CEMENT PROPERTIES:

                                    

                       CEMENT-BINDING MATERIAL

Portland cement is manufactured by crushing, milling and proportioning the following raw materials at a temperature of about 1450 degree centrigrade:

Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock. Silica, SiO₂: from sand, old bottles, clay or argillaceous rock, Alumina- Al₂O₃: from bauxite, recycled aluminum, clay. The raw material of clinker has an average  composition of CaO-67%, SiO2-22%, AI2O3-5% , Fe2O3-3%  and other components, like alkali, sulphates and calcium oxide are also present in minor amounts. :3%. These constituents  undergo chemical reactions during burning and fusion, and combine to form some compounds called alite, belite, aluminate phase and ferrite phase. 

The clinker obtained from the kiln is first cooled and then passed on to ball mills where a few per cent of gypsum, a naturally occurring mineral is added and finely ground to make Cement.  The gypsum controls the rate of setting and may be partly replaced by other forms of Calcium Sulphate. 

Role of Gypsum: Gypsum is required to slow down the hydration process. Once cement with less content of gypsum  is mixed with water it becomes hard quickly, thereby, enough 

time for mixing, transporting and placing is not obtained  to achieve a workable concrete. 

Adding gypsum slows down the hydration process, the cement mix remains plastic and workable for a reasonable period. For ordinary portland cement it is 3 to 4 %. For rapid hardening cement gypsum  is reduced to 2%, lower % age of gypsum allows concrete to attain early hardness as per demand  of the situation.

The main components of Ordinary Portland Cement are:

Tricalcium silicate, Ca3SiO3) (C3S), 50 to 70%, this phase(Alite) is responsible for the start of the setting process and early strength. It reacts relatively quickly with water. In normal Portland Cement it is the most important constituent for strength development at ages up to 28 days.

Dicalcium silicate (Ca2SiO3, C2S),this phase (Belite)constitutes 15-30% of normal Portland cement clinkers.  It reacts slowly with water, thus contributing little to the strength during the first 28 days. but it adds strength substantially  at later ages. By one year, the strengths obtainable from pure alite and pure belite are about the same under comparable conditions.

Tricalcium aluminate Ca3 AI2 06(C3A),This phase(Aluminate) constitutes 5-10% of most normal Portland cement clinkers. It reacts rapidly with water with generation of great amount of heat and can cause undesirably rapid setting unless a set-controlling agent, usually gypsum, is added. 

Tetracalcium alumino ferrite Ca2 AI FeO5 (C4AF), This phase(Ferrite) makes up 5 - 15% of normal Portland cement clinkers. The rate at which it reacts, in general,  is high initially and intermediate between those C3S and C2S at later ages.  Cement’s grey colour is due to the presence of iron in this compound.

 Calcium sulphate (CS) is added up to 5 % after the clinker has been obtained in order to delay the fast setting of the aluminates with the water.

Minimum water cement ratio for complete hydration; 

1. Volume of 1 bag of cement: 1.25 cft=0.0347cum.

2. Weight of 01 bag cement : 50kg

3. No. of bags in 1cum cement: 28.8 nos

4. Specific gravity of cement: 3.15

5.Bulk density of cement: 1440 Kg/Cum

8. Grades of OPC cement: 33, 43 and 53 grades

9. What does 43 grade cement or other  mean: 43 indicates 28 days compressive strength of cement mortar cube is  43 Mpa or N/mm2. 

Cement mortar cube is prepared with cement and ennore sand in proportion 1:3, size of mortar cube 70.6mm x 70.6mm x70.6mm.

Some Properties of OPC-43 and OPC-53 grade:

10. Initial Setting Time of OPC Cement: Minm. 30 min

11. Final setting time Cement: Maxm.10 hours

13. IS code for 43 grade cement: IS-8112.

14. IS code for 53 grade:  IS 12269.

15. IS code for different  test procedures: IS 4031 (Part 1. to Part-8)

16. Specific surface area of both = 225 m2/Kg minm, For Railway sleeper this value is 370 m2/kg

17. 672 hours Compressive strength of 43 grade: 43 N/mm2 Minm.

18. 672 hours Compressive strength of 53 grade: 53 N/mm2 Minm. 

Some Properties of Rapid Hardening Cement:

19. Initial Setting Time : Minm. 30 min

20. Final setting time : Maxm.10 hours

21. IS code for rapid hardening cement:  IS 8041.

22. IS code for different  test procedures of cement: IS 4031 (Part 1. to Part-8)

23. Specific surface area = 325 m2/Kg minm. 

24. 24 hours Compressive strength: 16 N/mm2 Minm.

