International Concrete Abstracts Portal

The construction of concrete slabs-on-ground in Australia requires often requires good control of setting times and bleeding rates in lean concretes using air entraining admixtures to minimize the potential for plastic cracking. Whilst this has been successfully achieved on the East Coast of Australia for decades with fly ash concretes a new source of fly ash from the Collie power station located on the West Coast proved difficult to use. The University of New South Wales undertook an investigation into the cause of sensitivity and variability exhibited by key properties of concretes that contained Collie fly ash. Testing of the basic properties of three Collie fly ash samples and one sample of fly ash from Eraring power station was used to characterize the materials and the samples separated into specific size fractions with further testing undertaken. Testing was undertaken included LOI, density, colour, fineness, carbon content, sulphur content, and physical features using SEM. The investigation concluded that the causes of the admixture sensitive behavior of Collie fly ash concrete is not simply due to the presence of carbon but the high surface area and low density of these carbon particles in association with the presence of oxidisable sulphur compounds.

This paper addresses kinetics and strength build-up of pozzolanic reactions in 2-inch-cubes of biomass, coal fly ash/Ca(OH)2 (CH) and sand mortar. A comprehensive experimental design of six biomass/coal fly ashes, three temperatures, three mass ratios of fly ash/CH and six test dates of up to 1 year was set up. The results show that the compressive strength of biomass fly ash samples exceeds that of coal ash samples by factors of 2-3 and rivals that of pure cement ones. Further investigations indicate that except CH reaction extent, other factors, such as fly ash type, mixing ratio of fly ash with CH and curing temperature, all have significant effects of compressive strength build up of fly ash samples.

The research goal of this project was to measure silica fume particle size distribution and conduct dispersion tests, using measured levels of ultrasound as a method to evaluate the relative agglomeration "strength" and de-agglomeration [dispersability] of the undensified and densified product forms of silica fume. Scanning Electron Microscopy (SEM) testing of silica fume samples was performed, showing combinations of individual particles (0.02 to 0.25µm) along with loose agglomerate clusters (25 to 120µm), which could not be quantified in size distribution analysis. A specially modified laser scattering particle size distribution analyzer, with a built in digitally controlled ultrasonic processor, was developed to measure particle size distribution statistics such as mean, medium and standard deviation. Ultrasonic energy levels were determined for complete de-agglomeration of undensified and densified material, which allows the measurement of the primary un-agglomerated material particle size. A test method was developed to evaluate the dispersability or relative agglomerate "strength" of the different silica fume forms by measuring the various particle size distributions, with and without ultrasound. Through the application of ultrasound, at specific energy levels and time periods, the relative agglomerate dispersability at different bulk density levels were determined. Mortar and shotcrete performance tests were conducted to evaluate the dispersability of different silica fume product forms, for different bulk loose density levels. The mortar laboratory evaluation tests included pozzolanic strength activity index ratios and electrical resistivity measurements. The test method’s ability to evaluate product dispersability and quality assurance was further verified through a field shotcrete test program, conducted with various bulk loose densities, measuring rebound percentages, thickness before bond break and compression strength.

The influence of silica fume (SF) on performance of cementitious systems like concrete is due to physical (i.e. improved particle packing) and chemical (i.e. pozzolanic reaction) effects. Since silica fume particles are much smaller than cement grains they will pack in the voids between cement grains. In this way the system is densified already at the time of setting. SF is an active pozzolana that leads to strength increase. When SF is not used to obtain equal strength at higher w/cm than reference, but to obtain high performance concrete with lower w/cm including plasticizers, no durability problems should occur unless adequate precautions are not taken to prevent cracking from autogenous shrinkage and high heat of hydration. In general, the use of SF in concrete mixtures reduces the permeability to waterborne aggressive ions (e.g. alkalis, chlorides, sulfates), thus mitigating the potential for sulfate attack, alkali-aggregate reactions, and corrosion of reinforcing steel in concrete.

Industrial waste clay materials, which are good pozzolans, can be used in place of cement in the manufacture of mortars and concretes. During the manufacturing process, which involves dehydration followed by firing at controlled temperatures ranging from 700 °C to 1000 °C, the clay minerals found in high proportions in the natural materials used to make bricks and similar products acquire pozzolanic properties. The present study examines the microstructure and morphology of the industrial concrete made with standard portland cement containing fly ash partially substituted with waste clay material, by backscattered electron images (BSE) and mercury porosimetry (MP) techniques. The microstructure of concrete test samples is not altered by the inclusion of clay discards and the pore size did not vary too, the only difference found was a slight decline in the proportion of larger diameter pores (over 0.1 micron).