Prevention of steel reinforcement corrosion in alkali-activated slag cement concrete mixed with seawater

. Concretes mixed with seawater are characterised by enhanced performances, but action of chlorides and sulfates ensures the risk of reinforcement corrosion. Application of high consistency fresh concretes ensures changes in hardened concrete structure that causes the problem of steel reinforcement passive state ensuring. Thus mixing of plasticized concretes by seawater actualizes the search for means of steel corrosion prevention. Alkali-activated slag cements (further, AASC’s) reduce effect of ions Cl - and SO 42-on steel reinforcement in concrete due to their exchange for ions OH - in the structure of zeolite-like alkaline hydroaluminosilicates. Complex additive «portland cement - calcium aluminate cement - clinoptilolite» was proposed to enhance the protective properties of AASC concretes to steel reinforcement. The results of DTA, X-ray diffraction, electron microscopy, microprobe analysis show that complex additive ensures to prevent steel reinforcement corrosion in AASC concrete mixed with seawater due to binding Cl - and SO 42- ions in Kuzel's salt in AASC hydration products and exchange of these aggressive ions with OH - ions in the structure of clinoptilolite. This effect of complex additive confirmed by surface state and the absence of mass loss of steel rebars embedded in plasticized AASC fine concrete mixed with seawater after 90 d of hardening.


Introduction
The actuality of green materials implementation is due to their conformity with modern tendencies in construction engineering concerning efficient consumption of raw materials and energy resources [1], as well as responsible attitude to ecology of the environment [2]. Cements, which contain inorganic additives, fully comply modern tendencies for sustainable development of mankind [3]. The ecological benefits of such cements are caused by reduction of CO 2 emission due to substitution of portland cement clinker by inorganic additives [4]. At that, materials based on the mentioned cements are characterized by high quality, functionality and durability. Efficiency of high early strength multicomponent cements based on ground blast furnace slags (further, GBFS), zeolite and fly-ash in mortars was shown [5]. Additives of polydisperse zeolite tuff and perlite provide positive effect on concrete strength in different conditions [6]. Addition of zeolite or highly dispersed chalk enhances strength of cements [7], as well as crack resistance [8] and freeze-thaw resistance of concretes [9]. Portland cements containing GBFS are characterized by increased corrosion resistance and can be used in protective materials [10].
Alkali-activated slag cement (further, AASC) can be consider as the most perspective environmentally friendly ones. Alkali activation of aluminosilicate raw materials is widely used [11,12]. The ecological benefits of AASC's are caused by reduction of CO 2 emission while consumption of by-products as well as waste products [13]. AASC mortars and concretes are characterized by increased strength [14], heat resistance [15,16], corrosion resistance [17], freeze-thaw resistance [18], waterproof [19] and fire resistance [20,21] in comparison with analogues based on traditional clinker cements. Beside of high performances, AASC's can be used in decorative materials [22]. Radioactive wastes [23] and manufacturing waters [24] are effectively used in safety building materials based on AASC.
As perspective direction being taken to improve the environmental friendliness of modern concretes, has been using of seawater. It's known, that the consumption of water in concrete is 9 % of the total consumption of industrial water [25]. Thus, the most recent substantial interest has been placed on seawater as an alternative solution for impeding the exploitation of freshwater.
However, corrosion of steel reinforcement under action of chlorides and sulfates is the main problem for concretes mixed with seawater. There are two main processes during corrosion action on steel reinforcement: carbonation and pitting corrosion, caused by chlorine-ions [35]. Sulfate-ions don't cause immediate depassivation of steel reinforcement, but determine formation of hydrogen sulfide (H 2 S), which catalyze oxidation (carbonation) of hydrate phases.
The stability of passive film on the surface of steel reinforcement in AASC concrete, mixed with seawater, depends on concentration of free chlorine-ions and ОНions in pore solution, primarily in contact zone between reinforcement and concrete [36]. The passive state of steel reinforcement is provided at molar ratio of Cl -/OH -≤ 0.6 in pore solution [37].
The modern requirements to high consistency fresh concretes are governed by practice. This way the disturbance of reinforcement passive state can be caused by changes in hardened concrete structure [38]. Problem with ensuring of steel reinforcement passive state in AASC concretes, mixed with seawater, is intensified. Thus, the means for protection of steel reinforcement in plasticized AASC concretes must be developed.
It was shown, that the modification by complex additive (further, CAD) based on SSA's ensures decreasing of AASC concrete drying shrinkage as well as enhancement of crack resistance [39]. This type of modification provides mitigation of steel reinforcement corrosion. Effect of CAD is caused by less water, acceleration of crystallization, alteration of porous structure as well as by changes in morphology of hydrated phases [33,34].
One of the ways to prevent the steel reinforcement corrosion in AASC concrete mixed with seawater is decreasing of chlorine ions in pore solution due to their binding in low soluble compounds. It's well-known that hydration products of AASC are able to bind chlorineions. Alkaline hydroaluminosilicates, as analogues of natural zeolites, can be refered as such phases [29]. Apart from alkaline hydroaluminosilicates, free chlorine can be binded in Cl-bearing phases like hydrotalcite [Mg 3 Al(OH) 8 ]Cl·3H 2 O and hydrocalumite 3CaO·Al 2 O 3 ·CaCl 2 ·10H 2 O [40]. These phases are suitable for chlorine binding both due to exchange processes and physical adsorption [41,42,43]. It was shown the binding of chlorine-ions by hydrosilicate C-S-H and hydroaluminosilicate C-A-S-H gel phases [44,45].
The above results cause reasonable application of calcium aluminate cement and portland cement as additives in AASC to ensure formation of highly-calcium hydroaluminates 3CaO•Al 2 O 3 •10H 2 O for prevention of steel reinforcement corrosion due to binding of Cland SO 4 2-ions.
Highly-calcium hydroaluminates 3CaO•Al 2 O 3 •10H 2 O can be formed due to interaction of portlandite Ca(OH) 2 (hydration product of portland cement) with low-calcium hydrosilicates (hydration product of calcium aluminate cement) [51]: binding Cland SO 4 2ions in low soluble AFm phases [52]: Application of aluminosilicate ionites such as clinoptilolite (natural zeolite) in AASC concrete composition, mixed with seawater, can be mean for prevention of steel reinforcement corrosion. Zeolites are known, first of all, as cationites which exchange Na + ions with Ca 2+ ions, but can also act like anionites to exchange ОНions with Cland SO 4 2ions [53]. Clinoptilolite supplements anionite function of alkaline hydroaluminosilicates (analogues of natural zeolites) during AASC hydration.
The above results allow predicting prevention of steel reinforcement corrosion in plasticized AASC concrete mixed with seawater due to application of CAD «portland cement -calcium aluminate cement -clinoptilolite».
The aim of this research was to investigate the effect of seawater on the performances of plasticized AASC concrete and to ensure prevention of steel reinforcement corrosion due to application of CAD «portland cementcalcium aluminate cement -clinoptilolite». Consistency class of fresh concrete was S4 (slump 160 -210 mm).

