It is well recognized that silica fume can contribute significantly to the compressive strength development of concrete. This is because of the filler effect and the excellent pozzolanic properties of the material, which translate into a stronger transition zone at the paste–aggregate interface.
The extent to which silica fume contributes to the development of compressive strength depends on various factors, such as the percentage of silica fume, the water/cement + silica fume ratio, cementitious materials content, cement composition, type and dosage of superplasticizer, temperature, curing conditions, and age.
Superplasticizing admixtures play an important role in ensuring optimum strength development of silica-fume concrete. The water demand of silica-fume concrete is directly proportional to the amount of silica fume (used as a percentage replacement for Portland cement) if the slump of concrete is to be kept constant by increasing the water content rather than by using a superplasticizer.
In such instances, the increase in the strength of silica-fume concrete over that of control concrete is largely offset by the higher water demand, especially for high silica-fume content at early ages. In general, the use of superplasticizer is a prerequisite to achieving proper dispersion of the silica fume in concrete and fully utilizing the strength potential of the fume.
In fact, many important applications of silica fume in concrete depend strictly upon its utilization in conjunction with superplasticizing admixtures. Silica-fume concretes have compressive strength development patterns that are generally different from those of Portland cement concretes.
The strength development characteristics of these concretes are somewhat similar to those of fly-ash concrete, except that the results of the pozzolanic reactions of the former are evident at earlier ages. This is due to the fact that silica fume is a very fine material with a very high amorphous silica content.
The main contribution of silica fume to concrete strength development at normal temperatures takes place between the ages of about 3 and 28 days. The overall strength development patterns can vary according to concrete proportions and composition and are also affected by the curing conditions.
Carette and Malhotra (1992) reported investigations dealing with the short- and long-term strength development of silica-fume concrete under conditions of both continuous water curing and dry curing after an initial moist-curing period of 7 days. Their investigations covered superplasticized concretes incorporating 0 and 10% silica fume as a replacement by weight for Portland cement and water/cement + silica fume ratios ranging between 0.25 and 0.40.
As expected, the major contributions of silica fume to the strength took place prior to 28 days; the largest gains in strength of the silica-fume concrete over the control concrete were recorded at the ages of 28 and 91 days, although this gain progressively diminished with age. For concretes with water/cement + silica fume ratios of 0.30 and 0.40, the gain largely disappeared at later ages.
Under air-drying conditions, the strength development pattern was found to be significantly different from that of water-cured concretes up to the age of about 91 days; thereafter, however, air drying clearly had some adverse effect on the strength development of both types of concrete.
The effect was generally more severe for silica-fume concrete, where some reduction in strength was recorded between the ages of 91 days and 3.5 years, especially for concretes with water/cement + silica fume ratios of 0.30 and 0.40.
These trends of strength reduction have not yet been clearly explained, but they appear to stabilize at later ages and therefore are probably of little practical significance.
Curing temperatures have also been shown to affect significantly the strength development of silicafume concrete. This aspect has been examined in some detail by several investigators in Scandinavia. In general, these investigations have indicated that the pozzolanic reaction of silica fume is very sensitive to temperature, and elevated-temperature curing has a greater strength-accelerating effect on silica-fume concrete than on comparable Portland cement concrete.
The dosage of silica fume is obviously an important parameter influencing the compressive strength of silica fume concrete. For general construction, the optimum dosage generally varies between 7 and 10%; however, in specialized situations, up to 15% silica fume has been incorporated successfully in concrete.
