Low Calcium Fly Ash Research Articles

Durability and microstructural studies on fly ash blended self-compacting geopolymer concrete

Effect of Fly ash blending on self-compacting geopolymer concrete (SCGC) in terms of durability and microstructure is discussed here. 10% Class C fly ash was incorporated into Class F fly ash in self-compacting geopolymer, to alter the conventional methods of heat curing and aiming for commercialisation in concrete production. Geopolymer (GP) synthesis of low calcium fly ash was prepared using combination of sodium hydroxide and sodium silicate, to achieve M30 grade concrete. Tests such as acid resistance, sulphate resistance, water absorption, sorption test, chloride penetration and accelerated corrosion were performed on account of durability, and interstitial characteristics were diagnosed using SEM analysis. Results prove that the inclusion of Class C fly ash did not show any detrimental effects in SCGC, and are viable for use in hazardous environmental conditions.

Multiphase pozzolanic reaction model of low-calcium fly ash in cement systems

The use of supplementary cementitious materials (SCMs) around the world to improve concrete performance has increased in recent decades. However, the application of SCMs remains limited by their heterogeneity and variability. Engineering practices demand a unified model for SCMs to predict and optimise the performance of blended concrete. A multiphase modelling scheme that covers various SCMs is therefore proposed in this study. Following this modelling scheme, three low-calcium fly ashes were selected and comprehensively studied by scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS) full-element mapping and image analysis. The pozzolanic reactivity of these three fly ashes was further investigated in alkaline and cement systems. A temperature-dependent two-phase pozzolanic reaction model (silicate and Al-silicate glass) considering fly ash material variations was proposed to predict the pozzolanic reaction and therefore the degree of reaction at different temperatures. The numerical results show that the proposed multiphase modelling scheme has great potential to cover various SCMs.

Pozzolanic reaction on alkali-activated Class F fly ash for ambient condition curable structural materials

Despite considerable efforts focused on the utilization of industrial wastes, the application of low-calcium fly ash to the construction industry is limited to the partial substitution to ordinary Portland cement. High-temperature curing is a method via which fly ash can be completely utilized as an alkali-activated construction material; however, additional energy is required. In this study, the dual reaction of alkali activation and the pozzolanic reaction was proposed to manufacture ambient-condition-curable structural mortars. For this purpose, calcium hydroxide was used in fly ash activated by sodium hydroxide and sodium silicate solutions. A series of experiments, including compressive strength tests, X-ray diffraction, thermogravimetric analysis, heat of reaction, and mercury intrusion porosimetry, were conducted. The continuous pozzolanic reaction on the pre-formulated geopolymeric skeleton was found to significantly enhance the material properties. By the addition of silica fume and a 7:3 mixture of sodium hydroxide and sodium silicate solutions, the material strength increased to greater than 60 MPa at 56 days. The added silica fume as well as the reduced alkali content of the solution enhanced the reactions due to the active participation of the calcium ion supplied by the added hydrated lime in a high pH environment.

The Effects of Water to Solid Ratio, Activator to Binder Ratio, and Lime Proportion on the Compressive Strength of Ambient-Cured Geopolymer Concrete

Low calcium fly ash based Geopolymer has been proven to be one of the potential alternatives substitutes to Portland Cement not only due to its high resistance to chemical attack but also because of the vast availability of class F fly ash for raw materials. However, one of the limitations of geopolymer as the alternative binders in concrete is that the strength develops slowly under ambient condition. This paper presented the investigation of water to solid ratio, activator to binder ratio, and lime proportion on the compressive strength of ambient-cured geopolymer concrete. To develop sufficient strength at an early age, class F fly ash and slaked lime (Ca (OH)2) were used as the binder with the proportion of lime to binder of 4%, 5%, 6%, and 7%. The blended binder was activated by sodium silicate and sodium hydroxide solution with the variation of activator to binder ratio of 0.45, 0.5, 0.55, and 0.6. The water to solid ratio of 0.30, 0.31, 0.32, and 0.33 was chosen to facilitate good workability which was done by adding water to the mix. The compressive strength tests were conducted at 7, 14, and 28 days on the cylindrical concrete specimens with a dimension of 100 mm diameter and 200 mm height. The results show that the activator to binder ratio of 0.50 to 0.55, and the proportion of lime to the binder of 6% to 7% were the optimum range value. It was also found that the lower the water to cement ratio the higher the compressive strength and the water to solid ratio as low as 0.3 produced the highest compressive strength while still maintaining good workability.

