1 Introduction

Concrete is the most used man-made material, with a per capita consumption of three tonnes per year [1]. Concrete is a mixture of aggregates and paste fractions, with the latter composed of binder and water. In 2012, the freshwater consumption for concrete production was equal to 9% of global industrial water withdrawal [2]. The World Resources Institute (WRI) identified and warned that 33 countries in the world could face extremely severe-water stress in 2040 based on predictions from climate models and socioeconomic scenarios [3]. By 2050, the countries experiencing severe-water stress will have 75% of the total water demand for concrete production in the world [2]. This scenario warrants the use of alternative water resources for concrete production. In recent years, several researchers have investigated the potential of incorporating alternative water resources such as seawater [4], grey water, treated sewage wastewater [5], and magnetised water [6] into concrete production. Seawater-mixed concretes (SWCs) are being recommended for niche applications with the view of reducing the overall freshwater requirement in concrete production for nations facing severe water stress [7].

Several international standards and specifications for producing reinforced concrete impose restrictions on the amount of total inorganic salt content in the mixing water for concrete production [8]. The presence of inorganic salts in mixing water can inflict pitting corrosion on the surface of embedded steel in reinforced concrete [9]. However, the usage of supplementary cementitious materials (SCMs) such as fly ash, ground granulated blast-furnace slag, and metakaolin in concrete improved the corrosion resistance of embedded steel in concrete by binding the free chlorides responsible for initiating corrosion above certain threshold limits [10]. Additionally, incorporating these SCMs in binary and ternary blended cementitious matrices enhanced the concrete’s resistance to ingression of moisture, chlorides, and carbon dioxide because of the dense microstructure produced by the pozzolanic reaction [11]. Concretes made with SCMs and alternative cements like limestone calcined clay are more durable [12] and sustainable [13] than concretes made with ordinary Portland cement. Furthermore, the usage of alternative reinforcement such as fibre reinforced polymer (FRP) bars, galvanised, and stainless steel rebars in concrete produced with seawater-mixing and sea sand [14] or coral stones as aggregates [15] showed an improvement in the strength and durability properties. Using seawater as an alternative to fresh water in concrete production, along with corrosion-resistant reinforcement can reduce its water footprint and make it more sustainable [16].

Unlike grey water and magnetised water, seawater does not require an extensive treatment process other than filtering out floating debris [17]. Salts in seawater-mixed concrete can alter the hydration products and mechanisms during the early age of the concrete [18]. More sulphates and chlorides in the cementitious systems can favour the formation of Friedel’s salt, Kuzel’s salt, and monosulfate over ettringite [19]. The formation of Friedel’s salt in the cementitious matrix can be attributed to the improved properties of the concrete. The substitution of clinker with SCMs can increase the chemical binding of chlorides in hydrated cement products, and amongst those metakaolin blends demonstrated the maximum ability of chloride binding [20]. The improvement in mechanical properties at an early age could be attributed to the formation of dense microstructures due to accelerated hydration in the presence of salts [21]. However, the later age (> 28 days) E-modulus, compressive and flexural strengths of the SWCs are typically lower than those of conventional concrete. The reduction in strength at later ages was found to be higher in CEM I concretes compared to the concretes produced with SCMs [17].

When fly ash was used as a partial substitution to clinker at a weight percentage of 25%, the drying shrinkage of SWCs was less than that of concretes made with CEM I [22]. Also, the oxygen permeability values of fresh water and seawater-mixed concretes containing fly ash and blast furnace slag are almost the same [23]. Furthermore, the frost resistance of seawater-mixed and sea sand concrete was reported to be better than that of ordinary concrete due to the dense pore structure of the former [24]. As a result of pore structure refinement and chloride immobilisation, SWC containing metakaolin showed improved resistance to chloride ingress in rapid chloride penetration and migration tests [25]. More hydroxyl ions (OH−) in the pore solution of SWCs can increase the chances of an alkali-silica reaction with reactive aggregates and higher levels of alkali in cementitious systems [26]. Concretes produced with slag as SCM demonstrated better corrosion resistance than concrete made with OPC when seawater was used as mixing water [27]. The synergetic effect of using SCMs and alternative reinforcement could enable the use of seawater for concrete production.

