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Article

The Effect of Sulfur Concentration on the Crystallization and Electrochemical Behavior of Portland Cement

by
Byung-Hyun Shin
1,†,
Jinyong Park
1,†,
Jeunghyeuon Cho
2,†,
Miyoung You
2,
Seongjun Kim
1,
Jung-Woo Ok
1,
Jonggi Hong
1,
Taekyu Lee
1,
Jong-Seung Bae
1,
Pungkeun Song
2,* and
Jang-Hee Yoon
1,*
1
Yeongnam Regional Center, Korea Basic Science Institute, Busan 46742, Republic of Korea
2
Department of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Crystals 2025, 15(4), 358; https://doi.org/10.3390/cryst15040358
Submission received: 10 March 2025 / Revised: 4 April 2025 / Accepted: 11 April 2025 / Published: 13 April 2025
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Abstract

:
Portland cement is a critical material widely used in the construction industry, where its crystallization and microstructure are key factors determining its physical and mechanical properties. This study investigated the effect of sulfur on the crystallization and microstructure of Portland cement. Sulfur acts as either an additive or an impurity during the cement production process, influencing the crystal size, distribution, and microstructure formation of major hydration products such as C3S (tricalcium silicate), C2S (dicalcium silicate), C3A (tricalcium aluminate), and C4AF (tetracalcium aluminoferrite). Through quantitative and qualitative evaluation using XRD, SEM, and EPMA analytical techniques, this study examined changes in the hydration characteristics, crystal structure, and microstructure of Portland cement with varying sulfur concentrations. The results revealed that increased sulfur content promotes the crystal growth of C3A and the formation of ettringite, which alters the density of the structure during the early stages of hydration and affects its long-term strength properties. These findings suggest that controlling the sulfur content plays a significant role in optimizing the performance and durability of Portland cement. This study highlights the potential for developing high-performance cement by regulating sulfur levels during the production process, contributing to advancements in construction materials.

1. Introduction

Portland cement is one of the most widely used materials in modern construction and civil engineering [1,2]. It serves as a fundamental component of the construction industry due to its excellent properties including compressive strength, durability, and workability [3,4]. It is primarily manufactured by calcining raw materials such as limestone, clay, and silica at high temperatures [5,6]. During hydration, the cement forms hydration products that determine its strength and stability [7,8]. In particular, calcium silicate compounds such as tricalcium silicate (C3S, 3CaO⋅SiO2) and dicalcium silicate (C2S, 2CaO⋅SiO2) play crucial roles in early and long-term strength development [9,10]. Additionally, tricalcium aluminate (C3A, 3CaO⋅Al2O3) and tetracalcium aluminoferrite (C4AF, 4CaO⋅Al2O3⋅Fe2O3) influence the setting time and coloration of cement [11,12]. These properties enable Portland cement to function as a high-performance material that is suitable for various environments and applications.
Sulfur is a critical factor that must be carefully considered in the Portland cement manufacturing process [13,14] and is introduced into the system through raw materials such as limestone, fuels, and recycled industrial waste [15,16]. Excessive sulfur content can significantly alter the chemical composition and crystalline structure of cement. Specifically, an excessive amount of sulfur can lead to the formation of ettringite (C6A⋅3CaSO4⋅32H2O, calcium aluminate sulfate hydrate) and other sulfate-rich phases, which can cause volume expansion, cracking, and the reduced long-term durability of cement pastes [17,18]. Moreover, sulfur oxide emissions from cement production pose environmental concerns, necessitating strict control during the manufacturing process.
Recent studies on Portland cement have focused on the development of high-performance and environmentally friendly materials. Garcia et al. investigated cement with low hydration heat, while Kamenskih et al. studied recycled cement materials [3,10]. Additionally, Douglas et al. explored cements designed for extreme environmental conditions [7]. As a result, research trends in cement science have increasingly emphasized low-heat hydration cement, low-carbon cement utilizing recycled materials, and specialized cement for harsh environments.
As part of these efforts, there is an increasing focus on understanding the effects of sulfur on the microstructure and mechanical properties of cement and developing methodologies to control its influence [19,20]. In particular, studies on the correlation between sulfur content and cement hydration products provide valuable insights for ensuring long-term material stability [21,22]. Research on sulfur not only aims to enhance cement quality, but also seeks to extend the material lifespan and improve sustainability [22,23]. Sulfur plays a crucial role in crystal growth and microstructural formation, directly affecting the physical and chemical properties of the final product. Therefore, systematically understanding sulfur’s influence and developing effective control strategies will contribute to the production of high-quality cement with superior strength and stability while minimizing its environmental impact.
Recently, a growing number of studies have focused on modifying the cement composition to reduce carbon emissions and improve performance under various environmental conditions. For example, Khedaywi investigated the effect of sulfur incorporation on the physical and mechanical properties of cement such as compressive strength and setting time [24]. Seo explored how different sulfur contents influence the hydration process and phase formation [25] while Zhang examined the applicability of sulfur-modified cement in structural applications, highlighting improvements in durability and cost-effectiveness [1]. While these studies provide valuable insights into the physical properties of sulfur-containing cement, there remains a significant gap in understanding the mineralogical and electrochemical implications of sulfur addition. In particular, the effects of sulfur on the crystallization behavior and corrosion-related electrochemical characteristics have not been sufficiently addressed in the existing literature. This study aims to fill that gap.
This study aimed to examine the impact of sulfur content (from 0 to 10 wt.%) on the crystallization and microstructure of Portland cement by analyzing both quantitative phase changes and qualitative microstructural modifications. To achieve this, various analytical techniques, including field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), electron probe micro-analysis (EPMA) and X-ray photoelectron spectroscopy (XPS), were utilized to examine the changes in cement hydration products and microstructure as a function of sulfur concentration (ranging from 0 to 10 wt.% in 2 wt.% increments). This research sought to explore the potential for high-performance cement development through sulfur control while enhancing the efficiency and sustainability of cement manufacturing processes.

