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Article

Effect of Sulfoaluminate Clinker Addition on Boron Removal During Water Softening

by
Marco Tagliabue
1,
Alessandra de Folly d’Auris
1,
Andrea Pacini
1,
Michela Bellettato
1,
Gianluigi Marra
2,
Sara Perucchini
2,
Cinzia Mazzara
3,
Emanuele Lagrotta
3 and
Marco Vocciante
4,*
1
Eni S.p.A., Research and Development, v. F. Maritano 26, I-20097 San Donato Milanese, Italy
2
Eni S.p.A., Research and Development, v. G. Fauser, 4, I-28100 Novara, Italy
3
Eni Rewind S.p.A., p. M. Boldrini 1, I-20097 San Donato Milanese, Italy
4
Department of Chemistry and Industrial Chemistry, University of Genoa, v. Dodecaneso 31, I-16146 Genova, Italy
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(16), 8890; https://doi.org/10.3390/app15168890
Submission received: 23 June 2025 / Revised: 2 August 2025 / Accepted: 8 August 2025 / Published: 12 August 2025
(This article belongs to the Special Issue New Approaches to Water Treatment: Challenges and Trends, 2nd Edition)
Browse Figures
Versions Notes

Abstract

Boron is frequently present in saline water (e.g., seawater, geothermal water, and hydrocarbon production water) due to the natural release of boric acid from minerals. While essential to life, excess boron is toxic, particularly to citrus plants, necessitating its regulation for safe water use. Current boron removal methods, such as reverse osmosis, chelating resin adsorption, and magnesium-based precipitation softening, increase water treatment complexity and cost. Ettringite, (Ca6Al2(SO4)3(OH)12·26H2O), is a clay and an effective anion adsorbent. It is also a key hydration product of Portland cement. This study explores boron removal via precipitation softening using sulfoaluminate clinker as an ettringite precursor. Raw water, a first-stage reverse-osmosis permeate from an Italian oil-and-gas site, contained approximately 15.0 mg/L of boron. Optimal removal required sulfoaluminate clinker in excess with respect to the stoichiometric dose and 150 min of contact time. The preliminary results demonstrate the feasibility of this approach, offering a viable alternative to existing methods.