Despite an increase in interest in research on SWC in the last decade, these studies were limited to research on concretes made with CEM I and binary blended concretes produced with SCMs [17]. Few articles demonstrate the possibility of producing SW-mixed concrete with improvements in the strength and durability properties of concretes made with fly ash, granulated blast furnace slag [28], limestone calcined clay [29], and high ferrite cement [30]. However, the studies on the effect of varying SCM content and ternary-blended binders on the properties of SWC are scarce and inconclusive [31]. The need for more research on understanding the effects of ions in seawater on cement hydration and strength development under actual exposure is necessary to cater to the need for alternative water for concrete production in water-stressed nations [32].

The early-age hydration and strength development of SW-mixed binary and ternary blended cementitious systems are assessed in this article. Also, the current study attempts to ascertain the effects of ions in seawater on the properties of cement pastes composed of CEM I that have been partly replaced at varying replacement levels with fly ash, slag, metakaolin, and limestone. The findings of this article may be helpful in identifying the best SCM to use in place of clinker and how much of it to replace to produce a seawater-mixed concrete that performs similarly to freshwater-mixed concrete. Furthermore, an approach for choosing suitable mixtures to produce seawater-mixed cementitious systems is also highlighted.

2 Materials and methods

2.1 Materials

The CEM I – 42.5 R conforming to EN 197–1 [33] was used as the primary binder in this study. Fly ash (F), ground granulated blast furnace slag (S), metakaolin (M), and limestone (L) were used as supplementary cementitious materials at different replacement levels ranging from 10 to 70%. Table 1 lists the chemical composition and physical properties of materials. The presence of a higher quantity of silica in fly ash indicates that the type of fly ash is siliceous. The ground granulated blast-furnace slag (termed as slag) used in this study consists of CaO and SiO2 in equal quantity, which exhibits the possible hydraulic and pozzolanic nature of the material. The metakaolin contains more alumina content, as indicated in Table 1.

Table 1 Chemical and physical properties of CEM I and SCMs in this study
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Amongst the physical properties of the binders, the specific gravity values were measured using a helium pycnometer and listed in Table 1. Also, the d50 values of these materials show that slag and fly ash have similar sizes of particles, but metakaolin and limestone have much finer particles than CEM I. Figure 1 consists of the particle size distribution of the binder systems measured using a particle size analyser (Mastersizer 2000, Malvern, UK) for the raw materials used in this study. The particle size distribution of blast furnace slag and CEM I is similar. Also, Fig. 1 shows the presence of coarser particles in fly ash and finer particles in metakaolin and limestone.

Fig. 1
figure 1

Particle size distribution of raw materials

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2.2 Methodology

Figure 2 explains the nomenclature of ternary-blended paste specimens based on the binder type, SCM content, and mixing water type. The letter ‘P’ stands for paste, followed by an alphabet indicating the type of binder used. The symbols ‘xx’ and ‘yy’ represent the amount of SCM content replacing the CEM I in the binder composition. For binary blends, the proposed nomenclature is modified by providing one alphabet representing the type of SCM and SCM content (e.g., PS10). Table 2 consists of the composition of paste specimens produced by mixing fresh water (FW) and seawater (SW). Binary and ternary blended cement pastes were prepared by mixing SCMs such as fly ash, slag, metakaolin, and limestone with CEM I. The replacement level of cement fraction with SCMs was between 10 and 70 wt%. A total of 31 binder combinations were selected and cement pastes were produced with both fresh and seawater for all combinations. A constant water-to-binder (w/b) ratio of 0.4 was used in this study. Potable water from the laboratory conforming to EN-1008 [34] was used as a mixing water for FW-mixed cement pastes. Artificial seawater was prepared in accordance with ASTM D-1141 [35] and used for producing SW-mixed cement pastes.

