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Article

Investigation of the Innovative Combined Reuse of Phosphate Mine Waste Rock and Phosphate Washing Sludge to Produce Eco-Friendly Bricks

by
Omar Inabi
1,2,*,
Abdessamad Khalil
1,2,
Abir Zouine
1,
Rachid Hakkou
2,3,
Mostafa Benzaazoua
2 and
Yassine Taha
2,*
1
Resources Valorization, Environment and Sustainable Development Research Team (RVESD), Department of Mines, Mines School of Rabat, Avenue Hadj Ahmed Cherkaoui—BP 753, Agdal, Rabat 10090, Morocco
2
Geology and Sustainable Mining Institute (GSMI), Mohammed VI Polytechnic University (UM6P), Lot 660, Hay Moulay Rachid, Ben Guerir 43150, Morocco
3
Laboratory of Innovative Materials, Energy and Sustainable Development (IMED-Lab), Faculty of Science and Technology Gueliz, Cadi Ayyad University (UCA), Marrakech 40000, Morocco
*
Authors to whom correspondence should be addressed.
Buildings 2024, 14(9), 2600; https://doi.org/10.3390/buildings14092600
Submission received: 26 July 2024 / Revised: 15 August 2024 / Accepted: 21 August 2024 / Published: 23 August 2024
(This article belongs to the Collection Sustainable and Green Construction Materials)
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Abstract

:
Phosphate mining generates substantial quantities of waste rock during the extraction of sedimentary ores, leading to significant environmental concerns as these wastes accumulate around mining sites. The industry is under increasing pressure to adopt more sustainable practices, necessitating considerable financial investments in remediation and technological advancements. Addressing these challenges requires a holistic strategy that balances social responsibility, environmental preservation, and economic viability. This study proposes an innovative, cost-effective, and environmentally friendly method to manufacture compressed stabilized earth bricks by combining the valorization of phosphate waste rock (PWR) and phosphate washing sludge (PWS). These bricks offer numerous advantages, including low embodied energy, robust mechanical performance, and excellent insulation and thermal properties. Initially, a Toxicity Characteristic Leaching Procedure (TCLP) test and radiometric surface contamination measurement, carried out on raw materials (PWR and PWS), showed that the results were below the permissible limits. Then, the chemical, mineralogical, and geotechnical properties of the raw materials were characterized. Subsequently, various mixtures were formulated in the laboratory using PWR and PWS, with and without cement as a stabilizer. Optimal formulations were identified and scaled up for pilot production of solid bricks with dimensions of 250 × 125 × 75 mm3. The resulting bricks exhibited thermal conductivity and water absorption coefficients that satisfied standard requirements. This method not only addresses the environmental issues associated with phosphate mining waste but also provides a sustainable solution for building materials production.

