Behaviour of sewage sludge based lightweight aggregate in geopolymer concrete

The global challenge of sewage sludge disposal has encouraged innovative solutions aimed at reducing environmental impact while simultaneously addressing the growing demand for sustainable construction materials. This study aimed to develop treated raw sewage sludge-based lightweight aggregates with strength comparable to commercially available aggregates. Two methods, namely cold bonding and sintering, were employed for the formation of aggregates. The sintering method produced well-formed and hard aggregates, while the cold bonded aggregates exhibited weakness and disintegrated under the slightest pressure. The optimal mix for quality aggregates was found to be 10%–20% sewage sludge, 70%–80% fly ash, and 10% lime using the sintering method. In the sintering method, an increase in sewage sludge content resulted in the reduction of bulk density and specific gravity by 13% and 4% respectively due to the high organic content in sewage sludge, volatile gas release, and porous structure formation. When 10% to 20% sewage sludge content was added, water absorption of the aggregates also increased by approximately 2%. Physical properties such as individual pellet strength. aggregate crushing value reduced by 18%, 20% respectively and the aggregate impact value increased by about 9%. These aggregates were then used to produce lightweight geopolymer concrete, which exceeded the design strength by 7% for the aggregate containing 20% sewage sludge and demonstrated excellent physical properties. The use of waste-based aggregates offers advantages including savings in cost, sustainability, resource conservation, waste reduction, and reduced environmental impact, making them a valuable alternative to natural crushed stone aggregates in specific applications.


Introduction
The construction industry stands at the forefront of innovation and evolution, driven not only by the need for infrastructure development but also by the pressing global concerns of sustainability and environmental responsibility.Traditional construction materials, such as Portland cement, river sand, and crushed stone aggregates, have been associated with significant carbon emissions and resource depletion over a long period [1].Cement production in particular is associated with six percent of overall greenhouse gas emissions [2].In response, there is a growing imperative to explore alternative materials and methodologies that not only reduce the environmental footprint in construction but also enhance the performance and durability of the materials.The quest for sustainability in construction has resulted in the development of geopolymers, which are the promising alternative to conventional Portland cement-based materials.Geopolymers offer several advantages, including reduced carbon emissions, improved resistance to harsh environmental conditions, and enhanced mechanical properties [3].As researchers struggle with the need to reduce the carbon footprint of construction, geopolymers have emerged as a promising alternative, guiding the way towards a more sustainable future [4].Geopolymer concrete is produced by using industrial byproducts which are rich in alumina and silica as binders.Fly ash, a byproduct of thermal power plants contains aluminosilicate minerals and is the most commonly used binder in geopolymer concrete.By using fly ash as a raw material, geopolymer concrete minimizes waste production from power plants and contributes to environmental protection.Geopolymer concrete exhibits high early strength and is resistant to aggressive atmospheres.The chemical activation of fly ash results in a cohesive matrix that binds coarse and fine aggregates effectively.Moreover, the production of geopolymer cement and concretes using fly ash is a better alternative to conventional concrete because it gives high early strength, is durable, economical, and emits less carbon [5].
Globally, the treatment and disposal of sewage sludge have posed formidable challenges.Approximately seven million tons of sewage sludge was generated across the world in the year 2017 [6].The huge volume of sewage sludge generated annually, coupled with the limitations of conventional disposal methods, has raised environmental, economic, and public health concerns [7].This multifaceted problem demands innovative methods that not only alleviate the burden of sewage sludge management but also harness its potential as a valuable resource.Many researchers have sought to convert sewage sludge into a useful construction material, creating a shift in perspective when it comes to waste management [8].

