Potential pathway for reusing Paper Mill Sludge Ash as a Supplementary Material in Alkali-Activated Bricks

doi:10.21203/rs.3.rs-3230803/v1

Alkali-activated bricks are considered a sustainable alternative in masonry construction. These bricks are produced by reacting the waste materials in an alkaline environment using alkali activators such as sodium silicate and sodium hydroxide. In this study, our objective is to explore the viability of manufacturing alkali-activated bricks using industrial waste materials, specifically by fly Ash, Paper Mill Sludge Ash (PMSA), and Crusher Sand Dust (CSD). To reduce the reliance on fly ash, alkali-activated bricks are manufactured by substituting PMSA in two different melds. The first meld is performed to optimize the PMSA and the second meld is carried out to optimize the Crusher Sand Dust (CSD) as filler material. Meld 3 is used to investigate the impact of the NaOH molarity of the bricks ranging from 6 M to 14 M for the optimum proportions obtained from Melds 1 and 2. XRD, FT-IR, TGA, and SEM analyses were carried out to understand the mineralogical and chemical characterization of the raw materials and brick specimens. Furthermore, various tests were conducted on the bricks to assess their physio-mechanical properties, following the relevant standards outlined in IS requirements. Overall, this study demonstrates the feasibility of producing alkali-activated bricks by utilizing industrial waste materials. These findings offer promising prospects for creating sustainable construction materials while effectively addressing the issue of waste disposal.

Alkali-activated bricks

geopolymers

paper mill sludge ash

fly ash

physio-mechanical properties

Bricks have been the fundamental and oldest construction material, with clay brick masonry being extensively used in India. The country produces approximately 250 billion bricks annually from approximately one million brick kilns, resulting in the depletion of nearly 600 million tonnes of clay soil each year (Kumarasamy et al. 2022). Although fired clay bricks are cost-effective and durable, their production has a substantial environmental impact as they consume and deplete environmental resources. Moreover, the manufacturing process of these bricks emits significant amounts of CO₂ and NO₂, making them environmentally unsustainable due to overconsumption and depletion of natural resources (Cicek and Tanrıverdi 2007; Gencel et al. 2013; Weyant et al. 2016). Consequently, utilizing waste materials as raw materials for brick production offers a favourable solution both economically and environmentally.

Previous research has explored the incorporation of waste materials in the construction industry, particularly in brick manufacturing. Various studies have investigated the production of bricks using waste materials as additives or replacements for conventional materials, with a specific focus on fly ash due to its excellent chemical properties (Zhang et al. 2012). In India, electricity generation relies heavily on thermal power plants, which consume approximately 150 million tonnes of coal annually. Fly ash, a major byproduct of these coal-fired power plants poses challenges in terms of its handling and disposal. However, its cementitious nature has made it a subject of study for various applications, which include using them as concrete additives, in brick production, for agricultural purposes, for heavy metal immobilization, and as a binder for sediments (Lin and Lin 1994; Hammermeister et al. 1998; Silitonga et al. 2009). Consequently, fly ash bricks have gained popularity due to their higher compressive strength (approximately 40–50% greater) and lower weight density (approximately 10% lesser) compared to conventional clay bricks (P. Bhangale and P. M. Nemade 2013). However, the availability of fly ash has decreased significantly as it is increasingly exploited as an alternative cementitious material in the construction industry (Balaguera et al. 2018). Thus, the supply of fly ash has been greatly reduced, leading to its diminished availability.

To combat pollution and create a sustainable environment, it is crucial to reuse and maximize the utilization of discarded waste (Zhang 2013). Recent literature emphasizes the documentation of construction materials that substitute conventional materials, giving priority to waste obtained from various industries. These wastes are dumped into landfills, which may affect agricultural land and ultimately affect vegetation, creating soil erosion. Agricultural industries generate solid waste feedstock, wheat straw ash, and rice husk ash and other industries generate significant amounts of byproducts such as metal slag, ground granulated furnace slag, and incinerated municipal waste ash, making them potential sources for material substitution (Al-Akhras and Abu-Alfoul 2002; Luukkonen et al. 2018; Vetturayasudharsanan et al. 2022). Blending these waste materials can modify the durability, physical properties, and chemical characteristics of cementitious materials. One such industrial waste material is paper mill sludge ash.

During the paper manufacturing process, large quantities of waste and sludge are produced, with wood and recycled paper being major contributors (Govindan and Kumarasamy 2023). These sludges typically contain organic (cellulose strands) and inorganic matter, which are used as fillers in cementitious material due to their rich calcium carbonate and kaolinite content (Mamat et al. 2018). The sludge undergoes dewatering and is subsequently incinerated to reduce waste volume and recover energy. This incineration process yields approximately 10–15 kg of paper sludge ash for every tonne of paper produced (Méndez et al. 2009). Utilizing PMSA in brick manufacturing can lead to a cleaner environment and minimize disposal costs (Ferrándiz-Mas et al. 2014). Therefore, this study focuses on the utilization of both fly ash and paper mill sludge ash (PMSA) in brick manufacturing. Considering the harmful emissions and resource depletion associated with clay bricks, it becomes imperative to adopt environmentally friendly methods. In this context, geopolymer technology has emerged as a solution for producing unfired bricks from waste materials, leveraging their chemical properties (Sutcu et al. 2015; Eliche-Quesada et al. 2018).

Geopolymers are inorganic polymers formed through the polymerization reaction of aluminosilicate raw materials in an alkaline environment (Davidovits 1991). They are used to produce unfired bricks with fewer energy requirements and with zero emission of hazardous gases (Komnitsas and Zaharaki 2007). They are generally formed from alumina and silica-rich materials such as fly ash, and GGBS under the alkali activation process (Davidovits; Khale and Chaudhary 2007). This reaction results in the formation and condensation of aluminosilicate gel, creating a three-dimensional polymeric structure (Davidovits; Pacheco-Torgal et al. 2008). The polymerization process involves two significant phases: the formation of sodium hydrosilicate gel (NASH) and calcium silicate hydrate gel (CSH). NASH gel forms a tetrahedral system of SiO₄ and AlO_4, which contributes to the cementitious properties, durability, and strength of alkali-activated materials (Waijarean et al. 2014). CSH gel is similar to the gel formed in Portland cement-based materials which acts as a binding material, enhancing the strength and bonding of the raw materials (Raut et al. 2012). Since fly ash contains a higher composition of silica and alumina, and paper sludge ash has a higher calcium content, they can both form NASH and CSH, making them suitable binders for polymerization.