1. Introduction

All over the world, phosphate mining faces several of crucial challenges. The environmental impact is a significant concern since the mining and processing of phosphate rock produce considerable amounts of waste, including waste rock and tailings that can contaminate water and soil. This waste often contains hazardous substances, posing risks to local ecosystems and communities. The extraction of lower-grade ores [1,2], which are more expensive and time-consuming to process, is also required due to the depletion of high-grade sources [3,4]. Thus, phosphate mining operations have a greater environmental [5,6,7] and radiologic [8,9,10] impact all over the world, especially on water and soil [11,12,13]. In addition, the business faces social and regulatory pressure to implement more environmentally friendly practices, which can entail large financial investments for remediation and advancements in technology. A comprehensive strategy is needed to address these issues, establishing an equilibrium between social responsibility, environmental preservation, and economic viability. In fact, phosphate mining is an essential sector for global food security as well as chemistry, medicine, and agriculture [14,15,16].
To tackle these complex issues, a novel rethinking of the traditional approach to phosphate mining and waste management is needed [17]. The conversion of these wastes from worthless materials into profitable and practical secondary resources is an environmentally responsible and sustainable solution [18]. In this context, The Kingdom of Morocco serves as an exemplary model, having conducted numerous studies over several years focused on the valorization of phosphate residues and by-products. These applications are wide-ranging and cover many different fields, like ore [19,20,21] and water [22,23,24] recovery, use of alkaline phosphate wastes for the control of acidic mine drainage [25,26], as lightweight aggregates [27,28], in the concrete sector [28,29], in fired [30] and compressed [31,32,33] bricks manufacturing, in the area of geopolymerization [34,35,36], and finally in civil engineering construction [37,38], especially for embankments [39,40] and roads construction [41,42,43]. In fact, because of their high carbonate content, phosphate waste rock (PWR) is geochemically inert and does not pose a risk of producing acidic mine drainage [18,44].
The increase in population and industrialization in recent years has led to a rise in the consumption and depletion of non-renewable natural resources used to make conventional building materials. Furthermore, the limitation of water resource utilization is particularly impactful on locations that are arid. The environment is adversely affected by these traditional materials. Using different elements to produce construction materials becomes necessary in the context of sustainable development. In this context, earth bricks have been regarded as one of the main building materials since antiquity. Utilizing locally accessible materials, they offered several environmental benefits, including reduced carbon emissions, low thermal conductivity, and low embodied energy [45]. Several applications dealing with waste valorization have been extensively studied all over the word, as raw materials for bricks manufacturing, using natural [46,47,48] raw materials, agricultural and natural fibers [49,50,51], and geopolymers and stabilizers [52,53]. For engineering properties, Dulal and al. [54] presented the main properties for compressed bricks in different countries all over the world. A minimum dry compressive strength of 1.30–3.00 MPa, a maximum water absorption capacity of 15–20%, and a bending strength of 0.3–0.5 MPa are required for soil-based unburnt bricks. These standards guarantee the bricks’ quality and longevity, as well as their applicability for a range of construction applications.
Reusing industrial waste and byproducts in conjunction with the advancement of contemporary earth construction techniques is emerging as a strategy in line with the principles of the circular economy and sustainability as society moves toward looking for more environmentally friendly construction practices [55]. Despite being relatively unfamiliar to many, stabilized compressed earth blocks offer significant advantages, including the preservation of heritage and their role as environmentally friendly building materials [46].
This study proposes an innovative, cost-effective, and environmentally friendly method to manufacture compressed stabilized earth bricks by combining the valorization of phosphate waste rock (PWR) and phosphate washing sludge (PWS). When resources are available locally, researching the manufacture of soil or soil–cement bricks can be beneficial, especially in areas where it is difficult to use other chemical binders or additives [54]. To assure practical application and sustainability in building practices, this study focuses on local materials without the use of any chemical additives other than cement. The impact of cement and fines content on the compressive strength, durability performance, and environmental behavior of compressed stabilized earth bricks (CSEB) is examined by laboratory testing. To produce a consistent brick that complies with current specifications for earthen construction materials [56], the best formulations at the laboratory scale was applied in a pilot size while maintaining the same preparation and curing conditions.

2. Materials and Methods

2.1. General Methodology

Figure 1 illustrates the comprehensive methodology employed in this work. The process begins with material sampling and preparation of raw materials, followed by an extensive laboratory testing program. Raw materials were analyzed to determine their environmental and health impacts. Additionally, their physical, geotechnical, and chemical properties, as well as their mineralogical composition, were thoroughly examined. Pilot-scale bricks were then manufactured from the optimal mixtures, both with and without conventional cement, to facilitate further laboratory tests on full-scale products.

2.2. Raw Material Sampling and Preparation

For laboratory testing, raw materials were sampled from phosphate waste rock (PWR) piles [57] in Benguerir’s Mine in Morocco. The phosphate washing sludge (PWS) was collected from the tailing’s ponds at the Youssoufia phosphate processing plant. Figure 2 present a site plan of the different sampled locations.
Samples were collected from multiple locations to ensure representative results. The PWR samples were homogenized, divided, and subsequently crushed using a jaw crusher to reduce the particle size to 10 mm (Figure 3). The PWS samples were also homogenized and divided. Chemical stabilization of the PWR samples was achieved using locally produced CPJ 45 class Ordinary Portland Cement.

2.3. Raw Materials Characterization

2.3.1. Environmental Behavior and Health Condition of Raw Materials

Given the waste generated by mining activities, it is crucial to consider raw materials when evaluating the environmental performance of bricks. For leaching solid waste samples, the US Environmental Protection Agency (US-EPA) has established a laboratory procedure called the Toxicity Characteristic Leaching Procedure (TCLP). A total of 20 g of the ground soil sample and 400 mL of the suitable leaching solution—a solid-to-liquid ratio of 1:20—are used in the test. The samples are placed into a plastic bottle. Subsequently, the bottle is sealed and subjected to an 18 ± 2 h leaching device rotation at a rate of 30 ± 2 revolution per minute. The leachate is collected and acidified before being subjected to ICP-OES analysis. Following leaching, the solid is left for use in filtration. The results are compared to US EPA regulatory limits for concentrations [58].
For evaluating the health condition of the raw material, an RDS-80 Surface Contamination Meter, commercialized by Mirion Technologies, was used to measure the surface contamination. The observed radiations include beta particles that have energies greater than 100 keV, alpha particles that have energies greater than 2 MeV, and gamma and X-rays that have energies between 5 keV and 1.3 MeV. Using a Pancake-type GM tube with a surface area of 1.5–2 cm2 and a MICA window diameter of 4.4 cm, detection was made easier. As a preliminary approach, ten samples were randomly taken out of the large bags for each material.

