Alkali-activated Fly Research Articles

Analysis of reaction degree and factors of alkali-activated fly ash/slag

The dissolution rates of alkali-activated fly ash (FA) and alkali-activated slag in hydrochloric acid (HCl) with various pH values were investigated. The geopolymerisation reaction degree of alkali-activated FA/slag (AAFS) can be determined based on the content of hydrochloric-acid-insoluble matter, so X-ray fluorescence, X-ray diffraction and Fourier-transformed infrared spectroscopy were used to analyse the hydrochloric-acid-insoluble matter of the alkali-activated FA and alkali-activated slag. Acid dissolution tests on the AAFS showed that the addition of slag increased the geopolymerisation degree of FA in most cases. For a certain slag content, the reaction degree of FA decreased with an increase in the modulus of the activator and increased with an increase in the liquid/solid ratio. An increase in activator concentration had a greater effect when the slag content was low. At low activator concentrations, the reaction degree of FA increased with an increase in slag content. The slag content corresponding to the highest reaction degree of the FA gradually reduced with an increase in activator concentration.

Effect of superabsorbent polymer and polypropylene fiber on mechanical performances of alkali-activated high-calcium fly ash mortar under ambient and elevated temperatures

This study investigated the mechanical properties of alkali-activated fly ash (AAF) mortars containing superabsorbent polymers (SAPs) and polypropylene (PP) fibers before and after exposure to elevated temperatures. AAF mortars were synthesized using sodium silicate and high-calcium fly ash as the solid activator and aluminosilicate source, respectively. This procedure classified the binder as a one-part AAF mortar. The mechanical properties of the AAF mortar, including compressive and flexural strengths, were investigated considering eight mixes with varying SAP contents (0%, 0.5%, 1%, and 2% by weight of binder) and fiber contents (0 and 0.2% by volume). Moreover, physical properties including the permeability and sorptivity were examined, and physiochemical analysis was performed to evaluate the behavior of AAF mortar at room temperature, 300, 500, and 700 °C. The results show that the compressive strength of specimens at room temperature slightly decreased owing to the increased macroporosity caused by SAPs through pre-swelling, while the flexural strength was maintained via an internal curing effect. The PP fibers significantly enhanced the flexural capacity of the cement mortar, while the compressive strength initially decreased at 28 d and later exhibited a negligible effect at 90 d. The specimens produced using SAPs were effective in preserving the residual compressive and flexural strengths compared with those without SAPs after exposure to 300 °C. Meanwhile, the residual strength of PP fiber specimens diminished after elevated temperature exposure because the melted fiber led to the excessive formation of cavities.

Stabilization of Aeolian Sand for Pavement Subbase Applications Using Alkali-Activated Fly Ash and Slag

Using local materials to construct building elements as well as transport road facilities, including highways, intercity roads, and roads, in remote areas is a top topic of scholarly research all over the world. The main reason for that is the fact that these kinds of materials not only ease the intensity of material transportation but are also cost-efficient. In desert areas, aeolian sand is a commonly used local material and it has been investigated in unbound and cement-stabilized pavement base/subbase applications. However, the production of cement is associated with a high carbon footprint, leading this research to seek alternative low-carbon binders. This research investigated the strength properties and the carbon footprint of fly ash (FA) and a ground-granulated blast-furnace slag (S)-based geopolymer-stabilized aeolian sand. Setting time, compressive strength, California bearing ratio (CBR), and temperature shrinkage measurements of the stabilized aeolian sand were carried out in this research. The maximum strength of the stabilized aeolian sand was found at the optimal ratio of Si/Al ratio of 2.5 and Na/Al ratio of 1.0. The compressive strength increased as the geopolymer stabilizer content increased. A stabilizer content ranging between 8% and 20% is recommended in practice. The carbon footprint of the geopolymer-stabilized aeolian sand was lower than that of cement-stabilized aeolian sand. This tendency became more evident in the samples with higher strength, indicating the effectiveness of geopolymer as an alternative green soil stabilizer to traditional Portland cement.

