Alkali-activated Fly Research Articles

Protection of galvanized steel using benzotriazole as a corrosion inhibitor in simulated concrete pore solution and alkali-activated fly ash solution

In this study, the natural passivation ability and chloride-induced subsequent increased levels of corrosion behaviour of galvanized steel in simulated concrete pore (SCP) solution and alkali-activated fly ash (AAFA) solution, with and without benzotriazole (BTA), were evaluated using corrosion potential (Ecorr), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic polarization (CPP). In addition, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) were used to observe the morphologies of passive layers and corrosion products of galvanized steel. It was highlighted that the hydrogen evolution could be prevented by AAFA solution to some extent, thus providing a new insight on the inhibition of hydrogen evolution for galvanized steel in concrete. Moreover, due to the formation of a complex protective film, galvanized steel in AAFA solution exhibited significantly higher corrosion resistance to chloride attack than that exposed to SCP solution, which could be further enhanced by the addition of BTA.

Fly Ash-Based Geopolymers as Lower Carbon Footprint Alternatives to Portland Cement for Well Cementing Applications

Ordinary Portland cement (OPC) is currently the preferred material for the creation of barriers in wells during their construction and abandonment globally. OPC, however, is a very carbon-intensive material with some inherent technical weaknesses. These include a low casing-to-cement bond strength which may allow for the formation of micro-annuli, which in turn can become a conduit for greenhouse gas transport (primarily of methane, a powerful greenhouse gas) to surface. Alkali-activated materials (AAMs), also known as geopolymers, have a much lower manufacturing carbon footprint than OPC and can be a good alternative to OPC for primary and remedial well cementing applications. This paper reports on a comprehensive study into the use of Class F fly ash-based geopolymers for a large variety of downhole well conditions, ranging from lower-temperature surface and intermediate casing cementing conditions to much higher temperature conditions (up to 204 °C (400 °F)) that can be encountered in high-pressure, high-temperature (HPHT) wells and geothermal wells. The rheological and mechanical properties of alkali-activated fly ash with six different sodium and potassium-based hydroxide and silicate activators were measured and compared to OPC. The results show that geopolymer formulation properties can be tuned to a variety of downhole cementing conditions. With the application of a suitable alkaline activator, geopolymers exhibit good compressive and tensile strength and an outstanding casing-to-cement bond strength of up to 8.8 MPa (1283 psi), which is more than an order of magnitude higher than OPC. This has important implications for preventing the creation of micro-annuli as a result of casing-to-cement interface debonding, thereby preventing the potential leakage of methane to the atmosphere on future wells that use geopolymers rather than OPC for barrier creation.

A multiscale study on gel composition of hybrid alkali-activated materials partially utilizing air pollution control residue as an activator

This paper provides a detailed underlying mechanism of gel composition of hybrid alkali-activated fly ash/slag (AAFS) pastes prepared by utilizing the air pollution control residue (APCr, a highly alkaline waste containing 38.3% of Na2O content by mass) as a partial replacement (8%, 16% and 24% replacement by mass) of energy-intensive commercial sodium silicate (CSS) activator. AAFS pastes containing APCr as a partial replacement of CSS showed the strength, micromechanical and microstructure properties equivalent to the reference AAFS paste without APCr (R-0). SEM-EDS, FTIR spectral subtraction, and nanoindentation results showed that reaction products in AAFS pastes were composed of geopolymer gel (N-A-S-H), cross-linked gel (C–N-A-S-H) and alkali-activated gel (C-A-S-H) contributing to 60–66% of paste volume. Reaction products were more dominated by calcium-rich gels (C-A-S-H or C-(N)-A-S-H) as compared to N-A-S-H gel in all AAFS pastes. The heat of hydration and thermogravimetric results confirmed the slow rate of reaction of APCr-AAFS pastes at the early age but a higher rate of reaction at the silater stage which contributed to achieve a similar strength as control paste (R-0).

