Protective Geopolymer Coatings Containing Multi-Componential Precursors: Preparation and Basic Properties Characterization

This paper presents an experimental investigation on geopolymer coatings (GPC) in terms of surface protection of civil structures. The GPC mixtures were prepared with a quadruple precursor simultaneously containing fly ash (FA), ground granulated blast-furnace slag (GBFS), metakaolin (MK), and Portland cement (OPC). Setting time, compressive along with adhesive strength and permeability, were tested and interpreted from a perspective of potential applications. The preferred GPC with favorable setting time (not shorter than 120 min) and desirable compressive strength (not lower than 35 MPa) was selected from 85 mixture formulations. The results indicate that balancing strength and setting behavior is viable with the aid of the multi-componential precursor and the mixture design based on total molar ratios of key oxides or chemical elements. Adhesive strength of the optimized GPC mixtures was ranged from 1.5 to 3.4 MPa. The induced charge passed based on a rapid test of coated concrete specimens with the preferred GPC was 30% lower than that of the uncoated ones. Setting time of GPC was positively correlated with η[Si/(Na+Al)]. An abrupt increase of setting time occurred when the molar ratio was greater than 1.1. Compressive strength of GPC was positively affected by mass contents of ground granulated blast furnace slag, metakaolin and ordinary Portland cement, and was negatively affected by mass content of fly ash, respectively. Sustained seawater immersion impaired the strength of GPC to a negligible extent. Overall, GPC potentially serves a double purpose of satisfying the usage requirements and achieving a cleaner future.


Introduction
Geopolymers, also known as inorganic polymers or alkali-activated materials (AAMs), are a cluster of materials synthesized with alkali-activation and subsequent geopolymerization at elevated or ambient temperature [1][2][3]. Davidovits distinguished geopolymers from AAMs in that the both pertain to different systems, respectively, from the perspective of material science [4]. Due to industrial by-products and/or other inexpensive aluminosilicate sources such as fly ash (FA), granulated blast interfacial agent that prevented reinforcing bars embedded in reduced-scale concrete members from corrosion [28]. Wiyono reported that surface soundness of pozzolan concrete can be enhanced through the application of GPC layer [29]. Lv and co-authors synthesized a powder GPC by mixing MK and solid water glass along with additives and fillers [30]. They claimed that the dry-mixed coating material has been successfully applied in indoor engineering. Wang and Zhao issued a tentative study on silica fume-based GPC for fireproof plywood that was modified with decanoic-palmitic eutectic mixture [31]. In addition, from a point of view of sustainability, the carbon footprint of GPC is several times lower than that of traditional organic-based coatings [32]. The GPC prepared from multi-componential precursors at ambient temperature and its applications on concrete substrate, however, has seldom been studied, thus hindering a better understanding towards potential utilization of geopolymers.
The objective of this study, hence, is to design and prepare green GPC mixtures comprised with a quadruple precursor at ambient condition and to correlate basic engineering properties of GPC to its several key influencing factors. This paper is structured as follows: In the first step, a series of GPC were formulated and prepared based on the proposed methodology. Secondly, setting time and mechanical strength alongside rapid chloride permeability of the prepared and optimized GPC mixtures were experimentally investigated. The correlations between these properties and key mixture parameters of GPC were interpreted. Moreover, mechanical strength of GPC cured in standard condition and in artificial seawater was compared. In the end, the adhesive strength of GPC to a typical concrete substrate was evaluated with a portable apparatus. As a complement, the potential sustainability of this type of geopolymer-based applications was briefly reviewed.

