Restriction of Cl- and SO4 2- Ions Transport in Alkali Activated Slag Cement Concrete in Seawater

The relevance of alkali activated slag cement (AASC) concretes for structures operated in seawater is due to their enhanced resistance in aggressive environments. The application of high consistency fresh concretes is accompanied by negative changes in their structure with higher penetration of aggressive environments. Thus, the methods to prevent corrosion of steel reinforcement in plasticized AASC concrete are actual for investigations. It is shown, that modification of plasticized AASC concrete (consistency class S4) by the complex «alumina cement - portland cement - clinoptilolite - trisodium phosphate (Na3PO4·12H2O)» restrict the transport of aggressive Cl- and SO4 2- ions. The results of DTA, XRD, electron microscopy, microzond analysis show that mentioned complex limits transport of the mentioned aggressive ions due to their binding by AFm phases in hydration products, exchange with OH- ions in the structure of clinoptilolite, formation of hydrated products of apatite group Ca5(PO4)3(OH, Cl). This was confirmed by qualitative reaction on Cl- and SO4 2- ions in concrete structure, as well as by assessing of surface and mass loss of steel bars embedded in AASC concrete after 9 months in seawater. It was ensured the advanced crystallization with densification of microstructure, which increases corrosion resistance of artificial stone.


Introduction
Current trends in construction engineering predetermine application of binder materials alternative to portland cement. It's well-known, that production of 1 ton of portland cement requires 1.5 tons of raw materials and is accompanied by 900 kg of CO2 emission [1]. World cement production causes 6…8 % of total CO2 emission and consumes of 12…15 % of total industrial energy [2]. Thus, production of low-emission cements, obtained by partial replacement of OPC clinker with additives, is demanded [3][4][5][6].
Alkali-activated slag cement (further, AASC) is the most perspective one in view of effective consumption of raw recourses and energy as well as responsible attitude to environment. The ecological benefits of AASC are caused not only by reduction of CO2 emission due to application of by-products as well as waste products [7,8,9], but also by possibility to utilize manufacturing waters in safety building materials [10]. AASC materials are characterized by advanced strength [11], heat resistance [12], corrosion resistance [13,14] and freeze-thaw resistance [15] in comparison with analogues based on traditional clinker cements.
The benefits of AASC cause the effectiveness of concrete constructions which are exploited in under the influence of seawater (foundations, berths, piers, coast-protecting structures, dams etc.). Durability of AASC concrete in such conditions was confirmed [16,17].
Peculiarities of AASC hydration products cause increased protective properties of AASC concretes to steel reinforcement in comparison with portland cement. It's well-known, that the aggressive ions can be chemically adsorbed by hydrosilicates C-S-H and hydroaluminosilicate C-A-S-H gel phases at the early stages of AASC hydration [18,19]. High content of gel-like low-calcium hydrosilicates causes advanced initial binding if compare with portland cement [20,21]. Moreover, occlusion of Cland SO4 2ions by alkaline aluminosilicates, which are analogues of natural zeolites, takes place [7]. Mentioned minerals form at late stages of structure formation [7,22] and can be characterized both as cationites and anionites [23]. Advanced protection of AASC concrete to steel reinforcement is also caused by presence of alkaline component which acts as cathode corrosion inhibitor [24,25].
Durability of AASC concretes, including reinforced concretes, which are obtained from harsh (low consistency) fresh mixes, was already proved by long-term exploiting experience [7,8,9]. However, the modern requirements to high consistency fresh concretes are governed by practice. This way, the increasing of porosity and, consequently, permeability can be caused by changes in hardened concrete structure. This causes the increasing transport of aggressive ions in AASC concrete and risk of disturbance of reinforcement passive state.
Thus, corrosion of steel reinforcement under action of chlorides and sulfates due to their penetration in concrete structure is the main problem for structures, which are exploited in seawater. There are two main processes during corrosion action on steel reinforcement: carbonation and pitting corrosion, caused by chlorine-ions [26]. The passive state of steel reinforcement is provided at molar ratio of Cl -/OH -≤ 0.6 in pore solution [27,28]. In one's turn, sulfate-ions don't cause immediate depassivation of steel reinforcement, but determine formation of hydrogen sulfide (H2S) and catalyze oxidation (carbonation) of hydrate phases.