The objective of this study is to investigate the physical, chemical, and mineralogical properties of PMSA and determine the optimal replacement proportion of PMSA in fly ash-based geopolymer bricks. Additionally, the study examines the performance of bricks when an additional silica source (crusher sand dust) is added along with the primary raw materials as filler material while varying the molarity of sodium hydroxide in the alkali activator solution. Mechanical tests are conducted on the bricks, and both the raw materials and bricks undergo chemical and microstructural analysis, including TGA, FT-IR, XRD analysis, and SEM imaging. The findings of this research contribute to the advancement of sustainable construction practices by demonstrating a viable pathway for transforming various waste materials into high-quality building products, thereby promoting resource efficiency and environmental conservation.

This research paper introduces a ground-breaking approach to produce alkali-activated bricks by incorporating a unique combination of paper mill sludge ash, fly ash, and crusher sand dust as primary constituents. The integration of these three materials in the brick manufacturing process presents a novel and innovative solution for utilizing multiple waste streams of different proportions from different industries by varying the molar concentration of the alkali-activator solution. By synergistically blending these components, the study not only addresses the challenges of waste management but also explores the potential synergistic effects and improved performance of the resulting bricks.

3.1 Materials

Alkali-activated bricks are manufactured primarily using three raw materials: fly ash, paper mill sludge ash (PMSA), and crusher sand dust (CSD). For this study, Class F-type fly ash was chosen, adhering to IS 3812 (part 1):2003 (of Indian Standards). Due to its higher silica and alumina and low calcium content compared to the Class C type of fly ash, Class F is used (Wardhono 2018; Govindan and Kumarasamy 2023). This composition is beneficial for polymerization and the formation of NASH and CSH networks (Madani et al. 2020). The fly ash used in the manufacturing process is sourced from the Mettur Thermal Power Plant in Tamil Nadu, India, adhering to the standards set by IS 3812 (Part-1):2003 (IS 3812 (Part-1):2003).

The residue obtained from the secondary treatment of the paper manufacturing process, known as paper sludge, was collected from Tamil Nadu Newsprint and Papers Limited (TNPL) in Karur, Tamil Nadu, India. This sludge undergoes a dewatering process and is then incinerated at various temperatures. Combustion of the sludge removes the organic matter, resulting in a residue called paper mill sludge ash. The properties and characteristics of PMSA can vary depending on the type of paper sludge and the specific combustion conditions employed (Bai et al. 2003; Vegas et al. 2006; García et al. 2008; Frías et al. 2008; Monte et al. 2009).

Therefore, to examine the chemical characteristics of PMSA under altered incineration temperatures, the collected paper sludge is burned at different temperatures (600°C, 800°C, 1000°C, and 1200°C) in a muffle furnace for two hours to convert them into ash (Furlani et al. 2008). This process helps to identify PMSA samples with higher silica content. Figure 1 illustrates the manufacturing process of PMSA and the visual differences in PMSA appearance at various incineration temperatures. Table 1 presents the variation in the chemical composition of PMSA corresponding to different incineration temperatures and the test methods employed to identify the chemical compounds. The silica and alumina contents in PMSA gradually increase as the incineration temperature rises, peaking at 10.46 wt% and 15.02 wt% at 1200°C, respectively. However, the calcium content in the PMSA decreased with an increase in its incineration temperature. This is due to the thermal decomposition of calcium compounds, leading to the release of water vapour and CO₂, resulting in decreased calcium content in the ash. Consequently, the paper mill sludge is incinerated at 1200°C, reducing it to ash used for brick production.

Table 1

Variation of the chemical composition of PMSA at different incinerating temperatures
S.NO.	CONSTITUENT	Chemical composition [in wt%]				TEST METHODS USED
S.NO.	CONSTITUENT	600^oC	800^oC	1000^oC	1200^oC	TEST METHODS USED
1.	Silica [SiO₂]	6.04	7.82	10.18	10.46	Gravimetric Method
2.	Aluminium Oxide [Al₂O₃]	12.45	13.98	13.83	15.02	EDTA Method
3.	Calcium Oxide [CaO]	50.49	44.37	41.02	36.65	EDTA Method
4.	Magnesium Oxide [MgO]	1.34	1.42	1.67	0.72	EDTA Method
5.	Iron Oxide [Fe₂O₃]	2.13	2.06	1.74	0.76	EDTA Method
6.	Potassium Oxide [K₂O]	0.58	0.53	0.32	0.11	Acid Base Titration
7.	Sodium Oxide [Na₂O]	1.35	0.75	0.67	0.43	Acid Base Titration
8.	Loss of Ignition [LOI]	25.62	29.07	30.57	35.85	Weight loss method

Before mixing, the solid raw materials were passed through a 90 µm sieve to ensure particle fineness and uniformity. In addition to the primary raw materials mentioned earlier, Crusher Sand Dust (CSD) is also used, which serves as a filler material and contributes an additional source of silica to enhance the polymerization reaction, which improves the mechanical properties of the bricks and reduces material costs. CSD is the residual waste obtained from the production of aggregates and is sourced from Konkan Aggregates Pvt. Ltd. in Erode, Tamil Nadu, India. Table 2 provides the physical properties of the raw materials used in brick manufacturing.

Table 2

Physical properties of Raw materials
Physical Properties	Fly Ash	PMSA	CSD
Specific gravity	2.73	1.84	3.11
Surface area in m²/kg	1020	4150	3260
Bulk density in kg/m³	1170	1250	1640

The alkali activators used in the alkali-activated brick manufacturing process typically consist of sodium or potassium hydroxides and silicates, which facilitate the polymerization reactions (Komnitsas and Zaharaki 2007). Sodium-based activators are generally preferred due to their higher alkalinity and reactivity. They release a greater number of silicate and aluminate monomers, promoting better polymer formation (Duxson et al. 2007a; Singh et al. 2015; Atabey et al. 2020). Sodium silicate is also employed in conjunction with sodium hydroxide to maintain a balanced SiO₂/Na₂O ratio, known as the silica modulus (KAYA et al. 2018). Figure 2 illustrates the solid raw materials involved in the production of PMSA bricks.

