Effects of load types and critical molar ratios on strength properties and geopolymerization mechanism

In this study, the effect of SiO2/Al2O3 (S/A), Na2O/Al2O3 (N/A) and H2O/Na2O (H/N) molar ratios on bending and compressive strength of geopolymer were investigated. The geopolymerization mechanism was also analyzed from microstructure difference by FTIR. The experimental results showed that compressive strength and bending strength of geopolymer has an opposite reaction under different criticalmolar ratios. The increase of S/Amolar ratio and the decrease of N/A and H/Nmolar ratios have resulted in an increase of the compressive strength. However, it caused a noticeable decrease in bending strength. Themicrostructure of geopolymer indicated that the degree of polymerization and cohesion of geopolymer have systematical depending on these critical molar ratios, making the mechanical properties of geopolymer susceptible to different types of loads. This paper reveals the relationship between the microstructure of geopolymer and different mechanical properties and helps to selectively prepare corresponding geopolymer for different loading patterns.

Davidovits et al. [1] proposed that the amorphous to semi-crystalline three-dimensional silico-aluminate structures, the products of polymerization reaction, are of the Poly(sialate) type, the Poly(sialate-siloxo) type, the Poly(sialate-disiloxo) type, whose formation has important connections with S/Al, Na/Al and H/Na molar ratios [9]. A lot of researches show that mechanical properties of geopolymer are also related to these three molar ratios. Duxson et al. [20,21] studied effects of Si/Al and Na/K on the microstructure and mechanical properties of geopolymer and found that as Si/Al increased, compressive strength and Young's modulus increased and then decreased, and reached the maximum when Si/Al reached 1.90 and Na/K was 0.5/0.5. He et al. [22] found that as the Si/Al increased from 2 to 4, the molecular structure also changed, resulting in a gradual increase in flexural strength and Young's modulus. Fletcher et al. [23] studied the microstructure and mechanical properties of geopolymer, when Si/Al changed from 0.5 to 300, and found that when Si/Al was greater than 24, no brittle failure occurred in the compressive specimens, and ductility gradually increased. Zhang et al. [24] [25] studied the effect of Na/Al molar ratio and water-solid ratio on geopolymer adhesion, setting time, microstructure and temperature stability. Yusuf et al. [26] studied the effect of H 2 O/Na 2 O molar ratio on microstructure and mechanical properties of geopolymer. It was found that High H 2 O/Na 2 O had a negative effect on the degree of polymerization thereby leading to low compressive strength due to the presence of excessive pores and microcracks within the matrix. Zhang et al. [27] studied the reaction kinetics, phase formation and mechanical properties of metakaolinbased geopolymer by varying Si/Al molar ratios of 1.2-2.2 and Na/Al molar ratios of 0.6-1.2.
In view of the microstructure of geopolymer, researchers used XRD\SEM and other microscopic techniques to study it. Duan et al. [28]. investigated durability and microstructure of fly ash and metakaolin based geopolymer, they concluded that geopolymer presented better durability and denser microstructure compared to ordinary Portland cement when exposed to elevated temperatures and acid attack. Subaer et al. [29]. investigated the structure of the geopolymer using XRD. The XRD patterns revealed that Na-PS geopolymer consists of zeolite-X in conjunction with amorphous aluminosilicate, while Na-PSS geopolymer was amorphous with a broad hump in the region 20 ∘ -38 ∘ (2θ). Duan et al. [30] investigated the effects of silica fume on properties of fly ash based geopolymer under thermal cycles. They concluded that the incorporation of silica fume optimizes the microstructure and improves the thermal resistance of geopolymer and the pores of geopolymer are also refined by the addition of silica fume. From the above results, as a new gel material, the microstructure analysis of geopolymers has yet to be further revealed.
Lots of studies have shown that Si 2 O/Al 2 O 3 , Na 2 O/Al 2 O 3 and H 2 O/Na 2 O molar ratios are the key parameters that influence the microstructure and mechanical properties of geopolymer. Most of them mainly focus on the effects of these ratios on compressive strength of geopolymer. However, researches are rare especially on effects and relationships of different load types on the mechanical properties of geopolymer. So in this paper, we prepare Na-PSS geopolymer to study effects of different molar ratios on mechanical properties and microstructure of geopolymer and relationships between microstructure of geopolymer and different mechanical properties.

Raw materials
Relatively pure powdery metakaolin (MK) from Hangzhou, Junyi Chemical Co. was selected in this study. NaOH with a purity of 99.8 wt% and sodium silicate solution (water glass) with S/A molar ratio of 3.2 and solid content of 37% were used as alkaline reagents. Chemical compositions of raw materials above are summarized in Table 1.

Mix proportions
Based on previous studies, the primary S/A, N/A and H/N molar ratios were adjusted on a small scale to study the effect on mechanical properties and microstructure of geopolymer. Mix proportions are shown in detail in Table 2.

Mixing, curing, and testing methods
Alkaline activator was made by dissolving solid NaOH in a solution of water glass and water, the liquid been allowed to cool to room temperature before mixing with metakaolin. The dry metakaolin was stirred for 1 minute by an adjustable speed mixer, then the alkaline activator was added slowly and mixed for 3 minutes in low speed then stirred for 2 minutes in high shear mode. The expansion test of geopolymer paste was conducted according to cement mortar expansion test. The paste was poured in 40×40×160 mm 3 molds. The molds sealed with plastic foil to prevent loss of water and left in room temperature for 24 hours then demolded and moved to the curing room with 90 humidity and temperature and kept there until appropriate testing age. According to Mehta and Siddique results [31], geopolymer did well in early strength and in 3-days the mix has already reached more than 90% of 28-days strength.

