General importance of carrying out test on cement
Cement is one of the most widely used materials in the built environment. It is a binding agent that sets and hardens to adhere to building units such as stones, bricks, tiles etc. Cement is used to make concrete as well as mortar, and to secure the infrastructure by binding the building blocks. Chemical properties, compressive strength of mortar prisms, fineness, soundness and setting time are important factors in testing cement. In Sri Lanka cements with SLS mark are commonly used. However, there could be batch to batch variation in properties of cement which should be checked and controlled. The chemical properties of the cement affect the ultimate performance in all of its applications. Many of the performance characteristics could be assessed by means of physical tests. Tests on cement is generally important to predict strength of concrete and mortar and sustainability of structures and finally leading to economical mix design.
Significance for testing each element property
According to the SLS 107 standard following mechanical, physical and chemical requirements should be mainly checked.
• Compressive strength
The compressive strength of hardened cement is the most important of all the properties. Therefore, cement is always tested for its compressive strength at the laboratory using standard sand and distilled water before it is used in important works. Strength of cement is determined from cement – sand mortar prisms made by mixing cement with standard sand in given proportion as specified in the standards. If the mortar is weak then also its compressive strength is very low but if the mortar is a strong one then its compressive strength is also very high.
• Setting time
Setting time is the time required for stiffening of cement paste to a defined consistency. It means the Initial setting time is the time when the paste starts losing its plasticity.
Tests for setting time of cement is important to determine the free time available to transport, place and compact it. In mixing, setting time should be enough to mix that cement with other constituents. It also should be enough to place concrete easily without effecting its workability. When considering compacting concrete should be in liquid state enough to be compacted easily by vibrators leaving no void or honeycombing and that is according to the setting time.
• Soundness
The soundness is a very important factor to ensure that the cement after setting shall not undergo any appreciable change of volume. Certain cements have been found to undergo a large expansion after setting causing disruption of the set and hardened mass. This will cause serious problems regarding the durability of structures when such cement is used. The testing of soundness of cement, to ensure that the cement does not show any appreciable subsequent expansion is of prime importance.
• Fineness
The fineness is also a very important physical property of cement. Rate of hydration of cement depends on its fineness which is related to particle size. Because of that fineness of cement is also responsible for the rate of gain of strength and also the rate of evolution of heat. Under the less fineness, cement particles will not absorb water properly and it will cause to decrease strength of mixture.
• Chemical Composition
The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another in the kiln at high temperature to form more complex compounds. The relative proportions of these oxide compositions are responsible for influencing the various properties of cement; in addition to rate of cooling and fineness of grinding. Hence tests on chemical compositions are important.
** Deviations between the testing procedures carried out and the specified standard procedure in SLS 107**
When comparing to SLS 107 Standard, there can be seen some variations in practical results that we obtained.
Reasons that generally affected to these deviations can be:
• Measuring instruments such as stopwatches can be less accurate
• Errors can be occurred when taking readings and observations
• Errors during calculations and conversions
• Natural changes in practical environment
Reasons for unsoundness of the cement:
• Cement can be consisted high percentages of elements such as MgO and CaO
• Non-uniformity of mixing
• Insufficient fineness of grinding
Reasons affected for compressive strength value:
• Environment condition is not exactly same as SLS 107 testing environment
• Problems of compressive strength machine
• The mixture can be not mixed well.
4Conformity of the tested cement sample to the requirements specified in SLS 107
Comprehensive Strength
SLS Standard: Not be less than 20.0 N/mm2 at 3 days
Initial setting time
SLS Standard: Initial setting time not less than 45 min
Soundness
SLS Standard: shall not have an expansion of more than 10 mm
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