3.2 Preparation of Alkali Activator Solution

The alkali activator solution involves the dissolution of sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) in water, which is used to activate raw materials to form alkali-activated materials (Duxson et al. 2007b). Sodium hydroxide is in pellet form, while sodium silicate is a transparent gel - like liquid with a purity of 98%. The physical properties of the alkali-activated solution are detailed in Table 3. To minimize the formation of gas bubbles and subsequent cracking, sodium silicate was mixed with sodium hydroxide 24 hours before usage. This delay helps in reduce the release of heat and gas bubbles.

Table 3

Physical properties of Alkali activators
Property	Sodium Hydroxide [NaOH]	Sodium Silicate [Na₂SiO₃]
Specific gravity	1.58	1.36
Density	2.15 g/cm²	1.50 g/mL
Molar Mass	40.10 g/mol	125.25 g/mol
Purity	98%	98%

The chemical reactions involved during the formation of the alkali activator solution can be provided in two steps:

Dissolution of sodium hydroxide (NaOH) in water: When NaOH gets dissolved in water, it releases sodium ions (Na⁺) and hydroxide ions (OH^-), which provide the alkaline environment required for the geopolymerization reaction.

NaOH + H₂O \(\to\) Na⁺ + OH^-

Dissolution of sodium silicate (Na₂SiO₃) in water: When Na₂SiO₃ is dissolved in water, it also releases sodium ions (Na⁺) along with orthosilicic acid (Si(OH)₄), which provides the silica required for the polymerization process.

Na₂SiO₃ + H₂O \(\to\) 2Na⁺ + Si(OH)₄

When this solution reacts with the silica (SiO₂) and alumina (Al₂O₃) present in the raw materials (fly ash, PMSA, and CSD), it initiates a geopolymerization reaction, which forms a three-dimensional geopolymer matrix, resulting in the formation of alkali-activated materials with cementitious properties. Additionally, the concentration of NaOH and the ratio of sodium silicate to sodium hydroxide [Na₂SiO₃/NaOH] in the alkali-activator solution significantly influence the polymerization reaction and the characteristics of alkali-activated bricks. When the silicate-to-hydroxide ratio is less than 2, there is insufficient availability of Na⁺ ions and an excessive amount of unreacted “Si ions” (Zawrah et al. 2016). Conversely, when the ratio exceeds 2, the mechanical properties of the bricks are affected due to an excess of unreacted Na⁺ ions (Zawrah et al. 2016; Youssef et al. 2019). Hence, throughout this investigation, the Na₂SiO₃/NaOH ratio is maintained at a constant value of 2. Previous studies in the literature have shown that molar concentrations of sodium hydroxide ranging from 6 M to 14 M yield better polymerization reactions (K and Babu 2018; Ghafoor et al. 2021). Consequently, alkali activator solutions are prepared in five different molarities (6 M, 8 M, 10 M, 12 M, and 14 M) to assess the impact on PMSA bricks under varying molarities.

3.3 Mix proportions

To maximize the utilization of PMSA in the manufacturing of alkali-activated bricks and minimize the quantity of fly ash, three different melds were made to determine the optimal combination. In Meld 1, the focus was on optimizing the PMSA quantity by completely replacing the fly ash with PMSA from 0 wt% to 100 wt%. Initially, the bricks were cast with 8 M molarity, as it is considered an ideal molar concentration (Youssef et al. 2019; Kumarasamy et al. 2022). From the preliminary test results, the bricks cast with 8 M molarity at 40 wt% replacement of PMSA, outperformed all the mix proportions from Meld 1. Hence, in Meld 2, the optimization of CSD was performed by fixing the PMSA content at 40% by weight. The replacement of CSD was carried out up to 30% by weight, where 10 wt% replacement gave better results. Meld 3 is executed to identify the performance of bricks (optimum from Meld 2), under varying NaOH molarity for achieving ideal PMSA bricks. The ratio of alkaline solution to binder is a critical factor in the production of alkali-activated bricks. An appropriate liquid-to-binder ratio ensures sufficient availability of the alkaline solution to activate the binder materials and promote the desired reaction kinetics. Several studies in the literature have concluded that a binder-to-liquid ratio lower than 0.3 or higher than 0.8 does not contribute to effective polymerization (Pacheco-Torgal et al. 2008; Sukmak et al. 2013). Therefore, the alkaline solution to binder ratio was fixed at 0.3, based on preliminary mix tests to ensure workability, shape development without deformation, and prevention of crack formation. Table 4 presents the different mix combinations used for manufacturing PMSA bricks, considering the varying percentages of PMSA, CSD, fly Ash, and NaOH molarity.

Table 4

a Meld 1 for optimization of PMSA
MIX NO.	MIX ID	NaOH Molarity	PMSA	Fly Ash	CSD	Na₂SiO₃/NaOH	Alkaline Solution/Binder
MIX NO.	MIX ID	NaOH Molarity	In per cent by weight			Na₂SiO₃/NaOH	Alkaline Solution/Binder
1	PMSAB 1	8 M	0	100	0	2	0.3
	PMSAB 2		5	95	0
	PMSAB 3		10	90	0
	PMSAB 4		15	85	0
	PMSAB 5		20	80	0
	PMSAB 6		25	75	0
	PMSAB 7		30	70	0
	PMSAB 8		35	65	0
	PMSAB 9		40	60	0
	PMSAB 10		45	55	0
	PMSAB 11		50	50	0
	PMSAB 12		55	45	0
	PMSAB 13		60	40	0
	PMSAB 14		65	35	0
	PMSAB 15		70	30	0
	PMSAB 16		75	25	0
	PMSAB 17		80	20	0
	PMSAB 18		85	15	0
	PMSAB 19		90	10	0
	PMSAB 20		95	5	0
	PMSAB 21		100	0	0

Table 4

b Meld 2 for optimization of CSD.
MIX NO.	MIX ID	NaOH Molarity	PMSA	CSD	Fly ash	Na₂SiO₃/NaOH	Alkaline Solution/Binder
			In per cent by weight
2	PMSAB 22	8 M	40 (optimum from Meld 1)	5	55	2	0.3
	PMSAB 23			10	50
	PMSAB 24			15	45
	PMSAB 25			20	40
	PMSAB 26			25	35
	PMSAB 27			30	30

