Fly ash and slag Giergiczny, Zbigniew
Cement and concrete research,
October 2019, 2019-10-00, Letnik:
124
Journal Article
Recenzirano
Low-calcium (FA) and high-calcium (HCFA) fly ash and granulated blast furnace slag (GBFS) are the most widely known, standardized and used SCMs in the composition of cement and concrete. In the last ...4 years, scientific work has focused on improving binder properties (e.g. long setting time, low early strength etc.) containing large quantities of FA, HCFA and GBFS. The main directions of activity are the introduction of high-level additives to concrete composition, such as nano-materials, chemical and mechanical activation. Due to the limited access to FA and GBFS, a large amount of studies is devoted to seeking synergies between FA, HCFA, GBFS and limestone. The research works focused on durability characteristics of composites cement containing FA, HCFA and GBFS. Moreover, attention was given to prospects of future application of other types of fly ashes and slags in cement and concrete.
•Slag/fly ash ratio and activator modulus show synergetic effects on reaction.•Activator modulus has a more significant influence on early age reaction.•Gel structures remain stable regardless of ...activator modulus and slag/fly ash ratio.•Slag content shows a dominating effect on compressive strength.
Room temperature cured alkali activated slag/fly ash blends have shown their advantages in field applications. Given that alkali activated materials are extraordinarily sensitive to the composition of the starting materials, identifying their influences is essential for their application. This paper focuses on the effects of two compositional factors: activator modulus (SiO2/Na2O from 1.0 to 1.8) and slag/fly ash mass ratios (between 90/10 and 50/50) on reaction kinetics, gel characters and compressive strength. The results show that when lowering the activator modulus, the early age reaction is significantly accelerated with a higher reaction intensity, and increasing the slag content also leads to an increased reaction rate, especially at low activator modulus. Regardless of the two influential factors, the main reaction products are chain structured C-A-S-H gels with similar water contents and thermal properties, and no typical N-A-S-H type gels are formed in the system. Slight differences in terminal SiO bonds and crystallization temperature are caused by the activator modulus and slag/fly ash mass ratios, respectively. The compressive strength results show that the optimum activator modulus changes with the slag/fly ash mass ratio, and higher slag/fly ash mass ratios prefer higher activator moduli in general, while either too high or too low activator modulus has detrimental effect on strength. Understanding the reaction, gel structure and strength changes are fundamental for determining key manufacturing parameters and tailoring the properties.
•The fly ash source has a significant effect on fly ash-based geopolymer concrete compressive strength.•The particle size distribution (PSD) has a direct effect on the compressive strength.•The fly ...ash source has a significant effect on fly ash-based geopolymer concrete microstructure.•The finer the fly ash particle size distribution, the more significantly permeable void ratio was reduced.•Fewer microcracks were observed when finer fly ash was used.
Geopolymer concrete has demonstrated promising mechanical and microstructural properties in comparison with conventional concrete; however, the variability found in fly ash sources and properties may be an obstacle to implementation. To better understand this variability, this study investigates the effects of particle size distribution and fly ash source on the mechanical and microstructural properties of fly ash-based geopolymer concrete. Two fly ash sources were studied including ordinary McMeekin and Wateree Station fly ash. McMeekin fly ash has three different fly ash particle grades, including the ordinary McMeekin fly ash (38.8 µm), Spherix 50 (17.9 µm), and Spherix 15 (4.78 µm). The Wateree Station is a thermally beneficiated fly ash, while McMeekin is a STAR Processed fly ash. A mixture of silica fume, sodium hydroxide, and water was used as an activating solution. The microstructure of fly ash-based geopolymer paste was observed using SEM. The density, absorption and permeable void ratios were estimated based on ASTM C642. Test results indicate that the resulting compressive strength is linearly affected by the average particle size distribution. The compressive strength of geopolymer concrete was decreased when McMeekin fly ash was used. In addition, the permeable void ratio and absorption after immersion ratio were decreased as a smaller particle size of fly ash such as Spherix 15 (4.78 µm) was used. The fly ash source influences the permeable voids, apparent density, bulk density, and absorption after immersion ratio.
•The effects of CRCA replacement level under the combined F–T and sulfate attack are analyzed.•The effects of LVFA and HVFA under the combined F–T and sulfate attack are analyzed.•The durability was ...more affected by the FA content than by the CRCA replacement level.•Interaction between F–T and sulfate attack to concrete with RCA and FA is discussed.•NMR and XRD analysis of concrete subjected to F–T cycles in sulfate solutions are conducted.