Effects of the Loss on Ignition of Fly Ash on the Properties of High-Volume Fly Ash Concrete

This study presents the experimental results of fresh and hardened properties of concrete incorporating high-volume fly ash (HVFA). Two kinds of low-calcium fly ash with loss on ignition (LOI) of 5% and 8% were used as replacement for cement and/or fine aggregate of 0% (control), 20%, 40%, 50%, 60% and 80% by weight of the total cementitious materials. The properties of fresh concrete tested included the slump, air content, unit weight and setting time; those of hardened concrete determined included compressive strength, modulus of elasticity, flexural strength and drying shrinkage. Test results indicate that the concretes made with high-LOI (8%) fly ash can be successfully produced for structural concrete, which contains fly ash of up to 60% of the total cementitious materials. The high-LOI fly ash-concretes with higher replacement levels presented longer setting times. However, although both the fresh and hardened properties of high LOI fly ash concretes were inferior to those of the low-LOI (5%) fly ash concretes, the high modulus of elasticity, the adequate strength development characteristics both at early and later ages (up to 365 days) and the low dry shrinkage were observed when compared to those of the control concrete with a comparable 28-day compressive strength of 30 MPa.

Mechanical properties and durability of unconfined and confined geopolymer concrete with fiber reinforced polymers exposed to sulfuric acid

Abstract This study reports the mechanical properties and durability of unconfined/confined sustainable geopolymer concrete (GPC) exposed to sulfuric acid attack under static and cyclic loadings. A low calcium (Class F) fly ash incorporating nano-silica was activated by a combination of sodium silicate and sodium hydroxide solutions to make the GPC. Unconfined/confined ordinary Portland cement concrete (OPCC) counterparts were also made for comparison purposes. The confined GPC/OPCC specimens were wrapped by one/three layers of basalt/carbon fiber-reinforced polymers (BFRP/CFRP). Effects of type and number of wrapping layers, along with wrapping and testing ages on the mechanical properties and durability of confined GPC/OPCC were investigated. In addition, microstructure of the unconfined/confined GPC specimens was compared with that of the unconfined OPCC counterparts. The results showed confinement of the GPC with BFRP/CFRP fabrics enhanced the strength, ductility and durability of the GPC exposed to sulfuric acid. The GPC confined with CFRP fabric was more ductile and durable than that confined with BFRP fabric. In addition, the GPC confined with three layers of BFRP/CFRP fabric was more ductile and durable than that confined with only one layer. Further, the unconfined/confined GPC was more durable than the corresponding unconfined/confined OPCC.

Effects of low- and high-calcium fly ash on magnesium oxysulfate cement

Abstract Magnesia-based cementitious materials have a reputation for being “green engineering materials in the 21st century”. By incorporating fly ash (FA) into magnesium oxysulfate cement (MOSC), a new type of high performance MOSC that achieves energy savings and environmental protection can be produced for industrial construction applications. In this paper, the performance of MOSC with low-calcium fly ash (L-FA) and high-calcium fly ash (H-FA) was investigated, including the setting times, compressive strengths, shrinkages, compositions and microstructures. The results show that adding FA to MOSC can accelerate the hydration reaction and improve the volume stability of the MOSC paste. The high compressive strength of MOSC is derived from the sound composition and compact microstructure of the hydration products. The large water requirement of the fine H-FA particles in H-FM and the resulting filling effect are beneficial to the optimization of the composition of the hydration product and the formation of dense microstructures, respectively, which make the MOSC with H-FA possess excellent setting characteristics, high compressive strength, good volume stability and a compact microstructure. These results confirm the feasibility of applying H-FA in MOSC. Therefore, both types of FA can be used as a major component of MOSC, and the dose can be up to 50% of MgO weight or more, which improves the technological, economic and environmental benefits.

Comparative study of geopolymer and alkali activated slag concrete comprising waste foundry sand

Abstract Concrete industry and current construction practices are highly unsustainable from the standpoint of energy consumption and their high dependence on natural resources. Using alternative binders to conventional cement such as geopolymers and alkali activated slags (AAS) can be one alternative. Besides, reusing industrial by-product and waste materials in construction industry can also be emphasized. These measures can substantially lower the carbon emissions and embodied energy of concrete along with solving the problem of industrial waste disposal. The current study reports the influence of inclusion of waste foundry sand (WFS), a by-product from foundry industries, on strength, permeability and microstructure of low calcium fly ash geopolymer concrete (GPC) and ground granulated blast furnace slag (GGBS) based alkali activated slag (AAS) concrete, both cured in ambient conditions. The mix proportions for both the concretes were kept same. The natural sand was replaced by WFS in the range of 0% to 100% at an interval of 20%. Tests on hardened concretes, to study, compressive strength, split tensile strength, capillary suction, SEM and EDS analysis, were conducted to assess the strength, permeability and microstructure of both types of concretes. Addition of WFS lowered the workability of GPC and AAS concretes and the effect was abrupt beyond 40% WFS replacement level. Strength and sorptivity of both concretes improved upto 60% WFS replacement in GPC, whereas, upto 20% replacement in AAS concrete mixes, besides more than 45% of strength of reference concretes (0% WFS) was achievable at 100% replacement by WFS.