Materials
The constituent materials used can be classified into three parts in terms of their functions, i.e., aluminosilicate precursors (or binders), alkali activators and additives. The precursor includes FA, GBFS, MK and ordinary Portland cement (OPC) as per Chinese standards and technical specifications. All raw materials used in the study are supplied by Zhejiang Huawei Building Materials Group, Co., Ltd., in Hangzhou, China. Compared to one single precursor, the quadruple combinations were expected to complement each other's advantages with regard to chemical compositions, specific properties (e.g., particle size distribution and reactivity), economical cost, as well as sustainability. X-ray fluorescent (XRF) analysis (Olympus, Tokyo, Japan) was carried out to obtain oxide compositions of the precursors as shown in Table 1. In order to make the GPC protective layer as compact as possible, the overall particle size distribution (PSD, Lab Synergy, New York, NY, USA) of the multi-componential precursors was optimized with the aid of EMMA (Version 1, Elkem, Olso, Norway) [33], a software package of particle packing calculation. Figures 1 and 2 respectively show scanning electron micrographs (SEM, Nanoscience, Phoenix, AZ, USA) and the laser-based PSD of the precursors. The FA mainly consists of spherical particles of glass, while the GBFS, MK and OPC are all composed of irregular and angular particles that are similar with previously published results [34][35][36]. The MK possesses the finest particles in all the precursors used, as clearly shown in Figures 1 and 2. It was expected to effectively fill spaces between coarser particles and densify the overall quadruple binder system. The GBFS and OPC with higher reactivity are beneficial to geopolymerization and development of mechanical properties of GPC at ambient temperature.
Materials 2020, 13, x FOR PEER REVIEW 4 of 23 particle packing calculation. Figures 1 and 2 respectively show scanning electron micrographs (SEM, Nanoscience, Phoenix, AZ, USA) and the laser-based PSD of the precursors. The FA mainly consists of spherical particles of glass, while the GBFS, MK and OPC are all composed of irregular and angular particles that are similar with previously published results [34][35][36]. The MK possesses the finest particles in all the precursors used, as clearly shown in Figures 1 and 2. It was expected to effectively fill spaces between coarser particles and densify the overall quadruple binder system. The GBFS and OPC with higher reactivity are beneficial to geopolymerization and development of mechanical properties of GPC at ambient temperature.   Materials 2020, 13, x FOR PEER REVIEW 4 of 23 particle packing calculation. Figures 1 and 2 respectively show scanning electron micrographs (SEM, Nanoscience, Phoenix, AZ, USA) and the laser-based PSD of the precursors. The FA mainly consists of spherical particles of glass, while the GBFS, MK and OPC are all composed of irregular and angular particles that are similar with previously published results [34][35][36]. The MK possesses the finest particles in all the precursors used, as clearly shown in Figures 1 and 2. It was expected to effectively fill spaces between coarser particles and densify the overall quadruple binder system. The GBFS and OPC with higher reactivity are beneficial to geopolymerization and development of mechanical properties of GPC at ambient temperature.   It is well-known that the geopolymerization process of geopolymer-based materials at ambient temperature is relatively slow and, therefore, a desirable strength is often difficult to obtain at early ages. Elevated temperature can accelerate the formation of geopolymer gel, and thus, enhance the strength. However, heat curing is often difficult to achieve in on-site applications. From the engineering and economical perspectives, therefore, it is essential to develop the GPC mixture with favorable mechanical strength under ambient condition. Based on the hypothesis that additional calcium silicate hydrate (C-S-H) gel coexists with geopolymer products, calcium-rich materials such as GBFS and OPC were introduced to shorten setting time and to improve mechanical properties of GPC. Additionally, heat release from hydration of the calcium-rich materials can accelerate the geopolymerization.
The alkaline activator was a blended aqueous solution of commercially available sodium hydroxide (NaOH pellets with 99.8% purity) and water glass (WG, with 27.5% SiO 2 , 8.7% Na 2 O and 63.8% H 2 O by mass) synthesized from industrial by-products. A polycarboxylate-based superplasticizer (SP, with 26.2% solid mass content and 25.8% water-reducing ratio) and polypropylene fiber (PPF, with length of 6-10 mm and aspect ratio of 400-600) were used as functional additives of GPC. The former is favorable to improve workability of fresh GPC mixture while the latter is beneficial to toughness and cracking resistance of the hardened GPC layer. It is noteworthy that, once cracking occurs, a penetration pathway for corrosive agents will be emerged in the GPC layer and the protected substrate will be deteriorated.

Experimental Parameters
In this experimental investigation, primary engineering properties involving setting time, compressive and adhesive strength, along with permeability of a number of GPC mixtures were explored and interpreted. It is essential that GPC set neither too rapidly nor too slowly. In the first case, there might be insufficient time to operate the fresh mixture before it becomes too rigid. In the second case, too long a setting period tends to delay the work unduly, also it might postpone the actual use of the coating layer because of inadequate strength at the prescribed age. Consequently, initial and final setting time of GPC must be designated in a definite and reasonable range.
Compressive strength is the most frequently-used parameter for characterizing the engineering properties of hardened geopolymer-based materials [37,38]. Adhesive strength refers to the capability of a GPC layer to stick to a substrate. It is often measured by assessing the ultimate tensile stress to detach or unstick the hardened GPC layer perpendicular to the substrate. It is essential that GPC possesses robust impermeability to moisture and deleterious substances. As a typical scenario, the penetration and transport of chloride ions often resulted in deterioration of concrete structures in marine atmosphere. As a trial and error, we employed electric charge passed based on rapid chloride permeability test to characterize the protection effect of GPC.
It is noteworthy to state that all test data in this article are average values from two or three replicate samples on the basis of identical experimental configurations, respectively. Relative standard deviations (i.e., the ratio of standard deviation to average value) of all the presented data in figures and tables are in the range of 3-8%. The scatter of test data is totally lower and meets the requirements of experimental design. For these reasons, the error bars and statistical analyses of the experimental results were uniformly omitted.

Mixture Formulation
In order to achieve proper performance of end product of GPC, the synergy of precursor and alkaline activator requires to be fully taken into account. The procedures of mixture proportioning of GPC are schematically presented in Figure 3. For the sake of achieving the balance between setting time and mechanical strength, total molar ratios of SiO 2 /Al 2 O 3 , SiO 2 /Na 2 O and H 2 O/Na 2 O as well as Si/(Na+Al) in the GPC system were calculated and regulated. According to Davidovits [39,40] and Rangan [41], the geopolymer pastes with these molar ratios in the ranges of 3.3-4.5, 0.8-2.2 and 10-25 are suitable for usage of protective coatings, respectively. The additives such as SP, PPF and retarder were adopted to further improve the particular performance of the GPC mixtures. Due to the mass proportion of the precursor being predominant, the economic cost and sustainability of GPC depend on the quadruple compositions of precursors. Less MK and more FA were suggested.
Materials 2020, 13, x FOR PEER REVIEW 6 of 23 retarder were adopted to further improve the particular performance of the GPC mixtures. Due to the mass proportion of the precursor being predominant, the economic cost and sustainability of GPC depend on the quadruple compositions of precursors. Less MK and more FA were suggested. Based on the principles of mixture design, a series of GPC mixtures (85 groups) with trial and error were examined. The mixtures were listed in Table 2. As stated previously, the GPC mixtures containing multi-componential precursors were highlighted. It should be noted that, in Section 3, the experimental results will be interpreted in terms of various factors that affected the properties rather than serial numbers of the GPC mixtures.  Mixture design procedure of GPC: a step by step methodology of mixture formulation involving particle packing, environmental impacts, engineering performance, as well as material chemistry.
Based on the principles of mixture design, a series of GPC mixtures (85 groups) with trial and error were examined. The mixtures were listed in Table 2. As stated previously, the GPC mixtures containing multi-componential precursors were highlighted. It should be noted that, in Section 3, the experimental results will be interpreted in terms of various factors that affected the properties rather than serial numbers of the GPC mixtures.

