Physical and Chemical characteristics of Cement

 Physical Characteristics of Cement 

Following are the physical characteristics of Cement:  

• Fineness 
• Setting time 
• Initial Setting Time 
• Final Setting Time 
• Soundness 
• Compressive Strength 

Fineness 

For normal people Cement is just a normal material, which acts as a binding property of mixing its sand and stones. But in Engineering terms Cement has many properties for its work of action. Fineness is one of the important property for its hydration so that it gains its strength and also evaluate the rate of heat. Finer the cement, offers a greater surface area for hydration and also the development strength is faster. 

Different types of cement are ground to different fineness. The disadvantages of fine grinding is that it is susceptible to air set and early deterioration. The particle size fraction below 3 microns has been found to have the predominant effect on the strength at one day, while 325 microns fraction has a major influence on the 7 days and 28 days strength. Increase in fineness of cement is also found to increase the drying shrinkage of concrete. In commercial cement it is suggested that there should be about 25-30 per cent of particles of less than 7 microns in size. 

Fineness is normally explained in Blaine (m²/kg). It means that if 1 kg of cement is taken, the sum total of the surface area of all its particles will be so many m². In Indian cements, normally it varies between 225 to 325 m²/kg, except for cement in concrete sleepers (53-S/43- S). 

Setting Time 

The term setting time is used to describe the stiffening of the cement paste. In order to allow sufficient time for applying the mortar or placing the concrete, the cement must not set too quickly. It is equally important that after mixing and placing, setting should be complete within a reasonable period. Accordingly, two terms initial & final - setting time are used to describe setting time. 

Initial Setting Time 

This is the time in which cement paste remains in plastic condition and can be moulded into any shape. Therefore, mixing transportation, placing and compaction shall be completed within initial setting time. The setting time of cement decreases with increase in temperature. Normally, initial and commercial setting time is kept 100-140 minutes against the minimum IS requirement of 30 minutes.  

Final Setting Time 

This is the duration after which the cement paste lose its plasticity and becomes rigid. It is during this time that the peak temperature of the paste is reached. The cement paste/concrete starts hardening after the lapse of the final setting time. The final setting is normally kept between 200-300 minutes against the requirement of 600 minutes as per IS code. 

Soundness 

Soundness refers to volume of hardened cement paste. It is necessary that a cement paste, once set, should not undergo any volume changes. The expansion may take place due to delayed or slow hydration, due to the presence of free lime, magnesia or calcium sulphate of the cement. Lime (CaO) externally added to cement does not produce unsoundness because it hydrates rapidly. Whereas free lime hydrates slowly and causes unsoundness. In good quality cement, free lime should not exceed 1 - 1.5 percent.   

Compressive Strength 

Compressive strength is the most important property of cement. Depending upon its class and type, a cement should attain the compressive strength stipulated in respective cement standard at 3, 7 and 28 days. Owing to the differences between the test methods, cement standards of different countries are not entirely comparable. In general a lower water-cement ratio and a higher proportion of cement will result in higher strength. 

There are several forms of strength test: direct tension, direct compression and flexure. As per IS direct compression method is used and cement-sand mortars are tested for 3, 7 and 28 days, as per standard testing procedure stipulated in IS specifications. 

Chemical Characteristics of Cement 

The various chemical characteristics specified in a test certificate and which influence the quality of cement are: 

• Lime Saturation Factor (LSF)  
• Insoluble residue 
• Magnesia (MgO) 
• Sulphuric Anhydride (SO) 
• Loss of Ignition 
• Alkalis 
• Chlorides 

Lime Saturation Factor (LSF) 

LSF is an indicator of the dgree of fixation of lime in cement, which in turn denotes the level of reactivity of the product. LSF is the ratio of the percentage of lime to the sum of percentage of silica and iron oxide. LSF should not be greater than 1.02 and shall not be less than 0.66. If this factor is higher towards 1.0, it means that the cement has higher tricalcium silicate (C3S). This will not only make the cement to gain early strength but also liberate higher heat of hydration. On the other hand, if the factor is less than 0.70, the cement has higher dicalcium silicate (C2S), this will make the cement to gain strength slowly. If LSF is 1.02, it indicates 100% C3A in cement and if it is 0.66, it indicates 100% C2S in cement.  

Insoluble Residue 

All the four compound of cement (C3S, C2S, C3A and C4AF), as well as MgO and CaSO4 are soluble in hydrochloric acid. But free silica (quartz), impurities in gypsum or some types of adulterants are insoluble. In India cement standards normally restrict the the maximum limit of these insoluble residue at 5 percent of OPC to check any possibility of adulteration. In case of PPC, the IR value will depend upon the content of fly ash or calcined clay added in the cement. Therefore, any value of insoluble residue higher than the prescribed values in the standard, indicates adulteration in cement. 

Magnesia (MgO) 

Excess MgO present in the form of crystalline mineral phase periclase, hydrates very slowly and the product of hydration expands when concrete is partially or fully set. Therefore most cement codes limits the amount of MgO in cement generally about 6%. 

Sulphuric Anhydride (SO3) 

Gypsum is added to clinker at the time of grinding to retard the flash setting effect of C3A. It is, therefore, termed as "set retarder" and serves to provide the required workability to cement paste before its application. The quantum of gypsum added depends on the amount of C3A present in clinker and the duration of retardation desired after mixing with water. The gypsum added to the cement is present in the form of sulphuric anhydride. Therefore, Indian Standard prescribed upper limit of SO3 in cement to 3.5%.   

Loss of Ignition (LOI) 

Loss of ignition of cement is determined by heating a cement sample to 900-1000°C, until a constant weight is obtained. Normally a high loss on ignition is an indication of pre-hydration and carbonation, which may be caused by prolonged and improper storage or adulteration during transportation. Most cement codes specifies a 4 to 5 percent upper limit for loss of ignition. 

Alkalis 

Alkalis in cement come from raw materials. The alkalies in cement are found in the form of sodium oxide (Na2O) and potassium oxide (K2O). These are expressed Na2O equivalent and BIS has laid down a limit of 0.6 percent alkalis (Na2O) for low alkalis OPC. A higher percentage of alkali in cement may cause alkali aggregate reaction in concrete, if reactive silica is present in the aggregate being used. The limits of alkalis in blended cements like PPC/PSC is different than OPC and normally it is specified 0.9 (Na2O equivalent)    

Chlorides 

Chlorides in cement again owe to raw material used in its manufacture. Excess presence of chlorides results in corrosion of reinforcement in RCC. As per IS-456 2000 code, the total acid-soluble chloride content in concrete to 0.6kg/m³ for RCC and 0.4 kg/m³ for pre-stressed concrete.