Influence of low calcium fly ash on compressive strength and hydration product of low energy super sulfated cement paste

This research aims to explore the effects of low calcium Class F fly ash (FFA) as an alumina supplier on the compressive strength and hydration products of an ecological super-sulfated cement (SSC) with fluidized bed combustion (CFBC) fly ash (CFA) as a crucial activator. Experimental results showed that partial replacement of ground granulated blast furnace slag (GGBFS/slag) by FFA in range of 10–30 wt% significantly improved the compressive strength of hardened eco-SSC paste. Microstructural examination using scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM/EDS), X-ray diffraction (XRD), thermogravimetric and derivative thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy obviously proved that the main hydration products of the hardened paste were ettringite/monosulfate (AFt/AFm) and calcium aluminum silicate hydrate (C-A-S-H). Increase in FFA resulted in higher degree of AFt/AFm precipitation and C-A-S-H with lower Ca:(Si + Al) ratio.

Sustainable alkaline activation of fly ash, aluminium anodising sludge and glass powder blends with a recycled alkaline cleaning solution

Introduction of new residues and wastes in the development of alkaline activated materials is a major concern regarding the spreading and dissemination of this technique. Such increase in the spectrum of available raw materials will decrease the need for long-distance transportation of the more traditional precursors that have been tirelessly used in the past two decades (e.g. fly ash, blast furnace slag). Additionally, the use of commercial activators severely hinders its environmental and financial performance, promoting the need to develop waste-based activators. In this paper, wastes from the aluminium anodising and extrusion processes and from the production of optical lenses were combined with low-calcium fly ash, to produce binders exclusively made from wastes and residues. The performance was assessed through uniaxial compression strength tests, which were accompanied by the monitorisation of several physical properties, including temperature, pH and weight; and by a thorough microscopic analysis, including scanning electron microscopy, coupled with energy dispersive spectroscopy, and x-ray diffraction, for mineralogy characterisation. Results show that the addition of glass powder and aluminium anodising sludge to class-F fly ash optimised the final precursor, which was then successfully activated with a disposed solution, originally used to clean the aluminium extrusion steel dies. The inclusion of soluble silicon and aluminium in the precursor raised the compression strength after short-term curing, compared with pastes prepared only with fly ash.

Mechanical behaviour of fly-ash-treated expansive soil

A major source of electrical energy is thermal power, which results in large quantities of coal ashes. Fly ash, which is produced from the combustion of pulverised coal, has an important place among industrial by-products. Currently, in India, more than 180 Mt of fly ash is being generated annually, posing the dual problem of environmental pollution and difficulty in disposal. This calls for establishing strategies to use the product as an engineered construction material in geotechnical applications for which the mechanical behaviour of soil–fly-ash mixes is an important consideration. In this study, high-calcium (class C) and low-calcium (class F) fly ashes in different proportions by weight were added to expansive soil to evaluate the effect of fly ashes on the strength behaviour of soil–fly-ash mixtures. Compacted clay–fly-ash samples were cured for 7 and 28 d and subjected to unconfined compression tests. It was found that 20% of class C fly ash is the optimum quantity to improve the characteristics of expansive soil, compared with 60% of class F fly ash. The main objective of the study was to illustrate the beneficial effects of fly ashes on the mechanical behaviour of fly-ash-treated expansive soil.

Impact of potassium cations on the light chemical stabilization of construction and demolition wastes

The ever-growing number of demolition materials that are landfilled has initiated several global incentives for reusing these wastes in industrial applications. Chemical stabilization is widely practiced for improving the mechanical properties of recycled aggregates. Although sodium silicate is the most popular alkali cation that has been extensively studied for stabilization and concrete production, the relatively high viscosity of the solution can be an obstacle to its usage in industrial applications. The second efficient alternative cation dissolution inducer that has been investigated for activation of low-calcium alkali activated systems is potassium. This research has evaluated the impacts of the higher enthalpy of potassium hydroxide and the overall lower viscosity of potassium dissolution for activation of low-calcium fly ash in order to stabilize demolition aggregates. Additionally, the impact of ground-granulated blast-furnace slag (GGBFS) on short-term strength improvement was also demonstrated. The resilient behavior of the stabilized demolition waste samples under repeated loading further indicates the viability of using stabilized demolition wastes for road base and subbase construction.

