Fly ash lightweight aggregates in high performance concrete

Abstract

Lightweight aggregates have been manufactured by sintering fly ash and crushing the product into suitable sizes. These aggregates possess unique characteristics that make them suitable for high strength and high performance concrete. Concrete produced using these aggregates is around 22% lighter and at the same time 20% stronger than normal weight aggregate concrete. Drying shrinkage is around 33% less than that of normal weight concrete. Moreover, the aggregates possess high durability characteristics required in high performance structures. The importance of the new aggregates lies mostly in the fact that superior qualities are achieved without having to increase the cement content. Thus it is possible to reduce the amount of cement by as much as 20% without affecting the required strength. Weight reduction may reduce precast concrete transportation costs as well as provide slender and spacious construction. Utilising fly ash to produce quality aggregates should yield significant environmental benefits.

Introduction

One of the more important of the industrial by-products, if not the most important, is fly ash. Fly ash is a by-product of the burning of coal for power generation. Its collection and disposal are necessary processes in power stations. It is estimated that the world production of fly ash in the year 2000 was about 600 Mt (million tonnes). Of this amount, only about 9% got to be utilised [1]. In Australia, in the late 1990s, about 9 million tonnes of fly ash was annually produced [2]. Less than 10% of this quantity was used [2]. In 2003, it was reported that Australia and New Zealand produced 12.5 Mt from which 4.1 Mt were effectively utilised [3]. This, is a significant improvement over the figures of the 90s but still falls very much short of what ought to be the case.

Fly ash, however, is a material that has many proven advantages even when used in its raw form [4], [5]. Such advantages, though very important and relevant, are not within the scope of this article.

The use of lightweight aggregate in concrete has many advantages [6], [7], [8]. These include: (a) Reduction of dead load that may result in reduced footings sizes and lighter and smaller upper structure. This may result in reduction in cement quantity and possible reduction in reinforcement. (b) Lighter and smaller pre-cast elements needing smaller and less expensive handling and transporting equipment. (c) Reductions in the sizes of columns and slab and beam dimensions that result in larger space availability. (d) High thermal insulation. (e) Enhanced fire resistance. Furthermore, certain structures, specifically offshore structures which are mostly used for oil production, require lightweight elements which can be towed easily and have the greatest buoyancy [9]. But perhaps the most significant potential advantage of the use of lightweight aggregates for concrete and building in general is the environmental value. When the raw materials needed for lightweight production are derived from industrial by-products, the environment and economy of the producing locality and country are deemed to benefit. Already, stringent environmental limitations are imposed on mining of natural aggregates in various parts of the world [8], [9], [10]. Such production would result in benefits to the community, the environment, and the building industry. Among these benefits are the following: (1) Efficient recyclable disposal of the fly ash. (2) Helping to conserve the natural and sometimes scarce materials of coarse aggregates and sand. (3) Sparing the country side, river beds and beaches from the scarring and damaging activities of aggregate mining. (4) Producing aggregates much lighter than the natural aggregates. This would result in the production of significantly lighter concrete whose advantages have been mentioned above. (5) Significantly reducing the emission of green house gases by reducing the need of large quantities of cement whose production is a major contributor to CO₂ emission. (6) Establishing an industry with export potential especially to countries where natural aggregates are depleted or, are of extremely inferior quality as in the oil rich Middle East states [11].

The fly ash aggregates reported here, have been manufactured using sintering but without pelletizing the aggregates as has so far been the usual procedure. Instead the aggregates were crushed from briquettes that were fired in a kiln [12]. This aggregate type will be referred to here as FAA.

In this paper, the performance of concrete made from this type of lightweight aggregates is examined and compared with conventional natural gravel and sand. The mix design was performed such that there is a constant cement content regardless of the type of aggregate in use.