Raw materials and methods
The performances plasticized of AASC concretes (AASC:sand = 1:3) were determined on specimens 40x40x160 mm.
The state of embedded steel rebars in plasticized AASC concrete, mixed with seawater, was estimated according to following method. The basic rebars, length (120±2) mm and diameter from 3 to 6 mm, were embedded in specimens 40x40x160 mm of AASC concrete. These rebars were degreased by acetone and weighted with accuracy of 0,001 g before embedding.
After hardening of specimens in normal conditions (t=202 °С, R.H.=95±5 %) the basic rebars were reached from AASC concrete and etched during (25±5) min in 10 % solution of hydrochloric acid with adding of urotropine (1 % by acid mass) to remove rests of cement stone and products of corrosion. The reference rebars, which weren't embedded in concrete, were weighted and etched simultaneously with basic rebars. After etching the basic and reference rebars were cleaned by distilled water and were immersed in fat solution of sodium nitrate for 5 min. Then the rebars were wiped by filter paper, dried up and weighted. The mass loss of rebars were calculated as a ratio of mean differences between masses before and after etching to surface area.
Monitoring the structure formation was carried out by X-ray diffraction (XRD), differential-thermal analysis (DTA) and electronic microscope with microanalyzer

Results and discussion
Effect of CAD «portland cement -calcium aluminate cement -clinoptilolite» on structure formation of AASC, mixed with seawater, was investigated.
The presence of Kuzel's salt in hydration products of AASC, modified by CAD and mixed with seawater, was confirmed by endothermic effects at t= 330 °C (dehydration) and 480 °С (departure of chloride) as well as by exothermic effect at t= 1000 °C (decomposition of sulfate) (Fig. 2, curve 2). Relocation of effects to higher temperatures ensures formation of CSH(B) and gyrolite with advanced crystallization. The modification of AASC by CAD causes formation of low-calcium hydrosilicates with a higher level of crystallization as well as hexagonal thin plates of Kuzel's salt (Fig. 4 a). The content of oxides confirms formation of Kuzel's salt, %: CaO -32,72, Al 2 O 3 -21,51, Сl -10,27, SO 3 -9,56) (Fig.4 b).

