The Effect of Various Si/Al, Na/Al Molar Ratios and Free Water on Micromorphology and Macro-Strength of Metakaolin-Based Geopolymer

materials

Article The Effect of Various Si/Al, Na/Al Molar Ratios and Free Water on Micromorphology and Macro-Strength of Metakaolin-Based Geopolymer

Hongguang Wang 1,2,* , Hao Wu 1, Zhiqiang Xing 1, Rui Wang 1 and Shoushuai Dai 1

1 School of Civil Engineering, Northeast Forestry University, Harbin 150040, China; [email protected] (H.W.); [email protected] (Z.X.); [email protected] (R.W.); [email protected] (S.D.) 2 Key Laboratory of Bio-Based Material Science and Technology (Ministry of Education), Northeast Forestry University, Harbin 150040, China * Correspondence: [email protected]; Tel.: +86-(451)-8219-2301

Abstract: The current work aimed to explore the effect of Na/Al ratios of 0.43, 0.53, 0.63, 0.73, 0.83, and 0.93, using NaOH to alter the molar ratio, on the mechanical properties of a geopolymer material, with fixing of the Si/Al molar ratio. While fixing the Na/Al molar ratio, alteration of the Si/Al ratios to 1.7, 1.75, 1.8, 1.85, 1.9, 1.95 was used, with silica fume and sodium silicate as a silica corrector. The influence on the micromorphology and macro-strength of samples was characterized through SEM, EDS, and compressive strength characterization methods. The results show that Si/Al and Na/Al molar ratios play a significant role in the microstructure and mechanical behavior of MK-based geopolymers, and revealed that the optimal molar Si/Al and Na/Al ratios for attaining maximum mechanical strength in geopolymers are 1.9 and 0.73, respectively. Under various Si/Al ratios, the macro-strength of the geopolymer mainly relies on the formation of NASH gel, rather than zeolites or silicate derivatives. The appropriate Na/Al molar ratio can contribute to the geopolymerization, Citation: Wang, H.; Wu, H.; Xing, Z.; but a ultra-high Na/Al molar ratio caused a high alkali state that destroyed the microstructure of the Wang, R.; Dai, S. The Effect of Various geopolymers. Regardless of the amount of water contained in the initial geopolymer raw material, Si/Al, Na/Al Molar Ratios and Free the water content of Si/Al = 1.65 and Si/Al = 1.75 after curing for 10 days was almost the same, and Water on Micromorphology and Macro-Strength of Metakaolin-Based the bound water content of the final geopolymer was maintained at about 15%. Structural water Geopolymer. Materials 2021, 14, 3845. exists in geological polymer gels in the form of a chemical structure. It has effects on the structural https://doi.org/10.3390/ma14143845 performance strength, while free water affects the volume stability of the geological polymer. Overall, the current work provides a perspective on the elemental composition analysis, combined with the Academic Editor: F. Pacheco Torgal molecular structure and micromorphology, to explore the mechanical performance of geopolymers.

Received: 7 June 2021 Keywords: metakaolin-based geopolymers; Si/Al; Na/Al molar ratio; morphology; free water; Accepted: 7 July 2021 mechanical properties Published: 9 July 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- In the 1940s, the Belgian scientist Purdon [1] found that ground blast furnace slag iations. mixed with NaOH solution created the phenomenon of coagulation and hardening during experiments. Based on this, he put forward the “alkali excitation” theory for the first time and initiated the prelude to geopolymer research. In the 1950s, former Soviet Union scholar Glukhovsky [2] and others further studied the alkali activated characteristics of Copyright: © 2021 by the authors. blast furnace slag, proposed the concept of “alkali activated cementitious material”, and Licensee MDPI, Basel, Switzerland. established the classical “Glukhovsky” polymerization model. Prof. Joseph Davidovits in This article is an open access article the 1970s made a systematic investigation of raw materials, the polymerization principle, distributed under the terms and and internal structure of geopolymers, and formed the concepts of “mineral polymer” [3] conditions of the Creative Commons Attribution (CC BY) license (https:// and “geopolymer” [4]. This presented the road map for the subsequent years of skyon creativecommons.org/licenses/by/ geopolymer science innovation and research. Palomo et al. [5], in the year of 2005, studied 4.0/). and observed the changes of phase microstructure of fly ash at different stages of alkali

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Materials 2021, 14, 3845 2 of 16 geopolymer science innovation and research. Palomo et al. [5], in the year of 2005, studied and observed the changes of phase microstructure of fly ash at different stages of alkali excitationexcitation reaction reaction by bymeans means of SEM of SEM and and TEM, TEM, and and proposed proposed a more a more detailed detailed descriptive descriptive reactionreaction mechanism mechanism model model at a at microscopic a microscopic level, level, based based on onthe the Glukhovsky Glukhovsky model. model. In In 2010,2010, Criado Criado [6] proposed [6] proposed a nanostructure a nanostructure geopolymer geopolymer model. model. Subsequently, Subsequently, in 2013, in My- 2013, ersMyers et al. [7], et al.based [7], on based the SGM on the model SGM [8] model proposed [8] proposed by Richardson by Richardson and Groves, and Groves, discovered discov- thatered hydrated that hydrated calcium calcium aluminosilicate aluminosilicate (sodium) (sodium) gel (C-(N)-A-S-H) gel (C-(N)-A-S-H) can canbe significantly be signiﬁcantly cross-linkedcross-linked by bythe the study study of alkali-activated of alkali-activated slag slag in the in thelaboratory, laboratory, and and proposed proposed the thecross- cross- linkedlinked substituted substituted tobermorite tobermorite model model (CSTM) (CSTM).. The The CSTM CSTM can betterbetter describedescribe the the spectrum spec- trumand and density density information information of of materials, materials, distinguish distinguish the the proportion proportion of of crosslinking crosslinking and and non- non-crosslinkingcrosslinking in in the the gel, gel, andand calculatecalculate the average average chain chain length length of of C-(N)-A-S-H C-(N)-A-S-H gel. gel. The The specificspeciﬁc representative representative time time course course is shown is shown in Figure in Figure 1. 1.

FigureFigure 1. Development 1. Development history history of geopolymers. of geopolymers.

GeopolymersGeopolymers are are a kind a kind of inorganic of inorganic cementitious cementitious material material with with a three-dimensional a three-dimensional networknetwork structure, structure, formed formed by by natural natural minerals minerals or or industrial industrial wastes wastes of of active aluminosilicatealuminosili- catematerials materials as as solid solid materialsmaterials underunder high alkali alkali conditions. conditions. Compared Compared with with traditional traditional inorganicinorganic cementitious cementitious materials, materials, such such as as Portland Portland cement, cement, a a geopolymer geopolymer has no hydrationhydra- tionreaction reaction with with calcium calcium silicate silicate and and mainly mainly consists consists of ionicof ionic bonds bonds and and covalent covalent bonds, bonds, which whichis similar is similar to naturalto natural zeolite zeolite mineral. mineral. The The geopolymer geopolymer of of each each system system showedshowed excellentexcel- lentdurability, durability, especially especiallythe the resistanceresistance to sulfate sulfate attack attack and and acid acid attack attack [9,10], [9,10 which], which was was significantlysignificantly better better than than traditional traditional cement-based cement-based materials. materials. Due Due to to the the excellent excellent mechan- mechanical icalstrength, strength, fire fire resistance, resistance,corrosion corrosion resistance,resistance, lowlow thermalthermal conductivity, solidificationsolidification of of heavyheavy metal metal ions, ions, and and energy energy conservation, conservation, its its application application was was no no longer longer limited limited to tothe the constructionconstruction field, field, but but began began to be to beused used in inhigh-performance high-performance composite composite materials materials [11], [11 ], fireprooffireproof and and high high temperature-resistant temperature-resistant materials materials [12], [12], rapidrapid repairrepair concrete concrete and and subgrade sub- gradeaggregate aggregate materials materials [13 ,[13,14],14], waterproof waterproof coating coating materials materials [15], [15], the solidificationthe solidification of heavy of heavymetal metal waste waste [16], [16], and and aerospace aerospace and and other other fields fields [17]. [17]. As can be seen from Figure2, the growth in cement demand in diverse countries will As can be seen from Figure 2, the growth in cement demand in diverse countries will increase steadily over the next 30 years or more. Although the rate of increase of demand for increase steadily over the next 30 years or more. Although the rate of increase of demand cement has slowed, it still keeps rising steadily, and the demand from developing countries for cement has slowed, it still keeps rising steadily, and the demand from developing is still very large. Worldwide, the production of Portland cement (PC) keeps increasing, countries is still very large. Worldwide, the production of Portland cement (PC) keeps by 9% per year. Due to the large amount of CO2 released into the atmosphere in the process increasing, by 9% per year. Due to the large amount of CO2 released into the atmosphere of cement production, this increasing rate poses a great danger to the environment. The Materials 2021, 14, x FOR PEER REVIEW 3 of 17 Materials 2021, 14, 3845 3 of 16

