Seawater-mixed concretes containing natural and sea sand aggregates

1. Introduction

The UN-Water Policy Brief states that improving the resilience of freshwater ecosystems is essential in adapting to climate change [1]. Freshwater is one of Earth's most precious resources, sustaining ecosystems, economies, biodiversity, and society. Based on United Nations (UN) General Assembly resolution 71/222, UN member states have adopted 2018–2028 as the international decade of Water Sustainable Development [1]. However, despite such initiatives poor water management strategies worldwide have led to inadequate access to safe drinking water for about 885 million people [2]. According to the Water Resources Institute (USA) “17 countries in the world face extremely high levels of stress,” which indicates the severity of declining freshwater resources in these countries. Among freshwater withdrawals for industrial purposes, 9% is used for producing concrete globally [3]. Furthermore, 75% of total freshwater withdrawals for concrete production occur in regions experiencing severe water stress [3]. Under these circumstances, using freshwater or potable water for concrete-making and curing processes in regions of extreme water stress, should be substituted with alternative or non-potable water.

The use of non-potable water sources for concrete mixing is regulated by prescribed limits on water quality parameters, such as the amount of chlorides, sulphates, total solids, total alkali content, total suspended solids, and pH [4]. Gokulnathan et al. (2021) [5] have summarized the recommended water quality parameters for concrete mixing present in several international standards. Accordingly, the strength and fresh property performance requirements of concrete made with non-potable water, shall not be more than 5 – 10% worse than that of concrete mixed with fresh water [6]. Most alternative sources of non-potable water, such as industrial wastewater, domestic sewage water, and washing water, require extensive treatment to remove harmful chemicals that can affect the hydrated cement phases in concrete [6,7]. Among the alternative sources of water, the usage of seawater in concrete production has, in recent decades, gained the attention of concrete technologists.

Seawater is a natural resource that does not require any treatment processes, other than filtering out floating debris, nor transportation charges near coastlines, and can thus be a suitable alternative to freshwater in concrete. However, the amount of chlorides and sulphates in seawater is much higher than the limits proposed by several international standards [8]. EN 1008 recommends that the maximum chloride content for prestressed concrete or grout, reinforced concrete with embedded steel, and plain concrete without reinforcement, should be lower or equal to 500 mg/L, 1000 mg/L, and 4500 mg/L, respectively. An Indian standard for plain and reinforced concrete (IS 456:2000) states that the permissible limits for chloride and sulphate ions inmixing water should not be greater than 500 and 400 mg/L, respectively. However, worldwide the concentration of chloride ions in seawater can be between 10000 mg/L and 26000 mg/L [9]. The presence of chloride ions in seawater can result in chloride-induced corrosion in the embedded steel of reinforced concrete exposed to certain conditions. Furthermore, the supersaturation of sulphate and chloride salts in the pore solution of a cementitious matrix can induce cracks due to physical salt attack. To mitigate these negative effects, special mix designs and additives are often used in the production of seawater-mixed concrete. Pozzolanic materials, such as fly ash, slag, and metakaolin can enhance the strength and durability properties of concrete mixed with salts. Superplasticizers, which are chemical admixtures, can be added to a mixture to improve the workability and fluidity of concrete. Chemical admixtures can also be added to a mixture to reduce the setting time of the concrete and to increase its durability.

Fig. 1 highlights the increase in research interest on the topic of seawater-mixed concrete and concrete exposed to seawater, in the last few decades. The result of an article search across research databases with the string ‘concrete and seawater’, includes articles related to concretes exposed to seawater, concretes cured with seawater, and seawater-mixed concretes. Fig. 1 (b) shows that 90% of publications are related to seawater-exposed or cured concretes, with the remaining 10% related to seawater-mixed and sea sand concretes with alternative reinforcements. The articles related to concretes exposed or cured in seawater deal with the possible mechanisms of ingress of aggressive ions in seawater, the repair and rehabilitation of marine structures subjected to chloride or sulphate attack, and preventive maintenance strategies for improving the service life of reinforced concretes in marine environments. Fig. 1(c) highlights the rapid increase in research publications related to seawater-sea sand mixed concretes (SW-SS).