2. Materials and Methods

2.1. Materials

In this study, commercially available Portland cement was used, with multiple samples prepared by varying the sulfur (Elemental Sulfur, S8; Flowers of Sulfur, Sulfur Powder; CAS No. 7704-34-9; Cyclooctasulfur) content [25,26]. The samples were formulated with a water to cement ratio of 1:2, and the sulfur concentration was controlled accordingly. The samples were categorized based on sulfur content, ranging from 0 wt.% to 10 wt.% in 2 wt.% increments. The curing process was conducted at room temperature for seven days.
Although the sulfur content in typical Portland cement is less than 1 wt.%, higher concentrations were intentionally applied in this study to investigate the full spectrum of structural and electrochemical changes. This approach helps to identify critical thresholds and potential effects under extreme or non-conventional conditions such as when sulfur-rich additives or waste materials are introduced.
The chemical composition of each sample was analyzed using inductively coupled plasma-mass spectrometry (ICP-MS, Thermo Fisher Scientific, Waltham, MA, USA) to quantify sulfur and other major elements [27,28]. The results, presented in Table 1, indicate that the cement naturally contained 0.6 wt.% sulfur. As the sulfur content increased, the concentrations of C, O, Si, Ca, and Al exhibited a decreasing trend. The commercial cement naturally contained approximately 0.6 wt.% sulfur. To achieve target sulfur concentrations of 2 to 10 wt.%, additional elemental sulfur (S8) was mixed into the cement in proportion to the total cement weight.
A w/c ratio of 0.5 was adopted in this study to ensure homogenous dispersion of sulfur within the cement paste and allow for reproducible testing of crystallization and electrochemical characteristics. The focus was not on mechanical performance but on material behavior at varying sulfur levels.
The curing period was limited to 7 days to focus on early-stage hydration behavior, during which most primary crystalline phases and electrochemical characteristics are known to develop. Since the objective of this study was not mechanical strength evaluation but microstructural and electrochemical analysis, a 7-day period was deemed appropriate.
Portland cement primarily consists of four major mineral phases: alite (C3S, 3CaO⋅SiO2), belite (C2S, 2CaO⋅SiO2), tricalcium aluminate (C3A, 3CaO⋅Al2O3), and brownmillerite (C4AF, 4CaO⋅Al2O3⋅Fe2O3). Changes in these phases were analyzed after the crystallization process.

2.2. Crystallization Analysis

The microstructure of Portland cement with varying sulfur concentrations was analyzed using field emission-scanning electron microscopy (FE-SEM, SUPRA 40VP system, Zeiss, Oberkochen, Germany) [29,30]. FE-SEM imaging was conducted to examine changes in the grain size, morphology, and distribution based on sulfur content [31,32]. The overall microstructure was observed at 1000× magnification, while fine crystal structures were analyzed at 100,000× magnification. Electron probe micro-analysis (EPMA) was performed to precisely analyze the chemical composition of the cement crystals. Each specimen with different compositions was analyzed at a magnification of 1000× to evaluate the quantitative composition and distribution of key elements including S, C, O, Al, Si, and Ca. This analysis allowed for the observation of changes in the elemental distribution within the microstructure as the sulfur content increased.

