Microstructure of Geopolymerized Building Brick Under Atmospheric Curing 1

This research work has been carried out to tailor the development of different type of microstructure in geopolymerized fly ash building brick. During the experimental, work alkaline activator containing both soluble sodium silicates (1:1 solution) and sodium hydroxide (1:1 solution) in presence of anions has been used to improve the reactivity of fly ash. Fly ash and alkaline activator in different weight % are mixed on 100 kg scale followed by water for preparation of the wet mix. Then the wet mixture has been used in casting of brick (230X75X11mm size) by hydraulic press applying 15-20 ton of compaction pressure and then shifted for atmospheric curing at 35 o C. Compressive strength, and microstructure of the bricks have been investigated. The raw mix consisting of 87% of fly ash and alkaline activator has been used by maintaining Na2O/(Al2O3+SiO2) ratio of 0.025 to 0.038 to manufacture building materials ranging in crushing strength of 6 to 12 MPa. The structural changes, i.e. crystalline structure of geopolymerized building brick under the scanning electron micrograph have been observed.


INTRODUCTION
Geopolymerization is an exothermic process that is carried out through oligomers (dimer, trimer) which provide the actual unit structures for the three dimensional macromolecular edifice (Davidovits, 1988b).It involves a heterogeneous chemical reaction between solid aluminosilicate and alkali metal silicate solutions at high alkaline conditions and mild temperatures yielding amorphous to semicrystalline polymeric structures, which consist of Si-O-Al and Si-O-Si bonds (Dimas et al., 2009).Geopolymerization is a geosynthesis (reaction that chemically integrates minerals) that depend on the ability of the aluminium ion (6-fold or 4-fold coordination) to induce crystallographical and chemical changes in a silica backbone (Davidovits, 2005).Fernandez-Jimenez & Palomo studied the composition and microstructure of alkali activated fly ash binder and found that the main reaction product of fly ash based geopolymer is an alkaline silico-aluminate gel (Fernandez-Jimenez et al., 2005).The OH -ion acts as a reaction catalyst during the activation process; and the alkaline metal (Na + ) acts as a structure-forming element.The structure of prezeolite gel contains Si and Al tetrahedral randomly distributed with the polymeric chains that are cross linked so as to provide cavities of sufficient size to accommodate the charge balancing hydrated sodium ions.The amorpohous to semi-crystalline three dimensional silico-aluminate structures of geopolymers based on silicoaluminate and also categorised the geopolymers structure based on the ratio of Si/Al (Davidovits, 2005).
From the SEM study of fly ash based geopolymer mortar, Fernandez-Jimenes and Palomo observed that the microstructure of geopolymer mortar mainly contain amorpohous aluminosilicate gel and the unreacted spheres of fly ash (Fernandez-Jimenez et al., 2005).However, they also detected some crystalline of the aluminosilicate gel and a little group of bright particles which they believe were zeolite crystals.Duxson, et al. found that the microstructure of geopolymers varied depend on the Si/Al ratio (Duxson et al., 2005).Specimens with Si/Al ratio >1.40 exhibit a microstructure comprising clustered dense particulates with large interconnected pores.Specimens with Si/Al ≥1.65 appear homogeneous with porosity distributed in small pores.Closer inspection of the microstructure of geopolymers with 1.40≤Si/Al≤1.65revealed that the evolution of the microstructure with increasing silicon content is rapid yet continuous within the small compositional region.Petermann et.al has also studied the advantages and disadvantages of using geopolymer concrete for industrial applications, the benefits and risks associated with binder variables (alkaline concentration, pozzolan ratios, curing methodologies), mechanical variance resulting from differences in microstructural evolution and performance in harsh environments.The dissolution of solid Al 2 O 3 regions by the activating solution controls the rate, stoichiometry, and extent of solution phase reactions and is dependent on several variables including pH level, binder temperature, the Si/Al ratio and alkali concentration (Petermann et al., 2010).For this reason, the composition and mineralogy of raw pozzolans is critical in the formulation of alkali-activated geopolymer materials.The increase in performance relative to the Si/Al ratio can be summarized by illustrating increasing densities with increasing Si/Al ratios (Vijaya Rangan, 2008).However, the heatcuring temperatures can be as low as 30 °C, which would be attainable in tropical climate conditions (Fernandez-Jimenez et al., 2008).The final reaction product of alkali-activated fly ash is an amorphous to semi-crystalline structure similar to a zeolite precursor.The activation process and degree of reaction within the geopolymer paste is directly related to the glassy content of the ash material (Vara, 2007).
Therefore it is a challenge in synthesizing fly ash based geopolymers at ambient temperatures, while it may be feasible to expose test specimens to elevated temperatures in a laboratory, a full scale pavement project would be difficult to efficiently heat during cure.The objective of the current research work is to investigate geopolymer designs with the potential to successfully harden under atmospheric temperature and observe the changes in the microstructure of the geopolymer building brick.In the present research work, low calcium, Class F dry fly ash obtained from one of the Thermal Power station of Odisha, was used as base material to make the geopolymer fly ash building brick.Chemical analysis of the raw material is given in Table -1.The fly ash used in this research is classified as class F fly ash according to the requirement of American Society for Testing and Materials (ASTM-C618-08, 2008) as given in Table -2.The percentage of carbon in the used fly ash is low as indicated by the low loss on ignition (LOI) therefore it has a higher pozzolanic activity and lower water demand.The SO 3 is less than 1% which will ensure high volume stability and also good for durability.The alkaline activator used in this study is a sodium silicate based solution which means that the alkaline activator contained sodium silicate and sodium hydroxide in presence of some Cl and SO 4 based anions (Nayak et al., 2009).The sodium hydroxide solution was prepared by dissolving sodium hydroxide pellets in water at least 1 day prior to mixing.Subsequently sodium silicate solution (1:1) was added to prepare a suitable alkaline activator solution, which was used for manufacture of building brick.Fly ash and alkaline activator in different weight % are mixed on 100 kg per batch to the pan mixer.The ratio of Na 2 O/(Al 2 O 3 +SiO 2 ) of different mix design for alkaline activator and fly ash has given in Table-3.Fly ash and alkaline activator as per the desired quantity was added to the pan mixer and allowed to mix for 15 minutes followed by water for preparation of the suitable mixture.Then the resulted wet mixture is discharged from the pan-mixer for feeding to the brick press machine for casting of brick of 230x110x75mm in size.After that the green bricks are kept exposed in the atmospheric air for hardening and development of strength shown in Figure -1a.The bricks are cured up to 25 days to observe the durability and mechanical strength.The crushing strength of the brick has been calculated as per the procedure IS. 3495 (IS 3495, (Parts 1 to 4): 1992.) and given in figure -1b.