Several researchers have studied the effects of partial and full replacement of fine and coarse aggregates, derived from riverbeds and quarries, with sea sand, dredged sand, coral sand, and coral aggregates locally available in the coastal environment [[10], [11], [12], [13], [14]]. These sands are rich in chloride and sulphate salts, which might have the same effects as using seawater in concrete mixing [15]. Furthermore, a significant amount of recent research has focused on using alternative reinforcement for concretes mixed with seawater and exposed to marine environments [16,17]. The development of special reinforcement bars, such as fibre-reinforced polymer (FRP) bars, stainless steel (Ss), and epoxy/cement-coated (EC/CC) rebars, could enable the use of seawater and sea sand in concretes, by circumventing the chloride limits proposed in international standards for producing reinforced concretes. Moreover, the addition of nano particles in SW-SS cementitious systems can enhance their mechanical performance and corrosion resistance [[18], [19], [20]]. Among these broad categories, the present work focuses on understanding seawater-mixed and sea sand (SW-SS) concretes (including dredged sand, coral sand, and coral aggregates).

Fig. 2 shows an in-depth analysis of raw materials used in various articles related to SW-SS concretes. CEM I 42.5 (Ordinary Portland Cement) is the most common cement type used for preparing SW-SS concrete specimens. Besides CEM I, CEM II, and CEM III, Portland composite cements (PCC) and calcium sulfoaluminate cements (CSA) are used for making cement paste, mortar, and concrete specimens. Fly ash, blast furnace slag, metakaolin, and silica fume are the most common types of supplementary cementitious materials (SCMs) used for producing SW-SS concretes. Researchers have also used alkali-activated cements based on slag and fly ash for producing SW-SS concretes. River sand, sea sand, coral sand, and marine sand are used as fine aggregates in producing SW-SS concrete, while crushed granite, coral aggregate, and gravel are used as coarse aggregates. Many research articles have outlined the benefits of using alternative reinforcements, such as glass fibre reinforced polymer (GFRP), carbon fibre reinforced polymer (CFRP), basalt fibre reinforced polymer (BFRP), and stainless-steel bars or wraps, to mitigate the serious threat of chloride-induced corrosion that prevails in typical carbon steel reinforcement.

Fig. 3 shows the geographical distribution of research articles related to SW-SS concretes. Countries which have been producing research publications related to SW-SS concrete, such as China, the USA, India, Indonesia, Australia, Qatar, and Egypt, will also face high or extremely severe water stress by 2040, according to estimates made by the World Resources Institute [21]. Thus, the need to develop a sustainable solution for producing concrete mixed with seawater, through the appropriate use of SCMs, alternative aggregates, and special reinforcements, is being realized by these countries. As such, the increase in research interest related to SW-SS concretes is clearly justified.

Previous review articles have highlighted the early-age hydration and microstructural development, flow characteristics, and mechanical properties of SW-SS concretes [[22], [23], [24]]. However, in the following sections, this paper focuses on quantifying the effects of seawater mixing, and the substitution of sea sand and coral aggregates for natural aggregates, on the properties of SW-SS. Furthermore, the article includes a meta-analysis of data from tests assessing the strength and fresh properties of SW-SS concretes, in published articles. Also, the effects of salts from seawater, sea sand, and coral aggregates, on the long-term durability of seawater-mixed and sea sand (SW-SS) concretes, are critically reviewed. In conclusion, the perceptions, challenges, and possible niche applications in the use of SW or SS concretes are summarized.

2. Seawater-mixed and sea sand concretes

2.1. Early age hydration and microstructure

Seawater is a complex mixture of water, salts, particulates, and traces of organic materials. Worldwide, the major constituents of seawater are chloride, sodium, sulphate, magnesium, calcium, and potassium ions [25]. The interaction of these ions with hydrated cementitious systems needs to be understood, in order to study seawater concrete's hydration at an early age. The presence of higher amounts of bound water and calcium hydroxide in cementitious pastes produced with seawater, could be attributed to an acceleration of hydration reactions at an early age [26]. Wang et al. [27] have observed a denser and stronger hydrated cement matrix in seawater cement paste, compared to de-ionized water-cement paste. Also, seawater cement pastes have a higher ratio of fine porosity and a narrower width of the interfacial transition zone (ITZ), compared to freshwater systems [28]. This decrease in porosity could be attributed to the accelerated formation of Al-Tobermorite (C–S–H) which fills the microstructure in the early days [26,29]. Seawater-mixed concretes have been found to exhibit better mechanical performance than conventional concrete at early ages and reduced strength at later ages [30]. The pore solutionof a seawater system has three to four times greater ionic strength than a freshwater system [26]. An increase in the salinity of mixing water enhances the rate of hydration heat generation and increases cumulative heat generation during hydration [31].