ANALYSIS REGARDING THE MECHANICAL PROPERTIES OF ALKALI-ACTIVATED FLY ASH-BASED GEOPOLYMER CONCRETE CONTAINING SPENT GARNET AS REPLACEMENT FOR SAND AGGREGATES

As research regarding the production and optimization of geopolymer materials is constantly increasing and is fundamentally motivated by the need, identified both in the global ecological context and at national level, new materials can be developed using this type of binder. Sustainable consumption of resources, focuses on the capitalization of existing waste and prevention of generating new ones, therefore adopting the Sustainable Development principles. The increase in the utilization of sand from rivers for various construction purposes, which also disturbs the environment can be considered another damaging factor for the environment. Recycling of garnets and their use as replacement for sand aggregates could provide an ecological solution for the production of the alkali-activated fly ash-based geopolymer binders. The aim of this paper is to present preliminary results regarding the use of spent garnet as replacement for sand aggregates in the production of alkali-activated fly ash-based geopolymer binders using Romanian local raw materials and to study its influence on the mechanical performances of the binder.

Shrinkage mitigation of alkali-activated fly ash/slag mortar by using phosphogypsum waste

Shrinkage in alkali-activated materials (AAMs) is relatively high, and its mitigation strategies are welcome to improve the performance of AAMs for practical applications. This study investigates the replacement of fly ash with phosphogypsum (PG) waste to mitigate shrinkage in alkali-activated fly ash/slag (AFS) mortar. A range of tests were carried out to ascertain the autogenous shrinkage, drying shrinkage, setting time, and compressive strength of the AFS. SEM/EDS and XRD characterized the microstructure and hydration products of the AFS paste. It was founded that using PG shortened the setting time of the AFS mixture cured at room temperature. Incorporating 5% of PG treated at 40 °C in the AFS mortar was optimal for improving its autogenous and drying shrinkage. Using an optimal addition of PG in the AFS mortar led to the formation of ettringite with a micro-expansive effect to compensate for shrinkage. However, a high PG content used may inhibit the gel phase formation in the AFS matrix, resulting in a reduction in the compressive strength of the AFS mortar.

Alkali-activated fly ash foam concrete with Yellow River silt: Physico-mechanical and structural properties

Developing innovative techniques to elevate the utilization efficiency of the Yellow River silt (YRS) is very important for the protection of the Yellow River in China. In this study, an approach to use the YRS in the fabrication of alkali-activated fly ash (AAFA) foam concrete was first attempted, and YRS was used as a kind of supplementary binder material. The effects of silt dosage and alkali content on the fluidity, water absorption, compressive strength, drying shrinkage and thermal conductivity of as-fabricated foam concrete with YRS were examined. Besides, the characteristic of pore structure and microstructures of foam concrete were characterized. The results show that the incorporation of YRS significantly affects the physico-mechanical properties of the AAFA foam concrete. With the increase of YRS dosage, the fluidity of fresh foam concrete and the dry density, mechanical properties, thermal conductivity and drying shrinkage of hardened foam concrete is reduced. However, the water absorption and pore structure of foam concrete is improved. In addition, the change of Ca(OH)2 content has a great influence on the physico-mechanical and structural properties of AAFA foam concrete. With the increase of Ca(OH)2 dosage, the dry density and thermal conductivity of AAFA foam concrete is increased obviously. Furthermore, the main hydration product of the AAFA foam concrete with YRS is C-S-H gels. The overall findings demonstrate the feasibility of using YRS as dependable and cost-effective building materials in the preparation of AAFA foam concrete.

