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Article

Tundish Deskulling Waste as a Source of MgO for Producing Magnesium Phosphate Cement-Based Mortars: Advancing Sustainable Construction Materials

by
Anna Alfocea-Roig
,
David Vera-Rivera
,
Sergio Huete-Hernández
,
Jessica Giro-Paloma
* and
Joan Formosa Mitjans
*
Departament de Ciència de Materials i Química Física, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain
*
Authors to whom correspondence should be addressed.
Resources 2025, 14(7), 107; https://doi.org/10.3390/resources14070107
Submission received: 20 May 2025 / Revised: 21 June 2025 / Accepted: 26 June 2025 / Published: 29 June 2025
Browse Figures
Versions Notes

Abstract

Currently, the cement industry stands as one of the sectors with the most significant environmental impact, primarily due to its substantial greenhouse gas emissions and energy consumption. To mitigate this impact, a roadmap has been followed in recent years, outlining a set of objectives aimed at diminishing the environmental footprint of the construction industry. This research focuses on the development of mortars with different water/cement ratios employing an alternative cement, specifically magnesium phosphate cement (MPC) formulated with secondary sources. The goal of this research relays in developing mortars based on MPC by using waste from the metallurgical industry, named tundish deskulling waste (TUN), as an MgO source. The results revealed the optimal water/cement (W/C) ratio for MPC-TUN mortars production through the assessment of various characterization techniques, which was 0.55. This ratio resulted in the highest compressive strength after 28 days of curing and the formation of a stable K-struvite matrix. Furthermore, it demonstrated the effectiveness of aluminum sulphate in preventing efflorescence caused by carbonates. The development of alternative masonry mortars for application in building materials represents a significant stride towards advancing the principles of a circular economy, in alignment with the objectives laid out in the 2030 roadmap.

1. Introduction

Currently, cement is one of the most widely used materials worldwide, serving as the primary component in the production of mortars and concrete. The cement industry accounts for 2% of global energy consumption and 5% of industrial electrical uses [1,2]. To mitigate its environmental impact to promote a future carbon-neutral cement, exploring alternative cements developed with secondary sources is a viable approach [1,3,4,5,6]. The possible alternative materials include the magnesium phosphate cements (MPCs) which belong to the chemically bonded ceramics (CBCs) family and are specifically classified under chemically bonded phosphate cements (CBPCs) [1,3,4,5,7,8,9]. To obtain MPCs, it is necessary to initiate a reaction between a magnesium oxide source and a phosphate salt in a water medium [10,11,12]. In this research, the developed MPCs were obtained by mixing MgO and MKP (KH2PO4). The reaction takes place in an acid–base reaction between MgO and KH2PO4, resulting in the formation of K-struvite (MgKPO4·6H2O) [13,14,15,16]. When examining the setting reaction steps of MPCs, they can be divided into five primary stages [13,17]:
(A)
Dissolution of MgO particles: in the initial step, MgO particles react with water, dissolving into Mg2+ and 2 OH− ions.
(B)
Formation of aquosols: cations can react with water to form positively charged species, such as [Mg ← OH2]2+(aq).
(C)
Acid–base reaction and condensation: in this step, the magnesium phosphate forms and subsequently condensates to produce the initial particle aggregates.
(D)
Gel formation and percolation: the newly formed salt particles interact to create a network, which subsequently leads to the formation of a gel.
(E)
Crystallization: the formation of the gel increases saturation, leading to the creation of crystals.
The incorporation of waste materials into MPCs can contribute to a circular economy aligned with the Sustainable Development Goals (SDGs), while also addressing the high energy consumption and CO2 emissions associated with the use of dead burned magnesite (DBM) by exploring solid waste alternatives as substitutes [18,19]. This practice has been increasingly applied in construction materials in recent years [20]. Common waste materials utilized in the construction industry include blast furnace slag, steelmaking dust, coal ash, and steel slag, all of which originate from of the steel industry. Steel is one of the most demanded materials worldwide, extensively produced due to its critical role in numerous industries and sectors [21,22]. In the metallurgical industry, the refractory material plays a pivotal function in withstanding high temperatures of melted steel during production processes [23,24]. The tundish mass, which is a refractory material, is an essential component in the steel continuous casting process, as it serves as the final vessel to regulate the flow of molten metal into the molds (Figure 1) [25,26,27,28,29].
The service life of the tundish mass relies on several factors, including the operating conditions and the performance characteristics of the material used. After reaching the end of its useful life, it must be replaced with new material, generating a large amount of waste, known as tundish deskulling (TUN), which currently ends up in landfills. Worldwide, an estimated 5–7 Mt of TUN are landfilled each year [30]. In the wider context of refractory use, the steel industry generates approximately 20 Mt of refractory waste annually, making up 60–75% of global consumption, with only 5–8% currently recycled [26,27,28,29,30,31]. Notably, tundish linings alone account for around 45% of monolithic refractories in steelmaking, underscoring the significant scale of TUN and the critical need for valorization strategies [31]. Besides, depending on production scale, the steel industry can generate between 5000 and 10,000 tons of tundish deskulling waste annually per plant. Likewise, deskulling operations contribute up to 3% of total refractory waste in continuous casting facilities.
The present research aims to use the industrial waste TUN as an MgO source instead of pure DBM for developing MPC with the objective of promoting the valorization of this waste and contributing to the circular economy. This study evaluates the use of TUN in MPC as an MgO source for synthesizing alternative mortars, focusing on properties such as mechanical performance, hydration behavior, and leaching, to determine its suitability for use as a building material [7,25,26,27,28,32]. Specifically, within the classification of mortars based on concept, manufacturing system, properties, applications, or binder type, this research focuses on the development of mortars based on their functional properties and intended application, particularly as masonry mortars.

