Quantitative Multi-Spectral Analysis of Portland Cement, Fly Ash and Slag Using Scanning Electron Microscopy and X-Ray ¿-Analysis | Nist (original) (raw)
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Assessment of the Glassy Phase Reactivity in Fly Ashes Used for Geopolymer Cements
Geopolymer Binder Systems, 2013
Geopolymer cements have not found widespread use as a portland cement replacement, in part due to the difficulty in proportioning mixtures in a reliable manner. Unlike portland cements, which are mixed with water, geopolymer cements contain caustic activating solutions, which must be selected carefully to optimize strength and durability. Geopolymer cement can be designed by tailoring caustic solution composition to the reactive phase composition of the solid component of the mixture; however, assessing which phases are reactive is challenging for complex and heterogeneous solids such as fly ash. The present work focuses on applying a scanning electron microscopy and multispectral image analysis (SEM-MSIA) method to identify and quantify the reactive glassy phases in fly ash and to determine how these phases dissolve over time in a caustic activating solution. The fly ash was selected based upon its oxide contents and was analyzed for phase content using x-ray diffraction and Rietveld analysis (RQXRD) and SEM-MSIA. The MSIA method identified multiple glassy phases in the fly ash. Next, the fly ash was suspended in 8 mol/L NaOH, and tested at various time intervals with SEM-MSIA to track changes in the amounts of each individual glassy phase initially identified in the fly ash. The results showed that an aluminosilicate glass (C-AS) with a moderate amount of calcium appeared to be the most reactive between 0 and 28 days for a Class F fly ash. Other phases that were identified in the fly ash included a high-Ca C-AS glass, two aluminosilicate glasses with different S/A ratios, two alkali-modified AS phases, and an iron-containing glass. Introduction: Geopolymers are an aluminosilicate-based cementing material that can be used instead of portland cement as a binder for concrete. The material is formed by mixing an aluminosilicate powder with a caustic activating solution, commonly an alkali hydroxide and/or alkali silicate (Davidovits 1991). A large body of the research on geopolymer cements has been completed using metakaolin as the aluminosilicate source (Barbosa et al. 2000, Rowles and O'Connor 2003, Duxson et al. 2007), which serves as a model geopolymer precursor due to its nearly pure aluminosilicate composition with little crystalline material. Fly ash can also be used as an aluminosilicate source for geopolymer formation since it contains a large proportion of reactive glassy aluminosilicates, and it has an added environmental benefit of being a recycled material. A major challenge in using fly ash as the sole geopolymer precursor material for concrete is that it is difficult to determine whether a given fly ash will react sufficiently with the activating solution. One chemical method for testing reactivity is hydrofluoric acid dissolution, which measures the reactive silica content of the fly ash by comparing the
Quantitative Characterization of Fly Ash Reactivity for use in Geopolymer Cements | NIST
2011
Geopolymer cements have not found widespread use as a portland cement replacement, in part due to the difficulty in proportioning mixtures in a reliable manner. Unlike portland cements, which are mixed with water, geopolymer cements contain caustic activating solutions, which must be selected carefully to optimize strength and durability. Geopolymer cement can be designed by tailoring caustic solution composition to the reactive phase composition of the solid component of the mixture; however, assessing which phases are reactive is challenging for complex and heterogeneous solids such as fly ash. The present work focuses on applying a scanning electron microscopy and multispectral image analysis (SEM-MSIA) method to identify and quantify the reactive glassy phases in fly ash and to determine how these phases dissolve over time in a caustic activating solution. The fly ash was selected based upon its oxide contents and was analyzed for phase content using x-ray diffraction and Rietveld analysis (RQXRD) and SEM-MSIA. The MSIA method identified multiple glassy phases in the fly ash. Next, the fly ash was suspended in 8 mol/L NaOH, and tested at various time intervals with SEM-MSIA to track changes in the amounts of each individual glassy phase initially identified in the fly ash. The results showed that an aluminosilicate glass (C-AS) with a moderate amount of calcium appeared to be the most reactive between 0 and 28 days for a Class F fly ash. Other phases that were identified in the fly ash included a high-Ca C-AS glass, two aluminosilicate glasses with different S/A ratios, two alkali-modified AS phases, and an iron-containing glass. Introduction: Geopolymers are an aluminosilicate-based cementing material