Cement Materials PDF

CE 201 Engineering Materials Construction Material : CEMENT  INTRODUCTION TO CIVIL ENGINEERING MATERIALS Civil Engineering encompasses the ❑ conception, design, construction, and management of residential and commercial buildings and structures, ❑ water supply facilities, and transp...

CE 201 Engineering Materials Construction Material : CEMENT  INTRODUCTION TO CIVIL ENGINEERING MATERIALS Civil Engineering encompasses the ❑ conception, design, construction, and management of residential and commercial buildings and structures, ❑ water supply facilities, and transportation systems ❑ control of the environment for the maintenance and improvement of the quality of life. Various types of materials are used for construction of buildings, highways, bridges, etc. Natural materials: →Unprocessed, raw, minimally processed (Stone, rock, sand, wood etc.) Synthetic materials →Industrial processing →Artificially composed (Cement, tiles, concrete, steel etc.) Material Selection Wood Lumber, Plywood, Composites Steel Shapes, Rebar, Pipes, Cold-Formed, Cable Material Selection Concrete Cast, Precast, Grout, Mortar Stone and Masonry Brick, Concrete Block, Cut-Stone Material Selection Asphalt Hot-mixed, Cold-mixed, Emulsions Aluminum Shapes, Pipe, Cold-Formed Material Selection Architectural Materials Tile, Glass, Acoustic Tile Soil Foundations, Fill, Embankment, Liner ▪ Selection of construction materials involves a wide spectrum of choices. ▪ The selection is often based on economic, environmental, availability, or technical considerations. ▪ Geography often determines the economic and availability considerations and engineering principals typically control the technical and environmental considerations. Civil Engineering design is based on relationships between- Stress vs. Strength Strain vs. Deflection Exposure vs. Durability Risk vs. Consequence Cost vs. Aesthetics & Life Cycle  CONSTRUCTION MATERIAL : CEMENT Cement is finely ground (powdered), inorganic & hydraulic material Cement is the active ingredient in concrete. → ▪ Cement acts as the binder material in construction works ▪ When mixed with water, forms a paste which sets and hardens by means of hydration reactions ▪ After hardening, retains specified strength levels and also long-term volume stability even under water History of Cement Cyclopean masonry of Greece - no cementing material Ancient Egyptian buildings - Stone & unbaked brick - covered with Nile mud Mesopotamians used bitumen as an adhesive in building structure ▪ First cements produced by early Greeks and Romans from volcanic ash mixed with slaked lime. ▪ This art was lost during the Middle Ages ▪ Portland cement – the modern artificial form of cement was developed in England by bricklayer Joseph Aspdin in 1824. ▪ Aspdin found that by heating clay and limestone at a very high temperature, then cooling, grinding and mixing it with water he had created particularly strong cement. ▪ This cement is named called “Portland” because concrete made with it resembled natural stone from the Isle of Portland. ▪ Today, portland cement is the most widely used building material in the world with about 3 billion tons produced each year. Classification of Cement ▪ Cement is the mixture of calcareous (calcite/ CaCO3), siliceous (consisting silica) , argillaceous (clay) and other substances ▪ According to source of raw material used cement can be classified as-: - Natural cements - Artificial cements  Natural cement ▪ Manufactured from natural stones by burning and then crushing to powder ▪ These stones contain 20~40% of argillaceous matter and the rest as calcareous matter ▪ As the chemical composition of natural cement stones varies considerably - the properties of natural cement also keep on varying. CE 201: Lecture on Cement by Snigdha Afsana, Asst Professor, Dept. of CE, BUET Artificial cement ▪ manufactured by burning at high temperature an intimate mixture of argillaceous and calcareous substances and lastly crushing the resulting clinkers to a fine powder after adding a little gypsum to it. ▪ Gypsum is added to delay the setting action of the cement Artificial cement is popular because of the following reasons: ▪ Can be manufactured in any desired color. ▪ Initial setting time can be easily regulated. ▪ Rate of hardening and heat evolution can be regulated. ▪ Quality cement can be easily controlled