Essay On Hydration

Introduction

In the hydration reaction of Portland cement, the absolute volume of hydration products formed is less than the combined volume of the reacted cement and water. This volume reduction is called the chemical shrinkage. After setting of the cement paste, microstructure of the paste is developed as a result that the paste is not able to change its bulk volume freely; therefore, only a fraction of total chemical shrinkage results in a decrease of its external volume which is known as autogenous shrinkage. While, the rest of total shrinkage presents as an increase of the internal porosity.

Another kind of bulk shrinkage of cement is called drying shrinkage. Drying shrinkage is caused due to the loss of moisture from cement paste to environment. To prevent these shrinkages, moist curing may be used. But, upon subsequent exposure to an atmosphere of normal humidity, the material slowly loses water and exhibits a shrinkage finally. In addition, during hydration reaction is going on without water curing, the paste undergoes shrinkage, chemical shrinkage combine with drying shrinkage create, which generate tensile stresses. These tensile stresses may cause cracks in the hardened paste. However, the cracking of the material may be avoided by using expansive additive.

In the absence of calcium hydroxide (〖Ca(OH)〗_2), hydrated produces are ettringite and an amorphous alumina gel (AH_3) as follows:

C_4 A_3 S ̅+2CS ̅H_x+(38-2x)H→C_6 AS ̅_3 H_32+2AH_3 with x = 0 for anhydrite, x = 0.5 for hemihydrate or plaster and x = 2 for gypsum.

When calcium sulfate is fully depleted in the system that absence of calcium hydroxide, the formation of monosulfate (C_4 AS ̅H_12) by tetracalcium trialuminate sulfate hydration becomes the dominant reaction according to the following