Table 4

b Meld 2 for optimization of CSD.
MIX NO.	MIX ID	NaOH Molarity	PMSA	CSD	Fly ash	Na₂SiO₃/NaOH	Alkaline Solution/Binder
			In per cent by weight
2	PMSAB 22	8 M	40 (optimum from Meld 1)	5	55	2	0.3
	PMSAB 23			10	50
	PMSAB 24			15	45
	PMSAB 25			20	40
	PMSAB 26			25	35
	PMSAB 27			30	30

3.4 Preparation of alkali-activated PMSA bricks

The alkali-activated PMSA bricks are fabricated based on the specified mix proportions by maintaining an alkaline solution-to-binder ratio of 0.3 and a silicate-to-hydroxide ratio of 2. The raw materials are initially dry-mixed for approximately 4 to 5 minutes to ensure uniformity. Subsequently, the prepared alkaline solution is added to the mixture while continuous mixing is performed until the desired workability is achieved. The resulting mixture is then shaped using a brick-making machine with a brick mould size of 230 x 105 x 75 mm. Hydraulic pressure is applied equally over the brick mould during the moulding process. Once the bricks are successfully cast, they undergo ambient curing for further investigation and development of their properties. Figure 3 provides a process flow diagram illustrating the steps involved in the production of alkali-activated geopolymer bricks using PMSA.

The alkali-activated PMSA bricks undergo various tests to evaluate their compressive strength, water absorption, efflorescence, particle size distribution, chemical composition, mineral composition, compound structure, thermal conductivity, drying shrinkage, and SEM morphologies. The testing procedures are conducted under relevant standards and guidelines. The number of specimens for testing is determined according to IS 5454:1976 (IS : 5454 2005). The water absorption, efflorescence, and compressive strength tests of the bricks were carried out following the guidelines specified in IS 3495(Part 1):1992 (IS 3495 (Parts 1 to 4):1992). A universal testing machine with a capacity of 1000 kN was used to identify the compressive strength. The voids present in the alkali-activated bricks are determined in accordance with ASTM C642 ([CSL STYLE ERROR: reference with no printed form.]). The impact resistance of the bricks is assessed by conducting a drop test of the bricks by dropping a 1000 g steel ball from a fixed height of 1000 mm. The number of blows required for the development of the first visible crack (N_i) and the specimen's final failure (N_f) are recorded to calculate the impact energy using a modified test setup as depicted in Fig. 4a. Drying shrinkage tests are conducted by following the guidelines provided in IS 4139:1989 (IS 4139:1989). The wet and dry lengths of the bricks are measured using a dial gauge and a drying shrinkage apparatus is set up as shown in Fig. 4b. The thermal conductivity of the bricks was measured using a quick thermal conductivity meter (QTM-500, Kyoto Electronics, New Castle). The measurements are performed under completely dry and saturated conditions at room temperature.

A laser scattering diffraction particle size distribution analyser (Partica LA-950) is employed to measure the particle size distributions, covering a range of particle sizes from 10 nm to 3 mm. X-ray diffraction (XRD) analysis was conducted to determine the chemical and mineral compositions, which utilized an Empyrean, Malvern Panalytical X-ray diffractometer with Co source Cu Kα (λ = 1.54 Å). Fourier Transform Infrared (FT-IR) spectroscopy (PerkinElmer) was employed to obtain the compound structure of PMSA bricks and their major raw materials. Dry powdered samples were scanned using FT-IR spectroscopy, utilizing KBr (potassium bromide) pellets made from powdered samples. Scanning electron microscopy (SEM) was used to examine the SEM morphologies of FaL-G brick specimens and raw material samples. The brick specimens are examined and photos are taken with an accelerated voltage of 15 kV using SEM VEGA3 TESCAN.

These comprehensive testing procedures provide insights into the mechanical, physical, microstructural, and chemical properties of PMSA geopolymer bricks, facilitating a thorough understanding of their characteristics and performance.

5.1 Chemical and mineralogical characterization, and microstructural analysis of raw materials

The chemical composition of the raw materials utilized in the production of alkali-activated PMSA bricks is presented in Table 5. Fly ash primarily consists of almost 60% silica (SiO₂) and aluminium oxide (Al₂O₃), with only 13.06% calcium oxide (CaO). In contrast, PMSA predominantly contains calcium oxide (CaO) accounting for 33.65 wt% (incinerated at 1200°C as discussed in section 3.1). The addition of Crusher Sand Dust (CSD) introduces a significant amount of silica, which was slighter in PMSA, thereby providing the necessary silica for polymerization. Other metal oxides present include magnesium oxide (MgO), iron oxide (Fe₂O₃), potassium oxide (K₂O), and sodium oxide (Na₂O).

Table 5

Chemical composition of raw materials
S.NO.	CONSTITUENT	Chemical composition [in wt%]
S.NO.	CONSTITUENT	Fly Ash	PMSA	CSD
1.	Silica [SiO₂]	41.34	10.46	89.74
2.	Aluminium Oxide [Al₂O₃]	22.27	15.02	6.11
3.	Calcium Oxide [CaO]	13.06	33.65	3.04
4.	Magnesium Oxide [MgO]	1.53	0.72	0.51
5.	Iron Oxide [Fe₂O₃]	8.92	0.76	0.60
6.	Potassium Oxide [K₂O]	2.45	0.11	-
7.	Sodium Oxide [Na₂O]	1.36	0.43	-
8.	Sulphur Trioxide [SO₃]	2.73	-	-
9.	Titanium dioxide [TiO₂]	0.51	-	-
10.	Loss of Ignition [LOI]	5.83	38.85	-

FT-IR analysis was conducted on both PMSA and fly ash to identify organic and inorganic materials by examining the absorption band patterns. The FT-IR pattern obtained is depicted in Fig. 5a. A sharp absorption peak was observed at 855 cm^− 1, corresponding to the OH group, along with C-O stretching vibrations at 997 cm^− 1. The presence of the OH group indicates hydroxyl-bearing minerals in the raw materials. The C-O bond specifies the presence of carbonates such as calcite and dolomite present in the raw materials. Additionally, an absorption peak at 1430 cm^− 1 indicated aliphatic stretching. Minor peaks were also identified at 1594 cm^− 1, 3699 cm^− 1, and 3038 cm^− 1, resulting from the stretching vibration of C-C bonds. These C-C bonds provide evidence of organic matter, such as lignin or cellulose, which can be present in PMSA (Coates 2006).