Durability of fly-ash geopolymer binder in deep-mixed expansive soils

This study investigates the durability and leachate behaviour of an alkali-activated fly ash (FA) as a binder to stabilise expansive soils in deep-mixing applications. To activate FA, a liquid alkali activator (LAA) to binder ratio (LAA–FA) was introduced and varied from 1.0 to 1.5. The effects on swelling–shrinkage, consolidation and unconfined compressive strength (UCS) characteristics under wetting–drying cycles (durability) were studied on deep-mixed expansive soils. Leachate studies were conducted to determine the environmental impact of alkali-activated FA. Heavy metal and sodium ion concentrations were measured in each leachate cycle. To examine the permanent microstructural and phase changes in the mixes, scanning electron microscopy (SEM) and x-ray diffraction (XRD) studies were conducted. The swelling–shrinkage behaviour of soil could be marginally controlled for LAA/FA = 1.0 and 1.25 due to heavy weight loss during the wetting–drying cycles. However, even after 12 durability cycles, the UCS of soil with LAA/FA = 1.5 retained 1.0 MPa and showed non-critical swelling–shrinkage behaviour. The heavy metals and sodium ion concentrations were in acceptable range for LAA/FA = 1.25 and 1.5. XRD and SEM studies revealed that soil treated with a binder ratio of LAA/FA = 1.5, showed high crystalline peaks and aggregated structures.

Effect of active MgO on the hydration kinetics characteristics and microstructures of alkali-activated fly ash-slag materials

The effects of active S-MgO contents, MgO types, Na2O concentrations, and alkali activator modulus on the hydration kinetics, compressive strength, hydration products, pore structure, and microstructure of alkali-activated fly ash-slag (AAFS) were studied. The results showed that adding active S-MgO can reduce the total hydration heat of AAFS pastes, and the total hydration heat of samples incorporating 9% R-MgO increased significantly. Krstulovic-Dabic model was not completely suitable for the hydration reaction process of AAFS pastes. When the value of hydration degree α was small, the α value corresponding to the peak value of dα/dt can be used as the basis of evaluating the hydration kinetics of AAFS; When the value of hydration degree α was larger, the reaction process can be explained by the Krstulovic-Dabic model. The addition of active MgO reduced the amount of unreacted raw materials, and the volume of micropores, macropores, harmful pores and porosity increased. The addition of active MgO enriched the micromorphology of hydration products. The changes in hydration products, pore structure, and microstructure caused by the addition of MgO were the important reasons for the change in compressive strength.

The Development of Composites Based on Alkali-Activated Matrices Resistant to Extreme Temperatures

The article presents the results of research focused on the behavior of composites with a matrix based on alkali-activated materials when exposed to extreme temperatures (up to 1200°C). The behavior of the material based on alkali-activated fly ash, blast furnace slag, metakaolin, their mutual combinations respectively was analyzed in detail. The effect of thermal exposure on the properties of the developed materials was assessed by a complex of physical, mechanical and chemical methods. A specific area of research was the examination of the rheological properties of the developed mixtures in the fresh state.

Mechanical and microstructural properties of alkali-activated fly ash-slag material under sustained moderate temperature effect

Alkali-activated materials are difficult to maintain strength under very high temperatures in the fire. But results are not available for those materials when subjected to sustained moderate temperature range (i.e. 60 °C–250 °C) which is a more common thermal condition in many engineering applications. In this paper, the mechanical and microscopic properties of alkali-activated fly ash-slag (AAFS) materials under sustained moderate temperature effects are studied. The microscopic properties are characterized by XRD, FT-IR, 29Si NMR-MAS, MIP and SEM-EDS. The results show that AAFS is able to maintain the compressive strength even being treated under 250 °C for 28d. The microscopic characterizations indicate that N-A-S-H gel exhibits greater thermal stability than C-A-S-H and C-S-H gels. However, after the gel loses water under sustained temperature effect, N-A-S-H is hard to reorganize. In other words, when more N-A-S-H is produced due to the addition of more slag, the AAFS becomes more porous with looser microscopic morphology leading to lower compressive strength. It is also found that, after sustained moderate temperature treatment, the gel reorganizes and agglomerates into cluster-like particles, while the matrix becomes more uniform and compact. The sustained temperature helps refine the pore size and improve the volume of connected pores and super-macropores which is helpful to maintain the compressive strength of AASF. This study provides experimental and theoretical basis for the application of alkali-activated materials subjected to sustained moderate temperature effects.

Microstructure evolution of interfacial transition zone between alkali-activated fly ash/slag matrix and aggregate

Microstructure evolution of interfacial transition zone between alkali-activated fly ash/slag matrix and aggregate

Workability and Strength Characteristics of Alkali-Activated Fly ASH/GGBS Concrete Activated with Neutral Grade Na2SiO3 for Various Binder Contents and the Ratio of the Liquid/Binder

Abstract Alkali-activated fly ash/GGBS concrete (AAFGC) is a new blended concrete that has been studied by many researchers due to its environmental advantages and better technological characteristics. However, the effect of various factors on AAFGC fresh and hardened properties has not yet been thoroughly studied. The literature mainly describes the combination of NaOH and Na2SiO3 as an activator for the AAFGC’s activation, but alkali-activated GGBS concrete (AAGC) prepared with this activator solution is less workable due to its rapid setting behaviour. In this study, AAFGC was prepared using neutral grade liquid Na2SiO3 with a SiO2/Na2O ratio of 2.92. An experimental program was performed for fly ash-GGBS combinations (100-0, 50-50, 0-100), solution/binder ratios (0.6, 0.65, 0.7) and binder contents (300, 400 and 500 kg/m3) to evaluate the workability and the compressive and tensile strength of AAFGC. The results of this study show that a water glass of 2.92 silica modulus used as an activator to prepare AAFGC under ambient curing is very useful in the construction industry.