Lightweight aggregates-an overview
Lightweight Aggregates (LWA) are a unique material in the construction sector due to their reduced weight when compared to natural rock-based coarse aggregates.This weight reduction can lead to substantial savings in foundational and reinforcement costs, making LWA an attractive option for the construction industry.Furthermore, these aggregates exhibit improved thermal properties and fire resistance, enhancing their applications.The environmental benefits are also significant, particularly when industrial waste products are repurposed to manufacture these aggregates [9].However, LWA also has its challenges.LWA tends to have lesser specific gravity than conventional aggregates and is more susceptible to water absorption.Other challenges include difficulty in placement and finishing, longer mixing times compared to conventional concrete, and a tendency for the aggregates to separate and float towards the surface in certain conditions.Despite the challenges associated with their use, the potential benefits of LWA make it a promising avenue for research in the pursuit of more sustainable construction materials [10].
LWA can be categorized into two types: natural and artificial.Natural LWA is derived from natural materials such as pumice, scoria, and volcanic ash.They are not of uniform quality and are not found in many countries [11].Out of all the natural aggregates, only pumice is used widely.The lightweight property of these rocks is due to the escaping of gas from the molten lava which erupts from beneath the earth's crust [12,13].Artificial LWA is made up of waste materials, such as fly ash, metakaolin, or volcanic ash, and ground granulated blast-furnace slag (GGBS) [14].Artificial aggregates can be produced either from natural resources or from industrial byproducts.They are produced through various methods such as sintering, cold bonding, and autoclaving.The key difference between the two lies in their origin and composition [15].Natural aggregates are derived from natural sources and require mechanical treatment before use, while artificial aggregates are manufactured using waste materials, offering a way to recycle these materials.
Fly ash, a byproduct of thermal power plants has been explored as a sustainable source for creating artificial LWA.The manufacturing process involve blending of fly ash with water and sintering at high temperatures, to obtain angular fly ash aggregates [16].These aggregates exhibit lightweight characteristics and can be used in structural concrete, reducing costs and addressing disposal challenges [17].Lime also plays a vital role in the production of LWA from industrial byproducts.Lime acts as a binder in both cold bonding and sintering processes, transforming waste materials into cohesive aggregates.The resulting LWA exhibits favorable properties, including reduced water absorption and improved mechanical performance [18].

Research significance
This study aims to bridge the gap between two critical domains: the utilization of sewage sludge and the adoption of geopolymers, with a specific focus on LWA.Lightweight Aggregates (LWA) play a pivotal role in the formation of lightweight concrete, which exhibits excellent properties while significantly reducing dead loads on structures.This characteristic has many positive implications for both the construction industry and the environment.Therefore, the primary objective of this study is to investigate the preparation of lightweight aggregates through sintering and cold bonding methods, incorporating a blend of lime, fly ash, and sewage sludge.Although fly ash and lime have been used to produce LWA in the past, the incorporation of sewage sludge in LWA has not been studied widely.By systematically varying the proportion of sewage sludge in the fly ash/ lime-based LWA composition, the impact on the key properties are explored, including bulk density, specific gravity, individual pellet strength, and water absorption.Also, the application of sewage sludge-based aggregate in the production of geopolymer concrete is an area that needs to be explored widely as geopolymer concrete may address the shortcomings usually associated with sewage sludge-based LWA.This study not only contributes to the development of LWA but also addresses the broader challenges of sewage sludge management and the adoption of geopolymers in the construction sector.

Fly ash
Fly ash is a by-product of coal combustion in thermal power plants which has proven to be a valuable resource in the construction industry due to its versatility and environmental benefits.It not only provides a sustainable solution for waste management but also contributes to the production of cost-effective and high-performance construction materials.Fly ash is rich in silicon and aluminum, which are the main constituents of geopolymers.When mixed with an alkaline solution, these elements undergo a polymerization process to form a solid binder.This binder can be used in concrete to replace traditional Portland cement, resulting in a more sustainable and environmentally friendly construction material [19].LWA can also be produced from fly ash through a process called pelletization.The resulting aggregates are sintered and crushed to produce angular lightweight aggregates.The production efficiency of fly ash aggregates is also found to be dependent on the binary blends of fly ash with clay binders and the concentration of alkali activator added to the fly ash [11].Fly ash procured from the North Chennai Thermal Power Station, Chennai was used in this study.The properties of fly ash are shown in tables 1 and 2.

Sewage sludge
Sewage sludge is a by-product of wastewater treatment processes, and its management has become a significant environmental concern worldwide.The disposal of sewage sludge in landfills, as practiced in many countries, is not a sustainable solution.Instead, it can be used as an alternative soil additive or fertilizer due to its richness in organic matter and other nutrients [20].In the construction industry, sewage sludge has found various applications.It is widely used in the production of building materials such as eco-cement, bricks, ceramic materials, and supplementary cementitious materials (SCMs).These applications are economically viable and help in the effective disposal of sewage sludge [7,21,22].The sewage sludge used in this study was obtained from the Koyambedu Water Treatment Plant, Chennai.The physical properties and chemical composition of the sewage sludge used in this study are shown in tables 1 and 2 respectively.Hydrated lime acts as a binding agent in the mix.It forms a cohesive matrix that binds the lightweight aggregate particles together thereby improving the pelletization process.This improved bonding enhance the overall cohesion within the concrete or mortar [23].By utilizing small quantities of lime, we can transform waste into resource, thereby enhancing sustainability, reducing reliance on natural resources, and addressing disposal challenges associated with industrial waste.The physical properties of lime are shown in table 1.The lime used in this study was composed of 86.45% CaO and 2.12% MgO with the other compounds making up the rest of the composition.

