Alkali Activated Fly Ash Concrete Research Articles

Numerical Study on Static and Fatigue Behavior of Alkali-Activated Fly Ash Concrete Modelled using Concrete Damage Plasticity (CDP) Model

Abstract Alkali-activated concretes (AAC) are considered as an alternative to Portland cement concretes because of their much lower carbon emissions. To study its characteristics is of utmost importance. In the present research, an attempt is made to create a numerical model to analyze the static and fatigue flexural behavior of alkali-activated fly ash concrete (FC) and compare them with ordinary Portland cement concrete (PCC). Three-point bending tests are simulated for the monotonic load to obtain the static behavior of a 2D notched rectangular beam. A 2D beam specimen of size 262.5 mm x 75 mm, with a notch of size 15 mm x 3 mm was provided at the bottom middle. Fly ash-based alkali-activated concrete (FC) and corresponding PCC of similar strength are modeled in ABAQUS CAE using the concrete damage plasticity (CDP) model. The ultimate loads, maximum deflection, and CMOD for each of them were obtained from the history output manager. The results of monotonic tests are utilized for creating a sinusoidal load cycle for fatigue tests with various loading frequencies and stress ratios. Different structural responses are examined under the fatigue test. The results showed that FC performs better than PCC under static and fatigue tests. Fatigue performance of both FC and PCC is influenced by the frequency of loading and fatigue life increases at higher frequencies. It is also found that stress reversal negatively impacts the fatigue life of both FC and PCC.

Strength and post-fire performance of fiber-reinforced alkali-activated fly ash concrete containing granite industry waste

The mechanical properties and the post-fire performance of alkali-activated fly ash (AAF) concrete containing granite industry waste (GW) and polypropylene (PP) fibers were investigated to promote the application of environmentally friendly concrete. The recycled GW proportions of 0, 25, and 50% by weight were used in fine aggregates to replace the natural river sand, and the PP fiber contents of 0, 0.5, and 1.0% by volume were added to the concrete mixtures. The mechanical properties investigated included compressive strength, flexural load–deflection response, and toughness. The post-fire performance was evaluated from the residual compressive strength and weight loss, together with the visual inspection and the scanning electron microscopy (SEM) investigation of the specimens after fire exposure. It was found that the inclusion of PP fibers was effective to improve the flexural performance of AAF concrete, with or without the GW. Based on the toughness of AAF concrete, the optimal percentage for the GW replacement was 25%. After exposure to the ISO-834 standard fire, the results showed that the degradation of AAF concrete manifested itself in the inner core in the form of extensive cracking, particularly for the fiber-reinforced concrete containing the GW. The residual compressive strengths of AAF concrete were reduced to a range of only 25–39% after the fire exposure of 30 min.

RETRACTED: Alkali-activated fly ash concrete with untreated coal aggregate: Fresh- and hardened state properties

The alkali-activated materials found in agricultural and industrial waste have less carbon impact on concrete production. This study intends to use unprocessed charcoal gravel (UCG) and unprocessed charcoal fine gravel (UCFG) appropriately as a substitution of normal gravel stack to manufacture self-consolidated construction material (SCCM). The SCCM contains pulverized fuel ash (PFA), two different alkalis- NaOH and Na 2 SiO 3 as alkali activator, and superplasticizer (SP). The measured features are density, workability, air content, mechanical stresses, modulus of elasticity, gas-based, water-based, and chloride-based endurance, resistivity of electrical power and strength of acid on ten different SCCM mixes in the work. In the mixes, the stack of normal gravel was substituted with the stack of unprocessed charcoal gravel at ten contents in terms of volume: 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%. In addition to the measured features, new math models were suggested to estimate the durability and mechanical features of SCCM produced with different stack of unprocessed charcoal gravel. It was shown with the results that the SCCM features were developed significantly by substituting of 20% normal gravel stack with unprocessed charcoal gravel stack. In addition, the suggested math models could obtain to estimate accurately the features of SCCM containing PFA, alkalis, unprocessed charcoal gravel, and superplasticizer.

