Preparation of Ecological Refractory Bricks from Phosphate Washing By-Products

Abstract

This research is to assess the potential use of phosphate sludge from the Gafsa (Tunisia) phosphate laundries as an alternative raw material for the manufacture of ecological refractory bricks. Feasibility was evaluated through comprehensive physico-chemical and mineralogical characterizations of the raw materials using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), and thermal analysis (TGA-DTA). Bricks were formulated by substituting phosphate sludge with clay and diatomite, then activated with potassium silicate solution to produce geopolymeric materials. Specific formulations exhibited mechanical performance ranging from 7 MPa to 26 MPa, highlighting the importance of composition and minimal water absorption values of approximately 17.8% and 7.7%. The thermal conductivity of the bricks was found to be dependent on the proportions of diatomite and clay, reflecting their insulating potential. XRD analysis indicated the formation of an amorphous aluminosilicate matrix, while FTIR spectra confirmed the development of new chemical bonds characteristic of geopolymerization. Thermal analysis revealed good stability of the materials, with mass losses mainly related to dehydration and dehydroxylation processes. Environmental assessments showed that most samples are inert or non-hazardous, though attention is required for those with elevated chromium content. Overall, these findings highlight the viability of incorporating phosphate sludge into fired brick production, offering a sustainable solution for waste valorization in accordance with the circular economy.

1. Introduction

Phosphate mines generate millions of tons of waste rock in open-pit mining [1]. Furthermore, during phosphate ore processing, fluorapatite is separated from associated gangue minerals through a combination of successive mineral processing steps, including crushing/sieving, washing, and flotation. These operations produce large volumes of phosphate—a mixture of water and fine mineral particles—which are discharged into large surface ponds. Over time, the solids in the mix settle and form phosphate tailings, while waste rock is dumped into the mine. In general, the mining industry is facing many environmental challenges resulting from the huge quantities of waste that mines engender, such as waste rocks, concentrator tailings, and phosphate sludge (fine-grained waste material generated during phosphate ore washing and beneficiation processes). These wastes are deposited or stockpiled within the mine site and constitute a potential source of pollution because of their chemical characteristics and grain size [2].

Tunisia, with its large phosphate reserves, is the fifth-largest producer of phosphates in the world, which have been mined for more than a century. Nationwide phosphate reserves are estimated at 325 billion tons. These phosphates are transformed into fertilizers for agriculture. The main phosphate deposits are divided into three areas: the northern phosphate area (Srâa El-Ouertane and Kalaâ-Khasba), the phosphate area of the center (Chérahil, Maknassy), and the phosphate area of the south (Gafsa). These three areas represent one of the most important phosphate complexes in the world. Thus, the Tunisian phosphate sector, represented by the Phosphates Company of Gafsa (CPG) and its subsidiary GCT (Tunisian Chemical Group), has stopped growing even representing a large part of the economic activity of Tunisia, 4.5% of GDP [3]. Tunisian natural phosphate is a rock formed approximately 50 million years ago (Eocene), characterized by a beige color, a sandy texture, and a soft consistency, with high porosity. Its microcrystalline structure is composed of carbonated fluorapatite.

This intense mining activity related to natural phosphate beneficiation generates large amounts of sludge. In Tunisia, for instance, 11 million metric tons per year of phosphate sludge were deposited in 2011 [3]. However, this activity, like any mining activity, generates environmental damages related to dust, wastewater, and atmospheric emissions from ore processing plants, which are not without consequences for water resources, wildlife, flora, and human health: air pollution, water, and soil pollution; chronic diseases; and degradation of the vegetation cover and wildlife habitat area [2]. Recently, with the growing environmental awareness and regulatory incentives, mining companies are looking for sustainable and feasible solutions to minimize the high amounts of waste. Consequently, many promising eco-friendly solutions have been established by numerous researchers [4]. Nowadays, several industrial and mining wastes have been studied for their potential use as alternative materials. Although the mining industry still considers most of its waste as materials without any value, recent trends favored by incentive legislation show that the potential mine waste reuse could be beneficial in many cases, especially when they are proven not to be acid-generating. Building materials are supposed to be the utmost efficient way to consume these substantial amounts of waste generated every day [4]. Therefore, the recycling of mining waste as raw materials for the construction industry allows the reduction of the quantity of this waste and also the preservation of natural resources [5].

For Obenaus-Emler et al. [6], alkali-activated materials can be made from mine tailings, geopolymers [7], or in the preparation of concrete blocks using purified tailings [8]. More recently, Boutaleb et al. [9,10] demonstrated that phosphate mine tailings improve the mechanical properties of ceramic tiles. They used industrial mine tailings at a rate of 33% in formulas based on local clays for ceramic material manufacturing. Bricks with up to 30% phosphate sludge exhibited acceptable mechanical properties, meeting industry standards. Environmentally, incorporating sludge reduced clay consumption and waste generation. At the same time, it increased open porosity and water absorption; for example, using 50% sludge resulted in 52% porosity and 32% water absorption [11]. For Inabi et al. [12], the bricks manufactured using phosphate washing sludge exhibit robust mechanical performance, low embodied energy, excellent insulation, and thermal properties. Additionally, it was demonstrated that bricks manufactured with 2.5% phosphate sludge demonstrated approved compressive strength and met environmental standards, classified as non-inert materials [13]. Valorization of tailings (the process of converting waste materials into valuable products or resources) could be a promising alternative not only to reduce their potential environmental impacts but also to create a path for an eco-commercial profit by making a good business opportunity.