1. Introduction

Earth is a watery planet, but freshwater is less than 3.0% of its total capacity. Thus, freshwater must be considered both limited and critical to any human activity [1]. Both intensive exploitation and climate change endanger the availability of raw materials and water security. This is exacerbated in highly anthropic settings, even in continents not commonly considered dry (e.g., Europe) [2,3,4]. Considering the ever-increasing global demand for water by industry and agriculture, the need for strategies to optimize water use and recovery has become more and more pressing [5]. Nevertheless, according to some scenarios, freshwater will approach one half of Earth’s total capacity by 2050, with terrifying effects on biodiversity [6]. Therefore, both science and technology are working hard to make low-grade water resources exploitable [7], with specific attention to dissolved solids removal (i.e., desalination) [8] and emerging-contaminant degradation [9,10,11].
Indeed, there is growing awareness of how almost every aspect of modern society has an impact on the environment, often resulting in depletion of resources and release of contaminants. These activities, frequently coupled with inadequate waste disposal and ineffective resource recovery [12,13], have profound effects on ecosystems, urban resilience, and human health [14,15]. A variety of physical–chemical and biological methods have been developed for the remediation of contaminated soils [16,17,18] and water streams [19,20,21,22]. Water importance is stressed also in United Nations Sustainable Development Goals (UN SDGs) # 06 on clean water and sanitation, # 12 on responsible consumption and production, and # 14 on life below water [23].
Boron (B) can be frequently found dissolved in water due to its release from minerals. It is particularly abundant in seawater, geothermal water, industrial effluents (e.g., from electronics and metallurgy), and hydrocarbon production water (i.e., saline water from hydrocarbon production, commonly referred to as produced water). In seawater, boron concentration can reach units in the order of mg/L, whilst in the latter sources, it can span to several hundred mg/L. Boron is mainly present as boric acid (B(OH)3) that hydrolyzes into the tetrahydroxyborate ion ([B(OH)4]−) due to its Lewis acid behavior. This equilibrium is outlined in Equation (1), with an associated acid dissociation constant (KA), typically expressed as pKA = −Log(KA), of 9.2 [24].
B(OH)3 + H2O ↔ [B(OH)4]− + H+
Boron is essential to life, but excessive amounts can be toxic. As a result, its concentration in water is regulated. The World Health Organization (WHO) currently recommends a maximum boron concentration of 2.4 mg/L in drinking water [25]. Italian regulations set a stricter limit of 1.5 mg/L [26]. For irrigation purposes, particularly when cultivating boron-sensitive crops (e.g., citrus trees), the safe boron concentration must be significantly lower, typically by an order of magnitude [27]. Boron removal is generally implemented as one of the final steps in the water treatment train. Reverse-osmosis membranes or chelating resin units, which require extensive raw water pretreatment, are generally used. Reverse-osmosis membranes operate at high pH, which promotes scaling [28]. Chelating resins are relatively costly (i.e., 25 EUR/kg, referring to the Italian market). Like reverse-osmosis membranes, they have to be protected against scaling and fouling. Additionally, their regeneration needs both acidic and basic treatments due to the simultaneous presence of acidic hydroxyl (-OH) and basic amine (-NH2) functional groups [29].
Globally, produced water accounts for millions of m3/d. Its management must be compliant with the most effective strategies for saving resources, protecting the environment, and recovering energy. Over half of the total volume is reinjected to sustain reservoir pressure, while the remainder is released into surface water bodies or used as a water resource after due treatment [30]. Produced-water treatment trains are operated according to the final user’s needs [31]. They entail operations that can remove both organic and inorganic contaminants [32].
Such treatments not only address economic viability but also raise concerns on environmental sustainability [33] in terms of energy, reagent needs, and waste disposal or recycle. These aspects are pointing to nature-based solutions (NBSs) [34] or methods allowing for energy recovery [35] in order to reduce the carbon footprint. In particular, constructed wetlands (CWs) [36], an approach also used in urban environments to face the effects of climate change [37], has been proposed as an effective alternative method for produced-water treatment [38,39].
Precipitation softening is one of the most employed treatment methods. It removes dissolved ions by forming their respective poorly soluble carbonates and hydroxides at high pH. Both calcium and magnesium salts, which are primary contributors to scaling, are removed through the reactions outlined in Equations (2)–(4) [32]. Calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) have solubility products (SP) expressed as pSP = −Log(SP) of 8.3 and 11.0, respectively [24].