Mechanical and durability properties of alkali-activated fly ash concrete with increasing slag content

Alkali-activated concrete is a promising alternative to conventional Portland cement-based concrete. However, further understanding in relation to the durability of alkali-activated concrete is required. This paper explores the resistance of alkali-activated concrete mixes to aggressive media that concrete is expected to encounter in-service. Findings indicate that the chloride, freeze–thaw and acid resistance of alkali-activated concrete increases as the slag content and activator increased. However, performance against chloride and freeze–thaw was not as good as that of Portland cement concrete. Acid resistance seems to be a more promising characteristic of alkali-activated concrete compared with Portland cement concrete.

Mechanical properties, shrinkage, and heat evolution of alkali activated fly ash concrete

Use of high calcium fly ash as a binder to replace the ordinary Portland cement possibly cause a false set for alkali-activated concrete. Therefore, the appropriate mixed proportion of alkali-activated concrete must be considered for further applications. This research investigated the mechanical properties, shrinkage, and heat evolution of concrete using high-calcium fly ash as a binder with two alkali activators: sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). Ratios of sodium silicate to sodium hydroxide (Na2SiO3/NaOH) of 0.3, 0.4, and 0.5 by weight, and NaOH concentrations of 2, 4, 6, and 8 Molar were used. The ratio of liquid to binder was 0.7, and the fly ash content was 400 kg/m3. The results indicated that increasing the Na2SiO3/NaOH ratio and NaOH concentration improved the compressive strength of the concrete. The maximum value for the compressive strength, 27.5 MPa, was observed for a Na2SiO3/NaOH ratio of 0.5 and NaOH concentrations of 2 Molar at 90 days. The modulus of elasticity of alkali-activated fly ash concrete was found to be similar to that of conventional concrete. Additionally, the shrinkage of concrete tended to decrease with increasing NaOH concentration, and increasing the Na2SiO3/NaOH ratio led to increased concrete shrinkage. Finally, the heat evolution of alkali-activated fly ash concrete was found to be two times lower than that of concrete made from Ordinary Portland cement at the same compressive strength. The increase of NaOH concentration and Na2SiO3/NaOH ratio resulted in higher temperature rise of alkali-activated fly ash concrete.

Effect of Ordinary Portland Cement and Water Glass on the Properties of Alkali Activated Fly Ash Concrete

This research presents the influence of ordinary Portland cement (OPC) and/or water glass addition on fly ash alkali-activated mortar and concrete. The results show that fly ash (FA) concrete activated with a NaOH solution and water glass mixture had better resistance to freeze and thaw, carbonation, alkali-silica reaction (ASR) and developed higher compressive strength and static elastic modulus compared with the FA concrete activated only with an NaOH solution. The addition of OPC contributes to the development of a denser microstructure of alkali activated concrete (AAC) samples. In the presence of water glass and OPC, the compressive strength (52.60 MPa) of the samples increased more than two times as compared with the reference sample (21.36 MPa) without OPC and water glass. The combination of OPC and water glass showed the increased strength and enhanced durability of AAC. The samples were more resistant to freeze and thaw, ASR, and carbonation.

Effect of activator, curing and humidity on drying shrinkage of alkali-activated fly ash

Alkali-activated fly ash (AAFA) concrete is receiving considerable attention due to its environmental benefits; however, its long-term performance still requires validation and laboratory verification. This research studied the drying shrinkage of AAFA pastes as a function of activator composition (water content and n-modulus), relative humidity (RH) and heat-curing duration. Four AAFAs activated by different sodium silicate solutions and a control ordinary Portland cement (OPC) were examined. The activating solution affected the pore structure, pore solution and shrinkage characteristics of AAFAs. Use of an activator with a higher n-modulus resulted in a finer pore structure, superior mechanical properties and a larger magnitude of shrinkage. The results indicate that heat-cured AAFA paste dried faster, and its shrinkage reached equilibrium earlier compared to that of OPC. Similar to OPC, a bilinear relationship between RH and shrinkage of AAFA was observed. The results indicate that a longer steam-curing duration can stabilize the matrix of AAFAs and reduce their shrinkage.

Atom probe tomography of an alkali activated fly ash concrete

The atomic structure of alkali-activated materials (AAMs) is comprised of aluminum and silicon tetrahedral units organized in a manner that is elusive to characterization by many experimental techniques. This letter reports the use of atom probe tomography (APT) to determine the atomistic partitioning of an alkali-activated fly ash, which lends new insights into the nanoscale chemical heterogeneity of the material. APT effectively provides 3D ionic-level resolution of the alkali-activated fly ash examined in this study, and could be useful for the analysis of other cementitious materials or ceramics as well.

