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Review

A Review of the Calcium Sulphoaluminate Cement Mixed with Seawater: Hydration Process, Microstructure, and Durability

by
Han Li
1,
Jing Meng
2,*,
Yang Liu
1,
Lilin Yang
1,
Yukai Wang
1,
Ning Xie
1,
Jinping Ou
2 and
Guoxiang Zhou
3,*
1
Shandong Provincial Key Laboratory of Green and Intelligent Building Materials, University of Jinan, Jinan 250024, China
2
Guangdong Provincial Key Laboratory of Intelligent and Resilient Structures for Civil Engineering, Harbin Institute of Technology, Shenzhen 518055, China
3
Chongqing Research Institute, Harbin Institute of Technology, Chongqing 401135, China
*
Authors to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(6), 1076; https://doi.org/10.3390/jmse13061076
Submission received: 13 April 2025 / Revised: 18 May 2025 / Accepted: 24 May 2025 / Published: 29 May 2025
(This article belongs to the Special Issue Monitoring and Evaluation of Marine Engineering Equipment and Structures)
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Abstract

:
The preparation of low-cost and high-durability cement-based material systems using seawater mixing has become an urgent task in marine engineering construction. The requirements have addressed key challenges, including high transportation costs for fresh water and raw materials, poor structural durability, and difficulty in meeting actual construction schedules. Sulfatealuminate cement (CSA) has become an ideal material for marine engineering due to its high corrosion resistance, rapid early strength, which is 35–40 MPa of 3-day compressive strength and is 1.5–2 times compared ordinary Portland cement (OPC), and low-carbon characteristics, reduced production energy consumption by 35–50%, and CO2 emissions of 0.35–0.45 tons/ton. The Cl− and SO42− in seawater can accelerate the hydration of CSA, promote the formation of ettringite (AFt), and generate Friedel’s salt fixed chloride ions, significantly enhancing its resistance to chloride corrosion. Its low alkalinity (pH ≈ 10.6) and dense structure further optimize its resistance to sulfate corrosion. In terms of environmental benefits, CSA-mixed seawater can save 15–20% fresh water. And the use of solid waste preparation can reduce environmental burden by 38.62%. In the future, it is necessary to combine multi-scale simulation to predict long-term performance, develop self-healing materials and intelligent control technologies, and promote their large-scale application in sustainable marine infrastructure.

1. Background

The durability of Portland cement concrete directly affects the normal service life of engineer structures and has always been a research hotspot in the engineering field. Seawater contains an amount of inorganic salts (such as sulfates, magnesium salts, chlorides, etc.). Marine engineering concrete structures are subjected to corrosion and deterioration due to seawater exposure during service leading to a decline in structural load-bearing capacity and failure [1]. Damage to concrete structures caused by insufficient durability is very common and has resulted in significant engineering losses. The durability issues of concrete structures lead to increasingly apparent losses, and it shows a trend of annual exacerbation. It is essential to find new cementitious, corrosion-resistant materials that can replace ordinary Portland cement in marine engineering [2].
China invented calcium sulfoaluminate cement (CSA) in the 1970s, which is primarily used for rapid repair of engineering projects, anti-seepage and leakage plugging, winter construction, and glass fiber reinforced cement (GRC) products [2]. Unlike ordinary Portland cement, CSA cement is characterized by high early strength, excellent impermeability, frost resistance, and superior corrosion resistance [3]. Due to the low CaO content in its clinker, the alkalinity of the cement hydration solution is relatively low, with a pH value of approximately 10.6. CSA cement has been widely used in China with advantages.
Although CSA cement is a high-performance alternative to ordinary Portland cement (OPC), cementitious materials remain extremely water-intensive. The manufacturing of 10 billion cubic meters of concrete consumes over 2 Gt freshwater [4]. Notably, concrete production accounts for approximately 18% of global industrial water use and 1.7% of total worldwide water withdrawals. A total of 75% of water demand in freshwater-stressed regions will be allocated to concrete production by 2050 [3]. In contrast to the escalating scarcity of freshwater, seawater is the earth’s most abundant natural resource. Consequently, utilizing seawater as mixing water for cement-based materials offers a viable strategy to mitigate freshwater shortages.
In recent years, the use of seawater as an alternative to freshwater for mixing CSA cement has garnered significant attention due to its potential in marine engineering and sustainable construction. The Cl− and SO42− ions in seawater accelerate CSA cement hydration and promote the formation of ettringite (AFt), which enhances early-age compressive strength [5,6,7]. Additionally, the SO42− in seawater reacts with the aluminum phases in CSA cement to form stable Aft, leading to improve sulfate resistance. Meanwhile, low-sulfur hydrated calcium sulfoaluminate (AFm) will be formed when gypsum is insufficient in the CSA cement, accompanied by the conversion of ettringite to AFm. AFm phase is a key product in the hydration process of CSA cement, and its chemical formula is Ca4Al2(SO4)(OH)12·6H2O. It can chemically bind chloride ions (Cl−), which reduce free chloride concentration and enhance resistance to the chloride corrosion [8,9,10,11].
Although CSA cement can immobilize a portion of Cl−, additional protective measures are still required in high-salinity environments [10]. And long-term performance data in tidal zones remain lacking. Research on producing CSA cement from solid waste could enhance the cost reduction [11]. Studies in the future should focus on using AI models to predict the mechanical/durability performance of CSA with seawater mixed and developing self-healing CSA cement with seawater mixed. In conclusion, CSA cement holds broader application prospects and untapped research potential.