Cold bonding method
The cold bonding method is a technique used in the production of LWA.This method involves the pelletization or agglomeration of mineral admixtures and cement in a tilted revolving pan at ambient temperature.The process begins with the uniform mixing of fly ash and a binder.This mixture is then thoroughly dry-mixed.After dry mixing, water is sprinkled in a pelletizer and the contents are thoroughly mixed until the formation of spherical shaped aggregates.The aggregates are then water-cured or oven-cured at a suitable temperature as required by the materials used in the mixing process.There are several advantages to the cold bonding method.
It is considered to be more conservative due to the utilization of minimum energy, compared to other methods like sintering which consume large amounts of energy.It contributes to less pollutant production, making it an environmentally friendly method.It also has low operating expenses, making it cost-effective.The lightweight aggregate produced through the cold bonding method has the potential to be applied in concrete production due to its comparable properties to other methods.However, there are some disadvantages associated with the cold bonding method.The main drawback of using cold-bonded aggregate is its increased density compared to sintered LWA.Cold bonded aggregates have lesser crushing strength compared to sintered LWA.To enhance the performance of cold bonded aggregates, dry density should be reduced, and crushing strength should be increased.Despite these challenges, the cold bonding method is still considered a promising technique for producing lightweight aggregates due to its environmental and economic benefits [24].

Sintering method
Sintering method is a widely used technique for producing LWA that involve heating of materials to high temperatures, causing the particles to bond together.The process commences with the preparation of raw materials, such as a mixture of a binder and fly ash.This mixture is prepared, after which it is formed into aggregates using a pelletizer.These aggregates are then sintered at temperatures ranging from 1190 to 1210 °C.
The sintering method offers several advantages.It can produce concrete with high strength performance and the LWA produced has low density, reducing the dead load applied to structural elements.Additionally, these aggregates exhibit good thermal conductivity (0.30 to 0.20 W mK −1 ).Sintered aggregates are less porous and have a well-developed microstructure compared to cold-bonded aggregates, resulting in lesser water absorption.However, there are also some disadvantages associated with the sintering method.The process needs to be maintained in a special atmosphere and it is also complex to procure or produce the large equipment that is required in the process.The specific procedure and conditions can vary depending on the specific materials used for the production of the final product.Factors such as the ratio of fly ash to water, sintering temperature, and sintering time can affect the performance of lightweight aggregates [10].

Tests conducted
Bulk density is the most important property to be considered in the production of LWA.As per ASTM C330-05, LWA must have a maximum bulk density of 880 kg m −3 .In this study, bulk density test was conducted as per the ASTM C29-97 code.Particle size distribution plays a crucial role in aggregates, impacting properties like stiffness, strength, workability, permeability, stability, and skid resistance.ASTM C136-06 was adopted to find the particle size distribution of LWA.Water absorption and specific gravity of LWA were found based on ASTM C127-15.Specific gravity influences aggregate strength, abrasion resistance, bonding capabilities, etc. and water absorption of aggregate is required as it is an important factor when designing concrete.Physical tests conducted included the individual pellet strength, aggregate crushing value (ACV), and aggregate impact value (AIV).
Individual pellet strength was done based on the recommendations of Tajra et al (2019) [24].ACV and AIV tests were done based on the Indian Standard code IS 2386 (Part 4).Microstructural studies were carried out using the Scanning Electron Microscope (SEM) and the elemental composition of various materials was found using the x-ray Fluorescence (XRF).SEM and XRF tests were conducted at the National Centre for Earth Science Studies, Kerala.SEM provides detailed images of the surface topography of LWA.It reveals features like pores, cracks, and irregularities and also helps identify the distribution of voids, particle shape, and surface roughness.

