Showing papers on "Geopolymer published in 2010"

Compressive strength and microstructural characteristics of class C fly ash geopolymer

Abstract: Geopolymers prepared from a class C fly ash (CFA) and a mixed alkali activator of sodium hydroxide and sodium silicate solution were investigated. A high compressive strength was obtained when the modulus of the activator viz., molar ratio of SiO 2 /Na 2 O was 1.5, and the proper content of this activator as evaluated by the mass proportion of Na 2 O to CFA was 10%. The compressive strength of these samples was 63.4 MPa when they were cured at 75 °C for 8 h followed by curing at 23 °C for 28 d. In FTIR spectroscopy, the main peaks at 1036 and 1400 cm −1 have been attributed to asymmetric stretching of Al–O/Si–O bonds, while those at 747 cm −1 are due to the Si–O–Si/Si–O–Al bending band. The main geopolymeric gel and calcium silicate hydrate (C–S–H) gel co-exist and bond some remaining unreacted CFA spheres as observed in XRD and SEM–EXDA. The presence of gismondine (zeolite) was also observed in the XRD pattern.

Influence of granulated blast furnace slag on the reaction, structure and properties of fly ash based geopolymer

Abstract: Ground granulated blast furnace slag (GBFS) has been used to alter the geopolymerisation behaviour of fly ash. The influence of varying amount of GBFS (5–50%) on the reaction kinetics has been studied using isothermal conduction calorimetry. It was observed that the reaction at 27 °C is dominated by the GBFS activation, whereas the reaction at 60 °C is due to combined interaction of fly ash and GBFS. The reaction product of geopolymerisation has been characterised using X-ray diffraction and scanning electron microscopy–X-ray microanalysis. Alumino–silicate–hydrate (A–S–H) and calcium–silicate–hydrate (C–S–H) gels with varying Si/Al and Ca/Si ratio are found to be the main reaction products. Coexistence of A–S–H and C–S–H gel further indicates the interaction of fly ash and GBFS during geopolymerisation. Attempt has been made to relate the microstructure with the properties of the geopolymers.

Chemical Research and Climate Change as Drivers in the Commercial Adoption of Alkali Activated Materials

Abstract: Portland cement production has been identified as a primary contributor to the world’s Greenhouse gas emissions, calculated at around 5–8% of all manmade emissions worldwide The majority of these emissions are inherent to the chemistry of cement and the high-temperature processing required for its synthesis, and so can only be avoided by radical changes in construction materials chemistry and synthesis pathways Inorganic polymer (including “geopolymer”) binders provide an alternative to traditional cements with approximately 80% less CO2 emissions, and are derived from industrial waste materials such as fly ash and metallurgical slags, which additionally provide a means of valorizing these wastes This paper reviews the technical and commercial factors driving the growing commercial adoption of geopolymer technology, and explains that an understanding of the chemistry and mechanisms of geopolymer synthesis is pivotal for the optimal mix design of “green” concretes in industry Demand pull by a carbon conscious market at a time of growing public awareness of climate change continues to be the key driver for the short term adoption of geopolymer concrete A detailed chemical understanding of the properties of geopolymers, such as setting time, workability and durability, plays an enabling role in the commercialization process

Effect of Alumina Release Rate on the Mechanism of Geopolymer Gel Formation

Abstract: The effect of the rate of alumina release during the reaction of a one-part (just-add-water) geopolymer mix on growing geopolymer gels is investigated by coupling time-resolved and spatially resolved infrared spectroscopic analysis. The rate of alumina release from different precursors has previously been identified as a critical controlling factor in the formation of mechanically strong and durable geopolymers; however, its influence on the nanostructure of the geopolymer gel has never before been directly analyzed. Gel microstructure and nanostructure are able to be observed by synchrotron radiation-based infrared microscopy (SR-FTIR) with hierarchical clustering analysis, conducted in conjunction with the in situ attenuated total reflectance (ATR) FTIR technique to provide temporal resolution. The SR-FTIR technique provides the opportunity to analyze the chemistry of the heterogeneous geopolymer binder at a level of detail that previously has not been available. Although spatially averaged (ATR-FTIR) i...