in the process of cement production, this increasing rate poses a great danger to the environment.annual greenhouse The annual gas greenhouse emissions gas from emission PC productions from PC are production about 1.5 billion are about tons, accounting1.5 billion tons,for 6% accounting of the average for 6% emissions of the average of many emis industriessions of many in the industries world. in the world.

FigureFigure 2. 2. GlobalGlobal cement cement demand. demand.

TheThe production production of of Portland Portland cement cement not not only only consumes consumes a alot lot of of resources resources and and energy, energy, itit also also brings brings serious serious environmental environmental problems. problems. Therefore, Therefore, it it is is urgent urgent to to develop develop a a new new typetype of of environmentally environmentally friendly friendly green green materi materialal to to replace replace Portland Portland cement. cement. Geopolymer, Geopolymer, asas the the most most promising promising new new green green inorganic inorganic cementing cementing material material in in the the 21st 21st century, century, has has greatgreat potential potential to to replace replace cement-based cement-based mate materials.rials. Compared Compared with with cement, cement, the the energy energy consumption and waste gas emission of geopolymers are extremely low, and its energy consumption and waste gas emission of geopolymers are extremely low, and its energy consumption for production is 1/6~1/4 of cement, 1/70 of steel, 1/20 of ceramic, and consumption for production is 1/6~1/4 of cement, 1/70 of steel, 1/20 of ceramic, and 1/150 1/150 of plastic. The production process of geopolymers can be reduced by 60% for of plastic. The production process of geopolymers can be reduced by 60% for energy con- energy consumption and 80% for the emission of greenhouse gases, and it is regarded sumption and 80% for the emission of greenhouse gases, and it is regarded as “green con- as “green concrete”. crete”. From the perspective of the microstructure, geopolymers are composed of cross-linked From the perspective of the microstructure, geopolymers are composed of cross- silico-oxygen and aluminum-oxygen tetrahedral, with charge balancing basic cations such linked silico-oxygen and aluminum-oxygen tetrahedral, with charge balancing basic cati- as Na+ or K+ ions. According to the Si/Al ratio, geopolymer structures can be divided ons such as Na+ or K+ ions. According to the Si/Al ratio, geopolymer structures can be into three types: silicon aluminum long chain PS type, double silicon aluminum long divided into three types: silicon aluminum long chain PS type, double silicon aluminum chain PSS type, and triple silicon aluminum long chain PSDS type. A geopolymer is an longamorphous chain PSS three-dimensional type, and triple silicon network aluminum polymer, long formed chain by PSDS the condensationtype. A geopolymer of silicon is anoxygen amorphous tetrahedrons three-dimensional and aluminum network oxygen polyme tetrahedrons.r, formed Its by empirical the condensation structural of formula silicon oxygen tetrahedrons and aluminum oxygen tetrahedrons. Its empirical structural+ + formula2+ is Mn (-(SiO2)z-Al2O3)n·wH2O, where M is alkali metal ions (such as Na ,K , Ca ); n is + + 2+ isthe Mn degree (-(SiO of2)z-Al polymerization;2O3)n·wH2O, zwhere is the M molar is alkali ratio metal of Si/Al; ions w(such is the as content Na , K of, Ca bound); n is water; the degreeand the of symbol polymerization; “-” indicates z is the the chemical molar ratio bond. of Si/Al; w is the content of bound water; and theRecent symbol studies “-” indicates have focused the chemical on evaluating bond. the properties of formulations of granu- latedRecent blast furnacestudies have slag [focused18,19], ﬂy on ashevaluating [20,21], andthe properties metakaolin-based of formulations geopolymers of granu- [22], latedshowing blast thefurnace enormous slag [18,19], potential fly ash of geopolymer [20,21], and developmentmetakaolin-based of an geopolymers environmentally- [22], showingfriendly constructionthe enormous material. potential In thisof geopol research,ymer the development emphasis was of not an theenvironmentally- optimization of friendlymetakaolin-based construction geopolymer material. In performance, this research, but the to emphasis explore thewas relationship not the optimization between theof metakaolin-basedmacro-strength and geopolymer micromorphology performance, of samples but to and explore various the Si/Al relationship (Na/Al) between molar ratios. the macro-strengthAnalyses of experimental and micromorphology results by SEM of samp and EDSles and were various conducted Si/Al to (Na/Al) present molar a systematic ratios. Analysesstudy of of the experimental development results of the by mechanical SEM and EDS properties were conducted of geopolymers to present between a systematic 7 and study28 days of the with development various Si/Al of andthe mechanical Na/Al ratio properties compositions, of geopolymers to serve as between a basis, correlated 7 and 28 daysto their with microstructure. various Si/Al and In addition,Na/Al ratio to compositions, further illustrate to serv thee role as a of basis, water correlated during and to theirafter microstructure. synthesis, metakaolin-based In addition, to geopolymer further illustrate samples thewith role variousof water Si/Al during molar and ratiosafter were prepared. Materials 2021, 14, 3845 4 of 16

2. Materials and Methods 2.1. Materials (1) Metakaolin: metakaolin was chosen as the aluminosilicate material in this research due to its high purity, reactivity of components, and being more active for geopolymerization, compared with other materials such as ﬂy ash, slag, etc. The metakaolin, made by calcining kaolin, was supplied by Jinao Refractory Materials Co., Ltd., China (Zhengzhou, China), and was used throughout this study. The general chemical composition of metakaolin powder is characterized in Table1, using X-ray ﬂuorescence (XRF) analysis and X-ray diffraction (XRD) experiment. The metakaolin was mainly composed of silica and alumina oxides, at the weight ratio of 1.36 (SiO2/Al2O3). The metakaolin used in this experiment was calcined at 650 ◦C to 850 ◦C low temperature with activity and physical properties as given in Table2.

Table 1. Compositions of metakaolin.

Compositions SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O Others w.t.(%) 53 39 2.71 1.76 0.17 0.9 0.3 2.14

Table 2. Physical properties of metakaolin.

Fineness Activity Index Speciﬁc Surface Area (kg/m2) LOI 1250 ≥110 437 1.62

(2) Sodium silicate: water glass, otherwise known as sodium silicate, is a kind of water- soluble silicate. The molecular formula is Na2O·nSiO2, with the n representing the ratio of SiO2 to Na2O, called the water glass modulus, which is a very important parameter. The larger the n value, the higher the viscosity and strength. The different modulus of the exciter was adjusted by varying the amount of NaOH, and the sodium silicate solution needed to be set for 4 to 6 h to achieve equilibrium and to cool down to ambient temperature before the experiment was performed. The detailed mass compositions of sodium silicate are given in Table3, as follows.

Table 3. Mass composition of sodium silicate.

Na O SiO H O Be Transparency Sodium Silicate 2 2 2 Modulus (w.t.%) (w.t.%) (w.t.%) (◦C) (%) Index values 8.5 26.5 65 40 3.2 ≥84 Note: The modulus of sodium silicate is the molecular ratio of silica and alkali metal oxides in sodium silicate, modulus = n (SiO2)/N (Na2O); Baume degree is used to characterize the concentration of sodium silicate.

