Showing papers on "Geopolymer published in 2012"

Effect of SiO2 and Al2O3 on the setting and hardening of high calcium fly ash-based geopolymer systems

Abstract: This study investigates the effect of silica and alumina contents on setting, phase development, and physical properties of high calcium fly ash (ASTM Class C) geopolymers. The characteristic rapid setting properties and, hence, limited workability range of high calcium fly ash geopolymers has restricted both development and potential application of these binder systems compared to conventional geopolymer binders derived from bituminous coal, i.e., (ASTM Class F) sources or from calcined kaolin feedstocks. For this study, control of setting and hardening properties were investigated by adjusting SiO2/Al2O3 ratio of the starting mix, via series of mixes formulated with varying SiO2 or Al2O3 contents to achieve SiO2/Al2O3 in the range 2.87–4.79. Foremost is the observation that the effect of varying silica and alumina in high calcium fly ash systems on setting and hardening properties is markedly different from that observed for traditional Class F geopolymer systems. Overall, increases in either silica or alumina content appear to shorten the setting time of high calcium-based systems unlike conventional geopolymer systems where increasing Al2O3 accelerates setting. The setting process was associated primarily with CSH or CASH formation. Furthermore, there appears to be a prevailing SiO2/Al2O3 ratio that prolongs setting, rather than Ca2+ ion content itself, while NASH primarily contributes to strength development. SiO2/Al2O3 ratios in the range of 3.20–3.70 resulted in products with highest strengths and longest setting times. These results suggest that initial predominance of Ca2+ ions and its reactions effectively help maintaining a SiO2/Al2O3 ratio at which amorphous geopolymer phase is stable to influence setting and initial strength development.

Efflorescence control in geopolymer binders based on natural pozzolan

Abstract: This paper addresses methods to reduce efflorescence in a geopolymer binder based on a pumice-type natural pozzolanic material from Taftan, Iran. Geopolymer pastes samples are analyzed for compressive strength and efflorescence formation after curing at 95% humidity for 28 days. To reduce the extent of efflorescence, Al-rich mineral admixtures such as metakaolin, ground granulated blast-furnace slag, and three types of calcium aluminate cements are incorporated into the dry binder at a range of concentrations. Hydrothermal curing at elevated temperatures also shows a positive effect in efflorescence reduction. Calcium aluminate cements show the greatest effect in efflorescence reduction, which is attributed to their dissolution in alkaline media releasing high amounts of alumina into the aluminosilicate geopolymer gel. These results confirm that it is possible to develop a more reliable geopolymer binder with improved properties either by adding a suitable amount of active alumina to precursors such as natural pozzolan, or by manipulating the curing conditions to enhance alumina release from less-reactive precursor phases.

Resistance of lignite bottom ash geopolymer mortar to sulfate and sulfuric acid attack

Abstract: This paper presents an investigation of the compressive strength and the durability of lignite bottom ash geopolymer mortars in 3% sulfuric acid and 5% sodium sulfate solutions. Three finenesses of ground bottom ash viz., fine, medium and coarse bottom ash were used to make geopolymer mortars. Sodium silicate, sodium hydroxide and curing temperature of 75 °C for 48 h were used to activate the geopolymerization. The results were compared to those of Portland cement and high volume fly ash mortars. It was found that the fine bottom ash was more reactive and gave geopolymer mortars with higher compressive strengths than those of the coarser fly ashes. All bottom ash geopolymer mortars were less susceptible to the attack by sodium sulfate and sulfuric acid solutions than the traditional Portland cement mortars.

Thermal analysis of geopolymer pastes synthesised from five fly ashes of variable composition

Abstract: This paper presents a study on the thermal properties of a range of geopolymers in order to assess their suitability for high temperature applications such as thermal barriers, refractories and fire resistant structural members. Geopolymers were synthesised from five different fly ashes using sodium silicate and sodium aluminate solutions to achieve a set range of Si:Al compositional ratios. The thermo-physical, mechanical and microstructural properties of the geopolymers are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. The amount and composition of the amorphous component (glass) of each of the fly ashes was determined by combining XRD and XRF results. It was found that the Si:Al ratio in the glass of the fly ashes strongly influenced the thermal performance of the geopolymers. Geopolymers synthesised from fly ashes with a high Si:Al (≥ 5) in the glass exhibited compressive strength gains and greater dimensional stability upon exposure to 1000 °C, whereas geopolymers synthesised from fly ashes with low Si:Al (