2. Materials and Methods

2.1. Materials

TUN was supplied by Magnesitas Navarras, S.A. (Navarra, Spain) after their conditioning processes to obtain the homogeneous non-magnetic fraction with the required particle size. The phosphate source, potassium dihydrogen phosphate (KH2PO4), commonly referred to as MKP, was supplied by Norken, S.L. (Barcelona, Spain) (food grade, 99.8 wt.% purity). Boric acid (H3BO3, HB) was supplied by Borax España, S.A, Castellón, Spain

2.2. TUN Characterization

To determine TUN composition accurately, XRF (X-ray fluorescence) was performed, and it is shown in Table 1. Moreover, XRD analysis was studied and evaluated in previous studies [26,29], published in the literature. Both analyses show that it is a suitable component to be used as a source of MgO.
Additionally, BET analysis determined a specific surface area (SSA) of 3.57 m2/g for the raw material, confirming TUN’s low reactivity due to its minimal surface area. PSD results indicated that particle size distribution followed these thresholds: 10% (d10) measured under 0.72 µm, 50% (d50) was below 3.31 µm, and 90% (d90) did not exceed 7.85 µm [26]. To assess the reactivity of this waste material and verify its suitability as an MgO source, a citric acid test was conducted [26,29,33,34]. Depending on its reactivity, MgO can be classified based on its reaction time as follows: soft-burned (below 60 s), moderately reactive (between 180 and 300 s), hard-burned (around 600 s), and dead burned (below 900 s) [35,36]. This characterization was executed in the previous studies, and the principal findings of them indicated that TUN is qualified as an MgO-rich waste, and that it possesses an adequate reactivity since it is classified as DBM for the advancement of MPC-TUN mortars [26,29,37].

