Deleterious materials in aggregate are those substances which detrimentally effect the fresh and hardened properties of concrete for instance strength, workability, and long-term performance of the concrete in which such are used. Deleterious materials and highly undesirable constituents.
Organic impurities, clay, silt and crushed dust, salts, unsound particles, and alkali aggregate reactions. Adverse effects of deleterious materials on concrete includes the increase of water demand in concrete, impair bond strength between cement and aggregate, reduce durability, result in concrete popouts, and impair wear resistance.
There are tests such as colorimetric test recommended by ASTM C 40-92 which are used to determine aggregate organic content. The colorimetric test does not show the adverse effect of deleterious materials in aggregate. This is because high aggregate deleterious substance content does not infer that the aggregate is not fit for utilization that is why strength test based on ASTM C 87-90 is recommended for mortars with questionable sand.
1. Organic Impurities
- Organic impurities interfere with the hydration reaction.
- Frequently, it is found in sand and consists of products of decay of vegetable matter.
- Organic matter may be removed from sand by washing.
- Colorimetric test recommended by ASTM C 40-92 can be used to determine aggregate organic content.
- The colorimetric test does not show the adverse effect of the organic impurity since high organic content does not necessarily mean that the aggregate is not fit for use in concrete.
- For this reason, strength test on mortar with questionable sand as per ASTM C 87-90 is recommended.
- This strength has to be compared with the strength of mortar with washed sand
- Clay may coat the surface of aggregates which impair bond strength between aggregate and cement paste. Consequently, it adversely affecting the strength and durability of concrete
- it is necessary to control the amount of clay in aggregate
- Since no test is available to determine separately the clay content, the limits of fine materials are prescribed in terms of the percentage of material passing sieve No. 200.
3. Silt and crusher dust
- Silt and dust, owing to their fineness, increase the surface area and therefore increase the amount of water necessary to wet all the particles in the mix.
- Impair wear resistance
- Reduce durability
- They may result popouts
- It is necessary to control the amount of silt and fine dust in aggregate.
- Since no test is available to determine separately the silt and dust, the limits of fine materials are prescribed in terms of the percentage of material passing sieve No. 200
- Salts are present in certain types of aggregates such as Sand from seashore, sand and Coarse aggregate dredged from the sea or a river estuary, and desert sand.
- Salts coming through aggregates cause reinforcement corrosion and also absorb moisture from the air and cause efflorescence.
- The BS 882:1992 limits on the chloride ion content of aggregate by mass, expressed as a percentage of the mass of total aggregate.
5. Unsound Particles
- Two major classes of unsound particles are materials fail to maintain their integrity, and substances lead to disruptive expansion on freezing or even on exposure to water.
- Shale, particles with low density, clay lumps, wood, coal, mica, gypsum, and iron pyrites are examples of unsound particles.
- Unsound particles if present in large quantities (over 2 to 5% of the mass of the aggregate) may adversely affect the strength of concrete.
- These materials should not be allowed in concrete which is exposed to abrasion.
- Mica is very effective in reducing strength (15% reduction in 28-d f’c with 5% mica).
- Gypsum and iron pyrites are mainly responsible for expansion of concrete
6. Alkali- Aggregate Reactions
- Reaction between alkali from cement and silica or carbonate from aggregate is called “alkali- aggregate reaction”.
- The most common reaction is that between the active silica constituents of the aggregate and that alkalis in cement, called as “alkali-silica reaction”
- Another type of the alkali-aggregate reaction is that between dolomitic limestone aggregates, containing carbonate, and alkalis in cement, called as “alkali-carbonate reaction”.
- Both types of the reactions cause deterioration of concrete, mainly cracking.
- The reactive forms of silica opal (amorphous, i.e. shapeless), Chalcedony (cryptocrystalline fibrous), and tridymite (crystalline).
- The gel formation on the surface of aggregate particles destroys the bond between the aggregate and cement paste.
- The swelling nature of the gel exerts internal pressure and eventually lead to expansion, cracking and disruption of the hydrated cement paste.
- In case of alkali-carbonate reaction also, gel is formed, which upon swelling cause expansion of concrete.
- Gel is formed around the active aggregate particles, causing cracking within rims and leads to a network of cracks and loss of bond between the aggregate and the cement paste.