If Cement satisfies all the field tests then it indicates that Cement is not bad for Construction but it also doesn’t show that Cement is really good for Mass work. For using Cement in big and massive works it is dependent on the part of the user to test the cement in the laboratory to confirm the requirements of the Indian Standard specifications respect to its physical and chemical properties.
All the Tests on Cement mentioned here have their own importance. These Cement Tests examines all properties and overall performance of Cement.
Cement is most important ingredient in any kind of construction in any aspect. So it’s very important to carryout certain test on cement to confirm the requirements of Indian Standards with respect to physical and chemical properties as we discussed earlier in this article.
Without any doubt, such tests have already done on cements at laboratory level but cement may get damaged in transportation and storage. These damages can be results of exposure to air, exposure to water and other conditions.
Also, another purpose of testing cement might be the determination of air content of the mix. Air entrainment is used to increase the workability and strength of concrete.
Following Tests are generally conducted in Laboratory :
The fineness of cement plays an important role in the rate of hydration and hence it also has an effect on the rate of gain of strength of Cement. Finer the Cement greater the surface area for hydration and faster development of strength.
More fineness will reduce the cementitious property of cement also increases the probability of drying shrinkage. So different types of Cements are grounded to different fineness as per their requirements. An increase in the fineness of cement reduces the amount of water required for getting standard consistency paste.
Most of the particles in a cement sample should be of size fewer than 100 microns. Also the size of the smallest particle should not be less than 1.5 microns. The average size of a cement particle can be taken as 10 microns.
Particle size 3 microns is predominant in strength development of day 1 and particle size 3-25 microns is predominant in strength development of 28 days strength. In commercial cement, 25-30% percent of particles should be less than 7 microns.
Fineness test of cement is done by
Finding out the factors like Initial setting time, Final setting time, and soundness of cement a parameter named Standard Consistency is used. Standard Consistency means the Amount of water required to make Cement Paste Plastic. Standard consistency of cement is also called as Normal Consistency.
Standard Consistency is defined as consistency which permits Vicat Plunger to penetrate 33-35 mm from top of the mould. The plunger is having a 10mm diameter and 50 mm in length. The instrument used to calculate Standard Consistency is the Vicat apparatus.
During Construction using concrete, cement paste, mortar certain time is required for mixing, transportation, placing, compacting and finishing. During this time concrete, mortar, cement paste should be in plastic condition and should not harden and it is mandatory that one should use the concrete before it starts to loose its plasticity. So How should someone know how long is concrete workable? and How much travel time is permitted for concrete?
Well its Setting time that tells us how long concrete is going to be workable. Now the question is What is Setting Time of Cement?
Time elapsed between water added and time at which paste jut starts loosing its Plasticity is called as Initial Setting Time. Normally for OPC Initial Setting Time is 30 minutes but should not be more than 60 minutes.
Time elapsed between water added and time at which cement paste completely loose its Plasticity is called as Final Setting Time. Normally for OPC Final Setting Time is 10 hrs and not more than that.
The compressive strength of hardened cement is the most important of all the properties. Therefore, it is not surprising that the cement is always tested for its strength at the laboratory before the cement is used in important works.
Why the compressive strength test of Cement is important? Compressive Strength Test on Cement mortar is calculated to find out whether the Cement conforms IS specifications or not and whether it can develop required strength or not when used in Concrete.
Any kind of strength testings are not done directly on Cement itself because of properties like shrinkage and cracking. So a Compressive strength test on Cement either done in the form of mortar or concrete.
It is necessary that Cement after hardening should not undergo any kind of change in volume. Certain cements are found to undergo large expansion on Hardening. This may cause serious problems in the durability of the structure. Testing of Cement for Soundness is done to ensure that the Cement does not show any appreciable subsequent expansion.
Unsoundness is due to the presence of an excess of Lime, Magnesia, Sulphates, and gypsum. Expansion is also due to Inadequate Burning, Insufficiency in Fineness, Insufficient Mixing.
Soundness Test is done by two methods :
The Le Chatelier test detects unsoundness due to free lime only. This method of testing does not indicate the presence and after effect of the excess of magnesia.
Indian Standard Specification stipulates that a cement having a magnesia content of more than 3 per cent shall be tested for soundness by Autoclave test which is sensitive to both free magnesia and free lime.
As you know the reaction of cement with water is an exothermic reaction, which liberates a considerable amount of heat. It can create problems when the heat is being evolved in mass concrete constructions. It is estimated that 120 calories of heat is generated on the hydration of 1 gram of cement.
For example, consider a big concrete dam if we assume 120 calories of heat for 1 gram of concrete then it can be estimated that rise in temperature due to hydration can go up to 50°C resulting in the expansion of dam on the other hand cooling due to water shrinkage occurs at the same time. Ultimately cracks start appearing due to simultaneous expansion and contraction.
The use of the lean mix, use of pozzolanic cement, artificial cooling of constituent materials and incorporation of the pipe system in the body of the dam as the concrete work progresses to absorb the heat, are some of the methods adopted to offset the heat generation in the body of dams due to heat of hydration of cement.
Test for the heat of hydration is essentially required to be carried out for low heat cement only. This test is carried out over a few days by Vaccum flask methods, or over a longer period in an adiabatic calorimeter. When tested in a standard manner the heat of hydration of low heat Portland cement shall not be more than 65 cal/gm. at 7 days and 75 cal/g, at 28 days.