2.3.2. Physical and Geotechnical Characterization

The raw material classification has a huge impact on the compressed bricks’ final properties. As specified by standard XP P94-041 [59], the coarse fraction (>80 µm) granulometry distribution was established in the present case using the wet sieving method. The Atterberg limits of the used soil were established in compliance with NF EN ISO 17892-12 [60]. To evaluate the activity of the fine fraction of the soil, the methylene blue value (MBV) was realized in compliance with AFNOR standard NF EN933-9 [61]. Proctor tests were performed in accordance with NF P94-093 [62] for determining the optimal compaction parameters. Lastly, Equivalent Sand Value (ESV) Test was performed according to EN N933-8+A1 standard [63] to measure the relative proportions of fine dust or clay-like materials in granular fractions. The test result indicates the cleanliness of the sandy fraction. Lastly, the materials have been classified according to earthen construction materials standard [56].

2.3.3. Chemical and Mineralogical Characterization

Prior to analysis, the material underwent laborious processes of grinding and pearling. To determine the main chemical composition of the raw materials, tests were carried out at GSMI’s Geo-Analytical Lab, using a RIGAKU (ZSX Primus IV, Rigaku, Tokyo, Japan) X-ray fluorescence (XRF) spectrometer, the major components CaO, SiO2, Al2O3, Fe2O3, MgO, K2O, Na2O, P2O5, and MnO were then fully chemically characterized. The carbonate content of milled materials was determined by calcining them at 950° due to the presence of carbonate rocks. A RIGAKU X-ray diffraction device (MiniFlex 600, Rigaku, Tokyo, Japan) was used to accomplish the mineralogical characterization of PWR and PWS samples. This made it possible to examine their detailed mineral composition.

2.4. Mixtures Definition and Characterization

Cylinder formulations (Figure 4) were employed before transitioning to hollow brick formulations. These cylinders facilitate preliminary assessments by ensuring sample consistency and adherence to standard testing protocols. This approach is both economical and effective, allowing for optimization before proceeding to more detailed and costly evaluations of the manufactured bricks.
Table 1 presents the various cylinder mixtures produced, categorized into four groups. The first group varies the percentage of washing sludge from 40% to 60% in 5% increments, with no cement (0%). The second group includes 5% cement, with washing sludge percentages ranging from 35% to 45%. The third group contains 10% cement, with washing sludge percentages varying from 30% to 50%. The fourth group maintains the PWR percentage at 0%, with varying quantities of washing sludge and cement.
To prepare the cylindrical molds, a manual compaction device designed for the Proctor test was used to compact a precisely measured amount of the prepared mixture, with quantities measured using an electronic balance with a precision of 0.1 mg. The cylindrical steel molds, with a diameter of 10 ± 1 mm and a height of 20 ± 1 mm, maintained a height-to-diameter ratio of 2.
For each formulation, compressive strength tests were conducted after 14 and 28 days in accordance with XP P13-901 [56], using a CONTROLS 3000 KN mechanical Press (Figure 5). During curing, the samples were kept at an average room temperature of 25 ± 2 °C and an air humidity of 23 ± 5%.

2.5. Laboratory Tests on Pilot Scale Bricks

2.5.1. Pilot Scale Bricks Manufacturing

Concerning the pilot-scale development of compressed stabilized earth bricks, hollow forms were manufactured mechanically, based on the most relevant formulations. Following the same process used to prepare laboratory bricks, utilizing an Eco Brava machine with a 15 MPa pressure and a 250 × 125 × 75 mm3 dimensions. Before being tested, the prepared bricks were left at room temperature for varying age times and covered with a plastic sheet to ensure that the cement had hydrated.
Once the best formulations have been defined by various characterization procedures (chemical, mineralogical, physical and mechanical) at the laboratory scale, the most optimal formulations that adhere to the specified specifications was scaled up. The used device in the pilot scale manufacture of compressed earth bricks (CEB) is the Eco Brava (Eco Máquinas, Brazil) automatic hydraulic press (AHP), as shown in Figure 6. To produce the blocks, the three best formulations from the optimization process were manufactured in the laboratory using identical preparation and pressure settings. The three top formulations from the optimization process were used to manufacture the blocks in the laboratory, ensuring that identical preparation and pressure settings were applied.

