International Concrete Abstracts Portal

The results of studies undertaken into the compositional and physical characteristics of a wide spectrum of UK fly ashes, and some from overseas, are examined in conjunction with data available from the literature. No direct relationship was found to exist between the compositional nature of a fly ash and its behaviour in concrete. The single most significant physical parameter characterising ash is shown to be fineness (as measured by 45 um sieve retention) and is used in developing two schemes classifying (a) the water-reducing and (b) the cement-saving abilities of an ash, which give a simple measure of ash suitability for use in concrete. It is demonstrated that for a correctly designed concrete, the grade of ash does not significantly affect its engineering properties, but only the cement savings that can be achieved.

Specifications for fly ashes should be based on a proper understanding of the factors controlling, and test methods for, water requirements for flow. The reduction of the water requirement for flow of concretes which results from partial replacement of portland cements by certain fly ashes is commonly attri-buted to the spherical shape of many of the fly ash particles. Critical re-examination of the literature does not support that conclusion. An alternative hypothesis is proposed: the water reduction is a result of adsorption of very fine fly ash particles on portions of the cement particle surfaces, with result-ing dispersion of the cement particles, similar to the action of organic water-reducing admixtures. This hypothesis is consistent with published data on fly ash-cement-admixture interactions. This effect should be separated in the test methods and specifications from that of the amount of fly ash coarser than 45 um, but is not in the present ASTM C 311 test for water requirement.

The fly ashes produced in Italy generally show good pozzolanic behaviour. Tests made on lime and cement mortars have shown that 4 of the 5 ashes give comparable or even superior performance to that of a natural pozzolan usually employed in the production of portland pozzolan cements. The chemical characteristics were: Si02 42.7-50.2%; Si02+A1203 66.5-77.2%; Si02+A1203+Fe203 75.8-83.5%; loss on ignition 3.9-12.9% and CaO 1.8-9.3%. The calcium oxide content is the parameter that greatly influences the technical behaviour. Glass content ranging from 63 to 75% does not have a dominant influence on mechanical strength. The particle size distribution, rather than the fraction <45pm determines the mechanical characteristics.

In order to reduce energy, save raw materials and improve mechanical properties, different pozzolans are now commonly used in cement and concrete production. A comprehensive research program was undertaken where cement and concrete properties, influenced by fly ash and condensed silica fume in different combinations, were investigated. This paper presents their influence on strength and heat development. The program included an ordinary portland cement and two blended cements with 10 % and 25 % fly ash respectively. The three cements were combined with 0 %, 5 % and 10 % condensed silica fume. Curing temperatures used were 5°C, 20°C and 35°C. Condensed silica fume is very finely graded and the content of amorphous Si02 is very high. The pozzolana reaction starts therefore early, at 20°C from around 7 days, at 35°C from around 2 days. At 5°C, no pozzolana reaction was observed for the first 28 days. The pozzolana reaction from fly ash was found to be slower than the reaction from condensed silica fume, probably due to the coarser grinding and the lower Si02-content. The compressive strength results indicated that the pozzo-lana reaction was more sensitive to the temperature than the reaction involving cement hydration alone. The slow strength development of concrete when using fly ash in blended cements can be avoided by grinding the cements to a higher fineness. The effect on strength development when using condensed silica fume was approximately the same in all three types of cement investigated. The heat development was higher in pure portland cement than in blended cements. However, when adding condensed silica fume, the heat development increased. Maturity functions were found to be valid up to maturities corresponding to curing in 20°C for approximately 2 days.

Condensed silica fume, at up to 30% by weight, was used as a partial cement replacement in lightweight aggregate concrete. The results of round and deformed bar cube pull-out tests, with and without applied lateral stress, show that condensed silica fume increases ultimate bond strength and affects the mechanism of failure. The influence of condensed silica fume on bond stress of round bars was similar at all lateral stresses, producing a 50% increase at 20% by weight replacement of cement. For deformed bars the increase in bond strength was more pronounced at higher levels of lateral stress, producing increases approaching 70% at 20% silica fume content. The improvements in ultimate bond strength with condensed silica fume are shown to only partly result from the associated increases in compressive strength, the greater part resulting from the modified properties of the concrete matrix.