The deleterious substances exist in aggregate are classified in three broad categories:
Substances that intervene in the hydration process of cement
Substances that impart coating which prevents development of a healthy bond strength between hydrated cement paste and aggregate
Contain certain particles which are unsound or weak. unsound aggregate show less durability under weathering.
A fraction of an aggregate or all aggregates can be detrimental to concrete in terms of interfering chemical reaction between cement paste and aggregate. It is worth mentioning that aggregate, in general, is considered as inert material and should be dispersed thoroughly in concrete.
Organic Content in Aggregate
Notice that we are always discussing about fine aggregates. This is due to washability of coarse aggregates which is not almost available in case of fine aggregates.
At first, we will learn about sources of organic content in fine aggregate. Most common fine aggregates is sand which is of two types
We are concerned about natural sand as manufactured sand is a relatively controlled product. We have provided some information about organic sources in glass powder sand in previous post.
The organic particles in fine aggregates are occurred due to decomposition of vegetable residue in sand (or other fine aggregates). In some regions tannic acid is found left by root of pine trees. This acid deposited on fine particles cannot be washed. Other organic acids that can impair concrete properties are:
a. Acetic acid
b. Citric acid
c. Formic acid
d. Humic acid
e. Lactic acid
Such organic matter found in aggregate as organic loam or humus.
Organic content is always suspected as harmful. Some type of organic content even as small amount in fine aggregate can retard or even prevent hardening of concrete and can produce severe impact on concrete reducing its strength.
Aggregate derived from nature may have enough resistance to wear and be strong enough; but having organic substances in aggregate may result serious interference with chemical reactions occur during hydration. Interestingly, some organic impurities may render temporary influence on concrete property. An investigation conducted by National Ready-Mix Concrete Association showed that when sand containing organic substances is used to make complete sample, after 24 hours it gained 53% strength of the concrete sample made with decontaminated sand. The results at 3 days were 82% and at 7days 92% and surprisingly, at 28 days is the effect of organic impurities diminished i.e the strength gained almost same.
How to Examine Organic Content in Aggregate?
All organic substances are not harmful to concrete. So, it is wise decision to ascertain their performance by testing specimens for compression test. Another important thing is that whether the amount of impurity is adequate to warrant more investigations.
ASTMC 40 guidelines are used to test organic impurities. It is basically a colorimetric test. As mentioned earlier, mainly acids exist as impurity in aggregate. These acids are neutralized by solution of NaOH (3%). According to requirement of ASTM C40, defined quantity of aggregate is shaken vigorously with solution so that they are mixed intimately to have a chemical reaction and the organic content exist in the aggregate is judged by the color obtained after 24 hours. The darker the color of the solution, the greater the amount of organic impurities in aggregate. If the color is not darker than the prescribed yellow color in ASTM standard, the sample is assumed to consist of organic content that are harmless for concrete.
If the solution color become brownish or brown i.e. darker than prescribed yellow color, the aggregate contain higher organic content. But this does not warrant aggregate is not suitable for concrete construction.
The substances that are harmful to concrete performance are:
a. Clay lumps and other friable particles
b. Materials that are finer than 75µm (No. 200 sieve)
c. Lignite and coal
d. Soft particles
e. Lightweight chert
Now we will discuss about impacts of each types on concrete properties and performance.
a. Clay lumps and other friable particles:
Clay particles exist in aggregate may coat it and interfere development of bond between cement paste and aggregate. To achieve sufficient strength and durability of concrete, a superior bond is essential. Thus clay coatings around aggregate is a pivotal issue.
Other particles may be in the form of
the size of silts remain in between 2~60 µm. When rocks are deteriorated by the natural process of weathering, silt formed. Thus aggregate derived from natural deposits may contain silts.
These are formed during crushing process of rock to obtain crushed stone. In case of crushing rock to form gravel (for fine aggregate) more crusher dust is formed. In modern well laid processing plant, such fine particles can be removed by proper washing. If loosely bonded or soft coating exist in aggregate can be removed during further processing of aggregate. Some well adherent coating of such materials, can not be removed.
The impacts on concrete are:
1. Upon breaking while mixing increase water requirements
2. Impair wear resistance
3. Reduce durability
4. May result popouts
b. Materials passing No. 200 sieve
These materials can affect concrete by
1. Increasing water demand
2. Effects bond strength (aggregate and cement paste)
3. Affects of air-entrainment
c. Lignite and coal:
These materials may
1. Result detrimental straining on concrete
2. Cause popouts
3. Impair ability to air entrainment
d. Soft particles:
2. less durability
3. less resistance to wear
e. Lightweight chert:
Lightweight means having specific gravity of less than 2.40.