Materials consist of crystallites, and the chemical composition of these crystallites determines their phase. Each crystalline phase produces a distinct diffraction pattern, which can be identified through X-ray diffraction (XRD) analysis. The differentiation in the XRD patterns of fly ash and PMSA is illustrated in Fig. 5b. This indicates that lime [CaO] and periclase [MgO] are the major constituents of PMSA, which was similar to the previous studies (Goel and Kalamdhad 2017a). It has weaker peaks corresponding to tricalcium aluminate [C₃A], larnite [Ca₂SiO₄], brownmillerite [Ca₂(AlFe)₂O₅], and dolomite [CaMg(CO₃)₂]. On the other hand, the major crystalline phase in fly ash is Quartz [SiO₂], with minor constituents of Anhydrite [CaSO₄] and Calcite [CaCO₃]. These findings demonstrate that the raw materials comprise a significant amount of Silica, Calcium, and Alumina rich compounds, which get reacted with an alkali activator solution to form geopolymer gels.

Figure 6 displays the particle size distribution of fly ash, PMSA, and CSD. The results reveal that the raw materials used exhibit a wide range of particle sizes. The particle size of fly ash ranges from 0 to 100 microns, while the particle size distribution of PMSA contains both fine and coarse particles with a significant portion falling within the range of 1 to 1000 microns. Although they both contain fine particles, PMSA is more finely divided due to the nature of the incineration process. CSD is usually composed of finely crushed stone particles and has a particle size distribution that primarily falls within 50 microns which falls under the Zone IV category as per IS 383–1016 ([CSL STYLE ERROR: reference with no printed form.]). Thus, these fine particles enhance reactivity and form a great geopolymer binder, facilitating a geopolymerization process leading to enhanced mechanical properties.

To understand the surface morphologies of the raw materials, SEM analysis was carried out, and the micrographs were studied. At magnifications of 500x and 2000x, the SEM images of fly ash demonstrate smooth, homogeneous, and spherical-shaped particles, resembling the texture of cement as shown in Fig. 7a-b. In contrast, the SEM images of PMSA at magnifications of 650x and 3000x as illustrated in Fig. 7c-d reveal the diverse shapes of its particles. They appear more irregular, porous, and rough due to the presence of unburnt residues of inorganic particles from paper mill sludge. These particles possess hydrophilic properties and undergo water absorption during their oxide hydration process (Grau et al. 2015).

5.2 Thermogravimetric analysis of raw materials

Thermogravimetric Analysis (TGA) is a technique used to investigate the thermal behaviour and composition of materials by measuring their weight changes as a function of temperature. Upon analysing the TGA curve, it is evident that the fly ash sample exhibits fewer conversions during heating, resulting in a total mass loss of 4.29%. The combustion of bituminous coal leads to the formation of fly ash, which retains calcium (CaCO₃). This calcium carbonate undergoes conversion into calcium oxide (CaO) between 430 and 700°C. Furthermore, the DTG curve displays an exothermic peak at 594°C, indicating the degradation of organic matter as depicted in Fig. 8a. Due to its high quartz content, fly ash demonstrates relative inertness when subjected to heat. Through the thermal analysis of PMSA, the mass loss stage can be divided into four distinct phases. In the first stage, a mass loss of 5.01% occurs up to 200°C, primarily caused by the evaporation of surface moisture or water molecules. The second stage (200°C–375°C) involves rapid devolatilization of organic compounds into combustion gases, resulting in a significant mass reduction of 22.49%. During the third phase (375°C–700°C), the mass loss amounts to 13.52%, attributable to the continued devolatilization process, decomposition of calcium carbonate, and partial combustion of fixed carbons. The fourth stage (beyond 700°C) signifies the combustion of more persistent or stable PMSA products, leading to additional mass loss. The decomposition of cellulose, hemicellulose, and calcium carbonate contributes to a mass loss of 349.0 and 305.0 mg/minute at temperatures of 349°C and 752°C, respectively in the case of PMSA, which is illustrated in Fig. 8b.

5.3 Mechanical properties

5.3.1 Compressive strength

The compressive strength of PMSA bricks was assessed following ambient curing, as it is a critical property for masonry bricks. In Meld 1, the compressive strength of the bricks gradually increased until reaching a 40 wt% replacement of PMSA, with the highest recorded value of 19.13 MPa observed for Mix ID PMSAB 9 at 8 M molarity. Figure 9a illustrates the variation in compressive strength for the bricks in Meld 1 with 8 M molarity. However, as the substitution of PMSA continued beyond 40%, the strength of the bricks started to decline with the increase in PMSA content, which was found to be similar to previous research (Goel and Kalamdhad 2017b; Singh et al. 2018a; Govindan and Kumarasamy 2023). This decrease can be attributed to incomplete polymerization reactions due to the insufficient presence of silicate-rich monomers, which form poor bonds leading to reduced overall strength.

In Meld 2, the strength of the bricks improved due to the addition of CSD alongside the 40% replacement of PMSA, as shown in Fig. 9b. CSD, acting as a filler and additional source of silica, contributes to the formation of a denser and stronger geopolymer matrix. The compressive strength of the bricks increased up to a 10 wt% addition of CSD, but thereafter, it began to decrease. Excessive addition of CSD can weaken the overall matrix, leading to reduced bonding and cohesion between particles of the mix, and may even dilute the alkaline activator solution. The brick specimen with Mix ID PMSAB 23 exhibited the highest compressive strength in Meld 2, reaching 28.03 MPa, which is 31.75% higher than the optimum brick obtained in Meld 1 (PMSAB 9).

In Meld 3, the performance of brick obtained as optimum from Meld 2 (40 wt% PMSA and 10 wt% CSD) is analysed under varying NaOH concentrations. The compressive strength results obtained by varying the molarities were comparable, with an increase in strength observed beyond 8 M molarity, as shown in Fig. 9c. However, there is a slight decline in the strength of the bricks beyond 10 M molarity. This may occur due to the formation of a weak geopolymer binder, as there might be a rapid initial reaction and geopolymerization due to higher NaOH molarity (Suksiripattanapong et al. 2015).

All the bricks produced using different mix proportions exhibited strengths higher than the minimum required compressive strength of 3.5 MPa. Notably, PMSAB 9 and PMSAB 23 bricks met the requirements specified for Class 17.5 and Class 25 bricks, respectively, as per IS 12894:2002 (IS 12894:2002 2002).