Formation of AFm phases (Al2O3-Fe2O3-mono) was proposed to enhance protective properties of AASC concrete, which is obtained from high consistency fresh mix, to steel reinforcement. It's wellknown, that these phases can chemically bind anions (Cl -, SO4 2-, CO3 2-, OHetc.) [30]. The effectiveness of mentioned mean was confirmed by work [31]. The AFm phases form at early hydration and are characterized by greater stability comparing to AFt phases (ettringite) while increasing alkalinity of hydration medium [32].
Application of aluminosilicate ionite such as clinoptilolite (zeolite) in AASC concrete can restrict transport of aggressive ions. Admixture of clinoptilolite enhances occlusion function of alkaline hydroaluminosilicates (analogues of natural zeolites), formed during AASC hydration [23].
Furthermore, application of trisodium phosphate Na3PO4•12H2O (further, TSP) was proposed to enhance the inhibitory action of AASC alkaline component. TSP is the most widespread mixed corrosion inhibitor [36,37]. Inhibitory effect of TSP is caused by dense protective film presented by iron phosphates FePO4 and FePO4•2H2O on steel surface [38], increasing polarization resistance of steel [39] as well as due to formation of chemical stable products like hydroxyapatite Са10(РО4)6(ОН)2 [40]. Formation of crystal hydroxyapatite causes densification of structure due to filling of micropores microcracks in AASC concrete to decelerate diffusion of aggressive Cland SO4 2ions [38]. At that, hydroxyapatite can bind Clions in chlorapatite Ca5(PO4)3Cl to decrease their content in porous solution, also while their transport from aggressive medium [41]. Effectiveness of TSP in complex with surfactants (sodium lignosulphonate, sodium gluconate) as well as calcium-containing additive (OPC clinker) for prevention of steel corrosion was shown [42]. Complex of additives (further, CA) based on salts of strong acids ensures multy-effects: reduction of water content, acceleration of crystallization, alterations in morphology of hydrated phases [43,44].
It can be predicted advanced protective properties of AASC concrete, which is obtained from high consistency mixes and exploited in seawater, to steel reinforcement, due to application of CA which ensures chemical adsorption of Cland SO4 2ions by gel-like hydration products (C-S-H and C-A-S-H), chemical binding (AFm phases abd hydroxyapatite) as well as occlusion (zeolite-containing components and zeolite-like alkaline hydroaluminosilicates). Proposed CA acts as corrosion inhibitor as well.
Thus, the aim of this research was to investigate the possibility of restriction Cland SO4 2ions transport in plasticized AASC concrete, which is exploited in seawater, due to modification by CA including portland cement, calcium aluminate cement, TSP and zeolite.
Polyorganohydridosiloxane (liquid 136-41) was used for milling intensification of GBFS and stabilization of AASC properties. Content of admixture was 0.1 by mass of GBFS.
The AASC was also modified by CA, which components were presented by: Content of CA was 15.00 % by mass of AASC. Contents of CA components, %: (portland cement + calcium aluminate cement) -5.00, clinoptilolite -5.00, TSP -5.00. The ratio between portland cement and calcium aluminate cement was like 2.17:1.00 to form 3CaO•Al2O3•10H2O.
The standard quartz sand according to EN 196-1 was used in AASC fine aggregate concretes (ratio AASC to sand = 1:3).
Normal consistency AASC pastes were mixed with seawater for simulation of chemical binding of Clі SO4 2ions by AASC hydrate phases, as it takes place in contact zone of concrete surface with aggressive medium. Cement pastes were prepared in Hobart mixer. Consistency of AASC pastes was determined according to the national standard of Ukraine DSTU B V.2.7-185:2009. Monitoring was carried out by X-ray diffraction (XRD), differential-thermal analysis (DTA) and electronic microscope with microanalyzer.
Fresh concretes were prepared in mixer «Raimondi Iperbet» (Italy). Consistency (workability) was determined by cone slump according to the national standard of Ukraine DSTU B V. The state of steel reinforcement in plasticized AASC concrete was estimated according to following method. The basic steel rebars, length 120 mm and diameter from 4.1 mm to 4.3 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 cleansed by distilled water and were immersed in saturated solution of sodium nitrate for 5 min. Then rebars were wiped by filter paper, dried up and weighted. Mean mass loss of the basic as well as the reference rebars were calculated as ratio of mean differences in their mass before and after etching to surface area. Mass loss was calculated as a difference between mean loss of the basic and the reference rebars.

Results and discussions
Restriction of Cland SO4 2ions from seawater in plasticized AASC concrete was considered in view of advancing both chemical binding and occlusion while application of modifiers.