Given the significant environmental challenges posed by the accumulation of phosphate washing by-products in the Gafsa-Métlaoui region, this research aims to explore a sustainable pathway for their valorization through the development of ecological refractory bricks. By systematically characterizing the physical, chemical, and mineralogical properties of local phosphate sludge and evaluating its performance as a primary precursor, alone and in combination with natural clay and diatomite, this study investigates the feasibility of producing high-quality geopolymer bricks activated with potassium silicate. The resulting materials are rigorously assessed for mechanical strength, water absorption, thermal conductivity, and environmental safety to ensure compliance with construction standards. Ultimately, this approach offers a promising solution for transforming mining waste into value-added construction materials, aligning with circular economy principles and contributing to both environmental protection and resource efficiency in the region.

Although extensive research has addressed the reuse of phosphate by-products in construction materials, most previous studies have been limited by relatively low substitution rates—typically between 30% and 50%—due to concerns over mechanical strength, increased porosity, and water absorption. For example, while some works have demonstrated that moderate additions of phosphate sludge or tailings can improve certain properties of bricks or ceramics, higher incorporation rates often result in products that do not meet industry standards for durability and performance. Furthermore, the majority of these studies have focused on traditional fired ceramics, which require significant energy input and contribute to the overall environmental footprint of the material. These limitations highlight the need for alternative approaches that can both maximize waste valorization and minimize environmental impact.

In this context, our research seeks to overcome these challenges by investigating the use of up to 100% phosphate washing sludge in geopolymer brick formulations activated with potassium silicate, entirely avoiding high-temperature firing. This strategy aims to enhance waste utilization, reduce energy consumption, and provide a more sustainable pathway for the production of high-performance, eco-friendly refractory bricks.

2. Mining Site

In Tunisia, the GPG oversees the processing of phosphate ores in ten washing stations, each discharging approximately 284,000 m³ of sludge into the rivers of the Gafsa region [5]. To optimize water recovery and reduce the volume of sludge for disposal, the GPG has implemented underground pipelines to transfer sludge from the washing stations to storage basins. By 2001, the accumulated sludge in these basins exceeded 11 million m³, with over 90% not reaching the residue basins but being dispersed outside through an artificial river system. Between 1998 and 2011, around 2.5 million tons of sludge were annually discharged into the Thelja, Sebseb, and El Malah rivers [3].

Our study area is located in the Gafsa mining basin, a region highly impacted by phosphate mining activities. This basin encompasses several key sites critical to understanding the environmental effects of mining sludge disposal. Specifically, we focused on three sampling zones (Figure 1): Métlaoui (also known as Thelja), the first city in the basin and a central hub for mining operations; Redeyef, another major mining town within the basin; and Oued Etfal, situated approximately 20 km from Gafsa along the regional road R201 connecting Gafsa to Moulares.

3. Materials and Methods

3.1. Materials Used

Sludge samples were collected from washing station 2 in Métlaoui Gafsa, after the flocculation process to characterize these by-products in the context of our ongoing study.

The materials employed in this study encompassed a selection of constituents: phosphate waste sludge (PWS), natural clay from Oued Tfal near Gafsa (20 km from Gafsa), diatomite from the Nagues quarry in Redeyef, corresponding to the interlayer between layers 6 and 7 of the phosphate series, and alkaline silicate (ALK) (with the product name Geosil). The phosphate sludge, sourced from industrial waste, served as a key precursor after calcination at 850 °C. The kaolinic clay underwent thermal transformation at 700 °C; this temperature was specifically selected based on literature and preliminary tests, as it is optimal for converting kaolinite into highly reactive metakaolin. Calcining at 700 °C ensures complete dehydroxylation of kaolinite, maximizing the availability of amorphous aluminosilicates necessary for geopolymerization, while avoiding excessive energy use or the formation of less reactive crystalline phases.

Geopolymerization refers to the chemical process in which aluminosilicate materials, such as metakaolin and calcined phosphate sludge, react with alkaline activators (here, potassium silicate) to form a three-dimensional amorphous aluminosilicate network. This reaction results in the formation of inorganic polymers known as geopolymers, which are characterized by their durable, cross-linked aluminosilicate structures and excellent mechanical and thermal properties.

Diatomite was calcined at 750 °C to enhance its reactivity and stability. Potassium silicate was employed to initiate geopolymerization, while ensuring the formation of a durable aluminosilicate matrix. This combination of materials aimed to produce sustainable refractory bricks with improved mechanical and thermal properties.

3.2. Mix Proportioning

The use of washing phosphate sludge (WPS), kaolinitic clay (KC), and diatomite (D) as precursors in alkali-activated geopolymer binders promotes sustainable construction practices by repurposing industrial and mining waste. These materials are abundantly available in regions such as Gafsa, Tunisia, where phosphate mining generates significant sludge by-products, while (KC) and (D) are locally sourced natural minerals. Previous studies have limited WPS substitution in refractory materials to 50% due to its lower reactivity [7,14,15]. However, this research expands the substitution range from 50% to 100% to optimize waste valorization. The formulations GBM (PS + KC), GBD (WPS + D), and GBDM (WPS + KC + D) (Figure 2) were designed using potassium silicate (K₂SiO₃) as the alkaline activator to initiate the geopolymerization reaction between the calcined phosphate sludge, clay, and diatomite. Its role is to dissolve aluminosilicate species from the precursors and promote the formation of a stable amorphous geopolymer matrix, which enhances the mechanical strength and durability of the resulting bricks, with its concentration fixed to 15% of the binder weight, as derived from preliminary trials and literature.

3.3. Mix Preparation Protocol

The refractory geopolymer bricks were formulated by combining washing phosphate sludge, kaolinitic clay, and diatomite in varying proportions, following a systematic mixing protocol adapted to ensure optimal homogeneity and performance. The preparation process consisted of several structured steps. First, the raw materials were weighed, and the dry components (PS, KC, and D) were mixed for 5 min to achieve a uniform blend. Subsequently, the potassium silicate solution was gradually added over 10 min while maintaining continuous mixing to ensure even distribution and activation of the binders.

