Geopolymer coating as a protection of concrete against chemical attack and corrosion

In order to mitigate and prevent chemical attack and concrete corrosion one has to choose the right concrete to make it less permeable or isolate it from the corrosive environment by using a suitable coating. Here, we present the use of geopolymer mortar made of local waste materials and sodium silicate as a chemo-resistive coating for concrete. In this paper, the development of geopolymer mortar working parameters (e.g. setting time and adhesion to concrete) is described. Here, we investigated the resistance of concrete samples coated with geopolymer to 10% inorganic/organic acids and saturated solution of sodium chloride. During the soaking in corrosive environment, weight loss and compressive strength were measured. Geopolymer coating described in this work showed great resistance to organic and inorganic acids. Coating of concrete samples reduced their weight loss after exposure to organic acids from 15% to 2% with concomitant compressive strength loss from 49% to 9% of the initial value. The difference in chemical resistance for inorganic acids was even more visible. Coating of concrete samples reduced their weight loss after exposure to inorganic acids from 73% to 0.8% with adequate compressive strength loss from 96% to only 3.5% of the initial value.


Introduction
Concrete, the composite made of cement, water, sand and coarse aggregate, is one of the most common and versatile building materials used around the world [1].Its contribution to transportations and especially to road construction is considerable.Bridges, tunnels, and concrete roads are several examples of successful cement applications [2].Their durability depends on many factors, one of which is their resistance to chemical attack and corrosion.The susceptibility of concrete to corrosive factors (e.g.chloride and sulfate anions) is connected with the presence of capillary pores responsible for water and gas migration into its matrix, and the presence of calcium hydroxide from alite and belite mineral phases of Portland cement [3].Deterioration of concrete performance can be connected with the decrease of concrete matrix pH induced by its carbonation due to the formation of calcium carbonate in the reaction of calcium hydroxide and CO 2 , and the formation of highly expansive compounds (e.g.ettringite) in the reaction of sulfates with hydrated aluminate phases [4].
Generally, there are two ways to mitigate or prevent chemical attack.One has to choose the right concrete to make it less permeable or isolate it from the environment by using a suitable coating or modify the environment to make it less aggressive to the concrete [5].As mentioned, one of the most common ways is to cover concrete with epoxide/polyurethane resins or vinyl ether barrier coatings to prevent its chemical degradation.Selection of proper coating for concrete gives the best performance, durability and long-lasting effect.An alternative way for coatings based on organic polymers is the use of barrier coatings based on mineral binder.One example of such are geopolymer materials made of local waste materials.Their microstructure resembles that known from organic polymers.Similarly, as in the case of mentioned resins, it may be successfully used as a coating with great adhesion to concrete and valuable durability performance.
Geopolymer binder is a two component system made of aluminosilicate waste material (e.g.blast furnace slag, fly ash) and aqueous solution of silicate, both potassium or sodium, as an activator of geopolymerisation process.The combination of these two ingredients lead to the formation of amorphous material with the range of specific properties i.e. high early strength, freeze-thaw resistance and higher chemical resistance as compared to traditional mineral binders [6,7].The application of geopolymer material and its specific properties depends on the ratio between SiO 2 /Al 2 O 3 molar ratio of aqueous silicate solution and the use of specific fillers.
The aim of the study was to show the applicability of geopolymer mortar as a coating for traditional concrete.In this work, the adhesion strength of geopolymer mortar coating to concrete surface was measured.Also, the chemical resistance of concrete coated with geopolymer mortar to 10% inorganic and organic acids, and a saturated solution of sodium chloride was measured after 28 days.Also, the compressive strength of coated and uncoated concrete samples was measured after 28 days of exposure to mentioned corrosive agents.

Portland cement concrete and geopolymer mortar mix
Portland cement concrete.Concrete samples were prepared by mixing quartz sand and coarse aggregates with a half part of mixing water.After 3 to 5 minutes of continuous mixing in a concrete mixer cement was added, and then the second half of water with superplasticizer.Mixing was continued for another 2 to 5 minutes.The water to  Geopolymer mortar.Geopolymer mortar samples were prepared first by mixing loose materials to gain molar ratios as described in tab. 3.Such prepared loose material was added to the water solution of sodium silicate with additional water and mixed about 5 minutes to gain a homogenous mix.Then, quartz sand fractions were added and mixed for another 5 minutes.Geopolymer mortar coating was then applied with a paint roller on concrete samples and left to dry at room temperature.

