International Concrete Abstracts Portal

The interface between the cement matrix and aggregate is mostly regarded as a weak link in concrete with respect to durability and strength. It is shown that the positive effects of pozzolans on the permeability of concrete are partly related to a decrease in the thickness of the weak, lime-rich, interfacial zone. Results for various mineral admixtures, such as ground granulated blast furnace slag, powder coal fly ash, silica fume, a synthetic colloidal silica, and metakaolinite are presented. It is shown that in the presence of mineral admixtures, the calcium hydroxide content in the interfacial zone is reduced substantially.

Within the context of atmospheric pollution prevention, fly ash producers are brought to better desulfurize combustion gases. As a result, there are important modifications in the composition of the residues that are enriched with sulfur, especially in the case of calcium sulfite fly ash. The sulfitic fly ashes studied result from desulfurization by quick lime addition in dust removers of a thermal plant. Paper presents the physical, chemical, and mineralogical characteristics of this type of fly ash. Their examination by x-ray diffraction shows the presence of crystallized minerals: lime, calcite, and calcium sulfite, and the existence of a glass, the composition of which is given by electron microprobe. A special emphasis is given to the sulfite stability. In a second part of the paper, the behavior of these fly ashes in paste with water and slaked lime has been studied. It is shown that they are a very weak hydraulic binder but they present, on the other hand, pozzolanic activity. At last, the study has allowed determination of the influence on workability and mechanical properties of hydraulic mortars and concretes in which they have been introduced as partial replacement of cement. Corrosion and carbonation aspects have also been studied in mortars.

Heavy damage due to alkali-aggregate reaction has been observed in concrete structure in and along the sea. An accelerated test is performed on mortar to evaluate effectiveness of fly ash for controlling alkali-aggregate reaction in the marine environment. Mortar bars using Pyrex as aggregate and cements with 0.6 and 1.1% of equivalent sodium oxide are made. The alkali content in the mixture is adjusted by adding NaOH or NaCl. Specimens are stored in distilled water, NaCl solution, and under more than 95% of relative humidity. The controlling effect of fly ash and the effect of internal and intruded chloride ion in mortar on alkali-aggregate reaction is studied by measuring the expansion of mortar. Expansion of mortar depends on the type of cement and chemical reagents used for alkali adjustment, the amount of fly ash used and the exposure condition. Even with the same equivalent sodium oxide in the mixture, mortar using NaCl for alkali adjustment shows higher expansion than mortar using NaOH. The highest expansion is revealed for mortar cured in NaCl solution. The controlling effect of fly ash also depends on the type of cement and the exposure condition.

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.