2.3. Crystalline Structure Analysis

The crystalline structure of the samples was analyzed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD was used to identify the hydration products and phase transformations in the cement samples. Scans were conducted from 20° to 60° (2θ) with a step size of 0.01° [33,34]. XPS was utilized for detailed surface chemical composition and bonding state analysis [35]. The scan range was 50 eV to 600 eV with a step size of 1 eV. These methods provided in-depth insights into the effects of sulfur on the crystal structure and chemical state of the cement.

2.4. Electrochemical Behavior Analysis

The corrosion resistance of Portland cement with varying sulfur concentrations was evaluated using electrochemical methods including open-circuit potential (OCP) measurements, potentiodynamic polarization testing, and electrochemical impedance spectroscopy (EIS) [36,37]. A three-electrode system was employed, consisting of cement specimens as the working electrode (WE), a saturated calomel electrode (SCE) as the reference electrode (RE), and a platinum (Pt) mesh (20 mm × 20 mm) as the counter electrode (CE) [38,39]. The tests were conducted in a 3.5 wt.% NaCl solution, prepared according to ASTM G61 standards, to simulate an aggressive corrosive environment [40].
OCP measurements were performed over 3600 s to monitor the electrochemical stability of the samples [41,42]. Since alloys and cement-based materials exhibit galvanic corrosion, direct potential calculation is not feasible, making OCP a crucial method for assessing material reactivity over time. Potentiodynamic polarization testing was conducted within a voltage range of −0.6 V to +1.2 V with a scan rate of 0.167 V/s to evaluate the corrosion behavior based on the current density variations with applied voltage.
EIS analysis was carried out over a frequency range of 106 Hz to 100 Hz to examine the changes in resistance due to sulfur content [36,43]. The results were analyzed using Bode plots, Nyquist plots, and equivalent circuit modeling to assess the effect of sulfur on surface passivation and protective layer formation. These electrochemical techniques provided comprehensive insights into the relationship between sulfur content and the corrosion resistance of the cement specimen.

