Cement concrete and glass

What are ceramics?

Inorganic, non metallic compounds which are formed by heating raw materials

What are key properties of ceramics?

• Hard - resistant to wear and abrasion

• brittle - fracture rather then deform under stress

• chemically inert

• high melting point

What is cement?

Powdered rock - calcium silicate, silicate aluminium and iron impurities

Steps to make cement

1. crushing

2. heating

3. cooling

   1. grinding

What happens in the crushing stage - making cement?

• starting material is limestone

• limestone aggregate is ground

• crushing process is demanding on the rollers which must be continuouslt maintained

• mixed with 25% clay

What happens during the heating stage of making cement?

• heated in a rotary kiln - 1000→ 1500

• calcium carbonate decomposes into lime, calcium silicates and alumminates reacts with the clay

• rotation ensures even heating so reproducible cement

What happens during the cooling stages of cement making?

• lime reacts with the clay to form clinkers 5-10cm

  • clinker →small nodules of calcium silicate and aluminates

What happens during the grinding stage of cement making?

• clinker is ground into a fine powder with calcium sulphate

• mixed with gypsum which controls setting times and improves durability

How does cement work?

• addition of water creates a gel structure, skeleton of the gel is made from calcium silicate hydrate fibrils

• over 2 hours the paste stiffents as the fibrils grow and mesh together locking in water

How does cement set?

• interlocking

• water levels build and it ruptures

• fresh surface is exposed for further hydration

• fibres grow, multiply and interlock

How does cement harden

• drives off all the water

• silicate fibrils continue to grow and interlock

What happens if cement has too much water?

not enough calcium silicate to react so water is left over

What happens if there is too little water in cement

Uncreacted cement left over

What are issues with cement?

• good under compression not tension

• shrinkage - use aggregates

• corrosion of steel reinforcements

• liming of surrounding soil

What is concrete?

Strong and versatile, made by combining cement, aggregates and water

How is concrete made?

Cement binds the aggregates together

aggregates - provides bulk and compressive strength

water - activates the cement causing it to harden

What is reinforced concrete?

Concrete is strong under compression

structurally useful when reinforced with steal bars

What are issues with reinforced concrete

Rusted steel has greater volume

causes cracks which allows in more water which further corodes

What is clay?

• made from the finest sedimentary particles from weathering and chemical attack on igneous rocks

What does clay consist of ?

• calcium alumino silicates which have a platelet structure making it soft

  • slide over each other when wet allowing shaping, firing causes them to turn into a rigid structure

How are pots formed?

• clay is mixed with sand and feldspar as binders

• its dried and the water evaporates, platelets form bonds through fusion to each other

  • fired in a kiln which drives off any remaining moisture, the temperature is just below mp, edges of the platelents melt fusing with the neighbours increasing rigidity

modern uses of clays

• bentonite to clear beer

• remove smell and pollutants

  • seperate materials

What are catastrophic failures in ceramics?

small air bubbles and occlusions weaken ceramics making them shatter

How do you prevent catastrophic failures in ceramics?

• transformation toughness - adding e.g. zircronium oxide which changes the crystal structure under stress preventing crack propagation

  • Ceramic fibre composites - adding strong ceramic fivres within the ceramic matric to distribure stress and prevent crack growth

What is glass?

• amorphous solids - lack long range order

  • transparent due to the disordered structure allowing light to pass through without scattering

What are the stages of making glass?

• mixing

• melting

• shaping

  • cooling

What happens during mixing in the glass making process?

• sand, soda ash, limestone and broken glass

• mixed to ensure uniform composition

  • soda ash reduces the melting temperature and limestone enhances chemical stability

    • broken glass lowers impurities and inclusions

What happens during the heating stage of melting glass?

• limestone reacts with sodium oxide to create sodium carbonate which reacts with sand

What is soda glass?

• general purpose glass

  • stable, non corrosive

What happens during the cooling stages of glass ?

• annealed, cooled slowly to avoid cracks and relive internal stresses, ensuring glass is usable and durable

What are the stages to make window glass?

1. mixing and reaction

2. float bath

3. specialist coatings

4. anneal

5. laser optical quality measurements

6. cutting

What happens in the reactor during making window glass?

• mix ingredients at 1500 for up to 50 hours to minimise inclusions and foreign bodies as would cause weakness to the structure

  • soda ash is used to lower the melting point

What happens in the float bath when making window glass?

• molten glass is drawn out into a molten tin bath

• the glass forms a flat surface

  • As it cools and solidifies the volume alters and stresses which must be removed by annealing

What happens during the specialist coating stage of making window glass?

• tints and textures are caused by using heated rollers to imprint a pattern on the moltern ribbon before it solidifies

  • tints and photochromics are applied using chemical vapour deposition.

What happens during the annealing stage of making window glass?

• as the ribon cools, it becomes highly stressed and annealed at 200 to relieve stress

What happens at the optical stage of making window glass?

• optical measurements are made using reflected and transmitted laser light to ensure the annealed glass is high quality

What happens during the cutting stage of making window glass?

• cut into intricate shapes by using computer controlled diamond cutters

What are the problems with glass?

• mechanical weakness - brittle

• thermal shock - glass expands and contracts due to the temperature causing it to crack

How do you prevent mechanical weakness within glass?

• mechanical fracturing - controlled break to minimise catastrophic failure

• polymer lamination - laminate with polymer layers to enhance toughness and shatter resistance

• metal mesh intergration - metal grids to distribute stress and prevent crack propagation

How do you prevent thermal shock within glass?

• boroscilicate glass - lower expansion coefficient enables resistance to sudden heating

  • very thin glass in theros - minimises thermal stress through thinner walls

What are specialty glasses?

• optical communications

• solar cells

  • fibre glass

what are speciality glasses?

• photochromics - coatings which change colour under light

• bulletproof