Review
Effect of coal bottom ash as partial replacement of sand on properties of concrete

https://doi.org/10.1016/j.resconrec.2012.12.006Get rights and content

Abstract

Coal bottom ash (CBA) is formed in coal furnaces. It is made from agglomerated ash particles that are too large to be carried in the flue gases and fall through open grates to an ash hopper at the bottom of the furnace. Bottom ash is mainly comprised of fused coarser ash particles. These particles are quite porous and look like volcanic lava. Bottom ash forms up to 25% of the total ash while the fly ash forms the remaining 75%. One of the most common uses for bottom ash is as structural fill.

Published literature shown that there is a strongly possibility of coal bottom ash being used as substitute/replacement of fine aggregate (sand). Its use in concrete becomes more significant and important in view of the fact that sources of natural sand as fine aggregates are getting depleted gradually, and it is of prime importance that substitute of sand be explored.

This paper presents an overview of the published literature on the use of coal bottom ash in concrete. Effect of coal bottom ash on the properties of concrete such as workability, bleeding, setting times, compressive strength, split tensile strength, flexural strength, shrinkage, and durability are presented.

Highlights

► This paper discusses about use of coal bottom ash (CBA) in concrete. ► Presents properties of coal bottom ash and its applications. ► Effects of CBA on properties of fresh concrete, mechanical and durability properties.

Introduction

On burning of coal in furnace of coal fired thermal power plants, the non combustible material present in it results in production of coal ash. The finer and lighter particles of coal ash escape with the flue gases and are extracted in the Electrostatic Precipitators before reaching the environment. The coal ash collected from the Electrostatic Precipitators is named as fly ash. Some melted ash accumulates on the boiler walls and against steam tubes and solidifies to form masses called clinkers. The clinkers build up and fall to the bottom of boiler/furnace and are cooled in the water sump before passing through clinker grinder. The coal ash collected at bottom of furnace is called bottom ash. Bottom ash particles are physically coarse, porous, glassy, granular and grayish in color. Bottom ash forms up to 25% of the total ash while fly ash forms the remaining 75%. In India, coal fired thermal power plants are the main source of power generation and about 70% electricity requirements are fulfilled by them. About 360 million tons of coal is burnt by the coal fired thermal power plants every year. Indian coals have high amount of inorganic inclusions with varying properties and on combustion result in high ash content up to 46%. About 100 million tones of fly ash and 25 million tone of bottom ash is produced by these thermal power plants annually. With the addition of 59,000 MW of power generation by the end of year 2012, annual production of bottom ash will shoot up to 50 million tones. Worldwide fly ash is used in large volume in production of cement as mineral additive and in construction industry as partial cement replacement in concrete. Bottom ash is used as land fill material and as base material in road construction. In India up till now a small volume of fly ash is utilized in production of cement but bottom ash is not used in any form. Bottom ash along with unutilized fly ash is disposed off in ponds spread over thousand acres of land. The disposal of bottom ash in ponds poses risk to human health and the environment. The hazardous constituents in bottom ash migrate and can contaminate ground water or surface water, and hence living organisms. Also there is danger of ash dyke spill and filling the surrounding area of pond with ash. Environment concerns are increasing day by day and land fill space is declining, therefore it becomes essential to initiate the effort to utilize the bottom ash. Bottom ash has the appearance and particle size distribution similar to that of natural fine aggregate i.e. river sand. Because of these properties it attracted to be used as sand replacement in concrete. Recently research works have been focused on usage of bottom ash as partial sand replacement in concrete. The published research data indicate that bottom ash is a viable material as sand replacement in concrete. Therefore its suitability as sand replacement material in concrete and the ways in which bottom ash affects the fresh, hardened as well as durability properties of concrete has been critically evaluated in this review.

Bottom ash can be beneficially utilized in a variety of manufacturing and construction applications. At present in America, coal bottom ash is predominantly used for the following applications:

  • Road base and sub-base

  • Structural fill

  • Backfill

  • Drainage media

  • Aggregate for concrete, asphalt and masonry

  • Abrasives/traction

  • Manufactured soil products

Section snippets

Chemical properties

Bottom ash is mainly composed of silica, alumina, and iron with small amounts of calcium, magnesium, sulfate, etc. Its chemical composition is controlled by the source of the coal. Table 1 shows the comparative study of Chemical composition of bottom ash obtained from different sources of coal.

Physical properties

The particles of coal bottom ash are angular, irregular and porous and have rough surface texture. The particle size ranges from fine gravel to fine sand. Coal bottom ash particles have interlocking

Workability

Water demand to achieve desired workability of concrete mainly depends on the number of fines and properties of fine aggregate in it. Natural river sand particles are dense and its shape, surface becomes smooth due to weathering affects. Weak minerals like mica are removed from it. Whereas bottom ash particles are angular and rough textured and are porous. Bottom ash has more number of particles of size smaller than 75 μm as compared to that in natural river sand. Therefore on use of bottom ash

Density

The research data reported by various researchers’ show that there is an appreciable decrease in unit weight of concrete when natural sand is substituted with bottom ash. The decrease in density of bottom ash concrete is attributed to the lower unit weight and porous structure of bottom ash as compared to natural sand. Another factor which is also responsible for lower density is the higher demand of mixing water by the bottom ash concrete which leaves behind more number of pores, larger size

Permeability

The permeability of concrete depends upon the size, distribution and continuity of pores present in cement paste and permeability of aggregates. The reports published by various researchers indicate that bottom ash concrete has higher permeability as compared to natural sand concrete. With the increase in bottom ash content in concrete its permeability increases. The main factors responsible for the increased permeability of bottom ash concrete are porous microstructure of bottom ash, increased

Observations and conclusions

The properties of fresh as well as hardened concrete are closely associated with the characteristics and relevant proportioning of its constituent materials. The research reports show that inclusion of bottom ash as sand replacement in concrete influences the workability, setting times, loss of water through bleeding, bleeding rate and plastic shrinkage of fresh concrete and density, strength, porosity, durability of hardened mass. The published research literature shows that the strength

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