Silica fume as porogent agent in geo-materials at low temperature

Abstract: The synthesis of geopolymers based on alkaline polysialate was achieved at low temperature (∼25–80 °C) by the alkaline activation of raw minerals and silica fume. The materials were prepared from a solution containing dehydroxylated kaolinite and alkaline hydroxide pellets dissolved in potassium silicate. Then the mixture was transferred to a polyethylene mold sealed with a top and placed in an oven at 70 °C for 24 h. For all geopolymer materials, following dissolution of the raw materials, a polycondensation reaction was used to form the amorphous solid, which was studied by FTIR-ATR spectroscopy. The in situ inorganic foam based on silica fume was synthesized from the in situ gaseous production of dihydrogen due to oxidation of free silicon (content in the silica fume) by water in alkaline medium, which was confirmed via TGA-MS experiments. This foam has potential as an insulating material for applications in building materials since the thermal measurement has a value of 0.22 W m−1 K−1.

The Effects of Temperature on the Local Structure of Metakaolin-Based Geopolymer Binder: A Neutron Pair Distribution Function Investigation

Abstract: Neutron pair distribution function (PDF) analysis is utilized to advance the understanding of the local atomic structural characteristics of geopolymer binders derived from metakaolin, specifically the nature and amount of the water associated with these materials. Samples were heated in air to temperatures up to 1200°C, then analyzed ex situ by high momentum transfer neutron total scattering and PDF analysis. Water contained in large pores, along with water associated with hydration of potassium cations in the geopolymer framework structure, comprise the majority of water in this material. The remaining water is situated in small pores and as terminal hydroxyl groups attached to the Si–Al framework. The Si–Al framework structure undergoes only subtle rearrangement upon heating, but maintains a tetrahedral aluminosilicate framework environment. After crystallization with heating beyond 1000°C, the geopolymer gel is predominantly converted to leucite, with small amounts of amorphous mullite and glassy silica, which have never before been observed in heated geopolymers. This demonstrates the value of neutron PDF analysis to probe the local structure of these important geopolymeric materials.

Thermal Character of Geopolymers Synthesized from Class F Fly Ash Containing High Concentrations of Iron and α-Quartz

Abstract: This article reports the thermal characteristics of geopolymers prepared with a class F fly ash containing 15 wt% iron oxide and 20 wt%α-quartz. The characterization techniques used included dilatometry, TGA, DTA, XRD, and SEM. Geopolymer specimens were prepared with nominal ratios of Si:Al=2.3 and Na:Al=0.85. Iron oxide in the fly ash precursor was found to play a critical role in the thermal expansion and morphology of geopolymers at temperatures >500°C. Volume changes of quartz on either side of the α–β phase transition cause only minor variations in thermal expansion.

Enhanced thermal stability in K2O-metakaolin-based geopolymer concretes by Al2O3 and SiO2 fillers addition

Abstract: Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K2O–Al2O3–SiO2 system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.

Performance of Fly ash Based Geopolymer Mortars in Sulphate Solution

Abstract: An experimental investigation was conducted to study the performance of fly ash based geopolymer mortar specimens in Magnesium Sulphate solution. Specimens were manufactured from low calcium fly ash by activation with a mixture of Sodium Hydroxide and Sodium Silicate solution and cured thermally. 10% by weight Magnesium Sulphate solution was used to soak the specimen up to 24 weeks. Performance of the specimens was evaluated in terms of visual appearance, variation of pH of solution, change in weight, and change in compressive strength over the exposure period. White deposits occurred on the surface of specimen which was initially soft but later converted to hard crystals. pH of solution increased noticeably during the initial weeks which indicate migration of alkalis from mortar specimens. At the end of 24 weeks samples experienced very little weight gain and recorded a loss of compressive strength by up to 56%.

Mechanical performance and hydration mechanism of geopolymer composite reinforced by resin

Abstract: A series of alkali-activated metakaolin (MK)/granulated blast furnace slag (GBFS)-based geopolymer composites by incorporating with different amount of resin were synthesized. The mechanical performance of MK/GBFS-based geopolymer was remarkably reinforced by doping amount of 1 wt% resin at the curing age of 28 d. The XRD results demonstrated that the geopolymer composite has both amorphous and semicrystaline structure characteristics. The hydration mechanism produced geopolymer composite was proposed in detail. The SEM and pore size distribution results revealed that the microcracks of MK/GBFS-based geopolymer were significantly modified and the volume fraction of micropore is obviously increased due to filling effect of resin. The TGA, DTG and DSC results implied that the resin has an effect of immobilizing water and effectively postpone the water evaporation resulting in the reinforcement of compressive and flexural strengths of MK/GBFS-based geopolymer composites.