3. Results

3.1. Microstructure with Added Sulfur

Sulfur, with an atomic number of 16, has a low density and weak bonding strength, making it highly reactive with other materials [20,31]. In cement manufacturing, a certain amount of sulfur is detected, which influences the crystallization of materials and their corrosion resistance [44,45]. To analyze the effect of sulfur composition on the microstructure, FE-SEM observations were conducted, and the results are presented in Figure 1 and Figure 2. As the sulfur content increased, the presence of voids and cracks also showed a tendency to increase.
When examining the fine microstructure, sulfur crystals promoted dendritic growth within the cement matrix. This dendritic structure, characterized by low density, is expected to result in reduced mechanical strength and corrosion resistance. Therefore, sulfur acts as a factor that disrupts crystal growth in cement, which subsequently affects both its strength and durability.
These crystallographic changes should be evaluated from a phase and crystal growth perspective [3,46]. Cement crystals generally grow at ambient temperature, forming a high density structure [7,12]. However, the presence of sulfur slowed down the crystal growth rate, leading to a dendritic growth pattern. These findings indicate that in the XRD analysis, the main peak shifted from 29° to 34° in the sulfur-containing cement, while the other peaks remained unchanged. Thus, sulfur acts as a disruptive factor in cement crystal growth.
Sulfur acted as a factor influencing the crystallization behavior of the cement. The elemental distribution of major components was analyzed, and the results are presented in Figure 3. According to the analysis of the primary cement components and sulfur, sulfur was found to exhibit a relatively low concentration and was generally distributed uniformly throughout the matrix. However, as the sulfur content increased, localized segregation was observed, and the sulfur appeared to not form bonds with silicon in the EPMA results.
Among the primary cement elements, aluminum (Al), silicon (Si), and calcium (Ca) showed tendencies to segregate, which also seemed to affect the distribution of oxygen (O). Since Al, Si, and Ca are dense elements that readily react with oxygen in the cement matrix, they were easily oxidized during the hydration process. On the other hand, carbon (C) remained uniformly distributed regardless of the sulfur content, suggesting that carbon was not significantly influenced by the presence of sulfur.
No specific element was observed to strongly react with sulfur, indicating that sulfur was not easily incorporated into the cement matrix. Consequently, unincorporated sulfur likely interfered with crystal growth, leading to delayed or inhibited crystallization.
To analyze the phase changes caused by the sulfur (S) reactions, XRD analysis was conducted [34,47]. The XRD results corresponding to different sulfur compositions are presented in Figure 4 and Table 2. As the sulfur content increased, noticeable shifts in the main peaks were observed. The primary peak at 28.7° was associated with ettringite and C3S, which are major components of cement. However, cement containing sulfur exhibited phase transformations. In sulfur-added cement, the main peak appeared at 34.8°, indicating a shift from 34.1°, thereby demonstrating a significant change in the primary peak. Additionally, XRD analysis revealed that the main peak of the sulfur-containing cement shifted by approximately 0.6° to 0.7°, suggesting structural modifications.
XPS analysis confirmed the changes in the chemical states of the major elements (Al, S, Si, C, Ca, O) in cement as the sulfur (S) content increased from 0 to 10 wt.% [10,12]. The XPS results are presented in Figure 5, showing significant variations in the S peak. Specifically, at 162 eV (S 2p3/2) and 164 eV (S 2p1/2), the S peak intensity increased from a baseline of 2500 counts to 4100 counts. Additionally, the main S peak at 168 eV (S-O (S 2p3/2)) and 169 eV (S-O (S 2p1/2)) increased from 4000 counts to 6200 counts. This increase indicates that as additional sulfur is incorporated into the existing cement matrix, the sulfur-related phases become more pronounced and intensified [4,35].
As the S peak intensity increased, sulfur was predominantly present in the form of sulfate (SO42−), potentially influencing the formation of ettringite (Aft, Ca6Al2(SO4)3(OH)12⋅26H2O) or monosulfate (Afm, 4CaO⋅Al2O3⋅SO3⋅12H2O). The typical hydration reaction of cement, where C3A (tricalcium aluminate, CaSO4·2H2O) reacts with sulfate to form ettringite, can be expressed as follows [10,12]:
3CaO⋅Al2O3 + 3CaSO4⋅2H2O + 26H2O → Ca6Al2(SO4)3(OH)12⋅26H2O (Ettringite)
Over time, ettringite can transform into monosulfate:
Ca6Al2(SO4)3(OH)12⋅26H2O + 2(3CaO⋅Al2O3) + 2H2O →
3(4CaO⋅Al2O3⋅SO3⋅12H2O) (monosulfate)
The identification of Aft and Afm phases supports the interpretation that sulfur, introduced into the system, is oxidized during hydration and contributes to the formation of these stable sulfate-containing phases.
However, the binding energy of Al and Ca remained relatively unchanged, suggesting that the formation of CaO3⋅Al2O3, Ca(OH)2, and CaCO3 was not significantly affected by the addition of sulfur [12,16]. This implies that while sulfur influences the sulfate-related phases, it does not substantially alter the primary formation of calcium and aluminum compounds in cement.
These results suggest that sulfur has a potential influence on the C-S-H and ettringite formation processes in cement. In particular, the observed variations in the Si and O peaks in the XPS data imply a possible interaction between sulfur and silica (SiO2), which may result in microstructural modifications [18,28]. Further investigations are needed to verify this mechanism.