Mixture Preparation
The preparation of the GPC mixtures in the laboratory consists of the following four steps. First of all, prescribed quantities of WG and NaOH were carefully blended and dissolved in the mixing water to obtain the activator solution with the required concentration. The desired concentration (or molarity) of NaOH is suggested to be within the range of 8-16 mol/L. However, 8-10 mol/L is adequate for most cases [38,42]. Too high alkalinity is prone to result in the efflorescence [43] and safety hazard of operations. The activator solution was cooled down and stabilized for at least 24 h before further mixing with the precursors. Secondly, the prescribed MK, GBFS, FA, and OPC combinations were fully mixed together to ensure the uniformity using a mechanical mixer. In the third step, the formulated alkaline solution was fully mixed with the specific additives. At this point, the liquid and solid components of the GPC mixture were prepared. Finally, the solid component was added to the liquid one and then the mixing process was performed to obtain the fresh GPC mixtures. All these steps were performed with constant temperature and relative humidity (RH) of 25 • C ± 2 • C, 60% ± 5%, respectively.

Experimental Procedures
Initial and final set of GPC mixtures was determined with Vicat needle method, which is in accordance with ASTM C191 [44] with necessary modifications [45]. The method requires casting the fresh mixture into a truncated ring with a height of 40 mm. The measurement started after 10-15 min of curing the GPC samples in an environmental chamber (25 • C ± 2 • C and 95% ± 5% RH) and continued with an interval of 5-15 min up to the final set.
Compressive strength of GPC was performed on 20 mm cubes at the ages of 3, 7 (or 10) and 28 days, respectively. The compacted specimens (six replicates for each test) with the steel molds were initially stored in the above-mentioned chamber (standard condition) until 24 h and then stripped, and placed again in standard condition up to the measured ages. The tests were performed on a mechanical testing machine with a capacity of 300 kN. To examine the latent influence of curing condition on compressive strength, a comparison group for each GPC mixture was continuously immersed in saline water until the test ages. The saline water was prepared with crude sea salt of a 7% concentration that is an analogue of marine environment.
Adhesive strength (more accurately, pull-off adhesion strength) of GPC was determined using a portable adhesion tester (HCTC-10, provided by Beijing HICHANCE Technology Co., Ltd. in China). This test method covers procedures for evaluating the adhesion of a coating on concrete. The test determines the greatest perpendicular force in tension that a surface area can bear before a plug of material is detached. Failure will occur along the weakest plane within the system comprised of the loading fixture, glue, coating system, and substrate, and will be exposed by the fracture surface. The fresh GPC was painted and coated on the surface of concrete substrate (with an area of 100 mm × 400 mm) that was previously polished and handled to a saturated and surface dry state. The thickness of GPC layer was 3 mm. As shown in Figure 4, after being exposed to standard conditions up to 5 and 26 days, five standard steel flanges with Φ50 mm smooth surface were glued on the GPC layer with epoxy resin (Figure 4a). After the resin was fully cured (~48 h), the flanges were detached using a portable tester and adhesive strength was displayed on the LCD screen ( Figure 4b). The failure interface in Figure 4c indicated that adhesive strength of GPC is comparably higher than that of the mortar part in concrete. In addition, the concrete substrate used was previously fabricated with the following mixture proportion (kg/m 3 ): 255 kg OPC, 206 kg FA, 846 kg river sand, 928 kg crushed stone, 160 kg water, and 9.5 kg SP. The concrete substrate was cast and cured in standard condition until the age of 28 days. The characteristic value of compressive strength is 40 MPa.  As the initial attempt, chloride permeability of GPC characterized through charge passed (Coulombs) was tested with a rapid method as per ASTM C1202 standard [46]. One cross section of the protected cylinder concrete specimen (Φ100 × 50 mm) was previously polished and coated with the GPC mixture to be examined. At the age of 28 days, the charge passed of the coated concrete specimen was determined with a specialized apparatus [46] in parallel with the control one without GPC. As the initial attempt, chloride permeability of GPC characterized through charge passed (Coulombs) was tested with a rapid method as per ASTM C1202 standard [46]. One cross section of the protected cylinder concrete specimen (Φ100 × 50 mm) was previously polished and coated with the GPC mixture to be examined. At the age of 28 days, the charge passed of the coated concrete specimen was determined with a specialized apparatus [46] in parallel with the control one without GPC.

Setting Time
The GPC mixtures solely prepared with GBFS precursor often give high early-age strength but very rapid set within 30 min. Commercial retarders for OPC have been proven ineffective in geopolymer systems [47]. Lv and Cui [30] reported that an addition of polycondensed aluminum phosphate can prolong the solidification of an inorganic geopolymer-based coatings. However, the authors were unable to achieve the observation in this study. It seems as if there is a dilemma between strength development and setting time of GPC. Replacing GBFS with other aluminosilicates and formulating multi-component precursors can prolong setting time to great extent. In this section, we will present and interpret a series of factors affecting the GPC's setting time.

