PORTLAND CEMENTS

The industrial uses of limestone and cements have provided important undertakings for chemists and engineers since the early years when lime mortars and natural cements were introduced. In modern times one need only mention reinforced – concrete walls and girders, tunnels, dams, and roads to realize the dependence of present – day civilization upon these products. The convenience, cheapness, adaptability, strength, and durability of cement products have been a foundation of these applications. PORTLAND CEMENTS

In spite of the modern concrete roads and building everywhere around us, It is difficult to realize the tremendous growth of the cement industry during the past century. Humans had early discovered certain natural rocks which through simple calcinations, gave a product that hardened on the addition of water. Yet the real advance did not take place until physiochemical studies and chemical engineering laid the basis for the modern efficient plants working under closely controlled conditions with a variety of raw materials. Table 1:Raw Materials Used in Producing Portland Cement

Raw Materials

1 - CalcareousLimestone (includes aragonite, marble, chalk)Cement rock (includes marl)Oyster shell

2 - ArgillaceousClayShaleOther (includes Staurolite, bauxite, aluminum dross, pumpice and volcanic material

3 - SiliceousSand

Sandstone and quartz

4 - FerrousIron ore, pyrites, mill – Scale and other iron – bearing material

5 - OtherGypsum and anhydriteBlast furnces slagFly ash

MANUFACTURING PROCEDURESTwo types of materials are necessary for the production of

Portland cement: one rich in calcium (calcareous), such as limestone, chalk, etc., and one rich in silica (argillaceous) such as clay. Formerly a large amount of cement was made from argillaceous limestone, known as cement rock. In addition to natural materials, some plants use blast – furnace slag and precipitated calcium carbonate obtained as by – product in the alkali and synthetic ammonium sulfate industry. Sand, waste bauxite, and iron ore are sometimes used in small amounts to adjust the composition of the mix. Gypsum (4 to 5%) is added to regulate the setting time of the cement.

These raw materials (Table 1) are finely ground, mixed, and heated (burned) in a rotary kiln to from cement clinke. Table 2 lists the predominant compounds that are formed during calcining. The cement industry uses the following abbreviations for these clinkerCompounds: CaO = C MgO = M CO2 = C

SiO2 = S SO3 = S H2O = HAl2O3 = A Na2O = N

Fe2O3 = F K2O = KThus Ca3SiO5 9S CaO SiO2) = C3S

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Table 2Clinker Compounds

Formula Name Abbreviation

2 CaO.SiO2 Dicalcium silicate C2S

3 CaO.SiO2 Tricalcium silicate C3S

3 CaO.Al2O3 Triclcium aluminate C3A

4 CaO.Al2O3.Fe2O3 Tetracalcium aluminoferrite C4AF

MgO Magnesium oxide in free state M

1 - At 900oC and above the following reactions occurs:CaCO3 CaO + CO2

CaO + Al2O3 CaO.Al2O32 - At 1200oC the main reactions between lime and clay:

4 CaO + 2 Al2O3 + CaO.Al2O3 5 CaO.3 Al2O3

2 CaO + SiO2 2 CaO.SiO2

3 - At 1250 – 1280oC and above the completion of formation of clinker cement compounds. 4 CaO + 5 CaO . 3 Al2O3 3 (3 CaO.AlO23)CaO + 2 CaO.SiO2 3 CaO.SiO2 4 CaO + Al2O3 + Fe2O3 4 CaO.Al2O3.Fe2O3

SETTING AND HARDENING OF CEMENT Although many theories have proposed to explain the setting

and hardening of cement, it is generally agreed that hydration and hydrolysis are involved. 1 - hydration: 3 CaO . Al2O3 . 6 H2O2 - hydrolysis: 3 CaO SiO2 + nH2O

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3 CaO.SiO2 + nH2O 2 CaO.SiO2.2H2O + Ca(OH)2 + (n-3) H2OCalcium hydroxide is responsible of hardening cement.The hydration products have very low solubility in water. If this were not true, concrete in contact with water would be rapidlyi attacked. Much attention has been given to the heat evolved during the hydration of cement. The various compounds contribute to the heat of hardening (basis, equal weights, i.e., gram for gram) after 28 days, as follows:

C3A > C3S > C4AF > C2STable 7 shows why low – heat – of – hardening cements are made low in C3A and C3S but high in C2S. This is accomplished (1) by adding more Fe2O3, which takes the Al2O3 out of circulation as C4AF, thereby diminishing the amount of C3A, and (2) by decreasing the CaO / SiO2 ratio. Notice these facts in the analyses a Table 5. Thus low – heat – of – setting cement is used in the construction of all large dams to avoid cracking the structure from heat stresses during setting and cooling. As an additional safeguard, the structures are cooled during setting by circulating cold water through lightweight 2.5 – cm pipes, placed in the concerte mass. Tables 8 and 9 present further facts regarding the functions of the different compounds

Table 8Strength Contribution of Various Compounds in Portland Cement (The Relative Strengths are the Apparent Relative Contributions of Equal Weights of the Compounds Listed)

1 day C3A > C3S > C4AF > C2S

3 days C3A > C3S > C4AF > C2S

7 days C3A > C3S > C4AF > C2S

28 days C3A > C3S > C4AF = C2S

3 months C2A > C3S = C3AF = C4AF

1 year C2S > C3S = C3A = C4AF

2 years C2S > C3S > C4AF > C3A

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Table 9Function of Compounds

Compound Function

C3A Causes set but needs retardation (by gypsum)

C3S Responsible for early strength (at 7 or 8 days)

C2S and C3S Responsible for final strength (at 1 year)

Fe2O3, Al2O3, Mg, and alkalies Lower clinkering temperature.

The setting and hardening of cement. To hold up the "flash set" caused by C3A, the gypsum added as a retarder causes the formation of C3A . 3 CaSO4 . 3 l H2O. Fig. (3).

(a) (b)

(c) (d) Fig. (3)

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Fig. 3. Four stages in the setting and hardening of Portland cement: Simplified diagrammatic representation of the possible sequence of changes. (a) Dispersion of unreacted clinker grains in water. (b) After a few minutes: hydration products eat into and grow out from the surface of each grain. (c) After a few hours: the coatings of different clinker grains have begun to join up, the get thus becoming continuous (setting). (d) After a few days: further development of the gel has occurred (hardening).

Admixtures (additives to cement for concrete formulations) extend the supply of cement and add other important properties. These usually give specific results from individual additions. One important group is the superplasticizer naphthalene derivatives. Calcium nitrite may be added to inhibit the corrosion of steel reinforcing bars in concrete.

Concrete based on cement is broadening in application and increasing in quantity each year. Low – heat – hardening, quick or retarded hardening, low and high density prestressed concrete beams give greater strength and save steel. The Portland Cement Association, which has offices in many cities, will furnish extensive data on uses.

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