huangyhg

concrete corrossion from aggregates containing sulphides
we are rehabilitating a dam by using unreinforced roller compacted concrete. the problem is that the dam is located
in a very remote area and the local sorce of rockfill (rhyolite and granite)used on the original dam contains some sulphide mineralisation. would this rock be suitable for a source of concrete aggregate.
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soluble salts, particularly chloride and sulphate salts, are commonly found in materials used in concrete. substantial levels of soluble salts in aggregate can cause efflorescence (deposition of salts on the exposed surface), corrosion of reinforcing steel or disintegration of concrete. limiting the salt presence is therefore important to concrete durability.
your country’s codes/guideline for concrete design and construction should specify limits for chlorides and sulphates.
in australian practice, aggregates containing sulphides or sulphate salts in proportions which result in sulphate content of the concrete exceeding 5% (by mass of portland cement) are not to be used.
aggregates that contain certain forms of silicas or carbonates may react with the alkalies prensent in cement (sodium oxide and potassium oxide). the reaction product cracks the concrete or may create pop-outs at the concrete surface. the reaction is more pronounced when the concrete is in a warm, damp environment.
similarly sulphides react with the cements, however, sulphides can desolve the cements paticularly when exposed to prolonged moisture.
the pontential reactivity of an aggregate with alkalies can be determined either by a chemical test (astm c289 or by mortar-bar method (astm c227). the mortar-bar method is the more rigorous test and provides more reliable results but it requires a much longer time to perform.
i have inspected many structures that have suffered from aggregate incompatability and internal and external sulfate attack. internal sulphate attack can reduce concrete to sand and gravel.
another consideration: what is the compressive strength of the aggregate?
aggregates must be strong, durable, and clean. should dust or other particles be present, they may interfere with the bond between the cement paste and the aggregate. the strength of the aggregate has an important effect on the stregth of the concrete, and the aggregates properties greatly affect the concrete's durability.

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