Alkali-activated Fly Ash Pastes Research Articles

Capturing the early-age physicochemical transformations of alkali-activated fly ash and slag using ultrasonic pulse velocity technique

Research has shown that reaction kinetics and physical transformation during alkali-activation of aluminosilicate materials are sensitive to both the compositions of raw precursors and the alkaline solution. Methods that allow for early observations relating to reaction kinetics, and that capture early-age physicochemical transformations with sufficient accuracy, can assist design engineers in bringing this new generation of binders to market. In this study, reaction heat and ultrasonic pulse velocity (UPV) evolutions of low calcium fly ash activated with alkaline solutions of different concentrations, and substituted with various dosages of ground granulated blast furnace slag (GGBFS), were systematically monitored using isothermal calorimetric and portable ultrasonic testing facilities. The incorporation of slag in a fly ash-based system influences the reaction kinetics, rate of physical transformation and morphological features. The study shows that UPV can capture the effect of slag addition on the microstructure densification, which is confirmed by measurements of heat evolution and SEM-EDS microstructural observations. A linear correlation of UPV was identified with the heat evolved. It was further shown that UPV tests can successfully capture workable time of alkali-activated fly ash paste blends that contain at least 10% of slag, noting that mixtures without any slag did not set within the first 24 h of activation. Further, improved dissolution of species, leading to an increased heat evolution, was observed with increasing activator alkalinity. SEM-EDS microstructural characterisation reveals an advanced dissolution of particles in a more alkaline environment, while higher silicate content leads to a more homogeneous morphology. The findings of this research will assist in the utilisation of alkali-activated materials in construction practice.

Mechanical Properties, Microstructure, and Chloride Content of Alkali-Activated Fly Ash Paste Made with Sea Water.

The aim of the present study is to investigate the potential of sea water as a feasible alternative to produce alkali-activated fly ash material. The alkali-activated fly ash binder was fabricated by employing conventional pure water, tap water, and sea water based alkali activating solution. The characteristics of alkali-activated materials were examined by employing compressive strength, mercury intrusion porosimetry, XRD, FT-IR, and 29Si NMR along with ion chromatography for chloride immobilization. The results provided new insights demonstrating that sea water can be effectively used to produce alkali activated fly ash material. The presence of chloride in sea water contributed to increase compressive strength, refine microstructure, and mineralogical characteristics. Furthermore, a higher degree of polymerization on the sea water-based sample was observed by FT-IR and 29Si NMR analysis. However, the higher amount of free chloride ion even after immobilization in sea water-based alkali-activated material, should be considered before application in reinforced structural elements.

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.

Pore solution composition of alkali-activated slag/fly ash pastes

The pore solutions of a series of hardened alkali-activated slag/fly ash pastes were extracted by the steel-die method, and analyzed using ICP-OES analysis technique. According to the saturation index from thermodynamic calculations, the pore solutions of alkali-activated slag pastes kept oversaturated with respect to solid reaction products with time. In the pore solutions of alkali-activated fly ash pastes, an increase of temperature (from 40 °C to 60 °C) led to decreases of the concentrations of Si, Al, Ca, Na, OH−, K, Fe and Mg, while the soluble silicate in the alkaline activator resulted in increases of the concentrations of these elements. Compared to the alkali-activated slag paste with the same alkaline activator, 50% replacement of slag by fly ash did not result in a substantial change of the pore solution composition. Based on the experimental results, conceptual models were proposed to describe the elemental concentrations in the pore solutions.

Influence of the alkaline solution and temperature on the rheology and reactivity of alkali-activated fly ash pastes

The aim of the present research is to ascertain the effect of the nature and concentration of the alkaline activator and temperature on the rheological behaviour of alkali-activated fly ash (AAFA) pastes. Furthermore, the reaction process of fly ash has been investigated from rheological measurements.Results have shown that the increase of the activator concentration and the temperature leads to an increase of yield stress and apparent viscosity, mainly at temperatures above 65 °C when dissolution of fly ash and precipitation of hydration products is raised. In general, similar activation energies were determined for alkali-activated fly ash pastes, regardless of the concentration and silica modulus of the activator, concluding that the underlying reaction mechanism is the same independently on the nature of the activating solution.

