Wk 3 Cement mortar and concrete

Cement

What is cement?

A powder that when mixed with water binds sand and aggregates into a stone like mass

What are the two broad types of cement?

Hydraulic - harden by reacting with water but also form a water resistant product

e.g portland cement has silicates and react chemically to form insoluble hydrates, remaining stable underwater

Non hydraulic - they react with water but don’t harden because of them, they harden through carbonation (react with CO2 in air) products are not resistant to water

What is cement made of?

Made of different silicates that hydrate at different speeds - have different effects on strength

How do we manufacture cement? (dry process) for hard raw materials

1. Limestone and clay are ground separately into fine powders and mixed

2. water is added to make paste that is shaped into cakes

3. fed into a rotary kiln heated to 1400-1500 degrees

4. chemical reactions take place - clinker is formed (precursor form of cement)

5. clinker is rapidly cooked to preserve mineral structure with 2-3% gypsum in tube mils to produce final cement product

6. the gypsum retards the setting process by regulating hydration

How do we manufacture cement? (wet process) for soft raw materials

1. mix crushed raw materials with water to form a slurry with around 40% moisture

2. slurry stored in silos where its chemical composition is precisely adjusted

3. placed in a rotary kiln where water evaporated

4. raw materials are calcined at 1500-1600 degrees - clinker formed

5. clinker is cooled and ground with gypsum to produce cement

Compare the dry and wet process for cement manufacturing

• Dry for hard raw materials

• Wet for soft raw materials

• Wet requires higher temps ( dry 1400-1500 whereas wet 1500-1600)

What type of reaction is hydration of cement?

exothermic

What are the stages of hydration?

Stage 1: initial surge - gyspum balance and controls flash, forms surface gel, ettringite

Stage 2: induction/dormant - slowed reaction maintains workability (window where cement is transported or placed)

Stage 3: acceleration - hydration forms bonds causing stiffening and early strength gain

Stage 4-5: deceleration/late ages - reaction slows as diffusion limits increase, long term formation and microstructure densifier

What are SCMs?

Supplementary cementitious materials

• they are added to improve physical properties like durability, long term strength and workability

• they typically slow early heat/hydration but enhance later strength, pore refinement and chemical resistance

Why are SCMs and admixtures needed for durability and thermal control in mass pours?

• they manage heat

• they manage setting

• and microstructures

What characterisation tests are done for portland cement?

• fineness

• consistency

• setting time

• soundness

• strength

• heat of hydration

• specific gravity

What are the key things engineers must remember when manufacturing cement?

1. Match cement type/grade to exposure, thermal and early strength requirements - can use SCMs to improve durability/ reduce embodied carbon dioxide

2. Control gyspum, fineness and clinker chemistry to avoid flash set, excessive heat or long term weaknesses

3. Implement routine quality control like (fineness, setting, soundness etc) and coordinated cement selection with mix design during and construction practice(

What is coordinated cement selection?

choosing the correct cement type in coordination with all project requirements