Fig. 2
figure 2

Nomenclature of the cement paste mixes

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Table 2 Mix proportion of cement pastes
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2.2.1 Isothermal calorimetry

The heat of hydration of cementitious samples was measured in an isothermal condition using an eight-channel calorimeter (TAM Air 3, TA Instruments). The eight-channel calorimeter was calibrated with quartz at an equivalent thermal mass. After calibration, the binder and water of mass 9 and 3.6 grammes respectively, were mixed in the ampoules using a rotary vibrator. Following that, the ampoules were inserted in the sample holders along with their corresponding reference channels. The temperature inside and outside the equipment was kept constant to avoid any major fluctuations during measurement. The heat flow at every instance and the cumulative heat of hydration up to 7 days were measured for every sample. The acquired signals were normalised according to the mass of the binder. Furthermore, the time of occurrence of the silicate peak, sulphate balance period, and sulphate dissolution peak were calculated using the Nadaraya–Watson (NW) Kernal estimate adapted from Canbek et al. [36]. The derivative of the rate of heat evolution was calculated and overlapped with the heat of hydration plots of every mix to determine the time of occurrence of the maximum silicate and sulphate dissolution peak.

2.2.2 Compressive strength

Cement paste cubes of size 2 cm were prepared for measuring the compressive strength of the binder combinations. A hand mixer with stainless steel blades was used to blend the cement paste in two steps. The CEM I, SCM, and water were mixed slowly for 60 s, and the sides of the container were scraped properly. After the slow mixing, the cement pastes were blended at high speed for 120 s, and the cube specimens for compressive strength tests were cast. The cubes were demoulded after 1 day and immersed in freshwater for moist curing. At the end of 2, 7, 14, and 28 days of curing, three specimens were tested in an electromechanical universal testing machine (Uniframe 250, Controls) using a load cell with a maximum capacity of 50 kN.

The results from the studies on compressive strength and isothermal calorimetry conducted on different paste compositions listed in Table 2 are compiled in the upcoming Sect. 3. Furthermore, the heat of hydration and cumulative heat curves obtained for 31 paste compositions in this study were analysed to understand the percentage reduction in setting time that occurred between FW-mixed and SW-mixed pastes. Also, mixes suitable for SW mixing were determined by calculating the percentage change in compressive strength and normalised strength, considering the effects of SW mixing and substitution of SCMs, respectively.

3 Results and discussions

3.1 Evolution of heat of hydration at early ages: binary blends

Figure 3 shows the evolution of heat of hydration in binary blended cementitious systems over a period of 7 days after mixing with fresh and seawater. The major stages in the heat evolution in calorimetry data consist of the initial reaction, induction, acceleration, deceleration, slow, and continued reaction phases [37]. Figure 3a clearly demonstrates the diluting effect of fly ash with an increase in replacement level to CEM I, and the drop in heat evolution peak is proportional to the replacement level [38]. Figure 3b indicates the shifting of silicate peaks towards the origin, indicating the acceleration of the hydration reaction with SW-mixing. The presence of sulphate ions and the tendency for faster dissolution of C3S in seawater result in the enhancement of hydration [39]. In both FW and SW-mixed cement pastes, 50–70% of CEM I with ground granulated blast furnace slag resulted in a significant retardation in the heat flow due to the clinker dilution effect, as shown in Fig. 3c and d.

Fig. 3
figure 3

Heat evolution during hydration up to 7 days in binary blends: PC-FW (a); PC-SW (b); PFxx-FW (a); PFxx-SW (b); PSxx-FW (c); PSxx-SW (d); PMxx-FW (e); PMxx-SW (f)

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The comparison between PS30 and PF30 reveals that at a similar replacement level, slag hydrates faster than fly ash due to the presence of finer particles as well as the lower requirement of activation energy at early stages [37]. Also, the presence of a higher proportion of Al2O3 in slag facilitated the formation of sulphate-bearing hydrated cement products such as monosulphate and ettringite. Also, the addition of seawater for mixing further reduced the sulphate dormant period and resulted in faster deposition of sulphate-bearing hydrated phases with bound chlorides (Friedel’s salt or Kuzel’s salt).