2.3. Samples Preparation

Figure 2 summarizes the formulation composition of the mortars developed. The cement (C) formulation consisted of 60 wt.% of TUN and 40 wt.% of MKP, a dosage that has been previously developed in earlier studies by the authors, which has proven to be the most promising formulation [26]. Additionally, 1 wt.% of H3BO3 (HB) was incorporated into the total cement content to retard the setting time and enhance workability, thereby counteracting rapid setting and hardening. The aggregate used was sand, added in a 1:3 cement/aggregates (C/A) mass ratio. Three different formulations were developed using water/cement ratios (W/C) of 0.55, 0.60, and 0.65 as shown in Figure 2 and Figure 3. As shown in Figure 3, the nomenclature used throughout the article refers to the W/C ratio, which is designated as 0.55, 0.60, and 0.65.
During the first trials, efflorescence was detected on the surface of the samples, attributed to the presence of carbonated salts, as shown in Figure 4 [38]. The efflorescence was characterized by using XRD, as shown in Figure 5. The main structures observed were carbonates such as buetschliite (K2Ca(CO3)2, 01-070-2051), dolomite (MgCa(CO3)2, 01-071-1662), and magnesite (MgCO3, 01-081-2273), sulphates such as epsomite (MgSO4·7H2O, 00-036-0419) and polyhalite (K2Ca2Mg(SO4)4·(H2O)2, 01-070-2158), silicates such as magnesium silicate (Mg2SiO4, 01-075-1447), calcium silicate (Ca2SiO4, 00-003-0753), and grossular (Ca3Al2Si3O12, 01-083-2209), and magnesium aluminum sulfide (MgAl2S4, 01-077-0289).
To avoid efflorescence, aluminum sulphate (AS, 52–57% Al2(SO4)3·16-18H2O) was incorporated into the mixing water at a concentration of 0.35 M, which has proven to be an effective method for preventing efflorescence [39]. This effectiveness is evident in Figure 5, where no efflorescence is observed. The presence of this efflorescence can lead to a loss of properties in the long term, such as a reduction of the mechanical strength [39].
MPC-TUN mortars were cast into prismatic specimens measuring 16 × 4 × 4 cm3. After 2, 7, and 28 days of curing, three specimens per curing period were tested to ensure reliable results and to comprehensively evaluate the mechanical performance and property evolution of MPC over time. The laboratory conditions were maintained at a constant temperature of 25 ± 1 °C and a relative humidity (RH) of 50 ± 5% throughout the experimental procedure.

2.4. Characterization of the MPC-TUN Mortars Formulation

MPC-TUN mortars were characterized to determine physical, mechanical, chemical, and leaching properties by different techniques, as shown in Table 2, to evaluate their suitability as masonry mortars for building material applications.
In the fresh state, workability was determined using the slump test, following the standard [40]. Additionally, apparent density was evaluated according to the standard [41].
In the hardened state, a mechanical characterization was performed by evaluating the Modulus of Elasticity (MoE) following the harmonic frequencies method, in accordance with the standard [42] an ultrasonic pulse velocity tester (C368 by Matest, equipped with a 55 kHz transceiver sensor) was used to conduct the test. This approach was employed to ascertain the material’s ability to tolerate deformation when subjected to external strength. The experimental procedure involves applying perpendicular strength to the reader and, subsequently, assessing the impulse excitation of vibration and the resulting analysis of the fundamental resonant frequency [43,44]. This measurement facilitates the estimation of MoE, considering Poisson’s coefficient.
Flexural strength (σf) and compressive strength (σc) were measured following the standard [45] using an Incotecnic MULTI-R1 universal testing machine, equipped with a 200 kN load cell. The specimens that were split in half during the flexural test were subsequently used for the compression test. The test speeds for the σf and σc were 5 kg·s−1 and 240 kg·s−1, respectively.
Chemical and structural characterization was also carried out specifically to the cement, as the aggregate was previously separated. Fourier-transformed infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR) was employed using the Spectrum TwoTM equipment from Perkin Elmer (Waltham, MA, USA). The data was collected with a resolution of 4 cm−1 in the range 4000–450 cm−1. This technique enables the determination of functional groups and chemical bonds in the samples. The analysis through X-ray diffraction (XRD) was conducted using Bragg-Brentano Siemens D-500 powder diffractometer device with CuKα1 radiation, to determine the mineral and crystalline phases. Afterwards, thermogravimetric analysis (TG) was performed using an SDT Q600 device from TA Instruments (New Castle, DE, USA) in an air atmosphere (50 mL⋅min−1) with a heating rate of 10 °C⋅min−1 ranging from 30 °C to 1100 °C.
An isothermal conduction calorimetry (ICC) experiment was conducted to evaluate the heat changes that could occur during the initial setting of the mortar, as well as the chemical reaction kinetics. This experiment was performed using an 8-channel TAM-Air calorimeter from TA Instruments (New Castle, DE, USA). The internal thermostat temperature was maintained at 20 °C throughout the entire experiment. For this experiment, fresh batches were prepared directly inside the calorimeter. Water was injected into a sealed glass ampoule which contained the mortar formulation and subsequently mixed using an Ad-Mix system equipped with a stirring rotor and a syringe for liquid injection. Data recording commenced immediately after the injection of water, marking the initial contact with the dry materials and the onset of the reaction. Following this, the paste was mixed for a duration of 180 s, and the heat flow was continuously recorded for a period of 140 h.
The microstructure characterization was performed through Scanning Electron Microscope (SEM) using EOL JSM-7100F equipment. The samples evaluated were polished and embedded in epoxy resin. SEM and energy-dispersive X-ray spectroscopy (SEM-EDS) were employed to analyze the elemental composition across various sections of the developed MPC-TUN mortars following a curing period of 28 days.
Finally, the environmental characterization was conducted through leaching tests at 28 days of curing, to evaluate the environmental risk associated with these mortars. This was achieved by quantifying the number of hazardous elements that were released upon exposure to water. The procedure for this analysis was carried out following the standard [46] using an OVAN RHDE rotator, rotating the containers which contained the mortars samples with a particle size below 4 mm and deionized water, spinning for 24 h at 40 rpm. The leaching process entailed extracting heavy metals from the solids utilizing deionized water at a liquid-to-solid ratio of 10 L·kg−1. Following this, the leachate was isolated and filtered utilizing a 0.45 mm pore membrane, then subjected to analysis through Inductively Coupled Plasma mass spectrometry (ICP-MS) using the NexION 350D Perkin Elmer (Waltham, MA, USA) equipment. The metals studied were the subsequent elements: As, Ba, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Se, V, and Zn.