Fly Ash-based Geopolymer Lightweight Concrete Using Foaming Agent

Abstract: In this paper, we report the results of our investigation on the possibility of producing foam concrete by using a geopolymer system. Class C fly ash was mixed with an alkaline activator solution (a mixture of sodium silicate and NaOH), and foam was added to the geopolymeric mixture to produce lightweight concrete. The NaOH solution was prepared by dilute NaOH pellets with distilled water. The reactives were mixed to produce a homogeneous mixture, which was placed into a 50 mm mold and cured at two different curing temperatures (60 °C and room temperature), for 24 hours. After the curing process, the strengths of the samples were tested on days 1, 7, and 28. The water absorption, porosity, chemical composition, microstructure, XRD and FTIR analyses were studied. The results showed that the sample which was cured at 60 °C (LW2) produced the maximum compressive strength for all tests, (11.03 MPa, 17.59 MPa, and 18.19 MPa) for days 1, 7, and 28, respectively. Also, the water absorption and porosity of LW2 were reduced by 6.78% and 1.22% after 28 days, respectively. The SEM showed that the LW2 sample had a denser matrix than LW1. This was because LW2 was heat cured, which caused the geopolymerization rate to increase, producing a denser matrix. However for LW1, microcracks were present on the surface, which reduced the compressive strength and increased water absorption and porosity.

Ambient Temperature Drying Shrinkage and Cracking in Metakaolin-Based Geopolymers

Abstract: Ambient temperature drying shrinkage in metakaolin-based geopolymer pastes exposed to low relative humidity environments has been investigated. The effect of varying the geopolymer composition (water content, Si:Al ratio, Na:Al ratio, and Na + or K + cations) on the sensitivity to ambient temperature drying shrinkage is reported. The definition of “structural” water as being the minimum water content required that prevents contractions in the gel structure, and thus drying shrinkage from occurring, is introduced. From the results presented, it is clear that the ionic charge density of cations, the total quantity of cations, and the relative quantities and stabilities of cation: AlO4 � pairs in the paste are major factors affecting the sensitivity of pastes to ambient temperature drying shrinkage.

Thermal Activation of Albite for the Synthesis of One‐Part Mix Geopolymers

Abstract: Precursors for the preparation of one-part geopolymers are synthesized by thermal activation of albite with sodium hydroxide and sodium carbonate, then cooling and crushing the resulting product. Albite is stable under thermal treatment up to 1000°C, but is able to be converted to depolymerized, disordered, and X-ray amorphous geopolymer precursors in the presence of sodium hydroxide or sodium carbonate at elevated temperatures. The geopolymer precursors react with the addition of water (i.e., form a “one part geopolymer mix”), forming geopolymers with acceptable compressive strength. One-part geopolymers synthesized via thermal activation of albite with NaOH show a higher compressive strength than those produced with Na2CO3 at the same dosage. Some crystalline sodium-aluminosilicate hydrates (zeolites) are also formed in addition to geopolymer gel in the geopolymers synthesized from albite activated by NaOH, compared to predominantly amorphous phases in the samples activated by Na2CO3. The activation of natural aluminosilicates including albite by thermal treatment with alkalis has great potential in the development of novel one-part mix geopolymers.

Quantitative study of the reactivity of fly ash in geopolymerization by FTIR

Abstract: Fourier transform infrared (FTIR) spectroscopy has been applied to analyse the environments of Al–O and Si–O bonds in fly ash, which are used as raw materials of geopolymer synthesis. It is noted that the relative intensities of the bands at around 1000, 910 and 700 cm−1 are much higher in fly ash with higher reactivity, as reflected by the compressive strength of geopolymer. Deconvolution analysis of the band from 400 to 1400 cm−1 shows that the cumulative area of these three resolved bands, together with the band at ∼1090 cm−1, which is assigned to the asymmetric stretching of Si(Al)–O–Si, is proportional to the reactivity of fly ash. If it is assumed that the area of the resolved bands is proportional to the concentration of the corresponding bonds, a general indication is therefore that fly ash containing more reactive bonds will exhibit higher reactivity in geopolymerisation. FTIR spectroscopy in combination with particle size analysis provides a fast approach to predict the reactivity of fly ash, fr...

Acid resistance of inorganic polymer binders. 1. Corrosion rate

Abstract: The resistance to acid-induced corrosion of inorganic polymer (including “fly ash geopolymer”) binders is examined, by exposing specimens to nitric and sulphuric acids at pH values between 1 and 3, and measuring the corroded depth as a function of exposure time. The inorganic polymer binders are shown to be affected by acid attack by surface corrosion, which contradicts some previous claims of extremely high acid resistance in such binders. Corroded depth is shown to be a more sensitive measure of the performance of inorganic polymer binders than change in mass, because acid attack on the highly-connected aluminosilicate network of an inorganic polymer binder leads to the formation of an apparently intact, but physically weak and porous, reaction product layer on the sample surface, rather than complete disappearance of the binder as is often the case for other binder types. A strong correlation between permeability and resistance to acid attack is noted across a wide range of inorganic polymer formulations, including samples based on fly ash, ground granulated blast furnace slag, and mixtures of the two. The presence of calcium (supplied either by a Class C fly ash or by slag) and of high alkali concentrations each show a positive influence on acid resistance, which is attributed to the reduction in mass transport rates through the finer and more tortuous pore networks of such binders.

