P. K. Das Poddar

Bio: P. K. Das Poddar is an academic researcher from University of Calcutta. The author has contributed to research in topics: Compressive strength & Spinel. The author has an hindex of 4, co-authored 13 publications receiving 128 citations.

Effect of preformed and in situ spinels on microstructure and properties of a low cement refractory castable

Abstract: Two types of magnesium-aluminate spinels, one commercially available as preformed powder and the other prepared from cheaper precursors by the sol–gel route as a semidried reactive mass, were characterized in terms of particle size distribution, XRD phase evolution and chemical composition. These two were incorporated in a high alumina based refractory castable in different concentrations and fired at elevated temperatures. Those preformed and in situ spinel bonded castables were characterized in terms of bulk density, apparent porosity, cold crushing strength, thermal shock resistance, volume shrinkage, flexural strength, XRD, SEM, EDS and slag corrosion resistance test to understand the effect of both kinds of spinel in the castable composition.

Thermal analysis of hydrated calcium aluminates

Abstract: Differential scanning calorimeter (DSC) has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively. Dehydration of CAH10 and C2AH8 (whereH=H2O) occur ∼ at 160–180°C and 200–280°C respectively. These two phases are unstable and ultimately get transformed to AH3 and C3AH6. Dehydration of AH3 and C3AH6 occur between 290 and 350°C and overlap at lower scanning rate. The activation energy for dehydration of the stable AH3 and C2AH6 phases has been found to be 107.16 and 35.58 kJ mol−1 respectively. The compressive strength of the hydrated calcium aluminates has been determined. The result shows that in the case of CA, almost 90% of ultimate strength has been attained in 1 day whereas in CA2, ultimate strength has been attained in 14 days and in C12A7 in 1 day. DSC results have been correlated with the rate of strength developments.

Crystal morphology of calcium aluminates hydrated for 14 days

Abstract: phase [2–4]. Many researchers [5–7] have investi-gated the development of these hydrated phases byelectron microscopy. The effects of crystal morphol-ogy on the hydration characteristics and setting timehave also been studied by some workers [8–9]. Theaim of the work reported here is to investigateexactly what the crystal habit and morphology are onthe fourteenth day of hydration of calcium alumi-nates, and what their role is in the development ofcompressive strength of the hydrated samples.Calcium aluminates such as CA, CA

Modified in situ spinel-alumina castables

Abstract: Modified in situ spinel-alumina castables were prepared by using 8.0 wt.% of calcined hydrated magnesium aluminate additives synthesized via facile sol gel and coprecipitation routes. These additives were characterized byparticle size analysis and transmission electron microscopy. The castables prepared by them were compared in terms of bulk density, apparent porosity, cold crushing strength and spalling resistance to identify the excellent performance of the sol gel additive having nanocrystalline feature. The quality of castable was illustrated again by comparing it with the same prepared by commercially available spinel fines having well established performance. Scanning electron microscopy, energy dispersive spectroscopy, pore size distribution and X-ray diffraction analysis were used to examine selected fired samples. It appeared that the spinel-alumina castable prepared by the sol gel additive can be a potential candidate for application in steel plants.

Hydraulicity and High Temperature Strength Retaining Capacities of Calcium Dialuminate in Relation to Refractory Aggregates

Abstract: Some hydraulic properties of synthetically prepared calcium aluminate (CA2) have been studied. Results show that strength development is dependent on curing temperature, time and nature of aggregate. CA2 based refractory castables heated at different temperatures have shown better strength in case of calcined bauxite than calcined kyanite and fireclay refractory aggregate. XRD technique has been applied to identify the phases formed in the prepared CA2 and also in castables heat treated at 1500°C. The changes in microstructure of hydrated and heat treated CA2 have also been studied using SEM.