(3) NaOH: The analytical pure sodium hydroxide was sourced from Tanli Chemical Reagent Co., Ltd. (Tianjin, China) with a purity of 99%. (4) Silica fume: Silica fume (SiO2 > 98%), purchased from Sichuan Langtian Resources Utilization Co., Ltd., China (Chengdu, China), was used as silica corrector to change Si/Al ratios. The particle size was measured at 50% of the cumulative under a size of 3 nm from SEM.

2.2. Experimental Preparation 2.2.1. Mix Design An experimental study was conducted for the purpose of evaluating the effects of various Na/Al and Si/Al molar ratios on the microstructure and mechanical properties of a metakaolin-based paste. Materials 2021, 14, 3845 5 of 16

In order to study the effect of Si/Al ratio on the structure and properties of geopolymer, the Na/Al was fixed at 0.83 (if it is more than 0.83, there will be transient solidification), and the content of Si in the structure was adjust using silicon powder. The aim was explore the influence of Na/Al ratio on the geopolymer; Si/Al = 1.85 was fixed, and the range of Na/Al was adjusted from 0.43 to 0.93 by adjusting the amount of NaOH. The specific mix design is illustrated in the Tables4 and5 below.

Table 4. The Si/Al mix design of the paste (kg/m3).

Si/Al Na/Al Water in GP Sample Metakaolin Sodium Silica NaOH (Molar (Molar GP Silicate Fume Ratio) Ratio) (w.t.%) MKSiAl1.65 1050 864 9.45 173.92 1.65 0.83 0.278 MKSiAl1.7 1050 864 33.54 173.92 1.70 0.83 0.298 MKSiAl1.75 1050 864 57.62 173.92 1.75 0.83 0.317 MKSiAl1.8 1050 864 81.72 173.92 1.80 0.83 0.334 MKSiAl1.85 1050 864 105.80 173.92 1.85 0.83 0.350 MKSiAl1.9 1050 864 129.89 173.92 1.90 0.83 0.365 MKSiAl1.95 1050 864 153.98 173.92 1.95 0.83 0.379 Note: MKSiAl1.65 means Si/Al = 1.65.

3 Materials 2021, 14, x FOR PEER REVIEWTable 5. The Na/Al mix design of the paste (kg/m ). 6 of 17

Si/Al Na/Al GP Sample Metakaolin Sodium Silica Fume NaOH (Molar (Molar Silicate Ratio) Ratio) Step 1: In order to prepare a suitable alkali activator solution, the modulus of sodium MKNaAl0.43 1050 864 104.81 43.34 1.85 0.43 silicateMKNaAl0.43 was adjusted1050 by adding pre-weighed 864 104.81analytically pure 75.45 NaOH particles 1.85 to 0.53the in- dustrialMKNaAl0.43 sodium silicate,1050 to meet the 864 needs of the 104.81 experimental 107.56 mix proportion. 1.85 During 0.63 the experiment,MKNaAl0.43 it was noteworthy1050 that 864 the NaOH 104.81 particles released 139.64 a lot 1.85of heat when 0.73 dis- MKNaAl0.43 1050 864 104.81 173.92 1.85 0.83 solvedMKNaAl0.43 in water, so 1050the alkali activator 864 solution 104.81 needed to 203.92be stilled in 1.85 advance to 0.93 reach Note:room MKNaAl0.43 temperature. means Otherwise, Na/Al = 0.43. the reaction process was intensified sharply due to the exothermic heat, resulting in the phenomenon of transient coagulation, the initial setting 2.2.2.time Experimentalwas too short (less Procedure than 5 andmin), Precautions and it was impossible to complete the molding. How- ever, the alkali activator solution should not be stilled for more than 6 h, or else conden- The metakaolin-based geopolymers were prepared from metakaolin, sodium silicate sation will occur due to too low a modulus. Therefore, it is recommend that the standing solution, alkali hydroxide, silica fume, and distilled water. Three group tests were carried time should be 3 to 6 h. out in this experiment. In the first group, while fixing the Na/Al ratio at 0.83, the Si/Al Step 2: Weigh the amount of distilled water and metakaolin needed in the experi- ratio varied from 1.7 to 1.95. The second group controlled the Si/Al ratio at 1.85, with the ment, and fully and evenly mix with the pre-disposed alkali activated solution. Then put Na/Al ratio varying from 0.43 to 0.93. The third group explored the influence of water the prepared solution in the planetary cement mortar mixer, and stir slowly for 120 s and composition content. The Na/Al ratio was fixed at 0.83, and the Si/Al ratio was 1.65, 1.75, quickly for 120 s. 1.85, and 1.95. The sealed and unsealed treatments were carried out at the early curing Step 3: After stirring, the fresh paste was then rapidly cast into 40 mm × 40 mm × 160 stage, and the changes of water molecules over 14 days were measured and recorded. All mm steel molds, while carrying out preliminary vibration, and then put on the vibration of the samples were cured at 60 ◦C for 6 h in an oven, and then cured at 25 ◦C at room table for about 3 min to liberate the bubbles and be compacted. temperature. The mechanical properties of the samples were tested for 14 days and 28 days. Step 4: The samples were put into the laboratory oven at 60 °C for 6 h and an ambient The experimental process of preparing metakaolin used in this study is described in temperature of 25 °C for 4 weeks. One group of samples were sealed with thin plastic at the following four steps. The diagram of geopolymer formation is shown in Figure3. the exposed portion of the mold during the 6 h curing stage.

Figure 3. Diagram of geopolymer formation. Figure 3. Diagram of geopolymer formation.

2.3. StepCharacterization 1: In order to prepare a suitable alkali activator solution, the modulus of sodium silicate2.3.1. Unconfined was adjusted Compressive by adding Strength pre-weighed Test analytically pure NaOH particles to the In the experiment, a 4 cm × 4 cm × 4 cm cubic steel mold was used. Compressive strength was tested by using an electro-hydraulic servo machine with a capacity of 300 kN, and the loading rate was 2 mm/min. The strength was measured as the average of three duplicated samples according to the standard JGJ/T70-2009.

2.3.2. Flexural Strength Test The flexural strength test method of geopolymer mortar refers to the standard test method of cement sand loading. The prism specimen (4 cm × 4 cm × 16 cm) was loaded to failure by three-point bending loading. The bottom of the specimen had a two support spacing. The top loading point was located in the middle of the two support points and acted on the center of the specimen along the length direction. Record the load value at failure and calculate the bending strength according to the following formula.

= 3Pl fb 3 (1) 2b

where fb is the flexural strength (MPa). P is the maximum failure load applied to the specimen in the middle of prism when broken (N). l is the spacing between the two fulcrums (mm). b is the side length of the square section of the prismatic specimen (mm). The average value of three test blocks was taken as the flexural strength test result. When one of the three-point flexural strength values deviates 10% from the average value, the data is removed and the average value of the other values is taken as the result. Materials 2021, 14, 3845 6 of 16

industrial sodium silicate, to meet the needs of the experimental mix proportion. During the experiment, it was noteworthy that the NaOH particles released a lot of heat when dissolved in water, so the alkali activator solution needed to be stilled in advance to reach room temperature. Otherwise, the reaction process was intensiﬁed sharply due to the exothermic heat, resulting in the phenomenon of transient coagulation, the initial setting time was too short (less than 5 min), and it was impossible to complete the molding. However, the alkali activator solution should not be stilled for more than 6 h, or else condensation will occur due to too low a modulus. Therefore, it is recommend that the standing time should be 3 to 6 h. Step 2: Weigh the amount of distilled water and metakaolin needed in the experiment, and fully and evenly mix with the pre-disposed alkali activated solution. Then put the prepared solution in the planetary cement mortar mixer, and stir slowly for 120 s and quickly for 120 s. Step 3: After stirring, the fresh paste was then rapidly cast into 40 mm × 40 mm × 160 mm steel molds, while carrying out preliminary vibration, and then put on the vibration table for about 3 min to liberate the bubbles and be compacted. Step 4: The samples were put into the laboratory oven at 60 ◦C for 6 h and an ambient temperature of 25 ◦C for 4 weeks. One group of samples were sealed with thin plastic at the exposed portion of the mold during the 6 h curing stage.

