Enhancement of alkali-activated slag cement concretes crack resistance for mitigation of steel reinforcement corrosion

The paper is devoted to mitigation of steel reinforcement corrosion in alkali-activated slag cement (further, AASC) concretes, based on soluble sodium silicates (further, SSS’s), obtained from high consistensy concrete mixes. Enhancement of AASC fine concretes crack resistance due to modification by complex shrinkage-reducing additives (further, SRA’s) based on surfactants and trisodium phosphate Na3PO4.12H2O (further, TSP) was proposed for mitigation of steel reinforcement corrosion. SSS’s were presented by sodium metasilicate (silica modulus 1.0, dry state) and water glass (silica modulus 2.9, density 1400 kg/m3). In case of sodium metasilicate the application of SRA composition “ordinary portland cement clinker – TSP – sodium lignosulphonate – sodium gluconate” provides enhancement of crack resistance starting from early age structure formation with restriction of drying shrinkage from 0,984 to 0,713 mm/m after 80 d. The effect is caused by reduction of water and by higher volume of crystalline hydrates. In turn, SRA presented by compositions “TSP – glycerol” and “TSP – glycerol – polyacrylamide” provide enhancement of AASC fine concretes fracture toughness during late structure formation with increasing ratio of tensile strength in bending to compressive strength up to 37 – 49 % if compare with the reference AASC when water glass is used.