5.3.2 Weight density, water absorption, percentage of voids, and efflorescence

The weight density of alkali-activated PMSA bricks decreased with the increasing addition of PMSA, which is evident from previous works in the literature (Goel and Kalamdhad 2017b; Goel et al. 2021). As water absorption is an indicator of porosity, the lines of water absorption and percentage of voids (porosity) run almost parallel to each other in Fig. 10. In Meld 1, the weight density of the bricks ranged from 1845 kg/m³ to 1373 kg/m^3, which is evident from Fig. 10a. The decrease in its strength is due to an incomplete polymerization reaction, because of the lower content of reactive silica and alumina in PMSA, which fails to form the binding phases and geopolymer matrix. They may also result in the incomplete filling of voids and gaps between particles, resulting in an overall increase in porosity and water absorption of the bricks. However, in Meld 2, the weight density of the bricks significantly increased due to the incorporation of CSD. The CSD particles can fill the void spaces and contribute to a greater packing and denser mix than Meld 1. This increases the weight density of the bricks and similarly reduces the porosity and water absorption of the bricks as depicted in Fig. 10b. Furthermore, the weight density of the bricks tended to increase with the increase in the molarity of sodium hydroxide in the alkali-activator solution, as observed in the results of Meld 3 shown in Fig. 10c. This occurs due to the increased amount of sodium hydroxide in the mix, which adds to the overall mass of the material. The water absorption for all the bricks did not exceed 20%, meeting the requirements for bricks of classes higher than 12.5 according to IS 12894:2002 (IS 12894:2002 2002).

Evidence of efflorescence was observed in the alkali-activated bricks as the molarity of sodium hydroxide increased. According to IS 12894:2002, for bricks with classes higher than 12.5, the efflorescence rating should be slight (IS 12894:2002 2002). No efflorescence was observed after the exposure period when the bricks were cast with a 6 M and 8 M molarity. However, bricks cast with a 10 M molarity showed a slight presence of efflorescence, and for higher molarities, the efflorescence varied from moderate to severe. The greater molar concentration introduces more alkali ions into the mix, which can lead to the formation of more soluble salts. These salts are most likely to migrate to the surface during the drying process, resulting in increased efflorescence. Thus, the optimum molarity for alkali-activated bricks was stated to be 10 M, as they have higher strength than bricks cast with other molarities. The observations of efflorescence are depicted in Fig. 11.

5.3.3 Impact test and drying shrinkage

The impact test and drying shrinkage test of the bricks were performed for all three melds and their corresponding impact energy values are tabulated in Table 6. It shows that the impact energy of bricks tends to decrease with the increase in PMSA replacement, as the bricks have lower weight density due to a higher percentage of voids, contributing to decreased impact resistance. However, the impact energy increases with the addition of CSD and increase in NaOH Molarity, due to improved polymerization. As per IS 12894:2002, the drying shrinkage of the bricks should not exceed more than 0.15% (IS 12894:2002 2002). The bricks exceeded this limit when the replacement of PMSA exceeded 45 wt%. In contrast, the drying shrinkage of the bricks increased with the increase in PMSA content of the bricks because they have greater porosity. During the drying stage, the evaporation of water from these pores can lead to the collapse of pore walls, resulting in higher shrinkage. Similarly, lower drying shrinkage is noticed when the CSD and molar concentration are increased in the bricks, as they have lower porosity.

Table 6

Test results of drying shrinkage and impact energy of the bricks
Meld 1 (Fly ash replaced by PMSA)
S.No	MIX ID	PMSA Replacement (wt %)	Drying Shrinkage (10^− 3) %	N_i	N_f	Impact Energy (I_e)
1	PMSAB 1	0	18.32	34.333	48.889	352.19
2	PMSAB 2	5	29.56	32.889	45.333	320.12
3	PMSAB 3	10	32.71	30.000	43.778	303.56
4	PMSAB 4	15	51.03	28.778	40.333	287.28
5	PMSAB 5	20	60.26	27.333	37.889	271.63
6	PMSAB 6	25	81.25	25.889	35.333	249.72
7	PMSAB 7	30	93.78	24.778	34.778	232.49
8	PMSAB 8	35	110.06	22.000	31.889	225.94
9	PMSAB 9	40	123.15	21.333	31.333	213.68
10	PMSAB 10	45	140.37	18.889	28.778	198.11
11	PMSAB 11	50	158.2	17.333	27.333	183.22
12	PMSAB 12	55	176.92	16.000	25.889	164.84
13	PMSAB 13	60	182.09	14.889	24.333	159.95
14	PMSAB 14	65	198.11	12.778	21.889	145.68
15	PMSAB 15	70	217.56	10.000	19.889	127.39
16	PMSAB 16	75	233.47	9.333	18.333	111.73
17	PMSAB 17	80	241.63	8.889	15.778	107.26
18	PMSAB 18	85	275.84	6.778	14.333	98.52
19	PMSAB 19	90	293.26	5.000	11.889	91.03
20	PMSAB 20	95	321.66	4.889	10.333	85.84
21	PMSAB 21	100	347.32	3.333	9.778	78.63
Meld 2 (Fly ash replaced by 40 wt % PMSA (optimum from Meld 1) and CSD)
S.No	MIX ID	CSD Replacement (wt %)	Drying Shrinkage (10^− 3) %	N_i	N_f	Impact Energy (I_e)
22	PMSAB 22	5	138.17	22.889	32.778	231.56
23	PMSAB 23	10	121.05	24.000	30.333	256.82
24	PMSAB 24	15	113.52	24.778	28.889	283.19
25	PMSAB 25	20	105.83	25.333	28.333	301.05
26	PMSAB 26	25	98.72	25.889	26.778	315.68
27	PMSAB 27	30	89.16	26.333	22.889	322.74
Meld 3 (Variation of NaOH Molarity)
S.No	MIX ID	NaOH Molarity	Drying Shrinkage (10^− 3) %	N_i	N_f	Impact Energy (I_e)
28	PMSAB 28	6 M	130.16	23.778	29.889	241.16
29	PMSAB 29	8 M	121.05	24.000	30.333	256.82
30	PMSAB 30	10 M	125.72	24.333	30.889	263.52
31	PMSAB 31	12 M	119.89	25.000	31.333	285.04
32	PMSAB 32	14 M	105.31	25.778	32.333	294.28

5.3.4 Thermal conductivity

The thermal conductivity of the materials greatly influences the heat transfer through the masonry unit, and ultimately the entire structure. The thermal conductivity can be measured in the dry state and saturated state, where the thermal conductivity is lower in the dry state only. The thermal conductivity of the bricks is directly proportional to their porosity, which is evident from the graph illustrated in Fig. 12. Less porous bricks have lower thermal conductivity, making them better thermal insulators. The test results of the thermal conductivity are tabulated in Table 7.