2.3. Characterization 2.3.1. Unconﬁned Compressive Strength Test In the experiment, a 4 cm × 4 cm × 4 cm cubic steel mold was used. Compressive strength was tested by using an electro-hydraulic servo machine with a capacity of 300 kN, and the loading rate was 2 mm/min. The strength was measured as the average of three duplicated samples according to the standard JGJ/T70-2009.

2.3.2. Flexural Strength Test The ﬂexural strength test method of geopolymer mortar refers to the standard test method of cement sand loading. The prism specimen (4 cm × 4 cm × 16 cm) was loaded to failure by three-point bending loading. The bottom of the specimen had a two support spacing. The top loading point was located in the middle of the two support points and acted on the center of the specimen along the length direction. Record the load value at failure and calculate the bending strength according to the following formula.

3Pl f = (1) b 2b3

where fb is the flexural strength (MPa). P is the maximum failure load applied to the specimen in the middle of prism when broken (N). l is the spacing between the two fulcrums (mm). b is the side length of the square section of the prismatic specimen (mm). The average value of three test blocks was taken as the flexural strength test result. When one of the three-point flexural strength values deviates 10% from the average value, the data is removed and the average value of the other values is taken as the result.

2.3.3. Scanning Electron Microscopy and Energy Dispersive Spectrometer Analysis The morphology of specimens at a microscale was observed with a scanning electron microscopy (SEM), with a Quanta 200 field emission scanning electron microscope, an acceleration voltage of 5 kV, and a maximum magnification of 1 million. Pieces with a size of 0.5 cm to 1 cm were selected from the crushed specimens and dried at 60 ◦C for 12 h. Before the morphology test, the sample was fixed on a metal carrier platform with conductive tape. Then, a gold spray was applied to the test samples for approximately 10 min, and conductive tape was used to connect the loading platform and the device base to increase the conductivity. The observation and freezing images were taken at rela- Materials 2021, 14, 3845 7 of 16

tively ﬂat locations on the unpolished surface. During the observation, the local element composition was also characterized with an energy dispersive spectrometer (EDS).

2.3.4. Calculation of Residual Water during Geopolymerization In order to explore the changes of bound water and free water during the geopolymerization reaction, the initial water content in the raw material was measured, Na/Al = 0.83 and Si/Al = 1.65 to 1.95, respectively, and the mass changes of the experimental samples from 1 day to 14 days were recorded. The percentage weight (Wtr) of residual water in the geopolymer was calculated using the equation as following.

0 WH2O − W H2O Wtr% = × 100% (2) Wn 0 where WH2O, W H2O, and Wn were the initial weight of the water in the specimen, total water weight change of the specimen during curing and aging, and total weight of the specimen at the moment of measurement, respectively.

3. Results 3.1. Effect of Various Si/Al ratios on Microstructure and Mechanical Properties In geopolymerization, the geopolymer formation is basically destructive, including dissolution and hydrolysis, followed by a coagulation process that occurs in an alkaline system; the concentrations of sodium and silicon determined the type of hydrolysis ion and the process of the condensation, showing prominent inﬂuence on the chemistry and physical properties of the geopolymer. Therefore, it is very important to investigate the effect of the Si/Al ratio on the micromorphology and mechanical behavior of MK- based geopolymers. It can be seen from Figure4 that the early strength of the geopolymer developed rapidly, and the strength basically reached more than 60% of its total strength in 7 days. This indicates that a geopolymer is a kind of early-strength cementitious material. When the Na/Al ratio was ﬁxed at 0.83 and Si/Al = 1.7, the compressive strength at 7 days and 28 days was 5.2 MPa and 7.94 MPa, respectively. With increasing the Si/Al ratios, the strength of metakaolin increases and then decreases, and the strength reached the maximum of 21.3 MPa when the Si/Al ratio was 1.90. This is in agreement with the previous research results of other scholars. Steveson et al. [23] reported that geopolymers could obtain the best mechanical properties at an Si/Al ratio of the range of 1.50 to 1.95. Furthermore, De Silva et al. [24] found that the strength reached the maximum when the Si/Al ratio was equal to 1.90. When the Si/Al ratio is less than 1.90, increasing the Si/Al molar ratio is conducive to increasing the number of precursor materials for the dissolution process of metakaolin. This can explain the experimental phenomena from the perspective of molecular structure. − In the geopolymer reaction system, the Al(OH)4 monomer cannot exist alone for a long period, and the polycondensation reaction with Si(OH)4 will occur quickly. However, the rate of polycondensation of Si(OH)4 monomers to form silicate oligomers is relatively slow. − In a low Si/Al ratio system, the free Si(OH)4 and Al(OH)4 monomers bond to each other and polycondensate in the form of hydrated ions to form a long chain (-Si-O-Al-O-) PS type geopolymer (Si/Al = 1) with low oligomeric dimer morphology and structural strength. When the Si/Al ratio is high, the continuous increase of Si content will enhance the content of dissolved elemental silicon and the increase of Si(OH)4 monomer will promote the formation of more -Si-O-Si- bonds, forming a more stable bond structure. Generally, the higher the Si content, the more stable it is, because the chemical bond strength of -Si- O-Si- bond is stronger than that of -Si-O-Al- and -Al-O-Al-, and due to the higher the energy required to break the chemical bond. With Si/Al increasing to a certain extent, the content of elemental silicon dissolved in the system is much higher than that of aluminum, meanwhile, a part of Si(OH)4 will form a dimer after condensation reaction and then react Materials 2021, 14, x FOR PEER REVIEW 8 of 17

other and polycondensate in the form of hydrated ions to form a long chain (-Si-O-Al-O-) PS type geopolymer (Si/Al = 1) with low oligomeric dimer morphology and structural strength. When the Si/Al ratio is high, the continuous increase of Si content will enhance the content of dissolved elemental silicon and the increase of Si(OH)4 monomer will promote the formation of more -Si-O-Si- bonds, forming a more stable bond structure. Gen- erally, the higher the Si content, the more stable it is, because the chemical bond strength

Materials 2021, 14, 3845 of -Si-O-Si- bond is stronger than that of -Si-O-Al- and -Al-O-Al-, and due to the higher8 of 16 the energy required to break the chemical bond. With Si/Al increasing to a certain extent, the content of elemental silicon dissolved in the system is much higher than that of aluminum, meanwhile, a part of Si(OH)4 will form a dimer after condensation reaction and − withthen Al(OH)react with4 Al(OH)to form4 a- to stable form long a stable chain long (-Si-O-Al-O-Si-O-) chain (-Si-O-Al-O-Si-O-) PSS polymer PSS polymer (Si/Al = (Si/Al 2) or more= 2) or stable more longstable chain(-Si-O-Al-O-Si-O-Si-O-) long chain(-Si-O-Al-O-Si-O-Si-O-) PSDS polymer PSDS polymer (Si/Al =(Si/Al 3). = 3).

