Use of Analytical Techniques for the Identification of the Geopolymer Reactions

Geopolymers are systems of inorganic binders that can be used as eco-friendly sustainable binder alternate to Portland cement and thereby causing significant reduction of Green house gas emission in construction activities. Geopolymer is often produced by the reaction of alumino-Silicate precursor with alkaline solutions. Geopolymer has shown many excellent properties such as high early strength, good resistance against acid and sulfate attacks, and good performance in high temperature. The key mechanical strength giving components of geopolymeric binders are chemically complex and of multiphase nature since most of the precursor materials used are also structurally disordered, being either glassy or thermally disrupted layer structure. Hence, geopolymers pose the most significant outstanding problem of micro structural characterization. In the present study, industrial by products such as fly ash and GGBS are used as geopolymeric precursor materials and activated by combined solution of sodium silicate.The reactivity of fly ash , GGBS, development of strength and structure of Geopolymer under optimized reaction conditions were examined using different techniques such as X-Ray Fluorescence, X-Ray Diffraction, Scanning Electron Microscopy, 29Si and 27Al MAS -NMR spectroscopy and FT-IR Spectroscopy. The thermal stability of Geopolymer formed studied through thermal analysis TGA/DTA. The average mass loss was found to be of 8% after 1000 0C exposure which was significantly lower than Metakaolin 750 based Geopolymer reported in literature. keywords: Geopolymer, Fly ash, FT-IR, 29Si and 27Al MAS-NMR, XRD.


INTRODUCTION
Geopolymer is an alumino-silicate inorganic polymer, developed by Prof Davidovits in 1970's but probably but more appropriately referred as inorganic polymers [1][2][3] .During the last decade, considerable researches have been directed towards the development of geopolymer due to the wide range of applications for these materials.Geopolymers are formed by alkali activation of solid aluminosilicate materials usually fly ash (FA), or Metakaolin (MK), rice husk, while alkali hydroxides and /or alkali silicate generally used as alkali activators.The main product of the is highly cross -linked 3D polymeric network chain or ring structure consisting of -Si-O-Al-O-(sialate) bonds of Geopolymer and the binding action is in form of Amorphous alumino silicate gel.Several reports can be found in the literature on the synthesis, properties and applications [4][5][6] .
Since fly ash represents industrial waste, can be found to all over the world and its employment in construction industry is still being a matter of numerous investigations, particularly in the Geopolymer synthesis [7][8][9] .Despite the fact that research in utilization of secondary source materials such as FA and GGBS in geopolymer research area is intense but FA based geopolymers are still far from practical applications on a large scale.One of the main reasons is the variability of FA composition, which differs from source to source even within the same source.The most important factors which influence in the FA-based Geopolymer performance are characteristics of precursor particle size distribution, content of glassy phase, reactive silicon and aluminium, presence of iron, calcium as well as nature and concentration of activators and reaction conditions 10 .If activation of FA performed by alkali hydroxide, a gel rich in aluminium formed whereas high ordered silicate species in sodium silicate activator resulted the formation silicon rich gel instead aluminiun-rich.Similarly, reaction conditions such as curing temperature in the range 75-95 0 C, reaction time (24-48h), relative humidity, amount of soluble reactive component in terms of Si/Al over time are changing the properties of geopolymers.So far there is no simple methodology or analytical tool for assessing precursor reactivity, controlling factor for the phase development to link geopolymersiation reaction and thereby mechanical performance characteristics of Geopolymer.In the present study, chemical, mineralogical and microstructure of FA, GGBS based Geopolymer characteristics using NMR, FT-IR, TGA/DTA and microscopic techniques have been investigated and the observations are presented.

Materials
Class F Fly Ash sample from Thermal Power Plant, Ennore, India , GGBS from Jindal steel plant were used as main aluminium and silicate source for synthesizing geopolymer binder.

Synthesis Of Geopolymer
The synthesis of geopolymer mortar was prepared according to the previously optimized procedure using FA and GGBS with liquid/binder ratio 0.45 1 the cylindrical specimens of size 50 mm x100 mm were casted and cured at different temperature.The 3,7 & 28 days compressive strength of mortars have been computed.

Characterization Of Source Material And Geopolymer
Chemical compositions of FA and GGBS were determined by means of Energy Dispersive X-Ray Fluorescence spectroscopy (EDXRF).(Bruker) showed in Table 1 The glassy phase content of fly ash was determined by dissolving fly ash in HF(1%) acid -Arjunan's method. 1 gram of FA dissolved in 100 ml of 1% HF with constant stirring for 6 hrs.Dried samples (110 0 C) were weighed and the content of glassy phase was determined by weight loss.HF dissolves the glassy phase of fly ash, while crystalline phases (usually qurartz, mullite, haematite and magnetite) remains intact.

RESULTS AND DISCUSSION
The X-Ray diffraction pattern analysis of the FA and GGBS precursors as shown in Figure 1a.b indicative the typical crystalline phases of FA such as: quartz (PDF#46-1045), mullite (PDF#85-1460) and hematite (PDF#88-2359) [3] The amorphous phase of GGBS has shown broad spectrum in the range of 2theta 20-40 o .In Flyash, total quantity of SiO 2 was found to be 48% wherein amorphous content was 35%.The SEM images FA and GGBS are shown in the Fig. 2 & 3.The majority of the fly ash particles are spherical in nature having different sizes.Although usually hollow still, some of these spheres may contain other particles of smaller size in their interiors.The presence of amorphous phase of GGBS were clearly seen in SEM picture