The presence of Cl− ions induces Friedel's salt formation when they combine with calcium aluminium hydrates, through absorption or ion-exchange mechanisms [22]. Friedel's salt binds chlorides in the cementitious matrix and reduces the amount of available free chlorides necessary for corrosion initiationin the embedded steel. However, Friedel's salt formation at later ages is diminished due to the physical adsorption of chlorides in the C–S–H interlayers[26]. The incorporation of chloride and sodium ions in C–S–H interlayers shortens chain length compared to C–S–H made with deionized water [32]. The higher chloride concentration in the cement matrix reacts with calcium hydroxide and produces calcium oxychloride. Calcium oxychloride is 303% larger than calcium hydroxide in the cementitious matrix, leading to volumetricexpansion [22]. The presence of sulphate and carbonate ions in the pore solution negatively affects the chloride binding ability of the cementitious matrix. Conversely, cations in the pore solutions, such as calcium and magnesium, can improve the binding ability of cementitious systems [11]. The interaction of these ions with cementitious systems results in an increase in early age compressive strength, a higher heat of hydration, increased shrinkage, and a shortened setting time.

Fig. 4 (a) shows the thermodynamic prediction of the evolution of hydrated phases over time, in Portland cement systems mixed with fresh and seawater. It is a clear indication of an increase in the quantity of major hydrated phases, such as amorphous C–S–H, hydrogarnet, and portlandite, at early age (< 10 days), in both experimental and model estimations. Also, cement pastes mixed with seawater consist of ettringite, Fridel's salt, and Kuzel's salt, instead of the monosulphate phases abundant in freshwater mixed cement pastes. This could be attributed to the excess supply of sulphates in seawater and the chemical binding of chlorides in hydrated phases. Fig. 4 (b) indicates an increase in the solid phases by 3.7% for seawater pastes and a decrease in the liquid phases by 12.2%, based on thermodynamic modelling in cement paste made with CEM I [33].

The partial substitution of clinker/CEM I with supplementary cementitious systems can further improve the durability and sustainability of SW-SS concretes [[34], [35], [36]]. The addition of SCMs to seawater-mixed pastes can alter the formation of additional hydrated phases due to the pozzolanic reactionand thus enhance the immobilization of chloride through binding [37,38]. The presence of metakaolin in seawater cement pastes encourages the formation of hydrocalumite, in addition to ettringite and Friedel's salt [39,40]. Similarly, alkali-activated slag systems produce calcium aluminate hydrate phases, in addition to hydrocalumite, due to the presence of a higher alumina content (>15%) [41]. Also, the incorporation of silica fume at lower contents in seawater cement pastes can enhance the hydration rate, compared to the presence of slag at higher concentrations (>50%) [42]. The addition of fly ash to seawater cement pastes results in denser C–S–H gels, compared to OPC pastes, but the total quantity of C–S–H gels is reduced with the higher replacement level of fly ash (45%) due to a decrease in OPC content [43]. The addition of limestone calcined clay can promote the formation of Friedel's salt and thereby further refine the porosity of seawater-mixed cementitious systems [43,44]. The incorporation of waste glass powder in SW-mixed cementitious systems can increase the hardness of the C–S–H produced, while the presence of ions in waste glass powder also enhances the acceleration of cement hydration [45]. In addition, seawater-mixed cementitious systems with nano silica refine the microstructure of the cement matrix, and the synergy between nano silica and seawater can enhance resistance to the transport of aggressive ions through the cementitious matrix [46]. More research is required to understand the combined effects of the addition of SCMs and seawater, in the hydration processes of cementitious and alkali-activated systems [23].

