What are the Naturally Occurred Pozzolanic Materials for Concrete?

We have already learned about pozzolana reaction in concrete. These materials may be natural and artificial. The artificial pozzolana that is well known is fly ash. We have published many posts about fly ash and will publish several posts regarding fly ash in this blog. Here our concern is natural pozzolana.

At first pozzolana meant to volcanic ash. This volcanic ash was used by Romans in Pozzuli. Pozzuli was a city of Naples province in Italy region. The original volcanic ash is pumicite.

Other than pumicite

-opanline

-shales

-santorin earth

-cherts

-burnt clay

-diatomaceous earth in calcined form

ASTM C618 defined above materials as class N. With these, rice husk can also be used as pozzolana for concrete. We have discussed about the application, properties and limitation of these in the previous post. There have pozzolanic cements which contains certain content of pozzolana which needs not to add separate pozzolanic materials. We will discuss this later.

These natural pozzolanas always have more or less some problems and these encourage to find and deriving artificial pozzolanas. We have mentioned that calcined diatomaceous earth as natural pozzolana. As an example we can include this pozzolana-

The physical properties of such pozzolanas results problems like these materials are angular and are also porous which requires more water to have optimum workability.

To avoid these problems some improvement techniques we applied. The most common is calcinations of them between (500-1100)0C; the temperature depends on the matter to be improved. Dear reader we are finishing this topic here. In the next post we will discuss calcined kaolinite clay to be treated an pozzolanic materials for concrete.

Application of Rice Husk for Pozzolanic Concrete

Dear reader we have discussed about pozzolanic reaction in the last post. Now we know that pozzolanas are siliceous or siliceous materials with aluminous content having some values to alter concrete properties. Here we will discuss about a siliceous material that is usually ignored in concrete production.

Dear reader we all know that rice husk is a waste product. This waste product is enriched with silica in very high percentage which can be used in concrete with some limitations. We will learn about properties and limitations in the next paragraph.

The rice husk is fired slowly and reached to (500-700)0C to produce an amorphous form of it which have porous structure. This ash derived from rice husk has very complex shape like origin of the plant. The specific surface of this ash is very high which can be reached even 50000 m2/kg; but has large particle size of between 10µm and 75µm.

These properties discussed above seek a greater water requirement when used in concrete. To have necessary workability and high strength, superplasticizers are used. But application of superplasticizers, reduce economy derived from using rice husk where processed rice husk is not easily available in any territory.

This ash is found effective in producing concrete strength at only (1-3) days. But there use in concrete may lead to increment in shrinkage.

Dear reader we are finishing this topic here, in the next post we will discuss about another amorphous natural materials having pozzolanic values, can be used in concrete advantageously.

Most Common Artificial Pozzolana for Concrete

In the last few post we have discussed about natural pozzolana that can be used in concrete to enhance it properties. We have discussed about volacanic ash, different siliceous clay, rice husk etc as natural pozzolana for concrete. Here, we are introducing an artificial pozzolanic material that is very common in concrete production. It is sometimes used in production of high performance concrete. We have published many posts regarding high performance concrete in this blog and other blogs of same authors. Follow the link to learn more.

Dear reader this is fly ash, also termed as pulverized fuel ash. The fly ash is derived from power station which uses coal as fuel. This is an ash that is precipitated mechanically or electrostatically form exhausted gases of such power station.

 The particle shape and size of flyash is very suitable for pozzolanic purposes. The particles are of spherical shape which is very important in regards of water requirement. The size is also very fine; diameter of majority particles lies between 100 µm to even less than 1 µm.

The specific surface of flyash particles are between 250 m2/kg and 600m2/kg when bline method is used in deriving them. The calcium hydroxides formed by the hydration of cement paste can react with flyash more effectively due to specific surface of such higher degree.

Dear reader we have discussed about the classification of flyash in our previous post. In the next post we will discuss about each classification elaborately.

What is Pozzolanic Reaction in Concrete?

Dear reader, we have used the term pozzolanic reaction in concrete in our many posts in our blog and many sites are using this term in discussing pozzolana related concrete properties like strength, durability etc. In this post we will learn about the pozzolanic reaction. Pozzolans are used in concrete to derive special properties, enhancing normal properties of concrete.

The pozzolana may be natural or artificial materials that are treated as cementitious materials in concrete remaining in latent form. These materials contain silica in the form of high reactive state. We are proving information here from ASTM 618.

The pozzolna is a siliceous or sometimes aluminous content may exist in it. The materials often have even no cementitious properties; if have, is in very little possession. Now why should we classified it as cementitious materials; why not filler materials in concrete? The answer is-these materials are always in finely divided state and when found moisture it reacts chemically with the calcium hydroxide at normal temperature and form compounds that have cementitious properties. Dear reader we will know the source of calcium hydroxide; we are escaping this formation. For detail understanding reaction that happened in concrete please follow the link below:

Pozzolanic Chemistry in Cement

We have emphasized about the condition “finely divided” as only in this state silica can react with calcium hydroxide formed by the hydration of cement. Another condition is moisture, that is, water. When these two condition satisfy calcium silicate, a stable form, is formed which have efficient cementitious properties, expected for pozzolanic concrete.

Regarding reactivity, the silica must be in amorphous form, as it is observed that crystalline silica shows lower reactivity, more precisely very low reactivity. Dear reader we are finishing this post here; in the next post we will discuss about natural pozzolana and next few posts will be about many aspects of pozzolana.