International Concrete Abstracts Portal

Conditioning coal-burning power-plant flue gases with ammonia reduces the emission of nitrous oxide compounds. But the ammonia often combines with available sulfur and other compounds that attach to the fly ash. If the ammoniated fly ash is then used in concrete, the high-pH environment causes a release of ammonia and a strong, objectionable ammonia smell. This can make the fly ash unmarketable. What’s the solution? Fly ash beneficiation processes that can remove ammonia and also reduce the unburned carbon content. Some of the processes are described in one of the 54 papers included in ACI SP-199, Seventh CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete. Other papers deal with effects of fly ash and admixture combinations on setting time, use of slag concrete to reduce corrosion of reinforcement, and the role of chemical and mineral admixtures in concrete made with recycled concrete as aggregate. 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. SP199

The objective of the study was to evaluate the influence of silica fume on the rate of heat liberation and the accumulated heat in high-performance concretes. Portland cement was replaced by silica fume in amounts from 10 % to 30 % in concretes with water-binder ratios varying between 0.25 to 0.45. A superplasticizer was used to maintain a fluid consistency. The heat of hydration was followed continuously by a semi-adiabatic calorimetric method for 10 days at 20°C. The calorimetric study indicates that the hydration is modified by the presence of silica fume. In the early stages, the silica fume shows a high activity and accelerates the hydration rate compared with the control concrete. The fine silica fume particles provide nucleation sites for hydrate growth. Then the pozzolanic activity takes over and increases the heat of hydration. A substitution of portland cement by 10 % silica fume produces greater cumulative heat of hydration compared with the control concrete.

It is now established that the incorporation of industrial byproducts, such as fly ash (FA), ground granulated blast furnace slag (GGBFS) and silica fume (SF), in concrete mixtures as partial cement replacement materials (CRMs) significantly enhances the durability related properties of concrete, besides improving its mechanical properties.. Such concretes are termed as High Performance Concretes (HPCs). In HPC mixtures, it is possible to have low W/C with reduced cement content by judicious choice of mineral and chemical admixtures. The assessment of the flexural behavior of reinforced HPC beams is a subject of ongoing research. Standards and codes of practice differ in the definition of compressive stress block parameters of reinforced HPC beams. Hence, investigations were undertaken at the Structural Engineering Research Centre (SERC), Chennai, to investigate the behavior of reinforced HPC beams under flexural loading. The beam specimens for the test programme were designed as under reinforced, balanced and over reinforced sections as per the guidelines given in the Indian code of practice for reinforced concrete, IS:451 978. The influence of supplementary cementitious materials (SCMs) used as cement replacement materials (CRMs), viz., fly ash (25% as CRM) and GGBFS (40% as CRM), on the flexural behavior of reinforced HPC beams was studied. The paper presents the details of the investigations and discusses the results obtained.

Two silica-manganese slags containing about 11 % MnO were ground to Blaine finenesses of 360 to 600 m*/kg. Their C/S (CaO/SiO2) modulus was very low (0.47 to 0.58) and, for this reason, these slags were considered likely to be unsuitable for use in the preparation of blended cements. They were therefore introduced in concrete instead of either fly ash or silica fume. The cement content of these concrete mixtures was in range of 2 1 0 kg/m3 t o 4 5 0 kg/m3. The quantity of each supplementary cementing material was adjusted according to the French standards and varied from 50 to 80 kg/m3. The slags behaved as well in concrete as fly ash or silica fume: no decrease in strength was observed and the water permeability was the same.

Synopsis: Ecocement is a new type of hydraulic cement made from incinerator ashes. In this study, the effect of blast-furnace on the corrosive behaviour of steel bars embedded in mortars made from two types of ecocement; rapid hardening type with high chloride content, and NPC type with low chloride content, was investigated. Cement-slag mortars were prepared with both types of cement. The mortar specimens, with mild steel and stainless steel bars embedded, were placed in an environmental chamber under severe saline conditions. The corrosion of the mild steel bars was monitored by electro-chemical measurements. Chloride concentration profiles in cylindrical mortar specimens were also measured after the exposure period. The results of the various tests clearly indicate the beneficial effect of slag on the time of corrosion initiation and the degree of corrosion of the steel bars.