Effect of Activator Composition
The influence of the alkaline activator on setting time of GPC mixtures can be separately explored from its triple compositional makeup, viz., the mass of water glass, NaOH and additional water. The experimental results are clearly plotted in Figure 5 respectively. In these scatter diagrams, the test data is based on the identical precursor combinations (MK = 125 g, FA = 225 g, GBFS = 125 g and OPC = 25 g) and the constant water-solid ratio (W/S = 0.55). In general, the relationships between the setting time and the activator composition were not quite monotonous. The intervals between the initial and final setting time of different mixtures were not long enough as those of the Portland cement paste did. However, for a first approximation, more water glass led to the extension of setting time while an increase of the quantity of NaOH or the mixing water resulted in a decrease of setting time, respectively. Keep in mind that the alkalinity and In general, the relationships between the setting time and the activator composition were not quite monotonous. The intervals between the initial and final setting time of different mixtures were not long enough as those of the Portland cement paste did. However, for a first approximation, more water glass led to the extension of setting time while an increase of the quantity of NaOH or the mixing water resulted in a decrease of setting time, respectively. Keep in mind that the alkalinity and the pH value of NaOH is much higher than water glass; their contributions on the activation and geopolymerization are different; and in turn the influence of setting time is somewhat distinguished. With regard to the mixing water, its impact manner and degree cannot seem to be explained with the dilution effect that is applicable to cement-based materials, even though the polycondensation of geopolymer produces water molecules [15,17,23,29].
As far as the effect of the precursors on setting time was concerned, it was observed that the style or mode of action of each component in the quadruple precursor failed to come to any agreement. Due to the fact that GPC is a complex mixture system, we should interpret physical mechanisms of various factors on its property from a more comprehensive viewpoint. Maybe, replacing the single components as impact factors with the total quantities/moles of specific oxides or chemical elements in the entire GPC system is more reasonable [23,26].

Effect of Key Molar Ratios
In this paper, the total molar ratios of several key oxides (i.e., SiO 2 , Na 2    According to the interrelation among setting time, η[Si/(Na+Al)] and alkaline silicate's molecule structure (ring or chain) of MK-based geopolymer developed by Arnoult et al. [48], as shown in Figure 6b, with a reference value of η[Si/(Na+Al)] = 1.1, were selected to explore a potential relationship between setting time and η[Si/(Na+Al)] of the GPC mixtures. The results are depicted in Figure 6a along with the data points collected from preceding studies [47,48]. It was obvious that the setting time Keep a constant value of η[Si/(Na+Al)] = 1.1 and the identical precursors; the relationships between setting time and the above-mentioned total molar ratios of key oxides are presented in Figure 7. Apparently, compared to the preceding correlations concerning setting time and single components, the setting time-total molar ratio relationships were definite and straightforward. It is also worth noting that the interval between initial and final setting time gradually increased with a decrease of η(SiO2/Al2O3) or η(Na2O/Al2O3) as well as an increase of η(SiO2/Na2O), respectively.
Ozer and Soyer-Uzun [49] characterized the effect of η(Si/Al) on reaction products of alkaliactivated MK with X-ray diffraction (XRD) and SEM technology. They found that the geopolymer product transformed from crystalline phase (zeolite-A) to amorphous microstructures (N-A-S-H gel) with the increase of η(Si/Al). The observation was recently further supported by Chen et al. [50]. The extension of setting time with the reduction of η(SiO2/Al2O3) was probably ascribed to this phase transformation, but the latent mechanism is still unclear. In the present usage of geopolymer, in order to obtain proper setting behavior, the appropriate η(SiO2/Al2O3) should be selected from the range of 3.7-4.0.

Effect of Preparation Method
It was generally suggested that the blended activator solution (water glass + alkali metal hydroxides + H2O) should be prepared at least 24 h in advance followed by mixed with the precursors. However, the reason why this emphasizes the procedure was not definitely stated. We took two sets of experiments randomly on two GPC mixtures to examine the effect of preparation of alkali-activator on basic properties such as setting time and compressive strength. In the control test, the activator was made as per the common procedures. Meanwhile, the mixed activator solution was immediately blended and stirred with the precursors in the parallel test. The results presented in Keep a constant value of η[Si/(Na+Al)] = 1.1 and the identical precursors; the relationships between setting time and the above-mentioned total molar ratios of key oxides are presented in Figure 7. Apparently, compared to the preceding correlations concerning setting time and single components, the setting time-total molar ratio relationships were definite and straightforward. It is also worth noting that the interval between initial and final setting time gradually increased with a decrease of η(SiO 2 /Al 2 O 3 ) or η(Na 2 O/Al 2 O 3 ) as well as an increase of η(SiO 2 /Na 2 O), respectively.
Ozer and Soyer-Uzun [49] characterized the effect of η(Si/Al) on reaction products of alkali-activated MK with X-ray diffraction (XRD) and SEM technology. They found that the geopolymer product transformed from crystalline phase (zeolite-A) to amorphous microstructures (N-A-S-H gel) with the increase of η(Si/Al). The observation was recently further supported by Chen et al. [50]. The extension of setting time with the reduction of η(SiO 2 /Al 2 O 3 ) was probably ascribed to this phase transformation, but the latent mechanism is still unclear. In the present usage of geopolymer, in order to obtain proper setting behavior, the appropriate η(SiO 2 /Al 2 O 3 ) should be selected from the range of 3.7-4.0.

Effect of Preparation Method
It was generally suggested that the blended activator solution (water glass + alkali metal hydroxides + H 2 O) should be prepared at least 24 h in advance followed by mixed with the precursors. However, the reason why this emphasizes the procedure was not definitely stated. We took two sets of experiments randomly on two GPC mixtures to examine the effect of preparation of alkali-activator on basic properties such as setting time and compressive strength. In the control test, the activator was made as per the common procedures. Meanwhile, the mixed activator solution was immediately blended and stirred with the precursors in the parallel test. The results presented in Figure 8 indicated that the activator after stabilizing 24 h led to a more prolonged setting time than its immediately mixed counterpart did. In other word, after the storage of a period of time, the reactivity of the activator reduced to some extent. This assertion would also be explained from the results of early-age mechanical strength of the both GPC mixtures made from the activators with different initial states: the GPC mixtures made from the immediately mixed activator obtained higher compressive strength at the ages of 3 and 7 days. The effects of the preparation method on the strength and setting time were also dependent upon the precursor combinations along with the specific age. The effect on the strength was more obvious at an earlier age [35,44].
Materials 2020, 13, x FOR PEER REVIEW 13 of 23 Figure 8 indicated that the activator after stabilizing 24 h led to a more prolonged setting time than its immediately mixed counterpart did. In other word, after the storage of a period of time, the reactivity of the activator reduced to some extent. This assertion would also be explained from the results of early-age mechanical strength of the both GPC mixtures made from the activators with different initial states: the GPC mixtures made from the immediately mixed activator obtained higher compressive strength at the ages of 3 and 7 days. The effects of the preparation method on the strength and setting time were also dependent upon the precursor combinations along with the specific age. The effect on the strength was more obvious at an earlier age [35,44].

