by
Azif Ali VK , QC -Construction Chemicals , Saudi BASF for Building Materials Co. Ltd.
Hydraulic cement have the ability to set and harden under water. Eg: Portland cements. It sets with hydration reaction. It reacts with water to form stable calcium silicate hydrates. Hydraulic cements can harden instantly too.
Non-hydraulic cement cannot be hardened when exposed to water. Non-hydraulic cement should be kept dry to attain strength, and be able to maintain its structure. It takes a substantially longer time to dry off and gain strength after being set. Example is lime. Here hardening takes place by carbonation- reaction of hydrated lime with carbon dioxide in the atmosphere. other examples are gypsum and magnesium cement.
Cements used in construction can be characterized as being either hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) harden because of hydration, a chemical reaction between the anhydrous cement powder and water. Thus, they can harden underwater or when constantly exposed to wet weather. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water. Non-hydraulic cements do not harden underwater; for example, slaked limes harden by reaction with atmospheric carbon dioxide.[1]
The most important uses of cement are as an ingredient in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material.
Non-hydraulic cement such as slaked limes (calcium hydroxide mixed with water), harden due to the reaction of carbonation in presence of the carbon dioxide naturally present in the air. Calcium oxide is produced by lime calcination at temperatures above825 °C (1,517 °F) for about10 hours at atmospheric pressure:
CaCO_3 \rightarrow CaO + CO_2
The calcium oxide is then spent mixing it to water to make slaked lime:
CaO + H_2O \rightarrow Ca(OH)_2
Once the water in excess from the slaked lime is completely evaporated (this process is technically called setting), the carbonation starts:
Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O
This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is small. The reaction of carbonation requires the air be in contact with the dry cement, hence, for this reason the slaked lime is a non-hydraulic cement and cannot be used under water.
Conversely, the chemistry ruling the action of the hydraulic cement is the hydration. Hydraulic cements (such as the Portland cement) are made of a mixture of silicates and oxides, the four main components being:
Belite (2CaO.SiO2);
Alite (3CaO.SiO2);
Celite (3CaO.Al2O3);
Brownmillerite (4CaO.Al2O3.Fe2O3).
The reactions during the setting of the cement are:
(3 \ CaO \cdot Al_2O_3)_2 + (x+8)H_2O \rightarrow4 \ CaO \cdot Al_2O_3 \cdot xH_2O +2 \ CaO \cdot Al_2O_3 \cdot8 \ H_2O3 \ CaO \cdot Al_2O_3 +12 \ H_2O + Ca(OH)_2 \rightarrow4 \ CaO \cdot Al_2O_3 \cdot13 \ H_2O4 \ CaO \cdot Al_2O_3 \cdot Fe_2O_3 +7 \ H_2O \rightarrow3 \ CaO \cdot Al_2O_3 \cdot6 \ H_2O + CaO \cdot Fe_2O_3 \cdot H_2O
And during the hardening (the chemistry of the reaction of hydration is still not completely clear):
(3 \ CaO \cdot SiO_2)_2 + (x+3)H_2O \rightarrow3 \ CaO2 \cdot SiO_2 \cdot xH_2O +3 \ Ca(OH)_2
(2 \ CaO \cdot SiO_2)_2+ (x+1)H_2O \rightarrow3 \ CaO2 \cdot SiO_2 \cdot xH_2O + Ca(OH)_2
The silicates are responsible of the mechanical properties of the cement, the celite and the browmillerite are essential to allow the formation of the liquid phase during the cooking.
When dry cement is blended with water, certain chemical reactions take place in the mixture. These reactions form chemical compounds which contain water, and the formation of these compounds causes the mixture to harden.The nature of the compounds formed in these reactions, they are insoluble in water. This means that the hardened cement will retain its strength and hardness even if immersed in water. This makes it perfect for brick buildings in wet climates, harbor structures that are in contact with sea water, and numerous other applications.
Non-hydraulic cement is cement which cannot harden while in contact with water.
Non-hydraulic cements are created using materials such as non-hydraulic lime and gypsum plasters and oxychloride, which has liquid properties.