DISCUSSION
During the experiment, fly ash has been used up to 87% by weight of the total mix by maintaining Na 2 O/(Al 2 O 3 +SiO 2 ) ratio between 0.025 and 0.078 in presence of an alkaline activator.In this type of fly ash mix, the solidified product under atmospheric curing attains a crushing strength as high as 48 MPa.The process has been implemented for economic production of high volume (87%) fly ash building bricks by maintaining Na 2 O/(Al 2 O 3 +SiO 2 ) ratio 0. 0 2 5 to 0.038.The fly ash building bricks of size 230×110×75 mm produced by hydraulic compaction after 25 days of atmospheric curing weigh in the range of 2.80 to 3.20 kg after 8 to 11% of water absorption and attain up to 10 MPa crushing strength, which sartisfies the BIS standard BIS-12894 (IS 12894:2002).It was observed that (Figure-1:a) the under atmospheric curing fly ash mix with Na 2 O/(Al 2 O 3 +SiO 2 ) ratio between 0.025 to 0.038 are suitable for manufacture of building bricks.
Inter-coordination of the polymerized minerals helps in build-up of the cementation property.Depending upon the crushing strength geopolymerisation process is flexible to make high strength building bricks ranging in crushing strength from 7 to 12 MPa.In geopolymerization process SO 4 ˭ is an effective ion of higher hydration energy which promotes hydration of silicates, aluminates and alkalis and enhances the interactions of monomers in formation of crystalline phases (Wang et al., 2006;Desbats-Le Chequer and Frizon, 2011).The Cl − ion acts as an accilator in gel formation and helps in crystallization (Lee and Van Deventer, 2002;Rattanasak, 2011).The polymerization process involves a substantially fast chemical reaction under alkaline condition with Si and Al bearing minerals that results in the three dimensional polymeric chain and ring structures of Si-O-Al-O monomers.Al and Si co-ordinates with oxygen atoms, and therefore, the presence of cations such as Na + is essential to maintain the electric neutrality in the geopolymeric matrix and formation of geopolymer minerals.
From the Scanning Electron Micrograpy study it is observed that Fly ash is fine grain in nature and mostly consists of spheres shaped particles and cenospheres, but in geopolymerized fly ash there is a formation of crystalline structures of sodium assisted cross linked alumino-silicate geopolymer phases.These alkaline phases occur in acicular form and seem to have cross linked into a matrix in the solidified product.The grains are mostly radiating, fibrous and elongated.SEM mapping studies given in Figure -3.areveals that the fly ash sample mostly consists of Si and Al with traces of Ca, where as the mapping study of geopolymer fly ash building brick reveals that the sample uniformly consists of Na-Al-Silicate with minor to trace amount of S, Mg, ca and K given in .Distribution of different elements of different spectrum of geopolymer fly ash has been shown in Figure -4 and the elemental distribution of the crystalline acicular structures of Na-crosslinked aluminum silicate of solidified geopolymer fly ash product shows the presence of Na, Al, and Si which correspond to the geopolymer phase of sodium aluminum silicate with minor to trace amounts of Fe, Ti, S, Mg, Ca, and K given in Table-4 and confirm the compositional analyses