The fresh geopolymer bricks were then poured into cylindrical molds (Ø50 mm × 50 mm) for compressive strength testing. This geometry, recommended by the NF EN 196-1 standard and supported by Mkaouar [16], ensures reliable and comparable mechanical results. Although the material is referred to as “brick,” cylindrical samples are commonly used in geopolymer research for standardized testing. Each sample was compacted using a uniaxial pressing force of 10 MPa to enhance density and reduce porosity. The molds were then sealed to prevent moisture loss and cured at ambient temperature (22 ± 2 °C) for 72 h, allowing the geopolymerization process to proceed. After curing, the bricks were demolded and oven-dried at 105 °C for 24 h to remove residual moisture and further improve mechanical properties Figure 3. The composite combines anatase-phase TiO₂ with calcite-phase CaCO₃, preserving their individual crystal structures. XRD analysis confirms successful integration without phase transformation. CaCO₃ reduces TiO₂ agglomeration, enhancing photocatalytic surface area and stability [17].

This meticulous procedure ensured consistent mixing, compaction, and curing, which are critical factors in evaluating the structural integrity and durability of geopolymer-based refractory bricks.

3.4. Experimental Methods

3.4.1. Material Characterization

The raw materials were characterized using various analytical techniques to determine their chemical and physical properties. The analyses included the following.

Loss on ignition (LOI) was determined in accordance with NF EN 15169, in order to evaluate the volatile matter content using a static furnace heated at 550 °C. The particle size distribution (PSD) was assessed following NF EN 933-10, providing insights into the granulometric profile of the samples. The = apparatus used is a LS13320 model.

Phase identification and structural analysis were carried out using the following techniques:

(a) X-ray diffraction (XRD) analysis was conducted with a Bruker D2 diffractometer (Bruker Corporation, Billerica, MA, USA) using Cu-Kα radiation (λ = 1.5406 Å), in accordance with NF EN 13925-1 and NF EN 13925-2.

(b) Fourier-transform infrared spectroscopy (FTIR) was performed using a Thermo Scientific Nicolet iS20 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), based on the methodology outlined in NF ISO 4650.

These analyses provided detailed insights into the mineralogical composition and microstructure of the raw materials and resulting brick specimens.

3.4.2. Mechanical Testing

Compressive strength tests were conducted on cylindrical specimens (Ø50 mm × 50 mm) using an INSTRON 5500 R universal testing machine (Instron, Norwood, MA, USA), following the NF EN 196-1 standard. For each brick formulation, three replicate specimens were tested to ensure reproducibility. Additional mechanical tests were also performed on fired bodies to evaluate the influence of thermal treatment.

3.4.3. Method for Measuring Water Absorption

Water absorption (W_s)was measured in accordance with the NF P51-302 standard. Three brick specimens per formulation were dried to constant mass (M_d), cooled to room temperature (20 °C), and immersed in water for 24 h. After immersion, excess surface water was removed, and the wet mass (M_s) was recorded. The water absorption was calculated as (Equation (1)):

$$W_S(\%)={\displaystyle \frac{M_S-M_d}{M_d}}\times 100$$

(1)

3.4.4. Thermal Conductivity Measurement Procedure

Thermal conductivity measurements were carried out using the transient plane source (TPS) method with a hot disk apparatus. A resistive sensor functioning as a planar heat source was positioned between two identical pellets. The temperature rise resulting from heat dissipation was monitored and analyzed using the Hot Disk Thermal Constants Analyser software version 7.6.15 to calculate thermal conductivity. The measurement is based on Fourier’s law of thermal conduction (Equation (2)):

$$q\left(r,t\right)=-k∆T\left(r,t\right)$$

(2)

where q represents the heat flux density, k the thermal conductivity, and $∆T\left(r,t\right)$ the temperature gradient.

3.4.5. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed using a NETZSCH STA 409 instrument (NETZSCH Group, Selb, Germany) under a nitrogen atmosphere. The heating program involved a constant rate of 10 °C/min, from 40 to 1000 °C. This test provided information on the thermal stability and decomposition behavior of the samples.

3.4.6. Chemical and Environmental Analysis

For environmental assessment, all samples were ground to a fine powder and then subjected to a leaching test. The powdered samples were agitated in distilled water for 24 h in accordance with the standard protocol. After agitation, the resulting solutions were filtered prior to analysis. Trace element concentrations were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES), for which 2% nitric acid was added to the filtrates to stabilize dissolved metals and prevent any precipitation or adsorption on the container walls before measurement. In contrast, for ion chromatography (IC), the filtered solutions were analyzed directly—without acid addition—in accordance with best practices for anion determination (chlorides, sulfates, fluorides, etc.), in order to avoid altering ion speciation. The concentrations of water-soluble ions such as sulfate, chloride, and fluoride were thereby quantified by IC. The environmental behavior of the geopolymer bricks was thus assessed via leaching tests and subsequent analyses, following protocols designed to ensure the reliability and reproducibility of results for both cationic and anionic species.

3.4.7. Durability Assessment

The durability of the bricks was evaluated using an efflorescence test, following the NF EN 772-5 standard. This test assessed the potential for the formation of surface crystalline deposits resulting from the migration of soluble salts.

3.4.8. Experimental Protocol

All tests were carried out at the IMT Nord Europe Center for Education, Innovation and Research. Unless otherwise stated, six specimens were prepared and tested for each formulation, in compliance with the relevant technical standards for mechanical, thermal, and physical characterization.

4. Results and Discussion

4.1. Raw Material Characterization

4.1.1. Physical Characterization

The sludges from the Gafsa phosphate washery exhibit distinct granulometric characteristics, with over 90% of particles being smaller than 200 µm (

Table 1. Physical characteristics of sludge.

