International Concrete Abstracts Portal

The importance of a low water cement ratio for enhancing durability of a concrete has long been accepted. Low water content leads to a low workability of the fresh concrete and if this concrete is not properly compacted, the durability of the structures will be impaired. In the mid seventies, superplasticizers were introduced to the market to produce rheoplastic concretes. These concretes had a high workability, slump of over 200 mm, but were at the same time cohesive and non segregating. Millions of cubic meters of such concretes have been used all over the world during this decade. Recently self-compacting concrete was developed in Japan and its use is spreading very rapidly to other countries. A proper design of a self-compacting concrete requires considerably more fines content as compared to the traditional concrete. Therefore, large volumes of fly ash, partially in substitution of cement and partially as filler, can be employed in producing self-compacting concrete. This paper compares the properties of fresh and hardened normal concrete and a self-compacting concrete with large volumes of fly ash. Significant advantages in the use of fly ash are demonstrated.

The durability of five mortar and concrete mixtures was investigated in this study. A normal portland cement and four blended cements incorporating different pozzolanic materials, two natural pozzolanas and two lignite fly ashes, were used. The properties measured were compressive strength development, sulfate and chloride resistance as well as the carbonation depth. The results available until the age of 1.5 years show that the replacement of normal portland cement by a pozzolanic material usually has beneficial effects on cement’s durability, especially when the sulfate resistance of mixtures is of primary interest. The carbonation depth of the pozzolanic mixtures was greater than the control’s at all ages up to 1.5 years. Among the four pozzolanic materials examined, the treated lignite fly ash from Ptolemaida gave the best performance in all the tests.

The effect produced by hydrothermal treatment of a mixture of two fly ashes from municipal solid wastes incineration (MSWIFA) is discussed in this work. Two kinds of fly ashes, from fluidized-bed combustion and cleaning-gas devices with different chemical composition, principally in terms of CaO and SiO2 contents, were mixed to synthesize a new low energy cement, like belite cement (Ca/Si > 2). The mixture was hydrothermally treated at 200°C and 1.24 MPa steam-pressure for periods of 1, 2, 4 and 6 hours. Changes of fly ashes compositions were characterized by X-ray diffraction (XRD), Infrared Spectroscopy (IR) and Thermal Analyses (TGA). The evolution of aqueous phase was followed by means of measurements on: pH, conductivity, chloride, sulfate and toxic metals concentration.

This research evaluated, from the ecological and economical points of view, the potential use of off-specification fly ash plus non-standard clinker ash (bottom ash or coal ash) in CLSM (Controlled Low Strength Materials). The effect of mixture proportions on the short-term as well as long-term compressive strength of CLSM is mainly investigated. A wide range of fly ash/clinker ash ratio was evaluated in order to provide a cost effective mixture design for various material costs. Two different sources of fly ash including off specification fly ash and three different sources of clinker ash were used with three levels of mixture combinations. A total of 20 mixtures was tested for the flowability, bleeding and short-term and long-term compressive strengths (strength developments up to 91 days were reported in this study). Test results showed that there is an optimum combination of fly ash clinker ash ratio on the physical properties of CLSM. Compressive strength improved with increasing the rate of replacement of clinker ash up to 5 0 percent in the case. It is found that there was no disadvantage of using off-specification fly ash and non-standard clinker ash in the physical properties of CLSM. CLSM with off-specification fly ash plus non-standard clinker ash showed excellent performance on the physical properties indicating the ecological and economical applicability to CLSM.

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.