International Concrete Abstracts Portal

Several new coal-fired combustion system modifications have been designed to improve the quality of air emissions from power plants. These plant modifications have led to changes in the character of fly ash, and presented challenges for many of it’s conventional uses. For example, low NOx burner systems improve air emissions but also have the side effect of increasing the carbon in the fly ash. Wisconsin Electric Power Company (WE) has developed three new coal ash beneficiation processes for carbon and/or ammonia removal. These new processes have been demonstrated at various Wisconsin Electric coal fired-power plants located in Michigan and Wisconsin. The processes take advantage of utilizing the residual energy in high carbon fly ash and bottom ash; while also producing high quality fly ash for use as a supplementary cementing material for the concrete industry. These beneficiation processes are also designed to remove any residual ammonia contained in the fly ash from advanced NOx reduction systems such as Selective Catalytic Reduction (SCR), Selective Non-Catalytic Reduction (SNCR), and Amine Enhanced Fuel Lean Gas Rebum (AEFLGR). These new ash beneficiation processes are designed both as stand alone systems or potential additions to existing power plants. In some cases it may be advantageous to rebum high carbon coal ash from one power plant by transporting it to another where more complete combustion normally occurs.

The potential use of Malaysian fly ash cement in concrete has been studied in terms of its influence on elastic properties, volume stability and durability properties. In durability studies, the resistance to chloride ingress, sulfate attack, and suppression of alkali-aggregate reactivity were examined. The results are presented and discussed. The role of the fly ash on heat of hydration, production of roller-compacted concrete, and high-strength, high-performance concrete have been proven in practice. Understanding the influence of fly ash on both fresh and hardened properties of concrete has led to its appropriate use in many important structures in Malaysia. They include the Petronas Twin Towers, the second Malaysia Singapore Causeway, and more recently considered for the construction of a major RCC dam. Properties of fly ash concrete are discussed with examples of application.

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.

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.

This investigation was performed to establish the effects of pozzolanic and chemical admixtures on setting behavior of cement paste at normal consistency. An ASTM Class C fly ash was used as a pozzolanic and cementitious admixture. Mixtures were proportioned to contain fly ash in the range of O-l 00% by mass of the cementitious materials using a cement replaced by fly ash in a proportion of 1: 1.25. One source of ASTM Type I cement was used. The effects of five admixtures, air-entraining agent (AEA), water reducer, retarder, high-range water-reducer (HRWRA), and hemihydrate gypsum (CaSO,. 1/2H2O) on setting characteristics of pastes, were investigated. Both initial and final setting times remained essentially the same or were slightly delayed up to 20% cement replacement with respect to the 0% fly ash mixture. Beyond this range, the times of setting were generally accelerated. Increased rate of setting occurred at cement replacement levels of 40 % and above irrespective of type of chemical admixtures used.