2.2. Fresh properties

The presence of chlorides in seawater accelerates the setting time of seawater-mixed cement pastes. The accelerating effect of seawater in cementitious systems has an impact on the flow characteristics of seawater-mixed concrete [27]. The initial slump flow of concrete mixed with seawater is 20% lower compared to concrete mixed with freshwater [47]. The presence of suspended solids in seawater leads to a decrease in packing density in seawater-mixed cementitious systems, compared to freshwater cementitious systems [48,49]. In addition to the flow characteristics mentioned above, the rheological properties of seawater cement pastes have also been evaluated [48,50]. Water film thickness (WFT) is a rheological parameter (being the ratio of excess water to the total surface area of solid particles) that governs the flow rate and adhesiveness of cement paste systems. In SW cement pastes, the WFT is lower than in freshwater-mixed (FW) cement pastes, due to lower packing density [48]. The plastic viscosity and yield stress of seawater cement pastes are higher than those of freshwater cement pastes, due to the presence of suspended solids in the mixing water [50]. Furthermore, the increase in viscosity and yield stress is significantly higher in fly ash mixes, due to the presence of more aluminium phases that bind the chloride ions to Friedel's salt [50]. Compared to freshwater cementitious systems, the thixotropic behaviour of SW-mixed cement pastes is higher, due to the increased probability of flocculation of cement clusters as a result of the high concentration of salts in seawater [50].

Fig. 5 (a) summarizes the percentage changes in the fresh properties, such as slump, spread, packing density, viscosity, water film thickness, thixotropic area, and yield stress, of SW-SS concretes with and without superplasticizers. A clear indication of a decrease in slump, flow spread, packing density, and WFT, and an increase in viscosity, yield stress, and thixotropic area, has been demonstrated in the literature [[47], [48], [49], [50]]. In SW cement pastes, the addition of poly-carboxylate ether (PCE)-based superplasticizer can improve absorption efficiency, due to complexation between carboxylate groups in PCE and divalent ions in seawater [45]. Fig. 5 (b) shows the influence of superplasticizer on the adhesiveness of freshwater and seawater-mixed cement pastes. The high adhesiveness in seawater pastes could be attributed to the acceleration of cement hydration and the viscosity of seawater pastes [49]. Furthermore, the stability and rapid setting of underwater concreting could be improved by using seawater cement pastes, along with anti-washout admixtures, such as polyacrylamide (PAM), that significantly enhance the viscosity of cement paste. Between the two dosages of PAM (0.5 & 1% by weight of water), cement pastes with 1% PAM have a higher viscosity and offer higher resistance against the washout loss that could occur in under-sea construction [49]. The fresh properties of SW-mixed cementitious systems can be affected by adding suitable chemical admixtures to achieve the desired performance. The rheological characteristics of SW-SS cementitious systems made with different SCMs are yet to be explored, so as to understand the interaction between the suspended solids in seawater, or sea sand and hydrated cement phases.

2.3. Mechanical strength and dimensional stability

SW-SS concretes develop notably higher compressive strengths than FW concretes, after 28 days of moist curing [26,[51], [52], [53]]. In general, the percentage increase in the compressive strength of SW-SS concrete is reported to be as high as 75% after 7 days and less than 10% after 180 days [53]. However, the results in the literature about the later-age strength of SW-SS concrete are contradictory and inconclusive [52]. Also, the influence of the salinity of seawater and concrete composition including type of cement, aggregates, and replacement levels of SCMs are not investigated adequate in the existing literature. Moreover, the level of salinity in seawater can influence the compressive strength of SW-SS concretes positively at early ages of up to 7 days, but slightly negatively after 14 days of casting [31,50,54]. Also, the higher salinity of mixing water can increase shrinkage strain at 90 days, compared to mortars mixed with fresh water [31]. The optimum replacement level of silica fume for achieving an improvement in the compressive strength of SW-SS concrete is 12% [55]. C–S–H gels with seawater mixing achieve a finer microstructure, with gel pore sizes less than 5 nm, leading to a higher elastic modulus value in the seawater concrete than in normal water concretes. Tensile strength at 28 days is reported to have increased by 10–20% in concretes mixed and cured with seawater [56].