Methodology
In this study, LWA was produced using the cold bonding method and sintering method.Mix proportions adopted in this study are shown in table 3. The main focus of this study was to prepare LWA using treated raw sewage sludge and fly ash.However, to improve the agglomeration and bonding of the aggregates, a small portion of lime was added to the mixture [25].
The steps involved in the cold bonding method are shown in figure 1.Initially, the mixtures were dry mixed and then fed into a pan pelletizer of 1.80 m diameter maintained at an angle of 45°to 50°.The pan was set to rotate at a speed of 42 revolutions/minute for about 10 min.The amount of water was fixed based on a trial and error method during pelletization.20% to 24% of the water taken by the weight of the feed mix was sprayed onto the pan pelletizer during the primary pelletization process.Water plays an important role as it improves the coagulation and also initiates the hydration process of the lime [25].The aggregates formed were packed in gunny bags and ambiently cured for about 24 h.After the ambient curing process, the aggregates were kept inside a water curing tank for 28 days to encourage the hydration process.
The mixes L0, L1, and L2 formed spherical pellets with good structure and consistency.However, as the sludge content reached 30%, there was no bonding between the particles and the pellets started to disintegrate upon handling and therefore could not be used to produce LWA.The observed phenomenon could be attributed to several factors such as sewage sludge composition, increased moisture content, porosity, and density of sewage sludge.Sewage sludge is a complex material with varying physical and chemical properties.It may contain impurities and organic matter that interfere with the binding process, leading to weak aggregate formation [26].Also, the high moisture content and water absorption of sewage sludge result in higher water content in the mixture, affecting the hardening process.The presence of sewage sludge supports the swelling of raw material due to its high porosity and weak density, thereby causing an increase in the porosity of aggregates [27].This leads to the formation of less dense and weaker aggregates.The image of the cold-bonded L2 aggregate is shown in figure 3.
In the sintering method, the aggregates ambiently cured for 24 h are kept inside a sintering oven at 1200 °C for one hour as shown in figure 2 [28].In cold bonding, the primary process of hardening of the aggregate is the initial coagulation and the subsequent hydration initiated by the lime content in the mix [25].However, in the  sintering process, the high-temperature process causes the raw materials to bond together, forming a hard, honeycombed structure of interconnecting voids within the aggregate formed by the mineralogical phases inside the aggregate [29].The physical appearance of the sintered aggregates was different from that of the cold bonded aggregates as shown in figure 3. Chemical analysis of sewage sludge showed the presence of Hematite (Fe 2 O 3 ) in its composition.Hematite, known for its distinctive reddish-brown to black metallic luster, influenced the color of sintered specimens.Hematite within the aggregate underwent oxidation and turned from ferrous state (Fe 2+ ) to ferric state (Fe 3+ ).This process generates rust (iron oxide) on the aggregate surface, contributing to the brownish color of LWA [30].

Physical inspection
Initially, physical inspection was carried out for the cold bonded and sintered aggregates.The color of cold bonded specimens was grayish and the color of the sintered aggregates was brownish as shown in figure 3.This was due to the ferrous deposits on the surface of the sintered aggregates during the sintering process, which is a common occurrence.Upon handling, the sintered aggregates were hard and well-formed when compared to the cold bonded aggregates.The cold bonded sewage sludge-based aggregates disintegrated when the slightest  pressure was applied due to its weak formation.The formation of LWA from raw treated sewage sludge in a cold bonded method might not be as effective as the sintering method for the following reasons: • The sintering process involves high-temperature treatment, which enhances bonding and crystalline structure, resulting in better mechanical properties.At temperatures above 1000 °C, liquid phase sintering occurs.The presence of a liquid phase (often from mineral components) facilitates particle bonding.
• Sewage sludge also contains P 2 O 5 , which tends to slow down the rate of hardening of the aggregate mix.Phosphorus pentoxide is also highly hygroscopic (absorbs moisture readily).Its presence can lead to weaker points within the aggregate structure, affecting overall strength [31].
• Some of the organic and inorganic components in sewage sludge may not be compatible with the cold bonding method, resulting in inadequate binding and poor aggregate formation.Sintering at high temperatures allows for better chemical reactions to occur, improving the binding of particles [32].
• Sewage sludge contains higher moisture content and high water absorption.The cold bonded method may require specific moisture content levels for proper aggregate formation.If the moisture content is not controlled adequately, it can affect the quality and strength of the aggregates [24].In sintering, moisture content is typically not a concern because the high temperatures will drive off any excess moisture.

Microstructural analysis
The dense microstructure of the sintered aggregates and the poorly formed microstructure of the cold bonded aggregates is shown in figure 4. In figure 4, the SEM image of the cold-bonded aggregates shows the presence of voids in the microstructure.The sintered aggregates display a dense microstructure when compared to cold bonded aggregates.Sintering involves heating the raw materials to high temperatures which results in the fusion of particles and the formation of solid aggregates.In cold bonding, the aggregates are formed at room temperature or slightly elevated temperatures, usually using binders such as cement or lime [10,24].The lack of elevated temperature in the cold bonding process can lead to incomplete fusion and weak aggregate formation, especially when dealing with materials like sewage sludge.Sewage sludge is a complex and heterogeneous material, and its properties can vary depending on its source and treatment process.The aggregates in cold bonded mixes L0, L1, and L2 were not properly formed and therefore could not be used to carry out the physical tests required to test its efficacy.The sintering method is generally more effective in the formation of LWA from raw treated sewage sludge due to its higher processing temperature, better chemical reactions, control over moisture content, and processing conditions.Sintering promotes better interlocking of particles, reducing voids and enhancing overall integrity as well as providing better chemical stability as shown in figure 4. The cold bonding method may still be feasible, but it requires careful consideration of the specific properties of the sewage sludge and the binder used to ensure proper aggregate formation.