Creep properties of cement and alkali activated fly ash materials using nanoindentation technique

This paper presents creep properties of cement and alkali activated fly ash (AAFA) paste and mortar determined from statistical analysis of nanoindentation data. Cement paste having 95 MPa compressive strength at 28 days was tested for comparison and validation with a conventional test. Using nanoindentation, the specific creep of the cement paste after one year was predicted as 18.32 microstrain/MPa. For AAFA samples, an experimental program was set up using Taguchi’s Design of Experiment method to consider four parameters, silica fume, sand to binder ratio, liquid to solid ratio, and superplasticiser, each with three variations.Using ANOVA, the percentage contributions of these parameters on the creep modulus of AAFA samples are: silica fume 26%, sand to binder ratio 21%, liquid to solid ratio 22%, and superplasticiser 31%. The results using deconvolution technique to identify the creep modulus of different phases of AAFA matrices show that partly-activated, non-activated slag and non-activated compact glass phases are leading the creep behaviour of AAFA samples due to their high creep modulus. Compare to other parameters, the liquid to solid ratio contributes the most to the creep property of partly-activated slag, non-activated slag and non-activated compact glass phases, that is, 51%, 89%, 68%, respectively. Sand to binder ratio and superplasticiser have minor effect on the creep behaviour. The results of the creep properties of AAFA paste were then compared with those of AAFA concrete using an upscaling process. The creep rate of AAFA concrete was defined by the creep properties of the matrix and the interface between aggregates and matrix assuming perfect bonding and slip bonding conditions. The results from the upscaling process show that the creep properties of AAFA paste from nanoindentation are representative of the long-term creep properties of AAFA concrete determined from a conventional test method.

A comparative study on the pore structure of alkali-activated fly ash evaluated by mercury intrusion porosimetry, N2 adsorption and image analysis

In this study, the pore structure of alkali-activated fly ash (AAFA) pastes characterized by different techniques, including mercury intrusion porosimetry (MIP), nitrogen adsorption and image analysis (based on backscattered electron images), was evaluated and compared critically. The degree of reaction of fly ash in AAFA pastes was derived from image analysis. It was found that due to a significant “ink-bottle” effect, the pore diameter of capillary pores derived from MIP was two orders of magnitude smaller than the size determined by image analysis. MIP and nitrogen adsorption results showed different peaks corresponding to the gel pores of AAFA pastes. Based on the experimental results, image analysis is regarded as a reliable technique for the characterization of large pores (> 1 μm) in AAFA pastes. Nitrogen adsorption is more suitable to characterize small pores (< 0.1 μm) in AAFA than MIP, and MIP data should be carefully interpreted, preferably in combination with other characterization techniques.

Estimation of fly ash reactivity for use in alkali-activated cements - A step towards sustainable building material and waste utilization

Abstract This paper addresses the detailed characterization of coal fly ashes with respect to their utilization in alkali-activated cement systems, thus maximizing the use of fly ashes in the construction industry. A technique was developed to estimate the reactivity of low calcium fly ashes in alkali-activated systems. The technique is based on a K-value which combines three characteristics of a fly ash. Two characteristics – amorphous phase percentage and specific surface area determined through Blaine measurement – are observed quantities. The third characteristic – degree of polymerization of silica in the amorphous phase of fly ash – is a calculated parameter. To take into account the water demand of fly ashes, which will influence the amount of water needed to get a workable mix, the shape factor was used to adjust the Blaine specific surface area. The relationship between the proposed K-value and compressive strength of alkali-activated fly ash pastes is approximated by a linear function. The correlation coefficient of the relationship varied from 0.961 to 0.833 for 1 and 91 days compressive strength respectively. The proposed K-value can firstly be used to rank fly ashes for their suitability to produce high strength alkali-activated materials and secondly when calibrated for a specific activator and curing conditions, to predict the compressive strength of alkali-activated fly ash binders.