In Equations (2)–(4) calcium hydroxide (Ca(OH)2) is considered a paradigm of a strong base, while calcium bicarbonate (Ca(HCO3)2), magnesium bicarbonate (Mg(HCO3)2), and magnesium chloride (MgCl2) are common dissolved salts (solids are underlined).
Mg(HCO3)2 + 2Ca(OH)2 → Mg(OH)2 + 2CaCO3 + 2H2O
MgCl2 + Ca(OH)2 → Mg(OH)2 + 2CaCl2
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
Silica (SiO2), an additional contributor to scaling, can also be removed by adsorption on magnesium hydroxide [40]. Similarly, iron (Fe) can be removed, like other noxious metal ions, through a process of precipitation softening, forming ferric hydroxide (Fe(OH)3), which has a pSP of 37.0 [24], as outlined in Equation (5), where ferric chloride (FeCl3) is exemplified as a paradigm of a dissolved salt. In turn, ferric hydroxide can adsorb arsenic (As) oxy-ions frequently present in raw water [41].
2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
Precipitation softening uses a straightforward reactor configuration and cost-effective reagents, offering broad applicability with minimal operational constraints. This process is frequently employed as the initial stage preceding more advanced treatment steps, such as separation through membranes or uptake on resins, which are necessary to achieve the requested water specifications. Its main drawback is the generation of caustic solids, which need proper disposal.
The removal of boron through this method remains a complex task. Nevertheless, notable results have been reported [42]:
  • At an industrial scale, effective boron removal has been observed when the magnesium-to-boron molar ratio exceeds 10 mol/mol. This process, however, results in the formation of gelatinous and hygroscopic solids [43]. Similar to silica removal, the mechanism is believed to involve adsorption. Notably, the preformed magnesium hydroxide is less effective than the in situ formed one [44].
  • At a laboratory scale, the addition of hydrogen peroxide (H2O2) has led to the formation of calcium perborate (CaB2O4(OH)4), a compound with low solubility that is inferred to facilitate boron removal. Enhanced performance has been reported when barium hydroxide (Ba(OH)2) is used instead of calcium hydroxide [45].
  • At an industrial scale, the use of ettringite (Ca6Al2(SO4)3(OH)12·26H2O) has also proven effective [46]. Ettringite, a clay and a key hydration product of Portland cement, can be synthesized from hydrated aluminum sulfate (Al2(SO4)3·18H2O) and calcium hydroxide [47]. Its structure comprises columns of (Ca6[Al2(OH)12·24H2O]6+) aligned along the c-axis, with sulfate (SO42−) and water molecules occupying the intercolumnar channels (Figure 1). The framework is stabilized by hydrogen bonding [48], and its open architecture allows for water mobility and ion exchange [49]. Specifically, ettringite can exchange one sulfate ion for one borate ion, turning into the clay mineral charlesite (Ca6Al2(SO4)2(B(OH)4)(OH,O)12·26H2O), which belongs to the same group [50]. Amorphous meta-ettringite, obtained from ettringite through thermal dehydration at temperature higher than 65 °C, has been claimed as more effective than ettringite as such [51]. Importance of meta-ettringite has also been confirmed in a study dealing with Portland cement waste as a low-cost boron adsorbent [52]. Charlesite cannot be considered an exploitable boron raw material because its theoretical boron content of 0.7% wt. is far less than the 15.0% wt. found in state-of-the-art minerals. According to that, other valorization routes alternative to landfilling have to be considered [53].
The characteristics of ettringite are exploited by using it as a Portland cement in environmental protection [54] for the stabilization of liquid [46,55,56] and solid [57] waste at an industrial scale. Conversely, its benefits concerning water treatment are less utilized [42], and the optimization of operational variables is mostly empirical, with specific methodologies often withheld as proprietary know-how. Therefore, a limited systematic analysis is presently available [47].
This study deals with boron removal via precipitation softening. Conditions promoting ettringite formation consistent with industrial-scale implementation were explored through tests with sulfoaluminate clinker. These conditions include simple reactor design, room temperature and pressure, reasonable contact time, and use of low-cost reagents. Sulfoaluminate clinker was used as the ettringite precursor, taking inspiration from the previous study performed using aluminum sulfate hydrated (Al2(SO4)3·16H2O), both together with calcium hydroxide [57]. The raw water used in this study was permeate from first-stage reverse-osmosis membranes from an Italian oil-and-gas site. The focus was placed on reagent dosing and boron-rich solids disposal or valorization. The previous study was considered for comparison to identify peculiarities of both ettringite precursors. A comparative preliminary economic evaluation was also performed.