maintaining the same composition of raw materials. ▪ Can be manufactured in very large quantity. Composition of Portland Cement The chief chemical components of portland cement are- calcium, silica, alumina and iron. Calcium is derived from limestone, marble or chalk, Silica, alumina and iron come from the sands, clays and iron ore respectively Other raw materials may include shale, shells and industrial byproducts such as mill scale etc. CE 201: Lecture on Cement by Snigdha Afsana, Asst Professor, Dept. of CE, BUET a) Limestone (CaCO3) is mainly providing calcium in the form of calcium oxide (CaO) CaCO3 (1000 0C) → CaO + CO2 b) Clay is mainly providing silicates (SiO2) together with small amounts of Al2O3 + Fe2O3 Clay (1450oC) → SiO2 + Al2O3 + Fe2O3 + H2O c) Iron ore and Bauxite are providing additional aluminum and iron oxide (Fe2O3) which help the formation of calcium silicates at low temperature. They are incorporated into the raw mix. Then Ordinary Portland Cement (OPC) produced by heating limestone and clay minerals in a kiln to about 1400 to 1600°C - the temperature range in which the two materials interact chemically to form calcium silicates Hydration of Cement (Cement Reaction with Water) ❑ When Portland cement is mixed with water its chemical constituents undergoes a series of chemical reactions that cause Setting & Hardening of cement. ❑ In short, - Setting is the process of retaining the shape - Hardening is the process of gaining strength ❑ The phase compositions in portland cement are denoted by ASTM as i. tricalcium silicate (C3S)- [3CaO.SiO2 ] ii. dicalcium silicate (C2S) -[2CaO.SiO2 ] iii. tricalcium aluminate (C3A)- [3CaO. Al2O3 ] and iv. tetracalcium aluminoferrite (C4AF) [4CaO.Al2O3.Fe2O] ❑ The behavior of each type of cement depends on the content of these components. i. Tricalcium silicate (C3S): Also known as “alite”. Hydrates and hardens rapidly. Largely responsible for initial set and early strength. Portland cements with higher percentages of C3S will exhibit higher early strength. C3S + 3H --> C-S-H + 2C-H (rigid gel) ii. Dicalcium silicate (C2S): Also known as “belite”. Hydrates and hardens slowly continues for several weeks. Largely responsible for strength increases beyond one week. C2S + 2H --> C-S-H + C-H (rigid gel) iii. Tricalcium aluminate (C3A): ▪ Hydrates and hardens the quickest. ▪ Liberates a large amount of heat almost immediately and contributes very little to early strength. ▪ Gypsum is added to portland cement to retard C3A hydration. Without gypsum, C3A hydration would cause portland cement to set almost immediately after adding water. iv. Tetracalcium aluminoferrite (C4AF): ▪ 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 C4AF. Strength development rate of major constituents of cement Setting Vs Hardening??  Heat of Hydration ▪ The Hydration of cement with water is exothermic (a chemical reaction that releases light or heat) ▪ The liberation of heat is called heat of heat of hydration. ▪ This process of hydration of cement results to the formation of minute crystals of calcium and gels from the solution of cement and water and it continues for a long time. ▪ As considerable amount of heat is evolved during hydration of cement, study and control of hydration becomes important in mass construction(e.g., piers, dams etc.)  As the crystals adhere to one another and to the surface of sand or inert particles of aggregate (with which cement is mixed), the entire mixture gets set and hardened resulting in gaining strength. The strength developed depends on the amount of gel formed, and the degree of crystallization. ▪ The hydration rate of cement constituents is highly variable. ▪ The rate of hydration during the first few days - C3A >C3S > C4AF >C2S ▪ Usually, the greatest rate of heat liberation occurs within the first 24 hours and a large amount of heat evolves within the first 3 days. ▪ However Portland cement evolves heat for a long time ▪ One gram of OPC Type-I gives out about 120 calories on setting over 13 years, out of which 80 calories are given out in the first 7 days. Calorimetric curve of Portland Cement Hydration Process Ettringite (C6AS3H2) Hydrous calcium aluminium sulfate mineral Ca6Al2(SO4)3(OH)12·26H2O Ettringite is formed in hydrated Portland cement system as a result of the reaction of calcium aluminate (C3A) with gypsum, C3A + 3 CaSO4 → ettringite  