2.5.2. Uniaxial Compressive Strength (UCA)

Using the process described in XP P 13 901 [56], the compressive strength of solid bricks was determined. By stacking the two wet sides of each brick with a layer of cement mortar, no thicker than 10 mm, between them, the dry compressive strength of each brick was evaluated. To make sure the compression test replicated real conditions, the assembled item was then covered with a plastic sheet.

2.5.3. Water Absorption (WA)

In accordance with the AFNOR standard XP 13 901 [56], a capillarity test was performed to verify the water absorption rates of samples following their curing ages. The samples were weighed both before and after they were submerged in a water bag for 10 min, with just 5 mm of the specimen submerged. The water absorption coefficient (Cb) was calculated using the following equation:
C b = 100 × ( m 1 − m 0 ) A   t
where Cb is the water absorption coefficient (g/cm2·min1/2);
m0 is the mass of the specimens before immersion (g);
m1 is the mass of the specimens after immersion (g);
A is the basic area of the specimens (cm2);
t is the time of 10 min.
Compressed earth bricks used for exterior walls can be divided into two categories based on the AFNOR XP P 13.901 standard [56]: extremely low capillarity absorption (Cb < 20 g/cm2·min1/2) and low capillarity absorption (Cb ≤ 40 g/cm2·min1/2).

2.5.4. Thermal Conductivity (TC)

Thermal conductivity measures a material’s ability to conduct heat. For the pilot-scale blocks, thermal conductivity (TC) was assessed using the HMF 446 Netzsch Lambda (Figure 7) at the Green Energy Park in Benguerir City, Morocco. During the measurement, a sample is placed between two plates equipped with highly sensitive heat-flow sensors that detect the heat flow into and out of the material. By knowing the measurement area and sample thickness, thermal conductivity can be calculated with specialized software.

2.5.5. Microstructural Analysis (SEM)

For the microstructural analysis of the three best formulations, the scanning electron microscope (SEM) and an integrated and automated mineral analyzer were used. SEM allows to visualize the morphology and the distribution of the raw material grains and the cementitious matrix, based on the prepared thin section.

3. Results and Discussion

3.1. Raw Materials Characterization

3.1.1. Environmental Behavior and Health Condition of Raw Materials

Table 2 presents the results from the ICP-OES analysis of PWR and PWS, following the TCLP protocol. The concentrations of pollutants such as As, Pb, and Cr were compared with the regulatory limits set by the US Environmental Protection Agency (EPA). The analysis shows that the levels of these pollutants in PWR and PWS are within the EPA’s regulatory limits, indicating that the materials are safe.
Using a simplified dosimetric model, values for surface contamination based on the document referenced SSR-6 (Rev. 1) of the International Atomic Energy Agency (IAEA) Transport Regulations [64] were taken into consideration. The observed values (which are 4 Bq/Cm2 for beta and gamma emitters and 0.4 Bq/Cm2 for all other alpha emitters and low toxicity alpha emitters) are below the upper limit overall, as shown in Figure 8.

3.1.2. Physical and Geotechnical Characterization

Table 3 summarizes the findings from the physical and geotechnical tests performed on PWR and PWS. The results reveal that PWR is a coarse-grained material, predominantly composed of larger particles. In contrast, PWS is identified as a fine-grained material with a plasticity index of 20%. This classification suggests that PWS is a clayey sand with low plasticity, containing a moderate amount of clay but exhibiting limited plastic behavior. Furthermore, PWR is characterized as silty sand with relatively low clay content, whereas PWS is categorized as clayey sand with fine particles and a modest plasticity index.
The optimal compaction parameters for each raw material have been defined. This involves determining the specific moisture content and compaction effort required to achieve the maximum dry density for each material.
For compressed bricks applications, the plasticity of the material should be within the recommended range of the plasticity diagram according to the XP P13-901 standard [56]. The materials whose plasticity is within this range generally give satisfactory results. However, even materials whose plasticity is not entirely within this range can sometimes give acceptable results. The plasticity range is illustrated in Figure 9. Upon projecting the results within this range, PWS shows a liquidity limit of 45%, a plasticity limit of 25%, and a plasticity index of 20%. This indicates that PWS falls within the acceptable plasticity range, making it suitable for brick manufacturing.
The second verification on PWR and PWS, according to the compressed earth block standard XP P 13-901 [56], is the grain size distribution envelope of raw materials. As shown in Figure 10a,b, the particle size distribution of PWR and PWS does not comply with the grain size distribution envelope required by the standard. The PWS shows an excess of fines, while the PWR shows a deficit of fines. However, mixing the two materials produces a particle–size curve (Figure 10c) that fits within the envelope, except for the 2 mm to 10 mm fraction, which falls outside the acceptable range. Therefore, a correction is needed, as shown in Figure 10d.