Fig. 6 (a–d) shows the evolution of the compressive strength, elastic modulus, and flexural strength of SW-SS concretes, with respect to time and binder type. The percentage (%) change in the above-mentioned hardened properties of SW-SS and FW concretes was calculated by collecting data from various articles [55,[57], [58], [59], [60], [61], [62], [63], [64], [65], [66]]. The box plots present the distribution (represented by lines indicating the 1.5 × inter quartile range (IQR)) of the % change in the hardened properties of the concrete. These plots also highlight improvements in the mechanical properties of the SW-SS concretes in the early days of hydration and their reduction at a later age. It is the median values of the percentage changes in compressive, flexural strength, and e-modulus that should be taken into consideration, as there was an asymmetrical distribution of data across the positive and negative quadrants. Also, the addition of SCMs resulted in a better improvement in the percentage increase in compressive strength, compared to SW-SS concretes made with OPC. Among the binder combinations considered, CEM I + fly ash (FA) performed better than SW-SS concretes produced with CEM I + blast furnace slag (SG), or CEM I + silica fume (SF).

Yang et al. [67] have suggested that the higher drying shrinkage in seawater concretes, at early ages, is due to the higher paste content than in concrete produced with freshwater. Despite the density of SW and FW concrete being the same, the presence of suspended solids and the acceleration in hydration can marginally increase the paste content of the former in comparison to the latter [67]. Also, the increase in drying shrinkage strain of SW-mixed concretes can be attributed to the formation of a finer pore structure, due to accelerated hydration at an early age induced by the presence of chlorides [68]. The addition of fly ash in SW concretes might further increase drying shrinkage, because of the finer pore structure resulting from the synergy between seawater and fly ash during cement hydration [69]. However, the addition of fibres can reduce drying shrinkage in SW-mixed cementitious systems with SCMs, by restricting moisture movement and arresting shrinkage-induced cracking [51]. The density of concrete measured with ultrasonic pulse velocity(UPV), in both fresh and seawater, has been found to be almost the same [70]. Like conventional concretes, the density and other mechanical properties of recycled aggregate concrete, made with SW, does not decrease below 5–15% [71]. However, more research is required to understand the density and shrinkage behaviour of SW-SS concretes containing supplementary cementitious materials.

2.4. Durability

The presence of sodium chloride in seawater can improve the pH of the cementitious matrix by increasing the hydroxyl ion concentration, thus improving the likelihood of an alkali-aggregate reaction (AAR) in the presence of reactive aggregate [72]. However, the role of seawater in increasing the likelihood of AAR is not significant in the presence of reactive aggregates at elevated temperatures [73]. The freeze-thaw resistance of SW-SS concrete has been found to be higher compared to ordinary concrete mixed with freshwater and made with river sand [74]. However, the addition of slag can improve the freeze thaw resistance of SW-SS concrete [11]. The water penetration depth of SW-SS concretes is less than that of ordinary concrete, due to the reaction between Friedel's salt and portlandite in the cementitious matrix, which forms CaCl2 and improves the compactness of concrete [75]. Among SCMs, the water penetration resistance of SW-SS concretes substituted with fly ash does not improve, compared to those of concretes made with limestone calcined clay [75]. Also, the addition of rice husk ash, together with nano-silica and nano-alumina, improves water penetration resistance in SW-SS concrete [76]. The presence of chlorides and sulphates in seawater and sea sand is a major threat to the long-term durability of SW-SS concrete [77].

Fig. 7 (a) shows that the total diffused chlorides in SW concrete with SCMs, has been found to be less than the diffused chlorides in FW concrete, due to the higher chances of precipitation of Friedel's salt and ettringite in the pore system, which leads to pore refining [66]. However, the number of free chlorides in coral aggregate seawater concrete has been observed to be higher than in ordinary concrete after 120 days of immersion in seawater, with this potentially being due to the fact that coral aggregates are light weight and porous in nature [78]. Fig. 7 (b) shows the carbonation rates of concrete mixed with seawater and with tap water, which have been reported as being almost the same [79]. The incorporation of SCMs, such as fly ash, metakaolin and blast furnace slag in SW-SS concrete, has been reported to cause a decrease in the penetrability of chloride ions through the refined pore structure of the cementitious matrix, due to the filling effect, pozzolanic reaction, and seawater accelerated hydration due to seawater-mixing [79,80]. SW-SS concretes with a higher replacement of BFS and fly ash have an increased resistance to oxygen permeability, but a decreased resistance to carbonation [79]. The initial presence of chloride in SW-SS concretes can influence the results of rapid chloride migration tests and thus silver nitrate solution cannot be used for observing chloride profiles [80]. New indicators based on K2SWO8 and H2O2have been proposed for use in rapid chloride migration (RCM) tests, for both concretes contaminated with chlorides, as well as SW-SS concretes [81]. More research on the durability of SW-SS concretes needs to be conducted to evaluate their durability parameters and to predict the long-term performance of these concretes in aggressive environments containing chlorides, sulphates, moisture, and carbon dioxide.

