Aggregate, which makes up 70 per cent of the concrete volume, is one of the main constituent materials of concrete. Thanks to relentless construction activity, naturally occurring materials like aggregate and sand are depleting very fast. Due to the high cost of natural sand used as a fine aggregate, and the rising emphasis on sustainable construction, there is a need for the construction industry to search for alternative materials. Slag, one of the most common industrial wastes, is a by-product of steel production, and can put to such use.
One tonne of steel implies the production of 150-180 kg of slag, depending on the composition of the steel and on the steel production process. Slag often appears as granulated material containing large clusters, with coarse and very fine particles. Serious environmental problems formerly originated from uncontrolled sand and gravel taken from rivers.
The strength of concrete assumes a greater significance because the strength is related to the structure of hardened cement paste and gives an overall picture of the quality of concrete. The strength of concrete at a specific age under given curing conditions is assumed to depend mainly on the water-cement ratio and degree of compaction.
The density of concrete is lower when the natural aggregate is fully replaced by blast furnace slag aggregate. This is beneficial when lower self weight of the final product is needed.
Experiments have been conducted in laboratories to produce M30 and M20 grade concrete with good degree of quality control and mild exposure. Encouraging results in the properties of concrete were observed during partial replacement of coarse aggregate and fine aggregate by steel slag. However, it will always be a commercial decision whether to use slag in place of aggregate, because if natural materials are cheaply available, then it is not worth to try out expensive substitutes. The properties tested as a part of experimentations were compressive strength, tensile strength, flexural strength and durability.
Here were some of the observations reported:
Various experiments suggest that optimum percentage of replacement for fine aggregate is 40 per cent, and for coarse aggregate it is 30 per cent, beyond which the compressive strength decreases on further replacement.
Workability of concrete decreases as the percentage of replacement increases. Fine aggregate replacement shows better workability compared to coarse aggregate replacement.
Concrete with slag showed a much more refined pore structure than ordinary Portland cement concrete. The changes in pore structure were reflected in strength and shrinkage values.
It was also reported that the partial replacement of fine aggregate by steel slag improves the compressive tensile and flexural strength of concrete. Improvement in strength property was slightly lower for coarse aggregate (CA) replacement when compared with fine aggregate (FA) replacement. Compressive strength increased by a larger magnitude when both CA and FA were replaced by steel slag. But the flexural strength slightly decreased for combined replacement.
The weight loss in cubes after immersion in acids was very low. So, the concrete with partial replacement of CA and FA by steel slag showed better resistance to hydrochloric acid than sulphuric acid. The penetration of chloride ions in rapid chloride penetrability is low for both conventional concrete and 40 per cent fine aggregate replacement concrete. On the other hand, 30 per cent coarse aggregate replacement came under moderate penetrability.
It has also been observed that 40 per cent FA replacement performs better than 30 per cent CA replacement. Further researches can be carried out to improve the strength and acid resistance by the addition of some admixtures.