International Concrete Abstracts Portal

The admixtures of condensed silica fumes (CSF) and phosphogypsum (neutralized and dehydrated at 400 C) were used together with fly ashes as blended cement components to improve early strengths and other properties. The cements with the initial 15 to 50 percent low-calcium PFA content (SiO2 + AL2O3 + Fe2O3 - 83.3 percent) or 15 to 70 percent high calcium PFA content (22.1 percent CaO) were mixed with the additional components just mentioned. Standard tests at normal curing were made, as well as measurements after the low-pressure steam treatment at 70 C. All cements mixed with CSF showed standard compressive strengths about 13 to 20 MPa higher than the reference mortars. More detailed studies of the hardening process were also carried out using calorimetry, DTA, TG, XRD, and porosimetry, which showed acceleration of the hydration process due to pozzolanic properties of CSF. Reduction of total porosity and pore size was also found. The same positive effect of CSF was observed in the case of mortars treated at 70 C. This additive improves significantly the pozzolanic properties of low-calcium PFA. At standard curing, activated phosphogysum addition brings about a decrease in the hydrated calcium silicates. A substantial amount of ettringite forms and partially inverts into monosulfate after 28 and 90 days of hardening. At accelerated curing, the mortars containing phosphogypsum show a significantly higher degree of hydration than the reference mortar. The results relating to pastes and mortars have been confirmed for concretes. Therefore, one can conclude that the admixtures studied, particularly CSF, have positive influence on the properties of PFA concretes and help to augment the effect of PFA content in these concretes.

Three of the major uses of silica fume (microsilica) additions to concrete have been to improve mechanical properties, improve corrosion resistance by reducing permeability to aggressive anions such as chlorides, and improve concrete resistance to chemical degradation. In the last two uses, the mechanical properties are also enhanced beyond those of ordinary portland cement concretes of the same mix proportions without silica fume. Thus, the production of durable concrete often leads to an improvement in mechanical properties. Long-term resistance in accelerated laboratory corrosion testing in sodium chloride solutions is documented. It is shown that silica fume significantly lowers chloride ingress with increasing efficiency as the water-cementitious ratio decreases. A clear improvement in corrosion performance with the addition of calcium nitrite corrosion inhibitor became evident in this long-term program. It is also documented that high concrete resistivities do not necessarily prevent severe corrosion from occurring. Chemical resistance of silica fume (microsilica) concretes to numerous acids, bases, and salts is also examined. The results show significant improvements with the addition of silica fume in the time to 25 percent mass loss in cyclic and continuous ponding experiments for most chemicals. For some highly alkaline solutions, there is no improvement with microsilica. Improvements in compressive strength are documented for the mixtures used in the corrosion and chemical resistance studies. Additional mixtures were examined to determine flexural strength and modulus of elasticity. These mixtures were similar in composition to those typically used for corrosion protection. The results showed that silica fume significantly increased strengths and the modulus of elasticity. The improvement in flexural strength was greater than that expected from formulas typically used for moderate strength concretes and the increase in modulus of elasticity was less. It is hoped that the design engineer will be able to utilize the data to take full advantage of the property improvements and not merely durability or strength improvements with silica fume.

The durability of alkali-resistant glass fiber in cement matrixes with and without silica fume was investigated. Several attack modes such as hydroxylation, mass dissolution, and notching by calcium hydroxide crystals were distinguished. The effect of silica fume addition was found to be slight; it greatly reduced the calcium hydroxide content of the cement matrix and inhibited notching attack, but it did not reduce the internal pH sufficiently to inhibit hydroxylation and mass dissolution. The flexural strength of cement pastes at 20 C with and without silica fume initially increased during the first month and thereafter started to decrease and eventually leveled off at longer ages. The addition of silica fume gave only a marginal improvement to the elastic properties of composites at 20 C. At 55 C, the flexural strengths of both formulations were observed to decrease very rapidly, approaching the flexural strength of the unreinforced matrix.

The short-term strength development and long-term durability of silica fume concretes is investigated for mixtures intended to be alternatives to those without silica fume currently used in precast prestressed concrete bridge girder production in Alberta, Canada, where accelerated curing at 65 C to obtain specified strengths of 28 MPa at 16 hr and 35 to 42 MPa at 28 days is normal industry practice. Concretes with 300 kg/m 3 and 450 kg/m 3 of normal cement, each with 10 percent silica fume by weight of cement, were examined for strength development, entrainment at air-void parameters, resistance to rapid freezing and thawing in water, resistance to deicer scaling in 3 percent NaCl solution, and chloride permeability by the applied-voltage technique. Silica fume was found to significantly enhance all aspects of durability relevant to exposure conditions involving freezing and thawing with deicers. Varying degrees of short- and long-term strength improvement and reduced cost in cementitious materials are also possible as a consequence of using silica fume.

Compressive strength, flexural strength, and fracture energy of ordinary portland cement concrete with and without fly ash have been determined over a 6-month period. Specimens were cured in water at various constant temperatures ranging from 7 to 80 C. Flexural strength and fracture energy were measured on notched specimens subjected to a constant rate of deformation. The influence of temperature on strength is complex, and does not always follow the trend of a higher initial rate and lower ultimate value as the curing temperature is raised. Compared with strength, fracture energy is less sensitive to curing temperature. For all concretes, general expressions are presented for relating flexural and compressive strengths, and facture energy and flexural strength. These expressions are independent of age and temperature, and suggest that approximate estimates of strength and fracture energy can be made only from a knowledge of strength of ordinary portland cement concrete cured at 20 C.