International Concrete Abstracts Portal

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.

Fly ash concretes with and without superplasticizers, hardened under normal conditions of temperature and by heat treatment. The influence of superplasticizers on the harde fly ash concretes was studied at ages (24-72 hours) days. ning process of and 28-730 Some technico-economic advantages resulting from the use of FA and superplasticizer admixtures in concrete are discussed in terms of design, production and standards development.

In recent years there has been a renewed interest in the utilization of oil shales as a source for energy production and this was accompanied by research and development aimed at finding ways to utilize the ash by-product as a building material. The present paper discusses the various types of oil shale ash and their possible applications, and reviews recent studies carried out in Israel to utilize the cementitious characteristics of oil shale ash. The composition and properties of the oil shale ash can vary widely, ranging from high SiO2 materials which are only pozzolanic in nature, to higher CaO oil shale ash which can be hydraulic and can serve as a cementitious matrix without any need for an activator. Therefore, each type of oil shale ash must be evaluated separately. The experience and knowhow gained in the application of one kind of oil shale ash may not be relevant to others. This is considerably different than with other residues, such as fly ash, which do not exhibit this extent of variable properties.

The use of fly ash as a mineral admixture for lean concrete (road base concrete) has aroused a rather limited interest until now. However this comparative study shows that there are so-me advantages in using low calcium fly ash in lean concretes. The compactibility of lean concrete is improved : the maximum level of compaction (Modified Proctor test) is achieved at about 5 % fly ash addition, whereas it is equal at 0 % and 10 % addition . The CBR-indexes of the mixes are similar at Proctor maximum, but the higher the fly ash content, the more sensitive the index is to an increase in moisture content. At an early stage, fly ash is not very effective in strength development : it is essentially the portland cement content (2 to 5 %) that governs the rate of strength evolution. On the other hand, at longer periods (more than six months), fly ash contributes very largely to strength : a factor of 1.5 between the weakest mix and the reference lean concrete without fly ash. Accordingly a reduction of the cement content in practice can be taken into consideration. Water stability which is obtained rapidly, is not much affec-ted by the presence of the admixture. On the other hand, resistance to repeated freezing and thawing cycles is delayed because of the slower strength gain for mixes containing more fly ash and less cement. The results on the whole show that the optimum low-calcium fly ash content in lean concrete for road base lies around 5 % by mass with the possibility of reducing the cement content appreciably.