2. Materials and Methods

2.1. Raw Water

Raw water was obtained from the permeate of the first-stage membranes of a reverse-osmosis process at an Italian oil-and-gas site. It had a specific electrical conductivity (EC) of about 400 μS/cm and a boron level of about 15.0 mg/L.
For comparison, a solution of Merck Pro AnalysisTM boric acid in Millipore deionized water (Merck Life Sciences, Milan, Italy) with a specific electrical conductivity of about 15 μS/cm was used.
Both solutions had negligible levels of aluminum and sulfate.
They were stored in 5 L polyethylene (PE) tanks at a temperature of 4 °C.
The level of boron in the raw water was verified prior to the conduction of every precipitation softening test following the indication given in Section 2.4.

2.2. Precipitation Softening

Tests for precipitation softening were carried out on 0.5 L samples in a polyethylene reactor under room-temperature and -pressure conditions, setting a stirring value of 630 1/min to ensure proper distribution of the slurry through a TeflonTM-coated bar driven by Ika BasicTM magnetic equipment (Ika Werke, Staufen, Germany).
Carlo Erba Reagents RPETM calcium hydroxide (Carlo Erba Reagents, Cornaredo, Italy) was added to obtain a pH of 11.5, before Heidelberg Materials Ali Pre GreenTM sulfoaluminate clinker addition (Heidelberg Materials, Peschiera Borromeo, Italy), which was not influent on the slurry pH level. Both reagents were used as received. Specifically, sulfoaluminate clinker dosing was based on the aluminum quantity needed to obtain the quantity of ettringite able to adsorb all dissolved boron, hypothesizing a 1.0 molar stoichiometric ratio between borate and ettringite [49], and a minimum aluminum level in sulfoaluminate clinker of 14.2% wt. (e.g., 510 mg/L of sulfoaluminate clinker for 15.0 mg/L of dissolved boron) [58]. This will be defined as the sulfoaluminate clinker stoichiometric dose, 1.0X. When it comes to NX, the recommended amount should be calculated as N-fold the stoichiometric dose.
It has to be noted that the aluminum sulfate hydrated used in the previous study (906 mg/L for 15.0 mg/L of dissolved boron) was poorer in aluminum (8.0% wt. vs. 14.2% wt.–17.5% wt. of sulfoaluminate clinker) but richer in sulfur (14.0% wt. vs. 4.0% wt.–5.6% wt. of sulfoaluminate clinker) [59]. Aluminum-driven stoichiometry was adopted in this study for the sake of continuity with the previous one and to avoid the addition of further reagents as sulfate providers (e.g., sulfuric acid, H2SO4).
The measurement of slurry pH, which is essential to the formation of ettringite, was performed using two different methods: Supelco MQuantTM indicator paper (Merck Life Sciences, Milan, Italy) and Hanna 9829TM multiparameter probe (Hanna Instruments Italia, Padua, Italy). In particular, the slurry pH was invariant during each test.
Stirring was carried out by assuming a contact time, t, from 90 min to 150 min, and ultimately, vacuum filtration of the slurry was carried out using Nalgene cellulose acetate septa 0.45 mm in pore size. Contact time was set according to a possible retrofitting of an existing water treatment train (a full kinetic study was outside of the scope of the present study).
The obtained boron-rich solids were dried in air at room temperature and pressure for 48 h. They were then weighed with a Mettler PE 1600TM analytical balance (Mettler Toledo Italia, Milan, Italy), and underwent physical–chemical characterization, following the process depicted in Section 2.3 and Section 2.4. Calorimetry studies have demonstrated that Portland cement hydration heat flow is almost invariant after 48 h, at which point a large part of hydration reactions are thus over [60].
Filtrates also underwent physical–chemical characterization.
The procedure was adopted considering equipment commonly available at the oil-and-gas production site (e.g., tanks, pumps, and filters).
The boron removal efficiency, ΔB, was calculated based on the difference between the final boron concentration (BF) and the initial concentration (BI), referred to as the initial value, as outlined in Equation (6):
ΔB = (BF − BI)·100/BI

2.3. Leaching Test

A leaching test was conducted in duplicate on the boron-rich solids derived from precipitation softening processes that achieved over 95.0% wt./wt. boron removal [61]. The boron-rich solids were mixed with Millipore deionized water (pH of 7.0, EC of 15 μS/cm) in a liquid-to-solid ratio of 10 mL/g. This mixture was shaken at room temperature and pressure through a Velp Scientifica Rotax 6.8™ rotary mixer (Velp Scientifica, Usmate Velate, Italy) set to 15 1/min for 24 h. Afterward, the slurry was filtered as described in Section 2.2. The resulting filtrate and residue were processed following the same procedure outlined for the precipitation of softening materials in Section 2.2.