M e c h a n i c a l S t r e n g t h O f G e o p o l y m e r s (GP100 :100% Fly ash & GP50: 50% Fly ash & 50% GGbS)
T h e m e c h a n i c a l s t r e n g t h s a t different curing time and temperature for the synthesized geopolymeric mixes FA-GP (GP100) FA-GGBS -GP (GP50) with the mix proportions a s N a 2 O / A l 2 O 3 : 0 .3.) Mechanical strength for FA based GP at hot curing conditions showed improved strength at longer age (28 days) whereas room temperature curing of Geopolymer having 50% GGBS resulting 56 MPa.The insoluble residues were found to be low after geopolymersiation process in both the mixes.[16]

Microstructure Of Geopolymers FT IR Analysis
Fig 4 depicts the FT-IR spectrum of precursor and Geopolymer products.Generally, the original Fly ash precursor contain both 'active' and 'inactive' bonds indicating their reactivity in the alkaline solution.The vibrational frequency assignment of FA precursor as given in Table 3, the active sharp absorption band at around 1093 cm -1 (s) related to asymmetric stretching of (Si, Al IV )-O-Si in glass and partially overlapped by phases of mullite and quartz.The vibrational.frequency at 990 cm -1 (m) which corresponds to the asymmetric stretching of (Si, Al IV )-O-Si in amorphous glasses which could be composed of higher Al concentration.The vibrational frequency at 793 cm -1 (w) and 555 cm -1 (w) was assigned to the symmetric stretching of Si-O-Si in quartz and Al VI ,  In the present study these inactive bond appears as weak but the active bonds are found to be sharp.and bending vibration of FA at 724 cm -1 and 460 cm -1 to lower wavenumber 571 cm -1 and 433 cm -1 respectively.C-O stretching frequency at 1413 cm -1 are also found to present.These shifts are interpreted as a consequence of aluminium incorportation in the Si-O-Si skeleton as observed analogously in zeolites.The more pronounced the shift, the higher the degree of the Penetration from the glassy parts of fly ashes into the [SiO4] 4-network obviously it confirms the condensation of Si-O tedrahedran in geopolymer paste 18 .

SEM ANALYSIS
The SEM Image of GP100 and GP50 is given in fig. 5 and 6 respectively.SEM Investigations of the microstructure of Fly ash activated Geopolymer hardened paste using SEM showed distinct changes in the morphology compared with precursors.Gradual dissolution of fly ash particles is evident in GP 100.Amorphous gel formation around fly ash particles are seen clearly along with residues of original unreacted materials of mullite and quartz.The characteristics products of s obtained by activation of 50% fly ash 50% GGBS (GP50) is predominantly hydrated components with relatively less amount of original fly ash particles.The gel formation over the surface of precursors is seen clearly.

Solid State MAS-NMR Analysis
The assignment of NMR components was based on reported literature values obtained in aluminosilicates. 29Si MAS-NMR spectra of FA and geopolymer paste are shown in Fig 7 .Spectrum of the initial FA has wide asymmetric signal indicates the heterogeneous distribution of silicon structural units.Resonance range between -80 to -108 ppm is mainly associated with initial glass phase .[Q 4 (4Al) (-88 ppm), Q 4 (3Al) (-94 ppm), Q 4 (2Al) (-101.2ppm) and Q 4 (1Al) (-104.9ppm).The resonances appearing above 108 ppm are assigned to different crystalline phases of silicon Q 4 (0Al) (-113 ppm) [18][19][20] .NMR peak at -76.4 ppm in GGBS precursor corresponds to Q 1 units of silica.In activated pastes, new peaks displaying a wide and asymmetric signal formed by several maxima were detected in comparison to unhydrated precursors of fly ash and GGBS in 29 Si NMR spectra.It shows that an important structural rearrangement has been produced.The substitution of Si by Al in the framework of silicate phases resulted in chemical shift of 3 to 5 ppm   4.
27 Al MAS-NMR spectra of precursors and geopolymer paste are shown in Fig 8 .The precursor FA shows chemical shift at around 60.8 ppm (Broad) which indicates the presence of tetrahedral aluminium Al(IV) and 0 ppm, corresponds to the Al(VI) from mullite.The precursor GGBS show the peak around 59 ppm.In GP100 the shift in the broad peak at 60.8 ppm to 59.9 ppm was observed in the case of geopolymerisation reactions, the rates of amorphous aluminosilicate dissolution and precipitation mostly dependent on factors such as temperature, pH, and concentration of soluble Si/Al ratio.The understanding of importance of aluminium in the formation of geopolymers and the determination of available aluminium in raw materials is critical for successful formulation of Geopolymer mixes.

Thermal Analysis (STA)
In thermo gravimetric analysis, the mass loss was measured while the specimens were gradually exposed to increasing temperatures.Powdered specimens were used in TGA to ensure the achievement of thermal equillibrium during transient heating.Fig. 9 shows the TGA and DTA analysis of fly ash based geopolymer cement.TGA . X-Ray diffraction (XRD) [Phillips pW 1710" (Cu K a = 1.54178.pattern are shown in Fig.1 Micro structural characterization carried out by Scanning Electron Microscopy and FT-IR by KBr pellet technique.[Perkin Elmer].The magic angle spinning MAS-NMR were measured in 300 MHz solid state NMR instrument using a Bruker 7mm probe tuned to 79.46 MHz for silicon and 104 MHz or alumina.The standard used for 29 Si NMR is TMS, for 27 Al NMR Aluminium Nitrate.The Simultaneous Thermal Analysis (STA) [ NETSCH 2500 Regulus] was used to study the thermal stability in the temperature range 30-1100 0 C in floating air/ Nitrogen with the heating rate of 10 0 C/min.

Table 2 : Compressive strength data
Si in Mullite or Mullite like structure.These bonds are said to be 'inactive'.