Pore Solution Composition and Solubility of Alkali-Activated Fly Ash

Pore solution of hardened alkali-activated fly ash paste was extracted by the steel-die method. The aqueous phase composition of pore solution was analyzed using ICP-OES analysis technique. The results show that the concentrations of Si, Al, Ca, K and OH- decrease with curing time regardless of the curing temperatures (40°C/60°C) and alkaline activators (sodium hydroxide with/without sodium silicate). On the contrary, the concentration of S increases with curing time. A higher temperature curing decreases the solubility of Si, Al, Ca and K in the alkali-activated fly ash, while it doesn’t show much influence on the solubility of S. The plot of the concentration of Al versus the concentration of Si displays a quasi-linear logarithmic relationship. This relationship implies congruent removal of Si and Al from the frameworks of fly ash.

An investigation on alkali-activated fly ash pastes modified with quartz powder subjected to elevated temperatures

In this study, the opportunity of employing quartz powder (QP) to improve the workability as well as the compressive strength of alkali-activated fly ash (AAFA) pastes before and after exposure to thermal loads has been investigated. Fly ash (FA) was partially substituted with QP at percentages ranging from 0% to 30% with an interval of 5%, by weight. After curing, specimens were exposed to different thermal loads ranging from 400°C to 1000°C with a step of 200°C with a heating rate of 6.67°C/min for 2h. Weight and compressive strength of the investigated specimens before and after being heating were exhaustively conducted. The different decomposition phases formed upon exposure to thermal loads were detected using X-ray diffraction (XRD) and thermogravimetric (TGA) techniques. The microstructure of the formed hydration products was determined by scanning electron microscopy (SEM). The results showed that the workability as well as compressive strength before and after firing improved by increasing QP content.

The shrinkage of alkali activated fly ash

In this study, the autogenous and drying shrinkage of alkali activated fly ash (AAFA) pastes prepared with different contents of sodium silicate solution are reported. A higher amount of both Na2O and SiO2 resulted in a larger autogenous and drying shrinkage. Although a large autogenous shrinkage was obtained during the first 1–3days, cracking was not observed in the ellipse ring tests. In AAFA pastes, water was not a reactant, but mainly acted as a medium. The experiment results indicate that the autogenous shrinkage of AAFA is not caused by the well-known self-desiccation process that happened in cement paste, but related to the continuous reorganization and polymerization of the aluminosilicate gel structure. AAFA pastes with a larger drying shrinkage exhibited a higher weight loss. The different microstructures lead to the different drying shrinkage of these AAFA mixtures.

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.

The effects of Na2O/SiO2molar ratio, curing temperature and age on compressive strength, morphology and microstructure of alkali-activated fly ash-based geopolymers

Fly ashes (FA) are byproducts of electricity production from mineral coal in thermoelectric power plants. The pozzolanic properties of FA have been utilized in various applications, including structural concrete, yet the large part of FA is still discarded into the environment. To promote greater FA usage, this study aims to produce a dense matrix, with mechanical properties satisfactory for civil engineering projects, from alkali-activated fly ash-based geopolymers . Three variables were studied: the Na 2 O/SiO 2 molar ratio (N/S 0.20, N/S 0.30 and N/S 0.40); curing temperature in the first 24 h (50, 65 and 80 °C); and age (1, 7, 28, 91 and 180 days). For this study, alkali-activated fly ash pastes and mortars were prepared. In pastes, morphology was studied using scanning electron microscopy (SEM/EDS) and microstructural properties with X-ray Diffraction (XRD) analysis. Mortars were evaluated according to their mechanical performance measured using compression strength tests. Compression strength results were analysed using ANOVA. The results show that the N/S molar ratio plays an important role in the mechanical and morphological characteristics of geopolymers. The mortars prepared with a N/S 0.40 molar ratio had the greatest compression strength. The analysis of paste morphology revealed that N/S 0.40 pastes had a denser appearance, which is in agreement with results of compression strength tests.