Figure 3e depicts the development of heat flow during the hydration process in cementitious systems with metakaolin substitution. The addition of metakaolin to freshwater-mixed cement pastes resulted in a decrease in heat flow during the silicate peak [40] and enhanced the formation of sulphate-bearing phases [41] with an increase in replacement level. This reversal in the heat evolution pattern in fresh water mixed cement pastes could be attributed to the availability of Ca2+ ions in the pore solution. At a lower replacement level of metakaolin, calcium aluminium hydrates and C-S-H gel are formed compared to the formation of stratlingite and C-S-H gel at a higher replacement level of metakaolin [38]. Figure 3f shows the formation of more stable phases such as hydrogarnet with the addition of 30% metakaolin in seawater-mixed cement pastes. Also, the formation of chloride-binding phases such as Friedel’s salt can be witnessed with the increased heat flow around 50th hour, as in Fig. 3f.

A higher silicate peak is characteristic of pastes produced with CEM I. However, at higher replacement levels of slag and metakaolin, the secondary peaks denote the hydration of alumina phases in the blended cementitious systems. This phenomenon leads to the deposition of more C-A-S-H and ettringite in the cementitious matrix [42]. Figure 3g and h shows the effect of the addition of limestone powder on the heat of hydration of freshwater and seawater-mixed cement pastes. The presence of limestone powder provides additional nucleation sites [43] and a large space between cement particles to promote hydration and thus, heat evolution [44]. The decrease in the time for achieving maximum heat flow in seawater-mixed pastes with limestone addition is observed at all replacement levels.

3.2 Evolution of heat of hydration at early ages: ternary blends

Figure 4 shows the evolution of heat of hydration in ternary blended cementitious systems mixed with fresh and seawater. Figure 4a and b shows that there is a significant reduction in the heat of hydration due to the dilution of clinker effect at a higher replacement level of CEM I with slag and fly ash. Also, the presence of higher amounts of slag and fly ash (45 and 60%) considerably delays the time for sulphate dissolution and the formation of sulphate-bearing hydrated phases. Moreover, the presence of higher alumina content in slag mixes leads to a delay in the sulphate dissolution observed in SW-mixed cement pastes, even higher than fresh water-mixed cement pastes [45]. On the other hand, a significant increase in heat flow and a reduction in the time of occurrence were observed at silicate peaks for SW-mixed cement pastes with a fly ash and slag content of less than 30%.

Fig. 4
figure 4

Heat evolution during hydration up to 7 days in ternary blends: PSxxFyy-FW (a); PSxxFyy-SW (b); PM/S/FxxLyy-FW (c); PM/S/FxxLyy-SW (d); PS/FxxMyy-FW (e); PS/FxxMyy-SW (f)

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Figures 4c and d present the results from cementitious pastes blended with metakaolin, slag, or fly ash in addition to limestone powder. However, the ternary blend with 30% of the replacement level PSxxFyy-SW mix showed an increase in heat of hydration almost similar to the mix PC-FW. Also, the accelerated hydration of the silicate peak led to the faster precipitation of C-S-H and calcium aluminate hydrate phases [46]. As indicated in Fig. 4d, the formation of cementitious phases during the 50th hour in PM30L15-SW cement paste produced with seawater demonstrates the formation of Friedel’s salt or hydrocalumite [22]. Metakaolin-limestone blends exhibited a faster rate of hydration due to the formation of stable carboaluminate phases, as compared to the ternary-blended slag-limestone and fly ash-limestone combinations at all replacement levels [47]. Figure 4e and f shows the evolution of heat of hydration in PSxxMyy and PFxxMyy mixes produced with fresh water and seawater, respectively. These ternary combinations offer additional reactive silica and alumina and promote the formation of chloride-binding hydration products. Furthermore, a reduction in the time taken for the maximum silicate peak was observed in SW-mixed cementitious systems.

