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A study has been made of the ability of a highly dense silica-cement mortar to protect steel reinforcement from corrosion. Corrosion of the steel is an electrochemical process in which the cement pore water acts as the electrolyte. The cement must, therefore, allow (i) passage of electrical current, (ii) diffusion of oxygen and other species from the environment to the steel and (iii) transport of the corrosion products away from the steel. The properties of the cement which are of interest are, thus, its electrical resistivity, its porosity and the chemistry of the pore water, particularly the pH. However, it is the combination of these properties which determines the corrosion rate and, as yet, not enough informatiou is available to allow a direct prediction of the corro-sion rate. Consequently, electrochemical polarization measurements have been made in order to determine the actual corrosion behaviour. In the electrochemical experiments, a steel rod embedded in the mortar is held at a constant potential with respect to a reference electrode and the current flowing between the embedded rod and an external steel electrode is measured. By repeating the process for different values of the potential, it is possible to extrapolate the data to the corrosion potential, thereby giving a direct estimate of the free corrosion rate. These experiments have been carried out in different environments (anaerobic solutions, NaCl solutions, H2C03 solutions, etc.) and the results are discussed in terms of the properties of the silica-cement mortar. Despite the Pozzolanic reaction in the cement, it appears to provide excellent protection to steel reinforcement.

This paper presents the results of an investigation to determine the performance characteristics of fiber reinforced concretes containing condensed silica fume. The plastic concrete properties studied are slump, air content, unit weight, and Vebe-time. The hardened concrete properties investigated are compressive strength, flexural strength, toughness index, tensile strength, pulse velocity, static modulus of elasticity, and absorption co-efficient. These hardened concrete properties are compared at ages 3 to 90 days. A total of 17 mixes were made using type I cement, with the addition of condensed silica fume, steel fibers, and superplasticizers. Seven cement contents varying from 160 to 560 kg/m3 (270 to 945 lb/cu.yd.) were investigated with W/C ratios varying from 0.28 to 1. In all mixes condensed silica fume was added at 20 percent by weight of cement and steel fibers at 47.5 kg/m3 (80 lb/cu.yd.). The results indicate that the addition of condensed silica fume and steel fibers had reduced the workability consid-erably, however the workability can be restored with the addition of an appropriate amount of superplasticizer. In general the finishability of fiber reinforced concrete is improved with the addition of condensed silica fume. The unit weight of fiber reinforced condensed silica fume concrete is higher than the normal concrete. The addition of condensed silica fume is beneficial for fiber reinforced concretes with low cement content; it was shown that high strength fiber reinforced concretes could be made with low cement contents. The moisture absorption capacity of the con-crete is reduced with the addition of condensed silica fume. For the selected 20 percent addition of condensed silica fume, there is an optimum cement content and water content that considerably improves the performance characteristics of the steel fiber rein-forced concrete.

This paper describes the use of condensed silica fume in the ready-mixed concrete plant at St. Eustache, P.Q., Canada. The effect of this admixture on the properties of fresh and har-dened concrete is discussed. Efficiency factors for the silica fume-modified concrete are given for different strength levels. Transportation of the fume and special installations at the ready-mixed concrete plant for the handling of condensed silica fume are described.

The utilization of agricultural residues for building mater-ials is particularly attractive in developing countries where indigenous resources for building are hard pressed to cope with the demand. In this paper the use of rice husks, essentially a waste material in most countries, as a source for a pozzolanic material is described. The behaviour of lime - rice husk ash mixes in terms of strength, shrinkage, durability and microstructure are examined. It is shown that although maximum strengths are achieved with low lime mixes, lime leaching could seriously impair the durability and indeed the long term strength of these mixes. Accordingly, it is proposed that the optimum ratio of lime : rice husk ash should be about 1 : 1. Further, it is shown that within the limits of carbon contents likely to be encountered in the field, strength development is unaffected, though some retardation in initial and final set may occur. Finally, examination of the microstructure through x-ray diffraction analysis indicates that the hydrated matrix consists of calcium silicate hydrate of the CSH (II) type.

At early ages, blended cement mortars with 30 % hydraulic slags or active pozzolans have lower mechanical strengths than mortars incorporating 100 % Portland cement. The action of 5 % condensed silica fume replacement for slag or pozzolan or material considered as inert has been studied from 7 days to 3 months by measuring the mechanical strengths of mortars made according to IS0 test method and by observing the microstructure of fractured samples. This action of the condensed silica fume is a function of the nature of the additive. With hydraulic slags, there is a competition between slag and silica fume for the available lime. With slightly or slowly reactive pozzolans (volcanic rock, fly ash) mechanical strengths and microporosity of mortars show improve-ment at 28 days. With inert materials (crystalline slag or quartz), the improvement is more marked. This is due to the formation of dense C-S-H, strong cement paste-aggregate bond and 20 % increase in mechanical strengths.