HOW TO PERFORM CONSISTENCY TEST OF CEMENT ?

Consistency test is used to estimate the quantity of mixing water to form a paste of normal consistency. The normal consistency defined as that percentage water requirement of the cement paste, the viscosity of which will be such that the Vicat’s plunger penetrates up to a point 5 to 7 mm from the bottom of the Vicat’s mould.

FACTOR AFFECTING FINENESS OF CEMENT 

1. The chemical composition and the degree of calcination influence the hardness of the clinker and consequently the fineness to which the cement is ground. 

2. Clinker, high in iron or silica, is apt to be hard and difficult to grind. The same is true with a hard burned clinker. 

3. Fineness is also influenced by the time of grinding and the character of the pulverizing machinery. It has been found that cement becomes finer with age provided it does not absorb too much moisture. This is probably due to the decrepitation of the coarser grains resulting from the hydration of the embedded lime particles.

TEST IMPORTANCE

Finer the cement, more is the strength since surface area for hydration will be large. With increase in fineness, the early development of strength is enhanced but the ultimate strength is not affected. An increase in the fineness of the cement increases the cohesiveness of the concrete mix and thus reduces the amount of water which separates to the top of a lift (bleeding), particularly while compacting with vibrators. However, if the cement is ground beyond a certain limit, its cementative properties are affected due to the prehydration by atmospheric moisture. Finer cement reacts more strongly in alkali reactive aggregate. Also, the water requirement and workability will be more leading to higher drying shrinkage and cracking.

TEST PROCEDURE

Objective

To determine the water requirement of the cement paste with normal consistency.

Apparatus

• Vicat’s Apparatus

Sample/Specimen

To makes a viscous paste as a sample, 300 g of cement is mixed with 25 per cent water.

Procedure

• The paste is filled in the mould of Vicat’s apparatus and the surface of the filled paste is smoothened and leveled.

• A square needle 10 mm x 10 mm attached to the plunger is then lowered gently over the cement paste surface and is released quickly. The plunger pierces the cement paste. 
• The reading on the attached scale is recorded. 

• When the reading is 5-7 mm from the bottom of the mould, the amount of water added is considered to be the correct percentage of water for normal consistency.

Result

The normal consistency of cement paste is achieved as a result.