Cement Chemistry and Manufacturing Quiz

Construction Materials - CENG 2092

• Course name: Construction Materials
• Course code: CENG 2092
• University: Addis Ababa Science and Technology University
• Year established: 2011

Chapter Two - Binders

• Binders are substances used to bind inorganic and organic components.
• Non-hydraulic binders only harden in the presence of air.
• Hydraulic binders require water to harden.
• Examples of binders:
  • Lime
  • Gypsum
  • Portland cement.
  • Mortar
  • Bituminous (Asphalt)
  • Synthetic

Binder Definition

• Binders are substances used to bind inorganic and organic components.
• Hydraulic binders are strong, hard, and/or flexible.
• Binder action is usually due to chemical reactions during heating, mixing with water/other materials, or exposure to air.

Non-hydraulic Binders

• Harden only in the presence of air.
• Cannot set or harden in water.
• Lime is the most common non-hydraulic binder.
• Gypsum is a naturally occurring soft crystalline rock, also a non-hydraulic binder.
• Hardening depends on combining with carbon dioxide (carbonation) to become limestone.

Hydraulic Binders

• Require water to harden and develop strength.
• Portland cement is the most common hydraulic binder.
• Hydraulic binders are usually fine powders.

Lime

• One of the oldest known cementing materials.
• Found naturally as a rock of varying degrees of hardness.
• Primarily composed of calcium oxide (CaO).
• In its pure form, it associates with CO₂ to form CaCO₃ (white).
• Often found mixed with impurities (CO₂, Fe₂O₃, MgCO₃).
• Impurities determine color variation.
• Generally a non-hydraulic binder but can be made hydraulic.
• Production: Burning raw material limestone (CaCO₃) which produces CaO and CO₂.
• Chalk and shell can contain over 98% CaCO₃.
• Product type (hydraulic or non-hydraulic) depends on heat and slaking method.
• Burning process occurs in either vertical shaft kilns or rotary kilns.

Lime Production - Vertical Shaft Kiln

• Raw materials are fed at the top.
• Finished product is drawn from the bottom opening.
• Stages include preheating, calcining, and cooling.

Lime Production - Rotary Kiln

• Rotating cylinder, inclined at 3-4 degrees to the horizontal.
• Limestone is fed in at the back end. Fuel and combustion air are fed into the front end.
• Product is discharged into a cooler.
• Process includes coal feed/pulverized coal, firing fan, gear, air blower, dual fuel, gas/air, and lime cooler.

Classification of Lime

• Commercial lime is classified into:
  • Quick lime (caustic lime)
  • Hydrated lime (slaked lime)
  • Hydraulic lime

Quick Lime (A)

• Obtained by calcining (burning) pure calcium carbonate.
• White in color with a specific gravity of approximately 3.40.

Hydrated Lime (B)

• Can't be used directly, needs to be mixed with water.
• Reaction is called slaking, resulting in calcium hydroxide (Ca(OH)2), also called slaked lime or hydrated lime.
• Forms of hydrated lime vary based on added water during slaking:
  • Dry hydrate (dry-slaking)
  • Milk of lime (wet-slaking)
  • Lime putty

Hydraulic Lime (C)

• Prepared by burning impure limestone that contains clay, producing compounds similar to Portland cement.
• Hardening occurs through internal reaction with water.
• Stronger but less plastic compared to non-hydraulic lime.
• Manufacturing is similar to quick lime production, but a slightly higher temperature is required during burning.

Setting and Hardening of Lime

• Slaked lime hardens by losing water and absorbing carbon dioxide from the air, transforming into calcium carbonate (CaCO₃) or limestone.

Uses of Lime

• Construction material:
  • Mortar (mixed with sand)
  • Cement mortar (improves workability)
  • Plaster
  • Whitewash
  • Lime concrete
  • Clay soil stabilizer

Gypsum

• Non-hydraulic binder that occurs naturally as a soft crystalline rock.
• Easily scratched by a fingernail.
• Naturally occurs as hydrated calcium sulfate (CaSO₄·2H₂O), generally 76% CaSO₄ and 24% H₂O.
• Pure gypsum is known as alabaster (white translucent crystalline mineral).
• Used in arts and building construction.
• Easily turns into powder when heated.
• Forms interlocking crystals when water is added to the powder.
• Building gypsum is made of semi-hydrate gypsum from processing at 150-160°C.

