Introduction
Experience has shown that it is practically impossible to make large quantities of cement without any variation in quality. To be sure, some mills working with raw materials which run very uniformly and using the best of equipment and methods of operation will have very few unsuccessful 'burns' in a year, whereas others will be less fortunate. Nevertheless the consumer has little chance of ascertaining how his particular consignment of cement was made; therefore, if he has under way a construction of any importance, he ought to satisfy himself regarding the quality of his purchase. He should test his cement not only to see that he gets what he has paid for but also to forestall the possibility of a failure through the use of defective material. In engineering construction the main qualifications demanded of a cement are permanency of structure, strength, and a rate of setting suitable to the demands of the work. To determine these qualifications, both physical and chemical tests are made, the former, on account of importance, more often than the latter.
As a result of long experience the physical tests which have come into general use in determining the acceptability of cement are:
(1) soundness or constancy of volume,
(2) strength,
(3) time of set or activity, and
(4) fineness. In order that the results of such tests made by different parties may accord as nearly as possible, it is necessary that a standard method be rigidly adhered to and that only experienced operators, who fully appreciate the necessity of eliminating personal equation from all manipulations, be employed.
Physical test (IS. :4031)
1. Fineness test
The degree of fineness of cement is the measure of the mean size of the grains in it. There are three methods for testing fineness: the sieve method—using 90 micron (9 No.) sieve, the air permeability method— Nurse and Blains method and the sedimentation method— Wagner turbidimeter method. The last two methods measure the surface area, whereas the first measures grain size. Since cement grains are finer than 90 micron, the sieve analysis method does not represent true mean size of cement grains. Also, the tiny cement grains tend to conglomerate into lumps resulting in distortion in the final grain size distribution curves. Considering these demerits, fineness is generally expressed in terms of specific area, which is the total surface area of the particles in unit weight of material.
Condition of affecting fineness.
The chemical composition and the degree of calcination influence the hardness of the clinker and consequently the fineness to which the cement is ground. Clinker, high in iron or silica, is apt to be hard and difficult to grind.
The same is true with a hard-burned clinker. 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.
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.
Air permeability
The fineness of cement is represented by specific surface, i.e. total surface area in cm2 per gram or m2 per kilogram of cement and is measured by Lea and Nurse apparatus or by wagner turbidimeter.
The Lea and Nurse apparatus shown in Fig. 5.8 essentially consists of a permeability test cell—where cement is placed and air pressure is applied, flowmeter—to determine the quantity of air passing per second through its capillary tube per unit difference of pressure, and manometer—to measure the air pressure.
The rate of flow is so adjusted that the flowmeter shows a pressure difference (h2) of 30-50 cm.
The reading (h1) in the manometer is recorded. The process is repeated till the ratio h1/h2 is constant. The specific surface is given by the expression
This is a test to estimate the quantity of mixing water to form a paste of 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.
Importance.
Importance.
The water requirement for various tests of cement depends on the normal consistency of the cement, which itself depends upon the compound composition and fineness of the cement.
Test procedure.
300 g of cement is mixed with 25 per cent water. The paste is filled in the mould of Vicat’s apparatus (Fig. 5.9) and the surface of the filled paste is smoothened and levelled. 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.
Determination of initial and final setting time .
When water is added to cement, the resulting paste starts to stiffen and gain strength and lose the consistency simultaneously.
The term setting implies solidification of the plastic cement
paste. Initial and final setting times may be regarded as the two stiffening states of the cement.
The beginning of solidification, called the initial set, marks the point in time when the paste has become unworkable.
The time taken to solidify completely marks the final set, which should not be too long in order to resume construction activity within a reasonable time after the placement of concrete. The initial setting time may be defined as the time taken by the paste to stiffen to such an extent that the Vicat’s needle is not permitted to move down through the paste to within 5 ± 0.5 mm measured from the bottom of the mould. The final setting time is the time after which the paste becomes so hard that the angular attachment to the needle, under standard weight, fails to leave any mark on the hardened concrete. Initial and final setting times are the rheological properties of cement.
Importance.
It is important to know the initial setting time, because of loss of useful properties of cement if the cement mortar or concrete is placed in moulds after this time.
The importance of final setting time lies in the fact that the moulds can be removed after this time. The former defines the limit of handling and the latter defines the beginning of development of machanical strength.
Condition affecting setting time.
The factors influencing the setting properties of cement are its composition, the percentage of retardant, degree of calcination, fineness of grinding, aeration subsequent to grinding clinker, percentage of water used to make cement paste, the temperature of the mixing water, cement and the atmosphere where the cement paste is placed, and the amount of manipulation the paste receives.
The effect of gypsum is to increase the setting time of freshly ground cement. It is usually mixed with the clinker before final grinding, or just after the clinker has received preliminary grinding. The addition of gypsum before calcination causes it to decompose into lime and sulphur trioxide. Since the latter is liberated in the kiln, there is resulting effect on the setting time. Often, an underlimed cement becomes quick setting after seasoning.
This can be avoided by adding to the cement 1 or 2 per cent of hydrated lime or the fraction of a per cent of Plaster of Paris. Setting time of cement is rapid with the increase in the fineness of cement. When the mixing water used in testing cement paste is increased by 1 per cent above that required for normal consistency, an increase of about 30 minutes or more is observed in the initial or final set.
Cements stored in warm rooms will, in general, be quick setting than those stored in cold places. Cold mixing water retards set while warm water accelerates it. Cement exposed to thoroughly saturated atmosphere will set much more slowly than those exposed to a dry atmosphere. If, however, a considerable proportion of moist CO2 is present in the air, the setting time is found to reduce greatly. By lengthening the time of mixing and by prolonged troweling of the surface mortars, it is also possible to considerably delay the setting time.
Test procedure
A neat cement paste is prepared by gauging cement with 0.85 times the water required to give a paste of standard consistency.
The stop watch is started at the instant water is added to the cement. The mould resting on a nonporous plate is filled completely with cement paste and the surface of filled paste is levelled smooth with the top of the mould.
The test is conducted at room temperature of 27± 2°C. The mould with the cement paste is placed in the Vicat’s apparatus and the needle is lowered gently in contact with the test block and is then quickly released. The needle thus penetrates the test block and the reading on the Vicat’s apparatus graduated scale is recorded.
The procedure is repeated until the needle fails to pierce the block by about 5 mm measured from the bottom of the mould. The stop watch is pushed off and the time is recorded which gives the initial setting time. The cement is considered to be finally set when upon applying the needle gently to the surface of test block, the needle makes an impression, but the attachment fails to do so.
0 comments:
Post a Comment