2.4. Physical–Chemical Characterization

Both raw water and filtrates were subjected to boron, aluminum, and sulfate quantification through optical absorption spectrometry:
  • Boron with a Hack Lange LCK 307TM kit (Hach Lange Italia, Lainate, Italy) based on the Azomethine-H methodology (certified for boron levels from 0.05 mg/L to 0.25 mg/L);
  • Aluminum with a Hack Lange LCK 301TM kit using the Chromazurol-S methodology (certified for aluminum levels from 0.02 mg/L to 0.5 mg/L );
  • Sulfate with a Hack Lange LCK 153TM kit based on barium sulfate (BaSO4) methodology (certified for sulfate levels from 50 mg/L and 140 mg/L).
For all three methodologies, a Hach Lange DR 5000TM ultraviolet (UV), visible (VIS), and near-infrared (NIR) self-calibrating spectrometer (Hach Lange Italia, Lainate, Italy) was employed.
Italian regulations on drinking water for these species forming ettringite and charlesite have been considered in the first instance [26].
Sulfoaluminate clinker elemental analysis was carried out through X-ray fluorescence (XRF) with a Panalytical Axios AdvantageTM spectrometer based on Panalytical WroxyTM calibration (Malvern Panalytical, Lissone, Italy). Samples were previously vitrified through a Panalytical EagonTM sample preparation device (Malvern Panalytical, Lissone, Italy).
The structural properties of the solid samples were analyzed using X-ray diffraction (XRD). Measurements were performed with a Panalytical X’Pert Alpha-1 Θ/2Θ™ Bragg-Brentano diffractometer (Malvern Panalytical, Lissone, Italy), employing Cu Kα radiation (λ = 1.5416 Å). The X-ray tube was operated at 40 kV and 40 mA. Diffraction patterns were recorded over a 2θ range of 5° to 90°, with a step size of 0.02° and a counting time of 15 s per step. Phase identification was conducted qualitatively using the Hanawalt methodology in conjunction with the PDF-2 database provided by the International Centre for Diffraction Data (ICDD). The full profile analysis was performed using the Rietveld method implemented in the Bruker TOPASTM Version #6 software package (Bruker Italia, Milan, Italia).
High-resolution micrographs were obtained using scanning electron microscopy (SEM) with a Jeol JSM 7600F™ instrument (Jeol Italia, Basiglio, Italy) operated at an accelerating voltage of 5 kV, utilizing the secondary electron signal for imaging. Samples were examined in their native state, without any metallization treatment. Prior to imaging, the powdered materials were ground, sonicated in Carlo Erba Reagents RPETM 2-propanol (C3H7-OH) (Carlo Erba Reagents, Cornaredo, Italy), and deposited as suspensions onto conductive carbon tape. Imaging was performed after complete solvent evaporation, covering the entire sample area to ensure statistically representative morphological characterization.
Energy dispersive X-ray analysis (EDX) is a tool for the quantitative and qualitative chemical analysis of materials. EDX makes use of the X-ray spectrum emitted by a solid sample bombarded with a focused high-energy beam of electrons to obtain a localized chemical analysis. Multielement maps of calcium (Ca), sulfur (S), and aluminum (Al), were collected with an acceleration voltage of 15 keV and a probe current of 800 pA by using the emitted Kα energy lines at 3690 keV, 2307 keV, and 1486 keV, respectively.