3.3 Understanding effect of seawater on setting time

Table 3 summarises the heat evolution parameters in the 31 binder compositions considered in this study mixed with fresh and seawater. The silicate peak and the time for the onset of sulphate dissolution were derived from the N–W Kernal Estimate [48]. The difference in the time taken to reach the maximum silicate peak is between 0.5 and 2 h. The acceleration in the time for achieving maximum heat evolution with a silicate peak can be beneficial to decrease the setting time and early strength gain. Also, the early onset of sulphate dissolution in the pore solution for producing stable hydration products such as hydrogarnet and monosulphate is due to the presence of higher amounts of sulphates in the pore solution due to SW-mixing [49].

Table 3 Heat evolution parameters in cementitious pastes mixed with fresh and seawater
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The acceleration of hydration in cementitious systems with high-volume replaced slag and fly ash is significantly higher compared to the mixes with metakaolin and limestone addition [50]. The narrowing of the main hydration peaks in metakaolin-blended cementitious systems could be attributed to the presence of more Al ions in the pore solution with the substitution of metakaolin mixes produced with seawater [51]. Also, the reactive silica in the SCMs reacts with hydrated products such as portlandite and ettringite to form C-A-S-H and C-A-H gels at a later age [52]. Furthermore, the increase in the peak representing silicate hydration due to seawater addition could be attributed to the acceleration effect of the 2% CaCl2 present in the mixing water. The presence of CaCl2 could accelerate the hydration of C4AF along with C3S during the first 2 h after the addition of water [39].

3.4 Understanding effect of seawater on total heat of hydration

Figure 5a and b shows the stages of cumulative heat evolution in binary and ternary blended cementitious systems. After 7 days of hydration, no substantial differences between FW and SW-mixed cementitious systems were found. However, the cementitious systems mixed with seawater exhibited an increase in the cumulative heat of hydration on the 1st and 2nd days of hydration. Figure 6a and b presents the percentage variation between the amount of total heat evolved in fresh water and seawater mixing on the 1st, 2nd, and 7th days of hydration. Figure 6a demonstrates the effectiveness of acceleration in hydration with seawater mixing in high-volume substituted fly ash mixes. This could be attributed to the compensation of the clinker dilution effect with accelerated hydration occurring in the presence of chloride salts in seawater-mixed cementitious systems. Cementitious systems with slag and metakaolin did not exhibit a significant increase in total heat at the 2nd day of hydration due to the possible effect of the formation of more sulphate and aluminate-bearing hydration products [53]. Also, the formation of Friedel’s salt and hydrogarnet around 50 h of hydration is very well documented in previous research works that explain the increase in the total heat emitted during hydration in cementitious systems with slag and metakaolin [42]. The filler effect of limestone powder and the effect of seawater mixing are quite similar to the Portland cementitious systems, as the limestone addition did not produce aluminate and sulphate-bearing hydration products similar to fly ash and slag additions [54].

Fig. 5
figure 5

Cumulative heat of hydration after 7 days from time of mixing cement and water: Binary blends (a); Ternary blends (b)

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Fig. 6
figure 6

Change in cumulative heat of hydration: binary blends (a), ternary blends (b)

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Figures 5b and 6b indicate that the effect of seawater mixing in ternary cementitious systems consisting of fly ash and slag on the cumulative heat of hydration at the 7th day is not more than a 10% increase at any replacement level. Ternary combinations of PSxxLyy, PFxxLyy, and PMxxLyy mixed with SW demonstrated a higher degree of hydration compared to the PSxxFyy blend. The total heat of hydration during the hydration process of the limestone-substituted ternary system (with a replacement level > 45%) does not reduce by less than 20% compared to the other ternary system. Cement pastes made with the PM20L10 and PM30L15 blends could produce more stable compounds such as hemicarboaluminate and monocarboaluminate [38].

The ternary combinations consisting of fly ash/slag and metakaolin showed that the percentage change in cumulative heat evolved on the 2nd day at higher replacement levels compared to other ternary mixes [42]. The presence of more reactive alumina in this combination contributed to the formation of monosulphates and Friedel’s salt during the 2nd day of hydration, as indicated by an increase in the heat evolved in higher metakaolin substitution in Figs. 3 and 4. From Figs. 5 and 6, the amount of cumulative heat hydration in both binary and ternary blended cement pastes was higher in SW-mixing compared to FW-mixing. These results indicate that the ions in seawater, including Cl−, Ca2+, Mg2+, Na+, and SO42− promoted the hydration of C3A and C3S [55].

