Impurities in cement are not completely avoidable; while some of them can be ignored, compounds like magnesium oxide are extremely dangerous for the integrity of the structure. Cement companies try to have minimum possible magnesium content in the product, to improve the strength and durability of the building.
Dalmia Bharat Cement is one of the leading cement manufacturers in India and has mastered the art of producing low magnesia cement products. CV Suvasan, Sr. General Manager from the Tech. Services & Product Development Dept. of Dalmia Bharat Cement, shares insights with readers of Indian Cement Review on why and how to restrict magnesia content. Excerpts from the interview.
All hydraulic cements have the cementatious compounds alite, belite, celite and ferrite, commonly called as phase components, in them.
These phase components constitute major proportion of OPC (85 to 92 per cent ) and they impart cementaitious properties to it. However, cement contains some minor unwanted constituents, too. Some of these adversely impact the quality and one such deleterious constituent in cement is magnesium oxide, present in the form of periclase. Magnesium oxide (magnesia) content in cement should be as low as possible.
How does magnesia enter the product?
This unwanted component normally finds its way into cement through limestone, which is the principal raw material in the cement manufacture. During pyro-processing of the raw meal (mixture of pulverised raw materials in the desirable proportion) and the air quenching of the resultant clinker, magnesia molecules exhibit a tendency to cling together and form macro-crystals known as periclase. This happens especially if the cooling of the clinker is insufficient.
What are the adverse effects of periclase?
Periclase has the unique property of remaining dormant in concrete for a long time, while other components get hydrated. Periclase crystals do not react with water easily in the normal hydration process, but gets activated after about five years or so and undergoes hydration forming expansive magnesium hydroxide. Magnesium hydroxide occupies more volume and puts stress on the hardened concrete. This leads to cracks in the concrete structure. As time goes by, these cracks grow deeper, causing leakages in or irreparable damage to the structure. Magnesia is one of the major culprits behind late expansion cracks in concrete.
Does it affect the compressive strength?
Yes, it does. Its presence in the raw material hinders the proportionate formation of useful phase components and consequently, it reduces the potential compressive strength of the concrete.
What is the safe limit set for magnesia content?
As there is a great threat to the safety of construction with high magnesia cement, the Bureau of Indian Standards has restricted the magnesia content to a maximum of 6 per cent in cement. The BIS limit of 6 per cent is justified only on the consideration that the technology involved in India is most modern. It assumes that plants will have high efficiency coolers to quench the clinker rapidly, and freeze the magnesia minerals into microcrystalline form or disperse them. However, cement plants that do not have these facilities or are operating at above the rated capacity, carry the risk of periclase formation. Periclase formation can occur if magnesia content is higher than 2 per cent. Besides this, operational inadequacy or shortage of cooling fans could lead to periclase crystals formation. This can occur even at lower levels of 3 to 5 per cent magnesia. Regrettably, the 6 per cent limit is not always safe for concrete. We can conclude that magnesia content below 2 per cent could help avoid the risk of periclase formation, even if the cooling is inadequate at times.
Tell us about magnesia content in your products.
The magnesia content in all Dalmia cement products marketed across the southern states of India is hardly 1.2 per cent. Limestone reserves judiciously acquired by Dalmia in the 1930s contain a negligible amount of magnesia. So the cement products manufactured from these raw materials are completely safe and carry no risk of late concrete expansion crack development. Moreover, the pyro-processing facilities and coolers at Dalmia are most modern, ruling out any possibilities of periclase macro crystals formation. This certainly is an important USP of Dalmia products and one that separates our offerings from competing products.
What is your take on the BIS' move restricting the magnesia content to a maximum of six per cent?
We accept BIS move on restriction of magnesia to max of 6 per cent in OPC to avoid failure of concrete structures because of expansion due to high magnesia.
What are the adverse impacts of high per cent of magnesia in cement on concrete?
Excess quantity of magnesia affects the Soundness of Cement. Free uncombined magnesia trapped inside the crystals of the silicates, hamper this as these compounds expand considerably on hydration and their hydration itself commences after a considerable time lag with respect to that of the other compounds.
How successfully have you addressed this issue?
Effect of high magnesia can be minimise by fast cooling clinker to avoid formation of magnesia crystals and keep it in glassy form. All the grades of cement offered by KCP contain less than 6 per cent of magnesia due to usage of low magnesia raw materials.
Does high content of magnesia have any impact on the compressive strength of concrete?
Yes, Due to change in geometric volume of concrete due to expansion, concrete loss its strength.
Tell us whether the use of low content magnesia has any contributory role in the durability of a concrete structure?
Yes. Due to formation of expansion cracks in concrete due to volume expansion of magnesia, the durability of concrete will be very poor.
At higher temperature of clinker from cooler discharge the MgO remains in periclase form, resulting in expansive cement. On the other hand if proper quenching at cooler occurs the MgO changes to glass phase with no expansion. So MgO remains in periclase it will affect the concrete volume stability. Entire VC Limestone deposit is of very low MgO, all the grades of cement offered by Vasavadatta Cement contain less than one percent of magnesia. The use of low content magnesia always contributes for durability of concrete. - CK Jain, Unit Head, Vasavadatta Cement