3.2. Results of Electrochemical Behavior Analysis

The electrochemical behavior was analyzed using open circuit potential (OCP), potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS), and the results were obtained accordingly. The electrochemical behavior was evaluated in a 3.5 wt.% NaCl electrolyte using a three-electrode cell and a potentiostat [36,48]. While the galvanic potential of pure metals can be calculated, the potential of alloys is more complex due to compositional variations. Therefore, OCP measurements were conducted to assess the potential variations as a function of sulfur (S) content, and the results are presented in Figure 6.
A lower potential generally indicates a higher corrosion susceptibility [49,50]. The 0 wt.% S sample (black curve) maintained the highest potential of approximately −0.2 V, indicating a more stable state. In contrast, as the sulfur content increased, the potential gradually decreased, and in the 10 wt.% S sample (green and blue curves), the potential dropped below −0.4 V, indicating an electrochemically active state. These results suggest that when an optimal amount of sulfur (from 2 to 4 wt.%) is added, the potential remains around −0.3 V, maintaining a relatively stable condition. However, excessive sulfur addition (over 6 wt.%) increases the risk of corrosion, likely due to the increased formation of sulfates (SO42−), which could enhance the probability of steel reinforcement corrosion within the cement matrix.
This section provides a comprehensive electrochemical evaluation to correlate the sulfur content with corrosion susceptibility, integrating the findings from the OCP, polarization, and EIS results.
The potentiodynamic polarization test is an effective method for evaluating the corrosion behavior of materials [51,52]. To further investigate the effect of sulfur on the corrosion resistance, polarization tests were conducted for different sulfur compositions, and the results are presented in Figure 7 and Table 3. The test results confirmed that sulfur increases the corrosion susceptibility. In the activation polarization region, the potential (Ecorr) decreased from −0.21 V to −0.39 V, while the current density (Icorr) increased from 5 × 10−7 to 1 × 10−6 A/cm2. This trend suggests that corrosion occurs more readily under activation polarization conditions. Additionally, the pitting potential (Epit) decreased from 0.71 V (0 wt.% S) to 0.51 V (8 wt.% S), and the 10 wt.% S sample exhibited two distinct corrosion events at 0.06 V and 0.62 V. The observed corrosion behavior was strongly influenced by the microstructure, with a higher sulfur content leading to increased corrosion susceptibility [53,54]. This is likely due to the formation of porous and loosely packed structures in sulfur-containing cement, which compromises its corrosion resistance by increasing the number of micro-voids and structural defects.
Electrochemical [36,55] spectroscopy (EIS) measures the resistance and phase angle variations as a function of frequency, allowing for the evaluation of surface coating layers and passive film thickness [56]. Additionally, circuit modeling can be derived from EIS data analysis. The EIS results corresponding to different sulfur compositions are presented in Figure 8 and Table 4. The solution resistance was measured at 6.1 ohms, which is a typical value for a 3.5 wt.% NaCl solution. When no sulfur was added, the highest potential was observed, while increasing the sulfur content led to a decrease in resistance from 10,500 ohms to 5400 ohms. This reduction in resistance indicates the degradation of the cement’s oxide layer, which primarily consists of calcium-based compounds formed on the ettringite surface. While cement with a well-formed oxide layer exhibits high resistance and superior corrosion resistance, the presence of sulfur disrupts its growth, leading to a decline in corrosion protection [36,57].
The electrochemical analysis results from the OCP, potentiodynamic polarization, and EIS demonstrate that sulfur negatively affects corrosion resistance [19,44]. Cement with added sulfur showed a decreasing trend in corrosion resistance as the sulfur content increased, which was attributed to the weakening of the surface oxide layer. This deterioration of the passive layer appears to have been influenced by crystallographic changes [58,59]. The addition of sulfur delayed the initial growth of cement, resulting in a fibrous structure. Consequently, cement that failed to form a dense microstructure developed a weaker passive layer, reducing its corrosion resistance. Therefore, to achieve high-performance cement, the sulfur content must be controlled. Furthermore, if elements promoting crystal growth are incorporated, both the strength and corrosion resistance of the cement could be significantly improved.

3.3. Discussion

The experimental results indicate that sulfur significantly affects the microstructure, electrochemical behavior, and corrosion resistance of cement. XPS analysis confirmed that as the sulfur content increased, the intensity of S peaks also increased, suggesting that sulfur primarily existed in the form of sulfates (SO42−) [44,59]. The presence of sulfur altered the phase composition, as observed in the XRD results, where the main peak shifted from 29° to 34° [7,60]. This suggests that sulfur disrupts the normal crystallization of cement, leading to a less stable phase transformation. FE-SEM images further revealed that the addition of sulfur promoted dendritic growth, resulting in an increased presence of voids and cracks. Such microstructural changes indicate that excessive sulfur can hinder cement densification, potentially reducing its mechanical strength and durability.
Electrochemical analysis, particularly OCP measurements, demonstrated that higher sulfur content led to a decrease in open circuit potential, suggesting increased corrosion susceptibility [27,30]. The potential shift from −0.2 V (0 wt.% S) to −0.4 V (10 wt.% S) indicates that sulfur promotes electrochemical activity, which may facilitate the chloride-induced corrosion of steel reinforcement in cement structures. While a moderate sulfur content (from 0 to 4 wt.%) maintained a relatively stable potential, excessive sulfur significantly increased the corrosion risks. The polarization test results supported this finding, as the increased sulfur content lowered the activation potential and increased current density, confirming a higher corrosion rate in the sulfur-rich cement samples.
EIS analysis further validated these observations by showing that the resistance of the cement’s passive layer decreased from 10,500 ohms to 5400 ohms as the sulfur content increased [56,61]. This reduction in resistance indicates the weakening of the surface oxide layer, which plays a crucial role in corrosion protection. Since the oxide layer in cement primarily consists of Ca-based compounds, its degradation suggests that sulfur hinders the formation of a stable passive film. The microstructural changes, including porous and fibrous formations, may have contributed to the loss of corrosion resistance, as cement with excessive sulfur failed to form a dense and protective structure.
Based on these findings, sulfur acts as a disruptive element in cement crystallization, altering the phase composition and reducing its corrosion resistance [22,31]. While a controlled amount of sulfur may not significantly deteriorate the cement properties, excessive sulfur disrupts crystal growth, densification, and passive layer stability, leading to weakened mechanical strength and increased corrosion susceptibility. To optimize cement performance, it is essential to regulate the sulfur content and incorporate elements that enhance crystal growth and passive layer formation.
In practical cement production, the sulfur content is largely influenced by the composition of raw materials and fuel sources used in the clinker manufacturing process. To ensure stable performance, the sulfur level can be managed by selecting low-sulfur raw materials, optimizing kiln operations, and incorporating supplementary cementitious materials (SCMs) such as slag or fly ash, which help dilute or stabilize sulfur-containing phases. These findings may serve as a valuable reference for developing sulfur control strategies in industrial cement manufacturing.
It should be noted that the XPS and XRD analyses provided information on the average chemical states and phase composition across the entire sample, confirming that sulfur existed predominantly as sulfate (SO42−) and contributed to the formation of AFt and AFm phases. However, the SEM-EDS analysis, which observes localized regions, revealed sulfur segregation in some areas. This phenomenon may be attributed to the limited capacity of the cement matrix to incorporate sulfate ions, especially under high sulfur concentrations. Excess sulfur that exceeds this incorporation limit may locally segregate without fully reacting with other major elements. Therefore, these observations do not conflict but rather complement each other, as they reveal both the bulk incorporation of sulfur as sulfate and the presence of locally segregated sulfur phases.

