Fly ash effects: III. The microaggregate effect of fly ash

In this paper, the microaggregate effect of fly ash is studied systematically by micromechanics, the hypothesis of center particle and pore size distribution. It is pointed out that the microaggregate effect is an important effect of fly ash. It is strengthened with the increase of content of fly ash, but weakened with age. At early age, fly ash cannot fine the pore structure. At late age, fly ash may fine the pore structure. However, in general, the fining role is only a relative fining role.     

Optimization of fly ash content in concrete: Part I: Non-air-entrained concrete made without superplasticizer

This paper outlines the preliminary results of a research project aimed at optimizing the fly ash content in concrete. Such fly ash concrete would develop an adequate 1-day compressive strength and would be less expensive than the normal Portland cement concrete with similar 28-day compressive strength. The results show that, in a normal Portland cement concrete having a 28-day compressive strength of 40 MPa, it is possible to replace 50% of cement by a fine fly ash (∼3000 cm²/g) with a CaO content of ∼13%, yielding a concrete of similar 28-day compressive strength. This concrete can be designed to yield an early-age strength of 10 MPa and results in a cost reduction of about 20% in comparison to the control concrete. In a case of a coarser fly ash (∼2000 cm²/g) with a CaO content of ∼4%, substitution levels of cement by this ash could be from 30% to 40%. This concrete yields a 1-day compressive strength of 10 MPa and a 28-day compressive strength similar to that of the control concrete. The total cost of this concrete is about 10% lower than that of the control concrete.     

The application of Raman spectrometry to the investigation of cement: Part II: A micro-Raman study of OPC, slag and fly ash

Various attempts to use Raman spectrometry to analyse the mineral phases in hydrated and unhydrated cement have been made and results were published on pure synthesized mineral phases as early as 1976. Limited investigations were reported in the past three decades on pure mineral cement phases, white cement and ordinary Portland cement (OPC) analysed with visible (VIS) excitation at 514.5 and 632 nm (mainly with normal Raman spectrometry) and near-infrared (NIR) excitation at 1064 nm (mainly Fourier Transform Raman Spectrometry — FT-Raman). Results were distinctly different for VIS and NIR lasers. Newman and co-workers assigned these differences to a fluorescence phenomenon. Previous work on the characterization of secondary cement materials (SCMs) by means of Raman spectrometry is even more limited. This work focused on the characterization of OPC, fly ash and slag, using ultraviolet-visible (UV-VIS), VIS and NIR excitation in micro-Raman spectrometry and the results obtained for OPC and fly ash will be compared with previously published results.     

Fly ash effects: I. The morphological effect of fly ash

The morphological effect is an important part of fly ash effects. The paper analyzes emphatically this effect and points out that it is composed of the filling role, surface role and lubricating role. For different fly ash, these roles are different. They must be considered synthetically when the morphological effect is analyzed. Analyzing result shows that the filling role is relative to the particle size, the surface role is relative to the specific surface area and the water affinity and the lubricating role is relative to the shape of particle. The morphological effect of fly ash is the synthetical embodiment of these roles.     

Effects of the incorporation of Municipal Solid Waste Incineration fly ash in cement pastes and mortars: I. Experimental study

This work falls within the scope of a general problem regarding the assessment of concrete manufactured from waste materials. The main objective is to study the long-term evolution of these materials during the leaching process, using the cellular automaton-based hydration model developed at the National Institute of Standards and Technology. The work is based on the analysis of mortars and cement pastes containing experimental waste: Municipal Solid Waste Incineration fly ash (MSWI fly ash). The study therefore aims to develop a methodology for assessing concrete manufactured from waste, and not to study a process or a formulation enabling the incorporation of the waste in concrete. The physical, chemical and mineralogical characteristics of MSWI fly ash were first analysed to introduce them into the model. A simplified quantitative mineralogical composition of the ash was proposed. The performance characteristics (setting times, compressive strengths, shrinkage, etc.) for mortars containing ash were then studied.     

Structure-texture correlation in ultra-high-performance concrete: A nuclear magnetic resonance study

The pore size distribution in three reactive powder concrete formulations has been studied by nuclear magnetic relaxation of protons. Confirming the discrete and fractal features of the distribution for this kind of concrete, each formulation is assessed a surface fractal dimension, which reveals the hierarchy of pores. The experimental results evidence a dependence between this dimension and both the filling ratio of cement grains and the reactivity of silica fume. ²⁹Si nuclear magnetic resonance (NMR) allows us to draw a relationship between the amount of calcium silicate hydrates (C-S-H) and this surface fractal dimension.     