Microstructure and macro properties of sustainable alkali-activated fly ash mortar with various construction waste fines as binder replacement up to 100%

Utilizing construction waste fines as eco-friendly binder for sustainable alkali-activated materials provides new approach to recycling construction waste. This work investigated the characterization of alkali-activated fly ash mortar (AAM) incorporating construction waste fines at varied types and replacement levels. The results showed that incorporating waste paste powder containing abundant hydrated products refined the pore size of AAM, but mixing waste mortar powder and waste concrete powder that contained massive inert quartz and calcite adversely affected the alkali-activated reaction and pore structure of AAM. The incorporation of construction waste fines was found to reduce the fluidity but increased the drying shrinkage of AAM. At identical replacement level of construction waste fines, the waste paste powder incorporated AMM has lower fluidity and greater drying shrinkage than the waste mortar powder or waste concrete powder incorporated AAM. Substituting waste paste powder for fly ash improved the mechanical strength of AAM, but the strength decreased as waste mortar powder or waste concrete powder was incorporated. Particularly, the AAM blended with 100% construction waste fines still owned good strength. Mixing construction waste powder reduced the chloride diffusion coefficient and chloride ingress depth of AAM, and the waste paste powder mixed AAM had better chloride resistance than the waste mortar powder or waste concrete powder mixed AAM. Therefore, the hydrated products in construction waste fines was revealed to benefit the micro-characteristics of alkali-activated materials, and optimizing the type and replacement level of construction waste fines can achieve sustainable AAM with good mechanical strength and chloride ingress resistance.

Effect of incorporating municipal solid waste incinerated bottom ash in alkali-activated fly ash concrete subjected to accelerated CO2 curing

The effect of incorporating bottom ash from municipal solid waste incineration (MIBA) as partial replacement for fly ash (FA) (0%, 15% and 30%) was studied on alkali-activated mortars under two curing environments: conventional climatic chamber (∼0.04% CO2) and carbonation chamber (5% CO2). The microstructural characterization showed that the use of MIBA resulted in a decrease of calcium silicate hydrate gel (C–S–H) and sodium aluminosilicate hydrate (N-A-S-H) observed as “rounded growths” worsening the macrostructural behaviour (compressive/flexural strength, ultrasonic pulse velocity, dynamic Young’ modulus, dry bulk density, shrinkage, depth of carbonation and porosity accessible to water). The accelerated CO2 curing stage produced high amounts of aragonite, observed as “needle-like” formations improving the macrostructural behaviour. One tonne of mixture with MIBA and CO2 curing could decarbonise approximately 3695, 7391 and 10163 m3 of air volume at the age of 7, 14 and 28 days. The combined substitution of FA with MIBA and CO2 curing can be a highly viable technique in plain precast products.

Minimizing the Global Warming Potential with Geopolymer-Based Insulation Material with Miscanthus Fiber.

Approximately 45% of global greenhouse gas emissions are caused by the construction and use of buildings. Thermal insulation of buildings in the current context of climate change is a well-known strategy to improve the energy efficiency of buildings. The development of renewable insulation material can overcome the drawbacks of widely used insulation systems based on polystyrene or mineral wool. This study analyzes the sustainability and thermal conductivity of new insulation materials made of Miscanthus x giganteus fibers, foaming agents, and alkali-activated fly ash binder. Life cycle assessments (LCA) are necessary to perform benchmarking of environmental impacts of new formulations of geopolymer-based insulation materials. The global warming potential (GWP) of the product is primarily determined by the main binder component sodium silicate. Sodium silicate’s CO2 emissions depend on local production, transportation, and energy consumption. The results, which have been published during recent years, vary in a wide range from 0.3 kg to 3.3 kg CO2-eq. kg−1. The overall GWP of the insulation system based on Miscanthus fibers, with properties according to current thermal insulation regulations, reaches up to 95% savings of CO2 emissions compared to conventional systems. Carbon neutrality can be achieved through formulations containing raw materials with carbon dioxide emissions and renewable materials with negative GWP, thus balancing CO2 emissions.