The scatter between the normalised total heat of hydration with the PC-FW mix and the change in cumulative heat of hydration at the time of the silicate peak between FW and SW-mixing is depicted in Fig. 7. Except for PS20M10, all other mixes showed an increase in total hydration at the time of setting, indicating enhanced hydration with seawater mixing. Out of the 31 binder compositions that were examined, 14 mixes highlighted with red rectangular boxes in Fig. 7 indicated a faster setting and a reduction in total heat of hydration by less than 10% after 7 days of testing. The binary blends with fly ash, metakaolin, and limestone content up to 20% had a substantial increase in the total heat at the time of setting compared to slag mixes.

Fig. 7
figure 7

Optimum binder compositions showing an acceleration in setting time and enhanced hydration of cement pastes containing seawater when compared to their freshwater counterparts

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In ternary blends, the combinations PSF, PSL, PFL, PML, and PFM having SCM content up to 30% showed an increase in the total heat of hydration compared to mixes having higher SCM content (> 45%). Furthermore, the mixes with a higher replacement level also showed a reduction in the setting time despite having a clinker dilution effect. Also, these mixes showed a reduction in time by 60–90 min for attaining maximum silicate and sulphate dissolution peaks with SW mixing, as indicated in Table 3. In general, mixes with lower SCM content showed an acceleration and improvement in total hydration with SW-mixing compared to mixes with higher SCM content. However, the mixes should be assessed for strength development up to 28 days before selecting the optimum binder combinations for producing SW-mixed concrete, and the following section presents the results from compressive strength tests on cement paste samples made with FW and SW-mixing.

3.5 Evolution of compressive strength

Figure 8 shows the strength development over a period of 28 days in FW and SW-mixed binary blends consisting of fly ash, slag, metakaolin, or limestone. Figure 8a showed that the replacement level of SCMs was higher compared to the strength development in SCMs until the age of 28 days. In all binder combinations, the SW-mixed cement pastes showed an increase in strength on the 2nd day compared to freshwater mixes. The higher compressive strength could be attributed to the formation of hydration products with a lower crystal size and better interlocking at early ages [56]. At later ages, the formation of Friedel’s salt is predominant in SW-mixed cement pastes [57]. The crystal structure of Friedel’s salt is hexagonal, having lower hardness and higher porosity compared to hydration products formed with FW-mixing [58]. The reduction in the strength of SW mixes could be attributed to the lesser degree of polymerization of the C–S–H chain due to the substitution of chloride ions in the C–S–H inter-layers. These changes in the C–S–H structure could lead to a reduction in bonding ability and degree of cross-linking and thus affect the load-bearing capacity [59]. This is consistent with the cement paste specimens produced with PC, PF, PS, and PM mixes at a lower replacement level (up to 30%). However, the substitution of SCMs beyond 30 and up to 70% does not show a significant reduction in the compressive strength in both FW and SW-mixed cement pastes.

Fig. 8
figure 8

Strength development in seawater-mixed cement pastes: binary blends—PF (a), PS (b), PM (c), and PL (d)

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The cementitious systems consisting of metakaolin up to 20% replacement do not show a significant reduction in compressive strength with seawater mixing, even at later ages. However, the cement pastes with 30% metakaolin showed a reduction in strength after 7 days, possibly due to internal stress developed because of the crystallisation of large hydration products [60]. The SW-mixing on a limestone powder-blended cementitious system improved the gain in compressive strength compared to the FW-mixing. The addition of limestone diluted the clinker proportions and led to a reduction in strength with the increase in replacement level. Nevertheless, the additional hydration products formed as a result of the nucleation sites created with limestone substitution caused the SW-mixed cementitious blends to show an increase in strength at 28 days [54]. The effect of clinker dilution is predominantly observed in all binary blended cementitious systems except metakaolin.