Supplementary cementing materials in concrete: Part I: efficiency and design

Many solid industrial by-products such as siliceous and aluminous materials (fly ash, silica fume, slags, etc.) as well as some natural pozzolanic materials (volcanic tuffs, diatomaceous earth, etc.) may be characterized as supplementary cementing materials (SCM) as they exhibit cementitious and/or pozzolanic properties. Due to plenty of these materials and their large variations on physical and chemical composition, the development of a general design for their use in concrete is required. In this work, the concept of an efficiency factor is applied as a measure of the relative performance of SCM compared with Portland cement. Artificial materials of various compositions and some natural pozzolans were studied. Compressive strength and accelerated chloride penetration tests were performed. With regard to these characteristics, efficiency factors for these materials were calculated. A mix design strategy to fulfil any requirements for concrete strength and service lifetime was developed and it enables concrete performance to be accurately predicted.     

Supplementary cementing materials in concrete: Part II: A fundamental estimation of the efficiency factor

For comparing the relative performance of various supplementary cementing materials (SCMs: silica fume, fly ash, slag, natural pozzolans, etc.) as regards Portland cement, the practical concept of an efficiency factor may be applied. The efficiency factor (or k value) is defined as the part of the SCM in an SCM-concrete that can be considered as equivalent to Portland cement. In the present work, an alternative procedure for experimental determination of the k value is proposed, using the concept of the pozzolanic activity index. For the first time, also, the k value for equivalent strength was correlated with the active silica content of the SCM through analytical expressions. Artificial pozzolanic materials of various compositions and some natural pozzolans were studied. It was found and verified by experimental comparison that these expressions are valid only for artificial SCMs (fly ash, slag), whereas in the case of natural SCMs the k value is overestimated. Thus, knowing primarily the active silica content of the SCM, a first approximation of the k value can be obtained and, further, the strength of a concrete incorporating artificial SCM can be predicted.     

Alkali sorption by C-S-H and C-A-S-H gels: Part II. Role of alumina

In Part I, it was shown that alkali partition between C-S-H gel and an aqueous phase can be represented by a partition function, R_d, the numerical value of which, at constant temperature, is defined by the Ca/Si ratio. This R_d value is constant or nearly so over wide ranges of NaOH and KOH concentrations up to ∼0.3 M. In the present paper, Al has been introduced to form C-A-S-H gels, and the influence of Al on alkali sorption properties was determined: Approximately 6-7% replacement of Si by Al was used. Microprobe evidence is presented to show that the Al is actually in solid solution. Introduction of Al into C-S-H markedly increases R_d, indicating enhancement of alkali binding. The results underpin and quantify the beneficial effects of alkali binding arising from the introduction of aluminous supplementary cementing materials, such as fly ash, into cement pastes.     

The effect of pozzolans and slag on the expansion of mortars cured at elevated temperature: Part I: Expansive behaviour

The expansive behaviour of heat-cured mortars containing pozzolans and slag was investigated. In most cases, the addition of any amount of these materials to the mixture typically reduced the long-term expansion, slowed the rate of expansion, and delayed the onset of expansion. However, the efficacy of a particular pozzolan or slag in controlling expansion may depend on its Al₂O₃ content. Metakaolin, which contains a high amount of reactive Al₂O₃, was the most effective at controlling expansion at relatively low cement replacement levels. Slag and fly ash, which are also sources of Al₂O₃, were also effective at suppressing expansion at higher replacement levels. Silica fume was less effective at controlling expansion at conventional replacement levels, and even at higher replacement levels expansion may only be delayed.     

Studies on blended cement with a large amount of fly ash

The influence of the contents of the clinker, activators and fly ash on the properties of blended cement with high fly ash content was studied. Experimental data from X-ray diffraction and pore size distribution indicated that the main hydration product of the fly ash blended cement was C-S-H gel, ettringite and a small amount of Ca(OH)₂. The volume porosity of the pores with diameter bigger than 0.1 μm was lower than that of the micro pores and gel pores with diameter lower than 0.05 μm. The amount of chemical combined water has increased with the curing age duration, while the content of Ca(OH)₂ has reduced after 7 days.     

Study of mineralogy and leaching behavior of stabilized/solidified sludge using differential acid neutralization analysis: Part I: Experimental study

In this work, differential acid neutralization analysis, chemical analysis of selected leachates and mineralogical study (XRD and SEM-EDS) are combined to investigate the relationship between mineralogy and leaching behavior of hydroxide sludge doped in Zn and Cr stabilized/solidified by hydraulic binders. The leaching behavior of stabilized hydroxide sludge is controlled by the dissolution of portlandite (pH ~ 12) followed by the dissolution of AFt/AFm and hydrogarnets (pH between 12 and 10) and finally the dissolution of ettringite and of the siliceous matrix of C-S-H (pH < 10). The zinc behavior is independent of the main components and is controlled by the dissolution of calcium hydroxizincate and its reprecipitation as hydroxyde or its adsorption on C-S-H. Chromium release is linked to the sulfate one due to a substitution in AFt/AFm phases.The proposed combination of tests is highly pertinent to establish the links between mineralogy and leaching behavior. Nevertheless, the hypotheses formulated are insufficient to implement a behavioral model to describe the leaching behavior.     

