Sustainable performance of alkali-activated blast furnace cement concrete with high freeze-thaw resistance

The application of blast furnace cements with minor clinker constituent is an actual task due to their conformity with modern tendencies of sustainable development. The alkali metal compounds were proposed to increase activity of CEM III/C. The aim of the research was to investigate the effects of technological factors on porous structure of alkali-activated blast furnace cement concrete (further, AABFC concrete) to ensure its sustainable performance by criterium of freeze-thaw resistance in NaCl solution. The effects of fresh concrete consistency, aggregate state of alkaline component and curing conditions on sustainability of AABFC concrete were investigated. Increasing of fresh concrete consistency from class S1 up to class S4 due to chemical plasticization as well as application of alkaline component in dry form, in contrast to liquid form, ensures negative changes in porous structure of AABFC concrete. These changes cause decreasing of freeze-thaw resistance from mark F500 down to F200. It was revealed that hardening of plasticized AABFC concrete under normal conditions (t = 20±2 ° C, RH = 95±5%), compared with hardening in water or under steam curing (t = 85±5 ° C), ensures more effective porous structure which causes maintained freeze-thaw resistance of F300 in contrast to F200 and F250 agreeable.


Introduction
To decrease the content of clinker constituent in cements due to use of ground granulated blast furnace slag (further, GGBFS) [1], fly ash [2], limestone [3], natural zeolites [4], biochar (the carbon negative product of pyrolysis) [5], etc. is an effective way to meet the requirements of sustainable development.Besides of ecological aspect (effective consumption of raw recourses, reduction of CO 2 emission), such replacement causes diminution of prime and logistics costs, as well as provides the means to use local raw materials [6,7].However, these cements suffer from slow hydration kinetics, resulting in less early strength gain, which is main drawback in construction applications.
The most known ways to activate hydraulic properties of the cements containing GGBFS is to apply calcium [8] and sulfate activators [9].It is also known that low alkaline [10], nearly neutral salts [11] as well as Na (K) salts of strong acids [12] can be used as activators as well.However, insufficient strength of blast furnace cements in accordance with [13] and the possibility to increase only early strength are disadvantages of these means [14,15].It was shown, that the application of oxides [16] or salts of alkaline metals (sodium aluminate [17], 1254 (2023) 012003 IOP Publishing doi:10.1088/1755-1315/1254/1/012003 2 sodium carbonate and silicate [18]), which provide an high alkaline reaction in water, can be a solution to increase the efficiency of blast furnace cements with minor clinker constituent without decrease of strength.Thus, blast furnace cements activated by alkali metal compounds are in compliance with mandatory requirements [19].
The alkali-activated blast furnace cements (further, AABFC), obtained due to mentioned activation, are the most perspective ones to ensure advanced service life of concrete structures.To ensure the durability of constructions is one more current world tendency of sustainable development of mankind.This fact can be confirmed by numerous scientific researches concerning durability of structures which are exposed to different aggressive effects, i.e. atmospheric [20], freeze-thaw cycles [21], alkali of aggregate [22], seawater [23], high temperatures [24], fire [25,26], etc. Proposed recommendations to ensure durability of structures while use of recycled concrete [27][28][29].
In general cases, alkali-activated blast furnace cement concretes are characterized by advanced performances in aggressive environments such as corrosion resistance [30], sulfate resistance [31] and freeze-thaw resistance [32], including salt scaling resistance [33], in comparison with analogues based on traditional clinker cements.
It is well known that the durability of concrete structures cannot be evaluated using only one performance.The most important properties of concrete for a specified case are used for evaluation of durability depending on destination and environment [34].Resistance of concrete to freeze-thaw cycles and sodium chloride scaling is one of the criteria.The mentioned environment can be classified as exposure class XF4 (road and bridge decks exposed to deicer agent, splash zones of marine structures, etc.) according to [35].It is well known that NaCl is the most demanded salt among deicers [34,36,37] as well as the predominant one in seawater [38,39].The above results have been defined the choice of sodium chloride as corrosion environment to evaluate the durability of concrete to freeze-thaw resistance.
