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قسمتی از متن پاورپوینت Supplementary Cementing Materials

Introduction

•Fly ash, ground granulated blast-furnace slag, silica fume, and natural pozzolans, such as calcined shale, calcined clay or metakaolin, are materials that, when used in conjunction with Portland or blended cement, contribute to the properties of the hardened concrete through hydraulic or pozzolanic activity or both. These materials are also generally categorized as supplementary cementing materials (SCMs) or mineral admixtures.

Supplementary cementing materials are added to concrete as part of the total cementing system. They may be used in addition to or as a partial replacement of Portland cement or blended cement in concrete, depending on the properties of the materials and thedesired effect on concrete.

Supplementary cementing materials are used to improve a particular concrete property, such as the mitigation of deleterious alkali-aggregate reactivity.

Traditionally, fly ash, slag, silica fume and natural pozzolans such as calcined clay and calcined shale were used in concrete individually. Today, due to improved access to these materials, concrete producers can combine two or more of these materials to optimize concrete properties. Mixtures using three cementing materials, called ternary mixtures, are becoming more common.

Fly Ash

•Fly ash is a finely divided residue (a powder resembling cement) that results from the combustion of pulverized coal in electric power generating plants (See Fig. 3-2). Upon ignition in the furnace, most of the volatile matter and carbon in the coal are burned off.

•During combustion, the coals mineral impurities (such as clay, feldspar, quartz, and shale) fuse in suspension and are carried away from the combustion chamber by the exhaust gases. In the process, the fused material cools and solidifies into spherical glassy particles called fly ash (See Fig. 3-3). The fly ash is then collected from the exhaust gases by electro-static precipitators or bag filters.

•Most of the fly ash particles are solid spheres and some are hollow cenospheres. The particle sizes in fly ash vary from less than 1 µm to more than 100 µm with the typical particle size measuring under 20 µm. The surface area is typically 300 to 500 m2 /kg.

For fly ash without close compaction, the bulk density (mass per unit volume including air between particles) can vary from 540 to 860 kg/m 3 , whereas with close packed storage or vibration, the range can be 1120 to 1500 kg/m 3 .

Fly ash is primarily silicate glass containing silica, alumina, iron, and calcium. Minor constituents are magnesium, sulphur, sodium, potassium, and carbon. Crystalline compounds are present in small amounts. The relative density (specific gravity) of fly ash generally ranges between 1.9 and 2.8 and is generally tan or grey in colour.