ICE - Institution of Civil Engineers - Virtual Library

The chemical, mineralogical and cement properties of Portland cement clinkers manufactured from a raw mixture of lime saturation factor (LSF) 0·91 by the vertical shaft kiln (VSK) process and the down draft sintering (DDS) process, and a low lime clinker manufactured from a raw mixture of LSF 0·79 by the DDS process have been examined.

At present the utilisaton of fly ash in cement and concrete in India is very minimal. Therefore, enhancing the activity of fly ash is the critical key to large scale applications. Physical, thermal and chemical activation methods have been effected and their performance characteristics tested by aggressive macro cell corrosion technique. The salient features of the investigations were: steel embedded in ordinary Portland cement (OPC) and unprocessed fly ash incorporated cements suffered severe corrosion under macro cell corrosion conditions. But OPC containing treated fly ashes improved the corrosion resistant properties to a greater extent. Chemically treated fly ashes showed superior performance when compared to thermally treated and physically treated systems. The tolerable limit of replacement level for various treated fly ashes under aggressive macro cell corrosion condition is found to be 20%–30%. Under 30% replacement level, chemically activated fly ash (CFA) system showed lowest corrosion rate, highest durability factor and lowest macro cell current under accelerated macro cell corrosion conditions.

The effects of the reactivity of the silica source on the formation and morphology of Al-substituted 1·1 nm tobermorite in a hydrothermally treated CaO-Al₂O₃-SiO₂-H₂O system were studied. Colloidal silica and fine quartz were used as the silica source and gibbsite, containing only octahedrally coordinated Al, was the aluminous source used in this investigation. From a combination of solid-state ²⁷Al MAS NMR, XRD, DTA-TGA and SEM, unequivocal evidence can be found that, with increasing silica reactivity, tetrahedrally coordinated Al in Al-substituted 1·1 nm tobermorite prevails over octahedrally coordinated Al species. Reaction sequences in this system as a function of the reactivity of the silica source are presented.

The usefulness was assessed of two ceramic fibres, α-Al₂O₃ and α-Al₂O₃/mullite mixture, for improving the toughness of sodium polyphosphate-modified fly ash/calcium aluminate blended (SFCB) cement, which is used for completing geothermal wells. At the hydrothermal temperature of 280°C, the reaction between the α-Al₂O₃ phase-based fibre's surfaces and silicate in the cement slurry was responsible for the development of a moderate bonding at the interfaces between the fibres and the SFCB cement matrix. This development improved fracture toughness to 2·7 times more than that of non-reinforced cements. In contrast, adding α-Al₂O₃/mullite mixed phase-based fibres did little to improve the fracture toughness because of the transformation of the mullite into Na-P type zeolite. This mullite → zeolite phase transition was caused by the hydrothermal reaction of mullite with the Na in the cement. This transition generated a strong linkage between the fibres and the matrix; however, it also converted the strong, flexible and tough fibres into fragile, brittle fibres. Correspondingly, cracks generated in the cement composites propagated through the fibres, but not at the fibre/matrix interfaces.

The influence of the chemical and phase composition of different mineral admixtures on pozzolanic activity was studied. Local kaolin clay, activated kaolin clay, porcellanite, activated porcellanite, pumice, fly ash, silica fume and ground quartz were used as components of blended binders for various mortars. Thermal treatments were performed as a means for activation of the minerals. The phase transformations during heat treatment were studied by X-ray diffraction services (XRD) and differential thermal analysis/thermogravimetric analyses (DTA/TGA). The pozzolanic activity of these materials as a function of heat treatment parameters was investigated.