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The use of blast furnace cement is an effective way to meet the requirements of sustainable development. However, CEM III/C is characterized by slow strength gain. The problem can be worse for plasticized reinforced blast furnace cement concretes mixed with sea water in view of shorter durability. The mitigation of corrosion in plasticized blast furnace cement concretes mixed with sea water can be provided through a composition of minor additional constituents, with percentage by mass of the main constituents: alkali metal compounds, 2…3; calcium aluminate cement, 1; clinoptilolite, 1. The alkali metal compounds are known to activate hydraulic properties of ground granulated blast furnace slag. A calcium aluminate cement promotes the accelerated chemical binding of Cl− and SO42−-ions with the formation of Kuzel’s salt. A clinoptilolite occludes these aggressive ions. The positive effects of the mentioned minor additional constituents in the blast furnace cement were supported by the increased early strength gain and the higher structural density, as well as by a good state of steel reinforcement, in the plasticized concretes mixed with sea water.
- blast furnace cement
- corrosion
- minor additional constituent
- sea water
1. Factors Affecting the Durability of Concretes
One of the most commonly used and widely accepted ways to improve the durability of concretes is the application of so-called blended cements, obtained by a partial replacement of Portland cement clinker by industrial by-products [4]. These by-products include a ground granulated blast furnace slag (further, GGBFS) and fly ash [5], silica fume, natural pozzolans and fillers like trass or limestone [6]. Although the use of various types of blended cements has been encouraged as a green solution, these cements suffer from slow hydration kinetics, resulting in a slower early strength gain, which is a drawback in many construction applications. In order to significantly accelerate the cement hydration process, a number of different methods have been proposed, including the incorporation of nanomaterials, such as, for example, nanosilica [7]. Various studies have confirmed the accelerating action of nanosilica in Portland cement [8] and blended cements [9].
The most known ways to activate latent hydraulic properties of the GGBFS is to apply calcium [10] and sulfate activators [11]. It is also known that low alkaline [12] and nearly neutral salts [13] can be used as activators of blast furnace cements as well. Na (K) salts of strong acids also ensure the increased hydraulic activity of the GGBFS [14]. However, insufficient strength of blast furnace cements and the possibility to only increase early strength are disadvantages of these means of activation: near-neutral salts [15], sulfate [16], etc. The application of oxides or salts of alkaline metals, which provide an alkaline reaction in water, can be a solution to the above-mentioned problem [17]. The increase of the GGBFS content in cement to substitute Portland cement clinker without a loss of strength is possible to achieve in such a way. Alkali activation of aluminosilicate raw materials is now widely used [18].
Concretes based on the GGBFS activated by alkali metal compounds are characterized by the increased strength gain, both early [19] and at 28 d [20], as well as good heat resistance [21], resistance corrosion [22], freeze–thaw resistance [23,24], high waterproofness [25], fire resistance [26,27], etc.
A promising direction being taken to improve the sustainability of concrete is to use sea water for its production. This is due to the fact that fresh water is now the most valuable commodity in the 21st century, as a result of over-exploitation of available deposits [28]. According to the data published by United Nations Organization [29], over 40% of the world’s population faces a scarcity of fresh water. At the same time, consumption of water for the production of concrete makes up 9% of the total worldwide industrial water consumption [30]. Great interest is now being taken in using sea water as an alternative solution in order to impede the exploitation of freshwater [31]. Furthermore, due to high costs for desalination, as well as a high demand for concrete production, there has been a keen interest in incorporating sea water in concrete [32].
2. The Effect of Sea Water on Performance Properties of Cement-Based Materials: The-State-Of-The-Art
Sea water can be used as an activator of the GGBFS as a result of the action of salts of strong acids, such as chlorides and sulfates, that are contained in sea water.
It was shown that sea water could promote the increase in early strength gain of the concretes based on both alkali-activated slag cement [33] and blast furnace cement [34]. The cements based on the GGBFS as a main constituent can, in most cases, reveal their high-performance properties due to a combined effect of alkali metal compounds and salts of strong acids, such as sodium sulfate [35], sodium chloride [36] and other chlorides (KCl, CaCl2 and MgCl2) [37].
However, a negative effect of sea water on the strength of the cements has also been reported. For example, the lower early strength (at 3 d) of the alkali-activated slag cement was reported in [38]. This effect was caused by a deceleration of this type of cement’s hydration process due to the formation, on the surface of the GGBFS particles, of low soluble or insoluble compounds such as brucite (Mg(OH)2), halite (NaCl), gypsum (CaSO4∙2H2O), etc. as a result of its interaction with the salts of strong acids. Brucite and gypsum could evidently bring some crystallization pressure into micropores resulting in the lower late strength [32].