2. Fundamentals of Calcium Sulphoaluminate Cement (CSAs)

2.1. Chemical Composition and Reaction Mechanisms

CSA cement consists primarily of calcium sulphoaluminate (C4A3Ŝ) as its key phase, along with other compounds, including Belite, Gypsum, and Ferrite [4]. The main hydration mechanisms are the following chemical reactions [5]:
C4A3Ŝ + 2CŜ + 38H→C6AŜ3H32
C4A3Ŝ + 18H→C4AH12 + AH3
C2S + AH3 + 5H→C2ASH8
C4A3Ŝ + 2CŜH2 + 34H→C6AŜ3H32+2AH3
where CŜ is the anhydrite; C6AŜ3H32 is ettringite; C4AH12 is the tetracalcium aluminate hydrate; AH3 is the aluminum hydroxide gel; C2S is the belite; C2ASH8 is the stratlingite; and CŜH2 is the gypsum.
These two equations represent the Ye’elimite hydration with or without gypsum. The main hydration products are needle-shape ettringite, C4AŜH12, C4AH12, and AH3 [6].
It is reported that the hydration process of the cement is divided into five characteristic stages through systematic research: initial dissolution stage, mineral phase transformation stage, self-drying process, dynamic equilibrium stage, and final acceleration stage [12]. It is indicated that the electrochemical testing techniques is used to simplify the hydration reaction into three main stages: reaction latency, acceleration and phase transition, and final stabilization [13].
The hydration kinetics and products of ye’elimite are influenced by the addition of calcium sulfate or calcium hydroxide. Ye’elimite reacts according to Equation (2) to form monosulfate and aluminum hydroxide when mixed with fresh water, the latter typically being X-ray amorphous. The kinetics of this reaction are quite slow, which exhibits a dormant period of several hours. The addition of gypsum or anhydrite accelerates the hydration kinetics, leading to the formation of ettringite along with aluminum hydroxide instead of monosulfate as per Equation (4). When calcium sulfate is consumed, monosulfate forms according to Equation (2). The ratio between ye’elimite and calcium sulfate determines the proportion of ettringite to monosulfate in the final product. When the molar ratio of calcium sulfate to ye’elimite is greater than 2, only the reaction shown in Equation (4) occurs. Compared to OPC cement, CSA-based cements react more rapidly, and most of the hydration heat release occurs between 2 to 24 h after hydration. The typical value of hydration heat measured by conduction calorimetry approaches 400 J/g after 72 h [7]. Figure 1 shows the phase development of a CSA cement (water/cement = 0.80) as a function of hydration time calculated by thermodynamic modeling [8].

2.2. Setting Time and Workability

CSA cement exhibits rapid setting and high early-age strength, making it ideal for time-sensitive construction projects [9]. The initial setting time typically ranges from 10 to 30 min, depending on the types of additives, curing temperature, and water/cement ratio, while the final setting time occurs within 30 to 60 min [10], due to the rapid hydration of ye’elimite (C4A3Ŝ) by forming the ettringite with the presence of calcium sulfate. However, the rapid setting time needs to be properly controlled to realize good workability via using appropriate admixtures or retarders [11]. Compared to OPC, the lower water demand makes it suitable for high-performance concrete applications like precast elements, repairs, and industrial flooring [14].
Due to the rapid setting of CSA cement, it is not suitable for some engineering applications because there is insufficient time for pouring before CSA cement hardens. The use of high-efficiency water reducers can increase the fluidity of the cement, while the use of retarders can extend the setting time of the cement to maintain good fluidity and workability. It is reported that the competitive adsorption between the citric acid (CA) and β-naphthalenelfonic acid-based superplasticizer (BNS) and minosulfonic acid-based superplasticizer (AS) significantly reduces the initial fluidity [15]. On the other hand, no competitive adsorption occurs between sodium gluconate (SG) and the superplasticizers BNS and AS. Additionally, the combination of these two retarders with the polycarboxylate acid-based superplasticizer (PC) shows a more positive effect on improving fluidity and reducing flow loss [6]. Figure 2 shows the effects of retarders on the PC adsorption amount of the cement pastes.
Although CSA cement sets quickly, its hydration rate slows down at low temperatures leading to prolonged setting times and decreased early strength. It has been observed that lithium carbonate-aluminum sulfate (LC-AS) can significantly improve the setting time and early strength of CSA at 0 °C. LC-AS not only promotes the formation of ettringite but also accelerates the hydration of belite. The amount of ettringite generated increases significantly at 0 °C, thereby reducing the setting time of CSA at this temperature [16].

2.3. Mechanical Properties

CSA cement exhibits high early strength, moderate long-term strength, and good flexural performance [17]. Its compressive strength can reach 20–40 MPa within 24 h and typically exceeds 50 MPa at 28 days, depending on the mix proportions [18]. This rapid strength development is primarily due to the fast hydration of ye’elimite (C4A3Ŝ), which promotes early ettringite formation [19,20]. CSA cement also demonstrates moderate tensile and flexural strength, making it suitable for structural and precast applications [21]. Additionally, it can be designed to exhibit controlled expansion or shrinkage-compensating properties, reducing internal stresses in concrete [22]. Its elastic modulus is comparable to that of OPC, but due to its rapid reaction, careful mix design is required to optimize mechanical performance for specific applications [23].
The mchanical properties of CSA cement are significantly influenced by the composition of the raw materials used in its production process. It has been observed that fly ash is utilized as the primary source of SiO2 in cement clinker to produce belite-calcium sulfoaluminate cement, and the results demonstrated that fly ash promoted the formation of clinker minerals C4A3Ŝ and C2S, which exhibited advantages such as rapid setting and early strength. The 1-day compressive strength is 18.0 MPa [24]. It is documented that the belite-calcium sulfoaluminate cement (B-SAC) uses electrolytic manganese residue and barium slag under 1300 °C for 30 min, and the findings show that the 7-day and 28-day compressive strengths of the produced B-SAC are 47.3 MPa and 60 MPa, respectively [25].
Fibers enhance the toughness of CSA matrix through bridging cracks and energy-dissipation mechanisms, and their effectiveness depends on the type and dosage of fibers. The addition of 1.6% steel fiber (SF) can increase the flexural strength of CSA concrete from 6.2 MPa to 9.8 MPa and increase the flexural toughness index by three times [26]. However, the high density of steel fibers may limit their application in lightweight structures. PVA fibers with a volume fraction of 0.8% can increase the ultimate tensile strain of CSA mortar from 0.02% to 0.15%, achieving strain-hardening behavior [27]. Its hydrophilic surface promotes interfacial bonding with CSA hydration products such as ettringite. When basalt fiber (BF) and polypropylene fiber are mixed and doped (0.5% each), the 28-day compressive strength increases by 18% and the drying shrinkage rate decreases by 30% [28].