Application parameters of geopolymer mortar
Consistence of fresh mortar.The fluidity of a fresh mortar was measured according to European standard PN-EN 1015-3:2000 "Methods of test for mortar for masonry.Determination of consistence of fresh mortar (by flow table)" [11].Each sample diameter was measured constant and equal to 160 mm.The consistence of each fresh mortar sample was measured for two independent test specimens.
Setting time.Setting time of geopolymer mortar was measured with the use of an automatic Vicat apparatus equipped with a cylindrical needle.All samples were measured according to European standard PN-EN 196-3:2016-12 "Methods of testing cement -Part 3: Determination of setting times and soundness" [12].The beginning of setting time was measured when needle of Vicat's apparatus penetrated the mortar by 6 ± 2 mm from the bottom of the mould.The end of setting time was measured when the needle penetrated mortar not more than 0.5 mm from the top of the mould.
Bond strength.Adhesion of geopolymer mortar to the surface of 30 × 30 × 4 cm concrete plate was measured according to PN-EN 1542:2000 "Products and systems for the protection and repair of concrete structures.Test methods.Measurement of bond strength by pull-off" [13].The concrete plate was covered with a 2 mm protective layer with the use of a paint roller and left for 6 days to dry.After the conditioning period 50 mm in diameter cylindrical shapes were drilled with a diamond core drill bit.The depth of drilling was around 15 ± 5 mm through the surface of mortar and concrete plate.The pull-head plate was bonded to the surface with the use of a suitable two-component epoxy adhesive.The pull-off test was performed 7 days after applying the protective layer.

Chemical attack and corrosion resistance
Samples treatment.Chemical resistance of geopolymer coating was measured against 10% inorganic acids (e.g.hydrochloric acid, nitric acid), 10% organic acids (e.g.acetic acid, lactic acid) and a saturated solution of sodium chloride.Concrete samples with and without a protective layer were placed in a chemically resistant tray and then filled up with the selected solution to half of the height of sample.Then, samples were soaked for one day and covered completely with the solution.Thereafter, samples were sealed to avoid evaporation of the solution.Samples were stored in this solution for 7, 14 and 28 days.
Weight loss measurement.Coated and uncoated concrete samples' weight was measured after 7, 14 and 28 days in the solution.Before measurement, the samples were rinsed with water and dried to constant weight.
Compressive strength.Compressive strength of covered and uncovered concrete samples was measured with the use of Compressive Strength Testing Machine (Toni Technik) according to European standard PN-EN 196-1:2016-07 "Methods of testing cement -Part 1: Determination of strength" [12].Compressive strength was measured after 28 days in the solutions.Samples were treated as described in section above.Before measurement, the samples were rinsed with water and dried to constant weight.

Results
Geopolymer mortar prepared according to the procedure described in the material and methods section possess suitable consistency for different application methods.It can be applied with the use of a trowel or paint roller.It sets within 4 hours from the beginning of the application.After the drying process, the bond strength to the surface is greater than 2 MPa (tab.4), which is comparable with commonly used epoxy resins [14].Weight loss.As described in the material and methods section 4 × 4 × 16 cm samples were placed in chemo resistant trays filled with a 10 % aqueous solution of organic and inorganic acids, and the saturated solution of sodium chloride.The weight loss of coated/uncoated samples was measured after 7, 14 and 28 days of soaking.Data presented in Figure 2 represent average weight loss ± standard deviation for n = 3 independent trials.Before the measurement of weight, samples were rinsed with water and dried to constant weight.The highest weight loss after 28 days was observed for uncoated concrete sample placed in hydrochloric and nitric acid and equal to 70% of basic weight (Fig. 2A).In the case of organic acids 15% weight loss was observed.The degradation of concrete was connected mainly with damage of dolomite grit and decomposition of C-S-H phase and formation of soluble calcium salts.In contrast, samples covered with geopolymer coating showed lower weigh loss during 28 days of soaking in corrosive environment.The weight loss was lower than 2%, both for organic and inorganic acids (Fig. 2B).Weight increase (< 3%) in the case of sodium chloride was probably connected with the precipitation of solid sodium chloride in capillary and micro-pores of concrete and the coating.Compressive strength.The loss of compressive strength of concrete samples was connected mainly with the degradation of concrete matrix (Fig. 3).As could be expected from weight loss observations, the compressive strength of samples was reduced by about 95% after soaking in inorganic acids and 50% after soaking in organic acids.As for sodium chloride, the observed strength decrease was below 10%.The reduction of compressive strength was inhibited when samples were covered with geopolymer coating.The reduction of compressive strength for keeping concrete in inorganic and organic acids was around 3.5% and 9%, respectively.Compressive strength loss for treatment with sodium chloride was equal both for coated and uncoated samples as tested with two-tailed t-test with P-value < 0.05 (Fig. 4.).The physical appearance of samples is presented in fig. 4.