Increased freeze-thaw resistance of AABFC concrete in NaCl solution is caused by several factors, including features of hydrated phases.It is known, that interaction of sodium chloride with hydration products of Portland cement ensures destruction of concrete.Particularly, decreasing of Ca(OH) 2 in hydration products in consequence of leaching as well as exchange reaction with sodium chloride NaCl with formation of CaCl 2 leads to decreasing of basicity of highly-calcium hydrosilicates (the main hydration products of portland cement) [34,40].Besides, participation of sodium chloride in hydration processes of portland cement ensures transformation of monosulfate 3CaO [41,42].Formation of secondary ettringite can cause destruction of concrete due to crystallization pressure on porous sides while volume increases.Advanced freeze-thaw resistance of AABFC concrete in solution of NaCl is caused by the absence of portlandite in hydration products [43] as well as by the absence of ettringite or due to changes in its morphology in highlyalkaline hydration medium from filamentous, needle to prismatic as well as plate shape [44].
Solution of NaCl can also provide steel reinforcement corrosion in constructions in consequence of transport of aggressive Cl − ions in concrete [45].AABFC concrete is characterized by increased protective properties to steel reinforcement due to high content of gel-like hydrosilicates and formation of alkaline hydroaluminosilicates (analogues of natural zeolites), which can bind Cl − ions [43,46,47].The enhancement of steel reinforcement protection in AABFC concretes, obtained from high consistency mixes, was proposed [12,18,23].
Increased freeze-thaw resistance of AABFC concrete compared with portland cement concrete is caused also by influence of alkaline component on decreasing of freezing temperature of solution in porous space [43].It is well-known, that increasing of water volume while formation of ice by 8. . .9 % ensures pressure on sides of pores and causes destruction of concrete [34].However, freezing of liquid in pores occurs at temperatures below 0 • C because of solution in pores of stone is not pure water and contains soluble substances (alkalis, oxides and hydroxides of alkaline-earth metals, sulfates, etc.) [48].Increased content of compounds of alkaline metals in porous solution causes increased freeze-thaw resistance of AABFC concrete compared with portland cement [43,49].
The peculiarities of porous structure cause advanced freeze-thaw resistance of AABFC concrete.In common case, the porous structure of cement stone is presented by gel (1.5. . .10.0 nm), capillary (0.01. . .1.00 nm) and closed (10. . .500 µm) pores [50].At that, freeze-thaw resistance and accordingly durability of concrete are caused mainly by capillary porosity in contrast to strength of concrete, which depends on total porosity [34].Capillary effect, which is caused by dependence of freeze point from the size of pores, causes the influence of porous structure on freeze-thaw resistance of concrete.Water firstly freezes in capillary pores whereas one remains in liquid form in smaller gel pores while freezing.Thermodynamic unbalanced state causes motion force for removal of water from smaller to larger pores and occurs because of the pressure under water is higher than under ice [34].Increased gel phase while decreased volume of capillary pores compared with Portland cement [43] causes advanced freeze-thaw resistance of AABFC concrete.
Porous structure of AABFC concrete in one's turn is caused by technological factors such as consistency of fresh concrete, form (state) of alkaline component, curing conditions, etc.The modern requirements to consistency fresh concretes are governed by practice [51][52][53][54][55]. Consistency of fresh concrete is regulated by surfactants.Principles for choice of surfactants as the bases of complex admixtures were proposed [56,57].Effectiveness of complex admixture "polyorganohydrosiloxane -sodium lignosufonate -polyethylene glycol" for AABFC concrete was determined while providing both electrostatic and steric mechanism of plasticization [56].However, increasing of consistency causes negative changes in concrete structure, which lead to increasing of porosity and consequently less freeze-thaw resistance.
The features of AABFC technology, which are caused by different aggregative state of alkaline components and chemical admixtures (dry form or liquid form), provide various intensity in formation of hydrosilicate gel and, consequently, different performances of AABFC concrete [43].Thus, the aim of this research was to investigate the effects of technological factors on porous structure of AABFC concrete to provide its high freeze-thaw resistance in solution of sodium chloride as criterium of sustainable performance.