Particle Diameter (µm)	Diameter	Volume %
D10	10	1.2
D25	25	6.0
D50	50	42.7
D75	75	136.7
D90	90	178.4
Bulk Density (γs)		2.85 g/cm3
Loss on Ignition (W_LOI)		6.23%
Water Demand (D_water)		0.243%

). Detailed analysis reveals a bright sandy texture, without the presence of particles exceeding 90 μm, indicating an absence of clay components. The particle size distribution, characterized by a conformity coefficient of 5 and a uniformity coefficient of 1.58, suggests marked heterogeneity in the soil composition but also a relative uniformity in particle sizes within the sample. These findings underscore the need for careful management of phosphate sludges to minimize their potential environmental impact. The density of the sludge was 2.85 g/cm³ and the loss on ignition was 6.23%.

4.1.2. Mineralogical Characterization

Raw Materials

X-ray diffraction (XRD) analysis was conducted to determine the mineralogical composition of the raw materials used in this study, revealing that the phosphate sludge (Figure 4) primarily consists of apatite (fluorapatite Ca₁₀(PO₄)₆F₂ or carbonated apatite) as the predominant crystalline phase, with minor components of calcite (CaCO₃), quartz (SiO₂), and clay minerals, consistent with previous research [5,18], with the XRD patterns suggesting that the clay, calcite, and quartz phases are predominantly associated with the exogangue components of the sludge, while their interference with the fluorapatite peaks may stem from their presence as endogangue constituents; the XRD patterns for the raw natural clay (Figure 4) reveal a complex mineralogical composition, confirming its heterogeneous nature and detecting minerals such as illite, kaolinite, hematite, dolomite, quartz, and calcite, consistent with findings by Bouaziz in a similar natural clay mineralogy study [19]. The XRD spectra for raw diatomite (Figure 4) reveal its composition primarily consists of amorphous silica (SiO₂) phases, along with crystalline quartz and calcium carbonate (calcite and dolomite), also identifying trace amounts of clay minerals and feldspars (albite, orthoclase).

Effect of Thermal Treatment on Raw Materials

Thermal treatment of raw materials induced notable changes in their mineralogical compositions. For phosphate sludge, calcination led to sharper XRD peaks, indicating improved apatite crystallinity, and the disappearance of calcite peaks, signaling its thermal decomposition into calcium oxide, while quartz remained stable (Figure 4). In natural clay, calcination transformed kaolinite into amorphous metakaolinite, evidenced by a broad peak at 27.9° 2θ, with illite, hematite, and quartz peaks persisting (Figure 4). Finally, calcination of diatomite promoted partial recrystallization, reduced clay impurities and calcite peaks, and caused structural rearrangements evident in cristobalite and quartz peak variations (Figure 5 and Figure 6).

4.1.3. Fourier-Transform Infrared (FTIR) Spectroscopy

Fourier-transform infrared was employed to reveal key functional groups and the impact of thermal treatment. The FTIR spectrum of raw phosphate sludge exhibited a broad absorption band between 3000 and 3600 cm⁻¹. That is indicative of significant water content, with the region between 1000 and 1200 cm⁻¹ suggesting the presence of phosphate groups (PO₄³⁻) and 1400–1600 cm⁻¹ hinting at carbonates or organic matter. After calcination, the water band diminished, and phosphate groups became more defined (Figure 5). Similarly, FTIR analysis of raw clay from Oued Tfal showed a broad band in the 3700–3000 cm⁻¹ region, characteristic of O-H stretching vibrations from water and hydroxyl groups within clay minerals, consistent with [19]. This band attenuated after calcination, while Si–O–Si bands initially located between 1000 and 1100 cm⁻¹ transformed into a wider, less defined band. The bands associated with kaolinite, such as the Si–O–Al vibration below 800 cm⁻¹ (notably around 750 cm⁻¹) and Al-OH (915 cm⁻¹), disappeared, in agreement with Louati et al. [20], indicating dehydroxylation and the disruption of the silicate network. Finally, FTIR spectra of diatomite before calcination displayed a broad, intense band around 3400 cm⁻¹, attributed to O-H stretching vibrations from adsorbed water and silanol groups (Si-OH), and a peak at 1630 cm⁻¹ corresponding to water bending; after calcination at 750°C, these bands nearly vanished, indicating the elimination of adsorbed and structural water, while the bands at 1000–1200 cm⁻¹, 800 cm⁻¹, and 460 cm⁻¹ representing Si–O–Si stretching and bending vibrations. Zheng et al. [21] showed that the band at 800 cm⁻¹ became narrower and more continuous, suggesting an improved silica crystallinity, although the overall silica network structure was retained, aligning with the XRD results.

4.1.4. Thermogravimetric Test

Phosphate Sludge

The TGA of phosphate sludge reveals three main stages of mass loss visible in Figure 6:

• 200 °C to 400 °C: 7.4% loss, attributed to the decomposition of organic matter. This is due to thermolyzing organic compounds in the sludge, such as proteins, lipids, and carbohydrates.
• 400 °C to 600 °C: 1.3% loss, attributed to the decarbonation of calcite. This occurs through dehydration, transforming fluorapatite into hydroxyapatite.
• 600 °C to 800 °C: 1.5% loss, associated with calcite decomposition. This stems from the thermal breakdown of calcite into calcium oxide.

The stability of fluorapatite up to temperatures above 1000 °C is consistent with both our results and the literature. These attributions are further corroborated by XRD and FTIR analyses, which show the disappearance of calcite and organic matter signatures after thermal treatment.

The total mass loss of 10.2% indicates a significant organic content in the phosphate sludge.