4. Conclusions

Portland cement was modified with sulfur, and its effects on the crystallization behavior and electrochemical properties were analyzed, leading to the following conclusions:
(1)
This study investigated the effects of sulfur addition (0–10 wt.%) on the microstructure, electrochemical behavior, and corrosion resistance of cement. The results indicate that sulfur alters the phase composition, as confirmed by the XRD and XPS analyses, leading to a shift in the main peaks and crystallographic transformations. FE-SEM analysis revealed that increased sulfur content resulted in dendritic growth, increased porosity, and crack formation, which may negatively affect the mechanical strength and long-term durability of cement.
(2)
Electrochemical tests demonstrated that the addition of sulfur lowered the open circuit potential and reduced passive layer resistance, indicating an increased corrosion susceptibility to corrosion. While a moderate sulfur content (from 0 to 4 wt.%) maintained a relatively stable electrochemical response, higher sulfur levels significantly (to 6 wt.%) weakened the passive layer and increased the corrosion rates. The EIS results supported that the sulfur-rich cement exhibited a decrease in resistance, suggesting the degradation of the surface oxide layer, which is crucial for corrosion protection.
(3)
Overall, these findings highlight the importance of precisely controlling the sulfur content. Although small additions of sulfur may not pose significant risks, excessive sulfur disrupts crystal growth, reduces densification, and weakens passive film stability, leading to diminished strength and corrosion resistance. Future research should focus on enhancing cement properties by incorporating elements that promote crystal formation and strengthen the passive layer, ensuring improved durability and structural integrity.