2.4. Durability

CSA cement exhibits excellent durability, particularly in aggressive environments. It has high sulfate resistance [29], as the main hydration product, ettringite (C6AŜ3H32), is stable in sulfate-rich conditions [30], making it ideal for marine structures [31], wastewater facilities [21], and sulfate-contaminated soils [32]. CSA cement also demonstrates low permeability [33], reducing the ingress of water and harmful ions, which enhances resistance to freeze-thaw cycles and chloride attacks [34]. Additionally, it has lower shrinkage compared to OPC, reducing the risk of cracking and improving long-term stability [35]. Its high resistance to carbonation [36] and moderate acid resistance [37] further contribute to its durability, particularly in industrial applications. However, in extremely acidic environments, protective coatings or supplementary materials may be required to extend service life [38].
CSA cement has outstanding sulfate resistance because no chemical interaction between hydration products and sulfates takes place. It is reported a significant amount of sulfate ions penetrated the specimens after 240 days of immersion, yet no sulfate damage was observed. However, the diffusion coefficient of sulfate ions in CSA mortar is similar to that in OPC cement mortar. Hence, it can be summarized that the enhanced sulfate resistance of CSA cement is not responsible for low permeability, but rather for the no chemical reaction between the hydration products and sulfate ions, and no expansive products are formed [21]. The initial concentration of sulfate ion in the mortar specimens and the sulfate ion concentration in the external solution are shown in the dotted line of Figure 3.
Polymers improve the performance of CSA cement-based materials through a dual mechanism of physical encapsulation and chemical bonding. Literature research shows that the addition of styrene acrylic lotion (SAE) and polyacrylate (PAE) can significantly extend the setting time, from 15 min to 40 min, and at the same time, it can cover the surface of hydration products by forming flexible polymer films, reduce porosity and improve impermeability. When the polymer content reaches 20%, the chloride ion penetration depth of CSA mortar decreases by more than 60% [10].

3. Role of Seawater in CSA Hydration and Microstructures

The core mineral of CSA cement is calcium sulphoaluminate, which reacts with gypsum to quickly generate AFt and aluminum gel, while OPC mainly relies on C3S hydration to generate C-S-H gel. The high concentrations of SO42− and Cl− in seawater have different effects on their hydration pathways: when CSA cement is mixed with seawater, SO42− can promote the formation of AFt, and Cl− reacts with aluminum to form Friedel salts, further stabilizing the structure [39]. The C3A in OPC is prone to form chloroaluminate in Cl− environment, but high concentrations of SO42− can react with C-S-H to form expansive gypsum or AFt, leading to microcracks [40,41]. Due to its rapid hydration reaction, CSA cement can achieve a compressive strength of 35–40 MPa after 3 days of seawater mixing, which is 1.5–2 times that of OPC. For example, pure CSA mortar has a 3-day strength of 38.2 MPa when mixed with seawater, while OPC only has a strength of 22.4 MPa [42,43]. This feature makes it suitable for rapid repair in marine engineering.

3.1. Hydration Process and Microstructure Evolution with Impacts of Seawater Ions

The seawater ions have complex and multifaceted effects on the hydration process of OPC. Mg2+ could form magnesium hydroxide precipitates in high alkaline environments, which cover the surface of cement particles and reducing the early hydration rate by more than 40% [44]. SO42− initially react with aluminum phase to form secondary gypsum, delaying the hydration of C3A and extending the initial setting time by 50% to 4–6 h. In the later stage, ettringite may form, which cause microcracks [29]. Chloride ions not only decrease the pH value of the pore fluid and affect the hydration reaction, but also increase the diffusion coefficient of chloride ions to twice compared with OPC mixed fresh water [45].
When CSA cement is mixed with seawater, the hydration process will be significantly different due to the alterations in the chemical interaction between the ions in seawater and the CSA cement [46]. First, the sulfate content in seawater will boost the formation of ettringite (C6AŜ3H32) which is a dominant hydration product in CSA cement [47]. This is a great benefit that further improves the sulfate resistance compared to OPC [29]. The reaction between SO42− in seawater and aluminate in CSA cement promotes the formation of secondary ettringite. The XRD quantitative analysis shows that the AFt content in seawater system is 12–15% higher than that in freshwater system after 28 days, but AFm decreases by about 30% [48]. Second, the presence of Cl− can react with the aluminate phases and form Friedel’s salt (C3A·CaCl2·H12). This will reinforce the binding of chloride and reduce its mobilization in concrete, thus improving the chloride-induced corrosion of concrete structures [44]. Cl− promotes the nucleation rate of ettringite by increasing the ion concentration of the solution, reducing the initial setting time by 30–50% [49]. Synchronous thermal analysis shows that the exothermic peak of CSA cement mixed with seawater appears about 1.5 h earlier than that of freshwater system, and the hydration heat release increases by 18% [45]. Despite the corrosion resistance, the low permeability of CSA cement reduces the content of calcium hydroxide (CH), which makes it more durable in marine environments compared to OPC [33], especially when properly designed with supplementary materials [50] or protective coatings [51]. However, prolonged exposure to seawater may lead to a magnesium attack, where Mg2+ ions react with calcium silicate hydrate (C-S-H) and ettringite phases, forming magnesium silicate hydrate (M-S-H), gypsum, brucite (Mg(OH)2) [21], potentially increasing porosity and reducing long-term durability [7]. Mg2+ reacts with OH− in the liquid phase to form Mg (OH)2 precipitate, which lowers the pH value of the system (from 12.8 to 11.5) and delays the dissolution rate of C4A3Ŝ. This dual effect results in a 5–7% lower hydration degree after 28 days compared to freshwater systems [48].
The presence of seawater can accelerate the hydration process. It is demonstrated that SO42− can promote the formation of AFt formation. Subsequently, in the later stages of the hydration process, AFt transforms into AFm. Additionally, AFm interacts positively with Cl−, resulting in the formation of Friedel’s salt which is beneficial for pore filling. On the contrary, high seawater concentration have disadvantages, as the rapid formation of AFt hinders the hydration of CSA cement, thereby impeding the Mechanical properties development. Figure 4 illustrates the hydration mechanism of the CSA cement when mixed with freshwater, normal concentration seawater, and high concentration seawater [52].