Table 7

Test results of thermal conductivity in the bricks
Meld 1 (Fly ash replaced by PMSA)
S.No	MIX ID	PMSA Replacement (wt %)	Thermal Conductivity (W/mK)
S.No	MIX ID	PMSA Replacement (wt %)	Dry condition	Saturated condition
1	FAB-CB	0	0.551	1.668
2	PMSAB 1	0	0.245	1.832
3	PMSAB 2	5	0.267	1.987
4	PMSAB 3	10	0.294	2.035
5	PMSAB 4	15	0.312	2.127
6	PMSAB 5	20	0.330	2.138
7	PMSAB 6	25	0.365	2.164
8	PMSAB 7	30	0.382	2.180
9	PMSAB 8	35	0.395	2.197
10	PMSAB 9	40	0.401	2.203
11	PMSAB 10	45	0.439	2.215
12	PMSAB 11	50	0.452	2.248
13	PMSAB 12	55	0.488	2.263
14	PMSAB 13	60	0.515	2.293
15	PMSAB 14	65	0.534	2.309
16	PMSAB 15	70	0.563	2.327
17	PMSAB 16	75	0.598	2.341
18	PMSAB 17	80	0.606	2.359
19	PMSAB 18	85	0.634	2.368
20	PMSAB 19	90	0.679	2.380
21	PMSAB 20	95	0.703	2.391
22	PMSAB 21	100	0.720	2.402
Meld 2 ( Fly ash replaced by 40 wt % PMSA (optimum from Meld 1) and CSD)
S.No	MIX ID	CSD Replacement (wt %)	Thermal Conductivity (W/mK)
S.No	MIX ID	CSD Replacement (wt %)	Dry condition	Saturated condition
23	PMSAB 22	5	0.398	2.247
24	PMSAB 23	10	0.381	2.269
25	PMSAB 24	15	0.365	2.283
26	PMSAB 25	20	0.342	2.299
27	PMSAB 26	25	0.319	2.316
28	PMSAB 27	30	0.292	2.347
Meld 3 (Variation of NaOH Molarity)
S.No	MIX ID	NaOH Molarity	Thermal Conductivity (W/mK)
S.No	MIX ID	NaOH Molarity	Dry condition	Saturated condition
29	PMSAB 28	6 M	0.397	2.251
30	PMSAB 29	8 M	0.381	2.269
31	PMSAB 30	10 M	0.378	2.289
32	PMSAB 31	12 M	0.363	2.295
33	PMSAB 32	14 M	0.349	2.303

5.4 Mineralogical characterization and microstructural analysis of alkali-activated PMSA bricks

XRD analysis was conducted to examine the structural and phase changes that occurred after the alkali activation process. Typically, geopolymer compounds exhibit a predominantly amorphous or semicrystalline structure. The semicrystalline nature arises from phase changes during the geopolymerization process, while the minor presence of crystalline phases can be attributed to unreacted raw materials during polymerization (Youssef et al. 2019). Figure 13a provides a comparison plot of PMSA geopolymer bricks with and without the addition of CSD (PMSAB 9 and PMSAB 23, optimum from Melds 1 and 2 respectively). The primary crystalline phases of PMSAB 23 are Quartz (SiO₂), which can be identified by their sharp diffraction peak, while the more amorphous structure of Brownleeite (MnSi), and Hauyne (Na₃Ca(Si₃Al₃)O1₂(SO₄)) is characterized by their broader range of diffraction peaks. However, the intensity of the crystalline peaks gradually decreased, indicating the formation of a new semicrystalline phase comprising NASH and CSH geopolymeric gels. These phases contribute to enhanced polymeric gel formation, thereby improving the strength of the geopolymer bricks. When alkali-activated PMSA bricks are made without the addition of CSD (PMSAB 9 from Meld 1), the diffractogram shows that calcite remains the major crystalline phase alongside quartz, and diffraction peaks corresponding to the semicrystalline aluminosilicate phases are not observed. This suggests that the bricks lacked sufficient silica content to form an aluminosilicate network during alkali activation, failing to impart strength to the bricks.

Figure 13b illustrates the comparison of FT-IR spectra of PMSA geopolymer bricks with and without the addition of CSD. The differing adsorption frequencies in the spectra analyse the extent of polymerization, which is influenced by the atomic arrangements of aluminosilicates and their surface functionalities. The major peak in the FT-IR spectrum of PMSAB 9 is identified at 3429 cm^− 1, corresponding to the stretching vibration of O-H. Other spectral peaks are observed at 1652 cm^− 1 (bending vibration of H-O-H) and 1429 cm^− 1 (carbonate bond). Prominent peaks indicating Al-O and Si-O bonds are seen at 1323 cm^− 1, 1013 cm^− 1, and 649 cm^− 1. The FT-IR spectrum of PMSAB 23 exhibits higher-intensity peaks at 3443 cm^− 1 (O-H stretching), along with significant peaks at 1790 cm^− 1 and 1005 cm^− 1 (formation of aluminosilicate network, Al-O-Si). Additional spectral peaks are observed at 1655 cm-1 (O-H stretching), 1440 cm^− 1 (Si-O-Si stretching - quartz), 876 cm^− 1 (O-Si-O bending), 794 cm^− 1 (asymmetric Si-O-Si stretching), as well as minor peaks at 646 cm^− 1 and 548 cm^− 1, indicating the development of geopolymeric rings.