The present study investigated the effect of the combined action of freeze–thaw (F–T) cycles and sulfate attack on the resistance of concrete containing low-volume fly ash (LVFA) and high-volume fly ash (HVFA) made with coarse recycled concrete aggregates (CRCAs). Concretes with a water–binder ratio of 0.50 containing fly ash (FA; LVFA and HVFA) and CRCA (i.e., 0%, 20%, 50% and 100% also by weight) as a replacement for coarse natural aggregates (CNAs) were exposed to water, 5% sodium sulfate solution and 5% magnesium sulfate solution under F–T cycles. The performance, including residual compressive strength, relative dynamic modulus of elasticity and concrete microstructure, was evaluated after being subjected to certain F–T cycles in sulfate solutions. Results indicated that the resistance of the concrete mixtures to the combined F–T cycles and sulfate attack increased with the increase in CRCA content as CNA replacement. Compared with the concrete without FA, the LVFA-based concrete showed excellent improvement in the resistance to the combined action of F–T cycles and sulfate attack; however, the HVFA-based concrete had an adverse effect on the resistance. Concrete deterioration was attributed to the interaction between F–T and sulfate attack. Moreover, the resistance of LVFA- and HVFA-based concretes against the combined F–T and sulfate attack increased during the entire test when the concretes were subjected to F–T cycles in 5% sodium sulfate solution. The sulfate attack exerted more positive effects than negative on the F–T cycles. However, the resistance of LVFA- and HVFA-based concretes against the combined F–T and sulfate attack increased during the initial F–T cycles and then decreased in the 5% magnesium sulfate solution. The 5% sodium sulfate solution produced similar improvements in the F–T resistance of the LVFA- and HVFA-based concretes, whereas the 5% magnesium sulfate solution evidently reduced the F–T resistance of the concrete with HVFA than that with LVFA.
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•The maximum amount of water to successfully form zeolite from fly ash via alkaline fusion was determined.•Five metals were simultaneously adsorbed in a single step ...process.•Adsorption selectivity decreased in the order: Pb(II) > Cu(II) > Cd(II) > Zn(II) > Co(II).•Similar levels of each of the five metals were present on the zeolite after adsorption.
This study reports the potential for the simultaneous removal of Cd(II), Co(II), Cu(II), Pb(II), and Zn(II) ions from aqueous solutions by FAU-type zeolites prepared from coal fly ash. The zeolite synthesis route was via alkaline fusion followed by the addition of deionised water and hydrothermal treatment using fly ash to water mass ratios of 4, 10, 15, and 20. XRD, XRF, SEM and N2 adsorption measurements were used to characterize the prepared zeolites. Adsorption experiments were carried out for variations in concentration, time, and adsorbent loading. The adsorption process followed pseudo second-order kinetics and Langmuir adsorption isotherm; intra particle diffusion model fitting indicated that diffusion within the pores affected the rate controlling steps and mass transfer across boundary layers for the adsorbate – adsorbent system. The efficacy of FAU – type zeolite for the quinary-metal ions adsorption studied decreased in the order Pb(II) > Cu(II) > Cd(II) > Zn(II) > Co(II).
The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main ...binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.
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•MSWIFA was used for manufacturing alkali-activated cementitious material.•Blending 10 % of MK significantly increased the compressive strength.•C-S-H was identified in AAFA, whereas ...C-(A)-S-H and ettringite was observed in AAFM.•Concentrations of heavy metal in AAFM were significantly reduced to the recommendation in Chinese standards.•AAFM reduced 64.8 % of total TEQ of PCDD/Fs in MSWIFA.
The proper treatment on hazardous municipal solid waste incineration fly ash (MSWIFA) is important. The application of alkali-activation technology to prepare alkali-activated MSWIFA (AAFA) material provides a potential not only to immobilise the heavy metals, but also to trigger its pozzolanic property in manufacturing building material. In this study, in addition to investigate the feasibility of alkaline activation technology in preparing AAFA with sodium silicate activator, the effect of metakaolin in AAFA (AAFM) was also explored to enhance its performance. The results showed that, compared to the AAFA, blending 10 % metakaolin in AAFA significantly improved both 28-day and 90-days compressive strengths, which was almost 200 % higher than that of AAFA. The compressive strength was increased with increasing the dosage of sodium silicate. The C-S-H gel was observed as the main hydration product of AAFA and AAFM. Moreover, the ettringite was observed in AAFM due to the reaction between the CaSO4 in MSWIFA and aluminate phase from metakaolin. Finally, the 28 and 210-day leaching behaviours of AAFM on Zn, Cu, Pb, Cd, Cr and Ni were successfully suppressed to less than 1 % of that originally from MSWIFA, which can meet the requirement from Chinese standards.