12-36 hrs 10-15 mins IST FST  The first heat peak is completed in 10 to 15 min and then the rate of heat evolution has been reduced to a very lower value due to the formation of the ettringite barrier. The heat of hydration remains at low value till the ettringite barrier is broken by transformation of ettringite to mono- sulfoaluminate after all the gypsum has been used to form the ettringite. The more gypsum there is in cement, the longer the ettringite will remain stable. In most cements ettringite remains in stable condition for a period of 12 to 36 hours. This is the reason why the concrete remains in plastic state for several hours. The rate of heat evolution starts increasing with start of ettringite conversion to monosulfo- aluminate and reaches to the second heat peak. By this time (4 to 8 h) final set has been passed and early hardening has begun and then again starts decreasing approaching to a steady-state condition.  Types of Portland cements ▪ Different types of portland cement are manufactured to meet different physical and chemical requirements for specific purposes, such as durability and high-early strength. ▪ Classification as per AASHTO M 85 and ASTM C 150, 1. Type I - Ordinary Portland cement 2. Type IA - Normal-Air Entraining 3. Type II - Moderate Sulfate Resistance 4. Type IIA - Moderate Sulfate Resistance Air Entraining 5. Type III - High Early Strength 6. Type IIIA – Air Entraining Early Strength 7. Type IV - Low Heat of Hydration 8. Type V - High Sulfate Resistance 1. Type I - Ordinary Portland cement ▪ Known as common or general purpose cement. ▪ Commonly used for constructions especially those concrete constructions not to be in contact with soil or ground water ▪ Fairly high C3S content causes good early strength development Table: Main Constituents in a Typical Portland Cement (Type-1) Chemical Name Shorthand % by Weight Notation Tricalcium Silicate (Alite) C3S 50 Dicalcium Silicate (Belite) C2S 25 Tricalcium Aluminate C3A 12 Tetracalcium C4AF 8 Aluminoferrite (Ferrite) Gypsum CSH2 3.5 2. Type IA - Normal-Air Entraining OPC ▪ An air-entraining modification of Type I. ▪ Use 0.001 to 0.1% by weight air entraining or foaming agents (e.g., vinsol, resin, darex etc) are added during grinding of clinkers. ▪ When water is added this air entraining agents create tiny air bubbles in the cement mix and prevent contact of cement particles with each other and thus delay setting. 3. Type II - Moderate Sulfate Resistance Cement ▪ Also known as expanding Portland cement ▪ Used as a precaution against moderate sulfate attack. ▪ Used for structures exposed to soil or water containing sulfate ions ▪ It will usually generate less heat at a slower rate than Type I cement. ▪ Low C3A content C 3S C2S C 3A C4AF 45 30 7 12 4. Type II - Moderate Sulfate Resistance Cement Sulfate attack is an important phenomenon that can cause severe damage to concrete structures. It is a chemical reaction between the hydration products of C3A and sulfate ions that enter the concrete from the outside environment. The products generated by this reaction have a larger volume than the reactants, and this creates stresses which force the concrete to expand and crack. 4. Type IIA - Moderate Sulfate Resistance Air Entraining - An air-entraining modification of Type II. 5.Type III - High Early Strength Cement ▪ A rapid hardening cement, is used when high early strength is needed (e.g., highway slab, pre-cast concrete, cold weather concreting). ▪ It has more C3S than Type-I cement and has been ground finer to provide a higher surface-to-volume ratio, both of which speed the rate of hydration. ▪ Strength gain is double that of Type I cement in the first 24 hours. C 3S C 2S C 3A C4AF 60 15 10 8 7. Type IV - Low Heat of Hydration Cement ▪ Used when hydration heat must be minimized in mass concrete structures such as bridge, abutment, dams, raft, retaining walls etc. ▪ Such structures have low surface to volume ratio therefore heat dissipation rate is much lower than that generated. ▪ Contains about half the C3S and C3A and double the C2S of Type I cement. C3S C2S C3A C4AF 25 50 5 12 8. Type V - High Sulfate Resistance Cement ▪ Used as a precaution against severe sulfate action - principally where the concrete is in contact to soils or groundwater having high sulfate content. ▪ High sulfate resistance is attributable to low C3A (