Chemical binding of Cland SO4 2ions
Effectiveness of CA components in AASC's for chemical binding of Cland SO4 2ions was investigated.
The presence of Kuzel's salt in hydration products of modified AASC, 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) (Figure 2, curve 2). Relocation of effects to higher temperatures is evidence of CSH(B) and gyrolite with advanced crystallization.  The presence of chlorapatite in hydration products of AASC co-modified by portland cement, high aluminate cement and TSP, while mixing with seawater, was confirmed by endothermic effects at t= 80 °C (dehydration) as well as by exothermic effect at t= 360 °C (Figure 2, curve 3). Advanced crystallization of specified phases was confirmed by relocation of effects to higher temperatures.
AASC based on sodium metasilicate XRD testifies to formation of low-calcium hydrosilicates such as CSH(B) and gyrolite in hydration products of the reference AASC mixed with seawater after 270 d of hydration (Figure 4, curve 1). The presence of zeolite-like minerals (similar to nosean, sodalite, concrinite etc. by composition), which can bind Cland SO4 2ions, may be assumed. The presence of mentioned hydrates was confirmed by DTA ( Figure 5, curve 1).
According to XRD (Figure 4, curve 2) and DTA ( Figure 5, curve 2) low-calcium hydrosilicates and Kuzel's salt were fixed in hydration products of AASC, co-modified by portland cement and high aluminate cement, while mixing with seawater. Displacement of mentioned effects to higher temperatures ensures formation of CSH(B) and gyrolite with advanced crystallization.   2co-modified by portland cement and calcium aluminate cement; 3co-modified by portland cement, calcium aluminate cement and TSP.
Thus, co-modification of AASC by poratland cement, high aluminate cement and TSP ensures binding of Cland SO4 2ions by calcium hydroaluminate 3CaO•Al2O3•10H2O and hydroxyapatite Са10(РО4)6(OH)2 in Kuzel's salt and chlorapatite agreeably. Specified structure formation of modified AASC ensures restriction of aggressive ions transport due to chemical binding in contact zone of plasticized AASC concrete with seawater.

Occlusion of Cland SO4 2іons
Occlusive additive of zeolite was applicated for additional restriction of aggressive ions transport in plasticized AASC concrete and enhancement of its protective properties to steel reinforcement. Effectiveness of CA including portland cement, calcium aluminate cement, TSP and clinoptilolite, was tested while consistency class S4 (slump 160 -210 mm) for AASC fresh concrete was provided. While mixing with seawater W/C ratios were 0.46 and 0.41 for cases of soda-ash and sodium metasilicate as alkaline components. The absence of Cland SO4 2ions transport in plasticized AASC concrete was confirmed by qualitative reactions by solutions of silver nitrate (AgNO3) and lead acetate (Pb(CH3COO)2) agreeably. The absence of white sediment on the surfaces of specimens confirmed this fact (Figure 7).

The state of steel reinforcement
The effect of CA on protective properties of plasticized AASC concrete was evaluated by mass loss tests of steel rebars. Mass losses of steel rebars, reached from plasticized AASC concrete (the reference as well as modified by CA) after 270 d of storing in seawater, were fixed (Table 1).
Generally, mass loss of steel rebars was in compliance with mandatory requirements (no more than 10 g/m 2 ) according to DSTU B V.2.6-181 that ensures protective properties of plasticized AASC concrete to steel reinforcement. In case of AASC concrete, based on soda ash, application of CA caused 3.7 times lesser mass loss than for the reference one. For AASC concrete, based on sodium metasilicate, application of CA resulted in 3.4 times smaller mass loss. The obtained results correlate with foregoing ones, concerning enhanced activity of AASC based on sodium metasilicate, if compare with soda-ash, that was caused by enhanced density of AASC concrete structure with advanced protective properties to steel reinforcement. 4. Conclusions 1. The enhancement of steel reinforcement protection in AASC concrete, which is obtained from high consistency mix and exploited in seawater, is possible due to restriction of Cland SO4 2 ions transport in structure while application of complex of additives including portland cement, calcium aluminate cement, trisodium phosphate and zeolite.
2. Restriction of aggressive ions transport from seawater into plasticized AASC concrete can be provided due to their chemical adsorption by gel-like phases, chemical binding in Kuzel`s salt 3CaO•Al2O3•0,5CaCl2•0,5SO4•10H2O and chlorapatite Ca5(PO4)3Cl as well as their occluding in structure of zeolite-containing admixture and hydrates presented by alkaline hydrosilicates. Advanced crystallization with densification of artificial stone restricts mentioned ions transport from seawater. 3. The 3.4…3.7 times lesser mass loss of steel rebars embedded in AASC concretes in comparison with non-modified ones, depending on anion of alkaline component, confirms advanced protective properties of plasticized AASC concrete, modified by proposed complex of additives after influence of seawater during 9 months.