that can be used instead of portland cement as a binder for concrete. The material is formed by mixing an aluminosilicate powder with a caustic activating solution, commonly an alkali hydroxide and/or alkali silicate (Davidovits 1991). A large body of the research on geopolymer cements has been completed using metakaolin as the aluminosilicate source (Barbosa et al. 2000, Rowles and O'Connor 2003, Duxson et al. 2007), which serves as a model geopolymer precursor due to its nearly pure aluminosilicate composition with little crystalline material. Fly ash can also be used as an aluminosilicate source for geopolymer formation since it contains a large proportion of reactive glassy aluminosilicates, and it has an added environmental benefit of being a recycled material. A major challenge in using fly ash as the sole geopolymer precursor material for concrete is that it is difficult to determine whether a given fly ash will react sufficiently with the activating solution. One chemical method for testing reactivity is hydrofluoric acid dissolution, which measures the reactive silica content of the fly ash by comparing the
Feasibility of use of a high percentage fly ash unburned material in alkaline activation processes
This study focused on the management and search for alternatives to the use of a fly ash with a high content of unburnt material, whose application as a pozzolanic addition is not possible. The alternative proposed in this paper is to use the fly ash as raw material in the manufacture of a geopolymeric material. Two fly ash samples were taken from paper mill boilers, with loss-on-ignition (LOI) values of 24.26 (FA-1) and 16.82% (FA-2). Because it is expected that carbon free fly ash based geopolymers have better mechanical properties than those produced with ash with a high content of carbon, two methods were developed. The first method consisted of fly ash subject to a calcination process at 690° C in order to remove the residual unburnt carbon up to 1.02%. Once the material was conditioned, it was produced a binary system made of fly ash and ground blast furnace slag (FA-1/GBFS) with the aim of increasing the mechanical properties. The ratio of FA-1/GBS in the mix was 50/50. In the second method was used virgin fly ash (FA-2) with an addition of 10% of Portland cement (OPC) obtaining the geopolymer FA-2/OPC. Both the first and the second method showed a high effectiveness in terms of increasing the mechanical properties of the geopolymers systems produced. The compressive strength values reach up to 43 Mpa at 7 days of curing. The above results demonstrate the feasibility of using this type of fly ash in the production of cementitious geopolymers with applications in the construction industry.
Waste and Biomass Valorization, 2010
The alkali activation of waste materials can produce a binder with similar properties as Portland cement but without the drawback linked to greenhouse gas emissions. Geopolymer pastes were made using coal fly ash as precursor and sodium hydroxide and solid sodium silicate powder as alkaline activators. One sample was activated with sodium silicate only while the second sample used a 50:50 mass% mixture of sodium silicate and 8 M NaOH solution. Fresh geopolymer paste was immediately placed in the Environmental SEM chamber for microstructure observation using the ESEM mode. The sodium silicate activator dissolves rapidly and begins to bond fly ash particles. Open porosity can be observed and is rapidly filled with gel as soon as the liquid phase is able to reach the ash particle. The liquid phase is important as a fluid transport medium permitting the activator to reach and react with the fly ash particles. During this ESEM experiment, the reaction was limited to the surface of the fly ash particles. The reaction products examined had a gel like morphology and no crystallized phase was observed. The effect of the activator on the gel composition was investigated by quantitative microanalysis. The gel analyzed in the sample activated with the mixture of sodium silicate and NaOH solution is enriched in Na and Al. In that sample, the fly ash reaction rate is more advanced considering that the gel is richer in Al and that this element results from the fly ash. In fly ash-based geopolymers, the aluminum content of the aluminosilicate gel is an indicator of the fly ash reactivity.
Fuel, 2006
A working procedure was developed for determining the degree of reaction of fly ash subjected to alkali activation (with 8 M NaOH) at mild temperatures. Since the reaction products dissolve in HCl, the residue left after this acid attack contains only the fraction of the original ash that failed to react with the basic solution. This residue was analysed with Rietveld XRPD quantification and NMR and the findings were compared to the results of the analyses run on the activated ash to obtain a very precise quantification of all of the (crystalline, vitreous and amorphous) phases present in the systems studied.