3. Results and Discussion

3.1. Slump Test

The slump test results shown in Table 3 reveal that the mixture presents greater workability for the 0.65 formulation, since it has a higher amount of water. Nevertheless, the three formulations exhibit closely matched consistencies, differing by less than 10%. This similarity indicates similar behaviors and workability in their fresh state, making them suitable for the same applications.

3.2. Apparent Fresh Density

The apparent fresh density exhibits similar values across all three formulations, as depicted in Table 4. The slight variations in the apparent fresh density results, despite a different W/C, are due to the significant impact of the compaction process depending on the consistency of the mortars.

3.3. Apparent Density

The calculated density for each formulation reveals a general decrease in density as curing time increases, as shown in Table 5, except for day 7. This trend can be attributed to the evaporation of free water during the curing process, which leads to the formation of pores. The exception observed on day 7 may be due to intermediate hydration processes, where some degree of consolidation or ongoing chemical reactions temporarily counterbalance the effects of evaporation.

3.4. Modulus of Elasticity, Flexural Strength, and Compressive Strength

Figure 6 illustrates the performance of the MoE, demonstrating a decreasing trend as the W/C ratio increases. This trend is associated with a reduction in density and an increase in porosity, attributable to the higher water content in the various formulations. However, concerning curing time, the MoE values show an opposite trend compared to the other mechanical properties (σf and σc), according to the data provided in Figure 7. This fact could be attributed to the presence of more free water within the structure during the early days of curing. This free water increases the apparent density of the specimen, significantly influencing its vibration during measurement and thereby affecting the results. In contrast, at later stages of curing, the evaporation of this water leads to the formation of a porous network, thereby reducing the density [47].
Both σf and σc results showed in Figure 7 elucidate, in general, the same trends: as the W/C ratio increases, the strength decreases, and as the curing time increases, the strength increases as well. The primary reason for the observed decrease in mechanical properties is attributed to the increased water content in the mortar formulations, which leads to a more extensive porous network. Additionally, extended curing times result in higher strength increases due to the continued hardening of the specimens, likely attributed to cement carbonation. The highest σc value, 14.3 ± 0.4 MPa, is associated with the 0.55 formulation after 28 days of curing, and its σf value is 5.2 ± 0.2 MPa. All formulation results indicate that, according to the standard [48], they can be classified as M-7.5 masonry mortars, making them suitable for building applications, as their 28-day σc exceeds 7.5 MPa. However, the 0.55 and 0.60 formulations could also be categorized as M-10, given their σc values exceed 10 MPa.

3.5. Fourier-Transformed Infrared Spectroscopy in Attenuated Total Reflectance (FTIR-ATR)

The FTIR-ATR spectra of each formulation at different test days (2, 7, and 28), showed no significant differences in the resulting bands at different ages. Therefore, only the FTIR-ATR spectra corresponding to day 28 are presented in Figure 8.
As a result, only the spectral bands associated with the MPC-TUN paste at 28 days are shown, facilitating a precise analysis of its composition. The functional groups identified in the formulations, shown in Table 6, were attributed to the presence of the O-H stretching and bending from H2O and the stretching from PO43− bands [49]. These groups are directly associated with the formation of K-struvite (MgKPO4·6H2O), previously mentioned as the product result from the acid–base reaction between MgO and KH2PO4 [15,50,51].