Particle size distribution
The particle size distribution of the sewage sludge-based LWA developed by the sintering method is shown in figure 5 and compared with natural coarse aggregates and commercially available LWA.The aggregate passing through 4.75 mm sieve and retained on the 20 mm sieve are used for the majority of construction purposes.It is clear from figure 6 that 70% of the LWA developed is feasible to be used in lightweight concrete, indicating a successful pelletization process.The rest of the pelletized aggregate can further be used as fine aggregate after the appropriate crushing/grinding process, thereby paving the way for effective utilization [33].

Specific gravity and bulk density
Bulk density is a fundamental physical property that plays a pivotal role in characterizing and evaluating LWA.It is defined as the mass of a material per unit volume, and it also provides essential information about the compactness, weight, and porosity of an aggregate.In the context of LWA, bulk density is a critical parameter because it directly influences their performance, applications, and suitability for various engineering and construction purposes [34].LWA, as the name suggests, are materials specifically engineered to possess a lesser density (maximum of 880 kg m −3 ) than traditional construction aggregates like natural stone, gravel, or sand [35].These aggregates are essential components in the production of lightweight concrete, lightweight concrete blocks, and various construction materials.Bulk density is crucial for several reasons when considering LWA.Firstly, bulk density is a key determinant of the overall density of a material.In the case of LWA, lesser bulk density means that the material is less dense, which is a desirable attribute for applications where reduced weight is essential [11].Lightweight concrete made with these aggregates exhibits lesser density, making it particularly useful in construction scenarios where structural load-bearing capacity needs to be balanced with weight considerations.Reduced bulk density results  in lightweight concrete that is easier to handle, transport, and erect, offering economic and logistical advantages.Secondly, bulk density impacts the thermal and insulating properties of LWA. Materials with lesser bulk density have a higher proportion of air voids, which serve as insulating spaces within the material.This makes LWA an attractive choice for applications where thermal insulation is important, such as in the construction of energyefficient buildings.The air-filled voids in LWA contribute to their superior thermal performance, making them an excellent choice for projects that require energy conservation.
The relationship between bulk density and specific gravity for the sintered L0, L1, and L2 samples is shown in figure 6.The relationship between specific gravity and bulk density of LWA can be described as follows: as specific gravity decreases (indicating lesser density compared to water), bulk density also decreases (indicating low mass per unit volume) [36].This relationship is a key factor in the use of LWA to reduce the weight of construction materials while maintaining adequate strength and insulating properties.It is clear from the figure 6 clear that increasing the sewage sludge content reduces the bulk density from 745 kg m −3 to 642 kg m −3 and specific gravity of the samples from 1.54 to 1.47, indicating a linear relationship.This is due to the high organic matter content in the sewage sludge, release of volatile gases during the production process, and the porous structure formation [37,38].While reducing bulk density is desirable for saving weight, it should not compromise the structural integrity of the material.An optimal balance is to ensure that the aggregates maintain sufficient mechanical strength to withstand the stresses and loads they will encounter in construction applications, which cannot be achieved by increasing the sewage sludge content.

Water absorption
Water absorption is an important parameter for LWA as it plays a significant role in determining the quality and suitability of these aggregates for various applications.Water absorption is an indicator of how susceptible the aggregates are to moisture-related damage.Aggregates with high water absorption can absorb and retain moisture, which can lead to issues like freeze-thaw damage, internal cracking, and reduced long-term durability [39].Aggregates with low water absorption are more resistant to such moisture-related problems, making them suitable for outdoor and harsh environmental conditions.LWA is commonly used in the production of lightweight concrete.The water absorption of these aggregates is critical in determining the water-cement ratio needed to achieve the desired workability and strength of the concrete mix, especially in geopolymer concretes where it affects the molarity of the alkaline solution.LWA with high water absorption might require adjustments to the mix design, affecting the overall performance of the concrete.In applications where thermal insulation is a consideration, such as in lightweight concrete blocks or insulating panels, low water absorption is desirable [40].LWA with low water absorption helps maintain the insulating properties of the material because they don't absorb and retain moisture that could compromise the insulation.The 24 h water absorption results of the sintered aggregates L0, L1, and L2 and their relationship with specific gravity are shown in figure 6.
Generally, aggregates with higher specific gravity values tend to have lesser water absorption.This means that denser aggregates are less likely to absorb water.Aggregates with lesser specific gravity values tend to have higher water absorption as less dense aggregates have more open pore space and can absorb more water [41].LWA often have relatively high water absorption due to their porous nature.It is clear from figure 7 that there is a gradual increase in the water absorption by the aggregates as the sewage sludge content increased.Sewage sludge typically has high water absorption due to its composition and characteristics, which include a significant organic content and a porous structure.Sewage sludge consists of organic materials, which can include various biodegradable substances such as plant matter, microorganisms, and organic chemicals.Organic matter can absorb and retain water, contributing to the high water absorption of sewage sludge.Sewage sludge often contains a porous structure with voids and interconnected spaces.These voids can hold and retain water, much like a sponge.The porosity of sewage sludge results from the decomposition of organic materials, gas release, and the formation of channels and gaps within the sludge [37].The presence of voids in the sewage sludge is shown in figure 7.

