Oxide Composition of Portland Cement

Oxide Composition of Portland Cement

Oxide Composition of Portland CementPortland cement is composed of four major oxides: lime (CaO), silica (SiO), alumina (AlO), and iron (FeO).Also Portland cement contains small amount of magnesia ( MgO ), alkalies (NaO and KO ), and sulfuric anhydrite ( SO ).

Approximate Composition Limits of Oxides in Portland CementOxideCommon NameContent, %CaOLime60-67SiOSilica17-25AlOAlumina3-8FeOIron0.5-6MgOMagnesia0.1-4NaO and KOAlkalies0.2-1.3SOSulfuric anhydride1-3These oxides interact with one another in the kiln at high temperature to form more complex compounds. The identification of the major compounds is largely based on R.H. Bogues work and hence it is called bogues compounds.Bogues CompoundsNameChemical formulaAbbreviationTricalcium silicate3CaO.SiOCSDicalcium silicate2CaO.SiOCSTricalcium aluminate3Cao.AlOCA Tetracalcium aluminoferrite4CaO.AlO.FeOCAFThe identification of the major compounds is largely based on R.H. Bogues work and hence it is called Bogues compounds.Significance of Compound CompositionMass PercentageCompoundCement 1Cement 2Cement 3CS653373CS8382CA14157CF41014Hydration of CementWhen Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden. This chemical reaction with water is called HYDRATION.OPC Hydration

Hydration starts as soon as the cement and water are mixed.The rate of hydration and the heat liberated by the reaction of each compound is different.Each compound produces different products when it hydrates.Tricalcium silicate (CS). Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Portland cements with higher percentages of CS will exhibit higher early strength.Tricalcium aluminate (CA). Hydrates and hardens the quickest. Liberates a large amount of heat almost immediately and contributes somewhat to early strength. Gypsum is added to Portland cement to retard CA hydration. Without gypsum, CA hydration would cause Portland cement to set almost immediatelyafter adding water.Dicalcium silicate (CS). Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.Tetracalcium aluminoferrite (CAF). Hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. Most Portland cement color effects are due to CAF.7Characteristics of Hydration of the Cement CompoundsCompoundsReaction RateAmount of LiberatedStrengthHeat LiberationCSModerateModerateHighHighCSSlowLowLow initially, high laterLowCAFastVery highLowVery highCAFModerateModerateLowModerate

1 DAY OLD MORTAR

Calcium Silicate HydrateC-S-H makes upto 50 to60% of the solids.

C-S-H appear as very small fibrous crystals which fill the space formerly occupied by water and the dissolved cement particles.

Calcium Hydroxide Constitutes 20 -25% of the volume of solids in the hydrated paste.Large prismatic crystals.Little contribution to strength of concrete.Chemically unstable.13

ETTRINGITE (CALCIUMSULFOALUMINATE)15-20% of solids volume.Minor role in structural behavior.Hexagonal plate crystals.

MONOSULFATE HYDRATEHexagonal plate crystalsSusceptible to sulfate attack.

UNHYDRATED CLINKER GRAINSome unhydrated clinker grains may be found in cement paste.

INTERLAYER SPACE IN C-S-HVoids are very small and does not cause any adverse effect on the strength & permeability.

CAPILLARY VOIDSSpace not filled by the solid components of the hcp.It ranges from 10 to 50nm in size in low w/c paste and from 3 to 5micrometer. In high w/c paste.

AIR VOIDSWater present in the voids but free from the influence of attractive forces exerted by solid surface.

ABSORBED WATERWater absorbed onto the surface of the solids in hcp.