An interpretation based on the mineral composition is: 74% organic matter: decomposes between 200 °C and 400 °C; 13% fluorapatite: decomposes between 400 °C and 600 °C; and 15% calcite: decomposes between 600 °C and 800 °C.

Natural Clay

The TG/DTG analysis reveals a thermal decomposition of the natural clay in several stages (Figure 7). Initially, a slight mass loss is observed up to approximately 200 °C, likely due to the desorption of water or residual solvents from the material’s surface. The most significant mass loss occurs between 400 °C and 600 °C, with a major peak on the DTG curve around 500 °C (Figure 7). This mass loss corresponds to the dehydroxylation of kaolinite and the formation of metakaolin. The initial mass loss below 200 °C can be attributed to the loss of water adsorbed by smectite and/or gypsum, while the slower mass loss above 600 °C could correspond to the decomposition of carbonates (calcite) or structural transformations of other mineral phases present. The total mass loss observed over the entire temperature range is approximately 12%, reflecting the total amount of volatile materials or decomposable compounds, including water released during the transformation into metakaolin.

Diatomite Mineral

The DTG thermograph reveals the presence of two endothermic peaks at around 90 °C and 750 °C, respectively. The first peak, which is associated with a mass loss of approximately 2 to 4% at low temperature, is typical of diatomite, which is known for its high porosity and ability to physically adsorb water, a characteristic confirmed in the literature on natural diatomites [22]. The second endothermic peak, correlated with a mass loss of approximately 8%, can be attributed to the decarbonation of calcite (Figure 8). This would contribute to decreasing its thermal conductivity, thereby improving its thermal insulation capacity [22].

The continuous decrease in the TG curve below 800 °C and the increase in the DTG curve after the 750 °C peak may be due to an exothermic reaction. Thus, the formation of compounds results from the destruction of diatomite. Alternatively, the mineralogical rearrangement of the siliceous phase following the transformation of opal-CT to opal-C is more ordered [23]. This phenomenon corresponds to recrystallization.

4.2. Environmental Characterization of Raw Materials

The environmental analysis of mining effluents from the phosphate washery, natural clay, and diatomite reveals concentrations of trace elements using inductively coupled plasma atomic emission spectrometry (ICP-AES). The detection limits for various elements indicate method sensitivity, while most heavy metals such as As, Ba, Cd, Cu, Ni, Pb, Sb, and Zn are present at concentrations far below the regulatory limits for inert (ISDI) and non-hazardous (ISDND) waste (see

Table 2. Heavy metals and anions of raw and calcined materials.

Elements	Phosphate Sludge (mg/l)	Clay Mineral	Diatomite	Calcined Clay	Calcined Phosphate Sludge	Calcined Diatomite	ISDI	ISDND	ISDD
As	<0.1	<0.1	<0.1	0.18	<0.1	<0.1	0.5	2	25
Ba	0.068	<0.008	0.026	<0.008	0.34	0.034	20	100	300
Cd	<0.009	<0.009	<0.009	<0.009	<0.009	<0.009	0.04	1	5
Cr	0.025	<0.004	0.01	15	79	17	0.5	10	70
Cu	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	2	50	100
Mo	0.55	0.18	2.0	0.88	7.3	6.1	0.5	10	30
Ni	<0.05	<0.05	0.076	<0.05	<0.05	<0.05	0.4	10	40
Pb	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03	0.5	10	50
Sb	<0.06	<0.06	<0.06	0.06	0.34	<0.06	0.06	0.7	5
Se	0.088	0.32	0.48	0.23	5.8	0.99	0.1	0.5	7
Zn	0.63	0.81	0.91	0.82	0.96	0.61	4	50	200
Sulfates	4760	644	724	353	450	1350	1000	20,000	50,000
Chlorides	386	487	80	259	70	13.5	800	15,000	25,000
Fluorides	14	2.0	1.5	2.1	13.4	1.8	10	150	500

). However, calcined phosphate sludge shows a significant increase in chromium (Cr) leaching (up to 79 mg/L), exceeding the ISDND limit (10 mg/L) and even surpassing the hazardous waste threshold (ISDD, 70 mg/L). Molybdenum (Mo) and selenium (Se) also increase after calcination, sometimes exceeding the ISDI limits. Sulfate, chloride, and fluoride concentrations are generally high, especially in phosphate sludge, but remain below ISDND thresholds.

4.3. Geopolymer Brick Characterization

4.3.1. Compressive Strength

The compressive strength of geopolymer bricks is an important property in material formulation. Higher resistance to compressive force also indicates a denser and more cohesive internal structure, which is typically associated with improved durability and reduced porosity in the bricks. Figure 9 shows the compressive strength of the developed bricks. As seen, the binary mixture GBM1 (90% calcined phosphate sludge and 10% calcined clay) achieved a remarkable strength of 25.9 MPa, significantly surpassing the 17.8 MPa observed for the G100 formulation (100% calcined phosphate sludge) due to enhanced geopolymerization from reactive aluminosilicates [7,15]. This result shows the efficacy of controlled clay addition in boosting mechanical properties. While higher clay proportions decreased strength (down to 6.8 MPa), the initial benefit of 10% substitution demonstrates a key threshold for maximizing performance. Ternary mixtures (GBDM) and diatomite-based formulations (GBD) exhibited moderate strengths (11.4–15.9 MPa), suggesting their suitability for applications prioritizing thermal insulation over extreme mechanical demands [24]. The study confirms that strategic clay incorporation can significantly elevate geopolymer performance for refractory applications.