Study on the characteristics of alkali-activated fly ash-slag improved by cenosphere: Hydration and drying shrinkage

Cenosphere is coal-based solid waste material, which has similar chemical compositions to fly ash. In this paper, the effect of the cenosphere on hydration kinetics and drying shrinkage characteristics of AAFS pastes were explored, and the action mechanism of the cenosphere was deeply revealed. The results showed that the hydration induction period was prolonged, the hydration exothermic rate was reduced or the peak value was delayed during the acceleration period, and the 72 h hydration degree of the sample was increased when mixed with the cenosphere. The incorporation of more than 20% cenosphere significantly reduced the drying shrinkage of AAFS pastes. Comparing the results of compressive strength, drying shrinkage, and pore structure, the most suitable content of the cenosphere is 20%. To significantly improve the drying shrinkage of alkali-activated materials, the content can also be increased. The action mechanisms of the cenosphere are shown as the filling effect, pozzolanic activity, and surface effect. The addition of cenosphere reduces the amount of C-S-H, C-A-S-H and N-A-S-H gel phases, but increases the formation of silicates (calcium hydrosilitates and calcium hydroaluminates).

Relationship between macro-properties and amorphous gel of FA-based AAMs with different curing conditions after elevated temperature

This study investigated the effect of curing conditions (40 °C, 60 °C, and 80 °C curing temperature, 3 h, 5 h, and 8 h curing time) on the macro-properties (compressive strength, surface properties, and length change) of alkali-activated fly ash materials (FA-based AAM) at elevated temperatures (300 °C, 600 °C, 800 °C, and 1000 °C). The microstructure properties of samples before/after high-temperature treatment were studied by the FTIR, XRD, XRD- Rietveld, SEM-EDS, and 29Si NMR experiments. The results show that the compressive strength of samples shows a decreasing trend beyond elevated 600 °C. A similar phenomenon was found in the surface and length properties. Unlike samples with lower curing temperatures (40 °C) exit the lower residual mechanical properties beyond 600 °C, samples with higher curing temperatures have stable performance in the compressive strength, surface properties, and length change. And high curing temperature is favorable for the formation of amorphous gels with a Si-rich phase in FA-based AAM. Lower diffusion coefficient of water and high skeleton stability were obtained in the higher Si/Al ratio of N-A-S-H amorphous gel decreasing the evaporative rate of water to exacerbate the damage of microstructure at a low elevated temperature, while increasing the stability at high temperature.

Alkali-activated fly ash cured with pulsed microwave and thermal oven: A comparison of reaction products, microstructure and compressive strength

The reaction products and microstructure of an 8 M NaOH-activated fly ash (AAFA) cured with microwave and thermal oven curing were comparatively investigated. The results show that the formation of an Al-rich N-A-S-H gel with a flaky morphology and crystalline chabazite-Na between fly ash particles were favoured under the microwave curing, whilst a Si-rich N-A-S-H gel with a fibrilllar morphology and crystalline hydroxysodalite on the surface of fly ash particles were dominant under the thermal oven curing. The formation of these unique reaction products and microstructure under the microwave curing is attributed not only to the thermal effect of microwaves which can increase the temperature of AAFA volumetrically, but also to the non-thermal effects of microwaves which can promote the dissolution and diffusion of silica and alumina during the AAFA reaction process. This explains why AAFA can achieve a high early strength within a short period of microwave curing.

Investigation of agro-forestry and construction demolition wastes in alkali-activated fly ash bricks as sustainable building materials

Production of burnt clay bricks, cement and burning of agricultural/forestry wastes are responsible for major greenhouse gases emission. The present work investigate the effect of treated rice straw and forestry leaves in alkali-activated fly ash bricks with construction & demolition wastes for non-load bearing partitions walls at elevated temperature of 800 °C. 1–4 wt% incorporated agro-forestry wastes fly ash bricks with 10 and 20 wt% ground granulated blast slag (GGBFS) addition shows compressive strength ranging from 8 to 15 MPa. The thermal conductivity of the panels with 1–4 wt% agro-forestry addition varies from 0.4 to 0.5 W/m.K. The sound transmission class (STC) of 20 mm thick agro-forestry waste fly ash samples depict moderate sound insulation properties over the range of 24 to 37 dB. The wallette units of burnt clay bricks and agro-forestry based fly ash bricks show load carrying capacity of 360 kN, 273 kN and 195 kN, 110 kN at temperatures of 35 °C and 800 °C respectively. The present study shows a potential green solution toward sustainable building materials in construction sector leading to reduced depletion of fertile soil used in production of burnt clay bricks.