3. Results and Discussion

The actual elemental composition of sulfoaluminate clinker is consistent with those declared by the provider [59], as reported in Table 1.
The test results are reported in Table 2.
Boron removal higher than 70.0% was obtained with a sulfoaluminate clinker dosing of at least 2.5X, for a contact time of 150 min (Tests C, E, F, and G). An excess dosing of sulfoaluminate clinker can be hardly explained with the presence of interfering dissolved ions. In fact, boron removal slightly decreases when utilizing boron solution prepared with deionized water where only hydroxyls (OH−), and carbonates (CO32−) are virtually present (Test G).
In the previous study, 2.5X aluminum sulfate hydrated was needed to obtain boron removal higher than 90% (Test P). Therefore, aluminum sulfate hydrated is a more effective ettringite precursor than sulfoaluminate clinker in terms of reagent dosing.
Aluminum levels surpass those in the Italian regulations on drinking water, corresponding to 0.2 mg/L [26], when sulfoaluminate clinker is used. Results were different with aluminum sulfate hydrated. In that case, a low boron level in filtrates was correlated to high sulfate and low aluminum levels.
These are consequences to different compositions of ettringite precursors as described in Section 2.2.
Aluminum levels can be abated by decreasing the pH to about neutrality [62], where a large part of soluble aluminum tetrahydroxoaluminate ([Al(OH)4]−) is turned into poorly soluble aluminum hydroxide (Al(OH)3), with a pSP of 32.3 [24], according to the reaction outlined in Equation (7):
Al(OH)3 + OH− ↔ [Al(OH)4]−
The use of carbon dioxide (CO2) for safe pH buffering at 6.4 has been proposed [46], according to the equilibrium outlined in Equation (8), with pKA of 6.4 [24].
CO2↑ + H2O ↔ [H2CO3] ↔ HCO3− + H+
The filtrate from Test E was saturated with food grade carbon dioxide, (Nippon Gases, Milano, Italia) and filtered a second time as described in Section 2.2. An aluminum level of 0.1 mg/L, compliant with the Italian regulations on water, was detected in the resulting filtrate. The detected level is overestimated with respect to the theoretical value probably due to the extensive presence of colloidal particles that are difficult to separate.
The boron-rich solids from Test E underwent the leaching test. Levels of pH and aluminum not compliant with the Italian regulations on surface discharge [63] were found in the leachate: a pH of 11.0 and an aluminum level of 363 mg/L were detected. The limits for surface discharge are 9.5 and 1.0 mg/L, respectively.
Conversely, 0.5 mg/L boron and negligible sulfate levels were observed in the leachate. Accordingly, boron-rich solids have to be disposed of cautiously in landfills because of potential aluminum release.
On the other hand, the presence of aluminum can be exploited in flocculation during due landfill leachate treatment.
As described in Section 2.2, the sulfoaluminate clinker composition is unbalanced with respect to aluminum and sulfate. A way to reduce the aluminum excess could be to use sulfoaluminate clinker only as the aluminum source, providing the sulfate needed for ettringite formation with another reagent (e.g., sulfuric acid).
The addition of boron-rich solids to building materials has been debated as a possible route to recycling: both an appreciable reduction in calcination temperature [64] and a worsening in mechanical properties [65] have been reported.
A comparative preliminary economic evaluation based on reagent and boron-rich solid disposal costs is outlined in Table 3. The input data refer to Test E and Test P, both with boron removal higher than 94%. The costs refer to the Italian market; aluminum sulfate hydrate is considered in the form of 25% wt. water solution; calcium hydroxide is considered in the form of flakes; boron-rich solid disposal is performed through landfilling.
The results show that both aluminum sulfate hydrate and sulfoaluminate clinker are equally viable as ettringite precursors. On the other hand, aluminum sulfate hydrate needs more calcium hydroxide for pH adjustment due to its Lewis acid behavior but produces less boron-rich solids than sulfoaluminate clinker.
Table 4 deals with structural characterization. Different crystal phases were identified in boron-rich solids obtained with sulfoaluminate clinker, including hydration [60] and carbonation [66] products (clinker and boron-rich solids from Tests A and E). Ettringite is already present in sulfoaluminate clinker, probably due to atmospheric moisture absorption.
The XRD patterns reported in Figure 2 show the conversion of the ye’elimite in clinker into ettringite in boron-rich solids from Test E (i.e., the best-performing one in terms of boron removal) [67]. The crystal-phase composition of boron-rich solids obtained with aluminum sulfate hydrated was simpler (boron-rich solids, from Test P), although in both cases, the higher the content of ettringite, the greater the removal of boron.
Hydration was observed in boron-rich solids from Test E’ collected after the leaching test with conversion of further ye’elimite into ettringite.
It must be stressed that the data outlined in Table 4 refer only to crystalline phases (i.e., the only ones directly detectable by XRD). On the other hand, the relevant aluminum release during leaching points to the presence of a labile and (probably) amorphous aluminum-rich phase.
The high-resolution micrograph of boron-rich solids from Test E, shown in Figure 3, shows the typical needle-like ettringite morphology with micrometer crystal lengths, which is confirmed by the multielement X-ray maps of calcium, sulfur, and aluminum in Figure 4.

4. Conclusions

Precipitation softening using sulfoaluminate clinker has demonstrated high efficiency in removing boron from pretreated produced water with an approximate content of boron of 15.0 mg/L. However, effective removal requires a minimum of 2.5 times the stoichiometric amount of sulfoaluminate clinker and a contact time of 150 min to supply sufficient aluminum and sulfate ions for the formation of an adequate quantity of ettringite. These preliminary results can be the starting data for possible optimization.
The aluminum level after precipitation softening in the presence of sulfoaluminate clinker surpasses Italian regulations on drinking water, although it can be adjusted by reducing the pH.
Slightly more boron-rich solids were obtained in comparison to precipitation softening in the presence of aluminum sulfate hydrated as an alternative precursor of ettringite.
The landfilling of boron-rich solids requires caution due to the possibility of aluminum emissions in leachate.