3.6. X-Ray Diffraction (XRD)

The XRD patterns obtained at different testing days (2, 7, and 28) showed no significant changes in the diffraction peaks, indicating stable phase formation over time. Therefore, only the data at 28 days are presented, confirming the formation of crystalline phases without significant alterations. The XRD analysis shown in Figure 9 presents only the most relevant crystalline peaks used to confirm the presence of seven common structures found in all formulations analyzed at different ages. On the one hand, the presence of magnesium oxide (MgO, 01-078-0430) is indicative of an incomplete dissociation reaction and an excess of this reagent on a molar basis, which needs to be added in larger quantities to ensure the completion of the MKP-TUN reaction. On the other hand, the presence of K-struvite (MgKPO4·6H2O, 01-075-1076) confirms the successful reaction between MKP and TUN [11,14,52]. The silicon oxide (SiO2, 01-078-2315) structure is derived from sand, which is used as an aggregate, being the main compound. The presence of both aluminum phosphate (AlPO4, 01-076-0228) and potassium sulphate (K2SO4, 00-005-0613) are related to the addition of aluminum sulphate (AS) to prevent the efflorescence and secondary phases of TUN, as reported in the literature [26,29,39]. Finally, calcium hydroxide (Ca(OH)2, 01-084-1275) and calcite (CaCO3, 01-083-1762), originating from the raw material composition, were identified in all the developed formulations. These compounds gradually hydrated and carbonated over time.

3.7. Thermogravimetric Analysis (TG)

The thermogravimetric analyses with derivative thermogravimetry (TG/DTG) carried out for each formulation at 2, 7, and 28 days do not exhibit significant differences among the different curing days. Figure 10 presents the results of the TG analysis for MPC-TUN mortar after a curing time of 28 days for the different formulations.
The results for the 28 day curing time in all the formulations showed the following decomposition profile: the first weight loss, observed between 25 and 260 °C, is attributed to K-struvite decomposition; the second weight loss, observed between 260 and 530 °C, is attributed to the water loss from Mg(OH)2; the third weight loss, occurring between 530 and 790 °C, is related to the decomposition of the present carbonates in the samples; and the fourth loss, observed between 790 and 1200 °C, is associated with the sulphate decomposition and other carbonates [53,54,55,56,57]. Furthermore, an overlap of the second peak may occur between Mg(OH)2 and the Ca(OH)2, previously observed in the XRD results. This overlap is expected, as the mass loss occurs within the 330–480 °C range [58]. However, in this study, it has been estimated to correspond to Mg(OH)2, as it is more abundant in the raw material.
Table 7 shows the estimated composition of the different formulations. The formation of K-struvite increases with a higher W/C ratio, as well as Mg(OH)2. Although Mg(OH)2 was not directly identified in the XRD results (Figure 9), the detection of MgO suggests that it may have undergone hydration, leading to the observed presence of hydroxide. In contrast, the concentration of CaCO3, decreases further as the W/C ratio increases. Finally, the increasing presence of K2SO4 as the W/C ratio increases is attributed to the larger volume of Al2(SO4)3 solution added, which promotes an increase in sulphate formation.
The weight loss values of the different formulations are shown in Figure 10. Focusing on the first loss related to the decomposition of K-struvite, it is possible to observe a trend towards a slight increase as the W/C ratio increases, which is related to a higher humidity in the analyzed samples.

3.8. Isothermal Conduction Calorimetry (ICC)

Isothermal conduction calorimetry was carried out to compare the thermodynamic behavior, focusing on the heat flow and the cumulative heat of the process.
Figure 11 shows the heat flow comparison, where the reaction starts with an endothermic peak, being larger for 0.60, followed by 0.65 and almost unappreciable for 0.55. This minimum is related to the dissolution of the MKP, which requires energy [59]. Additionally, it allows for the examination of the faster dissolution of MKP, along with the increased presence of K+, H+, and HPO42−.
After the first endothermic peak, it follows an exothermic maximum peak. The formulation with the larger exothermic peak is 0.65, followed by 0.55, and very close to 0.60. This peak is related to the dissolution process of MgO by H+ ions present in the medium, where the dissociation of magnesia to Mg2+ is a significant exothermic reaction [59,60]. There is also a second exothermic peak, less intense and practically unappreciable, related to the reaction between the present ions to produce MgKPO4·6H2O. All these three steps take place in the first experimental hour, and the experiment is considered completed after three hours.
The cumulative heat curves shown in Figure 12 confirm that the 0.65 formulation reaches the end of the reaction earlier, being much faster than the other two formulations. However, the formulation with a W/C ratio of 0.60 ultimately achieves the highest cumulative heat values. A possible explanation would be that the 0.65 formulation allows the correct dissolution of the different components by presenting a greater amount of water. Although formulations with more water typically exhibit slower reactions, the isothermal nature of the test allows the excess water to promote dissolution, accelerating the reaction process.