Improvement of the thermal properties of alkali-activated fly ash after high temperature by the Fe-based solid wastes

Compared with Portland cement, AAM was selected as the high-temperature resistant material. But there is still a trend toward a significant reduction in compressive strength at 600 ℃ and 800 ℃. Thus, in this paper, the Fe-based solid wastes, i.e., copper slag and iron tailing, were used to improve the compressive properties of fly ash-based AAM after high temperatures. The result shows that the residual compressive strength of the Fe-based solid waste samples is higher than that of the blank sample. The residual compressive strength increases by about 20 % at 800 ℃. Though the results of SEM-EDS, MIP, and XPS experiments, the mechanism of the effect of Fe-based solid wastes on the AAM could be summarized as follows: the iron element in the Fe-based solid waste samples is more concentrated after high temperature, which may be caused by the melting of the original iron mineral phase into a new mineral phase, filling the crack gap. And the formation of the Fe-O-Si amorphous phase would improve the densification of samples, resulting in the improvement of compressive strength.

Effects of Fly Ash Inclusion and Alkali Activation on Physical, Mechanical, and Chemical Properties of Clay.

This study investigated the improvement in the behaviour of a clay soil due to the addition of alkali-activated fly ash as a stabilising agent, and the effects of different activation factors such as alkali dosages and silica moduli. The alkali activator solution used was a mixture of sodium silicate and sodium hydroxide. Class F fly ash was used as the precursor material for the geopolymerisation process. Soil samples stabilised with non-activated class F fly ash were prepared and tested to compare the results with samples stabilised with alkali-activated fly ash. Compaction tests, unconfined compressive strength tests, X-ray diffraction analysis, and scanning electron microscopy analysis were carried out on samples cured 1, 7, and 28 days at room conditions. The results showed that the compressive strength of stabilised soil significantly increased when the fly ash was activated. The optimal activation parameters to stabilise the soil were found to be alkali dosages in the range of 12% to 16% and a silica modulus of 1.25. The highest compressive strength recorded was at 1293 kPa with an alkali dosage of 16% and a silica modulus of 1.25, while for the non-stabilised soil, it was at 204 kPa at 28 days of curing. Mineralogical analysis showed a decrease in the peak intensities of kaolinite and illite, while microstructural analysis indicated an alteration in soil texture with the addition of the alkali-activated fly ash.

Influence of heat treatment on the mechanical properties of alkali-activated fly ash-based binders with marble dust substitution

Marble waste contains a high level of calcium, which is obtained from the cutting process in marble production. The properties of geopolymer binders are influenced not only by the amount of alkali activators, their ratio, the molarity used, and the Si and Al content of the mineral additives used in the mixture, but also by the duration of the heat treatment and the heat treatment temperature. This study aimed to produce alkali-activated geopolymer binders based on fly ash and marble dust. Alkali-activated geopolymer pastes were made both with heat treatment for 24h at 70°C and without heat treatment at (23±2) °C. Structural analysis by optical microscopy revealed, in the case of heat-treated samples, the formation of pores with an average size of 1÷1.5 mm, much larger than in the case of samples without heat treatment.

The mechanism of pristine steel slag for boosted performance of fly ash-based geopolymers

The high autogenous shrinkage of geopolymers hampers their application as building materials. In this study, pristine steel slag was used as an expansion material to mitigate shrinkage in geopolymers. The influence of pristine steel slag on the chemical deformation of alkali-activated fly ash-based geopolymers is investigated. The microstructure of the expanded system was studied by X-ray diffraction (XRD), FTIR measurement, thermogravimetric measurement (TGA), scanning electron microscopy with energy dispersive X-ray (SEM/EDS) and mercury intrusion porosimetry (MIP). Finally, the mechanisms of expansion were elucidated. The experimental results show that the shrinkage of the geopolymer is eliminated with the addition of steel slag. Fly ash was replaced by 0, 5, 10, and 15% of steel slag, and the expansion rate ranged from −0.29% to 20.24%. Increased steel slag content reduced the bulk density of geopolymer. The incorporation of steel slag not only provided calcium for the information of C-A-S-H but also filled the pores. Steel slag produces different expansion effects in different porous media and thus refines the pore structure. Therefore, expansion by means of steel slag is an efficient approach for mitigating shrinkage in fly ash-based geopolymers.

