Shondeep L. Sarkar

Microstructural study of gypsum activated fly ash hydration in cement paste

Abstract The addition of 3 to 6% gypsum to a low alkali, low C3A cement blended with low (30 %) and high (60%) volume of a Class F fly ash (FA) led to a distinct increase in strength in comparision to the blends without additional gypsum. This is discussed in terms of the reaction of the FA with cement and gypsum. The role of FA alkalies is also described. The investigation included scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDXA) to study the microstructural development of FA replacement pastes with progressive hydration, and X-ray diffraction (XRD) for mineralogical identification of hydrated phases.

The superplasticizer adsorption capacity of cement pastes, pore solution composition, and parameters affecting flow loss

Abstract Studies are reported on the interaction of sodium salt of polynaphthalene sulfonate superplasticizer (PNS) with different cement types, its effects on the flow loss, and the chemistry of the pore solution at early time of hydration from mixing to presetting. Six commercial cements displaying a wide compositional range and fineness were selected. Results show that most of the superplasticizer is removed from the pore solution immediately after mixing. The adsorption capacity of the paste is mainly determined by the PNS molecular weight, cement fineness, and C 3 A content In turn, the rate of flow loss appears to be strongly governed by the ionic strength of the pore solution and marginally to C 3 A content and ettringite formation. Oversulfating the cements by addition of up to 7% of gypsum, reduces the flow loss but within the range of data available, does not enhance the uptake rate of the superplasticizer by the cement constituents.

An examination of fly ash carbon and its interactions with air entraining agent

Four fly ash samples, which had previously been found to effect concrete air entrainment in a manner inconsistent with their respective loss on ignition, were investigated using several physico-chemical techniques. This study focused on characterization of the high-carbon fraction of each fly ash, obtained by a triboelectric separation process. While the four samples displayed varying reactivities toward AEA adsorption, the BET specific surface area of all four samples was determined to be essentially the same. Thermal analysis and petrographic examination revealed that the higher demand for air entraining agents exhibited by two of the samples could be directly related to the presence of a higher proportion of optically isotropic, amorphous carbon. Liquid and vapor phase adsorption analysis suggested that the surface chemistry characteristics of the isotropic carbon resulted in a higher adsorption capacity for polar compounds such as air entraining surfactants.

The effects of simulated environmental attack on immobilization of heavy metals doped in cement-based materials☆

Environmental corrosion as is manifested in the form of CO2 attack has been found to have a profound effect on the leaching characteristics of heavy metals and the microstructure of the cement binder. An external peripheral leached zone was formed, followed by a calcium carbonate-rich layer, and an intact unleached core. The heavy metals did not substitute for calcium in the calcium silicate hydrate gel, rather nickel and cadmium gradually reacted to form hydroxides. This reaction is diffusion controlled, and depends on the initial grain size distribution of the heavy metal compounds. The incorporation of metals results in a decrease of the Ca(OH)2 content of the cement paste and increases its vulnerability to carbonation and resistance against acidic corrosion. A pH decrease to about 9 in the leached zone can account for the cadmium and nickel concentration in the leachates.

INTERDEPENDENCE OF MICROSTRUCTURE AND STRENGTH OF STRUCTURAL LIGHTWEIGHT AGGREGATE CONCRETE

Abstract The demand for lightweight concrete is steadily increasing because of economic and practical considerations. Hence, the inherent internal and external features of lightweight aggregates have been a subject of intense research in recent years. This study provides new insight into the micro-structural and chemical factors which influence the strength properties of structural lightweight aggregate concrete. These are described with respect to four expanded clay lightweight aggregates used in nine concrete compositions containing various types and proportions of dispersing agents such as water-reducing admixtures and superplasticizers, with silica fume and ground granulated blast-furnace slag as optional mineral admixtures. The microstructural characteristics of the paste-aggregate interface and the paste porosity of these concretes are discussed. The methods used include scanning electron microscopy-energy-dispersive X-ray analysis, X-ray diffraction analysis, optical microscopy and compressive strength testing.

Characterization of principal clinker minerals by FT-Raman microspectroscopy

Abstract Commercial and synthetic clinker minerals have been analyzed by Fourier Transform (FT) Raman microspectroscopy. The spectra of synthetic polymorphs of C 3 S and β C 2 S are compared to their counterparts in commercial clinker. Identification of the calcium silicate phases by FT Raman microscopy conforms with XRD diffractograms. C 3 S is easily distinguishable from C 2 S. The main band of the synthetic C 3 S polymorphs correspond to the out-of-plane Si-O bending ν 4 , and it is shifted to higher energy for the commercial C 3 S. The main band for the C 2 S minerals corresponds to the asymmetric Si-O stretching ν 3 . However, the reported spectra differ from other literature results designating the ν 1 SiO 4 vibration as the predominant lines characterizing calcium silicates. The analyses for synthetic C 3 A polymorphs and C 4 AF conform with the published data.