Effect of active components on strength development in alkali-activated low calcium fly ash cements

The compressive strength achieved in alkali-activated low-calcium fly ash depends on the total reactive oxide ratios in the activated system, the reactive alumina content in fly ash and the initial molarity of NaOH. The total reactive silica in the system is the reactive silica contributed by the fly ash and soluble silica from the alkaline solution. A minimum molarity of NaOH is required to ensure complete dissolution of the glassy phase present in fly ash. The maximum compressive strength depends on the amorphous reaction product content formed, which is determined by the reactive alumina content in the fly ash. The highest compressive strength is achieved for the ratio of the total reactive silica to sodium ratio in the activated system equal to 4.72 and lower. More Na is incorporated in the reaction product on increasing the Na2O content relative to the reactive oxides.

A study on the effect of nano clay and GGBS on the strength properties of fly ash based geopolymers

The role of reactive alumina and process variables such as sodium content and molarity on alkaline activation of different Nano clay and GGBS are explored. The reactive alumina content of fly ash is the key parameter which determines the maximum compressive strength achieved from the alkaline activation. In this paper, Effects of the incorporation of nano-clay in fly ash based geopolymer binders have been investigated. Low-calcium fly ash was used as the principal source of alumino silicate and it was blended with blast furnace slag in small percentages in order to accelerate the curing at room temperature. Nano-clay was added at different rates to the total binder on the 3, 7 and 28 day of curing.

The effect of aggressive environments on the properties of a low calcium fly ash based geopolymer and the ordinary Portland cement pastes

Abstract This paper analyzes the effects of four different aggressive environments, sulfuric acid solution (5%), magnesium sulfate solution (5 g/l), sodium sulfate (44g/l) and sodium chloride (164g/l), on the structure (durability) of fly ash based geopolymers and ordinary Portland cement. The samples were subjected to the aggressive solutions over a period of 120days. and the effects of the aggressive solutions were evaluated by visual, weight loss, and microstructural changes. It was found that the FA-GP geopolymer was poorly altered while the OPC cements are severely damaged after 120 days immersion in the 5% sulfuric acid solution. The decomposition of Portland cements in the acid solution is due to the formation of ettringite and gypsum which are very expansive materials which cause the cracking and bursting of the cement paste. The formation of ettringite and gypsum is confirmed by FT-IR and DRX analysis. The immersion in the magnesium sulfate solution, sodium sulfate and sodium chloride, caused a small change in the structure of the FA-GP and OPC, and we can conclude that the performance of FA-GP materials is higher than that of Portland cement in corrosive media.

Creep and Drying Shrinkage of Different Fly Ash-Based Geopolymers

Fly ash geopolymer concrete is a sustainable green construction material that has outstanding mechanical performance and is a low-energy material with a low carbon footprint. In this study, a detailed investigation of the long-term creep and drying shrinkage of three different 100% fly ash geopolymer concretes was carried out up to 1 year of age. Two geopolymers, produced from Gladstone and Pt. Augusta fly ashes, achieved approximately 700 microstrain at the end of 1 year-equivalent to the total creep strain displayed by portland cement (PC) concrete. Moreover, both geopolymer concretes displayed a lower creep coefficient than PC concrete. Hence, AS 3600 or the CEB-FIP model could be conservatively used to predict creep coefficient for two geopolymers. However, the Tarong fly ash geopolymer concrete differed significantly from the other geopolymers and achieved approximately 1900 microstrain after 1 year. The drying shrinkage of Gladstone and Pt. Augusta geopolymer concretes at 1 year are 175 and 190 microstrain, respectively, while Tarong geopolymer and PC concrete achieved 615 and 475 microstrain, respectively. All the fly ash geopolymer concrete showed lower drying shrinkage than the maximum permitted value recommended by AS3600. Incorporation of calcium-aluminasilicate-hydrate (C-A-S-H) gel with the sodium-alumina-silicatehydrate (N-A-S-H) geopolymeric gel was seen to positively affect the packing density of the gel phase. The degree of uniformity and compactness of aluminosilicate gel matrix together with the macroporosity in the 50 nm to 1 µm range was identified as determining the long-term creep and drying shrinkage of the 100% low-calcium fly ash geopolymer concrete.