Raw materials and testing techniques
The main constituents of the blast furnace cement (CEM III/C, in accordance with EN 197-1:2011) were presented by: • ground-granulated blast furnace slag (further, GGBFS) (% by mass: CaO -47.30 ) was introduced into a concrete mixer in dry form (powder) or in liquid form (water solution, 1180 kg/m 3 ) in such a way that its content in the concrete (as Na 2 O-equivalent) would be equivalent.The contents of alkali metal compounds (alkaline activators) were taken over 100 % of the aluminosilicate components in accordance with [19].
WA was used to intensify grinding and to prevent sorption of water from air and to retain the properties of AABFC.Contents of CPA components, % by mass of AABFC, were: LST -1.00, WA -0.06, polyethylene glycol -0.50.
The AABFC components and a half of aggregates together with LST and mixing liquid (water or solution of the alkaline component) were properly mixed in mixer for 1 min, then the remaining part of aggregates was added and mixed together for the next 2 min.
Consistency (workability) was determined by cone slump according to the national standard of Ukraine [59].
The prepared concrete mixtures were placed into moulds and compacted under vibration at a vibrating table, then covered with a plastic film and placed into a chamber for hardening under normal conditions (t = 18±2 • C and RH = 95±5 %), where it was stored for 2 days until demoulding.A part of the specimens after taken from the moulds was placed for further hardening in water, the other part was left for hardening under normal conditions, and some specimens were steam cured at t = 85±5 • C.
Water absorption and porosity of the AABFC concrete were tested in accordance to national standard of Ukraine [60].The concrete cubes (100 mm) after 28 days of hardening were dried up to a constant weight at t = 105±10 • C.Then, the specimens were saturated with water until a constant weight would be obtained at t= 20±2 • C. The values of porosity were calculated from the values of average density and water absorption.
Freeze-thaw resistance of AABFC concrete (figure 1) was studied according to the third test method prescribed by the national standard of Ukraine [61].According to this accelerated method, the concrete cubes (100 mm) were saturated with a 5 % solution of NaCl at t= 18±2 • C and after that were subjected to freezing at t = -50 • C. Thawing was done in a 5 % solution of NaCl.A class of concrete in freeze-thaw resistance was designated as a number of alternate freezing and thawing at which a mean compressive strength decreased by no more than 5 %.The freeze-thaw resistance of concrete was assessed by the correspondence between permissible number of freezing-thawing cycles by the mentioned method and by the first (basic) method prescribed in mentioned standard.

Results and discussions
The effects of fresh concrete consistency, aggregate state of alkaline component and curing conditions as technological factors on porous structure of AABFC concrete were investigated to ensure its sustainability.

Effects of fresh concrete consistency and aggregate state of alkaline component
The porous structure and corresponding values of freeze-thaw resistance of AABFC concrete were compared.AABFC concrete was obtained with different consistency (table 1, figure 2, figure 3, figure 4): class S1 (reference) and class S4 (plasticized by CPA).It was revealed, that consistency of fresh concrete is important factor of porous structure.Thus, increasing of   2a) and decreasing volume of conditionally closed pores by 41.2 % (figure 2b).Less volume of conditionally closed pores determined formation of minor dense and more permeable structure, which caused deterioration of physical and mechanical properties of AABFC concrete, including freeze-thaw resistance decrease from mark F400 down to mark F200 (figure 3a).Application of alkaline component in liquid form provided the similar dependence.Changes in consistency from class S1 up to class S4 ensured increasing volume of open capillary pores by 14.3 % (figure 2a) and corresponding decreasing of conditionally closed pores by 20.7 % (figure 2b), that was factor of reduction of freeze-thaw resistance from mark F500 down to mark F300 (figure 3b).Specified changes in porous structure and freeze-thaw resistance of AABFC concrete was caused by increasing values of water/cement ratio (W/C) from 0.34 up to 0.36 or solution/cement ratio (S/C) from 0.32 up to 0.34 while application of alkaline component in dry form or liquid form agreeably.Less freeze-thaw resistance of AABFC concrete was also caused by increased air-entraining due to CPA.That is explained by decreasing of surface tension between water and air [62].
Thus, increasing consistency caused negative effect on freeze-thaw resistance of AABFC concrete that is in dissonance with the modern requirements to high consistency of fresh concrete.
Application of alkaline component in dry form, in contrast to liquid form, ensured higher volume of open capillary pores by 5.0 % and 10.0 % at consistency classes S1 and S4 agreeably.Lack in filling intensity of porous space by hydrosilicate gel caused this phenomenon (figure 2a).
Decreasing volume of conditionally closed pores by 58.0 % and 43.3 % consequently occurred (figure 2b).Specified changes in porous structure ensured decreasing tendency in freeze-thaw resistance of AABFC concrete from F500 down to F400 (figure 4a) as well as from F300 down to F200 (figure 4b).
Obtained regularities confirm expediency of alkaline component exactly in the liquid form, that is on contrary to modern requirements concerning production of AABFC's namely under "all-in-one" technology.