Structure formation of alkali-activated slag cement based on sodium metasilicate and mixed with seawater
XRD ensures formation of slightly crystallized lowcalcium hydrosilicates such as CSH(B) and gyrolite (Fig.5, curve 1). The presence of zeolite-like minerals (similar to nosean, sodalite, concrinite etc. by composition), which can bind Cland SO 4 2ions, can be assumed.
According to XRD, low-calcium hydrosilicates and Kuzel's salt were fixed as hydration products of AASC modified by CAD (Fig.5, curve 2).
These hydrates can be confirmed by DTA (Fig. 6, curve 2). Displacement of mentioned effects to higher temperatures ensures formation of CSH(B) and gyrolite with advanced crystallization.
The phase composition of modified AASC (Fig.8 a) is represented by low-calcium hydrosilicates as well as by hexagonal thin plates of Kuzel's salt (content in probe, %: CaO -31.22, Al 2 O 3 -24.87, Сl -12.43, SO 3 -10.94) (Fig.8 b).  Hence, CAD «portland cement -calcium aluminate cement -clinoptilolite» ensures binding of Cl-and SO 4 2-ions in structure of hydrated AASC based on soda ash or sodium metasilicate while mixing by seawater. Interaction between hydration products of portland cement and calcium aluminate cement provides formation of high-calcium hydroaluminates like 3CaO•Al 2 O 3 •10H 2 O, which bind the mentioned ions in Kuzel's salt. Clinoptilolite, as component of CAD, supplements action of alkaline hydroaluminosilicates (typical hydration products of AASC) with replacement of ОН-ions to Cl-and SO 4 2ions.
Specified structure formation of AASC, mixed with seawater, ensures prevention of steel reinforcement corrosion due to minimization of free Cland SO 4 2ions in pore solution of artificial stone.

Effectiveness of complex additive in alkaliactivated slag cement plasticized fine concrete
The effect of CAD «portland cement -calcium aluminate cement -clinoptilolite» on protective properties of plasticized AASC fine concrete, mixed with seawater, to steel reinforcement was investigated. Mass loss of steel rebars, which were reached from plasticized AASC concrete after 90 d of hardening was fixed. AASC's, based on soda ash as well as on sodium metasilicate, were used as bases of fine concrete. AASC fine concrete was modificated by CAD «portland cement -calcium aluminate cement -clinoptilolite». The CAD composition, %: portland cement -34.2, calcium aluminate cement -15.8, clinoptilolite -50.0. AASC fine concrete was plasticized by LST and sodium gluconate.
W/C ratios in AASC concrete based on soda ash and sodium metasilicate were 0.46 and 0.43 agreeably. W/C ratios in AASC concrete while mixing with seawater were 0.44 and 0.41 agreeably. This is evidence of AASC fresh concrete workability increasing while mixing with seawater [54]. At that, increasing of AASC concrete strength while mixing with seawater was fixed. Thus, compressive strength of AASC concrete based on soda ash while mixing with seawater corresponds to values of 35.7 MPa that is on 21.8 % greater comparing with one mixed with water (29.3 МPа). Compressive strength of AASC concrete, based on sodium metasilicate and mixed with seawater, is on 10.9 % greater comparing with one mixed with water (45.8 and 41.3 МPа agreeably). Enhancement of AASC fine concrete strength while mixing with seawater comparing with one mixed with water is caused by effect of SSA's [30,33,34].
The effect of CAD on protective properties of plasticized AASC concrete, mixed with seawater, to steel reinforcement was investigated.
Mass loss of steel rebars weren't fixed. This is evidence of no effect of Cland SO 4 2ions on steel reinforcement in plasticized AASC fine concretes, modifided by CAD and mixed with seawater.
The state of steel rebars before embedding (Fig. 9, 10) and embedded in specimens after 90 d of hardening (Fig. 11, 12) were compared. The absence of corrosion processes on surface of steel reinforcement in plasticized AASC concrete, mixed with seawater, was also confirmed by visual control.
Thus, effectiveness of CAD in plasticized AASC fine concrete for enhanced protective properties to steel reinforcement was confirmed.

Conclusion
1. Increasing of fresh concrete consistency as well as advanced strength of concrete based on alkali-activated slag cement while mixing with seawater was shown.
2. Prevention of steel reinforcement corrosion under the action of chlorides and sulfates in plasticized alkaliactivated slag cement concrete mixed with seawater was ensured due to application of complex additive «portland cement -calcium aluminate cement -clinoptilolite».
3. Enhancement of protective properties of plasticized alkali-activated slag cement concrete, mixed with seawater, to steel reinforcement is explained by binding of ions Cland SO 4 2in Kuzel's salt (AFm phase) as well as by exchange of ОНions with these aggressive ions in structure of clinoptilolite.   Authors would like to acknowledge the contribution of the COST Action CA15202 «Self-Healing concrete: the path to sustainable construction» of the European Union's framework program HORIZON2020, http://www.cost.eu/COST_Actions/ca/CA15202. The authors also express their gratitude to the Ministry of Education and Science of Ukraine for financial support of this research that is carried out within the budgetary financing of topic with registration No 1020U001010 and implementation period 2020 -2022.