Individual pellet strength, aggregate crushing value (ACV) and aggregate impact value (AIV)
The individual pellet strength of LWA is a crucial parameter that has significant implications for its performance and applications.Pellets, in this context, refer to the individual granules or particles that make up aggregates.Understanding and assessing the strength of these pellets is essential for engineering and construction purposes.The individual pellet strength of LWA is a fundamental property that directly impact the structural integrity, load-bearing capacity, durability, and overall quality of materials in construction.It is a key factor in the successful utilization of LWA to meet both functional and weight reduction requirements in various applications [42].
As shown in figure 8, the individual pellet strength of the aggregates is tested in the compression testing machine at a loading speed of 0.5 mm/minute until it is disintegrated [24].The highest value is recorded, and the individual pellet strength of the aggregates is found according to the following equation, where σ is the single particle compressive strength of the aggregates (in MPa), P is the peak force (in N), and d is the distance between the upper and lower bearing platforms (in mm).A total of six pellets were selected based on the diameter of individual pellets to study the effect of diameter on the individual pellet strength.The pellets selected were 4 mm, 6 mm, 8 mm, 10 mm, 12 mm, and 14 mm.The results of the individual pellet strength are shown in figure 9.
The general trend observed was that as the diameter increased, the individual pellet strength decreased.As the diameter of the pellets increases, the volume enclosed by the pellet's structure also increases.This larger internal volume can result in a greater proportion of air voids within the pellet.These air voids create regions of weakness within the pellet, reducing its overall structural integrity.As the relationship between strength and diameter is linear, the individual pellet strength can be calculated by the following equations; where σ is the single particle compressive strength of the aggregates (MPa).Aggregate Crushing Value (ACV) and Aggregate Impact Value (AIV) are two important tests that evaluate the physical properties of aggregates [43].These tests are particularly relevant when considering LWA, which has specific considerations due to their porous and lightweight nature.ACV is a test that measures an aggregate's ability to resist crushing under a gradually applied compressive load.It is performed by subjecting an aggregate sample to a standard load and determining the percentage of fines produced as a result of the crushing action.The ACV is expressed as a percentage.ACV is essential because it assesses the aggregate's strength and durability.A high ACV indicates weaker aggregate, which may not be suitable for applications requiring strong aggregates, such as concrete for structural purposes.For LWA, a low ACV is generally desired, as it suggests a higher resistance to crushing, making them suitable for lightweight concrete and other applications where weight reduction is essential.AIV measures an aggregate's ability to resist impact or sudden shock.It involves dropping a standard steel hammer from a specified height onto an aggregate sample and measuring the percentage of fines produced as a result of the impact.AIV is expressed as a percentage.AIV is important for assessing the aggregate's toughness and its ability to withstand sudden loads or shocks.For LWA, a lower AIV is typically preferred because it indicates that the aggregate is less prone to breaking or shattering upon impact.LWA is often used in applications where impact resistance is necessary, such as in lightweight concrete for road pavements.The ACV and AIV results are shown in figure 10.
The ACV values increased as the sewage sludge content in the aggregate increased.As the sewage sludge particles are not uniform and may contain larger particles or hard inorganic materials, it increases the ACV because it can introduce weaker points in the aggregate structure and lead to a decrease in the resistance of the aggregate to crushing.The AIV value on the other hand was similar for all the three aggregate types.This is because sewage sludge often contains organic matter, which can make the aggregates more resilient to impact   [38].The organic matter can act as a binding agent and provide flexibility, making the aggregates less brittle and less prone to fracturing upon impact.