Author Contributions

Conceptualization, J.C., B.-H.S., M.Y., P.S., and J.-H.Y.; Methodology, S.K., and J.P.; Software, S.K. and J.P.; Validation, J.-W.O., J.H., T.L., and J.-S.B.; Formal analysis, J.-W.O., J.H., T.L., and J.-S.B.; Investigation, J.H., T.L., and J.-S.B.; Resources, M.Y., P.S., and J.-H.Y.; Data curation, J.H., T.L., and B.-H.S.; Writing—original draft preparation, J.C., B.-H.S., M.Y., P.S., and J.-H.Y.; Writing—review and editing, J.C., B.-H.S., M.Y., P.S., and J.-H.Y.; Visualization, J.-W.O., J.H., and T.L.; Supervision, S.K. and J.P.; Project administration, B.-H.S., M.Y., P.S., and J.-H.Y.; Funding acquisition, B.-H.S., M.Y., P.S., and J.-H.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a grant from the Korea Basic Science Institute (grant number C512220). This work was supported by a Korea Institute for Advancement of Technology (KIAT) grant, funded by the Korea Government (MOTIE) (RS-2024-00410787, HRD program for Industrial Innovation). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (20224000000090).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00358 g001
Figure 1. FE-SEM images of the Portland cement samples with the addition of varying sulfur contents: (a) 0 wt.%, (b) 2 wt.%, (c) 4 wt.%, (d) 6 wt.%, (e) 8 wt.%, (f) 10 wt.%.
Figure 1. FE-SEM images of the Portland cement samples with the addition of varying sulfur contents: (a) 0 wt.%, (b) 2 wt.%, (c) 4 wt.%, (d) 6 wt.%, (e) 8 wt.%, (f) 10 wt.%.
Crystals 15 00358 g001
Crystals 15 00358 g002
Figure 2. High-magnification FE-SEM images (100,000×) of the Portland cement samples with the addition of varying sulfur contents: (a) 0 wt.%, (b) 2 wt.%, (c) 4 wt.%, (d) 6 wt.%, (e) 8 wt.%, and (f) 10 wt.%.
Figure 2. High-magnification FE-SEM images (100,000×) of the Portland cement samples with the addition of varying sulfur contents: (a) 0 wt.%, (b) 2 wt.%, (c) 4 wt.%, (d) 6 wt.%, (e) 8 wt.%, and (f) 10 wt.%.
Crystals 15 00358 g002
Crystals 15 00358 g003aCrystals 15 00358 g003b
Figure 3. EPMA elemental mapping of the Portland cement samples with the addition of varying sulfur contents from 0 to 10 wt.%, showing the distributions of (a) carbon, (b) oxygen, (c) aluminum, (d) silicon, (e) sulfur, and (f) calcium.
Figure 3. EPMA elemental mapping of the Portland cement samples with the addition of varying sulfur contents from 0 to 10 wt.%, showing the distributions of (a) carbon, (b) oxygen, (c) aluminum, (d) silicon, (e) sulfur, and (f) calcium.
Crystals 15 00358 g003aCrystals 15 00358 g003b
Crystals 15 00358 g004
Figure 4. Counts, (au) and 2 theta (degree) curve of Portland cement with added sulfur concentrations from 0 wt.% to 10 wt.%.
Figure 4. Counts, (au) and 2 theta (degree) curve of Portland cement with added sulfur concentrations from 0 wt.% to 10 wt.%.
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Crystals 15 00358 g005
Figure 5. Counts, s vs. binding energy (eV) curve and the XPS results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
Figure 5. Counts, s vs. binding energy (eV) curve and the XPS results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
Crystals 15 00358 g005
Crystals 15 00358 g006
Figure 6. Potential (vs. SCE, V) vs. time (seconds) curve and the OCP results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
Figure 6. Potential (vs. SCE, V) vs. time (seconds) curve and the OCP results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
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Crystals 15 00358 g007
Figure 7. Potential (vs. SCE, V) vs. current density (A/cm2) curve and the potentiodynamic polarization curve with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
Figure 7. Potential (vs. SCE, V) vs. current density (A/cm2) curve and the potentiodynamic polarization curve with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
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Crystals 15 00358 g008
Figure 8. Electrochemical impedance spectroscopy results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement. (a) Z magnitude (Zmag, ohms) vs. frequency (Hz) curve, (b) phase of Z (degree) vs. frequency (Hz) curve, (c) Z real part of impedance (Zre, ohms) vs. Z imaginary part of impedance (Zim, ohms), and (d) electrochemical impedance spectroscopy circuit.
Figure 8. Electrochemical impedance spectroscopy results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement. (a) Z magnitude (Zmag, ohms) vs. frequency (Hz) curve, (b) phase of Z (degree) vs. frequency (Hz) curve, (c) Z real part of impedance (Zre, ohms) vs. Z imaginary part of impedance (Zim, ohms), and (d) electrochemical impedance spectroscopy circuit.
