive swelling of the stabilized soils leading to poor compressibility characteristics. In such conditions, either cement or lime needs to be rejected as the principal stabilizer or remedial measures need to be adopted to reduce the detrimental effects of sulphate attack on lime/cement stabilized soils. Literature indicates that solid wastes can play a vital role in reducing the detrimental effects of sulphate attack on lime/cement stabilized soils. For example, Ground granulated blast furnace slag (GGBS) can be used for effective control of swelling associated with sulphate rich soils stabilized with lime (Wild et al. 1998, 1999; Celik & Nalbantoglu 2013). Thus, further research on solid waste materials will bring out more materials that can be used for effective enhancement of soil stabilization with conventional stabilizers.

Utilization of solid wastes with lime in soil stabilization can result in more cost benefits in the project. Solid waste addition can result in improved strength and bearing which can enable reduced pavement thicknesses in subgrade stabilization applications. Beeghly (2003) states that combination of lime and FA can be more cost effective than lime only stabilization of subgrade. He estimated the cost of 3% lime with 6% FA to cost $1.35 per square yard against the cost of $2.20 per square yard for 6% lime stabilization.

There has also been a rising consciousness towards environmental pollution due to utilization of chemicals in soil stabilization activities. Eiswirth & Hotzl (2003) assessed the environmental risks of grouting with soft gels and found that the influence of soft gels on soil and ground water was comparatively over a shorter period and over a limited area. Kogbara & Al-Tabbaa (2011) and Kogbara et al. (2011) found that addition of slag to lime effectively reduced leaching of contaminants from contaminated soils. Shah et al. (2003) found that combination of FA with lime was more effective in remediation of fuel oil contaminated soil than lime alone. With rising concerns to the environment, there is an urgent need for an increased effort towards identifying potential solid wastes that can be adopted in soil modification while quantifying its environmental foot print in terms of pollution can go a long way in environmental safety in Civil Engineering projects. Identifying potential solid wastes in soil stabilization will pave way for researchers to further study the environmental impact of such potential waste materials in subsequent works.

The ultimate benefit of utilizing solid wastes in soil stabilization is achieving sustainable waste management. Disposal of wastes in landfills will result in utilization of large tracts of land while having a lifespan beyond which the fills will get used up. Identifying avenues for utilization of wastes in various areas of Civil Engineering will ensure that the quanta of solid wastes that find its way to landfills get reduced. Reutilization, if done effectively, can result in zero waste dumping in landfills. However, the various ways and means of reutilization of solid wastes can be identified only when research is taken up to study the uses of such materials in various fields of Civil Engineering

Thus, addition of solid wastes to lime in soil stabilization can result in enhanced performance, mitigation of performance drop under adverse soil conditions, cost effectiveness, reduced environmental concerns, better remediation of contaminated soils and last but not the least, sustainable waste management.

This research work envisaged the adoption of solid wastes in lime stabilization of an expansive soil, keeping in mind the aforementioned benefits. The novelty of the work when compared to the earlier research is the adoption of scientifically determined lime contents (rather than trial and error contents) for soil stabilization combined with solid wastes to study their effectiveness in improving the strength and index properties of the lime stabilized soil with the support of mineralogical and microstructural studies. Moreover, two hitherto unused/rarely used solid wastes in the field of soil stabilization have also been investigated along with three other solid wastes in effectively stabilizing an expansive soil with lime. The comparison of performance between three lime contents amended with solid wastes provides better insights into performance of solid wastes in combination with lime.

 AIM AND OBJECTIVES

The overall aim of the research is to study the effectiveness of utilizing solid waste materials as an additive in improving the strength and index properties of soil stabilized using lime, thereby revealing potential waste additives to stabilizers. This aim was achieved by setting the following objectives.

1. To evaluate the effect of addition of waste materials on the index properties of lime stabilized expansive soil.
2. To study the development of unconfined compressive strength of lime stabilized soil amended with wastd
3. To investigate the mineralogy and microstructure of lime stabilized soil modified with waste materials to understand the chemical and structural changes taking place at the micro level.
4. To obtain the optimum mixture proportion of the waste material enhanced lime stabilized soil for maximum

 SCOPE OF THE RESEARCH

Based on the various facets of chemical stabilization of soil, the scope of the work can be stated as follows. Soil stabilization using binders results in significant improvement in the properties of soil. The strength of the stabilized soil is affected by several different factors. The factors considered in this study are the quantity of the primary binder, the type and quantity of secondary additive and the curing period.

The investigation comprised of laboratory experimentation in two stages comprising of strength and index properties of stabilized samples in stage one and mineralogical and microstructural investigations of the same in stage two. An expansive soil was stabilized using combinations of lime (primary binder) and several different waste materials from various sources. The stabilized samples were cured for different periods of time and tested for their stabilized strength. The stabilized samples after strength test were also tested for changes in the index properties. This was followed by investigations leading to determination of changes in mineralogy and microstructure that are responsible for the macrostructural strength of the stabilized soil.

The tests performed were all laboratory tests performed on one type of expansive soil stabilized with hydrated lime and amended with five different types of solid wastes. The solid wastes were collected as deposited from various industries and used in the laboratory after preparation and sieving as per requirement. The lime adopted was readily available laboratory grade hydrated lime.

The results of the experiments were used to explain the changes taking place in the stabilized soil resulting in the performance changes due to stabilization. Comparisons with previous works were also done in order to complement the work done and understand the similarities and differences in the work and the results obtained.

 ORGANISATION OF THE THESIS

The thesis has been organized into seven distinct chapters to clearly explain the work that has been carried out. Chapter 1, Introduction, gives a background of the nature of expansive soil, its stabilization using chemical additives, need for solid wastes in soil stabilization and the aim of the study with objectives to achieve the same. Chapter 2 discusses the literature supporting the utilization of solid wastes in soil stabilization and the need for the present study based on deficiencies in literature. Chapter 3 lays out the methodology that was adopted to do this research work. Chapter 4 describes the characteristics of the materials used in the research. Chapters 5 and 6 lay out the results of index properties and strength (with mineralogical and microstructural changes) respectively and discuss the relevant inferences gained. Chapter 7 gives the conclusions and recommendations of the research work.