Compressive Strength
Mechanical strength of geopolymer presented in the literature varied widely because of the specific attributes of constitute materials, methods of preparation and exposed conditions [51]. However, there is very limited research that has been performed on comprehensive evaluation of the influencing parameters. Thermal curing is often difficult to achieve and inconvenient for in-situ applications; therefore, it is desirable to develop a GPC that can come into effect at ambient temperature. Furthermore, what we found was that, in comparison with the influence of alkaline activator, the dependence between compressive strength and precursor composition of the GPC mixtures was much stronger and more robust.

Effect of Precursor Composition
As the binder of the geopolymer, the precursors and their make-up are of vital importance to strength of GPC. Since GBFS, FA and MK have been widely utilized to synthesize geopolymers, we proposed a blended precursor system involving these typical aluminosilicates along with an addition of OPC, supposing that their advantages are complementary. The respective proportions of these aluminosilicates in the quadruple precursor system were intended to optimize on the basis of compressive strength, more accurately the balance of strength and setting time of the GPC mixtures [21,35,44].
As shown in Figure 9, compressive strength of the GPC at various ages (3, 7, 10 and 28 days) was positively and negatively affected by the mass content of GBFS and FA in the quadruple precursor, respectively, regardless of activator's composition, W/S and curing condition. Keeping identical mass of (MK + FA) and GBFS, with an increase of mass content of MK, the GPC's compressive strength increased. The influences of these binder components on compressive strength strongly pertain to their chemical reactivity. The mass content ranges of the precursor components were normalized on the one hand by the particle packing and were limited on the other hand by the attributes. For instance, more than 35% MK will result in an increase of mixing water while more than 50% GBFS

Compressive Strength
Mechanical strength of geopolymer presented in the literature varied widely because of the specific attributes of constitute materials, methods of preparation and exposed conditions [51]. However, there is very limited research that has been performed on comprehensive evaluation of the influencing parameters. Thermal curing is often difficult to achieve and inconvenient for in-situ applications; therefore, it is desirable to develop a GPC that can come into effect at ambient temperature. Furthermore, what we found was that, in comparison with the influence of alkaline activator, the dependence between compressive strength and precursor composition of the GPC mixtures was much stronger and more robust.

Effect of Precursor Composition
As the binder of the geopolymer, the precursors and their make-up are of vital importance to strength of GPC. Since GBFS, FA and MK have been widely utilized to synthesize geopolymers, we proposed a blended precursor system involving these typical aluminosilicates along with an addition of OPC, supposing that their advantages are complementary. The respective proportions of these aluminosilicates in the quadruple precursor system were intended to optimize on the basis of compressive strength, more accurately the balance of strength and setting time of the GPC mixtures [21,35,44].
As shown in Figure 9, compressive strength of the GPC at various ages (3, 7, 10 and 28 days) was positively and negatively affected by the mass content of GBFS and FA in the quadruple precursor, respectively, regardless of activator's composition, W/S and curing condition. Keeping identical mass of (MK + FA) and GBFS, with an increase of mass content of MK, the GPC's compressive strength increased. The influences of these binder components on compressive strength strongly pertain to their chemical reactivity. The mass content ranges of the precursor components were normalized on the one hand by the particle packing and were limited on the other hand by the attributes. For instance, more than 35% MK will result in an increase of mixing water while more than 50% GBFS will lead to very rapid set. Therefore, when one formulates the GPC mixture, it is critical to balance the strength and setting properties.
Materials 2020, 13, x FOR PEER REVIEW 14 of 23 will lead to very rapid set. Therefore, when one formulates the GPC mixture, it is critical to balance the strength and setting properties.

Effect of Key Molar Ratios
According to our observations, the effects of key molar ratios on compressive strength were less evident than on setting time did. With regard to the influences of η(SiO2/Na2O) and η(SiO2/Al2O3), several investigations have been carried out. Cheng et al. reported that the MK-waste catalyst-based geopolymer samples with higher η(SiO2/Na2O) appeared to possess more compact structures, higher strength and lower porosity [52]. Gao et al. highlighted that the nano-SiO2 and MK-based geopolymer