Introduction
The actuality of green materials implementation is due to their conformity with modern tendencies of building industry development in the part of efficient consumption of raw materials and energy resources, responsible attitude to ecology of the environment, while ensuring high quality, functionality and durability of building materials. Thus eco-efficient composite cements, based on slag, zeolite, fly-ash, limestone as well as microsilica, ensure high early strength of mortars [1,2]. Application of a highly dispersed chalk as an additive in these cements increases strength, waterproof and freeze-thaw resistance of concretes [3,4].
Alkali-activated slag cement (further, AASC) are the most perspective environmentally friendly one in view of the modern tendencies of sustainable development. The ecological benefits of AASC's are caused by reduction of CO2 emission while consumption of byproducts as well as waste products [5,6,7]. AASC mortars and concretes are characterized by increased strength [8,9], heat resistance [10,11], corrosion resistance [12,13], freeze-thaw resistance [14,15] and waterproof [16] in comparison with analogues based on traditional clinker cements.
Thus, AASC's are effective for strategic construction objects with special destination (fortifications, sea ports, bridge foundations, tunnels, etc.), which must be exploited in various aggressive mediums. AASC based on soluble sodium silicates (further, SSS) is the most interesting type, first of all due to comparative strength benefits. This fact is relevant for special concretes.
High durability of AASC reinforced concretes obtained from harsh (low consistency) concrete mixes was already proved by long-term exploiting experience [17][18][19][20][21]. However, 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. The means for protection of reinforcement in such AASC concretes must be developed. One of decisions is the enhancement of crack resistance during all stages of service life cycle.
AASC concretes along with high strength are characterized by increased drying shrinkage and fracturing [22,23,24] due to high content of gel during initial structure formation. The drying shrinkage causes heterogeneous structure of concrete during further structure formation and as a result the unstable characteristics of artificial stone [25].
There are number of means to prevent fracturing, for example fiber reinforcement of concrete [26]. It's also well known, that surfactants allow regulating consistency while ensuring high strength of concrete. Diminution of drying shrinkage is a result of decreased water. Most of surfactants are ineffective for AASC and therefore the principles for their choice were proposed [27,28]. The maximum water-reducing effect in this case can be provided by sodium lignosulfonate [29], sodium gluconate [30], polyhydric alcohols [27], etc. Triatomic alcohol glycerol and polyacrylamide are the most perspective modifiers of AASC's based on water glass. These admixtures bind surface groups of silicic acid oligomers by means of hydrogen links while ensuring high strength, waterproof and elasticity of AASC mortars and concretes [31,32,33].
Another mean for drying shrinkage mitigation is complex application of admixtures and additives. It was shown the effectiveness of salts-electrolytes, like Na2SO4 [34], and by-pass cement kiln dust, which typical components are free CaO and salts (KCl, NaCl, K2SO4, Na2SO4, CaSO4, K2CO3, Na2CO3, CaCO3, etc.) [35,36]. Trisodium phosphate Na3PO4 . 12H2O (further, TSP) is well-known retarder of AASC setting time [37,38]. TSP also ensures effect of water glass stabilizer to prevent early coagulation while interaction with slag in AASC [39]. Moreover, TSP can be used as inhibitor of steel reinforcement corrosion due to formation of chemical stable products like hydroxyapatite Са10(РО4)6(ОН)2 [37] with formation of a dense protective film on metal surface [40,41], increasing polarization resistance of rebar [42,43] as well as decrease of capillary pore network with time [44].
The fundamentals of mineralogy and chemistry can give some explanations concerning effect of the mentioned compounds on the structure of AASC stone, namely about isomorphism with replacing of silicate or aluminate anion, formation of solid solutions or additional crystalline formations, which reduce the proper deformations [45]. These compounds can be used as complex modifiers of AASC.
Increasing of AASC's crack resistance in consequence of drying shrinkage mitigation can also be accomplished by increasing crystallinity of hydrated phases when using high-calcium additives such as ordinary portland cement clinker (further, OPC clinker) [18,46] or lime [47,48].
The above results allows to predict increasing of AASC crack resistance due to complex shrinkagereducing additives (further, SRA's) in the system "additives -surfactants". Specified method was proposed for mitigation of steel reinforcement corrosion in high consistency AASC concretes, modified with SRA's based on salts-electrolytes, i.e. Na2SO4 and NaNO3 [24,49,50]. The effectiveness of these SRA's was explained by their co-acting in crystallization processes, alteration of porous structure as well as morphology of hydrated phases.
The aim of this research was to ensure crack resistance of AASC concretes, based on SSS and plasticized by SRA's, for mitigation of steel reinforcement corrosion during all stages of structure formation.
The content of TSP was accepted experimentally while providing allowable initial setting time (not less 15-20 min). TSP was dissolved in water glass to provide density= 1400 кg/m 3 .
Normal consistency of cement pastes and setting time were determined according to the national standard of Ukraine DSTU B V.2.7-185:2009.
The standard quartz sand in accordance with EN 196-1 was used as a fine aggregate to determine strength and proper deformations of AASC concrete.
Cement pastes and fine concrete mixes were prepared in mixer Hobart type.
Water-reducing effect of SRA's was evaluated by decreasing W/C ratio in AASC/sand mixes (1:3) with slump flow= 106-115 mm on flow table according to the national standard of Ukraine DSTU B V.2.7-187:2009. The strength and proper deformations of AASC fine concrete were determined on specimens 40x40x160 mm. After manufacturing and hardening in forms with an insulated surface for 1 day, the samples were stored for 7 d under the normal conditions (t= 20 ± 2 ᵒC, R.H. = 95 ± 5 %). Then the samples were stored over saturated solution of potassium carbonate (NH4NH3) at t= 20 ± 2 ᵒC till the control age. The length of samples after 1 d was taken as the initial one (zero) in calculations.
Analysis of elastic-deformed state of AASC concretes under load was realized by bending test while determination of deformations up to destruction of samples (Fig. 1). Fracture toughness was estimated by the values of tensile strength in bending to compressive strength ratio. This method allows to determine effect of cement composition as well as structural features of cement on the values of deformation and destruction [51]. Research of microstructure of artificial stone was carried out by differential-thermal analysis (DTA) and electronic microscope with micro analyzer.

Results and discussion
Effect of SRA's in the system "additives -surfactants" on crack resistance of AASC fine concrete (further, concrete) are shown on examples. Effect of SRA composition "OPC clinker -TSP -LST -sodium gluconate" on drying shrinkage of AASC concrete was researched (Fig. 2). The reference AASC concrete is characterized by high drying shrinkage during the initial structure formation. SRA composition "OPC clinker -TSP -LST -sodium gluconate" ensures formation of AASC concrete with preferable structure and mitigation of drying shrinkage from 0,984 down to 0,713 mm/m after 80 d. The enhancement of AASC concrete properties can be provided not only by water-reducing effect of SRA, but also due to effect on further structure formation. The features of AASC structure, modified by SRA, were fixed by DTA (Fig. 3) and electronic microscopy (Fig. 4, 6) as well as by microzond analysis (Fig. 5, 7). According to DTA, the phase composition of hydrated AASC in absence of SRA (Fig. 3) is represented low-calcium hydrosilicates CSH(B). The endothermic effect at t= 150 °C is determined by their dehydration and exothermic effects at t=840 °C are determined by recrystallization into wollastonite. Endothermic effects at t= 150 and 700 °C (dehydration) and exothermic effect at t= 800 °C confirm formation of slightly crystallized calcium hydrosilicates like gyrolite 2CaO•3SiO2•2H2O.