3.2. Curing Conditions

Generally, moist curing is required for CSA cement to facilitate proper hydration, especially the formation of ettringite, which contributes to rapid early-age strength [53]. However, when mixed with seawater, the curing parameters, such as temperature and humidity, must be carefully controlled, because the formation of expansive phases (ettringite or gypsum) will intrigue internal stress and cause micro, or even macro cracks [52].
Curing conditions significantly influence the formation of hydration products and the pore characteristics of CSA cement. For example, the initial and final setting times of CSA cement paste cured at 5 °C are significantly delayed, the dormant period in the calorimetry curve is prolonged, and the strength within 1 day is noticeably reduced compared to curing at 20 °C [54]. Generally, curing temperature does not alter the types of hydration products but affects their quantities [55]. As the curing temperature increases, the formation rate of ettringite and the degree of clinker mineral hydration within 24 h significantly enhance, which results in enhanced early compressive strength [23]. At the same time, due to the rise in curing temperature, the ratio of ettringite to monosulfoaluminate (Ms) hydration products decreases, indicating the formation of a large amount of Ms, especially when the initial gypsum content decreases. This can be explained by the fact that higher curing temperatures increase the stability of Ms relative to ettringite, while ettringite requires more gypsum for stabilization [56]. However, low temperatures (<20 °C) favor the formation of a denser and more uniform matrix, as hydration continues progressively [57]. And low-temperature and high-relative-humidity (RH) curing conditions are favorable for the formation of ettringite. Increasing curing temperature and RH can markedly accelerate the formation rate of aluminum hydroxide (AH3) and Ms. Low-temperature and high-RH curing also lead to a gradual reduction in the total porosity of CSA cement paste [58].

3.3. Mechanical Properties

Once mixed with seawater, CSA cement exhibits outstanding mechanical properties due to the presence of sulfate ions in seawater [45]. The rapid hydration of ye’elimite (C4A3Ŝ) leads to the formation of ettringite, which contributes to early-age strength development as high as 20–40 MPa within 24 h [52]. The sulfate ions in seawater can further reduce the mechanical properties degradation of CSA compared to OPC, due to its magnificent sulfate resistance [21]. Meanwhile, the low permeability of CSA cement in seawater helps prevent the ingress of aggressive elements like Cl− and Mg2+ [33].
CSA cement exhibits excellent mechanical properties mixed with seawater. Most studies indicate that the 7-day compressive strength of concrete prepared with sea sand and/or seawater is significantly higher than ordinary concrete, while the 28-day compressive strength is comparable to ordinary concrete. The long-term compressive strength is also similar to ordinary concrete [59]. Figure 5. shows the result of a 28-day strength comparison between ordinary recycled aggregate concrete (RAC) and RAC with sea sand and seawater.
The salts in seawater can react with CSA, resulting in the improvement of mechanical properties. It is communicated some of the salts brought in by seawater form crystals and react with CSA cement hydration products like AFm, CSH gel, and calcium hydroxide. Overall, the growth of AFt and gel in CSA cement with seawater incorporation is denser, which can enhance the compactness and mechanical strength of the structure [52]. Figure 6 shows the micromorphology of hydration products and corresponding EDS spectrum of the CSA cement paste when mixed with freshwater and seawater for 28 d.
Although the benefits of CSA mixed with seawater are predictable, several potential problems need to be considered in advance. First, long-term exposure to seawater results in negative impacts on the mechanical properties due to the leaching of Ca2+ [60]. In this case, Mg2+ substitutes the Ca2+ of the C-S-H phase and forms the M-S-H phase, which declines the stability of the mechanical properties. Second, the reaction of sulfate ions with calcium ions can lead to the formation of gypsum, which increases the porosity and reduces the mechanical properties. Last, except for the formation of gypsum, another important volume expansion phase, ettringite, also induces cracking and further compromises the mechanical properties [21].
CSA cement mixed with seawater can also hinder early hydration. A plentiful amount of AFt is generated in the early stages of hydration because of the rapid hydration rate of CSA cement whether using fresh water or seawater for mixing. However, slightly more AFt is generated in mixed fresh water than in seawater, due to the rapidly formed AFt in seawater precipitates on the surface of CSA particles, which impedes the hydration reaction [52]. Hence, it is essential to boost the early AFt content in CSA cement mixed with seawater. It is reported that mixing the OPC cement with CSA can effectively enhance the mechanical properties As the OPC content increases, the compressive strength initially rises and then declines, and reaches its maximum at the OPC content is 30%. The incorporation of OPC significantly increases the AFt content, resulting in a denser matrix [60].