Figure 9 shows the evolution of strength in ternary blended cementitious systems made with FW and SW mixing. When compared to mixes with lower SCM content, the ternary blended PSxxFyy mixes with higher SCM content (let’s say > 45%) showed a decrease in compressive strength. Moreover, the development of expansive hydration products is responsible for the strength loss of PSxxFyy mixes made with seawater at later ages (28 days), as shown in Fig. 9a. The ternary combinations PSxxLyy and PFxxLyy showed a reduction in strength at later ages compared to the early-age strength gain, as shown in Fig. 9b. PM30L15 showed a better increase in compressive strength, which could be attributed to the formation of hydration products such as hemicarboaluminate and monocarboaluminate. Figure 9c consists of the strength evolution of ternary combinations of PS/FxxMyy. The gain in strength of PSxxMyy at three different replacement levels showed that with seawater mixing, there is an increase in strength in all cases. However, the strength gain in PF20M10-FW, PF30M15-FW, and PF40M20-FW mixes is lower than the corresponding SW mixes.

Fig. 9
figure 9

Strength development in seawater-mixed cement pastes: Ternary blends—PSxxFyy (a), PF/S/MxxLyy (b), and PS/FxxMyy (c)

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Figure 10a and b shows the change in compressive strength between binary and ternary blended cement pastes produced with FW and SW after 2, 7, 14, and 28 days of curing. The increase in compressive strength is between 40 and 75% in binary blends and ternary blends, except for the PS40M20 mix, which showed a 120% increase in strength after 2 days of curing. Figure 10b shows a reduction in compressive strength at 28 days in limestone, including ternary blended fly ash and metakaolin mixes. The increase in compressive strength at a later stage is more than 20% in PS15F30, PM30L15, PS20L10, and PS40M20 mixes. The combined effect of substitution of SCMs and usage of seawater as mixing water shows a higher replacement level of CEM I with SCMs demonstrating a continuous improvement in strength with SW mixing [61]. This could be attributed to the continuous pozzolanic reaction and the densification of the microstructure in the available space for the growth of hydration products [62]. Hence, the optimum level of replacement of CEM I with SCMs for SW-mixed cementitious systems should be evaluated to achieve a similar strength as the PC-FW.

Fig. 10
figure 10

Percentage change in compressive strength: Binary blends (a), Ternary blends (b)

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3.6 Effect of seawater on strength development with time

Figure 11 summarises the general effect of seawater on the strength development of binary and ternary mixes up to 28 days of moist curing. The box plot represents the extent of strength improvement with seawater addition in SW-mixed cement pastes. The strength improvement factor (S) is calculated with the Eq. 1.

$${\text{Strength improvement factor}}, S=\frac{{f}_{SW}}{{f}_{FW}},$$
(1)

where fSW = compressive strength in seawater-mixed cement pastes and fFW = compressive strength in fresh water-mixed cement pastes. The inter-quartile range is the difference between the upper and lower medians of the percentage change in compressive strength of respective cement pastes mixed with fresh and seawater. The upper and lower medians of an increase in strength in binary and ternary blended cementitious systems lie between 1.3 and 1.6 times. As discussed in previous sections, the formation of hydration products such as Friedel’s salt in SW-mixed systems that to a reduction in strength due to the morphology of such phases. However, the range of reduction in strength in ternary blends exists between 0.8 and 1.25 times, which could be further analysed to identify the most suitable ternary blends for producing SW-mixed cementitious systems.

Fig. 11
figure 11

Strength improvement factor over a period in SW-mixed cement pastes compared to FW-mixed cement pastes a Binary and b Ternary blends

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3.7 Suitable binary and ternary blends for SW-mixed cementitious systems

Figure 12 shows the selected mixes for SW-mixing (marked with rectangular boxes in red colour) based on the strength improvement with seawater mixing and normalised compressive strength at 28 days compared to the reference CEM I mix (PC). The strength at 28 days of PC was used to calculate the normalised strength of mixes with SCM addition at same water-to-binder ratio and binder content as shown in Eq. 2.