25. 72 hours Compressive strength: 27 N/mm2 Minm. 

Some Important features:

1. Total chloride content in cement shall not exceed 0.05 percent by mass of cement used in prestressed concrete structures. 

2. Strength of Sand-cement mortar 1:4: 6 N/mm2 (As per NBCC Part 6 structural Design, Section 4, Cl-3.2.2.Strength of Sand-cement mortar 1:6: 3 N/mm2.

Silica Fume: 

Conforming to IS 15388 is used as part replacement of Cement usually 5 to 10% of cement Content in a Mix: Cl-5.2.1.2, IS-456-2000

FLY ASH: Fly ash is a waste residue produced after burning coal in a thermal power station. The term fly ash shall cover all types of coal ash such as pond ash, bottom ash and mound ash. It is a pozzolanic material that in the presence of water and calcium hydroxide produces cementitious compounds. Fly Ash is a byproduct of pulverized coal burnt in the the furnace for immediate combustion in thermal power plant. After the burning of coal the ash , a part being  heavier would fall down but another part being lighter would fly out and collected named as fly ash. Fly Ash is used in Ready-mix concrete, Cement manufacture, Mineral filler for asphalt roads, Soil stabilization, Structural fill, Waste stabilization/treatment etc. Fly ash and lime combination could reduce CO2 emission and it requires less energy to produce cement leads to green concrete. Fly ash is a filler in hot mix asphalt applications and improves the fluidity of flowable fill and grout because of its spherical shape and particle size distribution.

used as part replacement of ordinary Portland cement: Cl-5.2.1.1, IS-456-2000. Fly ash vary considerably with many constituents, but all fly ash includes substantial amounts of silicon dioxide (SiO_{2) ,  silica is a major component of glass, also in sand}, aluminium oxide (Al₂O_{3, Alumina is nothing but fine ceramic material}) and calcium oxide (CaO), Fly ash concrete increases the resistance to sulfate attack. The smaller pore size of fly ash concrete reduces the volume of ettringnite that may be formed. Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O. It is a colorless to yellow mineral crystallizing in the trigonal system. 

WHAT IS ETTRINGITE IN CONCRETE: Gypsum and other sulphate compounds available in cement or cementitious material/admixtures etc react with calcium aluminate present  in the cement to form ettringite within the first few hours after mixing with water. Most of the sulphate present  in the cement is normally consumed to form ettringite at early ages. The formation of ettringite in the fresh, plastic concrete is the mechanism that controls stiffening. At this stage ettringite is uniformly and discretely dispersed throughout the concrete. Ettringite formed at early ages is often referred to as “primary ettringite.” It is a necessary and beneficial component of portland cement systems.

What is Pozzolona?:

A pozzolan is a siliceous material which contain Sio2 parent material is nothing but sand formed out of different rocks or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties. Natural pozzolana (pozzolanic ash), silica fume (from silicon smelting), fly ash, and rice husk ash are examples of pozzolans. Silica fume (microsilica) is an amorphous form of silicon dioxide (silica).

Alumina is Aluminum oxide, the most well-known fine ceramic material for chemical and physical stability. clay is a mineral substance made up of small crystals of silica and alumina, that is ductile when moist; the material of pre-fired ceramics.

Ash is the inorganic residue remaining after the water and organic matter have been removed by heating in the presence of oxidizing agents, which provides a measure of the total amount of minerals within a food.

Hydration of cement:  is a set of reaction takes place when cement comes in contact with water. compounds of cemenl like 3CaOSio2 (C3S), 2CaOSio2 (C2S) reacting chemically with water, produces a gel like substance generally written in short form as C-S-H, Calcium Silicate Hydrate which is responsible for bonding. In fact C-S-H is a salt compound

For hydration of cement, another important reaction takes place with C3A or 3CaO AL2O3 when reacting with water, compound produced sets the cement paste very quickly which is undesirable as such concrete is not practicable to transport, place and compact. Gypsum (CaSO4,2H2O) is another compound added in cement making process to control the setting properties. C3A reacting with Gypsum independently produces a compound of Monosulphate termed as Ettringite which is useful for delaying the setting process to make it usable.

Due to hydartion reaction 98% of its strength is achieved in 28 days and still hydration continue to take place for a very long years ahead, however rate of increase in strength is very slow thereafter.