3.4. Durability Concerns Related to Seawater Ions

It has been proven that the SO42−, Cl−, and Mg2+ environments are all detrimental to the durability of OPC [12,61,62,63,64,65,66,67]. Compared to OPC, CSA cement has magnificent corrosion resistance with exposure to marine environments [29,30,33,37,47,50]. However, for coastal construction applications, the durability concerns of CSA mixed with seawater remain unknown due to the knowledge gap of the hydration mechanism, which determines the service life of the CSA structures. A few potential risks must be considered. First, long-term exposure to seawater introduces the leaching of Ca2+ ion due to the Mg2+ substitution in the calcium silicate hydrate (C-S-H) binder phase, leading to the formation of magnesium silicate hydrate (M-S-H), which weakens the service properties of CSA cement matrix over time [20]. Second, the debate about chloride ions’ impact on the durability of CSA with exposure to the marine environment has never been stopped. Several researchers believe that the hydration of CSA cement partially mitigates the penetration of Cl− by forming Friedel’s salt (C3A·CaCl2·H12) [68], while a disagreement point of view argues that the physical adsorption is the dominant mechanism rather than the chemical binding effect [44]. Last but not least, the existence of SO42− will lead to the formation of gypsum [29] and extra ettringite [21], potentially increasing porosity, thus reducing mechanical performances.
The strength and flexural strength of CSA concrete both enhance after mixing with seawater. The addition of seawater promotes the formation and growth of AFt, which leads to enhancing the compactness and macroscopic properties of the matrix. The fluidity of CSA cement mixed with seawater is generally boosted due to the increased ion concentration, which facilitates particle dispersion and hydration. Structures built with seawater-mixed CSA concrete exhibit excellent corrosion resistance, ensuring structural durability and reducing construction costs. Therefore, CSA cement holds significant advantages in marine environments and marine engineering applications [69].

3.4.1. Chloride Ingress

The presence of Cl− in seawater accelerates the setting time of CSA cement and enhances the early mechanical properties, which is crucial for marine engineering projects requiring speedy completion. Cl− can alter the microstructure of cement hydrates. Previous studies have shown that when the AFm phases in hydrated cement are exposed to high and low concentrations of Cl− respectively, the Cl−AFm can form, such as Friedel’s salt and Kuzel’s salt. The SO42− can weaken the binding capacity of chloride ions because it competes with Cl− to react with the AFm phases.
It has been demonstrated that the hydration products of CSA cement can effectively immobilize chloride ions through chemical binding. However this binding capacity exhibits a well-defined saturation threshold. When Cl− binding reaches saturation, the performance of CSA cement will decrease. In response to the performance degradation after Cl− saturation, it is demonstrated that CSA cement can form a secondary AFm phase in the later stage of hydration, which can supplement Cl− binding sites through continuous ion exchange. For example, the addition of limestone powder can promote the regeneration of AFm phase [70]. Nanoalumina can also be incorporated to fill pores and participate in Friedel salt formation, increasing Cl− binding by 30–50% while reducing permeability [71]. Cathodic protection or electrochemical desalination can also be applied to saturated CSA concrete to remove free Cl− and regenerate Friedel salts. But it is necessary to control the current density to avoid side reactions [72].
The effect of CSA cement mixed with seawater on chloride ions can also protect the embedded steel. Although the alkalinity environment of CSA cement is lower than OPC, the formation of a dense ettringite network and the combination of AFm and chloride ions can effectively delay the penetration of Cl−. It is reported that the passive film generated on the surface of steel bars in CSA concrete is mainly composed of Fe-O-S composite. Although its thickness, which is about 5–8 nm, is 30% thinner than OPC, its crystal structure is denser [73,74]. Electrochemical testing shows that the corrosion current density of steel bars in CSA cement is reduced by 42% compared to OPC. Especially in simulated seawater environment with Cl− concentration of 0.5%. The critical chloride ion threshold is increased to 0.68%, higher than OPC’s 0.4% [74].

3.4.2. Sulfate Attack

SO42− will lead to the formation of gypsum [29] and extra ettringite [21], potentially increasing porosity, thus reducing mechanical performances. Due to the seawater’s ability to promptly supply sufficient SO42− to the marine CSA cement specimens, the structural water content of AFt increases while its density decreases. It promotes the growth of AFt, and it causes its morphology to gradually transition from fine needle-like forms to tubular and elongated columnar structures. The development of AFt leads to more complete crystallization, thereby increasing the structural compactness of the cement mortar specimens.

3.4.3. Carbonation Resistance

CSA cement exhibits relatively high carbonation resistance when mixed with seawater, primarily due to its low calcium hydroxide (CH) content and the formation of stable ettringite and gypsum [69]. The relatively low free lime content in CSA reduces the susceptibility to carbonation [5]. When exposed to marine environments, CSA is less prone to be carbonized than OPC, as the low pH environment and the presence of sulfate ions in seawater [14].
The composition of CSA cement has a significant impact on its carbonation resistance. The content of anhydrite in CSA mortar influences the reaction kinetics of ye’elimite, which in turn plays a significant role in the carbonation resistance of CSA mortar. Adding calcium sulfate to CSA clinker during the production of CSA cement dilutes the clinker content and reduces the amount of carbon dioxide that the CSA cement can ultimately bind [75]. CSA cement-based materials with mixed seawater exhibit a lower pH, which reduces the dissolution of CO2 in the cement-based materials. Additionally, CSA cement-based materials mixed with seawater own a denser matrix and lower permeability, which enhance their resistance to CO2 erosion [69].