FigureFigure 4.4. CompressiveCompressive strength strength and and flexural flexural strength strength of geopolymer of geopolymer samples samples with various with various Si/Al ratios. Si/Al ratios. A structural diagram is shown in Figure5. Subsequently, the chemical system is grad- uallyA transferred structural fromdiagram mono-silicate is shown chains in Figure and cyclic5. Subsequently, trimers to species the chemical with larger system rings is Materials 2021, 14, x FOR PEER REVIEWandgradually more complextransferred polymer from structures,mono-silicate forming chains a and 3D polymer cyclic trimers skeleton to andspecies improving with9 larger of the 17

mechanicalrings and more properties complex of thepolymer resulting structures, geopolymer forming materials. a 3D polymer However, skeleton it can and be observed improv- froming the Figure mechanical4 that when properties the Si/Al of the ratio resultin continuesg geopolymer to increase materials. to 1.95, However, the compressive it can be strengthstrengthobserved of does from geopolymer not Figure increase 4 products that signiﬁcantly, when [25]. the Unre Si/Al butacted decreases ratio MK continues toparticles a certain to were increase extent, discovered withto 1.95, the usingthe ﬂexural com- an electronstrengthpressive microscope. instrength particular does showing not increase a sharp significan decline.tly, but decreases to a certain extent, with the flexural strength in particular showing a sharp decline. The reason may be that the dissolution of the reaction process reaches a saturated state in a certain alkaline environment. At the same moment, the high Si/Al ratio will inhibit the dissolution and release of the precursor in the alkaline environment, which will lead to a large number of unreacted metakaolin particles in geopolymer products. The agglomeration phenomenon is not conducive to the geopolymer reaction, which will affect the final strength of the geopolymer products. This can be confirmed by electron microscope, as shown in Figure 6, which shows more unreacted MK particles. It may be that the dissolution of the raw aluminosilicates in the reaction process reaches a saturation state under certain alkaline environments. At the same moment, the high Si/Al ratio will inhibit the dissolution and release of silicon oxide tetrahedron and aluminum oxide tetrahedron monomers of the precursor in an alkaline environment, resulting in a large number of unresponsive metakaolin particles remaining and agglomeration phenomenon occurring. This is not conducive to the synthesis reaction, which will affect the ultimate FigureFigure 5. 5. GeopolymerGeopolymer models models with with di differentfferent Si/Al Si/Al molar molar ratios. ratios.

The reason may be that the dissolution of the reaction process reaches a saturated state in a certain alkaline environment. At the same moment, the high Si/Al ratio will inhibit the dissolution and release of the precursor in the alkaline environment, which will lead to a large number of unreacted metakaolin particles in geopolymer products. The agglomeration phenomenon is not conducive to the geopolymer reaction, which will affect the ﬁnal strength of the geopolymer products. This can be conﬁrmed by electron microscope, as shown in Figure6, which shows more unreacted MK particles. It may be that the dissolution of the raw aluminosilicates in the reaction process reaches a saturation state under certain alkaline environments. At the same moment, the high Si/Al ratio

(a) (b)

(c) Figure 6. SEM images of geopolymers synthesized at various Si/Al ratios, (a) Si/Al = 1.7, (b) Si/Al = 1.9, (c) Si/Al = 1.9.

Figure 6 gives the SEM images of GP pastes cured at 60 °C for 6 h, 25 °C ambient for 14 days. As the micrographs (Figure 6a) show, when the Si/Al ratio is equal to 1.70, various holes and fissures can be seen, containing a small amount crystalline components of zeo- litic nuclei and sodalite phases, and most of these nuclei are not dispersed in the geopolymer binders, leading to weak geopolymerization and a mesoporous formation. Figure 6b, at the ratio of 1.9, reveals that the fewer the holes, the denser the structure, and the Materials 2021, 14, x FOR PEER REVIEW 9 of 17

strength of geopolymer products [25]. Unreacted MK particles were discovered using an electron microscope.

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will inhibit the dissolution and release of silicon oxide tetrahedron and aluminum oxide tetrahedron monomers of the precursor in an alkaline environment, resulting in a large number of unresponsive metakaolin particles remaining and agglomeration phenomenon occurring. This is not conducive to the synthesis reaction, which will affect the ultimate strength of geopolymer products [25]. Unreacted MK particles were discovered using an Figureelectron 5. Geopolymer microscope. models with different Si/Al molar ratios.

(a) (b)

(c)

FigureFigure 6. 6.SEM SEM images images of of geopolymers geopolymers synthesized synthesized at at various various Si/Al Si/Al ratios, ratios, ( a(a)) Si/Al Si/Al == 1.7, (b) Si/Al = = 1.9, 1.9, (c (c) )Si/Al Si/Al = =1.9. 1.9.

FigureFigure 66 givesgives thethe SEMSEM imagesimages ofof GPGP pastespastes curedcured atat 6060 °C◦C for 6 h, 25 °C◦C ambient for 14 days. days. As As the the micrographs micrographs (Figure 6 6a)a) show,show, when when the the Si/Al Si/Al ratio isis equal toto 1.70, various holesholes and ﬁssuresfissures cancan bebe seen,seen, containing containing a a small small amount amount crystalline crystalline components components of zeoliticof zeo- liticnuclei nuclei and and sodalite sodalite phases, phases, and and most most of these of these nuclei nuclei are notare dispersednot dispersed in the in geopolymerthe geopol- ymerbinders, binders, leading leading to weak to weak geopolymerization geopolymerization and and a mesoporous a mesoporous formation. formation. Figure Figure6b, 6b,at theat the ratio ratioof 1.9, of reveals1.9, reveals that that the fewerthe fewer the holes, the holes, the denser the denser the structure, the structure, and the and more the homogeneous the geopolymer gel formed. Combined with the compressive strengths at various Si/Al ratios (Figure4), it can be concluded that the formation of geopolymer gel NASH is the main factor inﬂuencing the strength of geopolymer, rather than the zeolites or silicate derivatives. In the EDS spectrum in Figure7, it can be seen that the content of silicon is the highest, but the content of silicon in metakaolin is lower than oxygen, which is due to the use of silicon powder as a Si/Al ratio regulator. The increase in sodium content occurs due to the use of an alkaline activator. From a quantitative point of view, we can roughly infer that Si/Al is equal to 1.90 and Na/Al is equal to 0.83 from the content of each element in EDS spectrum, which is consistent with the molar ratio used in the experiment; and Materials 2021, 14, x FOR PEER REVIEW 10 of 17

more homogeneous the geopolymer gel formed. Combined with the compressive strengths at various Si/Al ratios (Figure 4), it can be concluded that the formation of geopolymer gel NASH is the main factor influencing the strength of geopolymer, rather than the zeolites or silicate derivatives. In the EDS spectrum in Figure 7, it can be seen that the content of silicon is the highest, but the content of silicon in metakaolin is lower than oxygen, which is due to the use of

Materials 2021, 14, 3845 silicon powder as a Si/Al ratio regulator. The increase in sodium content occurs due10 to of the 16 use of an alkaline activator. From a quantitative point of view, we can roughly infer that Si/Al is equal to 1.90 and Na/Al is equal to 0.83 from the content of each element in EDS spectrum, which is consistent with the molar ratio used in the experiment; and the actual thecomposition actual composition can be calculated can be a calculatedccording to according the contents to theof SiO contents2 and Al of2O SiO3 in2 Tablesand Al 1 2andO3 in3 and Tables the1 silicaand3 fume.and the silica fume.

FigureFigure 7.7. EDSEDS spectrumspectrum ofof Si/AlSi/Al = 1.9. 1.9.

3.2.3.2. TheThe EffectEffect ofof VariousVarious Na/AlNa/Al MolarMolar RatiosRatios onon aa GeopolymerGeopolymer FigureFigure8 8 shows shows the the compressive compressive and and flexural flexural strength strength of of geopolymers geopolymers with with various various Na/AlNa/Al ratios ratios at at various various ages ages and and it itcan can be beseen seen that that the thecompressive compressive strength strength of MK-geo- of MK- geopolymerpolymer first first increases increases then then decreases decreases with with th thee increase increase of Na/AlNa/Al ratio. WhenWhen thethe Na/Al Na/Al ratioratio isis 0.73,0.73, thethe compressivecompressive strengthstrength andand flexuralflexural strengthstrength ofof MK-geopolymerMK-geopolymer achieveachieve peakspeaks ofof 32.5632.56 MPaMPa andand 6.36.3 MPa,MPa, andand theirtheir compressivecompressive strengthstrength andand flexuralflexural strengthstrength areare 4.524.52 timestimes and and 2.32 2.32 times times of of those those of Na/Alof Na/Al equal equal to 0.43 to 0.43 sample sample materials, materials, respectively, respectively, after 28 curing days. However, when the Na/Al ratio exceeds 0.73, the compressive strength after 28 curing days. However, when the Na/Al ratio exceeds 0.73, the compressive and flexural strength drop sharply, the compressive strength decreases to 18.49 MPa, and the flexural strength decreases to 3.2 MPa. This indicates that the Na/Al molar ratio has a considerable enhancing effect on the mechanical strength of MK-GP when the Na/Al is in a suitable range. Materials 2021, 14, x FOR PEER REVIEW 11 of 17

strength and flexural strength drop sharply, the compressive strength decreases to 18.49 Materials 2021, 14, 3845 MPa, and the flexural strength decreases to 3.2 MPa. This indicates that the Na/Al 11molar of 16 ratio has a considerable enhancing effect on the mechanical strength of MK-GP when the Na/Al is in a suitable range.