Effect of fly ash on the microstructure of cement mortar

A microstructural study of mortars prepared with a low-alkali, low-C3A cement and a Class F fly ash, both of Swedish origin, was carried out using the scanning electron microscopy-energy-dispersive X-ray analytical technique. Supplementary phase analyses were made by X-ray diffraction and thermogravimetry-differential thermal analysis. Normally, CH crystals in the transition zone grow with their c axis parallel (or the (0 0 1) cleavage plane perpendicular) to the aggregate surface. The encapsulation of the fly ash particles by the growing CH reduces the amount of orientated CH at the aggregate-paste interface. The growth mechanism of these crystals is discussed. The reduction of CH, most significant after 28 days of hydration, is mainly due to the reaction of CH with the fly ash glass phase. Initially, the replacement of cement by fly ash weakens the paste-aggregate interfacial zone due to reduction of contact points, and increases the local water-to-cement ratio. This, however, improves significantly when the fly ash has reacted. In order to enhance the reaction of fly ash, extra gypsum was added. The results show that gypsum can accelerate the fly ash reaction, but the products formed, and the beneficial effects of gypsum, are mainly determined by the total amount of gypsum in the paste.ResumeUne étude microstructurale de mortiers préparés à partir de ciment à faible teneur en alcali et en C3A ainsi que de cendres volantes de classe F (tous deux d’origine suédoise) a été menée en se servant du microscope électronique à balayage et de la technique d’analyse de rayons X à dispersion d’énergie. Les phases supplémentaires étaient étudiées par thermogravimétrie et analyse thermique différentielle.Normalement, les critaux de CH croissent dans la zone de transition en sorte que leur axe est parallèle à la surface du granulat (ou le plan de clivage (0 0 1) perpendiculaire à la surface). En croissant le CH recouvre les cendres volantes, ce qui réduit la quantité de CH orienté à l’interface pâte-granulat. La diminution de CH, très importante après 28 jours, semble dépendre surtout de la réaction entre les cristaux de CH et la phase vitreuse des cendres volantes.Au début de l’hydratation, l’addition de cendres volantes affaiblit la zone d’interface entre la pâte et le granulat en réduisant les points de contact. Le rapport eau-ciment local est aussi augmenté. Une fois que la réaction des cendres volantes commence, la situation s’améliore de façon significative. Afin d’augmenter la réactivité des cendres volantes, on a ajouté du gypse. Les résultats démontrent que, bien que le gypse puisse accélérer la réaction des cendres volantes, la formation de produits de réaction, ainsi que les avantages, dépendent de la quantité totale de gypse dans la pâte.

Dissolution rate of silica fume in very high strength concrete

A very high strength concrete, having a 91 day compressive strength of 113 MPa, was developed using Type III cement, limestone aggregates, sodium naphthalene superplasticizer and silica fume, with W/C ratio of 0.24. SEM-EDXA and AEM were used to study the rate of dissolution of silica fume in this concrete, with progressive hydration. The ultrafine particle size of silica fume (< 1 μm) makes it difficult to view the state of these particles in concrete under the SEM. With AEM, however, it was possible to observe the dissolution process of silica fume particles, which begin at an early stage. Within 28 days, most of the silica fume is consumed in the pozzolanic reaction. The initial reaction product is a silica rich gel which later transforms into different morphological types of C-S-H which are compacted together. This is a major contributory factor for the very high strength of this concrete. Some partly reacted silica fume particles, however, remain in the hardened paste; lack of water most probably inhibits their complete transformation.

Important properties of an ultrafine cement — Part I

Abstract Experiments were carried out to eliminate strength retrogression and unusually fast setting characteristics of an ultrafine cement with a Blaine surface area >7000 cm 2 /g. A solid retarder, a solid HRWRA, and a superfine fly ash were incorporated in the ultrafine cement to improve its properties. The changes in the important physical, chemical, and other associated properties that result from such additions are reported.

Retardation effect of superplasticizer on different cement fractions

Abstract Cement hydration is a complex chemical process strongly influenced by the proportions of the various minerals present in the cement, admixtures, and the size of the cement particles. In this note, it is shows that naphthalene based superplasticizer, used at high dosage, does not affect the total heat of hydration but retards the hydration process of portland cement. This retardation, however, is most effective on the medium size fraction of the cement (4 – 30 μm in our case). The superplasticizer does not affect the hydration of the fine fraction ( μ m) to that extent due to its richer concentration of SO 3 and alkalies. The coarse fraction of the cement (30 – 72 μm) does not participate (with or without superplasticizer) in the heat evolution process.