Fineness of Cement: 

The fineness of cement is a measure of cement particle size and is denoted in terms of the specific surface area of cement, having unit cm2/gm or m2/kg meaning summation of surface area of all particles containing in uit weight of cement. The Fineness Test of Cement is done by sieving cement sample through standard IS sieve of 90 microns. The weight of cement particle whose size is greater than 90 microns i.e. retained on 90 micron sieve is determined and the percentage of retained cement particle are calculated. This way fineness of cement is measured. For OPC cement fineness value shall be less than 10%.

Features of Fineness of cement:

1. Finer cement has greater surface area coming in contact with water leading to increased hydration process and faster strength development. 

2.   Increase in fineness of cement is also found to increase the drying shrinkage of concrete.

3.  In the laboratory fineness of cement is tested by sieving  and measured as the % weight retained on a 90µm IS sieve over the total weight of the sample and  also by the determination of a specific surface area.

4. Cement containing too much smaller particle results in quick setting, hardly any time for mixing, placing of concrete even can set in air also.

5. The specific surface of OPC cement should not be less than 2250 cm2/gm.

PROCEDURE OF FINENESS TEST OF CEMENT:

1. Collect a sample of cement and rub with your hands. The Fineness test sample should be free of lumps.

2. Taken 100 gm of cement sample and noted  as W1

3. The cement is kept in 90 µm sieve on a tray and close it with the lid.

4. Now, shaken the sieve by agitating the sieve in planetary and linear movements for 15 minutes.

5. After that taken weight of cement  that retained on 90 µm sieve noted as W2

6. Now calculate the fineness of cement from the formula, 100*W2 / W1

Ground Granulated Blast Furnace Slag (GGBS): 

Blast Furnace slag is a non-metallic by-product produced in the process of the making iron  in the Blast Furnace. The liquid slag consists primarily of the impurities of the iron ore as well as ash from coke and coal (mainly silica and alumina) combined with calcium and magnesium oxides from the flux. It consists primarily of silicates, alumino-silicates, and calcium-alumina-silicates. 

Granulated Blast Furnace  Slag is produced by quickly quenching (chilling) the liquid slag to produce a glassy, granular product. When this granular product crushed or milled to very fine cement-sized particles, it is known as the Ground Granulated Blast Furnace Slag. 

Cement is used as a binder material in conventional concrete involving heat of hydration .which leads to shrinkage of concrete so, that we replace GGBS as a binder material in concrete. Improves workability and reduce cement utilization, so cost will economical and reduce environment used pollution of industry waste [6]. The paper give the compressive strength by the replacement of cement by fly ash Material

CHEMICAL COMPOSITION OF GGBS, FLY ASH AND OPC

   COMPOUND       GGBS           FLY ASH        OPC      NATURAL  POZOLONA

    SIO2  %             4.27               59.3              21                   46.4          

    Al2O3 %            14.33              23.4               6                    17.5

    Fe2O3 %            1.11                4.8                4                     9.69

    CaO    %            37.02               8.6               63                    9.9

    MgO    %            8.41                0.6               2.5                  2.42

    SO3    %             0.85                0.1              1.5                    0.83

        

    K2O    %            1.28                  -                0.5                  1.51

    Na2O   %            1.32                0.32             0.5                   3.3

    LOI     %             1.41                  -                  -                    5.34

  

    TiO2    %               -                     -                 -                     2.1                         

 – Benefits

1.  As a cementitious component of concrete  with replacement of cement.

2. The slag cement use results in several advantageous concrete                              properties. 

3. Slag cements have a low heat of hydration.

 

4. Concrete made with  slag cement has the low capillary porosity                            ensuring high durability to the structure.

5. It is resistant to chloride penetration, sulphate  attack.

 

6. Protection against alkali silica reaction, a low risk of thermal cracking, a                high electrolytic resistance.

7. Further there is a better workability and an easier finishability.

  

8. Mixtures of Portland cement and  ground granulated slag gives high                      levels of durability. 

9. Excellent bond is provided with either hydraulic cements or bituminous                  binder materials.

10. Concrete incorporating ground granulated slag generally develop                       strength more slowly than concretes which contain only Portland                         cement but can have better long-term strength.

 

11. Release less heat during hydration, have reduced permeability, and                    normally show better resistance to chemical attack. 

12. Granulated slag brings down the cost of cement. 

13. In some countries, up to 80 % of the cement contains Granulated Blast 

      Furnace Slag.