3.1.3. Chemical and Mineralogical Characterization

Table 4 presents the chemical compositions of PWR and PWS. For the PWR, the pre-dominant oxides are CaO and SiO2, in the presence of Al2O3 and MgO, along with 14.21% P2O5 and traces of K2O and Na2O. On the other hand, for the PWS, the predominant oxides are also CaO and SiO2, with lower amounts than PWR, as well as Al2O3, MgO, K2O, and Na2O, with 12.95% of P2O5.
The mineralogical compositions are highlighted in Figure 11, showing that the primary phases identified in PWR include dolomite, calcite, quartz, calcium fluoride, and montmorillonite. In PWS, the various phases present are quartz, calcite, dolomite, fluorite, rutile, and montmorillonite. The findings from the mineralogical analyses are supported by the results of the chemical analyses performed.

3.2. Mixtures Characterization

Figure 12 presents the crushing results for cylinders after 14 and 28 days. Overall, for the first group (Figure 12a), the compressive strengths of the mixes at 28 days are higher than those at 14 days. Notably, the fifth formulation exhibits the highest strength, exceeding 2 MPa, and optimizes the valorization of PWS. Therefore, this formulation was chosen for the laboratory pilot scale. For the second group, the compressive strengths exceed 2 MPa after just 14 days of curing. This indicates that the formulations containing PWS, PWR and conventional cement rapidly gain strength, achieving significant compressive resistance in a relatively short period. This performance suggests the effectiveness of the mixture components in contributing to early strength development. Formulation 8 was chosen because it utilizes a lower percentage of cement and incorporates a higher amount of washing sludge. In the third group, although the strengths are generally high, the percentage of cement remains elevated. Finally, for the last group, formulations 14, 15, 16, and 17 all have compressive strengths greater than 2 MPa. However, given the priority for formulations with higher washing sludge content, formulation 17 was selected.

3.3. Laboratory Tests on Pilot Scale Bricks

3.3.1. Uniaxial Compressive Strength (UCA)

Figure 13 presents the results of dry uniaxial compressive strength tests on laboratory pilot-scale bricks, which reveal the following insights:
  • Reference 5: Comprising 60% PWS and 40% PWR, these bricks displayed variable compressive strengths, ranging from 3 to 4.7 MPa after 14 days and 3.5 to 5.3 MPa after 28 days. This variability suggests inconsistencies in the material’s behavior.
  • Reference 8: With a mix of 45% PWS, 50% PWR, and 5% conventional cement, these bricks showed more consistent and higher compressive strengths. After 14 days, strengths ranged from 4.4 to 5.6 MPa, and after 28 days, from 5.8 to 7.1 MPa. The addition of cement significantly improved the compressive strength and its development over time.
  • Reference 17: Containing 97.5% PWS and 2.5% conventional cement, these bricks exhibited the lowest compressive strengths, between 3.4 and 3.7 MPa after 14 days and 4.2 to 5.3 MPa after 28 days. The high sludge content likely contributed to the reduced strength.
  • Despite these variations, all formulations exceed the requirements of the standard [56] for dry uniaxial compressive strength. This indicates that each mixture, regardless of the differences in composition and resulting strength, is suitable for practical applications. The compliance with the standard requirements suggests that these bricks can be effectively used in construction, ensuring safety and durability. This finding is crucial for the utilization of alternative materials like PWS and PWR, promoting sustainable practices in the construction industry.

3.3.2. Water Absorption (WA)

Table 5 presents the results of water absorption properties of the three formulations by measuring their water absorption coefficients (g/cm2·min1/2) for exterior applications. Formulation 5 exhibited the lowest coefficient, indicating extremely low capillarity absorption. Formulation 8 showed a slightly higher coefficient of 13.8, yet was still classified as extremely low capillarity absorption. In contrast, formulation 17 recorded the highest coefficient at 26.9, denoting low capillarity absorption.
These results suggest that varying the proportions of PWS, PWR, and CC can significantly influence the water absorption properties, with formulation 5 demonstrating the greatest resistance to water penetration. These results indicate also that all tested formulations are suitable for use in exterior wall bricks, offering good resistance to water penetration.