Compressive strength and splitting tensile strength
The compressive strength and splitting tensile strength are conducted based on the recommendations from ASTM C330 [35].According to the standard, there is a linear relationship between the bulk density of lightweight concrete and its compressive strength, which is used to test the effectiveness of LWA.The required compressive strength and splitting tensile strength for LWA as specified in section 5.2.1 of ASTM C330-05 is shown in table 4.
In this study, lightweight geopolymer concrete will be used to test the efficacy of the LWA.As per the requirements, normal lightweight concrete (NLC) and lightweight geopolymer concrete (LGC) are designed according to ACI 211.2-98 [44].For geopolymer concrete, an alkaline solution is used instead of water to activate the Class F fly ash binder-based geopolymer concrete, and the same procedure mentioned in ACI 211.2 is adopted.The density, splitting tensile strength, and compressive strength of the resulting concrete are then validated as per table 4 by interpolating to confirm the effectiveness of the aggregates produced.The LGC is designed for M20 grade using the L0, L1, and L2 aggregates and three samples are taken for each mix.The mix design procedure is adopted from ACI 211.2-98 and the proportions are identified by assuming the data wherever required.The compressive strength test was conducted after one day of heat curing at 60 °C and ambiently curing the sample for 28 days and the results are shown in table 5 [45].
All three mixes exceeded the design compressive strength of 20 MPa as LGC generally tends to have a higher strength than normal concrete.The results in table 5 show that the LWA conformed to the specifications mentioned in ASTM C330 when used in LGC.The M20 LGC had a compressive strength of 28.04 MPa, 23.54 MPa, and 21.51 MPa for the mixes containing L0, L1, and L2 aggregates respectively.As per section 5.2.1 of ASTM C330-05, the required densities for the resultant compressive strength were 1840 kg m −3 , 1800 kg m −3 and 1760 kg m −3 .It is clear from table 5, that the density more or less conformed to the requirements.Similarly, the splitting tensile strength requirements for the corresponding concrete density exceeded by 43.91%, 35.02%, and 32.70% for the L0, L1, and L2 samples respectively, thereby indicating a wide range of applications.LGC generally tends to have higher compressive strength when compared to conventional concrete.This was the  basis for adopting geopolymer to test the efficiency of the sewage sludge-based LWA, which had slightly lesser physical properties when compared to commercially available LWA. Geopolymers are formed through the alkali activation of aluminosilicate materials and the resulting geopolymer gel has a stronger chemical bond.This robust bonding contributes to higher compressive strength.The alkaline environment in geopolymer concrete promotes silica polymerization leading to a dense microstructure.Similarly, LGC does not produce calcium hydroxide, which is a common byproduct in cement hydration.This reduces the voids present in the matrix and enhances the splitting tensile strength of the geopolymer concrete.The W/C ratio curve adopted from ACI 211.2-98 for this mix design is not suitable for LGC, as shown in the compressive strength results for L0 aggregate, where the LGC mix outperformed the target compressive strength.Therefore, to facilitate future mix design and to better understand the behavior of LGC, a new Alkaline solution to Binder (A/B) curve is proposed for designing LGC, as shown in figure 11.
An overall comparison between the sewage sludge-based aggregates and the commercially available LYTAG aggregates is shown in table 6.