Advantages of Gypsum as a Construction Material

• Incombustibility
• Superior surface finish
• Good fire resistance
• Resistance to insects
• Low energy consumption during burning
• Rapid drying

Major Shortcomings of Gypsum

• Low strength in wet state
• High creep under load

Plaster of Paris (A)

• Produced by incompletely dehydrating pure gypsum, finely ground, at a somewhat lower temperature than 185°C.
• This forms CaSO₄·½H₂O.
• At higher temperatures (around 200°C), gypsum loses all water of crystallization and turns into a hydrated gypsum.
• White powder with a specific gravity of 2.57.
• Used for small patching jobs on plastered walls.
• Excellent material for filling cracks, holes, and for painting/polishing wooden surfaces before painting.
• Sets rapidly (5-10 minutes) from mixing with water to form a plastic paste.

Hard Finish Plaster (B)

• Burning gypsum at a higher temperature than calcining cement plaster.
• Treated with solutions, like alum and Glauber's salt (Na₂SO₄), results in a very hard plaster with a slow setting time.
• Also known as anhydrous plaster or high-temperature gypsum derivative(CaSO₄· 2H₂O + High Heat = CaSO₄ + 2 H₂O).
• May be polished to form a smooth surface for interior walls.

Uses of Gypsum

• Wall plasters: gain half of their one-month strength in a day. Mortars of 1:1 proportions develop 80% of neat strength; 1:2 proportions generally have strength between one-half and two-thirds of neat strength. Gypsum to sand (1:3) sets in 2-32 hours, and 1.5-8 hours when mixed with wood fibers.
• Plaster boards: made from thin layers of cardboard or wood cemented together with wall plaster. Stronger with the addition of sisal or coconut fibres. 10 kg/m² of plaster; 250 g/m² of fiber; very light and fire-resistant.
• Non-load bearing gypsum partition blocks: solid or hollow, rectangular shape, straight and square edges; minimum compressive strength of 50 N/m².

Pyro-cell

• Finely ground powder with an admixture. Forms gas upon mixing with water and expands the mixture to 3-4 times its original volume.
• Hardens into a fire-resistant, light, cellular mass with good acoustical and insulating properties.

Cement

• A binder used for concrete.
• Dates back to 7000 BC, with a lime concrete floor found in Israel.
• Portland cement was first patented in 1824, named after natural limestone quarried on the Isle of Portland (English Channel).
• Primary components of raw materials: calcium, silica, alumina, iron.

Methods in Portland Cement Production

• Dry process: Proportioned, ground, blended, and fed into the kiln.
• Wet process: Adds water to the raw materials, grinding and blending in slurry form.

Portland Cement - Dry Process

• Four main steps: treatment of raw materials, burning of the dry mix, grinding of the clinker, packaging, and storage.
• Raw materials undergo crushing, drying, grinding, proportioning, and blending before burning in the kiln.
• Finely powdered mixture (raw meal) is charged into a long, steel cylinder called a rotary kiln.

Portland Cement - Reactions During Burning

• Complete dehydration: Water is driven off at low temperatures (about 400°C).
• Dissociation of carbonate: Calcium carbonates dissociate at 800-900°C, forming calcium oxide and carbon dioxide.
• Compound formation: Formation of compounds like tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF).

Portland Cement - Grinding of Clinker

• Completely burnt raw materials are converted into clinker lumps, drawn from the rotary kiln's lower end.

Portland Cement - Packing and Storage

• Cement is typically stored in specially designed concrete storage tanks (silos).
• It is drawn off mechanically and is commonly packaged in bags.
• Dry mixing and blending method has less popularity than the wet process.

Portland Cement - Wet Process

• Considered a better process for cement manufacture where a soft-variety limestone is the abundant raw material.
• Steps in wet process:
  • Preparation of slurry
  • Burning or calcination
  • Clinker grinding

Portland Cement - Clinker Production

• Stages during clinker production
  • Raw materials (at 700°C) are free-flowing
  • Particles remain solid up to 700-900°C
  • Nodulization: Solid particles; no reaction, Nodulization, particles stay solid as calcification continues
  • Clinkering reactions: Formation of liquid phase; more heat is needed for various reactions
  • Reactions happen at 1150-1200°C, particles become sticky, aglomerate, rotation initiates coalescing and layering of particles
  • 1200-1350°C, capillary forces hold particles; small belite crystals forms, liquid phase; and reaction between belite and free CaO forms alite crystal formation.
  • 1350-1450°C, Particles aglomerate, layer particles by falling, nodules form, and insufficient liquid produces dusty clinker. -Cooling causes C₃A and C₄AF crystallize, resulting in lamellar structure.