Landfill leachate concentrate (LLC) is a high-salinity organic wastewater produced in the process of membrane separation and has become an urgent problem because of its complex composition and high ...biological toxicity. Solidification/stabilization (S/S) technology using geopolymers provides a feasible and economical zero-discharge LLC treatment method. This study developed one-part fly ash-based geopolymers using LLC as the only solvent. The results showed that the solid alkali activator significantly increased the LLC treatment capacity with a high liquid−solid ratio of 0.55. The high-salinity LLC significantly promoted the substitution of Al for Si and improved the compressive strength of the geopolymers. Three leaching experiments showed that the prepared geopolymers had excellent stability, high S/S rates for organic matter (92.9%) and NH3-N (91.4%), and a high contaminant removal rate to reduce acute biological toxicity. Microscopy tests showed that sodium aluminosilicate hydrate (N-A-S-H) gel was the main component of the fly ash-based geopolymers after the addition of LLC. Cl− attached to the surface of the geopolymer gels through exchange with OH−, while SO42− was bound within the geopolymer structure in the form of Na2SO4 crystals. There was no evidence that organic matter and NH3-N were chemically combined with the geopolymer material, rather, they were physically encapsulated within the geopolymer structure. This study proposes a technical strategy for LLC S/S using fly ash-based geopolymer. The geopolymer can be used as a new building material, which provides a new perspective for the treatment of LLC and the utilization of fly ash resources.
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•Landfill leachate concentrate (LLC) can replace water to prepare one-part geopolymer.•High-salinity concentration of LLC accelerates the substitution of Al for Si.•Fly ash-based geopolymers can effectively immobilize organic matter and NH3-N.•The leaching toxicity of the immobilized LLC decreases from 72.5% to 13.8%.•Geopolymers save 20% in costs and reduce 2/3 of CO2 emissions compared to cement.
•4% of Pb is successfully immobilized in fly ash-based geopolymer.•Curing conditions influence both strength and the effectiveness of Pb immobilization.•High Pb addition promotes the formation of ...aluminum-deficient aluminosilicate gel.•The formation of depolymerized aluminosilicate gel leads to the high loss of strength.•The mechanism of Pb immobilization is predominantly of chemical nature.
Preventing or reducing negative effects on the environment from the waste landfilling is the main goal defined by the European Landfill Directive. Generally geopolymers can be considered as sustainable binders for immobilization of hazardous wastes containing different toxic elements.
In this paper the influence of addition of high amount of lead on structure, strength, and leaching behavior (the effectiveness of Pb immobilization) of fly ash-based geopolymers depending on the geopolymer curing conditions was investigated. Lead was added during the synthesis of geopolymers in the form of highly soluble salt – lead-nitrate. Structural changes of geopolymers as a result of lead addition/immobilization were assessed by means of XRD, SEM/EDS, and 29Si MAS NMR analysis. Investigated curing conditions significantly influenced structure, strength and leaching behavior of geopolymers. High addition of lead caused a sizeable decrease in compressive strength of geopolymers and promoted formation of aluminum-deficient aluminosilicate gel (depolymerization of aluminosilicate gel), regardless of the curing conditions investigated. According to the EUWAC limitations, 4% of lead was successfully immobilized by fly ash-based geopolymers cured for 28 days in a humid chamber at room temperature.
The limited application of fly ash-based geopolymers in large-scale construction is primarily due to the slow evolution of compressive strength and low early-stage compressive strength. Our research ...counters this by introducing a method that harnesses the innate reactivity of finer fly ash (FF), significantly enhancing the compressive strength. Experimental and characterization results suggest that the superior performance of FF-based geopolymer is attributable to its higher content of reactive silica-alumina components, an optimal ratio of soluble silica to alumina, and a greater specific surface area, which collectively facilitate the geopolymerization process. Adding just 15 wt% NaOH, the FF-based geopolymer attains a compressive strength of 58.81 MPa after 14 d and an additional 10 wt% NaAlO2 can increase compressive strength to 70.5 MPa, showcasing a method that circumvents the need for energy-intensive milling or activation treatments. For the coarser fractions, thermal activation at 550 °C for an hour markedly improves strength to 39.40 MPa, aligning it with traditionally processed fly ash. This breakthrough paves the way for cleaner, more energy-efficient, and more substantial geopolymer materials in modern construction.
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•Post-screened fine fly ash with amorphous aluminosilicates enhances the potency of low-alkali geopolymers.•NaAlO₂ enrichment optimizes silica-alumina ratio, boosting geopolymer strength.•Room temperature curing boosts sustainability and efficiency of fly ash-based geopolymers.•Screening tech enables clean, efficient fly ash utilization in geopolymer synthesis.
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