International Concrete Abstracts Portal

In 2004, the Canadian Centre for Mineral and Energy Technology (CANMET), in association with the American Concrete Institute, the Electric Power Research Institute, Palo Alto, CA, UWM Center for By-Products Utilization, Milwaukee, WI, and several other organizations in Canada, sponsored the Eighth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete. The conference was held in Las Vegas, Nevada, U.S.A., May 23-29, 2004. The proceedings of the conference containing 56 refereed papers from more than 20 countries were published as ACI Symposiuml Publication SP-221. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP221

Chloride ingress into concrete is commonly modeled using Fick's second law of diffusion. The common form of Fick's second law was derived for the boundary condition of one surface being in contact with a reservoir of constant concentration, such as the submerged portion of a marine seawall. However, in structures exposed to cyclic wetting and drying, this constant concentration boundary condition is not satisfied. Essentially, in cyclic exposures the surface concentration is not constant, but increases as a function of time. Chloride ingress occurs through sorption and diffusion in such exposures, but solely through diffusion in saturated environments. This paper compares the difference between chloride ingress in concrete exposed to cyclic wetting and drying environments with that of submerged concrete. This multi-year evaluation consisted of six different concrete mixtures, five of which contained different supplementary cementing materials. In addition, each concrete mixture was tested with and without a surface applied silane sealer. The results of these evaluations illustrate the relative rates of chloride ingress in the different environments and the effectiveness of supplementary cementing materials when combined with a silane sealer to reduce chloride ingress in cyclic exposures.

The paper shows the influence of mineral additions (in form of fly ash, slag and ground limestone) replacing portland cement on the COz penetration rate of concretes manufactured at a given water-cementitious material ratio (w/cm). The results indicate that at a given w/cm there is an increase in the carbonation rate in concretes with mineral additions, except when the amount of portland cement replacement is relatively low (15%). On the other hand, when the comparison of the carbonation rate is made on concretes at the same strength level, there is no significant difference between concretes with portland cement and those with replacement by mineral addition up to 50%.

The influence of silica fume (SF) on the plastic viscosity, yield point and gel strength of cementitious paste has been studied. A super-plasticizer based on polyacrylate with grafted polyether chains (SSP) was used. The effect of delayed addition of super-plasticizer versus addition with the mix water was investigated as well as the effect of densified versus untreated silica fume. Inert limestone slurries with SF replacement were used as comparison. The results showed that there was not much to gain in terms of lower shear stress in flow curves by delayed addition of this particular SSP. The plastic viscosity had a de-creasing tendency with increasing SF replacement, while yield stress was rather constant. Plastic viscosity increased with increasing time. There was a substantial increase in 10 min gel strength with increasing SF replacement. Gel strength was lower for mixes with densified SF versus untreated SF. Delayed SSP addition reduced gel strength for cementitious pastes with 10% SF. Neutral limestone pastes with increased SF replacement had increasing gel strength from 6 vol% replacement. Densified silica fume gave 10 times less gel strength than untreated SF. Increasing pH from 8 to 13.2 in limestone slurries with 10 vol% SF increased the shear stresses substantially.

The work presented herein is a laboratory study on the mechanical properties of ternary blended cement built with various combinations of two different high calcium (ASTM Class C) fly ashes and an amorphous silica. A commercial amorphous silica agent of great specific surface was added in binary fly ash-cement (FC) systems in order to compensate for the certain shortfalls associated with the presence of high lime ashes in cementitious blends and furthermore to provide a benchmark for utilizing supplementary cementing materials rich in active silica (such as silica fume, metakaolin and rice husk ash) in resembling ternary systems. For the purpose of this study, both fly ashes were substituted by 5, 10 and 20% amorphous silica and the new blends replaced 20 and 30% of cement. The generated ternary blends were examined in terms of compressive strength development, efficiency factors (k-values) and strength gain. It was found that in the case of 20% cement replacement, up to 10% amorphous silica addition accelerated the strength development of all blends. For higher cement replacement (30%), 5% amorphous silica introduction was the optimum percentage for the blends incorporating high-calcium ashes. Amorphous silica acts immediately after inclusion in the mixture, improving the early strength of all binary systems, while in the later stages of the hardening process, the behavior of all mixtures is highly depended on the active silica content of the ashes they incorporate.