Conclusion
Mineral based geopolymer coating was prepared from local waste materials -class F fly ash from coal burning plant Ostroleka, and ground granulated blast-furnace slag from the steel plant in Katowice.According to data presented in this paper, geopolymer coating possess promising protective properties in terms of barrier coating for standard Portland cement concrete.Nevertheless, one has to remember that long-term durability tests and additional evaluation at construction sites under real working conditions are needed to confirm this statement.As shown, geopolymer coating protects concrete from corrosive factors (e.g.hydrochloric acid, nitric acid and acetic acid).As an advantage to the commonly used resins, it does not contain volatile organic compounds.As reported previously by Davidovits, high resistance of geopolymer materials to chemical attack and corrosion is connected with their dense amorphous phase consisting of semi-crystalline 3D alumino-silicate microstructure and the presence of tetrahedral frameworks linked by shared oxygens as poly(sialates) or poly(sialate-siloxo) or poly(sialate-disiloxo) depending on the SiO 2 /Al 2 O 3 ratio in the system [15].The degradation and weight loss of geopolymer materials, where Si-O-Si and Si-O-Al structures are present, occurs after leaching of protective cations (e.g.Na + , K + and Ca 2+ ).More susceptible to degradation against proton attack are Si-O-Si structures.Here in the case of hydrochloric acid exposure, the bond cleavage process follows two steps: attachment of the proton to the lone electron pair of the siloxane oxygen, and decomposition to form silanol unit Si-OH and silicon-anion Si-Cl bond.In the case of nitric acid the degradation of geopolymer networks proceed also step by step.Firstly, unstable aluminum Al-end units with none bridging oxygens are extracted, and secondly, a zeolitic structure is formed, which causes samples strength loss [16].As showed in this work, further development of geopolymer mortar as a coating against corrosive factors can bring an alternative solution to commonly used polymeric resins.
Grain size distribution.Grain size distribution of CEM II and aluminosilicate materials used for geopolymer mortar were measured with the use of laser diffraction method.Light scattering by raw material grains in propan-2-ol was measured and analyzed with Malvern Mastersizer 2000 (Malvern Instruments) with optional Hydro 2000MU accessory.Data shown in Fig. 1.

Fig. 1 .
Fig. 1.Grain size distribution of raw materials used for the preparation of concrete and geopolymer mortar: (A) Class F fly ash; (B) ground granulated blast-furnace slag; (C) silica fume; (D) fly-ash Portland cement.

Fig. 2 .
Fig. 2. The comparison between weight loss of concrete samples (A) -uncoated, and (B) -coated with geopolymer coating (2 mm thin layer).Data represents results obtained after 7, 14 and 28 days of soaking in different solutions.Data are presented as mean ± standard deviation (n = 3).

Fig. 3 .
Fig. 3.The comparison between compressive strength loss of concrete slabs uncoated, and covered with geopolymer coating (2 mm thin layer).Data represent measures obtained after 28 days of soaking in different solutions.Data are presented as mean ± standard deviation (n = 3).

E3SFig. 4 .
Fig. 4. The comparison of appearance between concrete slabs with and without protective geopolymer coating applied.Image taken 28 days after soaking in 10 % solution of (A) acetic acid, (B) lactic acid, (C) hydrochloric acid and (D) nitric acid.Pictures taken after water purging.

Table 1 .
The chemical composition and loss on ignition (LOI) of waste materials used for the preparation of geopolymer concrete (data in table responds to wt%).FA -Class F fly ash, GGBFS -ground granulated blast-furnace slag, SF -silica fume, bdl -below detection limit.
After obtaining a homogenous mix 4 × 4 × 16 cm and 30 × 30 × 4 cm concrete samples were formed.The concrete composition made of Portland cement is described in table 2. The day after mixing and forming, samples were placed in water and seasoned for 28 days.

Table 2 .
Mix proportions of cement concrete and geopolymer mortar (data in table responds to percent by weight, wt%).

Table 3 .
Molar ratios of the geopolymer binder dry mix and geopolymer slurry.

Table 4 .
Bond strength of geopolymer mortar coating to the surface of concrete slab.