Mechanical, thermal and acoustic properties of cellular alkali activated fly ash concrete

This article characterizes a cellular system prepared from alkali activated Class C fly ash mixed with a preformed foam. The resulting mixture was cast at three densities between 1000 and 1400 kg/m3 and all tests were conducted under ambient drying conditions resulting in densities between 940 and 1310 kg/m3. A phase analysis of the components in the cell wall was performed using X-ray diffraction. The compressive strength ranged from 3 to 9 MPa, while the corresponding modulus of elasticity was between 850 and 1700 MPa. The thermal constants were measured using the Transient Plane Heat Source (TPS) technique per ISO 22007-2. Sound absorption and noise reduction coefficients were evaluated using the impedance tube method per ASTM C384-04. The results show that at an oven dry density near 1000 kg/m3, this alkali activated system is a viable alternative for thermal and sound insulation with properties comparable to those of current commercially available options.

Crack growth resistance in fibre reinforced alkali-activated fly ash concrete exposed to extreme temperatures

This paper examines the crack growth resistance of alkali-activated fly ash concrete under extreme temperatures. Plain and hybrid fibre reinforced alkali activated concrete prepared with fly ash were subjected to a range of temperatures from − 30 to 300 °C, sustained for 2 h. The alkali activation was effected with a blend of sodium hydroxide and sodium silicate. A fibre blend of steel to polypropylene in the volume ratio of 4:1 and a total as high as 1% by volume fraction reinforced the mixtures. The resulting systems were examined for residual strength under compression and splitting tension. Further, notched prisms were loaded under 4-point flexure to evaluate the residual fracture toughness. Based on the results, four different stages for fracture behaviour were identified with superior fibre efficiency seen at sub-zero temperatures. Across the breadth of temperatures examined, adding fibres led to higher residual fracture toughness for those systems that displayed a narrow range of thermal conductivity.

Comparison of chloride permeability methods for Alkali-Activated concrete

This paper presents the results of an experimental study of chloride permeability in alkali-activated fly ash, alkali-activated slag, and Portland cement concrete. Test methods include the rapid chloride permeability test (RCPT), AC and DC electrical resistivity, and the 90-day salt ponding test. We hypothesize that differences in pore solution chemistry between alkali-activated and Portland cement binders render electrical methods unable to accurately estimate chloride permeability in alkali-activated concrete. The present study seeks to evaluate this hypothesis by comparing results from electrical tests with diffusion coefficients from salt ponding tests. Contrary to previous claims, the RCPT provided a good estimate of chloride permeability in both alkali-activated slag and alkali-activated fly ash concrete. RCPT results showed excellent correlation with diffusion coefficients determined from salt ponding tests. Resistivity-measurements exhibited poor correlation to diffusion coefficients and overestimated the resistance to chloride ion penetration. Furthermore, AC and DC resistivity measurements showed significant disagreement for alkali-activated concrete. Finally, evidence form salt ponding tests suggests differences in chloride binding potential between alkali-activated and Portland cement concretes.

Environmental assessment of green concretes for structural use

This paper presents a comparative environmental assessment of several different green concrete mixes for structural use. Four green concrete mixes were compared with a conventional concrete mix: recycled aggregate concrete with a cement binder, high-volume fly ash concrete with natural and recycled aggregates, and alkali activated fly ash concrete with natural aggregates. All five concrete mixes were designed and experimentally verified to have equal compressive strength and workability. An attributional life cycle assessment, based on the scenario which included construction practice, transport distances, and materials available in Serbia, was performed. When treating fly ash impacts, three allocation procedures were compared: ‘no allocation’, economic, and mass allocation, with mass allocation giving unreasonably high impacts of fly ash. Normalization and aggregation of indicators was performed and the impact of each concrete mix was expressed through a global sustainability indicator. A sensitivity analysis was also performed to evaluate the influence of possibly different carbonation resistance and long-term deformational behavior on the functional unit. In this specific case study, regardless of the choice of the functional unit, the best overall environmental performance was shown by the alkali activated fly ash concrete mix with natural aggregates and the high-volume fly ash recycled aggregate concrete mix. The worst performance was shown by the recycled aggregate concrete mix with a cement binder.