Author Contributions

Conceptualization, M.T.; methodology, M.T.; validation, M.T., A.d.F.d. and M.V.; formal analysis, M.T. and M.V.; investigation, A.d.F.d., A.P., M.B., G.M. and S.P.; resources, A.d.F.d., A.P., M.B., G.M. and S.P.; data curation, M.T., A.d.F.d. and M.V.; writing—original draft preparation, M.T. and M.V.; writing—review and editing, M.T. and M.V.; visualization, M.T. and M.V.; supervision, M.T., E.L. and M.V.; project administration, E.L. and C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research study received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

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

Acknowledgments

Eni S.p.A. colleagues Massimiliano Baric, Paolo Filtri, Erica Montanari, and Nerea Zubin are acknowledged for their precious technical assistance and helpful discussion. Eni Rewind colleagues Massimo Paglia and Luciano M. Zaninetta are acknowledged for their suggestions. Heidelberg Materials Italia Cementi S.p.A. are acknowledged for providing the sulfoaluminate clinker sample.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 15 08890 g001
Figure 1. The crystal structure of ettringite, as viewed along the (001) plane, features calcium atoms, represented by green spheres in the ball-and-stick model. Aluminate groups are shown as gray octahedra, sulfate groups as yellow tetrahedra, and hydrogen atoms in light gray. The structure consists of repeating units of Ca6[Al2(OH)12·24H2O]6+ aligned along the c-axis, with sulfate ions and water molecules occupying the intercolumnar channels.
Figure 1. The crystal structure of ettringite, as viewed along the (001) plane, features calcium atoms, represented by green spheres in the ball-and-stick model. Aluminate groups are shown as gray octahedra, sulfate groups as yellow tetrahedra, and hydrogen atoms in light gray. The structure consists of repeating units of Ca6[Al2(OH)12·24H2O]6+ aligned along the c-axis, with sulfate ions and water molecules occupying the intercolumnar channels.
Applsci 15 08890 g001
Applsci 15 08890 g002
Figure 2. XRD patterns of clinker (top) and boron-rich solids from Test E (bottom). For the sake of clarity, only the main crystal phases are evidenced: ye’elimite in clinker (orange bars) and ettringite in boron-rich solids from Test E (red bars). The detailed crystalline phase composition is reported in Table 4.
Figure 2. XRD patterns of clinker (top) and boron-rich solids from Test E (bottom). For the sake of clarity, only the main crystal phases are evidenced: ye’elimite in clinker (orange bars) and ettringite in boron-rich solids from Test E (red bars). The detailed crystalline phase composition is reported in Table 4.
Applsci 15 08890 g002
Applsci 15 08890 g003
Figure 3. High-resolution micrograph of boron-rich solids from Test E (×10,000 magnification).
Figure 3. High-resolution micrograph of boron-rich solids from Test E (×10,000 magnification).
Applsci 15 08890 g003
Applsci 15 08890 g004
Figure 4. High-resolution micrograph of boron-rich solids from Test E (×8000 magnification) and multielement X-ray maps of calcium (red), sulfur (green), and aluminum (blue).
Figure 4. High-resolution micrograph of boron-rich solids from Test E (×8000 magnification) and multielement X-ray maps of calcium (red), sulfur (green), and aluminum (blue).
Applsci 15 08890 g004
Table 1. Actual and declared compositions of sulfoaluminate clinker. Only relevant elements are reported. ---, not detected.
Table 1. Actual and declared compositions of sulfoaluminate clinker. Only relevant elements are reported. ---, not detected.
ElementCaAlSSiMgFeK
[-][% wt.][% wt.][% wt.][% wt.][% wt.][% wt.][% wt.]
Actual27.916.24.64.02.81.00.6
Declared25.7–29.314.2–17.5 4.0–5.60.0–4.30.0–3.00.0–1.4---