2.5. Corrosion characteristics

Corrosion of steel is the major roadblock in the implementation of SW-SS concretes. Kaushik and Islam [82] have recommended that seawater not be used as mixing water for reinforced and prestressed concretes, as they have observed an extremely high degree of deterioration in reinforced concrete with SW as mixing water, after 18 months of exposure. Active corrosion of steel embedded in SW-SS concrete has been observed, after two years of natural exposure, with a higher [Cl−/OH−] ratio compared to concrete mixed with freshwater [11]. However, the use of seawater as mixing water for concrete was found to have a lesser influence on corrosion activity, compared to the use of seawater as curing water [83,84]. Also, the application of seawater-mixed reinforced concrete in seawater immersion conditions, does not result in any serious deterioration, even after 20 years of exposure [85]. Partial substitution of concrete with SCMs such as fly ash, slag, and silica fume has been recommended in order to produce a dense matrix which resists corrosion activity [85]. However, the addition of fly ash in larger quantities (>50%) can reduce the formation of Friedel's salt, due to the lesser availability of the clinker fraction, thus reducing the corrosion resistance of steel embedded in concrete. Conversely, lower corrosion rates have been observed in SW reinforced concrete specimens with 30% fly ash, compared to a higher replacement of 50% [86]. Among the typical SCMs, metakaolin-blended SW-SS concretes demonstrate better corrosion protection than fly ash and slag specimens, due to an enhancement in chloride binding capacity [87]. The presence of more chloride ions in SW-SS concretes can result in the formation of rust products which are more expansive than magnetite and haematite phases, like lepidocrocite (ϒ-FeOOH) and akaganite (β-FeOOH), in the layer of embedded-steel [88]. Service life assessment of SW-SS concretes with fly ash and slag, in regard to chloride-induced corrosion, has showed that the time for corrosion initiation is shorter and that the time for corrosion propagation is longer than for conventional concretes [79]. Thus, the sum of initiation and propagation time, as the total service life of seawater concrete, is marginally shorter compared to freshwater concretes. Table 1 summarizes the effects of seawater-mixing and sea sand addition on concrete properties, as well as the influence of the partial substitution of SCMs and fibres in SW-SS concretes.

Table 1. Effect of salts and ions in SW-SS concrete with SCMs and fibres.

Change in properties of concrete.	Early-age hydration		Fresh properties			Strength & E-modulus		Shrinkage	Long-term durability
Change in properties of concrete.	Heat of hydration	Setting time	Slump/Spread	Viscosity	Shear stress	Early age	Later-age	Shrinkage	Water Permeability	Chloride resistance	Steel corrosion
Effect of salts present in SW-SS concrete	↑	↓	↓	↑	↑	↑	↔	↑	↑	↓	↓
Substituting SCMs in SW-SS concrete	↓	↑	↔	↔	↔	↑	↔	↑	↑	↑	↔
Addition of fibres in SW-SS concrete	↔	↔	↓	↑	↑	↑	↔	↓	↑	↑	n/a

Note: ↑- increases; ↔ - no difference; ↓- decreases; n/a – not applicable.

3. Ensuring the durability of sw-ss concrete

From Section 2 above, it can be summarized that the strength, fresh, and durability properties of SW-SS concrete are not inferior in any way, excluding a marginal reduction in workability and an increased risk of embedded-steel corrosion in reinforced concretes. Accordingly, in order to encourage the use of SW-SS concrete by enhancing corrosion resistance, the following approaches are justified: improving chloride binding, incorporating corrosion inhibitors, adapting cathodic protection techniques, and using special reinforcements such as FRP and stainless steel.