Effect of reactive magnesium oxide in alkali-activated fly ash mortars exposed to accelerated CO2 curing

The effect of reactive magnesium oxide (MgO) as partial fly ash replacement in alkali-activated fly ash mortars (0, 10 and 20 %) was studied under two different environments: conventional climatic chamber and climatic carbonation chamber with 0.04 and 5 % level of CO2 respectively. A complete characterisation of all raw materials was performed. The effect of replacing fly ash with MgO resulted in a decline of compressive (up to 52.3 % at 28 days of age and 20% of MgO replacement) and flexural strength, ultrasonic pulse velocity and dynamic Young’s modulus mainly due to the formation of weak and expansive brucite, detected by X-ray diffraction (XRD). Carbonation depth and shrinkage also were obtained. CO2 curing improved the physicomechanical properties, due to the formation of nesquehonite in samples with MgO. An improvement of CO2 absorption of 2 g CO2/kg mixes with the CO2 curing and the substitution of fly ash with MgO was obtained (determined by thermogravimetric and differential thermal analysis). The increase in mass due to CO2 capture was of 6.91%, 9.39% and 12.26% at the age of 7, 14 and 28 days, respectively, when compared to mixes cured with 0.04 (reference) and 5 % (CO2 curing) level of CO2.

Effect of Sodium Hydroxide Molarity on the Alkali-Activated Fly Ash and Granulated Slag Geopolymer Binders with Water Glass – Part I

Effect of Sodium Hydroxide Molarity on the Alkali-Activated Fly Ash and Granulated Slag Geopolymer Binders with Water Glass – Part I

Life cycle assessment of alkali-activated concretes under marine exposure in an Australian context

Alkali-activated concretes have been shown to be an environmentally advantageous construction material as they utilize waste or by-products as precursors, such as fly ash and ground blast furnace slag. These concretes also have the ability to achieve strengths suitable for structural applications. However, each mix design of an alkali-activated concretes is unique and requires a detailed life cycle analysis to determine the environmental impact and cost viability. This study evaluates the feasibility of alkali-activated fly ash or fly ash geopolymer and alkali-activated slag concrete developed for application in Australia currently subject to long term site performance studies. This paper reports a detailed life cycle assessment analysis of these concretes to assess their environmental footprint. The paper considers three distinct allocation methods: baseline, mass, and economic allocation, with two system boundaries: manufacture and construction. The study shows AAS concrete has a lower environmental impact than an equivalent strength PC concrete in two impact categories, global warming potential and eutrophication, while acidification and human toxicity depend on the allocation method applied in the manufacturing stage. The global warming potential (100-years) of AAS is 5.25–35% less than PC concrete. The FAGP concrete has more negative impacts on the environment than PC-based concrete, regardless of which allocation method is applied. The report highlights alkaline activators and transportation of raw materials as the main environmental impact contributors to concrete manufacture. The global warming potential of FAGP is 22–34% higher than the equivalent PC concrete in the baseline method. For the construction stage boundary, machinery appliances for handling concrete and transportation only contributes a small environmental impact (<4%) compared with concrete manufacturing. Cost estimation for concrete production suggests that the cost of the alkali-activated concrete is competitive with the conventional concrete market, dependant on the proximity of the feedstocks and the cost of sodium silicate. Neglecting the transportation cost of feedstocks, the cost of AAS concrete can be 4.8% cheaper, whereas the cost of FAGP concrete is 2.7% more expensive than PC concrete.

Study of particle packing and paste rheology in alkali activated mixtures to meet the rheology demands of 3D Concrete Printing

The rheological behavior of concrete mixtures made with alkali-activated fly ash-slag (AAFS) binder paste is investigated for varying aggregate content. The static yield stress, plastic viscosity and thixotropy of the mixtures are evaluated and related with the performance in 3D Concrete Printing (3DCP). The thixotropy of the concrete mixture is primarily influenced by AAFS binder paste composition and does not change with the aggregate content. Rheology control of the AAFS binder paste is achieved using additives like nano-clay, which enhance the buildup of internal structure and provide thixotropy in the concrete mixtures. The paste content, the surface area of aggregate, and the packing of solids in a concrete mixture influence its yield stress. The static yield stress increases sensitively in relation to the proportion of the surface area of the solids to the paste volume content in the concrete mixture. The minimum requirement of the paste content from packing of solids in the mixture is given by the Fuller-Thompson curve. Excess paste content in proportion to the demand determined from the surface area of aggregate and packing of solids in the concrete mixture produces a decrease in the static yield stress. For producing AAFS concrete mixtures suitable for 3DCP, the paste content in the concrete mixture should meet the requirement of paste demand, which depends on the aggregate content and packing of solids.