3.3.3. Thermal Conductivity (TC) Properties

Table 6 shows the thermal conductivity measurements for the three selected mixtures. The values obtained are between 0.553 and 0.632 W·m−1·K−1. The thermal conductivity values measured are better than the requirements presented by the extensive literature [67,68,69], which are between 0.8 and 1.04 W·m−1·K−1.
All studied mixtures present lower thermal conductivity, which means better insulation properties, which is essential for energy efficiency in buildings.

3.3.4. Microstructural Analysis

Scanning Electron Microscopy (SEM) images of formulations 8 and 17 (with conventional cement addition), taken after 28 days, are presented in Figure 14. These images were analyzed to investigate the microstructure of the manufactured blocks. The findings are consistent with the compressive strength results. The SEM images reveal that Calcium Silicate Hydrate (C-S-H) is the dominant phase in the hydrated cement [34,70,71], accounting for most of the calcium and silicon. Furthermore, Portlandite (Ca(OH)2), a common phase in hydrated cement, was observed. Portlandite, a mineral form of calcium hydroxide, typically forms during the hydration process of Portland cement and is a significant component of cementitious materials [71,72].

4. Conclusions

In this study, the potential for recycling phosphate waste sludge (PWS) and phosphate waste rock (PWR) to mitigate environmental impacts and reduce waste volumes was investigated through the production of compressed earth bricks.
Based on literature reviews and the results from an extensive characterization program, PWS and PWR were found to be inert and exhibit similar properties to those of raw materials commonly used in the construction industry. These findings indicate a significant potential for these phosphate wastes to be utilized in building materials. The laboratory-scale study focused on exploring various methods for valorizing phosphate wastes, both with and without the addition of cement.
This study demonstrated the feasibility of producing bricks from phosphate waste sludge (PWS) and phosphate waste rock (PWR), both with and without the addition of conventional cement:
  • The PWS and PWR leaching characteristic test (TCLP) conducted in this study demonstrate no existence of contaminant. Despite being made with phosphate waste, compressed earth bricks (CEBs) do not pose any threats to the environment or human health;
  • Following a preliminary study including seventeen distinct formulations, three best formulations were developed to meet the specified requirements, namely 5 (S60-R40-C0), 8 (S45-R50-C5) and 17 (S97.5-R0-C2.5);
  • The effect of scale-up, marking the transition from laboratory preliminary characterization to laboratory pilot-scale on real products, was also studied;
  • All laboratory pilot-scale formulations achieved dry compressive strength values exceeding 2 MPa after 28 days;
  • Related to water absorption, they exhibited very low to low capillary absorption and are within the requirements indicated by international standards;
  • The thermal conductivity of the scaled up best three mixtures was about 0.6 W m−1 K−1 at laboratory scale;
  • The SEM analyses of the elaborated bricks confirmed the formation of Calcium Silicate Hydrates (C-S-H) and Portlandite, which allowed to improve the mechanical properties of the elaborated CEBs.
Based on the scale-up results, the properties of the best mixtures studied suggest that these bricks can provide the necessary construction safety and thermal performance to create a comfortable living environment. With evidence now showing that bricks can be made from a combination of phosphate waste (PWR) and a by-product (PWS), it is crucial to further investigate this method. Additional research should evaluate the long-term effects on construction durability, environmental impact, and health condition. Future studies could include building pilot houses with PWS and PWR CSEBs to assess their durability and explore their effects, particularly on human health and in terms of fire resistance, which are critical for both safety and environmental considerations.
Finally, to provide a comprehensive assessment of the environmental benefits associated with the use of phosphate waste rock (PWR) and phosphate washing sludge (PWS) in the production of compressed stabilized earth bricks, it is recommended to conduct a detailed Life Cycle Assessment (LCA) on the raw materials prior to evaluating the bricks themselves. This preliminary LCA should focus on the environmental impacts of raw materials, including their carbon footprint, resource consumption, and waste generation. Following this, an LCA of the bricks should be carried out to compare their environmental performance against traditional brick manufacturing methods. By taking this two-step method, sustainable building practices will be much improved and a greater understanding of the environmental benefits of employing these waste products will be provided.