Enhancement of crack resistance of alkaliactivated slag cement concretes based on sodium water glass
The effect of SRA based on TSP and surfactants on enhancement of crack resistance of AASC mortars based on sodium water glass was investigated.
The AASC, modified only by TSP (further, the reference AASC), was characterized by initial setting time - 19  The ratio Rbend/Rcomp increases up to 0.152 after 28 d. Thus SRA causes maintenance of Rbend/Rcomp ratio on 37-49 % in comparison with the reference AASC concrete. This is evidence of higher fracture toughness as well as crack resistance during late structure formation [51].
The features of AASC structures, modified by SRA of the mentioned compositions, were investigated by electronic microscopy (Fig. 8, 9, 10). Unlike to the reference AASC (Fig. 8), SRA "TSPglycerol" ensures increasing volume of hydrates presented by submicrocristalline compounds like spherolites as well as united into agglomerates and block-agglomerates (Fig. 9). SRA composition "TSPglycerol -polyacrylamide" increases depressiveness of hydrated phases and ensures increasing of phase's boundary area in artificial stone (Fig. 10). . Fig. 9. SEM images of alkali-activated slag cement, modified by shrinkage-reducing admixture composition "trisodium phosphate -glycerol", after 28 d of hydration.
Specified structures of modified AASC concretes testify their higher ability for relaxation and dissipation of outside energy.
Diagrams of AASC mortars elastic-deformed state in co-ordinates "stress -deformations" are shown on fig. 11, 12, 13. Some increasing of rigidity of AASC concrete, modified by SRA's, during initial load stage was fixed. Increasing of tangent angle for AASC concretes, modified by SRA compositions "TSP -glycerol" and "TSP -glycerol -polyacrylamide", in comparison with the reference AASC concrete from 65° (Fig. 11) to 66° (Fig. 12) and 67° (Fig. 13) agreeably confirm this fact.   SRA ensures higher specific micro cracking (Wn) under subsequent gradual increasing of load until destruction point of samples. Increasing of load resistance for AASC concrete, modified by SRA compositions "TSP -glycerol" and "TSP -glycerolpolyacrylamide" from 3310 N (for the reference AASC concrete) up to 3680 N and 3720 N agreeably was fixed. As well AASC concrete, modified by SRA's, can be characterized by increased specific energy of elastic deformation. Increasing in area of sections Wu by 39-52 % and maintenance of total bending deflection of samples up to 310 µm confirm this fact. Both mentioned SRA's ensure enhancement of fracture toughness, crack resistance and higher ability to deformation of AASC concretes.

Conclusion
1. The crack resistance enhancement of alkali-activated slag cement concretes, based on soluble sodium silicates and obtained from high consistency concrete mixes, was shown from initial up to late stages of structure formation. It was realized due to shrinkage-reducing admixtures based on trisodium phosphate and surfactants. This way the higher crack resistance ensures mitigation of steel reinforcement corrosion. 2. Modification of alkali-activated slag cement fine concretes by shrinkage-reducing admixture composition "ordinary portland cement clinker -trisodium phosphate -sodium lignosulphonate -sodium gluconate" causes less drying shrinkage during early age structure formation and mitigation of drying shrinkage from 0,984 to 0,713 mm/m after 80 d as well as enhancement of crack resistance during late structure formation. Effect of shrinkage-reducing admixtures is caused by reduction of water content and increasing crystallinity of AASC hydrates. 3. Shrinkage-reducing admixtures compositions "trisodium phosphate -glycerol" and "trisodium phosphate -glycerol -polyacrylamide" provide fracture toughness enhancement of alkali-activated slag cement fine concretes during late stages of structure formation. As result, the ratio between tensile strength in bending and compressive strength of AASC concrete can be increased up to 37 -49 % when water glass is used. At that increasing of bending deflection under load ensures higher elasticity of modified alkali-activated slag cement fine concretes. Specified effects of the shrinkagereducing admixtures are caused by increasing depressiveness of gel-like hydrates, providing redistribution of stresses in artificial stone and thereby advanced fracture toughness under load.
Authors express their gratitude to the Ministry of Education and Science of Ukraine for financial support of the research, that was performed in the framework of budget funding, as well as for the development of the theme of research according to the program of scientific cooperation COST Action CA15202 "Self-Healing concrete: the path to sustainable construction" of the European Union's framework program HORIZON 2020.