Cement - PDF

# Cement ## by Aouatef Bouali ## INTRODUCTION - Definition: "Cement is a crystalline compound of calcium silicates and other calcium compounds having hydraulic properties". - The product is obtained by pulverizing clinker consisting essentially of hydraulic calcium aluminates, silicates of varying...

# Cement ## by Aouatef Bouali ## INTRODUCTION - Definition: "Cement is a crystalline compound of calcium silicates and other calcium compounds having hydraulic properties". - The product is obtained by pulverizing clinker consisting essentially of hydraulic calcium aluminates, silicates of varying composition, which hydrates and becomes hard like stone in contact with water or moist air. ## HISTORY - Lime and clay have been used as cementing material on constructions through many centuries. - Romans are commonly given the credit for the development of hydraulic cement, the most significant incorporation of the Roman's was the use of pozzolan-lime cement by mixing volcanic ash from the Mt. Vesuvius with lime. - Best know surviving example is the Pantheon in Rome (128 AD) - In 1824 Joseph Aspdin from England invented the Portland cement. The set product of cement, sand and water has got similar color and strength to that of a natural stone obtained at Portland of England. ## RAW MATERIALS OF CEMENT ### Calcareous Materials - Limestone, cement rock, marine shells are examples of calcareous materials. - Should contain less than 3.3% MgO. - Limestone containing high percentage of MgO and the percentage is reduced to less than 5%. ### Argillaceous Materials - Rich in silica, supply Silica (SiO2), Alumina (Al2O3), Iron Oxide (Fe2O3). - Clay, shell, blast furnace slag, ash, cement rock etc. ## CEMENT MANUFACTURING - CaCO3 = CaO+CO2 (at 700°-1000° Celsius) - 2 CaO + SiO2 = 2CaO SiO2 (C2S) (at 1300°-1500° Celsius) - 3 CaO + SiO2 = 3CaOSIO₂ (C3S) (") - 3 CaO + Al2O3 = 3CaOAI2O3 (C3A) () - 4 CaO + Al2O3 + Fe2O3 = 4 CaO Al2O3 Fe2O3 (CAF) - These materials are combined together to form hard, grayish pellets, which is known as cement clinker. - The lime saturation factor (CaO/2.8 SiO2 +1.2 Al2O3+0.65 Fe2O3) should be in the range of 0.66 to 1.2. This will ensure the formation of C2S,C3S and C3A which are responsible for giving strength of concrete. - Silica modulus (SiO2 / Al2O3 + Fe2O3) should be within 2.2 to 3.5 - MgO should be below the specific limit which ensures the cement is sound. ## COMPOSITION OF CEMENT | SI | Name of Compound | Oxide Composition | Abbreviation |%| Function | |---|---|---|---|---|---| | 1 | Tri Calcium Silicate | 3CaO.SiO2| C3S | 45-55 %| Responsible for early and later strength| | 2 | Di Calcium Silicate | 2CaO. SiO2 | C2S | 20-30 % | Responsible for later strength | | 3 | Tri Calcium Aluminate | 3CaO.Al2O3 | C3A | 6-10 % | Increases rate of hydration of C3S. It gives flash set in absence of Gypsum | | 4 | Tetra Calcium Aluminoferrite | 4CaO.Al2O3Fe3O3 | CAF | 15-20% | Hydrates rapidly but contribution to strength is uncertain and very low | - Other components of cement are 1. Gypsum 2. Free Lime 3. Chlorides 4. Alkalis 5. Magnesium ## COMPOSITION OF CEMENT - Tri Calcium Silicate & Di Calcium Silicate 1. Tri Calcium Silicate hydrates and hardens rapidly and responsible for early strength and initial set. 2. Higher percentage of C3S exhibits higher strength. 3. When C3S increases it decreases C2S. ## INCREASING TRI CALCIUM SILICATE & DECREASING DI CALCIUM SILICATE - Heat of Hydration Increases: - _2Ca3SiO5 +7H2O=3CaO.2SiO2.4H2O +3Ca(OH)2 + 173.6 kj_ - Tricalcium Silicate - Calcium Silicate hydrate (Non Cementous compound) - (Cementous glue) - _2Ca2SiO4 +7H2O =3CaO.2SiO2.4H2O +3Ca(OH)2 + 58.6 kj_ - Dicalcium Silicate - Calcium Silicate hydrate - (Cementous glue) - (Non Cementous compound) ## TRI CALCIUM ALUMINATE (C3A) - Tri Calcium Aluminate hydrates and hardens the quickest. Liberates a large amount of heat and contributes somewhat to early strength. - Gypsum is added to retard the hydration of C3A. - In absence of Gypsum Portland cement sets immediately after adding water ## INCREASING C3A & DECREASING CAF - Bleeding decreases - Chloride permeability decreases - Heat of hydration increases - Setting time decreases - Strength increases - Sulfate resistivity decreases - Water requirement increases - Workability decreases ## TRI CALCIUM ALUMINOFERRITE (C4AF) - It hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperature in Portland Cement Manufacturing. - The most Portland cement color effects are due to C4AF. - The higher the CAF more darker the color of cement. ## GYPSUM - Gypsum (CaSO4:2H2O) is a setting time retarder, withouf it cement will sef immediately. - To prevent Sulfate expansion in cement the Sulfate (SO3) content in cement is limits to 3-3.5%. That's why Gypsum is kept less than 5% in cement. - The higher the Gypsum content the longer is setting time of cement. - Excess Gypsum leads to the formation of Calcium Sulfoaluminate, (Ettringite) and it results in a volume increase in the concrete, resulting expansion and cracks. ## SULFATE ATTACK - An illustration of Sulfate attack on concrete ## FREE LIME (CALCIUM OXIDE) - Lime (CaO) which is not combined with other constituents in clinker is known as free lime. - Free lime is harmful for cement, in presence of higher free lime causes expansion in cement and drop early strength. - Free lime normally generated in burning process. - Higher free lime causes cement to change of its color to brown. - Higher lime cement is not a good binder. ## CHLORIDE - Higher chloride content leads to corrosion of steel. - The origin of chloride is mainly from cement raw material. - An illustration of chloride corrosion in concrete. ## ALKALIS - Higher presence of alkalis leads to internal expansions and cracking in concrete due to reaction between silica and alkali. - The origin of alkali is mainly from cement raw material and fuels. - An illustration of Alkali damage in concrete. ## EFFECT OF ALKALIS - Air content increases - Bleeding decreases - Heat of hydration increases - Reactivity of SCM increases - Alkali Silica reaction increases - Changes in setting time - Shrinkage decreases - Early Strength up, late strength downs - Water requirement increases - Workability decreases ## INSOLUBLE RESIDUE - A material which is not soluble in acid and alkali is known as insoluble residue. - Insoluble residue is non cementing material which is present in Portland cement. - It decreases the early strength of cement. - The limit of insoluble residue is 0.75 percent of cement. - It comes from limestone, fly ash, volcanic clay etc. - A diagram illustrating insoluble residue in cement. ## PHYSICAL PROPERTIES OF CEMENT - Fineness 1. The fineness of cement effects the hydration rate of cement and thus the rate of strength gain. 2. Greater fineness increases surface area so that increases hydration rate, heat of hydration and greater early strength. 3. The effect on strength can be seen in 7 days. 4. The coarser cement will result in higher ultimate strength and lower early strength. 5. Coarse cement results in creating porosity in concrete thus reduces durability. 6. Fineness can be measured by - Sieve analysis, turbid meter, Blain Air Permeability apparatus - Effect of Fineness 1. Increases water requirement 2. Increases Strength 3. Shrinkages Increases 4. Setting time decreases 5. Heat of hydration increases 6. Bleeding decreases 7. Air Content decreases 8. Workability Decreases - A diagram illustrating the Effect of Fineness - Setting Time - Initial Set - Occurs when the paste begins to stiff considerably. Should be greater than 45 minutes. - Final Set - Occurs when cement is hardened enough to sustain some load. Should be less than 420 minutes. - Setting Time affects by 1. Fineness (Setting time Decreases with increasing of fineness) 2. Water Cement Ratio (Setting time increases when water cement ration increases) 3. Gypsum content increases setting time 4. Admixtures - Strength - ASTM Standard for Compressive Strength of Cement 1. For 3 Days: 1890 psi 2. For 7 Days: 2900 psi 3. For 28 Days: 3620 psi - Strength - It depends on various factors 1. Water-Cement ratio 2. Cement-Fine aggregate ratio 3. Type and grading of aggregate 4. Manner of mixing 5. Curing conditions 6. Moisture Content 7. Fineness of Cement - Specific Gravity 1. It is used in mixture proportion calculating. 2. The specific gravity of cement is normally 3.15 - Heat of Hydration 1. Increases when water cement ratio, fineness and curing temperature increases. 