Figure 14 shows the SEM images that reveal the surface morphology of the PMSA geopolymer bricks. Figure 14a-b illustrates the SEM image of PMSAB 9 at 800x and 3000x magnifications respectively. The presence of minor cracks between unreacted raw materials and the polymeric matrix is observed, resulting from the higher porosity due to an unfinished polymerization reaction. Conversely, the SEM image of PMSAB 23 which is depicted in Fig. 14c-d, at the same magnifications, displays a denser appearance with very few cracks, indicating a higher degree of polymerization. Moreover, PMSAB 23 exhibits minimal voids and pores, which can be attributed to the beneficial impact of incorporating CSD. The particles of CSD fill the void spaces and improve the particle packing in the bricks, enhancing the overall strength and mechanical properties of the bricks. The micrographs also provide evidence of NASH and CSH gel formation during polymerization which is relatively lower in PMSAB 9, indicating a lower degree of polymerization. PMSAB 23 shows the presence of zeolites, which are three-dimensional frameworks made up of tetrahedral units of silicon and oxygen. They have a high surface area and the ability to selectively adsorb cations and exchange molecules, which improves the performance and durability of bricks. The chemical reaction for the geopolymerization process is given below:

2NaOH + Na₂SiO₃ + SiO₂-Al₂O₃-M₂O + 3H₂O \(\to\) NASH + CSH + Na₂O.3SiO₂.4H₂O + NaOH

where,

M represents the metal oxides present in the raw materials, such as MgO and CaO.
NASH ((Na₂O).(Al₂O₃).(x)SiO₂._(y)H₂O) represents Sodium Alumino-Silicate Hydrate gel.
CSH ((CaO).(x)SiO₂.(y)H₂O) represents Calcium Silicate Hydrate gel.
NaOH represents the excess sodium hydroxide that may not fully participate during the reaction.
Na₂O.3SiO₂.4H₂O, a type of zeolite known as “Sodium Zeolite A.”

5.5 Comparative analysis of alkali-activated PMSA bricks with clay brick and fly ash brick

Table 8 presents a comparison between the manufactured alkali-activated PMSA bricks, burnt clay bricks and fly ash bricks. The characteristics of clay bricks were related to the specifications provided by IS 1077:1992 (of Indian Standards). The fly ash bricks and alkali-activated PMSA were compared from standards of IS 12894:2002 (IS 12894:2002 2002). The test results were analysed to determine the optimal material between alkali-activated PMSA bricks (from Melds 1 & 2) and the conventional clay and fly ash bricks. The optimum bricks from Meld 1 and Meld 2 (PMSAB 9 and PMSAB 23) satisfy the requirements of Class 17.5 and Class 25 bricks, respectively, as per IS 12894:2002 (IS 12894:2002 2002). The water absorption and drying shrinkage of these bricks are less than the permissible limits of 15% and 0.15% respectively. The optimum bricks have 27.22% and 30.85% lower thermal conductivity than clay bricks of 0.551 W/m K (Singh et al. 2018b). In every aspect, the alkali-activated bricks with PMSA surpass traditional clay and fly ash bricks. Moreover, PMSAB 23 outperforms PMSAB 9 due to the inclusion of CSD, which enhances its properties.

Table 8 Comparative Analysis of clay, fly ash and alkali-activated PMSA bricks

S.No	Property	Clay brick	Fly ash brick	Alkali-activated PMSA brick
		IS 1077:1992 [Class 15]	IS 12894:2002 [Class 15]	IS 12894:2002
		IS 1077:1992 [Class 15]	IS 12894:2002 [Class 15]	PMSAB 9 [Flyash+PMSA]	PMSAB 23 [Flyash+PMSA+CSD]
1.	Compressive Strength	≥ 15 N/mm²	≥ 15 N/mm²	19.13 N/mm²	28.03 N/mm²
2.	Water Absorption	< 15%	< 15%	9.65 %	11.32 %
3.	Efflorescence	Moderate	Slight to Moderate	Moderate	Slight to Moderate
4.	Drying shrinkage	-	< 0.15 %	0.123 %	0.121 %
5.	Thermal conductivity	-	-	0.401 W/m K	0.381 W/m K

From the mechanical and microstructural analysis, the following conclusions can be drawn regarding the utilization of paper mill sludge ash (PMSA) as a partial replacement for fly ash in fly-based geopolymer bricks:

PMSA exhibits a higher content of calcium and alumina, and its active silica is obtained when the paper mill sludge is incinerated at 1200°C.
The X-ray Diffraction (XRD) pattern of PMSA reveals major crystalline phases of Lime (CaO) and Periclase (MgO). Scanning Electron Microscopy (SEM) imaging of PMSA showcases its heterogeneous nature, unlike the more uniform particles of fly ash.
The compressive strength of PMSA geopolymer bricks shows a progressive increase up to a 40 wt% replacement of PMSA, with the highest recorded strength of 19.13 MPa observed for PMSAB 9. This strength surpasses that of fly ash bricks by 56.41% and conventional clay bricks by 71.15%. Further addition of 10 wt% Crusher Sand Dust (CSD) enhances the compressive strength of 40 wt% PMSA bricks. The maximum compressive strength is achieved by PMSAB 23, reaching 28.03 MPa, which is 31.75% higher than that of PMSAB 9.
The optimal molarity for the alkali-activated solution is determined to be 10 M. Increasing the molarity beyond this point results in decreased strength and leads to efflorescence on the surface of the bricks.
The increase in PMSA content results in a decrease in weight density and an increase in porosity and water absorption of the bricks. The water absorption for PMSAB 9 and PMSAB 23 is measured at 9.65% and 10.24%, respectively, providing satisfactory requirements with IS 12874:2002 (IS 12894:2002 2002).
The therrmal conductivity and drying shrinkage of the alkali-activated PMSA bricks increased with an increase in its pores and its PMSA, however, PMSAB 9 and 23 had drying shrinkage within the permissible limit of 15%.
SEM observations of the PMSAB 23 brick sample reveal that they are comparatively denser than PMSAB 9 due to a higher degree of polymerization resulting from the availability of silica through CSD. The presence of NASH and CSH gels, along with zeolites, indicates the occurrence of polymer formation with the elements found in the precursor materials.

These findings support the viability of utilizing PMSA as a partial replacement for fly ash in the manufacturing of alkali-activated bricks, demonstrating improved compressive strength and other desirable mechanical and microstructural characteristics.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Acknowledgement: Not Applicable

Funding Information: Not applicable

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Standard Test Method for Density, Absorption, and Voids in Hardened Concrete 1
IS (2016) 383-

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Potential pathway for reusing Paper Mill Sludge Ash as a Supplementary Material in Alkali-Activated Bricks

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Statement of Novelty

3. Materials and methods

3.1 Materials

3.2 Preparation of Alkali Activator Solution

3.3 Mix proportions

3.4 Preparation of alkali-activated PMSA bricks

4 Testing methods

5 Results and Discussion