Figure 8.8. CompressiveCompressive strength strength and and flexural flexural strength strength of geopolymer of geopolymer samples samples with variouswith various Na/Al Na/Al ratios. ratios. Figure9 shows stress–strain curves of sample with various Na/Al molar ratios. All curvesFigure manifest 9 shows linear stress–strain elasticity and curves then brittleof sample failure, with except various the Na/AlNa/Al equalmolar to ratios. 0.83 and All 0.53.curves The manifest reason linear may be elasticity that when and the then Na/Al brittl ise relativelyfailure, except high, the excessive Na/Al equal sodium to ions0.83 willand increase0.53. The thereason alkaline may environmentbe that when the in the Na/Al solution. is relatively The high high, alkali excessive solution sodium can promoteions will theincrease coagulation the alkaline and hardening environment of the in polymer.the solution. However, The high the excessivealkali solution rate of can coagulation promote willthe coagulation lead to premature and hardening crystallization of the polymer. precipitation However, of aluminosilicate the excessive gel,rate retained of coagulation in the formwill lead of particles to premature in the system,crystallization and the precipit raw MK-materialsation of aluminosilicate cannot be fully gel, dissolved,retained in thus the reducingform of particles the performance in the system, of the and geopolymer the raw MK-materials materials. In addition, cannot be when fully thedissolved, Na/Al ratiothus Materials 2021, 14, x FOR PEER REVIEWisreducing high, too the much performance alkali will of the also geopolymer react with COmaterials.and carbonize In addition, in when the air, the reducing Na/Al12 ratioof the 17 2 compressiveis high, too much strength. alkali will also react with CO2 and carbonize in the air, reducing the compressive strength. As shown in Figure 10a, at the Na/Al ratio of 0.43, the micrograph shows flaky and colloidal particle clusters. These granular particles may be unreacted RM powder or silicon powder. The reason for the existence of granular powder is that the alkali concentration strength is too low, so that the alkali activator is not sufficient to dissolve all the precursor materials. In addition, it is noted that the microstructure is relatively loose and the pore structure is large at Na/Al = 0.43, because the geopolymer gel reaction is not sufficient to form a dense three-dimensional network structure. Compared with the Na/Al = 0.73 in Figure 10b, the microstructure of Na/Al = 0.73 is denser, showing a large number of NASH geopolymer gels. Nevertheless, when the alkali content exceeds a certain range, it is not conducive to the geopolymer reaction. As shown in Figure 10c, when the Na/Al ratio is 0.93, it shows a flaky and needle morphology. This is because the increasing Na/Al will increase the alkali content in the system, resulting in a very high alkalinity environment, which destroys the formation of geopolymer gels and reduces the structural strength.

Figure 9. Stress–strain curves of MK-geopolymer with variousvarious Na/AlNa/Al ratios ratios at at 28 28 days days of of curing. curing.

As shown in Figure 10a, at the Na/Al ratio of 0.43, the micrograph shows ﬂaky and colloidal particle clusters. These granular particles may be unreacted RM powder or silicon powder. The reason for the existence of granular powder is that the alkali concentration

(a) (b)

(c) Figure 10. SEM micrographs of geopolymers synthesized at Na/Al, with fixing the Si/Al molar ratio, (a) Na/Al = 0.43, (b) Na/Al = 0.73, (c) Na/Al = 0.93.

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strength is too low, so that the alkali activator is not sufficient to dissolve all the precursor materials. In addition, it is noted that the microstructure is relatively loose and the pore structure is large at Na/Al = 0.43, because the geopolymer gel reaction is not sufficient to form a dense three-dimensional network structure. Compared with the Na/Al = 0.73 in Figure 10b, the microstructure of Na/Al = 0.73 is denser, showing a large number of NASH geopolymer gels. Nevertheless, when the alkali content exceeds a certain range, it is not conducive to the geopolymer reaction. As shown in Figure 10c, when the Na/Al ratio is 0.93, it shows a flaky and needle morphology. This is because the increasing Na/Al will

increase the alkali content in the system, resulting in a very high alkalinity environment, Figurewhich 9. destroys Stress–strain the formation curves of MK-geopolymer of geopolymer with gels va andrious reduces Na/Al theratios structural at 28 days strength. of curing.

(a) (b)

(c)

Figure 10. SEM micrographs micrographs of of geopolymers geopolymers synthesized synthesized at at Na/Al, Na/Al, with with fixing ﬁxing the the Si/Al Si/Al molar molar ratio, ratio, (a) (Na/Ala) Na/Al = 0.43, = 0.43, (b) Na/Al = 0.73, (c) Na/Al = 0.93. (b) Na/Al = 0.73, (c) Na/Al = 0.93.

3.3. Investigation into the Changes to Water in Geopolymerization A simpliﬁed reaction process of a geopolymer model is shown in the Figure 11. First, the raw aluminosilicates were dissolved under the attack of the alkali activator and the monomer is released. Then, the silica-aluminum monomer reacted with the alkaline solution to generate a series of products, which continuously accumulated and produced oligomeric polymers. With the reaction processing, the oligomeric state was transformed to high-polymer state, and ﬁnally the geopolymer products were formed. Materials 2021, 14, x FOR PEER REVIEW 13 of 17

3.3. Investigation into the Changes to Water in Geopolymerization A simplified reaction process of a geopolymer model is shown in the Figure 11. First, the raw aluminosilicates were dissolved under the attack of the alkali activator and the monomer is released. Then, the silica-aluminum monomer reacted with the alkaline solution to generate a series of products, which continuously accumulated and produced oligomeric polymers. With the reaction processing, the oligomeric state was transformed to high-polymer state, and finally the geopolymer products were formed. From Figure 11, the whole reaction seemed to be a linear process, but the actual reaction process was almost synchronous in time and involved dissolution, reconstruction, and polycondensation simultaneously. Throughout the reaction process, water molecules were essential components. In the initial hydrolyzing stage of the reaction, the water molecules acted as reaction media as a solvent for hydroxyl ions, promoting the dissolution Materials 2021, 14, 3845 of silicoaluminate. However, excessive water would dilute the concentration of hydroxide13 of 16 ions, thereby reducing the reaction rate. In the latter stage of the second reaction, water molecules were treated as reaction products and released.

Figure 11. Structural model of geopolymerization.