Bending and compressive strength
The specimens were moved from the curing room and wiped by dried towel after curing for the specific ages. Three specimens for flexural strength and six specimens for compressive strength of each group were tested according to JGJ/T233-2011 [32].

FTIR test
Samples dried for 24 h, ground and sieved to pass sieve No. 0.3 mm. Then they were mixed with KBr (inert and does not show absorbance for radiation) to get better resolution for peaks. AVATAR370 machine was used to performed FTIR test. The resolving power is 4 cm −1 , and the scanning frequency is 32 times with a wavelength of 450-4000 cm −1 . Samples were tested using attenuated total reflectance.

NMR-MAS test
The specimens were kept at 20 ∘ C, and the hydration was stopped by pure alcohol. Before the test, the specimens were broken and ground on a 45 mm sieve to a residue ratio of less than 2%. The NMR-MAS were performed by using Avance III HD Solid state nuclear magnetic resonance spectrometer. Cross polarization was used for 29 Si and one pulse was used for 27 Al. With fixed S/A and H/N molar ratios, as N/A molar ratio varies from 0.95 to 1.00 and 1.20, the fluidity increased, that lead to a gradual increase for the expansion degree of the geopolymer paste and bending strength of hardened geopolymer, while the compressive strength showed a dropping trend, Figure 1(b). For N/A molar ratio 1.00 the bending strength increased by 17% and compressive strength decreases by 6.5% compared to N/A 0.95. Other samples with N/A molar ratio 1.2 followed the same behavior where the bending strength increased by 27% and compressive strength decreases by 13%. Figure 1(

FTIR analysis
FTIR absorption spectroscopy is well known for its sensitivity in characterizing materials with short-range structural order and has been useful for characterizing geopolymer.  Figure 2(b) [33][34][35].
As per Figure 2(a), the stretching vibration is extremely sensitive to Si/Al composition ratio in structure for that by increasing the S/A ratio from 4.3 to 4.5 the corresponding peak may shift to lower frequency due to the more Si substitution by Al in IV fold coordination, which indicates that the partial replacement of SiO 4 species by AlO 4 will result in a change in the local chemical environment of Si-O bond [21]. Si-O bond has higher bond energy than Al-O bond, leading to higher cohesion in structure, which resulted in higher compressive strength for S/A molar ratio of 4.50 than 4.30 as mentioned before. The broad bands in the region of 1645-3448 cm −1 characterized the spectrum of stretching and deformation vibrations of OH and H-O-H groups from weakly bound water molecules which are adsorbed on the surface or trapped in the large cavities between rings of geopolymeric products [35]. So the broader bands in the region for S/A molar ratio of 4.50 than 4.30 caused more weak regions and lead to lower bending strength.
With the increase of N/A molar ratio from 0.95 to 1.20, asymmetric stretching vibration (T-O-Si) shifted from 1020 cm −1 to 1014 cm −1 and bending vibration of Si-O-Al shifted from 578 cm −1 to 574 cm −1 leading to the lower compressive strength of geopolymer. The vibration in the region of 900-1300 cm −1 for N/A molar ratio of 0.95 is broader than 1.20 indicating that the increase of alkali concentration stimulated the dissolution and recombination of silicate components in raw materials so that more uniform geopolymerization products be generated, resulting in higher bending strength of geopolymer [24].
With a high H/N molar ratio, stretching vibration (T-O-Si) shifted from 1020 cm −1 to 1018 cm −1 and the region of 900-1300 cm −1 became broader leading to lower compressive strength due to the presence of excessive pores and microcracks within the matrix. At the same time, with high H/N molar ratio, formation of more gel phase [Nax(AlO 2 )y·nNaOH·mH 2 O] made the polymerization between alkali silicate solution and aluminum-silicon complicated, contributing to the improvement of bending strength [25].

NMR-MAS analysis
Magic Angle Spectroscopy Nuclear Magnetic Resonance (MAS NMR) is an important microscopic technique for studying Si-Al structures, which can provide effective structural data for the study of geopolymers, especially MAS  29 Si and 27 Al. 27 Al MAS NMR can determine the coordination number of aluminum atoms in geopolymer and the corresponding basic unit, but cannot distinguish whether the geopolymer unit is PS type, PSS type or PSDS type, which requires 29 Si MAS NMR. Studies have shown that the chemical shift of 29 Si increases with the degree of polycondensation of the silicon tetrahedron, and with each alum tetrahedron connecting to silicon tetrahedron, the chemical shift of 29 Si increases by about 5 ppm. Therefore,  29 Si MAS NMR can be used to determine the structure and dynamics of geopolymer systems.
The 27 Al MAS NMR results of geopolymers in Figure 3 showed that molar ratios had some influence on geopolymer structure. The extremely narrow peak at 58-61 ppm indicated a well-defined single Al environment which exists in the form of tetrahedral Al Q 4 (4Si), and there were no low relative molecular mass like dimers and trimers in geopolymer structure, which mean geopolymer was a kind of silicon-aluminum compounds with spatial threedimensional networks. 29 Si MAS NMR spectrum showed a broad resonance between −75 ppm and −120 ppm associated with very strong resonance at about −87 ppm and a small peak at about −96 ppm ( Figure 4). Resonances in the 29 Si MAS NMR spectrum, namely −85 ppm, −87 ppm and −96 ppm, can be assigned to Q 4 (4Al), Q 4 (4Al) and Q 4 (3Al) respectively. The spectrum showed that with Na/Al ratio and H/Na increasing, Q 4 (3Al) disappeared, which indicated to some degree that there were more Al in the geopolymer structure.

Conclusions
This paper firstly studied the effect of load type on properties of geopolymers and use FTIR and NMR to explain the reason for different development of compressive strength and bending strength of geopolymer. The detailed experimental results were shown as follows. (