Geopolymers

The reaction of alkaline solution (usually hydroxides or alkali metal silicates) with solid aluminosilicates can produce a group of cementitious binders called "geopolymer cement". The geopolymer binder structure is included fundamental of Si-o-Si and Si 'o-Al tetrahedral bonds arranged in an aluminosilicate gel network where Al3+ sites charged ' balanced by alkali cations.
Geopolymers have an extensive range of applications compared to usual construction materials due to their unique properties such as high resistance to salt and acid environments and high resistance to evaluated temperature and fire and their production greatly is reduced co2 emissions compared to Portland cement production.

The mechanism of geopolymer formation includes three stages:
1) Liberation (release) of silicate and aluminate species from of a solid source by dissolving of raw material in alkaline solution.
2) Hydrolyzation and depolymerization of solid particle.
3) Polycondensation of oligomers to form network gel structure.

The properties of geopolymer binders can depend on different synthesis parameters such as raw material, curing conditions and mix properties. The concentration of the alkaline activator plays an important role in the geopolymerization reaction and the densification of the microstructure. The literature has been reported that strength after one day curing increase from 40 to 90 MPa when alkaline activator containing of sodium silicate and NaOH was used instead of NaOH. Moreover, the use of sodium silicate accelerates the dissolution of the raw material.
The sodium aluminasilicate geopolymers can suffer from unpleasant efflorescence due to excess sodium oxide remaining unreacted in the material, particularly when they are synthesized with a high Na2O/Al2O3 ratio.
Whenever, there is motion of water (moisture) within gel structure, the Sodium cations are movable within the pores. This migration depends on the permeability, moisture content and voids. For example, when a concrete is in contact with moist soil, the capillary force causes to move water upward through the concrete column. Then, the water is evaporated from concrete surface and remains a surface richened in the alkali cations. At the surface, atmospheric carbon dioxide reacts with deposited alkalis to form a white carbonate known as efflorescence. It should be noted that efflorescence is different from atmospheric carbonation when atmospheric dioxide carbon react with calcium or sodium in alkali ' activated material. The carbonation usually reduces of PH and deposit of carbonate reaction productions in the bulk of the sample. Whereas, the efflorescence form a visible surface deposits which may or may not companioned with further degradation of the binder. In addition, the high alkali concentration or a lower extent of reaction due to open microstructure of some materials and also due to weak connection of Na in the gel network structure causes tendency towards efflorescence. Some ways have been made to reduce off efflorescence formation, such as use of potassium hydroxide due to more strongly bound to the aluminasilicate gel instead of sodium hydroxide in the activator.

There are various phases in the geopolymer structure containing partly is reacted particles, unreacted particles, a recently formed (alumina) silica gel, water and alkali ' metal hydroxides which is dissolved. Attenuated total reflectance Fourier transforms infrared (ATR ' FTIR) spectroscopy is one technique to study nanostructural evaluation of geopolymer binders and analyze all these phases by using infrared spectra without wrecking sample preparation. Initially, an evanescent was transmitted through an internal reflection element (IRE) of high refractive index. Then, in contact with the IRE, it penetrates a short distance into the sample and spectra are obtained to analyze sample.
The Ferna'ndez ' Jime'nez, Palomo and co-workers used of FTIR to investigate the formation of different two separate gel phases in fly ash geopolymer systems during curing which are Al ' rich and Si ' rich, respectively. More recently, it has been shown that hardened fly ash geopolymers analyze by using an attenuated total reflectance Fourier infrared (ATR-FTIR) spectrpscopy and their evaluation up to 200 days has been observed. In the FTIR literature is mentioned to correlate between position of asymmetric Si-O-T (T= Si or Al) stretching vibration and gel Al content so that the Si-O-T asymmetric stretch is shifted to lower wavenumbers to increase in the amount of Al. Moreover, the effect of high temperature processing on metakaolin-derived geopolymers has been studied by using ATR ' FTIR technique. Moreover, the FTIR information regarding geopolymer formation mechanism relies on spatially averaged results. So, the spatial distribution of gel connectivity within geopolymer binders has never been analyzed. Recently, synchrotron X-ray florescence has been used for the analysis of heterogeneous fly ash, because geopolymer binders are well known to be a heterogeneous on a length scale of nanometers to micrometers. At X-ray florescence, the hard X-ray nanoprobe instrument with high spatial resolution (30 nm) used to provide detailed elemental distribution data, while spectroscopy information regarding Si/Al structures within the geopolymer binders has not been achieved using X-ray florescence.
In recent years, the possibility of utilizing of wide range of materials has investigated as raw materials in production of geopolymer cements. Many researchers have been reported to use of aluminasilicate industrial waste materials such as fly ash and metakaolin or artificial pozzolans.
In this work, the pumice ' type pozzolan from Taftan mountain, Iran, used as source material which can activated with alkaline solution.
In this paper, firstly, the effect of efflorescence control admixture (Secar 71) on the strength mechanically has been investigated. Then, the development of ATR-FTIR analytical tools is used for the in situ analysis of reacting natural pozzolan based geopolymer gels. For this reason, data sets collected with a time travel of just 1 min at near ' ambient reaction temperatures for up to one day. The collected small samples set have been obtained fundamental insight into the processes when efflorescance control admixture added to natural pozzolan.

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