The effect of activator concentration on the residual strength of alkali-activated fly ash pastes subjected to thermal load

This investigation reports on a comparative study of the residual compressive strength at different temperatures of alkali-activated fly ash (AAFA) pastes chemically activated using sodium silicate with three different concentrations named 20, 30 and 40 (wt.%). The behaviour of different mixtures in conditions of rapid temperature changes was studied. Water quench test was applied to determine thermal shock resistance. The traditional Portland cement pastes were used as a reference. The temperatures ranging from 200 °C to 1000 °C with an increment of 200 °C has been examined. Pore solution pH and compressive strengths before and after exposure to elevated temperatures were determined. The various decomposition phases formed were identified using X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis (DTG) and scanning electron microscopy (SEM). The results indicated that fly ash activated by sodium silicate is more able to resist degradation caused by exposure to elevated temperature than Portland cement system as its relative strengths are superior. In the hardened AAFA pastes, as activator concentration dosage increased as the relative compressive strengths and thermal shock resistance decreased. The AAFA system is able to maintain a sufficient high pH to retain the passive protective layer on the surface of any reinforcing steel present.

Alkali activated fly ash: effect of admixtures on paste rheology

In this paper, an investigation related to the rheological behaviour of alkali-activated fly-ash pastes (AAFA) is described. Those pastes were prepared by mixing the fly ash with an alkaline dissolution containing 85% of a 12.5 M NaOH solution and 15% of waterglass and adding some commercial chemical admixtures usually used in the Portland cement concrete fabrication, like lignosulphonates, melamines (first and second generation products) and polycarboxylates (latest generation). The fly ash rheological data were determined by rotational viscometry measurements as well as by the use of the flow table test. Results indicate that chemicals admixtures used do not work the same in the Portland cement systems than in alkali-activated fly ash systems. As a general rule, it seems that the most efficient admixtures for these new cementitious pastes (AAFA) are those based in polycarboxylates.

Durability of alkali-activated fly ash cementitious materials

The study described in the present paper addresses the durability of alkali-activated fly ash (AAFA) cement under different conditions: specifically, cement performance is measured in a number of aggressive environments (deionized water, ASTM seawater, sodium sulphate and acidic solutions) and with respect to alkali–silica reaction-induced expansion. The chief parameters studied are: weight loss, compressive strength, variations in volume, presence of the products of degradation and microstructural changes. The results show that AAFA pastes perform satisfactorily in aggressive environments and that degradation in these materials is distinctly different from such processes in OPC paste. These mortars are also compliant with the 16-day expansion limit stipulated in ASTM standard C1260-94 on potential alkali–silica reactivity.

04/00730 Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes: Puertas, F. and Fernandez-Jimenez, A. Cement & Concrete Composites, 2003, 25, (3), 287–292

This investigation reports on a comparative study of the residual compressive strength at different temperatures of alkali-activated fly ash (AAFA) pastes chemically activated using sodium silicate with three different concentrations named 20, 30 and 40 (wt.%). The behaviour of different mixtures in conditions of rapid temperature changes was studied. Water quench test was applied to determine thermal shock resistance. The traditional Portland cement pastes were used as a reference. The temperatures ranging from 200 °C to 1000 °C with an increment of 200 °C has been examined. Pore solution pH and compressive strengths before and after exposure to elevated temperatures were determined. The various decomposition phases formed were identified using X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis (DTG) and scanning electron microscopy (SEM). The results indicated that fly ash activated by sodium silicate is more able to resist degradation caused by exposure to elevated temperature than Portland cement system as its relative strengths are superior. In the hardened AAFA pastes, as activator concentration dosage increased as the relative compressive strengths and thermal shock resistance decreased. The AAFA system is able to maintain a sufficient high pH to retain the passive protective layer on the surface of any reinforcing steel present.