Behaviour of alkali-activated concrete at elevated temperatures: A critical review

Alkali-activated concrete (AAC) is recognised as a novel sustainable construction material to substitute Portland cement concrete with superior thermal and mechanical performance. However, AAC would suffer significant deterioration when subjected to elevated temperatures due to different damage mechanisms, including thermal incompatibility caused by different thermal coefficients between matrix and aggregates, pore pressure build-up and phase transformation. This paper presents a systematic and comprehensive review on the behaviour of different types of AAC such as alkali-activated fly ash, alkali-activated slag, alkali-activated metakaolin and alkali-activated fly ash-slag systems at elevated temperatures in terms of phase stability and microstructural evolution as well as thermal and mechanical performance. The effective strategies for improving the high-temperature resistance of AAC are reviewed and discussed from the perspectives of AAC matrix, aggregates and fibre incorporation, with special focus on how these strategies can tackle different damage mechanisms. This paper summarises the recent advances in the field and identifies the remaining challenges and opportunities for future research.

A numerical study on chloride transport in alkali-activated fly ash/slag concretes

Alkali-activated binders for concrete composites are emerging as the available low-embodied-carbon alternatives to ordinary Portland cement (OPC). Despite decades of research, the durability of alkali-activated concrete remains a concern, especially the issues related to chloride penetration and the potential for chloride-induced corrosion of embedded steel bars. In this study, a multi-phase, multi-component ionic numerical model is developed to investigate the chloride transport in alkali-activated fly ash/slag (AAFS) concretes. The model framework considers the porosity of concrete, the chloride binding, and the electrochemical coupling between multi-species. Based on the proposed model, the parametric studies of a series of influencing factors on both chloride transport and initiation time of reinforcement corrosion are performed and discussed. The results indicate that the slag content and the water to binder ratio are the dominant factors affecting chloride penetration and corrosion initiation time of steel bars in AAFS concretes due to their greater contributions to reduce the porosity, while the aggregate volume fraction presents a limited effect compared with other factors. The transport model proposed in this study brings insights to the prediction of the service life of alkali-activated reinforced concrete structures.

Chloride transport resistance of alkali-activated concrete exposed to combined chloride, sulfate and carbonation environment

The chloride transportation in alkali-activated fly ash (AAFA), alkali-activated slag (AAS) and alkali-activated fly ash/slag (AAFS) concrete exposed to the environments of single chloride and combined of chloride, sulfate and carbonation were investigated. The iodide migration, chloride diffusion and capillary absorption tests were carried out before and after exposure, as well as the analysis of their microstructure and compositions. It was found that the initial chloride transport of AAFA concrete was the highest but decreased gradually with the increase of exposure period. For AAFA and AAFS specimens with a relatively lower strength, chloride and sulfate salt crystallization led to the surface spalling and strength reduction. The coupling of carbonation would further accelerate the deterioration of AAFA, but the inhibitory effect was acted for AAFS specimen. Although the appearance of AAS specimens had little change under various aggressive environments, carbonation led to the decalcification of C-(A)-SH gels and deterioration of pore structure, both the porosity and the harmful 10–50 nm pores content of the specimens were higher compared with that before exposure, and their chloride transport increased with time. However, the deterioration of AAFS specimens resulted from carbonation was less serious, due to the formation of C-(N)-ASH gels with a higher degree of cross-linking and polymerization, thereby it had the best long term performance under the combined environment of chloride, sulfate and carbonation.