SomeFrom scholars Figure 11 thought, the whole that the reaction number seemed of water to bemolecules a linear involved process, butin the the dissolu- actual tionreaction process process was was approximately almost synchronous equal to inthe time number and involved of water dissolution, molecules released reconstruction, in the polymerizationand polycondensation process simultaneously. and that the water Throughout molecules the maintained reaction process, the phase water equilibrium molecules werein the essentialwhole process. components. As shown In in the Figure initial 11, hydrolyzing a change of stage water of molecules the reaction, in the the reaction water processmolecules can acted be observed. as reaction In media order asto aexplore solvent the for alteration hydroxyl ions,of bound promoting water theand dissolution free water inof silicoaluminate.the process of geopolymer However, excessive reaction,water the samples would dilutewith Na/Al the concentration = 0.83, Si/Al of = hydroxide1.65, 1.75, 1.85,ions, and thereby 1.95 reducingwere cured the in reaction the oven rate. for In6 theh. During latter stagethis period, of the secondone group reaction, was sealed water withmolecules plastic were film treated and the as other reaction was products not sealed, and released.and then the film was removed during demolding.Some scholars The mass thought change that of the the number sample ofs was water monitored molecules with involved ambient in the temperature dissolution processcuring for was 14 approximatelydays. equal to the number of water molecules released in the poly- merizationThe initial process water and content that the of water the geopolym moleculeser maintained raw materials the phaseand each equilibrium component in the is wholeshown process.in Figure As 12, shown and calculated in Figure 11accord, a changeing to of the water residual molecules water inEquation the reaction (2). process can beIn observed.first curing In day order of 6 to h, explore it was shown the alteration that before of bound the film water was and removed, free water the inwater the processevaporation of geopolymer in the sealed reaction, sample the was samples relatively with slow, Na/Al which = 0.83, was Si/Al reflected = 1.65, in 1.75, the 1.85, slope and of 1.95the graph were curedbeing inrelatively the oven smooth, for 6 h. Duringwhile the this water period, evaporation one group in was the sealed unsealed with sample plastic wasﬁlm greater, and the showing other was a notsteep sealed, straight and line then of thedevelopment ﬁlm was removed in Figure during 13. Probably, demolding. the plas- The ticmass film change prohibits of the rapid samples water was evaporation, monitored re withsulting ambient in a lower temperature initial water curing evaporation. for 14 days. Materials 2021, 14, x FOR PEER REVIEW The initial water content of the geopolymer raw materials and each component14 of is17 After curing for 6 h, the film was removed and demolded. It was observed that both sealed andshown unsealed in Figure samples 12, and had calculated similar outcomes, according th toat the is, residuala large amount water Equation of water (2).evaporation occurred on the first 10 days or so, and the mass changed significantly. During this time, the evaporated water was mainly free water. After 10 days, the mass loss of Si/Al = 1.65 and Si/Al = 1.75 of water tended to be stable and the mass remained basically unchanged, showing a smooth linear line segment. At this time, non-evaporated water mainly existed in the geopolymer, which played a vital role in the structural properties of the geopolymer.

FigureFigure 12.12. InitialInitial geopolymergeopolymer constituentsconstituents andand water.water.

In first curing day of 6 h, it was shown that before the film was removed, the water evaporation in the sealed sample was relatively slow, which was reflected in the slope of the graph being relatively smooth, while the water evaporation in the unsealed sample was greater, showing a steep straight line of development in Figure 13. Probably, the plastic film prohibits rapid water evaporation, resulting in a lower initial water evaporation. After

(a) (b) Figure 13. Water remaining in samples with various Si/Al ratios during curing in sealed and unsealed molds for 6 h and aging for 14 days in ambient environment, (a) curing without film, (b) curing with sealed film.

In addition, Figure 13 shows that the difference between the sealed and unsealed samples was large only in the early stage of water evaporation, while the diversity was relatively small with the remaining amount of non-evaporated water after 10 days in the final stage, presenting a basically similar state; while the mass fluctuated within a small range of 1% after 14 days. Moreover, no matter how much water was contained in the initial geopolymer raw material, the water content in all samples after 16–20 days of curing was almost the same, and the bound water content of the final geopolymer was maintained at about 15%; it could also be seen that the two curves of Si/Al = 1.65 and Si/Al = 1.75 were almost overlapping after 10 days. This phenomenon was consistent with the theory proposed by previous scholars. That is, water molecules remained unchanged during the whole process of geopolymer reaction, the water molecules consumed in the dissolution process were equal to the water molecules released in the polymerization process, and the water molecules maintained a dynamic phase equilibrium during the whole reaction process. According to the mass calculation of free water and non-evaporated water in the experimental process, the content of bound water and non-bound water in the process of geopolymer reaction can be roughly calculated. The existing forms of water molecules in the geopolymer paste were mainly as follows: Materials 2021, 14, x FOR PEER REVIEW 14 of 17

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curing for 6 h, the film was removed and demolded. It was observed that both sealed and unsealed samples had similar outcomes, that is, a large amount of water evaporation occurred on the first 10 days or so, and the mass changed significantly. During this time, the evaporated water was mainly free water. After 10 days, the mass loss of Si/Al = 1.65 and Si/Al = 1.75 of water tended to be stable and the mass remained basically unchanged, showing a smooth linear line segment. At this time, non-evaporated water mainly existed Figurein the 12. geopolymer, Initial geopolymer which played constituents a vital and role water. in the structural properties of the geopolymer.

(a) (b)

Figure 13. Water remaining in samples with various Si/Al ratios ratios during during curing curing in in sealed sealed and and unsealed unsealed molds molds for for 6 6 h and agingaging for 14 days in ambient environment, ( a) curing without ﬁlm,film, (b) curing with sealedsealed ﬁlm.film.

InIn addition, addition, Figure Figure 1313 showsshows that that the the difference difference between the the sealed and unsealed samplessamples was was large large only only in in the the early stage of water evaporation, while the diversity was relativelyrelatively small small with with the the remaining remaining amount amount of of non-evaporated non-evaporated water water after after 10 10 days days in in the finalfinal stage, stage, presenting presenting a a basically basically similar similar st state;ate; while while the the mass fluctuated fluctuated within a small rangerange of of 1% 1% after after 14 14 days. Moreover, Moreover, no no ma mattertter how how much much water water was was contained contained in in the initialinitial geopolymer geopolymer rawraw material,material, thethe waterwater content content in in all all samples samples after after 16–20 16–20 days days of of curing cur- ingwas was almost almost the same,the same, and and the boundthe bound water water content content of the of final the final geopolymer geopolymer was maintainedwas main- tainedat about at 15%;about it 15%; could it alsocould be also seen be that seen the that two the curves two curves of Si/Al of =Si/Al 1.65 = and1.65 Si/Al and Si/Al = 1.75 = 1.75were were almost almost overlapping overlapping after after 10 days. 10 days. ThisThis phenomenon waswas consistent consistent with with the the theory theory proposed proposed by previousby previous scholars. scholars. That Thatis, water is, water molecules molecules remained remained unchanged unchanged during during the whole the processwhole process of geopolymer of geopolymer reaction, reaction,the water the molecules water molecules consumed consumed in the dissolution in the dissolution process were process equal were to the equal water to moleculesthe water moleculesreleased in released the polymerization in the polymerization process, and process, the water and the molecules water molecules maintained maintained a dynamic a dynamicphase equilibrium phase equilibr duringium the during whole the reaction whole process.reaction process. AccordingAccording to to the the mass mass calculation calculation of offree free water water and and non-evaporated non-evaporated water water in the in ex- the perimentalexperimental process, process, the the content content of ofbound bound wa waterter and and non-bound non-bound water water in in the the process process of of geopolymergeopolymer reaction reaction can can be be roughly roughly calculated. The The existing existing forms forms of of water water molecules in thethe geopolymer geopolymer paste paste were were mainly mainly as as follows: follows: (1) About 60% of the water was free water, also known as pore water, which mainly existed between the gel particles and pore structures of the geopolymer, while the pore diameter was generally several hundred nanometers. It was basically not affected by the physical interaction, so it could participate in the reaction of the hydration process. (2) Another part of about 35% was combined water, also called the interlayer water, its binding force relied on capillary tension and strong hydrogen bonding, hydrating with C-A-S-H and adhering to the pore surface. The pore diameter was at a nanometers scale. This part of bound water had a significant influence on the shrinkage of the Materials 2021, 14, 3845 15 of 16

microscopic pore structure and would combine with active silicon and aluminum cations to form hydration ions. (3) The remaining part of 5% was structural water [26], which was an integral part of the chemical structure of the hydration structural products of geopolymers. They existed in the form of hydroxyl groups in the geopolymer products, with a strong chemical bond binding ability. Destroying their chemical bonds would consume a lot of energy and also change the molecular structure and morphology, so the structure water of this part was the most stable.

