Metakaolin is a supplementary cementitious material with pozzolanic properties. Its precursor, kaolin, does not occur in natural conditions as a pure phase but is often mixed in various proportions with many secondary minerals. Two natural clays from the same deposit, mainly composed of kaolinite and quartz have been burnt at a suitable temperature and the resulting calcined products have been blended with laboratory normal portland cement for which the nature of added calcium sulfates as well as free lime content have been varied. Subsequent properties of cement pastes and mortars such as rheology, setting time and compressive strength have been compared. Concurrently, early hydration products have been characterized by means of DSC and combined water content has been evaluated by C02-H20 analyser. The difference of behavior that has been observed as a function of the artificial pozzolan mineralogy and cement chemical parameters is discussed in the frame of the mechanism of early hydration of metakaolin blended cement.

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

Blending fly ash with normal portland cement enhances the durability and service life of concrete structures. Fly ash-based blended portland cements containing 1530% fly ash by mass are being increasingly used worldwide. These cements have been used in high-performance concrete for modem structures such as Petronas Towers, Euro-Tunnel, and Akashi-Kaikyo Bridge. High-volume fly ash concrete is another recent development according to which, large volumes of fly ash, 50-60% by mass of the total cementitious material, can be used in combination with a superplasticizer to overcome the slow rate of strength development in blended portland cements. The chloride permeability of unsuperplasticized, high-volume fly ash concrete at early ages is rather high, but it can be greatly reduced by the incorporation of either silica fume or a highly pozzolanic rice-husk ash. Rice-husk ash is a siliceous material produced by furnaces that use rice-husks as fuel. The rice-husk ash containing amorphous silica in cellular microstructure is a superpozzolan, and has proven to be a valuable material for making highly durable concrete. It also contributes to concrete strength at early ages. This paper describes a preliminary research study on the role of rice-husk ash in enhancing the properties of high-volume fly ash concrete, particularly the early-age strength and chloride impermeability.

Amorphous silica with high whiteness decomposing the basic mineral olivine, solution of magnesium and ferrous salt washing and filtering or decanting. And (Mg s and surface can be produced by simply Fe)SiO2 in any acid. The result is a silica slurry that can be purified by Such silica has been produced by treating an olivine mineral residue, a by-product of nickel ore recovery, by hydrochloric acid. The free flowing silica residue, after drying at 105C, is proven to have pozzolanic activity (consumption of calcium hydroxide) by thermal analysis (DTA/TG) and by strength measurements of mortars where cement is replaced with silica. The reactivity and strength gain was comparable to conventional silica fume obtained from ferrosilicon plants. The abundant mineral olivine can be a valuable source of amorphous silica for concrete technology, while the waste product lye could be used as a CO2 free magnesium chloride source for magnesium metal production, after purification. The process could also use waste acids, from paper pulp industry.

This paper provides results about the effect of using different types of curing on the compressive strength of concrete both with and without large volumes fly ash (FA). In all the concrete mixtures, the portland cement content was 200 kg/m3. The FA amount was varied from zero to 33,43,50 and 56 percent by mass of the total binder, and a superplasticizer was used to obtain 200-220 mm slump. The compressive strength was tested at the age of 3,7, 14,28,56 days and 6 months. The compressive strength of the Portland-cement concrete made at 35°C was reduced by about 11% at 28 days when compared to that of concrete made at 23°C with ASTM standard curing. With continuous moist-curing of fresh concrete, there was no strength loss of concrete made at 35°C. FA concrete specimens that were under intermittent spray-water curing at 35°C in the laboratory (every four hours) for 7 days and then under ambient conditions gave increased compressive strength up to the time of testing i.e. 6 months. Reduced strength was obtained for 3 days intermittent curing. Higher strength was obtained as the amount of FA was increased for a given amount of the portland cement. The FA concrete mixtures cast at 35°C were cured by covering the specimens with membrane curing compound and placed under ambient conditions until age of testing, the strengths were lower than reference concrete by about 20% to 30% at 28 days, and 30% to 50% at 56 days. It is necessary that enough curing water to promote the pozzolanic reaction is used. The membrane curing did not allow the ingress of water to the concrete mass.