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 Materials 2021, 14, 3845. https://doi.org/10.3390/ma14143845 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 17 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 the to 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