$$\mathrm{Normalized strength},\mathrm{ N}=\frac{{f}_{c, SCM-28d}}{{f}_{c, PC:FW-28d}}$$
(2)

where fc,SCM-28d = Compressive strength at 28 days in cementitious blends with SCMs, fc,PC-28d = Compressive strength at 28 days in PC-FW mix. The mixes with normalised strength not less than 0.90 and % change in compressive strength with SW-mixing greater than 0% are considered as most suitable mixes for SW-mixed cementitious blends. Most of the binary and ternary blends are exhibiting a strength improvement with seawater mixing. Amongst these mixes, fly ash and slag substituted at 40–50% showed the maximum strength improvement with SW-mixing but lower normalised strength (less than 35%). The binary and ternary blends with replacement levels of 10 to 30% showed a strength improvement and 10–25% of lower normalised strength.

Fig. 12
figure 12

Suitable mixes for producing SWC without a substantial reduction in compressive strength compared to reference concrete made with PC-FW

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The metakaolin blended cementitious systems (PML and PSM) showed a better strength improvement and a lesser reduction in normalised strength compared to other SCMs. The reduction in gel/space ratio with the addition of metakaolin is significantly higher than the other SCMs and that leads to a denser microstructure with seawater mixing [41]. The substitution of limestone with metakaolin at an optimum level led to the formation of these stable carbo-aluminate phases contributing to the formation of a stable cementitious matrix [38]. Furthermore, the addition of metakaolin leads to the immobilisation of free chlorides in the seawater mixing with the formation of hydrocalumite [42].

In summary, Sect. 3 demonstrated that the cement pastes mixed with SW had a reduction in setting time and improved hydration at an early age compared to FW-mixed cement pastes. Moreover, the comparison between the strength development between FW-mixed and SW-mixed cement pastes countered the conflicting opinions in the literature on the reduction in compressive strength observed at a later age [63]. This article presents suitable binary and ternary blended compositions of binder which could be useful for producing SW-mixed cementitious systems. Prior to this, there was a limited research data available on the hydration behaviour and strength development of SW-mixed cementitious systems with supplementary cementitious materials [17]. Hence, future research works shall be carried out to evaluate the mechanical and long-term durability properties of SWC made with identified binder combinations. Furthermore, numerical and analytical models shall be developed for predicting the strength and durability properties of SWC.

4 Conclusions

A detailed investigation with several binary and ternary blended combinations of cementitious materials were evaluated. The results prove that the addition of seawater significantly accelerate the hydration at early age and leads to the formation of denser microstructure. Following conclusions are drawn from the results of isothermal calorimetry and compressive strength tests in this study.

  • The heat of hydration of seawater-mixed cementitious blends was increased at an early age and could decrease the setting time of the concrete demonstrated by the earlier occurrence of silicate and sulphate dissolution peaks.

  • The cumulative heat of hydration in SW-mixed cementitious blends increased from 10 to 20% at 1st and 2nd day of hydration compared to an almost equal cumulative heat of hydration at 7 days in most of the mixes considered in this study. Thus, the presence of salts in seawater enhances the degree of hydration in blended-cementitious systems with high-volume SCM content.

  • The calculated strength improvement factor is significantly higher in mixes with high volume SCM content. However, the normalised strength in high volume substituted mixes with SCMs are lesser than the reference PC mix at same water-to-binder ratio and binder content.

  • An approach was suggested to ensure a reduction in compressive strength of not more than 10% at 28 days in SW-mixed cementitious systems compared to the reference mix PC-FW. From this approach, the metakaolin blended cementitious systems are found to be suitable in producing SW-mixed blended cementitious systems having a higher replacement level up to 45%.

However, the above conclusions are primarily based on the evaluation of early-age hydration behaviour and strength development in SW-mixed cementitious systems. Future works shall focus more on assessment of chloride binding ability of metakaolin-substituted blended cementitious systems for producing durable and sustainable SW-mixed concretes. Such studies can provide more insights into the idea of adapting seawater for concrete mixing in water-scare regions for niche applications and thereby reduce the impact of the water crisis in regions that can face extremely severe water stress.