3.4.4. Potential Solutions

Although the benefits of CSA seem distinctive; the durability concerns must be considered once it is mixed with seawater. Ongoing exposure to aggressive atmospheric CO2 levels may still require protective measures to further improve long-term carbonation resistance. The use of protective coatings [76], supplementary cementitious materials (SCMs) [77], and optimized mix designs [14] are recommended to mitigate seawater-induced degradation and enhance the service life of structures.
To further enhance the carbonation resistance of CSA cement, the surface of CSA cement-based materials can be treated. It is suggested that hydrophobic poly-methyltriethoxysilane (PMTS) used on its surface can significantly reduce pore sizes smaller than 50 nm. It is also found that the carbonation rate is positively correlated with the volume of pores smaller than 50 nm, because pores smaller than 50 nm may not provide channels for water transport, Thus, the hydrophobic PMTS can effectively restrict CO2 transmission [76].
In summary, CSA cement exhibits significant performance advantages over OPC under seawater mixing conditions. Its core mineral, calcium sulfoaluminate, reacts with gypsum to form AFt and aluminum gel through a unique hydration mechanism, allowing the SO42− in seawater to promote AFt formation. Cl− reacts with aluminum to form Friedel salt, and this dual synergistic effect allows CSA cement to achieve a compressive strength of 35–40 MPa within 3 days, which is 1.5–2 times that of OPC. At the same time, the corrosion resistance is significantly improved by reducing porosity, harmful pores > 50 nm only account for 8.5%, and chemical curing mechanism. However, long-term exposure should pay attention to the M-S-H phase transformation caused by Mg2+ erosion. The concentration of seawater increased may hinder the problem of later hydration. By adding 30% OPC to optimize the formation of AFt, using nano alumina or surface hydrophobic treatment, such as PMTS coating to seal < 50 nm pores, and other modification methods, the durability of CSA can be further improved. These characteristics make CSA cement more durable CSA cement is particularly suitable for marine engineering restoration projects that require rapid hardening and long-term durability, providing innovative solutions for sustainable marine construction.

4. Environmental and Economic Considerations

4.1. Sustainability Benefits

When mixed with seawater, the sustainability benefits of CSA cement will be significantly boosted compared to OPC [78]. The merits of sustainability of CSA mixed with seawater are fourfold. First, CSA cement requires less limestone and is produced at lower kiln temperatures (~1250 °C compared to ~1450 °C for OPC), resulting in reduced CO2 emissions [79]. Second, the outstanding early-age strength development allows high construction efficiency, which reduces energy consumption [69]. Third, when mixed with seawater, CSA cement demonstrates high sulfate resistance and low permeability, which ensure its service life in marine environments and reduce the need for frequent repairs and material replacement, minimizing carbon footprint compared to OPC [21]. Last but not least, seawater mixed with CSA will significantly reduce the consumption of freshwater, which makes the most important contribution to sustainable development [59].
Ye’elimite, the primary clinker in CSA cement, contains 27.6 wt% of CaO, which is significantly lower than the alite in OPC cement, which contains 73.7 wt% of CaO. The lower CaO content reduces the amount of limestone required in the raw materials for producing CSA clinker. Ultimately, while alite in OPC emits approximately 0.58 g CO2/g during the cementation phase, ye’elimite in CSA cement releases only 0.22 g CO2/g during the same phase, resulting in 35% lower CO2 emissions compared to OPC [7]. The unique clinker phase composition of CSA cement contributes to its rapid hydration reaction, thereby favoring the development of high early strength. These physicochemical properties of CSA cement make it suitable for applications requiring rapid early strength development, including precast concrete, 3D printed concrete, and emergency repair agents [80]. However the production of concrete consumes vast amounts of raw materials, particularly river sand and freshwater, leading to significant environmental issues. The extraction of river sand as fine aggregate negatively impacts river ecosystems, navigation, and flood control. Therefore, using seawater to mix cement-based materials mixed with seawater is meaningful [59]. The ions in the seawater can promote the hydration reaction of CSA cement, while using seawater to mix CSA cement, resulting in a denser matrix and a more corrosion-resistant structure. It is possible to replace OPC cement with CSA cement in marine engineering applications [69].

4.2. Cost Analysis

From the perspective of sustainable development in modern concrete, CSA may be one of the most promising low-CO2 and energy-efficient alternatives to OPC because of its low cost and energy consumption. It is documented the cost of replacing OPC Type III cement with CSA in rapid strength concrete (RSC) is $1422/tonne of CO2-eq reduction [26]. It is also summarized CSA cement has a significantly smaller environmental impact compared to OPC, with its emissions reduced to 436~555 kg/t of clinker. This represents a substantial decrease of 35~48% compared to OPC emissions. The average energy consumption is reduced by 13% to 16%, and the average CO2 emissions are decreased by 35% to 48%, which demonstrates its significant contribution to mitigating climate change. The most notable energy consumption occurs during the combustion process, and a large portion of CO2 emissions is due to calcination and fuel use, which is similar to OPC production [81]. Utilizing solid waste to produce CSA can further reduce energy consumption. The paper applies the life cycle theory to analyze different production processes, and the result suggests that the comprehensive utilization of waste materials to produce sulphoaluminate clinker can reduce resource consumption by 92.89% compared to the traditional process of preparing sulphoaluminate clinker. Additionally, it can decrease global warming potential, chemical oxygen demand, particulate matter, primary energy consumption, acidification, water eutrophication, and solid waste generation by 40.95%, 36.48%, 25.25%, 12.6%, 8.3%, 7.91%, and 5.29% respectively, compared to the traditional technique. The comprehensive utilization of waste materials to produce sulphoaluminate clinker can reduce the total environmental burden by 38.62% compared to the traditional approach [82].
It is reported that when using seawater, the early hydration rate of CSA is accelerated, and the hydration heat release within 24 h is about 15% higher than that of pure water system [49], and the production of AFt increases by 20–30%. This reduces the theoretical water demand of CSA in seawater by about 8–12% compared to pure water systems [10]. Mg2+ in seawater will combine with OH− in OPC to form Mg(OH)2 precipitate, which hinders the hydration of C3S and increases the effective hydration water demand of OPC in seawater by about 10–15% compared to pure water to maintain reaction equilibrium [83]. In addition, Cl− can interfere with the densification of C-S-H, requiring additional water to fill the pore structure [84]
The use of CSA cement can reduce the consumption of raw materials. CSA concrete can reduce freshwater consumption by 15–20% per cubic meter, and can save approximately $3–5/m3 in water treatment costs in coastal areas. SO42− in seawater can replace 30–50% of gypsum addition, reducing raw material costs by about $8–12/ton [85]. CSA produces CO2 emissions of 0.35–0.45 tons/ton, which is more than 50% lower than OPC (0.85–0.95 tons/ton) [26].