An examination of the reactivity of fly ash in cementitious pore solutions
Materials and Structures, 2012
Fly ash is frequently used to replace cement in concrete, but it is difficult to predict performance based only on the oxide composition, which is typically the only compositional information available. In order to better utilize fly ash in concrete, it is important to develop more meaningful characterization methods and correlate these with performance. The research presented here uses a combination of analytical methods, including X-ray powder diffraction, scanning electron microscopy coupled with multispectral image analysis, and solution analysis to determine the compositions of the glassy phases in a specific fly ash and to examine the fly ash's reactivity in late-and early-age cement pore solutions, ultrapure water, and sodium hydroxide. The dissolution of individual glassy phases in the fly ash was tracked over time and the precipitation of reaction products monitored. A high-calcium aluminosilicate glass was the most reactive, a low-calcium aluminosilicate glass was of intermediate reactivity and a medium-calcium aluminosilicate glass had the lowest reactivity in the solutions tested for a specific fly ash. This result suggests the glass composition has a strong effect on reactivity, but that that there is not a strict correlation between calcium content and glass reactivity.
An improved basis for characterizing the suitability of fly ash as a cement replacement agent
Journal of the American Ceramic Society, 2017
Fly ash is a critical material for partial replacement of ordinary portland cement (OPC) in the binder fraction of a concrete mixture. However, significant compositional variability currently limits fly ash use. For example, the performance of OPC-fly ash blends cannot be estimated a priori using current characterization standards (e.g., ASTM C618). In this study, fly ashes spanning a wide compositional range are characterized in terms of glassy and crystalline phases using a combination of X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy with X-ray energy dispersive spectroscopy (SEM-EDS) techniques. The compositional data are distilled to a unitless parameter, the network ratio (N r), which represents
Journal of Cleaner Production, 2018
This paper addresses the detailed characterization of coal fly ashes with respect to their utilization in alkali-activated cement systems, thus maximizing the use of fly ashes in the construction industry. A technique was developed to estimate the reactivity of low calcium fly ashes in alkali-activated systems. The technique is based on a K-value which combines three characteristics of a fly ash. Two characteristics e amorphous phase percentage and specific surface area determined through Blaine measurement e are observed quantities. The third characteristic e degree of polymerization of silica in the amorphous phase of fly ash e is a calculated parameter. To take into account the water demand of fly ashes, which will influence the amount of water needed to get a workable mix, the shape factor was used to adjust the Blaine specific surface area. The relationship between the proposed K-value and compressive strength of alkali-activated fly ash pastes is approximated by a linear function. The correlation coefficient of the relationship varied from 0.961 to 0.833 for 1 and 91 days compressive strength respectively. The proposed K-value can firstly be used to rank fly ashes for their suitability to produce high strength alkaliactivated materials and secondly when calibrated for a specific activator and curing conditions, to predict the compressive strength of alkali-activated fly ash binders.
Comprehensive phase characterization of crystalline and amorphous phases of a Class F fly ash
Cement and Concrete Research, 2010
Amorphous material (b) Characterization (b) Image analysis (b) Fly ash (b) SEM (b) X-ray powder diffraction A comprehensive approach to qualitative and quantitative characterization of crystalline and amorphous constituent phases of a largely heterogeneous Class F fly ash is presented. Traditionally, fly ash composition is expressed as bulk elemental oxide content, generally determined by X-ray fluorescence spectroscopy. However, such analysis does not discern between relatively inert crystalline phases and highly reactive amorphous phases of similar elemental composition. X-ray diffraction was used to identify the crystalline phases present in the fly ash, and the Rietveld quantitative phase analysis method was applied to determine the relative proportion of each of these phases. A synergistic method of X-ray powder diffraction, scanning electron microscopy, energy dispersive spectroscopy, and multispectral image analysis was developed to identify and quantify the amorphous phases present in the fly ash.