Portland cements are commonly characterized by their physical properties for quality control purposes.Their physical properties can be used to classify and compare Portland cements.The challenge in physical property characterization is to develop physical tests that can satisfactorily characteristic key parameters.
Physical properties of Cement:
- Fineness or particle size of Portland cement effects hydration rate & thus the rate of strength gain.
- The smaller the particle size the greater the area to volume ratio and thus the more area is available for water cement interaction/unit volume.
- The effect of greater fineness on strength is generally seen during the first 7 days.
- when cement particle are coarser hydration starts on the surface of particle, so the coarser particle may not be completely hydrated.this causes low strength and low durability.
- for rapid development of strength a high fineness is necessary.
Test for fineness:
- There are various methods for determining the fineness of cement particles.
- the Blaine's air permeability method is one of the most commonly used method.
- The given volume of air is passed through the prepared sample of definite density. the number and size of pores in a sample of given density is a function of a particle and a size distribution.
- Rate of air flow through the sample is determined and calculations are made and fineness is calculated.
- It is the ability of concrete to not to shrink after hardening
- this can be tested using Le Chatelier test.
- This is usually caused by lime and magnesia content.
- cement must always regain its volume without showing any expansion upon hardening
Test for soundness:
Le Chatelier test:
- cement paste of normal consistency is taking between glass slides and merged in water for 24 hours.
- its taken out and distance between indicator is measured.
- returned to water and bought to boil for an hour.
- the distance between indicator is measured again.
- this method tests the expansion of cement due to lime.
- this expansion measurement after boiling must not exceed 10 mm.
- Strength can be effected by a number of factors like W/C ratio , C/FA ratio , type & grading , Fine aggregate , curing condition , size & shape , loading conditions & age.
- Duration of testing -
- 1 day for high early strength cement
- 3 , 7, 28, 90 days for monitoring strength progress.
- 28 days strength is recognized as a basis for control in most codes.
- when considering the cement paste strength test- 2 things to be considered are
- Cement mortar strength is not proportional to concrete strength.
- Strength test are done on cement mortar(cement+water+sand) not on cement paste.
Compression strength of mortar:
- Compressive strength of Portland cement is determined by BIS method.
- The cement paste can be of any combination
- ex: 1 part of cement + 3 part of sand +water can also be the combination for mortar
- the above combination is poured in 7 cm molds and the specimen is water cured for ages of testing.
- the mortar specimen are taken out of the molds are subjected to compression determines the strength.
- Finding the breaking load of mold. strength is calculated by relation Load/Area ( area= 50 m2 for 7cm mold)
- the average of the results found by testing 6-specimens is the compression strength of mortar cubes.
- Cement paste setting time is effected by cement fineness, W/C ratio , chemical content(gypsum) and admixture.
- setting time is used to characterize how a particular cement paste sets for construction purpose.
- the initial set must not be too soon and final set must not be too late
Initial set: Occur when paste begin to stiffen.
Final set : occur when cement has hardened to the point at which it can sustain some load.
False set: occurs due to the conversion of unhydrous/semihydrous gypsum to hydrous gypsum.
Flash set : it happens in the absence of gypsum.
its usually used for under water repair.