Author Contributions

Conceptualization, O.I. and A.K.; methodology, O.I.; validation, O.I., A.K. and Y.T.; formal analysis, O.I. and A.Z.; investigation, O.I. and A.Z.; data curation, O.I. and Y.T.; writing—original draft preparation, O.I., A.Z. and A.K.; writing—review and editing, A.K., R.H., M.B. and Y.T.; visualization, O.I. and A.Z.; supervision, A.K. and Y.T.; project administration, A.K. and Y.T.; funding acquisition, M.B. and Y.T. All authors have read and agreed to the published version of the manuscript.

Funding

This study received partial funding from the APRD research program 2021, which was developed through a partnership involving the Moroccan Ministry of Higher Education, the National Center for Scientific and Technical Research (CNRST), the OCP Foundation, and University Mohammed VI Polytechnic.

Data Availability Statement

Data available upon request from the authors.

Acknowledgments

The authors extend their gratitude to K. Mouih from IMED LAB, Cadi Ayyad University (Marrakech), and S. Oubaha from Green Energy Parc (Benguerir) for their invaluable support. Special thanks are also due to B. Zayi, E. EL Ghizlany, and the GAL/2GML laboratory team at GSMI/UM6P for their assistance with this work. Special thanks go to N. Attalir and M. El Ghorfi for their assistance with the microscopy analyses.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. General methodology followed for carrying out the study.
Figure 1. General methodology followed for carrying out the study.
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Figure 2. Site plan of PWR and PWS sampled locations.
Figure 2. Site plan of PWR and PWS sampled locations.
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Figure 3. Raw material sampling and preparation for laboratory testing.
Figure 3. Raw material sampling and preparation for laboratory testing.
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Figure 4. Photographic illustration showing a part of cylinders made from different formulations.
Figure 4. Photographic illustration showing a part of cylinders made from different formulations.
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Figure 5. Illustration of the 3000 KN mechanical press used for UCS tests.
Figure 5. Illustration of the 3000 KN mechanical press used for UCS tests.
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Figure 6. Details of pilot scale bricks: (a) Eco Brava machine used for the manufacturing pilot scale compressed bricks; (b) example of prepared bricks using pilot scale dimensions.
Figure 6. Details of pilot scale bricks: (a) Eco Brava machine used for the manufacturing pilot scale compressed bricks; (b) example of prepared bricks using pilot scale dimensions.
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Figure 7. Pilot-scale laboratory set-up for thermal conductivity test.
Figure 7. Pilot-scale laboratory set-up for thermal conductivity test.
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Figure 8. Results of surface contamination of PWR and PWS.
Figure 8. Results of surface contamination of PWR and PWS.
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Figure 9. PWS plasticity properties fall within the permissible range for CEB, as recommended by Houben and Guillard [65], AFNOR [56], and CRATerre [66].
Figure 9. PWS plasticity properties fall within the permissible range for CEB, as recommended by Houben and Guillard [65], AFNOR [56], and CRATerre [66].
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Figure 10. Grading curve of the employed soil, within suitable limits for CEB according to AFNOR [56] and CRATerre [66]: (a) PWR granularity, (b) PWS granularity, (c) 50% PWR, 50% PWS mixture granularity, (d) mixture granularity after correction.
Figure 10. Grading curve of the employed soil, within suitable limits for CEB according to AFNOR [56] and CRATerre [66]: (a) PWR granularity, (b) PWS granularity, (c) 50% PWR, 50% PWS mixture granularity, (d) mixture granularity after correction.
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Figure 11. XRD diffractograms: (a) XRD diffractograms of PWR, (b) XRD diffractograms of PWS.
Figure 11. XRD diffractograms: (a) XRD diffractograms of PWR, (b) XRD diffractograms of PWS.
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Figure 12. Results of dry uniaxial compressive strength on cylinder mixtures: (a) for formulations 1 to 5 (with no cement addition); (b) formulation 6 to 8 (with 5% CC); (c) formulation 9 to 13 (with 10% CC); (d) formulation 14 to 17 (with variable addition of CC).