3.9. SEM-EDS

The three formulations were evaluated by SEM-EDS (Figure 13 for the 0.55 formulation, Figure 14 for the 0.60 formulation, and Figure 15 for the 0.65 formulation). There were no significant differences among them. The elemental distribution mapping (Figure 13, Figure 14 and Figure 15) shows the presence of K, P, and Mg as main elements in the matrix, which is attributed to K-struvite structure [61]. Furthermore, it is evident that the matrix surrounds the embedded particles primarily composed of SiO2, attributed to the aggregate, while unreacted MgO particles originating from TUN are also noticeable. The presence of S and Al in the sample are related to the addition of aluminum sulphate (AS) to avoid efflorescence. Additionally, the observed fracture surfaces are attributed to the rapid exothermic reaction due to thermal expansion, commented on the isothermal conduction calorimetry section [62].

3.10. Dynamic Leaching Test

As shown in Table 8, the concentration values for the elements studied are within inert limits except for Se, which slightly exceeds the inert limit. Consequently, the MPC-TUN mortars are classified as non-hazardous.
Comparing the values of the mortars with the MPC-TUN pastes (obtained in previously published studies), a dilution effect is observed, attributed to the lower concentration of TUN in the mortar due to the addition of the aggregate [27]. This effect reduces the concentrations of Se and Sb in MPC-TUN cement, categorizing it as hazardous. However, in the case of the MPC-TUN mortars, the concentration of Sb does not exceed the inert limit, while Se surpasses the inert limit but does not reach the hazardous limit. In the case of Ba, its concentration in the mortars increases compared to the MPC-TUN paste, probably due to the influence of the aggregate.
Similarly, the increase in pH observed in the mortars compared to the MPC-TUN paste is attributed to the previously mentioned dilution effect. With a lower cement content, fewer OH− ions are produced, resulting in a more acidic pH.

4. Conclusions

The main conclusions of this study are as follows:
  • The utilization of TUN waste from the metallurgical industry as a residual source of MgO allows the production of a binder, named MPC-TUN, which is adequate for developing mortars. MPC-TUN mortars exhibit properties that make them suitable for their utilization as precast material. Consequently, this research marks the beginning of a path toward exploring alternative mortars, which may play an important role in the development of alternative construction materials.
  • There is a correlation between the progression of curing days and the increase in mechanical strengths, with the 0.55 formulation yielding the best mechanical performance at 28 days among the tested compositions. The consistency results further support this, as the 0.55 formulation achieves an optimal balance between adequate fluidity and adequate strength development. Moreover, this formulation meets the compressive strength requirements to be classified as M-10 masonry mortars, making it suitable for building applications.
  • The characterization of MPC-TUN mortars, including FTIR-ATR, XRD, TG, ICC, and SEM-EDS, was essential for a comprehensive analysis. These techniques provided key insights, confirming K-struvite as the main phase and identifying secondary phases derived from TUN.
  • The development of mortars using TUN waste demonstrates the potential for future MPC applications, promoting the principles of the circular economy, sustainability, and implementing proper waste management practices. By incorporating solid waste materials as substitutes for pure DBM, these strategies address the high energy consumption and CO2 emissions typically associated with MPC production. The implementation of waste recovery strategies not only promotes the efficient management of natural resources but also highlights a paradigm shift towards sustainable practices, contributing to the preservation of environmental ecosystems and the mitigation of climate change.