4.3.2. Water Absorption

Low water absorption is desirable for refractory bricks, as it helps to reduce the risk of cracking and delamination. A recent study examined the influence of formulation composition on the water absorption of refractory bricks. Figure 10 shows that compositions with 100% calcined phosphate (G100) exhibit the lowest water absorption (~8%), indicating a dense microstructure with enhanced water resistance, slightly below values reported by [5]. Additives such as diatomite (GBD series) and clay (GBM series) increase absorption up to 17% (GBDM3/GBM4), attributed to diatomite’s inherent porosity and clay’s post-calcination structural changes [14]. Key formulations (G100, GBM1-GBM3) comply with NF EN 772-21 standards (17–22% absorption for frost-resistant bricks), aligning with findings that waste-derived additives can stabilize absorption [25], highlighting the need for precise additive control to optimize durability.

Table 3. Comparative studies of some geopolymers brick formulations and results.

References	Valuation Ways	Raw Materials	Origine	Firing Conditions	Water Absorption (%)	Compressive Strength (MPa)
[5]	Fired bricks	100% phosphate sludge	Tunisia Kef schfeir	Air drying for 24 h Oven drying at 60 °C for 24 h Firing at 900 °C, 1000 °Cand 1100 °C for 3 h (heating rate of 120 °C)	12.5–17.2	_
[14]	Ceramics bricks	Indonesian sludge (Banten Province)	25–50% of phosphate sludge + kaolin	Dried in an oven at 110 °C for at least 24 h. a heating rate of 5 °C/min to 500 °C, at 10 °C/min from 500 to 925 °C and at 15 °C/min from 925 °C	>30.23	>à 25
[18]	Ceramics products	Tunisian phosphate Kef eddour	0–50% phosphate sludge + kaolin	Dried at 105 °C for 24 h. The dried pellets were heated at 900, 1000, and 1100 °C for up to 2 h	_	_
[26]	Ceramics industries	Marrocan sludge	0–100% sludge + 0–100% clay	Heating ramp 5 °C/min up to the selected firing temperature (600, 900, 1000, 1100, and 1120 °C) 2 h dwell time at the temperature selected
[27]	Geopolymer	Marroc phosphate industry	Alkaline solution, metakaolin-in and thermally untreated phosphate sludge (UPS)(of 50%)	Liquid to solid ratio of L/S = 1.2 Left drying at 60 °C for 24 h Hardened matrices for 28 days	_	28.05–46.83
[28]	geopolymer	Fly ash came from the heat and power plant in Skawina (Poland), the metakaolin came from the Czech Republic, and the diatomite came from Jawornik Ruski	Fly ash (FA) + metakaolin (MK) + 1–5% diatomite	Alkaline solution consisted of technical sodium hydroxide flakes with aqueous sodium silicate (a ratio of 1:2.5 was used) and tap water	Not specified	15–31.7
[15]	Geopolymer		Phosphate washing waste and alkaline solution	PPW calcined at 700 °C or 900 °C, activated with NaOH (7 M) and sodium silicate		15–22
[29]	Geopolymer		Geopolymers based on fly ash or metakaolin			20–70
[30]	Fired bricks	China	84% hematite tailings, fly ash, and clay mixed with 12.5–15% water	20–25 MPa of forming pressure, and a suitable firing temperature ranged from 980 to 1030 °C for 2 h	16.54–17.93%	20.03–22.92 MPa
[31]	Hybrid brick	India	70–90% clay + 5–15% ceramic waste powder + 5–15% bagasse ash	The bricks were cast using molds without any pressure being applied to them. In India, the bricks were left to dry in the sun for two days at a temperature of 35 to 40 °C for an 800 °C firing	11.4–18%	20–27.2
Current Study	Geoploymer	Phosphate washing by-product	50–100% phosphate washing by-product + (10–50% calcined clay (GBM)/10–20% Calcined diatomite (GBD)/calcined diatomite + calcined clay (GBDM)) + potassium silicates solution	The materials were calcined at 700 °C, 750 °C, and 800°C, activated with potassium silicate solution, then pressed, cured at ambient temperature for 72 h, and oven-dried at 105 °C for 24 h	7.7–17.8%	7–26 MPa

compares selected results from other studies. That allows us to compare different properties.

4.3.3. Thermal Conductivity

The study of thermal conductivity in refractory bricks highlights significant trends influenced by composition. The G100 formulation (100% phosphate sludge) exhibits the highest thermal conductivity (0.55 W/m·K) (see Figure 11). This could be related to its phosphate content. Phosphate-rich sludges, such as phosphogypsum, show high electrical conductivity due to their soluble ion content, mainly Ca²⁺, Na⁺, and PO₄³⁻. Studies [32,33] report conductivity increases from hundreds of µS/cm to several mS/cm with higher phosphate sludge concentrations, confirming their strong impact on salinity levels. Incorporating calcined diatomite (GBDM series) reduces conductivity to 0.38 W/m·K, while clay additions (GBM series) yield intermediate values (~0.50 W/m·K). These results align with the study of Bories et al. [34], who reported conductivity around 0.53 W/m·K for bio-based porous agents in fired clay bricks, and Phonphuak et al. [35] have reached lower conductivity (0.22–0.47 W/m·K) using sawdust, albeit with higher porosity (22.8–32.4%) [35]. This proves the critical role of pore agent type and quantity in modulating thermal and structural properties. Compared to commercial dense (1.0–2.0 W/m·K) and insulating (0.2–0.8 W/m·K) refractory bricks [34], the formulations developed in this study fall within the insulating range (0.38–0.58 W/m·K). Notably, the use of local, sustainable materials reduces the carbon footprint.

