Lightweight Aggregate Concretes 1: Materials and Properties
THE increasing attention given in recent years to the thermal insulation
of structures and to reduction in weight has stimulated interest in the use of lightweight concretes of all kinds. One variety, aerated concrete, was described in Volume 16, No. 5; the present article, with others to follow, deals with another type-concrete made with lightweight aggregate. In this field there have been many recent developments, both in the aggregates available and in the use of the concretes made from them.
AGGREGATES
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For many years furnace clinker... - a waste product and pumice naturally occurring but not a native product were the only lightweight aggregates of any importance. Some thirty years ago, they were supple- mented by foamed blastfurnace slag. which is now produced in Britain on a large scale.
Supplies of clinker. however, are fast diminishing and little pumice is imported and. since the demand for lightweight concrete continues to in- crease, it has been necessary to find new ways of producing lightweight aggregates cheaply and in quantity.
Materials of suitable quality, for use in concretes of various kinds. are now being produced by process- ing clays and shales. pulverised fuel ash, and vermiculite. The following notes describe these aggregates and also certain others--expanded slate and expanded perlite which are in use in many countries.
Furnace clinker
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It is important to distinguish clearly between clinker and breeze. The latter, which is simply a finely divided coke, is not recommended for use as a concrete aggregate. The term 'breeze blocks.' still commonly used for blocks made with clinker aggregate, is a misnomer and likely to lead to confusion. For the purpose of definition as a concrete aggregate, the term 'clinker' applies only to well-burnt furnace residues that have been fused or sintered into lumps.
The presence of unburnt or par- tially burnt coal of certain kinds can cause serious expansion of con- crete in which the clinker is used. and even if the clinker is not actual-
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ly 'unsound' in this way any exces- sive content of combustible matter is liable to affect the quality of the concrete adversely. All clinker ag gregates, therefore, should conform to B.S. 1165, which specifies limits for the combustible content of clink- ers for different grades of concrete. A limit for sulphate content and a test for soundness are also includ- ed.
The combustible content of clinker may be reducel by screening through a mesh of about in.- aperture and discarding the fines. which usually contain the greater part of the combustible matter. The coarser material can then be crush- ed and graded as required. Alter- natively, unburnt fuel can be moved by reburning the clinker on sinter-strand with a forced-air draught. The heat generated in this way can be usefully employed, for example for heating water or gener- ating steam.
a
re-
Some clinkers have been found to contain particles of material (usual- ly hard-burnt lime) that expand slowly when wetted. and if these particles are near the surface of the concrete they may cause popping of an applied plaster coating. The risk, if suspected, can be overcome by wetting the clinker and allowing it to stand in a moist state for a few weeks before it is used. Occa- sionally small pieces of iron occur in clinker and these may cause rust- staining of the concrete. Much of the iron can be removed by magnets at the crushing stage.
In England, clinker has been used as an aggregate for more than 60 years and is still widely used. As a waste product, it was formerly cheap and plentiful. In recent times. however, supplies of clinker have been greatly curtailed owing to the ever-increasing use of powdered coal at the electricity generating stations which have always been the principal source of clinker. Because of this increasing scarcity, in many areas clinker is no longer cheap.
Foamed blastfurnace slag
to a
Foamed or expanded blastfurnace slag is produced by allowing the molten slag to come into contact with a limited amount of water; al- ternatively, jets of steam and com pressed air are directed on stream of molten slag. The slag expands into a spongy mass resem- bling pumice, which is crushed and graded to the sizes required for ag- gregate. The density of the materi al ranges from 20 to 55 lb/ft. according to the grading. The re- quirements for foamed slag aggre gate are covered by B.S.877.
In thirty years. foamed slag has become the most widely used light- weight aggregate, next to clinker, in Britain. The output is now about 250,000 tons per annum, approxim ately half of which is used in in situ concrete for the thermal insulation of roofs and the remainder mainly in lightweight concrete blocks. Foamed concrete and has been used in this way abroad and to a limited extent. slag is also suitable for reinforced in Britain.
Expanded clay and shale
When certain clays or shales are heated nearly to softening point. they expand or 'bloat", owing to the gen ération of gases within the material. The cellular structure is retained on cooling, giving a lightweight material suitable as a concrete. For satisfac tory commercial production the raw material should soften at an economi cally low temperature and should
THE HONG KONG & FAR EAST BUILDER — VOLUME 17. NUMBER 5
Because of the corrosive effect of certain impurities on steel. the use of clinker for reinforced concrete is prohibited.