Effect of Key Molar Ratios
According to our observations, the effects of key molar ratios on compressive strength were less evident than on setting time did. With regard to the influences of η(SiO 2 /Na 2 O) and η(SiO 2 /Al 2 O 3 ), several investigations have been carried out. Cheng et al. reported that the MK-waste catalyst-based geopolymer samples with higher η(SiO 2 /Na 2 O) appeared to possess more compact structures, higher strength and lower porosity [52]. Gao et al. highlighted that the nano-SiO 2 and MK-based geopolymer with the η(SiO 2 /Na 2 O) value of 1.5 exhibited higher strength and less porosity [53]. Wang et al. presented a comprehensive study on molecular structure and mechanical properties of geopolymers with different η(SiO 2 /Al 2 O 3 ) by means of experiments and simulation. They indicated that, with the increases of η(SiO 2 /Al 2 O 3 ), both the stability of molecular structure and mechanical properties of resultant geopolymer decreased [31]. Various zeolites were main products in geopolymers with a relatively low η(SiO 2 /Al 2 O 3 ) = 2.0-2.5. The formation of these zeolitic structures resulted in low-strength MK-based geopolymers [54]. These findings basically coincided with the relevant results in this work, so there is no need to repeat.
In with the η(SiO2/Na2O) value of 1.5 exhibited higher strength and less porosity [53]. Wang et al. presented a comprehensive study on molecular structure and mechanical properties of geopolymers with different η(SiO2/Al2O3) by means of experiments and simulation. They indicated that, with the increases of η(SiO2/Al2O3), both the stability of molecular structure and mechanical properties of resultant geopolymer decreased [31]. Various zeolites were main products in geopolymers with a relatively low η(SiO2/Al2O3) = 2.0-2.5. The formation of these zeolitic structures resulted in lowstrength MK-based geopolymers [54]. These findings basically coincided with the relevant results in this work, so there is no need to repeat.
In the case of the effects of η(H2O/Na2O), η[Si/(Na+Al)] and W/S, the corresponding experimental results are plotted in Figure 10. With the identical η(SiO2/Na2O), η(SiO2/Al2O3) and η[Si/(Na+Al)], the correlation between compressive strength and W/S was elusive. The observation was basically different from the findings reported by Juengsuwattananon et al. who indicated that the geopolymer's strength decreased with an increase of W/S [54]. They ascribed the phenomenon to dilution effect by adding more water to the system. More mixing water led to a reduction in pH value of the system and adversely affected the reaction rate. In nature, W/S or liquid-solid ratio are the practical forms of η(H2O/Na2O). However, the influences of η(H2O/Na2O) on strength varied with experimental configurations. When η(SiO2/Na2O), η(SiO2/Al2O3) and η[Si/(Na+Al)] were held constant, higher η(H2O/Na2O) led to greater strength. Otherwise, when the same precursors were used, the strength slightly decreased with the increase of η(H2O/Na2O). The effect of η[Si/(Na+Al)] was also inapparent.

Effect of Exposed Conditions
In Figure 11, we compared the standard cured and seawater-immersed GPC's compressive strength. It was easy to find that, although continuous seawater immersion impaired the strength, the loss of 7% was more or less negligible from a viewpoint of practical applications. That is to say, water resistance and anti-corrosion of the GPC mixtures are desirable, even though the exposed condition is harsh. This observation coincided with the results presented by Zhang et al. [27]. The strength reduction of the seawater-immersed specimens could be attributed to dilution of excessive water rather than the corrosion of seawater. Water reservoir also inhibited the polycondensation to some extent [36,51]. Due to excellent impermeability of the GPC, the dilution effect was limited to the surface layer.

Effect of Exposed Conditions
In Figure 11, we compared the standard cured and seawater-immersed GPC's compressive strength. It was easy to find that, although continuous seawater immersion impaired the strength, the loss of 7% was more or less negligible from a viewpoint of practical applications. That is to say, water resistance and anti-corrosion of the GPC mixtures are desirable, even though the exposed condition is harsh. This observation coincided with the results presented by Zhang et al. [27]. The strength reduction of the seawater-immersed specimens could be attributed to dilution of excessive water rather than the corrosion of seawater. Water reservoir also inhibited the polycondensation to some extent [36,51]. Due to excellent impermeability of the GPC, the dilution effect was limited to the surface layer. Figure 11. Correlating compressive strength of GPC immersed in artificial seawater to its counterpart stored in standard condition.

Several Critical Observations
With the exception of all above-discussed factors that affected basic properties of GPC, there are some critical experimental results that are worthwhile to discuss or interpret here.

A Threshold of Total Molar Ratio
In this subsection, with the same W/S, WG and precursor combination, the relationship between setting time/early-age strength and mass of NaOH/η(SiO2/Na2O) was discussed. In this experimental configuration, the only independent variable was the mass of NaOH.
From a phenomenological perspective, it seems as if there was a threshold of the mass of NaOH (30 g) that divided its influence on the strength as well as setting time into two distinct segments (see Figure 12a,b). In the first part, the setting time and strength decreased with an increase of the mass of NaOH, and in the second part, the reverse happened. Because η(SiO2/Na2O) consistently decreased with the increasing addition of NaOH, the correlations between setting time or early-age strength and η(SiO2/Na2O), as shown in Figure 12c,d, highly resembled one another. The threshold of η(SiO2/Na2O) equals to 4.56. Hamidi indicated that the geopolymer paste activated with 12 M NaOH solution obtained the greatest mechanical strength in an experimental study exploring the effect of molarity of NaOH (4-18 M) [55]. In addition, the molarity or content of NaOH in the alkali-activator affected the fluidity of the fresh GPC mixtures and solid particle size [56]. Higher concentration of NaOH solution often leads to better flowability and less particle agglomeration. Similar test results

Several Critical Observations
With the exception of all above-discussed factors that affected basic properties of GPC, there are some critical experimental results that are worthwhile to discuss or interpret here.