Crystals 15 00358 g008
Table 1. Chemical composition of Portland cement samples with the addition of varying sulfur concentrations from 0 wt.% to 10 wt.% by ICP-MS (unit: wt.%).
Table 1. Chemical composition of Portland cement samples with the addition of varying sulfur concentrations from 0 wt.% to 10 wt.% by ICP-MS (unit: wt.%).
(a)(b)(c)(d)(e)(f)
S0.62.54.56.58.610.5
C0.70.70.70.60.60.6
O45.344.543.742.942.141.3
Si5.35.14.94.74.54.3
Ca47.146.245.344.443.542.6
Al1.11.00.90.80.70.7
Table 2. Chemical composition of the Portland cement samples with the addition of varying sulfur concentrations from 0 wt.% to 10 wt.% (unit: wt.%).
Table 2. Chemical composition of the Portland cement samples with the addition of varying sulfur concentrations from 0 wt.% to 10 wt.% (unit: wt.%).
No.DegreeShifted DegreeMineralogical PhaseChemical FormulaAbbreviationCrystal
Structure		Miller Index
123.8 EttringiteCaO⋅Al2O3⋅SO3⋅12H2OAFtHexagonal(114)
226.6 SilicaSiO2Silicon dioxideHexagonal(101)
327.0 SilicaSiO2Silicon dioxideHexagonal(100)
427.5 SilicaSiO2Silicon dioxideHexagonal(102)
528.5 CalciteCaCO3Calcium carbonateTrigonal(104)
628.729.4AragoniteCaCO3Calcium carbonateTrigonal(104)
729.430.1DolomiteCaCO3Calcium carbonateTrigonal(104)
829.7 Aluminite3CaO·Al2O3C3ATetragonal(110)
929.7 CalciteCaCO3Calcium carbonateTrigonal(104)
1032.2 Alite3CaO·SiO2C-S-H, calcium silicate hydrateMonoclinic(200)
1132.9 Belite2CaO·SiO2C2S, dicalcium silicateMonoclinic(200)
1233.3 Belite2CaO·SiO2C2S, dicalcium silicateMonoclinic(201)
1334.134.8Belite2CaO·SiO2C2S, dicalcium silicateMonoclinic(001)
1434.134.8Slaked LimeCa(OH)2Calcium hydroxideHexagonal(201)
1541.9 Aluminite3CaO·Al2O3C4AFTetragonal(202)
1643.4 Aluminite3CaO·Al2O3C3ATetragonal(211)
1747.147.8Slaked LimeCa(OH) 2Calcium hydroxideHexagonal(211)
1847.147.8Aluminite3CaO·Al2O3C4AFTetragonal(001)
1950.851.5Slaked LimeCa(OH)2Calcium hydroxideHexagonal(211)
2050.851.5Aluminite3CaO·Al2O3C3ATetragonal(001)
2151.652.0Aluminite3CaO·Al2O3C4AFTetragonal(212)
2252.3 Aluminite3CaO·Al2O3C3ATetragonal(213)
2353.8 Aluminite3CaO·Al2O3C4AFTetragonal(213)
2453.355.0Aluminite3CaO·Al2O3C3ATetragonal(214)
2557.0 Aluminite3CaO·Al2O3C4AFTetragonal(216)
Table 3. Major values on the potentiodynamic polarization curve with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
Table 3. Major values on the potentiodynamic polarization curve with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
0 wt.%2 wt.%4 wt.%6 wt.%8 wt.%10 wt.%
Ecorr, vs. SCE, V−0.21−0.30−0.31−0.32−0.33−0.39
Icorr, A/cm25 × 10−72 × 10−65 × 10−66 × 10−67 × 10−61 × 10−6
Epit, vs. SCE, V0.710.630.600.600.510.06, 0.62
Table 4. Major values on the electrochemical impedance spectroscopy results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
Table 4. Major values on the electrochemical impedance spectroscopy results with added sulfur concentrations from 0 wt.% to 10 wt.% of Portland cement.
0 wt.%2 wt.%4 wt.%6 wt.%8 wt.%10 wt.%
Rs, ohms6.16.16.16.26.26.1
n of CPE13,12512,25010,50010,00095006750
P of CPE0.80.80.80.80.80.8
Rp, ohms10,50098008400800076005400
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Shin, B.-H.; Park, J.; Cho, J.; You, M.; Kim, S.; Ok, J.-W.; Hong, J.; Lee, T.; Bae, J.-S.; Song, P.; et al. The Effect of Sulfur Concentration on the Crystallization and Electrochemical Behavior of Portland Cement. Crystals 2025, 15, 358. https://doi.org/10.3390/cryst15040358

AMA Style

Shin B-H, Park J, Cho J, You M, Kim S, Ok J-W, Hong J, Lee T, Bae J-S, Song P, et al. The Effect of Sulfur Concentration on the Crystallization and Electrochemical Behavior of Portland Cement. Crystals. 2025; 15(4):358. https://doi.org/10.3390/cryst15040358

Chicago/Turabian Style

Shin, Byung-Hyun, Jinyong Park, Jeunghyeuon Cho, Miyoung You, Seongjun Kim, Jung-Woo Ok, Jonggi Hong, Taekyu Lee, Jong-Seung Bae, Pungkeun Song, and et al. 2025. "The Effect of Sulfur Concentration on the Crystallization and Electrochemical Behavior of Portland Cement" Crystals 15, no. 4: 358. https://doi.org/10.3390/cryst15040358

APA Style

Shin, B.-H., Park, J., Cho, J., You, M., Kim, S., Ok, J.-W., Hong, J., Lee, T., Bae, J.-S., Song, P., & Yoon, J.-H. (2025). The Effect of Sulfur Concentration on the Crystallization and Electrochemical Behavior of Portland Cement. Crystals, 15(4), 358. https://doi.org/10.3390/cryst15040358

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