4.3.4. X-Ray Diffraction Results

The X-ray diffraction patterns of geopolymer formulations in Figure 12 reveal the mineralogical composition of consolidated materials. While geopolymers are generally considered amorphous materials by XRD, a broad hump centered around 20° to 35° (2θ) indicates the dissolution of SiO₄ and AlO₄ species and the formation of an amorphous phase compared to calcined clays, reflecting polycondensation reactions [19,36,37]. However, the significant intensity of crystalline peaks, particularly quartz, suggests limited reactivity and incomplete dissolution of raw materials [38,39]. Compared to metakaolin-based geopolymers, which typically exhibit more complete amorphization [19], our materials show similarities to fly ash-based geopolymers, which often retain residual crystalline phases [40]. These crystalline phases may contribute to thermal stability, making the materials potentially suitable for refractory applications. Thus, while geopolymerization is evident, optimization of synthesis conditions is needed to enhance precursor reactivity and achieve improved performance for refractory uses.

4.3.5. FTIR Analysis

Figure 13 represents the FTIR spectra of the GBM (phosphate sludge + clay), GBD (sludge + diatomite), and GBDM (sludge + diatomite + metakaolin) series and reveals distinct structural signatures. The Si–O–Si/Al band (1100–950 cm⁻¹) shifts to 980 cm⁻¹ (GBM), 950 cm⁻¹ (GBD), and 960 cm⁻¹ (GBDM), reflecting differential Al incorporation into the silicate network [19,41]. The marked reduction of the 450 cm⁻¹ band (Si–O) in GBDM indicates advanced consumption of clay phases [42], while residual peaks at 800 cm⁻¹ (illite) [19] and 700 cm⁻¹ (quartz) stabilize the matrix at high temperatures [43]. The hydroxyl region (3300–3600 cm⁻¹) shows increased residual porosity for GBD (linked to diatomite) and improved condensation for GBDM (via metakaolin) [44] compared to fly ash-based geopolymers (960 cm⁻¹, low hydroxyl absorption) or slag-based systems (dominant amorphous halo) [45]. These formulations combine residual crystalline phases and complete carbonate decomposition (1470 cm⁻¹) at 400 °C, offering superior thermal stability. The GBDM series, optimizing the Si/Al ratio through metakaolin, outperforms conventional geopolymers in structural stability and dehydration resistance, confirming the efficacy of hybrid systems [46].

4.3.6. Thermogravimetric Analysis

The study of the thermal behavior of synthesized geopolymer materials, based on the thermograms of the GBM, GBD, and GBDM series (Figure 14), highlights significant differences depending on the composition and treatment of the raw materials. For the GBM series, the main mass loss occurs between 25 and 600 °C, corresponding to the removal of free water, dehydroxylation of –OH groups, and decomposition of carbonates, with a moderate total mass loss (4–6%) (Figure 14), which could lead to higher porosity at elevated temperatures, indicating good consolidation of the geopolymer network [38]; (Dabbebi et al., 2018) [15]. The GBD formulations (Figure 14) show contrasting behaviors: GBD1 is exceptionally stable, GBD2 combines low mass loss (~3%) and strong thermal robustness, while G100 (100% calcined phosphate sludge) exhibits a more pronounced mass loss due to carbonate decomposition [39]. Finally, the GBDM series, which incorporates calcined diatomite and clay, exhibits excellent thermal stability, with all samples losing less than 3% of their mass up to 1200 °C. Among them, GBDM2 stands out by offering the best compromise between minimal mass loss and structural robustness, making it particularly suitable for refractory applications. This superior performance is attributed to the optimized combination of calcined diatomite and clay, which enhances the formation of a dense, stable geopolymer network capable of withstanding high temperatures without significant degradation [28,44]. These results confirm the importance of initial composition and the use of calcined diatomite and clay to achieve lightweight, stable, and high-performance geopolymers at high temperatures.

4.3.7. Environmental Assessment of Heavy Metal and Anion Leaching from Geopolymer Formulations

Across all formulations in

Table 4. Heavy metals and anions of geopolymers.

Elements	GBM1	GBM2	GBM3	GBM4	GBD1	GBD2	GBDM1	GBDM2	GBDM3	G100	ISDI	ISDND	ISDD
As	<0.1	<0.1	0.13	0.35	<0.1	0.12	<0.1	0.13	<0.1	<0.1	0.5	2	25
Ba	<0.008	<0.008	<0.008	<0.008	<0.008	<0.008	<0.008	<0.008	0.009	0.012	20	100	300
Cd	<0.009	<0.009	<0.009	<0.009	<0.009	<0.009	<0.009	<0.009	<0.009	0.01	0.04	1	5
Cr	10	13	16	19	13	18	14	16	20	41	0.5	10	70
Cu	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	2	50	100
Mo	0.91	1.7	2.2	2.1	1.7	2.0	1.1	2.1	2.3	2.9	0.5	10	30
Ni	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	0.4	10	40
Pb	<0.03	<0.03	<0.03	<0.03	0.05	<0.03	<0.03	<0.03	<0.03	<0.03	0.5	10	50
Sb	<0.09	0.10	0.10	0.08	0.09	0.12	0.065	<0.06	0.079	0.18	0.06	0.7	5
Se	0.87	2.2	2.7	2.5	2.2	1.9	1.1	2.8	2.2	2.8	0.1	0.5	7
Zn	<0.01	0.74	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	4	50	200
Sulfates	243	772	734	151	1033	841	298	1042	1100	595	1000	20,000	50,000
Chlorides	50	70	98	151	34	25	54.5	74.4	40.3	40.8	800	15,000	25,000
Fluorides	4.9	8.4	7.2	7.0	11	6.6	5.2	8.0	7.5	7.0	10	150	500

, most heavy metals remain well below ISDND limits, indicating effective immobilization within the geopolymer matrix. Sulfate, chloride, and fluoride levels are sometimes elevated (e.g., sulfate up to 1195 mg/L in GBDM1), but still comply with non-hazardous waste regulations.