Sea water can also change the pore structure of concrete. As was shown in [39], the alkali-activated GGBFS pastes mixed with sea water were characterized by an increased total porosity and a denser structure, resulting in a lower water absorption. However, the use of sea water can increase the open capillary porosity due to the less-densified gel microstructure as well as the transformation of large pores into pores of a smaller size due to precipitation of the mentioned hydration products as a result of interaction of the GGBFS with salts of strong acids [38]. The greater volume of open capillary pores in concrete based on alkali-activated GGBFS and mixed with sea water predetermines their lower durability due to the lower resistance to the ingress of aggressive substances [40].
The abovementioned controversial results regarding the effect of sea water on the pore structure and strength of the GGBFS containing cement materials can support an assumption about their dependence from a chemical nature, on the alkali metal compound and its content [17].
Another considerable factor affecting performance properties of the cement materials is the water content required to provide a desirable consistency, determined by the chemical structure of the plasticizing admixtures used. However, the increase in the GGBFS content and the presence of alkali metal compound leads to a diminution in the effectiveness of plasticizing admixtures [41]. The principles to be considered for the choice of suitable admixtures depending on the content of GGBFS and the alkali metal compound in cement have been proposed: plasticizing admixtures [42,43], shrinkage reducing admixtures [44,45,46] and expanding admixtures [47,48]. For example, the highest plasticizing effect in the case of concretes based on the mentioned cement can be provided thanks to sodium lignosulfonate, sodium gluconate, polyols and by other acyclic low and high molecular compounds [42].
However, the corrosion of steel reinforcement resulting from the action of chlorides and sulfates is another concern with regard to the concretes mixed with sea water [49]. The corrosion of steel reinforcement in concretes mixed with sea water can be prevented by decreasing the contents of Cl−-ions in a pore solution through their chemical adsorption by hydrates of cement [50]. It is also well-known that low soluble hydration products of GGBFS activated by alkali metal compounds are able to bind Cl−-ions, both in a physical and chemical way [51]. The alkaline aluminosilicate hydrates, being analogues of natural zeolites [17], can occlude Cl−- and SO42−-ions. Moreover, Cl−-ions can be chemically bound by hydrotalcite ([Mg3Al(OH)8]Cl·3H2O) and hydrocalumite (3CaO·Al2O3·CaCl2·10H2O) [52].
The AFm phases (Al2O3-Fe2O3-mono) are characterized by a greater stability, in comparison to the AFt phases (ettringite), when increasing the alkalinity of the hydration medium [53,54]. The AFm phases can include various anions: Cl−, SO42−, CO32−, OH−, etc. The AFm phases can be represented by monocarboaluminate, hemicarboaluminate, stratlingite, hydroxy-AFm and monosulfoaluminate [55]. The presence of the nitrate-containing AFm phase, as well as those containing Cl− and SO42−-ions, has also been demonstrated in [56,57].
In this regard, the use of complex additives based on salts of strong acids has been proposed to minimize drying shrinkage, as well as in order to enhance crack resistance of the plasticized alkali-activated slag cement concretes, as a result of the lower water requirement, accelerated crystallization, alteration of pore structures and morphology of the hydrated phases [56].
The above results show that the use of sea water as a mixing water can lead to insufficient durability of the reinforced concrete structures due to the lower strength and deterioration of the pore structure, as well as the corrosion of steel reinforcement, under the action of Cl− and SO42−-ions. All this requires finding effective solutions to mitigate the content of the Cl− and SO42−-ions in the pore solution.
A solution is to use, as admixtures, the calcium aluminate cement and natural zeolite in order to bind the Cl− and SO42−-ions both physically and chemically and to enhance the durability of concretes mixed with sea water. Calcium aluminate cement was chosen to initiate the formation of high-calcium aluminate hydrates (3CaO∙Al2O3∙10H2O) due to their interaction with Portland cement clinker [58]. In its turn, 3CaO∙Al2O3∙10H2O ensures the binding of Cl− and SO42−-ions by the AFm phases, like 3CaO∙Al2O3∙CaCl2 (SO4)∙10H2O [15]. The application of natural zeolite (clinoptilolite) can also be a means to enhance the durability of concrete due to the occlusion of the Cl− and SO42−-ions. Clinoptilolite adds to the occlusion function of the alkaline aluminosilicate hydrates (analogues of natural zeolites) during hydration of the blast furnace cement.
This entry is adapted from the peer-reviewed paper 10.3390/ma15093003
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