A Threshold of Total Molar Ratio
In this subsection, with the same W/S, WG and precursor combination, the relationship between setting time/early-age strength and mass of NaOH/η(SiO 2 /Na 2 O) was discussed. In this experimental configuration, the only independent variable was the mass of NaOH.
From a phenomenological perspective, it seems as if there was a threshold of the mass of NaOH (30 g) that divided its influence on the strength as well as setting time into two distinct segments (see Figure 12a,b). In the first part, the setting time and strength decreased with an increase of the mass of NaOH, and in the second part, the reverse happened. Because η(SiO 2 /Na 2 O) consistently decreased with the increasing addition of NaOH, the correlations between setting time or early-age strength and η(SiO 2 /Na 2 O), as shown in Figure 12c,d, highly resembled one another. The threshold of η(SiO 2 /Na 2 O) equals to 4.56. Hamidi indicated that the geopolymer paste activated with 12 M NaOH solution obtained the greatest mechanical strength in an experimental study exploring the effect of molarity of NaOH (4-18 M) [55]. In addition, the molarity or content of NaOH in the alkali-activator affected the fluidity of the fresh GPC mixtures and solid particle size [56]. Higher concentration of NaOH solution often leads to better flowability and less particle agglomeration. Similar test results were also observed in this research. The effects of the concentration of alkali metal hydroxide on properties of geopolymer are dependent on the change of zeta potential in the alkaline activator [57]. were also observed in this research. The effects of the concentration of alkali metal hydroxide on properties of geopolymer are dependent on the change of zeta potential in the alkaline activator [57].

Effect of OPC Addition
In the binder of geopolymer, OPC, which can cause environmental load such as CO2 emission, is often not recommended. However, as an important calcium source, the hydration of OPC has two roles: accelerate geopolymerization and produce calcium silicate hydrate (C-S-H) gel [58]. The coexistence of sodium aluminosilicate hydrate (N-A-S-H) and C-S-H will enhance the engineering properties of geopolymer-based materials [59]. In the quadruple precursor, the dosage of OPC should be determined in accordance with the balance between compressive strength and setting behavior of the prepared GPC.
As clearly depicted in Figure 13, setting time and strength properties separately shortened and raised with the increase of OPC replacement ratio in general. Besides, the effects were implicated with the molarity of NaOH in the activator solution; the higher the molarity, the stronger the effects. When the mass ratio of OPC was greater than 10% by total mass of precursor, the GPC's strength tended to decrease, especially the ones activated with higher concentration NaOH solution. Other calcium-rich compounds such as calcium hydroxide or calcium oxide can also be utilized to regulate setting time and mechanical properties; their replacement ratios should be tailored in terms of particular usages [16,22,34].

Effect of OPC Addition
In the binder of geopolymer, OPC, which can cause environmental load such as CO 2 emission, is often not recommended. However, as an important calcium source, the hydration of OPC has two roles: accelerate geopolymerization and produce calcium silicate hydrate (C-S-H) gel [58]. The coexistence of sodium aluminosilicate hydrate (N-A-S-H) and C-S-H will enhance the engineering properties of geopolymer-based materials [59]. In the quadruple precursor, the dosage of OPC should be determined in accordance with the balance between compressive strength and setting behavior of the prepared GPC.
As clearly depicted in Figure 13, setting time and strength properties separately shortened and raised with the increase of OPC replacement ratio in general. Besides, the effects were implicated with the molarity of NaOH in the activator solution; the higher the molarity, the stronger the effects. When the mass ratio of OPC was greater than 10% by total mass of precursor, the GPC's strength tended to decrease, especially the ones activated with higher concentration NaOH solution.
Other calcium-rich compounds such as calcium hydroxide or calcium oxide can also be utilized to regulate setting time and mechanical properties; their replacement ratios should be tailored in terms of particular usages [16,22,34].

Effect of GBFS Type
The properties of geopolymer are affected not only by mixture proportions of precursor and activator, but also by type and source of constituent materials. Figure 14 shows the compressive strength of GPC mixtures prepared from four different GBFS with the same recipes. GBFS-B and GBFS-D separately produced the highest and the lowest strength. It appears that this observation was not easy to interpret from key parameters of GBFS such as reactivity index, Al2O3 content or total content of Al2O3 and SiO2. The geopolymerization process is complicated and related with the phase morphology of the precursors apart from their combinations and oxide compositions. By the way, the influence of type of GBFS on setting behavior was also ignorable. At this point, the optimized 5 GPC mixtures presented in Table 3 with both practicable setting time (not earlier than 120 min) and desirable compressive strength (not less than 35 MPa) were selected from the investigated 85 mixtures listed in Table 2. In the course of seeking the optimized mixtures, a series of factors affecting the basic properties of GPC were explored and discussed. In particular, an eye was kept on the balance between strength and setting behavior at ambient temperature as well as the usage of quadruple precursors.

Effect of GBFS Type
The properties of geopolymer are affected not only by mixture proportions of precursor and activator, but also by type and source of constituent materials. Figure 14 shows the compressive strength of GPC mixtures prepared from four different GBFS with the same recipes. GBFS-B and GBFS-D separately produced the highest and the lowest strength. It appears that this observation was not easy to interpret from key parameters of GBFS such as reactivity index, Al 2 O 3 content or total content of Al 2 O 3 and SiO 2 . The geopolymerization process is complicated and related with the phase morphology of the precursors apart from their combinations and oxide compositions. By the way, the influence of type of GBFS on setting behavior was also ignorable.

Effect of GBFS Type
The properties of geopolymer are affected not only by mixture proportions of precursor and activator, but also by type and source of constituent materials. Figure 14 shows the compressive strength of GPC mixtures prepared from four different GBFS with the same recipes. GBFS-B and GBFS-D separately produced the highest and the lowest strength. It appears that this observation was not easy to interpret from key parameters of GBFS such as reactivity index, Al2O3 content or total content of Al2O3 and SiO2. The geopolymerization process is complicated and related with the phase morphology of the precursors apart from their combinations and oxide compositions. By the way, the influence of type of GBFS on setting behavior was also ignorable. At this point, the optimized 5 GPC mixtures presented in Table 3 with both practicable setting time (not earlier than 120 min) and desirable compressive strength (not less than 35 MPa) were selected from the investigated 85 mixtures listed in Table 2. In the course of seeking the optimized mixtures, a series of factors affecting the basic properties of GPC were explored and discussed. In particular, an eye was kept on the balance between strength and setting behavior at ambient temperature as well as the usage of quadruple precursors. At this point, the optimized 5 GPC mixtures presented in Table 3 with both practicable setting time (not earlier than 120 min) and desirable compressive strength (not less than 35 MPa) were selected from the investigated 85 mixtures listed in Table 2. In the course of seeking the optimized mixtures, a series of factors affecting the basic properties of GPC were explored and discussed. In particular, an eye was kept on the balance between strength and setting behavior at ambient temperature as well as the usage of quadruple precursors.