Chloride induced corrosion in different fly ash based geopolymer concretes

This study investigates the corrosion of reinforcement in geopolymer concrete manufactured from three different low calcium fly ashes from Australian power plants. The long term corrosion condition of embedded rebar in fly ash geopolymer concretes containing cast in chlorides (0–5%) subjected to wet-dry cycles, together with specimens exposed to ponding in 3% NaCl were examined. Half-cell potential and linear polarisation resistance techniques were used to measure corrosion up to 540 days of age, and compared with results of a similar binder content PC concrete. Increased levels of corrosion were observed in the cast-in chloride geopolymer specimens compared with the equivalent PC concretes. However, in the case of the ponded specimens the reinforcement in the geopolymer concrete specimens displayed lower corrosion levels than the PC concrete. The higher corrosion rate in the cast-in specimens is attributed to a lower pH in the geopolymer specimens resulting in a higher Cl−/OH− ratio. In the ponded specimens the formation of three-dimensional N-A-S-H and C-A-S-H cross linking in the gel matrix reduces chloride diffusion to rebar depth, resulting in a lower corrosion rate being observed for ponded geopolymer specimens compared to the PC concrete.

Activity of sedimentary-fly-ash-aerated concrete

The key technology for the production of aerated concrete, using sedimentary fly ash to replace the new fly ash, is of enormous interest to the concrete industry. The chemical composition, mineral composition and microstructure of the sedimentary fly ash in Qian-Xi, Gui-Yang and Qing-Zhen power plants are presented. The activity of the sedimentary fly-ash-aerated concrete was tested in detail by employing grinding, mineral admixtures and chemical admixtures. The results show that the sedimentary fly ash is essentially low-calcium fly ash. The mineral composition of the sedimentary fly ash is basically the same as that of the new fly ash. The compressive strength and flexural strength of the ground fly ash specimens are larger than that of untreated fly ash. The use of mineral admixtures SH1 and SH2 for sedimentary fly-ash-aerated concrete can increase the compressive and flexural strength. The excitation effects of SIII activator, SIV activator and SV activator are better than those of other activators. The best chemical admixture is calcium oxide (CaO)–SIV activator. These studies are of significance for the use of sedimentary fly ash in aerated concrete. This study provides a new way to eliminate sedimentary fly ash.

Influence of processing temperature on the reaction product and strength gain in alkali-activated fly ash

The roles of temperature and curing periods on product formation and strength gain in alkali activation of low-calcium siliceous fly ash are evaluated. Higher temperature favors the kinetics of amorphous reaction product formation and inclusion of Si in the reaction product, which accompanies the densification of the microstructure. Following initial curing at 60 °C, the reaction product continues to form in the activated system on decreasing the temperature to 25 °C. Initial curing at 25 °C does not lead to formation of reaction product. Reaction product forms in the system for initial curing temperature of 40 °C and higher. The rate of the solid-based reaction, after hardening, is increased by temperature. The total amorphous reaction product content formed in the system depends on the water to solids ratio of the activated mix. The amorphous product composition, however, varies with temperature. With time there is an increase in the Si/Al ratio in the reaction product formed 40 °C and higher. For initial curing at 60 °C, there is no increase in the Si/Al ratio in the reaction product when the temperature is decreased to 25 °C. Reaction product with a higher Si/Al atomic ratio achieves a higher ultimate compressive strength. On increasing the Si/Al ratio in the reaction product from 2.5 to 3.3, the ultimate compressive strength achieved increased from 60 MPa to 74 MPa. Early curing, immediately after mixing, at a higher temperature is critical in achieving a higher Si/Al atomic ratio in the reaction product. Optimum initial curing time at high temperature is based on achieving a high Si/Al atomic ratio in the reaction product.

Some Studies on Sustainable Utilization of Iron Ore Tailing (IOT) as Fine Aggregates in Fly Ash Based Geopolymer Mortar

This study presents the sustainable utilization potential of Iron Ore Tailings (IOT) as a replacement material against natural fine aggregates in the preparation of fly ash based geopolymer mortar. Low calcium fly ash is used as the source material and a mixture of sodium hydroxide and sodium silicate is used as the alkaline activator in the mix. Systematic studies such as setting times and compressive strength of the various mixes with different alkali binder ratio’s were investigated in detail. It is to be noted that setting times of the mixes were found to be increasing with the increase in alkali binder ratio (0.4 to 0.8). Alkali binder ratio of 0.6 is found to be the optimum with respect to the compressive strength, irrespective of the type of fine aggregate. Scanning electron microscopy also reveals that microstructure of the fly ash based geopolymer mortar produced had a dense matrix with utilization of the Iron ore tailing. It can be concluded from the study that IOT is found to be a best alternative against the natural sand as a fine aggregate.