Conclusions
This experiment was aimed at developing sewage sludge-based Lightweight Aggregates (LWA) that exhibited strength comparable to commercially available aggregates, and has yielded promising results.This achievement has significant implications for sustainable and environmentally responsible construction practices.
• Treated raw sewage sludge-based aggregates were formed using a combination of lime and fly ash using cold bonded method and sintering method.The cold bonded aggregates displayed a gray color, while the sintered aggregates appeared brown in color due to the ferrous deposits from the sintering process.Sintered aggregates were harder and well-formed despite weighing less than the cold bonded aggregates.Cold bonded aggregates disintegrated under slight pressure due to their weak formation.The high temperature sintering method led to solid, well-formed aggregates.Cold bonding, which occurs at lower temperatures, resulted in incomplete fusion and weak aggregate formation, particularly with sewage sludge.Sewage sludge's complex and variable composition made it less compatible with the cold bonding method, affecting the binding and aggregate  quality.High temperature sintering allowed for better chemical reactions and improved particle binding.Moisture content was another critical factor, as sewage sludge has high water absorption.The cold bonding method required precise moisture control while sintering naturally removed excess moisture.Some coldbonded mixtures were improperly formed and unsuitable for testing.In summary, sintering is generally more effective for forming LWA from sewage sludge due to temperature control, chemical reactions, and moisture management.Cold bonding remains feasible but demands meticulous consideration of sewage sludge properties and binder use for successful aggregate formation.
• In the sintering method, the study found that an increase in sewage sludge content led to a reduction in both bulk density and specific gravity of the samples, with a linear relationship between the two.The bulk density of L0 sample, which did not contain any sewage sludge, was 749 kg m −3 and the bulk density of L2 sample, which contained 20% sewage sludge was 641 kg m −3 .This reduction was attributed to the high content of organic matter in sewage sludge, the release of volatile gases during production, and the formation of a porous structure.While decreasing bulk density is desirable for weight savings, it is crucial to strike a balance to ensure that the aggregates maintain sufficient mechanical strength for construction applications, which cannot be achieved by increasing sewage sludge content.
• As the sewage sludge content increased, there was also a gradual rise in water absorption by the LWA.There was a difference of 2% in the water absorption value of the sample containing 20% sewage sludge (16.25%) and the sample without sewage sludge (14.25%).This increase is primarily due to the high water absorption characteristics of sewage sludge, driven by its organic content and porous structure.Sewage sludge's composition, including organic matter and interconnected voids, enable it to absorb and retain water, much like a sponge.
• The Aggregate Crushing Value (ACV) tended to rise as the sewage sludge content in the aggregate increased.ACV value of ordinary fly ash aggregates was around 60.46%, whereas for the L2 mix containing 20% sewage sludge it was 48.23%.This is attributed to the non-uniform nature of sewage sludge particles, which may include larger particles or hard inorganic materials.These irregularities introduced weak points in the aggregate structure, leading to reduced resistance against crushing.
• In contrast, the Aggregate Impact Value (AIV) remained consistent across all three aggregate types at around 30% to 32%.The presence of organic matter in sewage sludge contributed to this stability.Organic matter acts as a binding agent, enhancing aggregate resilience to impact.Consequently, the aggregates become less brittle and less prone to fracturing upon impact.
• The individual pellet strength of the 4mm LWA decreased from 7.10 MPa to 5.80 MPa for the samples without sewage sludge and the aggregate sample containing 20% sewage sludge.This was due to the irregular size and the presence of organic matter in sewage sludge as discussed above.This study also observed a consistent trend where, as the diameter of the pellets increased, the individual pellet strength decreased.This relationship is explained by the larger internal volume created within larger pellets, resulting in a higher proportion of air voids within the structure.These air voids act as regions of weakness within the pellet, ultimately reducing its structural integrity.The linear relationship between strength and diameter allows for the calculation of individual pellet strength using the derived equations without having to test the specimens physically.Based on the above results, it was concluded that a mix of 10% lime, 70 to 80% fly ash, and 10 to 20% raw sewage sludge can be used to develop LWA of commercial quality by sintering method.Finally, the derived aggregates were used to produce lightweight geopolymer concrete (LGC).The results showed that the LGC exceeded the design requirements and displayed excellent properties, thereby indicating a wide range of applications.LWA made from waste materials, offer several advantages over natural crushed stone aggregates in specific applications and environmental contexts.These include cost benefits, sustainability, resource conservation, waste reduction, reduced energy, carbon footprint, weight reduction, etc.

Future recommendations
• Further investigation can be done on the sintering process by varying parameters such as temperature and duration which could yield the best combination of strength, color, and overall aggregate quality.
• Alternative binders beyond fly ash and lime can be explored.
• Long-term durability tests on sintered aggregates can be conducted their performance under various environmental conditions (e.g., freeze-thaw cycles, chemical exposure) can be studied.
• Assessing the economic feasibility of large-scale production of sewage sludge-based LWA and optimizing the cost for widespread adoption in construction practices.

Figure 3 .
Figure 3. Physical appearance of cold bonded and sintered aggregates.

Figure 6 .
Figure 6.Relationship between bulk density and specific gravity for the sintered samples.

Figure 7 .
Figure 7. SEM image of raw sewage sludge.

Figure 11 .
Figure 11.Comparison between ACI W/C ratio to compressive strength relationship for NLC and A/B ratio to compressive strength for LGC.

Table 1 .
Properties of fly ash and sewage sludge.

Table 2 .
Chemical composition of fly ash and sewage sludge.CaO) was used in this study for the production of LWA by pelletization.When lime is mixed with water, it undergoes a chemical reaction to form calcium hydroxide (Ca(OH) 2 ).

Table 4 .
Requirements for LWA and sand-based concrete.

Table 5 .
Compressive strength and splitting tensile strength results.

Table 6 .
Properties of sewage sludge-based LWA and LYTAG.
a LYTAG is a commercially available LWA that has been procured and used as a reference for this study.bTheconcrete properties of L0, L1, and L2 mixes are for lightweight geopolymer concrete, and LYTAG is for normal lightweight concrete.