Author Contributions

Conceptualization, J.G.-P. and J.F.M.; data curation, A.A.-R. and D.V.-R.; formal analysis, A.A.-R.; funding acquisition, J.G.-P. and J.F.M.; investigation, A.A.-R., D.V.-R. and S.H.-H.; methodology, J.F.M.; resources, S.H.-H.; supervision, J.G.-P. and J.F.M.; visualization, A.A.-R., D.V.-R., S.H.-H., J.G.-P. and J.F.M.; writing—original draft, A.A.-R.; writing—review and editing, D.V.-R., S.H.-H., J.G.-P. and J.F.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Spanish Government with Grant, PID2021-125810OB-C21, funded by MCIN/AEI/10.13039/501100011033, by “ERDF A way of making Europe”, Grant CNS2022-135800, funded by MICIU/AEI/10.13039/501100011033, and by the “European Union NextGenerationEU/PRTR”. The authors would like to thank the Catalan Government for the quality accreditation and funding given to their research group DIOPMA (2021 SGR 00708). DIOPMA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. Furthermore, the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) contributed through A. Alfocea-Roig’s PhD grant (FI-DGR 2021). This work is also partially supported by Magnesitas Navarras, S.A. (FBG312160).

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank Magnesitas Navarras, S.A. company, for the information supply.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Continuous casting: 1. ladle; 2. tundish; 3. mold; 4. plasma torch; 5. stopper; 6. straight zone.
Figure 1. Continuous casting: 1. ladle; 2. tundish; 3. mold; 4. plasma torch; 5. stopper; 6. straight zone.
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Figure 2. Experimental design. W = water; C = cement; A = aggregate; AS = aluminum sulphate; HB = boric acid.
Figure 2. Experimental design. W = water; C = cement; A = aggregate; AS = aluminum sulphate; HB = boric acid.
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Figure 3. Three formulations developed.
Figure 3. Three formulations developed.
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Figure 4. Sample with efflorescence.
Figure 4. Sample with efflorescence.
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Figure 5. XRD of the efflorescence present on the surface.
Figure 5. XRD of the efflorescence present on the surface.
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Figure 6. MoE results of MPC-TUN mortars at different ages of curing.
Figure 6. MoE results of MPC-TUN mortars at different ages of curing.
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Figure 7. Flexural strength (σf, (left)) and compressive strength (σc, (right)) of MPC-TUN mortars at different ages of curing.
Figure 7. Flexural strength (σf, (left)) and compressive strength (σc, (right)) of MPC-TUN mortars at different ages of curing.
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Figure 8. FTIR-ATR of 28 days MPC-TUN mortars.
Figure 8. FTIR-ATR of 28 days MPC-TUN mortars.
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Figure 9. XRD analysis after 28 days of curing for the MPC-TUN mortars.
Figure 9. XRD analysis after 28 days of curing for the MPC-TUN mortars.
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Figure 10. Thermogravimetric analysis of MPC-TUN at 28 days.
Figure 10. Thermogravimetric analysis of MPC-TUN at 28 days.
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Figure 11. Heat flow of reaction of MPC-TUN mortars.
Figure 11. Heat flow of reaction of MPC-TUN mortars.
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Figure 12. Cumulative heat of reaction of MPC-TUN mortars.
Figure 12. Cumulative heat of reaction of MPC-TUN mortars.
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Figure 13. SEM-EDS elemental distribution mapping of 0.55 formulation.
Figure 13. SEM-EDS elemental distribution mapping of 0.55 formulation.
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Figure 14. SEM-EDS elemental distribution mapping of 0.60 formulation.
Figure 14. SEM-EDS elemental distribution mapping of 0.60 formulation.