Effect of Specimen Size and Curing Condition on the Compressive Strength of Alkali-Activated Concrete

Alkali-activated concrete is a rapidly emerging sustainable alternative to portland cement concrete. The compressive strength behavior of alkali-activated concrete has been reported by various studies to a limited extent, but these discussions have been based on minimal evidence. Furthermore, although it is known that specimen size has a distinct effect on the apparent compressive strength of concrete, this effect has yet to be modeled for alkali-activated concrete. This paper presents the results of a comprehensive study of the effects of curing condition (i.e., moist-cured at ambient temperature for 28 days or heat-cured at 50çC for 48 h) and specimen size on the compressive strength of sodium silicate–activated fly ash and slag cement concrete. The heat-cured strength of alkali-activated slag cement concrete was linearly related to the moist-cured strength; the former was about 5% greater than the latter. Heat curing also improved the strength of alkali-activated fly ash concrete, although the effect was greatly magnified for lower-strength mixtures and was much less significant at higher strengths. Existing size effect laws employed for portland cement concrete proved reasonably accurate in describing the effect of specimen size on the apparent strength of alkali-activated slag cement concrete. However, these existing models greatly underestimated the size effect in alkali-activated fly ash concrete; the authors suggest that this finding was the result of significant microcracking in the alkali-activated fly ash concrete.

On drying shrinkage in alkali-activated concrete: Improving dimensional stability by aging or heat-curing

The problem of excessive drying shrinkage in alkali-activated concrete (AAC) is well-documented in the literature. The magnitude of drying shrinkage is often three or more times that in portland cement concrete. This study investigates the effects of binder type, activator concentration, strength, age, and curing method on the manifestation of drying shrinkage in alkali-activated fly ash and slag cement concrete. Early-age shrinkage strains in excess of 1200με (0.12 percent strain) are observed in AAC. This is attributed to delayed hydration, microstructure refinement, and strength development. The resulting damage is far more significant than in portland cement concrete. Shrinkage and resulting damage are greatly reduced when specimens are dried at later age and after heat-curing. Alkali-activated slag cement concrete is more sensitive to water loss than portland cement or alkali-activated fly ash concrete. This results from a finer pore structure in alkali-activated slag binders.

Direct electric curing of alkali-activated fly ash concretes: a tool for wider utilization of fly ashes

Utilization of the fly ashes is a major problem in many developing countries and in South Africa only about 7% of the fly ash produced annually by coal-fired power stations, is being utilized. Although, fly ashes can be used as an alternative binder in alkali-activated concretes, strength development of these concretes at room temperature is slow limiting application of the material. Direct electric curing is proposed for heat curing of alkali-activated fly ash concrete which will open new opportunities for in-situ applications of these concretes in the construction industry thus increasing the amount of beneficially utilized fly ash. Alkali-activated fly ash concretes containing unclassified low calcium fly ash, sodium hydroxide and sodium silicate solutions were cured at 60 °C by means of direct electric curing. The electric resistivity and compressive strength development of the concretes were investigated. The resistivity strongly depends on the type of activator used. Compressive strength up to 33.8 MPa and 48.5 MPa at 2 and 28 days respectively, can be achieved after a short period of direct electric curing. This opens new opportunities for wider application of alkali-activated fly ash concretes and for more extensive utilization of fly ash.

The role of activating solution concentration on alkali–silica reaction in alkali-activated fly ash concrete

To enable commercial use of alkali-activated fly ash concrete, its durability must be better understood. Alkali–silica reaction is a primary concern since highly alkaline solutions are generally used for activation. This study investigated the effect of NaOH activating solution concentration on pore solution alkalinity and subsequent alkali–silica reaction in alkali-activated fly ash concrete. It was found that pore solution alkalinity increased with increasing activating solution NaOH concentration, and this effect was amplified at concentrations above an optimum, defined as the concentration that resulted in the highest mortar compressive strength. Expansion of concrete prisms containing highly reactive fine aggregate and activating solution concentrations above the optimum concentration was approximately three times that of concrete with optimum activating solution concentrations, but only about 5% of the expansion observed in the ordinary portland cement control. The low expansion may be attributed to the low calcium levels in the alkali-activated fly ash concrete.