Figure -4: Elemental analysis of different spectrum of geopolymer fly ash
The SEM mapping study reveals that the sample uniformly consists of Na-Al-Silicate with minor to trace amount of S, Mg, ca and K.These grains are mostly fibrous and elongated.These mineral phases with different cation and anion substitution grow into amorphous to crystalline structures by dissolution and solidification.The basic raw materials play a vital role in the geopolymerization reaction and controlthe chemical composition and microstructure of the final geopolymeric products.It differs basing on their amorphous phases, Si/Al ratios, solubility in alkali solution, reactivity, and even the nonreactive crystalline phases.These factors may result in the variability of curing condition and mechanical properties of geopolymers, which are two important factors, should be considered in practical applications.The knowledge of the micromorphology of these raw materials can help identify those nonreactive and unreacted phases present in the geopolymer binder, as discussed in the text.Development of microstructure with a Na 2 O/(Al 2 O 3 +SiO 2 ) ratio of 0.078 consists of clear porous, gel like spine-shaped particles while the EDX analyses of selected reveals that both Al, Si, Na with with minor to trace amounts of Fe, Ti, S, Mg, ca and K which are the major elements of the geopolymer backbone.As the fly ash contains very few impurities, other elements should not be expected in the final geopolymeric product.Therefore, it is reasonable to conclude that the continuous, gel-like area is geopolymer.After atmospheric curing some unreacted NaOH may appear.

Conclusion
It is clear that the microstructure of fly ash geopolymer is very difficult to be understood.A series of geopolymer fly ash building brick was manufactured to investigate, the microstructure of fly ash geopolymer.The elemental distribution of the crystalline acicular structures of Na-crosslinked aluminum silicate of solidified geopolymer fly ash product shows the presence of Na, Al, and Si w with ca, Mg, Fe, Ti etc. as minor element, which correspond to the geopolymer phase of sodium aluminum silicate looks mostly fibrous and elongated.As the sodium silicate activator dissolves rapidly and reacts with fly ash particles, the crystalline spine structures are formed by alkaline reaction under atmospheric curing with a high mechanical crushing strength and suitable for manufacture of building brick.

Figure
Figure-1: a. atmospheric curing of geopolymer fly ash building brick (at 35o C)

Table - 2 Raw material mix design Table-3 Quantitative elemental analysis of different spectrum of the crystalline phases of EDX image of Na- crosslinked fully reacted geopolymer Element At different spectrum in %
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