4. Discussion and Conclusions This article used a sodium silicate solution and sodium hydroxide as an alkali activator, with silicon powder as a regulator, and metakaolin as the raw material for a geopolymer. The influence of various Si/Al and Na/Al ratios on the mechanical properties of the sample was evaluated by fixing Na/Al and Si/Al molar ratios, respectively; with explanation using SEM, EDS, and other macroscopic characterization methods. The variation of free water and bound water during the geopolymerization reaction was also discussed in this article. Based on the results of the study and characterization, the following conclusions could be drawn: (1) The mechanical properties of the geopolymer attained a peak value of 21.3 MPa when Si/Al was equal to 1.90. Under various Si/Al ratios, the macro-strength of a geopolymer mainly relies on the formation of NASH gel rather than the zeolites or silicate derivatives. (2) Increasing the alkalinity within a suitable range in the polymer reaction system was beneficial to the dissolution rate of the MK and geopolymerization. Whereas, a ultra- high Na/Al molar ratio caused a high alkali environment, it would enhance the crystallization and inhibit the polyreaction, resulting in significantly worse geopolymer performance. (3) No matter how high the initial water content in the geopolymer, the discrepancy of water evaporation was only large in the early stages. The water content of all samples was almost the same after 16–20 days curing, and the bound water content in the final geopolymer was near to 15%. (4) On account of the influence of various external factors and raw aluminosilicate material activity, the optimal mixture ratio of geopolymer may vary. It was necessary to explore the mixing ratio to achieve an optimal performance. This paper studied the influence of Si, Al, and Na, the most important elements in MK-Geopolymer, on the performance of the geopolymer and explained the reasons for the influence on geopolymer performance from the perspective of chemical bond formation, making up for the deficiency in micro-mechanism research. Geopolymer, as an environmentally friendly material, is the most promising material for the future. The frost resistance, refractory performance, shrinkage, and material informatics of geopolymers are worthy of further research and exploration.

Author Contributions: Supervision, H.W. (Hongguang Wang) and Z.X.; investigation and original draft preparation, H.W. (Hao Wu) and S.D.; resources, R.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Fundamental Research Funds for the Central Universities of China (Grant No. 2572019BJ01), and China Postdoctoral Science Fund Project (Grant No. 2018M631894). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conﬂicts of Interest: The authors declare no conﬂict of interest. Materials 2021, 14, 3845 16 of 16

References 1. Purdon, A.O. The Action of Alkalis on Blast—Furnace Slag. Soc.Chem. Ind. Trans. Commun. 1940, 59, 191–202. 2. Glukhovsky, V.D. Soil Silicates: Their Properties, Technology and Manufacturing and Fields of Application; Civil Engineering Institute: Kiev, Ukraine, 1965. 3. Davidovits, J.; Davidovics, M. Geopolymer: Room-Temperature Ceramic Matrix for Composites. Ceram. Eng. Sci. Proc. 1988, 9, 835–842. [CrossRef] 4. Davidovits, J. Geopolymers and geopolymeric materials. J. Therm. Anal. 1989, 35, 429–441. [CrossRef] 5. Fernández-Jiménez, A.; Palomo, A.; Criado, M. Microstructure development of alkali-activated fly ash cement: A descriptive model. Cem. Concr. Res. 2005, 35, 1204–1209. [CrossRef] 6. Criado, M.; Fernández-Jiménez, A.; Palomo, A. Alkali activation of fly ash. Part III: Effect of curing conditions on reaction and its graphical description. Fuel 2010, 89, 3185–3192. [CrossRef] 7. Myers, R.; Bernal, S.; San Nicolas, R.; Provis, J. Generalized Structural Description of Calcium-Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model. Langmuir ACS J. Surf. Colloids 2013, 29.[CrossRef][PubMed] 8. Richardson, I.G.; Groves, G.W. The incorporation of minor and trace elements into calcium silicate hydrate (C S H) gel in hardened cement pastes. Cem. Concr. Res. 1993, 23, 131–138. [CrossRef] 9. Mehta, A.; Siddique, R. Sulfuric acid resistance of fly ash based geopolymer concrete. Constr. Build. Mater. 2017, 146, 136–143. [CrossRef] 10. Aiken, T.A.; Kwasny, J.; Sha, W.; Soutsos, M.N. Effect of slag content and activator dosage on the resistance of fly ash geopolymer binders to sulfuric acid attack. Cem. Concr. Res. 2018, 111, 23–40. [CrossRef] 11. Alshaaer, M. Synthesis and characterization of self-healing geopolymer composite. Constr. Build. Mater. 2020, 245, 118432. [CrossRef] 12. Lahoti, M.; Tan, K.H.; Yang, E.-H. A critical review of geopolymer properties for structural fire-resistance applications. Constr. Build. Mater. 2019, 221, 514–526. [CrossRef] 13. Laskar, S.M.; Talukdar, S. A study on the performance of damaged RC members repaired using ultra-fine slag based geopolymer mortar. Constr. Build. Mater. 2019, 217, 216–225. [CrossRef] 14. Xiao, R.; Polaczyk, P.; Zhang, M.; Jiang, X.; Zhang, Y.; Huang, B.; Hu, W. Evaluation of Glass Powder-Based Geopolymer Stabilized Road Bases Containing Recycled Waste Glass Aggregate. Transp. Res. Rec. J. Transp. Res. Board 2020, 2674, 22–32. [CrossRef] 15. Liang, G.; Zhu, H.; Zhang, Z.; Wu, Q.; Du, J. Investigation of the waterproof property of alkali-activated metakaolin geopolymer added with rice husk ash. J. Clean. Prod. 2019, 230, 603–612. [CrossRef] 16. Tan, T.H.; Mo, K.H.; Ling, T.-C.; Lai, S.H. Current development of geopolymer as alternative adsorbent for heavy metal removal. Environ. Technol. Innov. 2020, 18, 100684. [CrossRef] 17. Davidovits, J. Geopolymer Chemistry and Applications; Institut Géopolymère: Saint-Quentin, France, 2008; Volume 171. 18. Poornima, N.; Katyal, D.; Revathi, T.; Sivasakthi, M.; Jeyalakshmi, R. Effect of curing on mechanical strength and microstructure of fly ash blend GGBS geopolymer, Portland cement mortar and its behavior at elevated temperature. Mater. Today Proc. 2021. [CrossRef] 19. Ishwarya, G.; Singh, B.; Deshwal, S.; Bhattacharyya, S.K. Effect of sodium carbonate/sodium silicate activator on the rheology, geopolymerization and strength of fly ash/slag geopolymer pastes. Cem. Concr. Compos. 2019, 97, 226–238. [CrossRef] 20. John, S.K.; Nadir, Y.; Girija, K. Effect of source materials, additives on the mechanical properties and durability of fly ash and fly ash-slag geopolymer mortar: A review. Constr. Build. Mater. 2021, 280, 122443. [CrossRef] 21. Ababneh, F.A.; Alakhras, A.I.; Heikal, M.; Ibrahim, S.M. Stabilization of lead bearing sludge by utilization in fly ash-slag based geopolymer. Constr. Build. Mater. 2019, 227, 116694. [CrossRef] 22. Ayeni, O.; Onwualu, A.P.; Boakye, E. Characterization and mechanical performance of metakaolin-based geopolymer for sustainable building applications. Constr. Build. Mater. 2021, 272, 121938. [CrossRef] 23. Steveson, M.; Sagoe-Crentsil, K. Relationships between composition, structure and strength of inorganic polymers. J. Mater. Sci. 2005, 40, 4247–4259. [CrossRef] 24. De Silva, P.; Sagoe-Crenstil, K.; Sirivivatnanon, V. Kinetics of geopolymerization: Role of Al2O3 and SiO2. Cem. Concr. Res. 2007, 37, 512–518. [CrossRef] 25. Riahi, S.; Nemati, A.; Khodabandeh, A.R.; Baghshahi, S. The effect of mixing molar ratios and sand particles on microstructure and mechanical properties of metakaolin-based geopolymers. Mater. Chem. Phys. 2020, 240, 122223. [CrossRef] 26. White, C.E.; Provis, J.L.; Proffen, T.; Van Deventer, J.S.J. The Effects of Temperature on the Local Structure of Metakaolin-Based Geopolymer Binder: A Neutron Pair Distribution Function Investigation. J. Am. Ceram. Soc. 2010, 93, 3486–3492. [CrossRef]