Influence of Erosion Induced by NaCl on the Mechanical Performances of Alkali-Activated Mineral Admixtures

In this paper, the influence of NaCl freeze-thaw (F-T) cycles and dry-wet (D-W) alternations on the flexural, compressive and bonding strengths of alkali-activated fly ash (FA) and a blast furnace slag powder (BFS) is investigated. The considered NaCl concentration is 3%. The effect of polypropylene fibers on the mechanical strengths is also examined. Scanning electron microscopy (SEM), thermogravimetry (TG) and X-ray diffraction (XRD) are selected to discern the mechanisms underpinning the NaCl-induced erosion. The obtained results indicate that the best results in terms of material resistance are obtained with admixtures containing 60% BFS and 40% FA in terms of mass ratio and 3% polypropylene fibers in terms of volume ratio. The maximum rates of decrease of the flexural, compressive and bonding strengths after 300 NaCl F-T cycles are 21.5%, 20.3% and 22.6%, respectively. The corresponding rates of decrease due to NaCl D-W alternations are 28.1%, 26.1% and 31.5%, respectively. The TG curves show that the alkali-activating activity of BFS is higher than that of FA. Moreover, in the first case, the microstructure of the hydration products is more compact. The results also show that NaCl F-T cycles lead to increasing cracks in the alkali-activated BFS.

Strain-hardening alkali-activated fly ash/slag composites with ultra-high compressive strength and ultra-high tensile ductility

This study designed and developed strain-hardening alkali-activated fly ash/slag composites (SH-AAFSC) with both ultra-high compressive strength and ultra-high tensile ductility for the first time. The developed SH-AAFSC showed a compressive strength of 94.4–180.7 MPa and a tensile strain capacity of 8.1–9.9 %, which successfully pushed the performance envelope of alkali-activated materials. A multi-scale investigation was conducted to get an in-depth understanding of the obtained mechanical properties. Results showed that higher GGBS content increased the Ca/Si ratio of C(N)ASH, leading to a further refined microstructure with reduced porosity. As w/p ratio decreased from 0.27 to 0.22, the compressive strength significantly increased but the tensile ductility slightly decreased. Notably, a strong linear relationship was observed between fiber-bridging strength and the average elastic modulus of matrix obtained from nanoindentation. The study provided an avenue to produce SH-AAFSC towards ultra-high compressive strength and tensile ductility, which are promising for resilient and sustainable infrastructures.

Alkali activation of recycled concrete and aluminum salt slag aggregates for semi-rigid column inclusions

Stone columns develop their load-bearing capacity from the circumferential confinement offered by the surrounding soils. In highly plastic soils, the circumferential confinement provided by the surrounding soft soils may not be sufficient to develop the required bearing capacity. Meanwhile, the ground improvement industry has been keen to explore substitution backfill materials due to the depletion of natural aggregate resources typically used to construct stone columns. This research investigated the viability of using recycled concrete aggregate (RCA) and recycled aluminum salt slag (RASS), stabilized with alkali-activated fly ash (FA), slag (S) and a binary precursor (FA + S), as a novel semi-rigid inclusion column system for ground improvement of soft soils. The effect of the RCA/RASS ratios, additive types and dosages along with different curing conditions on the unconfined compressive strength (UCS) of geopolymer-stabilized mixtures, were evaluated. Various reinforcing mechanisms were suggested through the microstructure characterization of the mixtures. In addition, the permanent deformation responses of mixtures under cyclic loading were studied using repeated load triaxial (RLT) tests that simulate the cyclic moving traffic loads imposed on the stone columns system. The results demonstrated that increasing the RASS contents resulted in a subsequent decline in the UCS of the mixtures. Increasing the additive dosages on the other hand, in general led to higher strengths. The effects of both curing duration and temperature were found to play a vital role in the strength development of the mixtures. Furthermore, 100RCA stabilized by as low as 5% alkali-activated FA, S or (FA + S), met the minimum UCS requirement of 1.034 MPa for ground improvement. The highest reduction in permanent strain of 87.3% was recorded for the stabilized 100RCA + 10%(FA + S) mixture compared to the unbound 100RCA (control group). The practical implications of the findings were also discussed in detail throughout the paper.