Figure 12. Results of dry uniaxial compressive strength on cylinder mixtures: (a) for formulations 1 to 5 (with no cement addition); (b) formulation 6 to 8 (with 5% CC); (c) formulation 9 to 13 (with 10% CC); (d) formulation 14 to 17 (with variable addition of CC).
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Figure 13. Results of dry uniaxial compressive strength on laboratory pilot-scale bricks.
Figure 13. Results of dry uniaxial compressive strength on laboratory pilot-scale bricks.
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Figure 14. SEM imaging of cement-enhanced formulations after 28 days: (a) magnification of 20× for formulation 8; (b) magnification of 20× for formulation 17; (c) magnification of 100× for formulation 17.
Figure 14. SEM imaging of cement-enhanced formulations after 28 days: (a) magnification of 20× for formulation 8; (b) magnification of 20× for formulation 17; (c) magnification of 100× for formulation 17.
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Table 1. Mixture definition on laboratory scale.
Table 1. Mixture definition on laboratory scale.
ReferenceMixturePWS (Wt %)PWR (Wt %)CC (Wt %)
1S40-R60-C040600
2S45-R55-C045550
3S50-R50-C050500
4S55-R45-C055450
5S60-R40-C060400
6S35-R60-C535605
7S40-R55-C540555
8S45-R50-C545505
9S30-R60-C10306010
10S35-R55-C10355510
11S40-R50-C10405010
12S45-R45-C10454510
13S50-R40-C10504010
14S90-R0-C1090010
15S92.5-R0-C7.592.507.5
16S95-R0-C59505
17S97.5-R0-C2.597.502.5
Table 2. Results of the Toxicity Characteristic Leaching Procedure (TCLP) of PWS and PWR samples.
Table 2. Results of the Toxicity Characteristic Leaching Procedure (TCLP) of PWS and PWR samples.
SampleTrace Elements (mg/L)
AsBaCdCrPbAgZn
PWR<0.10.550.15<0.1<0.1<0.10.25
PWS<0.10.430.1<0.1<0.1<0.10.21
US EPA Standards510015552
Table 3. Results of the main physical and geotechnical characterization of the raw materials.
Table 3. Results of the main physical and geotechnical characterization of the raw materials.
Properties ValueUnitPWRPWS
Particle size analysis
D50mm14.340.051
D80mm18.940.108
Dmaxmm250.5
Plasticity index%unmeasurable20
Methylene blue valueg/100 g0.70.22
Sand equivalent%419
Proctor test
Optimal dry densitykg/m318201960
Optimal moisture content%137.3
Densitykg/m31610.361476.7
Table 4. Chemical compositions of PWR and PWS (%).
Table 4. Chemical compositions of PWR and PWS (%).
CaOSiO2Al2O3Fe2O3MgOK2ONa2OTiO2P2O5MnOLOICaF2SO3CO2
PWR34.2723.770.750.593.190.130.5<0.0114.21<0.0115.652.980.842.22
PWS31.7322.022.210.983.410.570.70.2112.95<0.0116.154.710.812.71
Table 5. Results of WA test carried out on laboratory pilot-scale bricks.
Table 5. Results of WA test carried out on laboratory pilot-scale bricks.
FormulationsWater Absorption Coefficient (g/cm2·min1/2)Obtained Category for Exterior Walls
Formulation 5
(60% PWS + 40% PWR)		10.3Extremely low capillarity absorption
Formulation 8
(45% PWS + 50% PWR + 5% CC)		13.8Extremely low capillarity absorption data
Formulation 17
(97.5% PWS + 2.5% CC)		26.9Low capillarity absorption
Table 6. Values of thermal conductivity for various mixtures.
Table 6. Values of thermal conductivity for various mixtures.
FormulationsThermal Conductivity (W·m−1·K−1)
Sample 1Sample 2Sample 3Average
Formulation 5
(60% PWS + 40% PWR)		0.6270.581 0.604
Formulation 8
(45% PWS + 50% PWR + 5% CC)		0.5770.562 0.597
Formulation 17
(97.5% PWS + 2.5% CC)		0.5630.6320.5530.583
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Inabi, O.; Khalil, A.; Zouine, A.; Hakkou, R.; Benzaazoua, M.; Taha, Y. Investigation of the Innovative Combined Reuse of Phosphate Mine Waste Rock and Phosphate Washing Sludge to Produce Eco-Friendly Bricks. Buildings 2024, 14, 2600. https://doi.org/10.3390/buildings14092600

AMA Style

Inabi O, Khalil A, Zouine A, Hakkou R, Benzaazoua M, Taha Y. Investigation of the Innovative Combined Reuse of Phosphate Mine Waste Rock and Phosphate Washing Sludge to Produce Eco-Friendly Bricks. Buildings. 2024; 14(9):2600. https://doi.org/10.3390/buildings14092600

Chicago/Turabian Style

Inabi, Omar, Abdessamad Khalil, Abir Zouine, Rachid Hakkou, Mostafa Benzaazoua, and Yassine Taha. 2024. "Investigation of the Innovative Combined Reuse of Phosphate Mine Waste Rock and Phosphate Washing Sludge to Produce Eco-Friendly Bricks" Buildings 14, no. 9: 2600. https://doi.org/10.3390/buildings14092600

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