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Figure 15. SEM-EDS elemental distribution mapping of 0.65 formulation.
Figure 15. SEM-EDS elemental distribution mapping of 0.65 formulation.
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Table 1. XRF analysis of TUN, expressed in wt.%.
Table 1. XRF analysis of TUN, expressed in wt.%.
CompoundsMgOCaOSiO2Al2O3Fe2O3SO3P2O5Cr2O3Na2OLOI
wt.%68.709.438.674.683.560.150.110.070.024.61
Table 2. Characterization of MPC-TUN mortars.
Table 2. Characterization of MPC-TUN mortars.
TechniquesMPC-TUN Mortars
Fresh stateSlump test---
Apparent fresh density---
Hardened stateTechniques2d7d28d
Apparent density🗸🗸🗸
MoE🗸🗸🗸
CS (σc)🗸🗸🗸
FS (σf)🗸🗸🗸
FTIR-ATR🗸🗸🗸
XRD🗸🗸🗸
TG--🗸
Isothermal calorimetry🗸🗸🗸
Leaching tests--🗸
SEM--🗸
Table 3. Base expansion of slump test of MPC-TUN mortar formulations.
Table 3. Base expansion of slump test of MPC-TUN mortar formulations.
FormulationBase Width (mm)
0.55112.53
0.60120.88
0.65126.78
Table 4. Apparent density of each MPC-TUN mortar formulation.
Table 4. Apparent density of each MPC-TUN mortar formulation.
FormulationApparent Fresh Density (g·cm−3)
0.552.044 ± 0.034
0.602.042 ± 0.025
0.652.054 ± 0.019
Table 5. Apparent density of MPC-TUN mortars at different curing ages.
Table 5. Apparent density of MPC-TUN mortars at different curing ages.
Apparent Density (g·cm−3)
Formulation2d7d28d
0.552.064 ± 0.0052.034 ± 0.0401.954 ± 0.030
0.602.024 ± 0.0032.030 ± 0.0201.932 ± 0.020
0.651.985 ± 0.0092.015 ± 0.0021.947 ± 0.010
Table 6. Assignation of FTIR-ATR bands.
Table 6. Assignation of FTIR-ATR bands.
Band (cm−1)Assignation
2940–3250ν O-H from water
1590–1680δ O-H from water
1000–1010ν1 P-O4 from K-struvite
575–560ν4 P-O4 from K-struvite
Table 7. Estimated composition of MPC-TUN mortars in weight percentage.
Table 7. Estimated composition of MPC-TUN mortars in weight percentage.
wt.%
FormulationMgKPO4·6H2OMg(OH)2CaCO3K2SO4
0.5527.771.271.035.66
0.6028.721.051.015.55
0.6528.831.330.617.15
Table 8. Leaching test for TUN, MPC-TUN paste, and mortars.
Table 8. Leaching test for TUN, MPC-TUN paste, and mortars.
SamplepHAsBaCdCrCuHgMoNiPbSbSeZn
TUN12.54<0.012.20<0.020.020.05<0.010.290.03<0.050.010.49<0.10
MPC-TUN (paste)11.280.41<0.05<0.02<0.05<0.05<0.011.80<0.05<0.050.090.99<0.10
0.5510.740.0210.86<0.01<0.01<0.01<0.020.33<0.02<0.01<0.010.35<0.10
0.6010.630.019.18<0.01<0.01<0.01<0.020.40<0.02<0.01<0.010.42<0.10
0.6510.54<0.019.69<0.01<0.01<0.01<0.020.38<0.02<0.01<0.010.42<0.10
Inert limit (mg/kg)-0.5200.040.520.010.50.40.50.060.14
Non-hazardous limit (mg/kg)-2100110500.21010100.70.550
Hazardous limit (mg/kg)-25300570100230405057200
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Alfocea-Roig, A.; Vera-Rivera, D.; Huete-Hernández, S.; Giro-Paloma, J.; Formosa Mitjans, J. Tundish Deskulling Waste as a Source of MgO for Producing Magnesium Phosphate Cement-Based Mortars: Advancing Sustainable Construction Materials. Resources 2025, 14, 107. https://doi.org/10.3390/resources14070107

AMA Style

Alfocea-Roig A, Vera-Rivera D, Huete-Hernández S, Giro-Paloma J, Formosa Mitjans J. Tundish Deskulling Waste as a Source of MgO for Producing Magnesium Phosphate Cement-Based Mortars: Advancing Sustainable Construction Materials. Resources. 2025; 14(7):107. https://doi.org/10.3390/resources14070107

Chicago/Turabian Style

Alfocea-Roig, Anna, David Vera-Rivera, Sergio Huete-Hernández, Jessica Giro-Paloma, and Joan Formosa Mitjans. 2025. "Tundish Deskulling Waste as a Source of MgO for Producing Magnesium Phosphate Cement-Based Mortars: Advancing Sustainable Construction Materials" Resources 14, no. 7: 107. https://doi.org/10.3390/resources14070107

APA Style

Alfocea-Roig, A., Vera-Rivera, D., Huete-Hernández, S., Giro-Paloma, J., & Formosa Mitjans, J. (2025). Tundish Deskulling Waste as a Source of MgO for Producing Magnesium Phosphate Cement-Based Mortars: Advancing Sustainable Construction Materials. Resources, 14(7), 107. https://doi.org/10.3390/resources14070107

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