Chemical Compounds of Portland Cement

• C₃S (tricalcium silicate): Highly desirable for its high early strength. Proportion ranges from 25-60%.
• C₂S (dicalcium silicate): Hardens slowly, contributing to strength increase beyond one week. Proportion ranges from 13-50%.
• C₃A (tricalcium aluminate): Generates a large amount of heat during hardening (first few days). Acting as a retarder and lowering heat of evolution when gypsum is added. Leads to calcium sulfoaluminate formation. Proportion ranges from 5-15%.
• C₄AF (tetracalcium aluminoferrite): Decreases clinkering temperature to improve Portland cement manufacturing. Contributes little to strength, good resistance to chemical attack. Proportion ranges from 8-15%.

Summary of Portland Cement Clinker Characteristics

• Table summarizing the properties (rate of hydration, strength, amount of heat liberated, and resistance to chemical attack), for each chemical compounds (C₃S, C₂S, C₃A, C₄AF)

Types of Portland Cement

• IA (Normal): General purpose, used where special properties are not required.
• II (Moderate): Suitable for structures needing moderate sulfate resistance or moderate heat of hydration (e.g., abutments, piers).
• III (High Early Strength): Ideal for high early strength requirements, often used in quick construction.
• IV (Low Heat): Intended for mass structures where low heat of hydration and slower strength development is needed (e.g., large concrete pour).
• V (Sulfate Resisting): High sulfate resistance; for use where concrete is exposed to harsh sulfate conditions.
• IA, IIA, IIIA: Improved resistance to freezing and thawing.

Other Types of Cement

• Portland Pozzolana Cement (PPC): Blended with 20-30% by weight of pozzolanic material (e.g., a naturally occurring or artificial siliceous material) to get improved cementations properties.
• Acid-resistant cement: Composed of acid-resistant aggregates and additives, increasing acid resistance from mixing with sodium silicate or soluble glass.
• Blast furnace cement: Utilized slag from blast furnace, cheaper/economical, with nearly the same properties as ordinary cement, but unsuitable for dry arid zones; longer curing time.
• Colored cement: Coloring via mineral pigments, the percentage usually varies from 5-10% but shouldn't exceed 10% for strength implications.
• Expanding cement: Produced by adding an expanding medium to ordinary cement. Used for water-retaining structures and damaged concrete surfaces.
  • Used in repairing damaged/cracked concrete surfaces.

Mortar

• A mixture of sand or inert particles and a binding agent (typically cement and/or lime) with water added in predetermined ratios.
• Mortar = Binder + Sand + H₂O

Mortar Uses

• Jointing medium in masonry construction.
• Wall plaster: even, smooth surfaces over construction.

Types of Mortar

• Mud mortar: Soil and water.
• Lime mortar: Lime, sand, water.
• Cement mortar: Portland cement, sand, water.
• Compo mortar: Combines lime and cement for improved properties.

Mortar Properties

• Workability: Ease of transporting, placing, and finishing the mortar mix. Lime-sand mortars typically have better workability than cement-sand mortars.
• Strength: Affected by ingredients, proportions, and curing method. Cement-sand mortars generally develop greater strength.
• Water tightness: Portland cement mortars are often preferred for watertight applications due to their hydraulic properties. Strength and degree of water tightness increases with the density of the mix, keeping all other factors consistent.

Factors Affecting Mortar Properties

• Mixing water amount
• Binder properties
• Cement content, fineness, and composition.
• Sand characteristics and grading

Materials for Mortar

• Binder: OPC, PPC, RHC, lime types.
• Sand: Well graded, clean (free from dust, loam, clay, vegetable matter).

Mortar Proportioning/Recipe

• Proportions vary based on application; e.g., Masonry cement, compo mortar, for plastering, etc.

Batching and Mixing

• Accurately measuring components (cement by weight, sand by volume).
• Specific volume measurements for different quantities in multiples of 35 liters. Standard boxes for mortar batching 40x35x25cm, 40x50x18cm.

Other Information (Miscellaneous)

• Silt test (Jar test) for determining silt content of sand - 3mm or greater silt or 6%, then not suitable for mortar job.

Test your knowledge on the chemistry and manufacturing processes of Portland cement. Explore questions on the roles of various chemical compounds, the impact of additives like gypsum, and key processes such as slaking and calcining. Perfect for students studying civil engineering or material science.