5. Current Research Gaps, Challenges, and Future Trends

CSA cement offers several advantages when mixed with seawater, including rapid early strength development, high sulfate resistance, and low permeability, which contribute to its overall durability in marine environments. The formation of stable phases like ettringite (C6AŜ3H32) during hydration enhances its resistance to sulfate attack, making it a suitable choice for marine structures and environments exposed to aggressive chemicals. However, there are still some problems that have not been studied in the CSA cement mixed with seawater.

5.1. Combinations of Seawater and Chemical Admixtures

The main types of chemical admixtures used in calcium sulphoaluminate (CSA) cement include superplasticizers, retarders, accelerators, air-entraining agents, and pozzolanic materials [20,52,81]. Superplasticizers are commonly added to improve workability and fluidity while reducing the water-to-cement ratio, leading to higher strength and density. Retarders are used to delay the setting time, allowing for extended workability in hot weather or large-scale applications where longer processing times are needed. In contrast, accelerators are utilized to speed up the hydration process, promoting early strength development and faster setting times, which is advantageous in cold-weather conditions or for rapid construction. Air-entraining agents introduce microscopic air bubbles into the mix, improving freeze-thaw durability and enhancing the workability of CSA cement. Finally, pozzolanic materials, such as silica fume, fly ash, or blast furnace slag, can be incorporated to enhance long-term strength, improve the microstructure, and boost resistance to sulfate and chloride attack, contributing to greater durability in harsh environments. Together, these admixtures allow CSA cement to be tailored for specific performance requirements, optimizing its properties for diverse construction applications.
However, the synergistic effects of chemical admixtures along with seawater on the CSA remains unclear. Because chemical admixtures may react with ions in seawater resulting in the invalidity of chemical admixtures, or synergistically influence the hydration of CSA cement [26,47,53,84]. Therefore, the actual impact of admixtures on the performance of CSA cement mixed with seawater and their alterations of the hydration process require further experimental validation, particularly quantitative analyses targeting specific admixtures in seawater-CSA cement systems.

5.2. LCA of CSA Mixed with Seawater

The Life Cycle Assessment (LCA) of calcium sulphoaluminate (CSA) cement reveals its significant environmental advantages over traditional ordinary Portland cement (OPC), particularly in terms of carbon emissions, energy consumption, and resource efficiency. CSA cement requires lower kiln temperatures (around 1250 °C vs. 1450 °C for OPC), reducing CO2 emissions by 30–40% during production. Additionally, CSA cement uses less limestone, resulting in reduced raw material extraction and a smaller environmental footprint. The ability to utilize industrial by-products like gypsum or fly ash further promotes a circular economy, decreasing waste and reducing the demand for virgin materials. CSA cement also offers lower water consumption by allowing mixing with seawater, which is especially advantageous in coastal or arid regions. Its durability in aggressive environments, such as marine and sulfate-rich areas, leads to longer service life and fewer repairs, reducing overall environmental impacts over the life cycle. Despite higher production costs per ton, the lower long-term maintenance costs, reduced water and energy usage, and extended lifespan make CSA cement a more sustainable alternative for construction, particularly in infrastructure exposed to harsh conditions [30,67,70].
However, the LCA study of CSA mixed with seawater is quite limited. The CSA cement system using seawater as mixing water lacks systematic data on aspects such as carbon emissions, energy consumption, and resource utilization. Moreover, the life cycle assessment (LCA) of seawater-mixed CSA must account for seawater extraction/pretreatment, admixture production, and variations in maintenance cycles due to durability differences [55]. Additionally, the composition of seawater (salinity and impurities) varies by location, which may also influence LCA outcomes. Most existing studies are based on idealized laboratory conditions, but they lack region-specific analyses.

5.3. Modification Strategies via Nanotechnology

Nano-modification of CSA cement enhances its mechanical properties, durability, and microstructure by incorporating nanomaterials such as nano-silica (NS), nano-alumina (NA), graphene oxide (GO), and carbon nanotubes (CNTs) [40,61]. These nanomaterials act as nucleation sites, accelerating hydration reactions and leading to a denser and more refined microstructure with reduced porosity. Nano-silica, for example, enhances the formation of calcium silicate hydrate (C-S-H) gel, improving early strength and long-term durability. Graphene oxide and carbon nanotubes contribute to crack resistance and flexural strength by reinforcing the cement matrix at the nanoscale. Additionally, nano-modification improves chloride and sulfate resistance, making CSA cement even more suitable for marine and aggressive environments. However, challenges such as material dispersion, cost, and processing complexity must be addressed for large-scale applications. Overall, nano-modified CSA cement offers superior performance, making it an attractive option for high-performance and durable infrastructure [10,11,12,13,14,44,84].
However, the study of nano-modified CSA mixed with seawater is barely investigated. How do nanomaterials interact with ions in seawater (Cl−, SO42−, Mg2+) to regulate the hydration kinetics and product stability of CSA cement? How might they affect material durability if reactions occur between nanomaterials and seawater ions? Could seawater ions induce nanoparticle agglomeration? How can nanomaterials achieve better dispersion in seawater? These are critical questions that must be systematically considered in future research.