Chromium (Cr) is the main limiting factor in the formulations; its concentration ranges from 10 to 41 mg/L, exceeding both the inert (ISDI: 0.5 mg/L) and often the non-hazardous (ISDND: 10 mg/L) thresholds, but remaining below the hazardous waste limit (ISDD: 70 mg/L). This elevated Cr content is directly linked to the use of calcined phosphate sludge, which is very rich in leachable Cr. However, the reduction in Cr concentration compared to the pure calcined sludge is due to dilution effects and partial immobilization by the geopolymer matrix. To conclude, the geopolymer formulations can be classified as non-hazardous waste for most elements, but the Cr content remains above the thresholds for inert and non-hazardous waste, limiting their valorization without further treatment. Optimizing the formulation or adding specific stabilizing agents for chromium is recommended to improve environmental performance [47].

5. Comparative Analysis and Selection of Optimal Geopolymer Brick Formulations

To identify the most promising geopolymer brick formulations for refractory applications, a comprehensive comparative analysis was performed, considering mechanical strength, thermal stability, heavy metal immobilization, and thermal conductivity. While all formulations exhibited potential for waste valorization, two distinct categories of high-performing bricks emerged, depending on the specific application requirements: dense and high-strength, or balanced and environmentally optimized.

The GBM1 formulation (with 10% calcined clay) is unequivocally the best choice for producing dense, high-strength refractory bricks. This formulation demonstrated the highest compressive strength in our study, achieving 25.9 MPa (Figure 9). This performance not only significantly surpasses other formulations tested within this study (whose overall mechanical performance ranged from 7 MPa to 26 MPa), but it also renders GBM1 highly competitive with commercial refractory bricks and standard geopolymer materials reported in the literature [15,45]. Its thermogravimetric analysis (TGA) (as presented in Section 3.4) reveals good thermal stability, with a moderate mass loss of approximately 12%, primarily due to dehydration and dehydroxylation processes, which is crucial for maintaining material integrity at high temperatures. The thermal conductivity of GBM1 is 0.55 W/m·K (Figure 11), a value characteristic of dense refractory bricks, providing a suitable balance between thermal resistance and heat transfer. Furthermore, environmental assessments (Table 4) showed that GBM1’s leachate concentrations of heavy metals are among the lowest, classifying it as “non-hazardous” and very close to “inert” for most elements (with the exception of chromium, which, while slightly above the inert limit of 0.5 mg/L, remains well below the non-hazardous threshold of 10 mg/L).

Conversely, the GBDM (phosphate sludge + clay + diatomite) formulation represents the best compromise for a versatile and highly environmentally friendly refractory brick. While its compressive strength (15 MPa, Figure 9) is lower than GBM1, it is still adequate for many non-structural or semi-structural applications. GBDM’s main advantage lies in its outstanding thermal stability, exhibiting the lowest TGA mass loss (~9%) among all our formulations (Figure 14), indicating exceptional durability at high temperatures. Moreover, GBDM demonstrates the best environmental performance (Table 4), with the lowest concentrations of leached heavy metals, confirming its “non-hazardous” classification and minimal environmental impact potential. Its thermal conductivity of 0.45 W/m·K (Figure 11) positions it as a good insulator, performing better than GBM1 in this specific property.

In conclusion, the selection of the “best” formulation depends on the specific application requirements. If maximum mechanical performance is the predominant criterion for dense refractory bricks, GBM1 is the most appropriate choice. However, if a balance between good strength, improved thermal insulation, superior thermal stability, and minimal environmental impact is desired, GBDM represents the most balanced and promising solution from our study for a versatile and sustainable refractory brick. These distinctions provide valuable insights for future industrial applications of these eco-friendly materials.

6. Economic Feasibility and Scalability Considerations

The process developed in this study offers several economic and practical advantages compared to traditional fired refractory bricks. All raw materials—phosphate sludge, clay, and diatomite—are abundantly available and locally sourced from the Gafsa mining basin, with diatomite and phosphate sludge being mining by-products or considered as waste. This ensures low material costs and reduces the need for waste management.

Unlike conventional refractory bricks, our method does not require high-temperature firing, but only calcination of raw materials and drying at 105 °C. This significantly lowers energy consumption and production costs and reduces greenhouse gas emissions. The limited use of potassium silicate as an alkaline activator further minimizes chemical input costs. The preparation process (mixing, compacting, ambient curing, and drying) is simple and compatible with existing industrial equipment, facilitating easy scale-up. The abundance of raw materials and the straightforward process make large-scale production feasible and sustainable.

In summary, our approach provides a cost-effective and environmentally friendly alternative to traditional fired bricks, with strong potential for industrial applications in regions with similar resources.

7. Conclusions

This study presents a novel and sustainable approach for the valorization of Tunisian phosphate washing sludge as a primary raw material for ecological refractory bricks. Unlike previous research limited to partial substitution, our work demonstrates for the first time the successful use of up to 100% calcined phosphate sludge, in combination with locally sourced kaolinitic clay and diatomite, to produce geopolymer bricks activated by potassium silicate and cured at ambient temperature.

The developed bricks exhibit outstanding mechanical performance, with compressive strengths reaching up to 25.9 MPa, low water absorption, and favorable thermal insulation properties. These results confirm that careful optimization of precursor composition and activator concentration can yield materials that meet or exceed industry standards for refractory applications.

From an environmental perspective, this process offers significant benefits by transforming mining waste into high-value construction materials. The method reduces the volume of hazardous waste, mitigates risks of soil and water contamination, and substantially lowers both energy consumption and greenhouse gas emissions compared to conventional fired bricks.

Overall, our findings highlight the dual advantage of this approach: it provides a technically robust solution for producing high-performance, durable refractory bricks, while also contributing to environmental protection and circular economy objectives. Future research should focus on scaling up the process, further optimizing formulations for industrial applications, and ensuring the safe management of trace elements to fully realize the environmental and economic potential of this technology.