Table 2. Results from Tests A–G (this study) and Test P (previous study). The subsequent information is given dosage of sulfoaluminate clinker, removal of boron (ΔB), and final levels of boron (BF), aluminum (AlF), sulfate (SO4F), and boron-rich solids referred to the volume of raw water. See text for the meaning of other headings. Slight oscillations in boron balance are due to oscillations in initial boron levels. ---, not available.
Table 2. Results from Tests A–G (this study) and Test P (previous study). The subsequent information is given dosage of sulfoaluminate clinker, removal of boron (ΔB), and final levels of boron (BF), aluminum (AlF), sulfate (SO4F), and boron-rich solids referred to the volume of raw water. See text for the meaning of other headings. Slight oscillations in boron balance are due to oscillations in initial boron levels. ---, not available.
TestECDosingtΔBBFAlFSO4FSolids
[-][mS/cm][-][min][%][mg/L][mg/L][mg/L][g/L]
A4002.0X90 −12.313.220.7---1.7
B4002.0X150−19.612.145.3---2.0
C4002.5X150−70.44.462.03.06.5
D4009.0X90−43.98.458.1---5.3
E4009.0X150−99.70.0557.26.06.6
F40013.0X150−94.30.9120.07.09.4
G1513.0X150−77.13.4104.0---8.7
P4002.5X150−94.60.80.2534.03.8
Table 3. Preliminary economic evaluation based on reagent and boron-rich solid disposal costs. ---, not applicable.
Table 3. Preliminary economic evaluation based on reagent and boron-rich solid disposal costs. ---, not applicable.
ItemTest ETest P
TypeSpecific Cost [EUR/kg]Amount [kg/m3]Cost [EUR/m3]Amount [kg/m3]Cost [EUR/m3]
Aluminum sulfate hydrated0.60------2.271.36
Alipre GreenTM0.324.961.59------
Calcium hydroxide0.600.980.592.631.58
Solids disposal0.356.602.313.801.33
Total------4.49---4.27
Table 4. Formulas of identified crystalline phases for sulfoaluminate clinker and boron-rich solids from Tests A, E, and E’ of this study and P of the previous study. Hc, hemicarboluminate. ---, not available.
Table 4. Formulas of identified crystalline phases for sulfoaluminate clinker and boron-rich solids from Tests A, E, and E’ of this study and P of the previous study. Hc, hemicarboluminate. ---, not available.
PhaseFormulaClinker
[% wt.]		A
[% wt.]		E
[% wt.]		E’
[% wt.]		P
[% wt.]
AragoniteCaCO3---------13.1---
BredigiteCa14Mg2(SiO4)831.029.528.714.6---
CalciteCaCO3---24.613.323.223.7
DellaiteCa6(Si2O7)(SiO4) (OH)2---3.64.4------
DolomiteCaMg(CO3)2---3.2---------
EttringiteCa6Al2(SO4)3(OH)12·26(H2O)20.84.431.145.876.3
Hc(Ca4Al2(OH)12)(OH(CO3)0.5·4(H2O))---7.4---------
KuzeliteCa3Al6(OH)18(SO4)1.5·9(H2O)---6.2---------
PericlaseMgO---2.61.92.8---
SilviteKCl---------0.5---
Ye’elimiteCa4Al6O12(SO4)48.218.520.6------
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Tagliabue, M.; de Folly d’Auris, A.; Pacini, A.; Bellettato, M.; Marra, G.; Perucchini, S.; Mazzara, C.; Lagrotta, E.; Vocciante, M. Effect of Sulfoaluminate Clinker Addition on Boron Removal During Water Softening. Appl. Sci. 2025, 15, 8890. https://doi.org/10.3390/app15168890

AMA Style

Tagliabue M, de Folly d’Auris A, Pacini A, Bellettato M, Marra G, Perucchini S, Mazzara C, Lagrotta E, Vocciante M. Effect of Sulfoaluminate Clinker Addition on Boron Removal During Water Softening. Applied Sciences. 2025; 15(16):8890. https://doi.org/10.3390/app15168890

Chicago/Turabian Style

Tagliabue, Marco, Alessandra de Folly d’Auris, Andrea Pacini, Michela Bellettato, Gianluigi Marra, Sara Perucchini, Cinzia Mazzara, Emanuele Lagrotta, and Marco Vocciante. 2025. "Effect of Sulfoaluminate Clinker Addition on Boron Removal During Water Softening" Applied Sciences 15, no. 16: 8890. https://doi.org/10.3390/app15168890

APA Style

Tagliabue, M., de Folly d’Auris, A., Pacini, A., Bellettato, M., Marra, G., Perucchini, S., Mazzara, C., Lagrotta, E., & Vocciante, M. (2025). Effect of Sulfoaluminate Clinker Addition on Boron Removal During Water Softening. Applied Sciences, 15(16), 8890. https://doi.org/10.3390/app15168890

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