Granular-bed filtration assisted by filter-cake formation: 2. The panel bed gas filter with puffback renewal of gas-entry surfaces

Laboratory studies, confirmed by field studies at both large and small scales, elucidated behavior of a granular panel bed for gas filtration with accumulation of filter cakes on gas-entry surfaces (filtering occurring by a sieving mechanism). Filtration is cyclic: intermittently, flow of dusty gas is interrupted, and the panel is subjected to a sharp “reverse” puff of gas (a “puffback”), producing a body movement of sand toward gas-entry faces, dumping both dust cake and a relatively small quantity of sand from each face.     

Alkali-silica reaction: A method to quantify the reaction degree

We propose a new chemical method for quantitative measurement of the reaction degrees of the alkali-silica reaction (ASR). We apply this method to a crushed natural reactive aggregate kept in contact with an alkaline solution, lime saturated by an appropriate amount of portlandite. This chemical system is designed to model the concrete capillary pores alkaline solution, in contact with reactive aggregates. Two reaction steps are taken into account in the mechanism: formation of Q₃ sites made by breaking up siloxane bonds of the reactive silica and dissolution of these Q₃ sites. The dissolution degree is measured by a selective acid treatment, and the nature of silica into solution is characterised by liquid NMR spectroscopy. The remaining silica is composed of Q₄ tetrahedrons and Q₃ protonated sites identified by solid NMR spectroscopy. These Q₃ protonated sites are measured by thermogravimetry analysis. We show that the formation Q₃ sites prevails on dissolution as the reaction progresses and contributes to an internal silica gel generation. The limiting step is the siloxane breaking up. Petrographic observations show that the reaction front penetrates in the aggregate through its porosity.     

Effect of steam curing on class C high-volume fly ash concrete mixtures

The effect of steam curing on concrete incorporating ASTM Class C fly ash (FA), which is widely available in Turkey, was investigated. Cement was replaced with up to 70% fly ash, and concrete mixtures with 360 kg/m³ cementitious content and a constant water/binder ratio of 0.4 were made. Compressive strength of concrete, volume stability of mortar bar specimens, and setting times of pastes were investigated. Test results indicate that, under standard curing conditions, only 1-day strength of fly ash concrete was low. At later ages, the strength values of even 50% and 60% fly ash concretes were satisfactory. Steam curing accelerated the 1-day strength but the long-term strength was greatly reduced. Setting time of fly ash-cement pastes and volume stability of mortars with 50% or less fly ash content were found to be satisfactory for standard specimens. In addition, for steam curing, this properties were acceptable for all replacement ratios.     

Water-entrained cement-based materials : II. Experimental observations

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].     

Effects of the incorporation of Municipal Solid Waste Incineration fly ash in cement pastes and mortars: II: Modeling

This work falls within the scope of the general problem of the assessment of concretes manufactured from waste materials. The main objective is to study the long-term evolution of these materials during leaching using the cellular automata-based hydration model developed at the National Institute of Standards and Technology (NIST). The work is based on the analysis of mortars and cement pastes containing experimental waste: Municipal Solid Waste Incineration fly ash (MSWI fly ash). After having determined the mineralogical composition of the MSWI fly ash and its interactions with cement during hydration, presented previously as Part I, the phases comprising the fly ash have been incorporated into the hydration model. The increase in porosity of cement pastes containing MSWI fly ash during leaching has then been simulated. Finally, a simplified leaching model has been developed to study the influence of the changes in microstructure on the release of calcium and sodium.     

Replacement of “coarse” cement particles by inert fillers in low w/c ratio concretes: II. Experimental validation

It has been suggested that in low water-to-cement ratio (w/c) concretes, the “coarser” cement particles could be replaced by an “inert” filler with little loss in performance in terms of hydration and strength development. This communication presents the results of an experimental study conducted to validate this hypothesis, using a coarse limestone filler and a classified cement. The cement and limestone powders were both classified with a cutoff diameter of about 30 μm. The coarse limestone was then blended with the fine cement, and water-to-solids ratio=0.3 pastes and mortars were prepared to compare to reference (original cement powder) systems. The results for chemical shrinkage for the pastes were consistent with a simple dilution of the cement by the limestone and also with the results predicted by the CEMHYD3D hydration model. In mortars, the predicted compressive strength loss in the filled system at 7 days was consistent with model predictions, and furthermore, at 56 days, no detectable difference in strength was measured. Thus, this study further supports the idea that coarse limestones could be used to replace equivalent size cement particles in low w/c concretes with little loss in hydration and strength performance.     

Technology update: Vacuum infusion

Vacuum, or resin, infusion techniques are becoming increasingly popular, especially in the marine industry where they can produce high quality boat hulls quickly and cleanly (see Reinforced Plastics' Marine Supplement, October 2003).