Effect of curing conditions
Normal curing conditions of AABFC concrete was more advisable if compared with curing in water or under steam curing in the view of formation of effective porous structure (table 2).
Porous structure of plasticized AABFC concrete, at consistency of class S4 and after hardening during 28 d under normal conditions, was characterized by decreased volume of open capillary pores by 5.0 % and 1.3 % as well as by increased volume of conditionally closed pores by 35.7 % and 20.0 % compared with analogues under water or steam curing (figure 5).Specified changes in porous structure contributed to formation of AABFC concrete with more dense and impermeable structure, which is able for self-healing.This phenomenon provided advanced freeze-thaw resistance mark F300 in contrast to marks F200 and F250 of analogues (figure 6).
Thus, effective technological decisions to ensure advanced freeze-thaw resistance of AASC concrete in solution of sodium chloride as criterium of its sustainability were determined.

Figure 1 .
Figure 1.A freezing chamber for testing freeze-thaw resistance.

Figure 2 .
Figure 2. The influence of consistency and form of alkaline component on the volumes of open capillary pores (a) and conditionally closed pores (b) of alkali-activated blast furnace cement concrete.

Figure 3 .
Figure 3. Open capillary porosity and freeze-thaw resistance of alkali-activated blast furnace cement concrete vs. form of alkaline component: dry form (a), liquid from (b).

Figure 5 .
Figure 5. Porous structure of alkali-activated blast furnace cement concrete vs. curing conditions.

Figure 6 .
Figure 6.The volume of open capillary pores and freeze-thaw resistance of alkali-activated blast furnace cement concrete vs. curing conditions.

Table 1 .
The porous structure and freeze-thaw resistance of alkali-activated blast furnace cement concrete versus form of alkaline component and consistency.

Table 2 .
The porous structure and freeze-thaw resistance of alkali-activated blast furnace cement concrete versus curing conditions.
its sustainable performance by criterium of advanced freeze-thaw resistance in solution of sodium chloride.Fresh concrete consistency, aggregate state of alkaline component and IOP Publishing doi:10.1088/1755-1315/1254/1/01200310 20 % and decreasing of conditionally closed porosity by 67 % that was accompanied by diminution of freeze-thaw resistance the concrete by 60 %. 3. It was revealed, that negative effect of plasticization can be compensated while dry form of alkaline component and sustainable performance of alkali-activated blast furnace cement concrete can be ensured due to hardening under appropriate temperature-humidity conditions.In that way, freeze-thaw resistance of the plasticized concrete while hardening in normal conditions, in contrast to hardening in water or under steam curing, increased up to 1.3. . .1.5 times in consequence of reduced open capillary porosity by 5.0 % and increased volume of conditionally closed pores by 36.0 %. and Cl − during transport in Portland cement concrete with steel reinforcement in hydraulic structures", project code S-LU-22-7.ORCID iDs P Krivenko http://orcid.org/0000-0001-7697-2437I Rudenko http://orcid.org/0000-0001-5716-8259O Konstantynovskyi https://orcid.org/0000-0002-7936-5699A Razsamakin https://orcid.org/0000-0001-5130-6059