6. Conclusions

The mineral components in CSA cement, such as anhydrous calcium sulfoaluminate, can react with ions in seawater to accelerate hydration, it can enhance early-age mechanical properties, and meet the rapid construction demands of marine engineering. Moreover, seawater promotes the formation of stable hydration products and a denser matrix resulting in effectively blocking the penetration of corrosive agents and improving the durability of concrete structures in marine environments. Additionally, CSA cement-based materials exhibit excellent volume stability, which helps prevent cracking caused by dimensional changes, thereby avoiding seawater intrusion and further extending the service life of structures. It is highly suitable for using CSA cement to build marine engineering applications because of these advantages, which provide significant potential for coastal and island construction. It can substantially reduce freshwater consumption and carbon emissions while enabling the utilization of industrial solid waste in CSA cement production, which can reduce material costs and energy consumption. In the future, interdisciplinary collaboration could overcome practical challenges. For instance, by developing multiscale simulation algorithms (combining molecular dynamics and machine learning) to predict the long-term durability of CSA cement mixed with seawater. Such approaches could partially replace exposure tests and accelerate research progress to facilitate faster engineering adoption. Furthermore, environmental science researchers could assess the potential ecological impacts of CSA cement applications in marine ecosystems to maximize its benefits in sustainable marine construction. Through coordinated efforts across disciplines, it is possible to overcome barriers in the widespread application, unlock its full potential in green building application, and make the construction industry more sustainable.

Author Contributions

H.L. prepared most of this manuscript, J.M. organized the main challenges and figures, Y.L. revised the manuscript, L.Y., Y.W. and N.X. reviewed the manuscript, J.O. and G.Z. organized the outline and the whole contents. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Shandong Provincial Medium/small Enterprise Hatching R&D Project (2023TSGC0252), Lianyungang Science and Technology Transformation Program (CA202204), Lianyungang Haiyan Plan program (2019-QD-002). The authors also acknowledge the financial support from the Guangdong Provincial Key Laboratory of Intelligent and Resilient Structures for Civil Engineering (2023B1212010004), Shenzhen Science and Technology Program (KQTD20210811090112003).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Phase development of a CSA cement (water/cement = 0.80) as a function of hydration time calculated by thermodynamic modeling [8].
Figure 1. Phase development of a CSA cement (water/cement = 0.80) as a function of hydration time calculated by thermodynamic modeling [8].
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Figure 2. Effects of retarders on the PC adsorption amount of the cement pastes [6]. (a) BNS + CA, (b) BNS + SG, (c) AS + CA, (d) AS + SG, (e) PC + CA, (f) PC + SG.
Figure 2. Effects of retarders on the PC adsorption amount of the cement pastes [6]. (a) BNS + CA, (b) BNS + SG, (c) AS + CA, (d) AS + SG, (e) PC + CA, (f) PC + SG.
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Figure 3. Distribution of SO42− in CSA mortars after 240 days exposure [21].
Figure 3. Distribution of SO42− in CSA mortars after 240 days exposure [21].
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Figure 4. The hydration mechanism of CSA mixed with freshwater and seawater [52].
Figure 4. The hydration mechanism of CSA mixed with freshwater and seawater [52].
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Figure 5. 28-day strength comparison between ordinary RAC and RAC with seasand and seawater [59].
Figure 5. 28-day strength comparison between ordinary RAC and RAC with seasand and seawater [59].
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Figure 6. Micro-structure and EDS spectrum of the CSA cement paste at 28 d [52].
Figure 6. Micro-structure and EDS spectrum of the CSA cement paste at 28 d [52].
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MDPI and ACS Style

Li, H.; Meng, J.; Liu, Y.; Yang, L.; Wang, Y.; Xie, N.; Ou, J.; Zhou, G. A Review of the Calcium Sulphoaluminate Cement Mixed with Seawater: Hydration Process, Microstructure, and Durability. J. Mar. Sci. Eng. 2025, 13, 1076. https://doi.org/10.3390/jmse13061076

AMA Style

Li H, Meng J, Liu Y, Yang L, Wang Y, Xie N, Ou J, Zhou G. A Review of the Calcium Sulphoaluminate Cement Mixed with Seawater: Hydration Process, Microstructure, and Durability. Journal of Marine Science and Engineering. 2025; 13(6):1076. https://doi.org/10.3390/jmse13061076

Chicago/Turabian Style

Li, Han, Jing Meng, Yang Liu, Lilin Yang, Yukai Wang, Ning Xie, Jinping Ou, and Guoxiang Zhou. 2025. "A Review of the Calcium Sulphoaluminate Cement Mixed with Seawater: Hydration Process, Microstructure, and Durability" Journal of Marine Science and Engineering 13, no. 6: 1076. https://doi.org/10.3390/jmse13061076

APA Style

Li, H., Meng, J., Liu, Y., Yang, L., Wang, Y., Xie, N., Ou, J., & Zhou, G. (2025). A Review of the Calcium Sulphoaluminate Cement Mixed with Seawater: Hydration Process, Microstructure, and Durability. Journal of Marine Science and Engineering, 13(6), 1076. https://doi.org/10.3390/jmse13061076

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