Adhesive Strength
The adhesive strength of the optimized GPC mixtures was measured through a portable tester, as listed in Table 3. All the test results were higher than 1.5 MPa, regardless of the measured age. Previous investigations indicated that the adhesive strength of coatings for most civil applications should not be less than 1.4 to 1.75 MPa, which varied from one testing method to another. According to the provisions in Chinese technical standards, the adhesive property of theses mixtures fully meets the requirement of surface protection. It is easy to understand that higher compressive strength often responds to higher adhesive strength.

Rapid Chloride Permeability
In order to characterize the compactness of GPC protective layer, as an easily determined indicator, charge passed based on rapid chloride permeability test was explored herein. The preferred mixtures were separately applied to the concrete substrate, and their respective permeability was compared to that of the naked substrate in Figure 15. Apparently, the coated substrate with the GPC obtained improved resistance to deleterious water-soluble ions. The average charge passed of the coated concrete specimens was approximately 30% lower than that of the uncoated ones. The transport mechanism of chemical species in the geopolymer will be further studied.  The adhesive strength of the optimized GPC mixtures was measured through a portable tester, as listed in Table 3. All the test results were higher than 1.5 MPa, regardless of the measured age. Previous investigations indicated that the adhesive strength of coatings for most civil applications should not be less than 1.4 to 1.75 MPa, which varied from one testing method to another. According to the provisions in Chinese technical standards, the adhesive property of theses mixtures fully meets the requirement of surface protection. It is easy to understand that higher compressive strength often responds to higher adhesive strength.

Rapid Chloride Permeability
In order to characterize the compactness of GPC protective layer, as an easily determined indicator, charge passed based on rapid chloride permeability test was explored herein. The preferred mixtures were separately applied to the concrete substrate, and their respective permeability was compared to that of the naked substrate in Figure 15. Apparently, the coated substrate with the GPC obtained improved resistance to deleterious water-soluble ions. The average charge passed of the coated concrete specimens was approximately 30% lower than that of the uncoated ones. The transport mechanism of chemical species in the geopolymer will be further studied. Figure 15. Rapid chloride permeability of coated and uncoated concrete substrate characterized through accelerated charge passed: "Non-GPC" stands for uncoated concrete substrate, and "OP-x" stands for the optimized GPC mixtures listed in Table 3.

An outlook on Sustainability
Apart from the engineering properties, the cost, availability and environmental compatibility of GPC are also critical to its potential application. The complexity of aluminosilicate sources, especially aluminosilicate-containing industrial by-products, is a major concern for GPC production. The multi- Figure 15. Rapid chloride permeability of coated and uncoated concrete substrate characterized through accelerated charge passed: "Non-GPC" stands for uncoated concrete substrate, and "OP-x" stands for the optimized GPC mixtures listed in Table 3.

An Outlook on Sustainability
Apart from the engineering properties, the cost, availability and environmental compatibility of GPC are also critical to its potential application. The complexity of aluminosilicate sources, especially aluminosilicate-containing industrial by-products, is a major concern for GPC production. The multi-componential composite precursor is superior to the single precursor, from the perspective of comprehensive utilization of various resources. Due to strong corrosivity and consequent operation safety of alkali metal hydroxides, certain alkaline by-products from chemical industry are more suitable to prepare GPC. Moreover, the end product of GPC possesses preferred life-cycle performance as opposed to organic coating materials. Several authors calculated carbon footprint (or emission factor) and economic cost as well as embodied energy consumption of geopolymer-based product in terms of laboratory or industrial scale [60][61][62][63]. Although these analyses referred to different original databases, the results generally indicated that geopolymer is a sustainable alternative material. The preparation of GPC relies on the alkali-activation and the subsequent polycondensation without extra inputs [6]. This process kills two birds with one stone by meeting usage requirements as well as by achieving a cleaner future [64].

Conclusions
Apart from as a partial alternative to Portland cement, this paper reports an experimental study on designing, preparing and characterizing a cluster of sustainable GPC that can be potentially utilized to surface the protection of civil infrastructures. Based on the results presented, the following conclusions can be drawn: • Optimized GPC mixtures with both reasonable setting time and desirable compressive strength were selected from the investigated 85 mixtures. Balancing strength and setting behavior at ambient temperature was viable with the aid of quadruple precursors and mixture design.

•
Adhesive strength of preferred GPC mixtures was successfully measured with a portable tester. The test results ranged from 1.5 to 3.4 MPa and fully meet the requirements of surface protection.

•
Characterizing GPC's permeability based on a rapid test is feasible and easy to handle. The charge passed of coated concrete specimens with the optimized GPC was 30% lower than that of the uncoated ones. There existed a threshold of mass of NaOH that divided its influence on GPC's strength and setting time into two distinct segments; in the first part, setting time and strength decreased with an increase of NaOH mass, and in the second part, the reverse happened.

•
Replacing mass contents of the single components with the total molar ratios of specific oxides or chemical elements as design parameters in the overall GPC system is more reasonable. • GPC is an innovative material for surface protection and a viable alternative to conventional synthetic polymer coatings for use in civil engineering applications.