Durability and mechanical long-term performance of reclaimed asphalt pavement stabilized by alkali-activation

The application of alkali-activated industrial by-products for the stabilization of reclaimed asphalt pavement (RAP), can become a sustainable solution to reduce the carbon footprint of road construction and maintenance activities. Furthermore, this approach can also reduce the increasing depletion of natural resources. Thus, the durability and long-term mechanical performance of RAP stabilized with alkali-activated fly ash were assessed in this study. The alkaline activator was a solution composed of sodium hydroxide and sodium silicate. To this extent, unconfined compressive strength (UCS) and durability tests were conducted in this research. The proposed alkali-activated binder significantly increased the UCS of RAP mixtures, with long-term (365 days) results reaching values up to 32 MPa; fulfilling the strength requirements for cement-stabilized soil mixtures and even stable inorganic binder materials for road base and sub-base layers of pavements. These results indicate that when stabilized with an alkali-activated fly ash binder, RAP presents several applications for road engineering; even when subjected to seasonal variations in humidity and temperature, as shown by the durability tests.

Effect of Chloride on Accelerated Corrosion of Steel Rebar in Alkali-Activated Fly Ash and Paper Sludge Ash–Reinforced Concrete

Effect of Chloride on Accelerated Corrosion of Steel Rebar in Alkali-Activated Fly Ash and Paper Sludge Ash–Reinforced Concrete

Effect of activator properties on drying shrinkage of alkali-activated fly ash and slag

Alkali-activated fly ash and slag (AAFS) pastes with a CaO content of 10%–20% are cementitious materials with high strength and good durability. The correlation between the drying shrinkage and microscopic characteristics of the pastes is investigated. By affecting the reaction degree of the pastes, the alkali content and activator modulus cause different pore-size distributions and deformation abilities, leading to different capillary pressures and shrinkage of the pastes during drying. The elastic deformation of the pastes under the capillary pressure during drying only accounts for a small part of the whole drying shrinkage. The long-term development of drying shrinkage mainly depends on the viscous behavior of the pastes under capillary pressure. The change law of the elastic deformation part with the activator modulus and alkali content is positively correlated with that of the corresponding capillary pressure. Moreover, the change law of the creep contribution part with the activator modulus and alkali content negatively correlates with that of the corresponding strength.

The effect of reinforcement ratio on the flexural performance of alkali-activated fly ash-based geopolymer concrete beam

Alkali activated fly ash (AAFA) based geopolymer concrete structure is getting attention due to its eco-friendly construction characteristics and improved engineering properties. However, comprehensive studies on the structural performance of hardened properties of AAFA geopolymer concrete is not well addressed, especially on non-linear fracture behavior. This paper aims to present the reinforcement ratio effect on the flexural performance and non-linear fracture characteristics of alkali activated fly ash based geopolymer concrete beams. Sixteen finite element (FE) and four experimental models were used to study the effect of reinforcement ratio on the flexural performance and non-linear fracture characteristics. Four groups of concrete specimens with an average compressive strength of 19.30 MPa, 32.60 MPa, 38.2,0 MPa, and 41.70 MPa were utilized under this study. To investigate the effect of reinforcement ratio on flexural performance of the beams, reinforcement ratios of 0.03, 0.042, 0.045, and 0.063 were used for each compressive strength class. The result showed that the ultimate load carrying capacity of the beam showed significant improvement by about 36.38% by increasing of reinforcement ratio from 0.03 to 0.063 by keeping the compressive strength of concrete constant. However, it was observed that the effect of compressive strength was not such substantial as reinforcement ratio in enhancing the ultimate load bearing capacity. The experimental result showed that the increase in ultimate load by keeping the reinforcement ratio constant is about 12.20% for different compressive strength. Furthermore, the crack formation in the concrete was highly associated with the tensile reinforcement ratio, i.e., smaller reinforcement ratio led to higher strain growth in the concrete. Moreover, the validation study between the numerical simulation and test results showed a good agreement.