3.1. Chloride binding

Chloride binding in cementitious systems can occur through chemical binding in hydrated phases and physical adsorption in the C–S–H interlayers [89]. Fixed chloride ions in the cementitious matrix can have a lesser affinity for ion exchange with steel, compared to the free chlorides present in the pore solution [79]. In general, chloride binding capacity gradually increases with a decrease in the water-to-binder ratio from 0.4 to 0.2, at later ages [90]. Also, the chloride binding capacity can be significantly improved by the incorporation of metakaolin, compared to the addition of silica fume, due to the presence of higher Al-phases in the former [90]. The presence of higher Al2O3 phases in cement and SCMs has a positive impact on chloride binding capacity [91]. Furthermore, the incorporation of nano-Al2O3 by up to 4% can immobilize the chloride ions in the cementitious matrix, even at later ages [92,93]. Moreover, the addition of barium sulphate can enhance the fixation of chloride ions in hardened cement mortars by improving the dissolution of monosulphate in the presence of gypsum [94]. The addition of fumed silica can enhance the consumption of portlandite and densify the matrix, so that the free chlorides in it can be bound through physical adsorption on the C–S–H interlayers formed [95]. The role of sulphate availability in enhancing chloride binding has also been highlighted in the literature. The presence of higher sulphate ions in the solution can encourage the formation of ettringite, over the conversion of monosulphate to Friedel's salt [96].

The addition of an artificial aggregate made with CaO–NaAlO2, by up to 5%, can improve chloride binding in cementitious systems by forming brucite, Friedel's salt, and Kuzel's salt [97,98]. The addition of clintophile (natural zeolite), along with an alkali-activated slag system, facilitates the binding of chloride and sulphate ions in the pore structure with the formation of Kuzel's salt [99]. Furthermore, the addition of mineral admixtures with calcium sulpho-aluminate cement pastes mixed with seawater, can improve the physical adsorption of chloride ions to the CSA pastes, corresponding to the ratio of Ca/Al [100]. More research on the optimum levels of mineral admixture additions and dosages of additives enhancing chloride binding, is required to produce seawater-mixed concrete with a longer service life against chloride-induced corrosion.

3.2. Corrosion inhibitor

Corrosion inhibitors are chemical admixtures added to a cementitious system to reduce free chlorides, which are responsible for initiating corrosion in embedded steel in concrete, in the pore solution. In SW-SS concretes, corrosion inhibitors such as imidazoline, at 0.75%, and triethylenetetramine, at 1%, have been used to extract chloride ions on the surface of steel rebars [101]. A combination of corrosion inhibitors, including triethanolamine, dimethylethanolamine, lithium nitrate, and ethenyl triethoxysilane, at a dosage of 1.5%, has been used to enhance the stability of the passive film of the rebar by adsorbing on its surface. The efficiency of this corrosion inhibitor in SW-SS is good enough to retard the corrosion process in cementitious systems with pre-mixed chlorides [102]. In coral aggregate concretes, the addition of aluminiumtri-polyphosphate can improve the quality of the protective passive layer on the surface of steel bars, by increasing polarization resistance and interacting with hydration products, to form phases filling the concrete's micro and macro pores [63]. A novel corrosion inhibitor, made with silver nanoparticles, can be included in SW-SS concrete to block the diffusion of chloride ions at the level of the steel, by blocking capillary pores [103]. It is clear that corrosion inhibitors, together with the appropriately chosen reinforcement (as discussed in detail in Section 3.3), can improve the resistance of SW-SS concrete against corrosion-induced failure.