2. Higher heat of hydration causes considerable loss of strength and regression at later ages. - A diagram illustrating Heat of Hydration ## TYPE OF CEMENTS - Factors affecting choice of cement: - Durability Characteristics - Functional requirement - Deflection, crack width etc. - Design parameters- Strength, fineness, setting time requirement etc. - Speed of construction- Time for construction etc. - Environnemental Conditions- Ground conditions, Soluble Salt, Sulfate, Chemical plants etc. - 1. Ordinary Portland Cement (OPC) - 2. Portland Pozzolana Cement (PPC) - 3. Portland Blast Furnace Slag Cement (PBSF) - 4. Rapid Hardening Cement - 5. Low Heat Cement - 6. Sulfate Resisting Cement - 7. Super Sulfated Cement - 8. White Cement - A table summarizing the different types of cement and their physical characteristics ## SUITABILITY OF DIFFERENT CEMENTS - 33 grade OPC Cement - It is used for normal grade of concrete up to M-20, plastering, flooring, grouting of cable ducts etc. The fineness should be between 225 and 280 m²/kg. - 43 grade OPC Cement - It is the most widely used general purpose cement. - For concrete grades up to M-30, precast elements. - For marine structures but C3A should be between 5 - 8%. - 53 grade OPC Cement - For concrete grade higher than M-30, PSC works, bridge, roads, multistoried buildings etc. - For use in cold weather concreting. - For marine structures but C3A should be between 5-8%. - Portland Pozzolana Cement: (using fly ash): - It gives low heat of hydration and reduces the leaching of calcium hydroxide. This cement should be used only after proper evaluation. - It is used for : - Hydraulic structures- dams, retaining walls - Marine structures - Mass concrete works- like bridge footings - Under aggressive conditions - Masonry mortar and plastering. - SRC- Sulfate Resisting Cement: (C3A < 5%) - Sulfate Resisting Portland Cement is a type of Portland Cement in which the amount of tri calcium aluminate (C3A) is restricted to lower than 5% and 2C3A + C4AF lower than 25%. - The percentage of C4AF is comparatively higher in this cement. - The SRC can be used for structural concrete wherever OPC or PPC or Slag Cement are usable under normal conditions. - SRC- Sulfate Resisting Cement: (C3A < 5%) - The use of SRC is recommended for following applications: - Foundations, piles - Basement and underground structures - Sewage and water treatment plants - Chemical factories - Suitable for underground works where Sulfate is present in the Soil and water. - Attention: - Sulfate resisting cement is not suitable where there is danger of chloride attack. This will cause corrosion of rebar. - If both Chlorides and Sulfates are present, Ordinary Portland Cement with C3A between 5& 8 should be used. - Recommendations of different cements (for application) based on SO3 in soil. - A table summarizing the different types of cement and their applications. - Low Heat Cement - Heat liberated during setting is low, due to presence of C4AF and C₂S. - Contains low percentage of C3S and C3A. - Shrinkage and cracks are reduced. - This type of cement is used for mass scale concrete work where low liberation of heat is desired such as dams, roofs etc. - Rapid Hardening Cement - Lime saturation factor is relatively higher and the final product is ground to more fineness. - Contains high percentage of C3S and C3A. - More expensive. - This type of cement is used for emergency construction where high early strength is desired. - White Cement - Raw materials are free from iron impurities. - More expensive. - This type of cement is used for fare face concrete, cement paints, colored cements etc. - An illustration of white cement. ## STORAGE OF CEMENT - Portland Cement is a moisture sensitive material. - If kept dry it will remain its quality. - When stored in moist and damp condition it will set more slowly and has less strength. - When storing bagged cement, a shaded area or ware house is preferred. - When storing cement bags at outdoor it should be stacked on pallets and covered with waterproof covering. - Storage of bulk cement should be in bin or silo. - An illustration of cement bags stacked.