Properties of inorganic polymer (geopolymer) mortars made of glass cullet

Abstract: This study presents the results of experiments aiming to produce geopolymers from glass cullet, a non-traditional material compared to those usually found in the manufacture of geopolymers (e.g., metakaolin and fly ash). The study gives the principal formulation parameters affecting the behavior of glass cullet geopolymers. The glass used comes from recycled glass bottles. The parameters studied are the fineness of the glass (Blaine of 1000 to 4000 cm2/g), the temperature of synthesis (20, 40 and 60 °C), and the nature and concentration of the activation product (KOH, NaOH). The properties are evaluated in terms of compressive strength and durability. The results show that cullet of soda-glass can be used as a base material for the production of geopolymers and, contrary to metakaolin-based geopolymers, no waterglass is necessary for its setting and hardening since cullet glass already contains a high proportion of alkalis. Thermal activation at 40 or 60 °C is necessary but sufficient to obtain strength of more than 50 MPa, especially for the finer glass (4000 cm2/g). The durability of glass cullet geopolymers is affected by water conservation.

Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin‐Based Geopolymers

Abstract: In this study, we evaluate the effects of chemistry and curing and aging conditions on water loss kinetics, porosity, and the structure of geopolymers. Geopolymer samples were prepared by mixing metakaolin precursor with K or Na silicate solutions having SiO2/A2O3 molar ratios from 2.5 to 4 and H2O/ (SiO2 + Al2O3) molar ratios from 2 to 4. The samples were cured in sealed and unsealed molds at 60°C for 24 h and then aged at room temperature in open containers. The weight changes were monitored during curing and aging until a steady-state weight was observed. This study shows conclusively that the amount of water [H2O/(SiO2 + Al2O3)] in the initial geopolymer mixture is the most dominant factor affecting density and open porosity of geopolymers after curing and extended aging. The results also indicate that regardless of the amount of water in the initial mixture and curing conditions (i.e., in sealed or unsealed molds), ~6–10 wt% of all K-activated samples after aging for 21 days is water whereas in the case of all Na-activated samples that amount ranges between ~15 and 20 wt%. Furthermore, the results demonstrate that SiO2/Al2O3 molar ratio does not have a direct effect on density and open porosity of geopolymers.

Structural Evolution during geopolymerization from an early age to consolidated material.

Abstract: Time-resolved rheology, small angle X-ray scattering (SAXS), and electron paramagnetic resonance (EPR) techniques were used to study the polymerization of geopolymers. These polymers are inorganically synthesized by the alkaline activation of an aluminosilicate source (metakaolin) in aqueous solution. The influence of the alkali activator (Na+, K+, and Cs+) was investigated at room temperature. As observed through the variation of the viscoelastic moduli (G′, G″), curing proceeds in steps that are well pronounced when NaOH is used. These steps correspond to a specific dissolution/polycondensation mechanism and are smoothed when the size of the alkali cations increases. This size effect also has an impact on the gelation time (maximum of tan δ). Structural analysis through SAXS experiments allows us to characterize these mechanisms on the nanoscale and to show that the growth of the geopolymer is due to the aggregation of oligomers with a size that is even smaller than the cation is chaotropic. Finally, wa...

Improved geopolymerization of bottom ash by incorporating fly ash and using waste gypsum as additive

Abstract: This research studied the improvement of the geopolymerization of bottom ash (BA) by incorporating fly ash (FA) and using flue gas desulfurization gypsum (FGDG) as additive. The BA:FA ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 were used as the blended source materials. The source materials were then replaced with 0%, 5%, 10%, and 15% of FGDG. NaOH, sodium silicate and temperature curing were used to activate the geopolymer. Test results indicated that the increase in FA content in the BA–FA blends improved the strengths of geopolymer mortars owing to the high glassy phase content and high reactivity of FA compared to those of BA. The use of up to 10% of FGDG as additive also significantly increased the strengths of geopolymer. In this case, the compressive strength enhancement was due to the increase in the Al3+ leached from BA in the presence of SO 4 2 - and the formation of additional calcium silicate hydrate.

Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

Abstract: In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO2 mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H2O/Al2O3 molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.