3.3. Special reinforcement

3.3.1. Stainless steel and coated rebars

The higher critical chloride threshold values of stainless steel over carbon steel, make it possible to use the former in SW-SS concrete with a higher initial chloride content [104]. Previous work has demonstrated the possibility of designing reinforced SW-SS concretes with a service life greater than 150 years, by providing a reasonable cover depth of 30–40 mm [105,106]. In alkaline conditions, the use of duplex stainless steel has been found to be feasible in SW-SS concrete, as a result of the presence of a stable passive layer at a very high chloride ion concentration of 2 M [107]. However, the reduction in pH, due to carbonation, can result in depassivation of embedded steel rebars. As such, more experiments are required to understand the effects of low pH on corrosion initiation in reinforcing steel within SW-SS concretes. The suitability of using low alloy steel rebars in SW-SS concrete has been evaluated and found to be suitable for use in concretes having a pre-mixed chloride content of less than 0.05 M [108]. Under stress corrosion cracking, austenitic stainless steel can be used in SW-SS concretes, even at a higher temperature of 60 °C, as no cracks occur at such exposure conditions [109]. Epoxy coated rebars should be avoided in SW-SS concretes, despite their higher chloride threshold, due to the possibility of pinholes or microdefects forming on the surface of the coating [110,111]. Such defects can lead to more severe crevice corrosion than the pitting corrosion expected to occur in chloride-induced corrosion [111]. Therefore, austenitic and duplex grade stainless-steels are suitable for SW-SS concretes, as their critical chloride threshold values are much higher than the chloride content in concrete mixed with seawater [112].

3.3.2. FRP rebars, tubes, and wraps

Fibre-reinforced polymers are made with fibres such as basalt, carbon, glass, and steel embedded in resin matrices such as epoxy, vinyl ester, and polyester [113]. Fig. 8 shows the FRP wraps, bars, and tubes that have been used in various SW-SS concretes [113,114]. Due to their mechanical properties, GFRP and BFRP rebars are the most commonly used FRP bars in concrete made with seawater and sea sand, for applications that require adequate corrosion resistance in severe exposure conditions [[115], [116], [117], [118]]. Research has shown that the initiation of cracks caused by flexure in GFRP reinforced SW-SS beams can be mitigated, and that this leads to an increase in the ultimate bearing capacity of the beams [119]. CFRP bars exhibit better durability performance in aggressive environments such as alkaline cementitious matrices [120]. Combinations of steel-FRP composites and rebars (SFCB) are used in SW-SS concretes, and the bond performance of SFCB composites is superior to ordinary concretes made with river sand and natural aggregates [121]. In order to avoid poor performance, adequate care should be given to FRP composites by protecting them from aggressive exposure conditions, such as extreme temperature, moisture, and ultraviolet radiation [122]. The inclusion of BFRP needles, as an alternative to bars, can enhance the fracture toughness of SW-SS concrete beams without adverse effects on their other mechanical properties [123].

Composite tubes, with BFRP and CFRP wrap and steel tubes confining SW-SS concretes, have been developed to enhance the performance of SW-SS columns under axial compressive load [126]. In comparing BFRP and CFRP confinements, the former show better deformation effects, while the latter show better reinforcement effects; both enhance the ultimate stress and strain values of a column [[126], [127], [128]]. Partial confinement of SW-SS concrete with CFRP wraps also provides excellent protection against sudden failure, by improving lateral-to axial strain relationships [129]. Models have been developed to predict the long-term behaviour of such FRP confinements and composite beams under load and environmental effects [130,131]. There is a clear advantage in using FRP as an alternative reinforcement in SW-SS concretes in offshore structures, as the mechanical performance and durability of FRP composites have been thoroughly assessed in the existing literature.

3.4. Other techniques

Researchers have demonstrated the suitability of using cathodic protection and monitoring with anodes for improving the service life of reinforced concrete with carbon steel. An increase in charge transfer resistance and pH near the surface of a steel rebar decreases the corrosion rate of seawater concrete [132,133]. Also, the impressed current cathodic protection method has been used to impede further corrosion in SS concrete beams [134]. but more research is required to optimize such strategies, in order to protect SW-SS concrete beams reinforced with steel. Furthermore, steel, polyethylene, polyoxymethylene, polyvinyl alcohol, and sisal fibres are used in SW-SS concrete beams for producing ultra high-performance concretes, so as to improve their fresh, mechanical, microstructural, shrinkage, durability, and corrosion performance. Table 2 summarizes the materials and approaches available for enhancing the durability of SW-SS concretes. The durability of SW-SS concretes could be improved by enhancing chloride binding, using anti-corrosive alternate reinforcements, adapting corrosion preventionmethodologies, and adding fibres to the cementitious matrix [[135], [136], [137], [138], [139], [140], [141]].