Testing of cement
Testing of cement can be brought under two categories –
- Field testing
- Laboratory testing
Read more about types of cement.
1. Field testing of cement
It is sufficient to subject the cement to field tests when it is used for minor works. The following are the field tests:
- Open the bag and take a good look at the cement. There should not be any visible lumps. The colour of the cement should normally be greenish grey.
- Thrust your hand into the cement bag. It must give you a cool feeling. There should not be any lump inside.
- Take a pinch of cement and feel-between the fingers. It should give a smooth and not a gritty feeling.
- Take a handful of cement and throw it on a bucket full of water, the particles should float for some time before they sink.
- Take about 100 grams of cement and a small quantity of water and make a stiff paste. From the stiff paste, pat a cake with sharp edges. Put it on a glass plate and slowly take it under water in a bucket. See that the shape of the cake is not disturbed while taking it down to the bottom of the bucket. After 24 hours the cake should retain its original shape and at the same time it should also set and attain some strength.
2. Laboratory for testing of cement
- Fineness test
- Setting time test
- Consistency test
- Strength test
- Soundness test
- Heat of hydration test
- Chemical composition test
(i) Fineness Test
The fineness of cement has an important bearing on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat. Finer cement offers a greater surface area for hydration and hence faster the development of strength. The fineness of grinding has increased over the years. But now it is stabilized. This is considered most important among all testing of cement.
Fineness of cement is tested in two ways –
(a) By sieving
(b) By determination of specific surface by air-permeability apparatus
(a) Sieve Test
Weigh correctly 100 grams of cement and take it on a standard IS Sieve No. 9 (90 microns). Break down the air-set lumps in the sample with fingers. Continuously sieve the sample giving circular and vertical motion for a period of 15 minutes. Mechanical sieving devices may also be used. Weigh the residue left on the sieve. This weight shall not exceed 10% for ordinary cement. Sieve test is rarely used.
(b) Air Permeability Method
Testing of cement covers the procedure for determining the fineness of cement as represented by specific surface expressed as total surface area in sq. cm/gm. of cement. It is also expressed in m2/kg.
The apparatus can be used for measuring the specific surface of cement. The principle is based on the relation between the flow of air through the cement bed and the surface area of the particles comprising the cement bed. From this the surface area per unit weight of the body material can be related to the permeability of a bed of a given porosity. The cement bed in the permeability cell is 1 cm, high and 2.5 cm. in diameter.
Specific surface Sw is calculated from the following formula
Knowing the density of cement the weight required to make a cement bed of porosity of 0.475 can be calculated. This quantity of cement is placed in the permeability cell in a standard manner. Slowly pass on air through the cement bed at a constant velocity.
Adjust the rate of air flow until the flow-meter shows a difference in level of 30-50 cm. Read the difference in level (h1) of the manometer and the difference in level (h2) of the flow-meter. Repeat these observations to ensure that steady conditions have been obtained as shown by a constant value of h1/h2.
Fineness can also be measured by Blain Air Permeability apparatus. This method is more commonly employed in India.
(ii) Standard Consistency Test
For finding out initial setting time, final setting time and soundness of cement, and strength a parameter known as standard consistency has to be used. It is pertinent at this stage to describe the procedure of conducting standard consistency test. The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate to a depth of 33-35 mm from the top of the mould. The appartus is called Vicat Appartus. This appartus is used to find out the percentage of water required to produce a cement paste of standard consistency. The standard consistency of the cement paste is some time called normal consistency (CPNC).
The following procedures is adopted to find out standard consistency.
Take about 500 gms of cement and prepare a paste with a weighed quantity of water (say 24 per cent by weight of cement) for the first trial. The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes. After completely filling the mould, shake the mould to expel air. A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of the paste in the test block and quickly released allowing it to sink into the paste by its own weight. Take the reading by noting the depth of penetration of the plunger.
Conduct a 2nd trial (say with 25 per cent of water) and find out the depth of penetration of plunger. Similarly, conduct trials with higher and higher water/cement ratios till such time the plunger penetrates for a depth of 33-35 mm from the top. That particular percentage of water which allows the plunger to penetrate only to a depth of 33-35 mm from the top is known as the percentage of water required to produce a cement paste of standard consistency This percentage is usually denoted as ‘P. The test is reguired to be conducted in a constant temperature (27° ± 2°C) and constant humidity (90%).
(iii) Setting Time Test
An arbitrary division has been made for the setting time of cement as initial setting tine and final setting time. It is difficult to draw a rigid line between these two arbitrary divisions.
For convenience/ initial setting time is regarded as the time elapsed between the moment that the water is added to the cement, to the time that the paste starts losing its plasticity The final setting time is the time elapsed between the moment the water is added to the cement, and the time when the paste has completely lost its plasticity and has attained sufficient-firmness to resist certain definite pressure.
In actual construction dealing with cement paste, mortar or concrete certain time is required for mixing, transporting, placing, compacting and finishing. During this time cement paste, mortar, or concrete should be in plastic condition. The time interval for which the cement products remain in plastic condition is known as the initial setting time.
Normally minimum of 30 minutes is given for mixing and handling operations.