Incorporation of Al in C-A-S-H gels with various Ca/Si and Al/Si ratio: Microstructural and structural characteristics with DTA/TG, XRD, FTIR and TEM analysis

Abstract: Systems of different Ca/Si and Al/Si molar ratios were investigated. Incorporation of Al increases main basal spacing, amount of bounded water and decreases crystallinity of C-(A)-S-H (calcium (aluminium) silicate hydrate). Transmission electron microscope observations showed that aluminium results in formation of more compacted, foil-like microstructure. FTIR revealed the presence of rings within the structure of C-(A)-S-H. Low Ca/Si ratio promotes Al incorporation into C-(A)-S-H, while in case of high Ca/Si ratio aluminium is also incorporated into AFm. The results show, that Ca/Si ratio is of key significance deciding on Al incorporation into C-(A)-S-H in hydrating SCMs bearing blended systems.

Dehydration of a layered double hydroxide – C2AH8

Abstract: Thermal dehydration of dicalcium aluminate hydrate, C2AH8, has been investigated by simultaneous differential thermal and thermo gravimetric analysis (DTA/TGA), powder X-ray diffraction (XRD), temperature-dependent infrared spectroscopy (FT-IR), and BET method of surface area measurement. The temperature-dependent infrared measurements were studied by two-dimensional infrared (2D-IR) correlation spectroscopy. The structure of aluminum-oxide polyhedron, characterized by 27Al solid state NMR spectrum method and FT-IR, shows tetrahedron and octahedron as the main forms of aluminum-oxide polyhedrons in C2AH8 sample. From the results obtained a variety of structural transformations observed are explained as a consequence of the removal of loosely held interlayer water molecules at lower temperatures, followed by grafting process of the interlayer [Al(OH)4]− anion. Structural model of a grafting process of the interlayer [Al(OH)4]− tetrahedron onto hydroxylated octahedrons of [Ca2Al(OH)6]+ layers has been proposed in order to explain observed loss of one water molecule, shrinkage of interlayer spacing and qualitative changes of FT-IR spectra. At higher temperatures the dehydroxylation of the lattice and decomposition of the interlayer species occurs, yielding amorphous material that crystallizes into C3A and C12A7 at 885 °C. Those findings provide improvement in the interpretation of thermo-analytical results of calcium aluminate cements (CAC) hydration products, and better understanding of CAC conversion process.

Microstructure development in mixes of calcium aluminate cement with silica fume or fly ash

Abstract: The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal calcium aluminate hydrate to cubic form. This conversion is usually followed by an increase in porosity determined by the different densities of these hydrates and the subsequent loss of strength. Mixes of calcium aluminate cement (CAC) and silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with mixtures CAC-SF and CAC-FA and its evolution over time. Thermal analysis (DTA/TG), X-ray diffraction (XRD) and mid-infrared spectroscopy (FTIR) have been used to assess the microstructure of these formulations.

Solidification/stabilization of toxic metals in calcium aluminate cement matrices.

Abstract: The ability of calcium aluminate cement (CAC) to encapsulate toxic metals (Pb, Zn and Cu) was assessed under two curing conditions. Changes in the consistency and in the setting time were found upon the addition of the nitrates of the target metals. Both Pb and Cu caused a delay in CAC hydration, while Zn accelerated the stiffening of the mortar. Compressive strengths of the metal-doped mortars, when initially cured at 60 °C/100% RH, were comparable with that of the free-metal mortar. Three different pore size distribution patterns were identified and related to the compounds identified by XRD and SEM. Sorbent capacities of CAC for the toxic metals were excellent: a total uptake was achieved for up to 3 wt.% loading of the three metals. In this way, CAC mortars were perfectly able to encapsulate the toxic metals, allowing the use of CAC for waste management as proved by the leaching tests.

Chemical activation of calcium aluminate cement composites cured at elevated temperature

Abstract: The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH10 and C2AH8 to form the stratlingite (C2ASH8). The later reactions prevent the strength loss by preventing the conversion of CAH10 and C2AH8 to the cubic C3AH6 phase. The acceleration effect of Na2SO4 on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH10 and C2AH8 phases.