Sadananda Sahu

Preparation of sulphoaluminate belite cement from fly ash

Sulphoaluminate belite cements containing the phases C[sub 2]S, C[sub 4]A[sub 3][bar S], C[sub 4]AF, C[bar S] were synthesized from limestone, fly ash and gypsum at 1,200 C. The correspondence between the predicted phase composition and real phase composition were checked. The influence of quantities of different phases in the hydration behavior and strength development were verified. Results show that an optimum proportion of phase quantities help in high strength development in early age. These cements fulfill all the requirement of Portland cement and have very high early strength. Porosity measurements show that the total pore volume in early period is comparatively less than that of Ordinary Portland Cement. Thus these cements can be usable for special purposes.

Identification of thaumasite in concrete by Raman chemical imaging

Abstract Identification of thaumasite (CaSiO3·CaO3·CaSO4·15H2O) in concrete undergoing external sulfate attack by X-ray powder diffraction or by microscopic techniques is difficult due to its crystallographic and morphological similarity with ettringite. Widefield Raman chemical imaging via liquid crystal tunable filter (LCTF) technology has been used in a preliminary study to determine the presence of thaumasite in association with ettringite (3CaO·Al2O3·3CaSO4·32H2O) and gypsum (CaSO4·2H2O). Raman chemical imaging combines Raman spectroscopy with optical microscopy and digital imaging to provide images with molecular-based contrast. Thaumasite has three major peaks at 658, 990, 1076 cm−1 and three minor peaks at 417, 453, 479 cm−1. Ettringite has major peaks at 990, 1088 cm−1. Gypsum has a major peak at 1009 cm−1 and minor peaks at 417, 496, 621, 673, 1137 cm−1. When these minerals are presented together, Raman chemical imaging provides an excellent way to determine their molecular composition and spatial distribution within the sample.

Evidence of thaumasite formation in Southern California concrete

Abstract Ettringite is one of the main reaction products formed during external sulfate attack on concrete. During this process, the microstructure of the original cement paste is altered. The formation of excessive amounts of ettringite may, under some conditions, have deleterious effects leading to expansion and cracking of concrete. Depending on the severity of attack, local environmental conditions, and availability of sulfate, gypsum may be formed in the paste and at the paste aggregate interfaces. Sometimes the presence of a third mineral, thaumasite (CaSiO3·CaCO3·CaSO4·15H2O) is overlooked due to its morphological similarity to that of ettringite. Examination of concrete obtained from various structural locations of residential homes of Southern California by scanning electron microscopy (SEM) and optical microscopy revealed the presence of thaumasite in association with ettringite and/or gypsum. Morphologically, both thaumasite and ettringite have very similar structure. Analysis of the chemical composition using energy dispersive X-ray spectroscopy (EDS) shows the presence of calcium, silicon, sulfur and oxygen in thaumasite and calcium, aluminum, sulfur and oxygen in ettringite as the major components. Thaumasite deposits are usually associated with gypsum deposits in highly carbonated pastes.

Thaumasite formation in stabilized coal combustion by-products

Abstract The by-products of the desulfurization process in a spray drier usually contain a mixture of hannebachite (CaSO3·1/2H2O), gypsum (CaSO4·2H2O), and the finer fraction of the fly ash. This material was mixed with an additional fly ash and stabilized by adding about 3 wt% lime kiln dust (LKD). The stabilized product was used either as a structural fill or was left in the storage yard for several years. Samples extracted from these sites were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The analytical results show the formation of thaumasite (Ca3Si(OH)6(CO3)(SO4)·12H2O), ettringite (Ca6Al2(SO4)3(OH)12·26H2O), and an intermediate phase with varying chemical composition of calcium, aluminum, silicon, and sulfur.

Mechanism of thaumasite formation in concrete slabs on grade in Southern California

Abstract Thaumasite formation has been observed in residential concrete slabs on grade in Southern California. The concrete examined did not contain any carbonate bearing aggregates or fillers. Microstructural analyses showed a carbonated layer with calcite and gypsum at the bottom of the concrete. Above the carbonation layer, deposits of intermixed gypsum and thaumasite were observed. Further into the concrete towards the upper surface, deposits of thaumasite alone or in combination with ettringite were observed. Most of the thaumasite deposits were observed in air voids. SEM–EDS analysis showed deposits of ettringite, thaumasite and intermediate phases within the same air voids. The formation of thaumasite, ettringite and gypsum was caused by ingress of sulfate and carbonate ions from ground water. The presence of thaumasite, ettringite and intermediate phases in the same air void indicates that ettringite is first formed followed by thaumasite with a series of solid solutions. In this reaction process the pH of the local environment and the balance between sulfate, silicate and carbonate ions are important parameters.

PETROGRAPHIC ANALYSIS OF CONCRETE DETERIORATION BY FEATURE RELOCATION

Petrographic analysis of concrete specimens that uses both optical microscopy and scanning electron microscopy (SEM) with backscatter electron detector and energy-dispersive X-ray spectrometer (EDS) provides a unique opportunity to obtain information on mineralogical, chemical, and microstructural features. Combining optical microscopy and SEM-EDS provides complementary information of concrete suffering from alkali silica reaction or sulfate attack. Feature relocation software enables the petrographer to identify an object of interest with one technique and confirm its identity with another, thereby using the strength of both techniques.

Backscattered electron imaging to determine water-to-cement ratio of hardened concrete

A methodology has been developed for the determination of the water-to-cement (w/c) ratio in hardened concrete using backscattered electron imaging (BEI) within a scanning electron microscope. Reproducible quantitative data are obtained by using standards from polished sections as well as by controlling brightness and contrast on the instrumentation. The method is based on concrete sections that have been vacuum impregnated with epoxy and polished to a flat surface. During impregnation, epoxy fills capillary porosity as well as cracks, voids, and other defects. The w/c ratio is directly related to the capillary porosity and degree of hydration and therefore can be measured. The epoxy-impregnated porosity appears black in BEI, whereas other phases such as calcium silicate hydrate, ferrite phases, and aggregate appear as brighter phases. Signal production, brightness, contrast, sample preparation, general methodology, and resolution are discussed.