Evaluation of short- and long-term properties of heat-cured alkali-activated fly ash concrete

Short- and long-term properties of alkali-activated fly ash concrete, namely, compressive, flexural and splitting tensile strengths, modulus of elasticity, Poisson ratio, early-age and drying shrinkage, creep and durability performance have been investigated in this paper. Blended ordinary Portland cement–fly ash concrete was used as a reference. The average compressive and flexural strengths of alkali-activated fly ash concrete at 28 d were 45 MPa and 5·8 MPa, respectively; drying shrinkage and creep values were about 360 and 200 microstrain, respectively, at 1 year. The total early-age shrinkage during the first 24 h of curing was lower for alkali-activated fly ash concrete at 1460 microstrain compared to 2800 microstrain for blended cement concrete. The use of this non-Portland cement containing material for producing precast concrete units can utilise high volumes of fly ash and potentially reduce the carbon footprint of the final product.

Air-Cured Alkali Activated Binders for Concrete Pavements

The present study focuses on the possibility of use of alkali activated binders for use in concrete pavements. Alkali Activated Slag Concrete (AASC) and Alkali Activated Slag Fly ash Concrete (AASFC) are prepared and the properties are compared with Ordinary Portland Cement Concrete (OPCC). The Ground Granulated Blast Furnace Slag (GGBFS) and Fly Ash (FA) are blended in the ratios 100:0, 75:25, 50:50 and 25:75 as binder and activated using strong alkaline solution. Trial mixes are carried out to identify the optimal Activator Modulus (Ms) for each combination of GGBFS and FA. The mix design for the optimal activator modulus is optimised to achieve sufficient strength for Pavement Quality Concrete (PQC) and the fresh and mechanical properties are studied in detail. The results indicate the properties of AASC and AASFC are similar or slightly better than conventional OPCC and satisfy the minimum strength requirements for concrete pavements. The application of alkali activated binders will minimise the environmental hazards occurring from augmented OPC production, along with effective utilisation of industrial waste materials and conservation of natural resources.

CONCRETE BASED ON ALKALI ACTIVATED FLY ASH FROM ONE POWER PLANT IN SERBIA

Alkali activated fly ash concrete (AAFAC) prepared with a class F fly ash from power plant „Nikola Tesla B“ and a mixed alkali activator of sodium silicate and sodium hydroxide solutions was investigated. Firstly, eight alkali activated fly ash pastes (AAFAP) with different activator concentration were tested in order to investigate the effect of this concentration on the compressive strength and workability. These pastes were prepared with different mass ratios of sodium silicate and sodium hydroxide solutions and a mass ratio of total alkali activator to fly ash equal to 0.6. Analyzing the compressive strength, cost and environmental effect of these mixtures, the mixture made with 10M sodium hydroxide solution, and a mass ratio of sodium silicate to sodium hydroxide equal to 10 was chosen as the optimal one for further analysis on concrete. To analyze the influence of additional water on the slump and compressive strength of AAFAC six different mixtures were made and tested. All samples were cured in the oven at the temperature of 80°C for six hours. The results of compressive strength and workability of tested concretes showed that fly ash from power plant „Nikola Tesla B“ can be used as a binder in AAFAC made with sodium silicate and sodium hydroxide as alkali activators.

Durability of alkali-activated fly ash concrete: Chloride penetration in pastes and mortars

Durability of alkalis-activated fly ash (AAFA) concretes is of vital concern for their commercial application. This study investigates the durability in the aspect of chloride penetration in AAFA pastes and mortars in comparison with Portland cement. A coarse fly ash with 0–40% substitution by a granulated blast furnace slag were used to prepared AAFA pastes and mortars at liquid/solid ratio of 0.6–0.8 (in mass). It was observed that at the same grade of compressive strength, even higher, the unsaturated AAFA paste and mortar had higher Cl− penetration rate than Portland cement paste and mortar. Only the mortars with the liquid/solid ratio of 0.6 and the mortar with 40% slag substitution exhibited similar Cl− penetration rate as the cement mortar at w/c of 0.5. The pore structure analysis showed that the porosity and tortuosity are the two most significant factors affecting the Cl− penetration. This study underlines the important roles of the natural properties of fly ash, liquid/solid ratio and slag substitution in the development of durable AAFA concrete.