The constituents and fineness of cement is maintained in such a way that the concrete remains in plastic condition for certain minimum time. Once the concrete is placed in the final position, compacted and finished, it should lose its plasticity in the earliest possible time so that it is least vulnerable to damages from external destructive agencies. This time should not be more than 10 hours which is often referred to as final setting time.
The Vicat Apparatus is used for setting time test also. The following procedure is adopted. Take 500 gm. of cement sample and gauge it with 0.85 times the water required to produce cement paste of standard consistency (0.85 P). The paste shall be gauged and filled into the Vicat mould in specified manner within 3-5 minutes. Start the stop watch the moment water is added to the cement. The temperature of water and that of the test room, at the time of gauging shall be within 27°C + 2°C.
(a) Initial Setting Time
Lower the needle (C) gently and bring it in contact with the surface of the test block and quickly release. Allow it to penetrate into the test block. In the beginning, the needle will completely pierce through the test block. But after some time when the paste starts losing its plasticity, the needle may penetrate only to a depth of 33-35 mm from the top. The period elapsing between the time when water is added to the cement and the time at which the needle penetrates the test block to a depth equal to 33-35 mm from the top is taken as initial setting time.
(b) Final Setting Time
Replace the needle (C) of the Vicat apparatus by a circular attachment (F). The cement shall be considered as finally set when, upon, lowering the attachment gently cover the surface of the test block, the centre needle makes an impression, while the circular cutting edge of the attachment fails to do so. In other words the paste has attained such hardness that the centre needle does not pierce through the paste more than 0.5 mm.
(iv) Strength Test
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. Strength tests are not made on neat cement paste because of difficulties during the testing of cement of excessive shrinkage and subsequent cracking of neat cement.
Strength of cement is indirectly found on cement sand mortar in specific proportions. The standard sand is used for finding the strength of cement. It shall conform to IS 650-1991. Take 555 gms of standard sand (Ennore sand), 185 gms of cement (i.e. ratio of cement to sand is (1:3) in a non-porous enamel tray and mix them with a trowel for one minute, then add water of quantity P/4 + 3.0 per cent of combined weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform colour. The time of mixing should not be less than 3 minutes nor more than 4 minutes.
Immediately after mixing, the mortar is filled into a cube mould of size 7.06 cm. The area of the face of the cube will be equal to 50 sq cm. Compact the mortar either by hand compaction in a standard specified manner or on the vibrating equipment (12000 RPM) for 2 minutes.
Moulding of 70.7 mm Mortar Cube Vibrating Machine
Keep the compacted cube in the mould at a temperature of 27°C + 2°C and at least 90 per cent relative humidity for 24 hours. Where the facility of standard temperature and humidity room is not available, therefore cube may be kept under wet gunny bag to simulate 90 per cent relative humidity. After 24 hours the cubes are removed from the mould and immersed in clean fresh water until taken out for testing of cement.
Three cubes are thus tested for compressive strength at the periods. The periods being reckoned from the completion of vibration. The compressive strength shall be the average of the strengths of thus three cubes for each period respectively.
(v) Soundness Test
It is very important 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 difficulties for 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.
The unsoundness in cement is due to the presence of excess of lime than that could be combined with acidic oxide at the kiln. This is also due to inadequate burning or insufficiency in fineness of grinding or thorough mixing of raw materials. It is also likely that too high a proportion of magnesium content or calcium sulphate content may cause unsoundness in cement.
For this reason the magnesia content allowed in cement is limited to 6 per cent, It can be recalled that, to prevent flash set, calcium sulphate is added to the clinker while grinding. The quantity of gypsum added will vary from to 5 per cent depending upon C3A content. If the addition of gypsum is more than that could be combined with C3A, excess of gypsum will remain in the cement in free state. This excess of gypsum leads to an expansion and consequent disruption of the set cement paste.
(vi) Heat of Hydration
The reaction of cement with water is exothermic. The reaction liberates a considerable quantity of heat. This can be easily observed if a cement is gauged with water and placed in a thermos flask. Much attention has been paid to the heat evolved during the hydration of cement in the interior of mass concrete dams. It is estimated that about 120 calories of heat is also generated in the hydration of 1 gm. of cement. From this it can be assessed the total quantum of heat produced in a conservative system such as the interior of a mass concrete dam. A temperature rise of about 50°C has been observed. This unduly high temperature developed at the interior of a concrete dam causes serious expansion of the body of the dam and also with the subsequent cooling considerable shrinkage takes place resulting in serious cracking of concrete.
The use of lean mix, use of pozzolanic cement, artificial cooling of constituent materials and incorporation of pipe system in the body of the dam as the concrete work progresses for circulating cold brine solution through the pipe system 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.
Testing for heat of hydration is essentially required to be carried out for low heat cement only. Testing of cement is thus 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.
(vii) Chemical Composition Test
A fairly detailed discussion has been given earlier regarding the chemical composition of cement. Both oxide composition and compound composition of cement have been discussed. At this stage it is sufficient to give the limits of chemical requirements.
Ratio of percentage of lime to percentage of silica, alumina and iron oxide, when calculated by the formulae
(Cao – 0.7 SO3) : (2.8 SiO2 + 1.2 Al2O3 + 0.65 Fe2O3 : Not greater than 1.02 and not less than 0.66.
The above is called lime saturation factor percent.
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