Non-metals Flashcards
Why are unreinforced masonry walls not used in seismically active regions?
Unreinforced masonry has very little resistance to shearing forces.
What are the positives and negatives of using masonry in construction?
\+) use locally sourced stone/ clay \+) high thermal mass \+) durable (>500 years) -) manual labour, hard to mechanise -) difficult to make tall structures- heavy, low tensile
Which is more expensive, a dry pressed brick or a soft mud process brick? Which is more porous?
dry pressed cost more. less porous porous due to less water content.
What makes many bricks red?
iron, after being baked ≈950’c
Does a high-quality engineering brick show high or low water absorption?
Lower, <4.5% compared to normal, ≈20-30%
Which is more likely to show efflorescence: a brick that is kept completely dry, partially wetted, or immersed in water?
moisture travels through masonry and evaporates, depositing salts on the surface. Avoided by keeping water out.
Why does rapid cooling give a glass in some cases, while slow cooling gives an ordered structure?
Slow cooling allows equilibrium phase change, giving a crystal with ordered structure. Fast cooling creates a non-equilibrium transition.
Float glass is prepared using a bath of liquid tin. What key property does this give to the glass?
Flat surface as it sets on top of the liquid
Why can glass rarely achieve more than 1% of its theoretical strength?
Flawless glass could be very strong. Very weak in tension (always some tension in compression). H2O attack on chemical bonds in cracks makes it weaker under long-term loading (static fatigue).
Why is bullet-proof glass often laminated?
Polymer layers are much more elastic. On impact the glass will shatter but the polymer layer will deform elastically.
Why does a double-glazed window need the same thickness of glass on the inside and outside panes?
- To prevent shearing
- temperature mismatch can cause cracking
- dimensional changes can break seals, allowing moisture to get inside and cause fog and mould
Name the 4 major Portland cement clinker compounds, and describe briefly the role of each in Portland cement hydration
Tricalcium Silicate, C3S (Alite): early strength development, <7 days.
Dicalcium Silicate, C2S (Belite): longer term gain of strength, 28 days.
Tricalcium Aluminate, C3A (Aluminate): controls initial setting time.
Tetrecalcium Aluminoferrite, C4AF: reacts slower giving low early strength. controls melting in the kiln. gives grey colour.
Clinker is interground with another component to make cement. What is this component, and with which of the clinker compounds does it mainly react?
When cement is hydrated, gypsum reacts with tricalcium aluminate, C3A, to hinder hydration and delay the setting time of cement.
What happens when fresh cement paste is dried?
Slows or stops reactions causing cracking and loss of strength.
Identify the 4 main types of hydrate product which are important in Portland cement. Which of these are mainly responsible for: (a) setting, (b) final strength?
(a) Ettringite
(a) Calcium hydroxide (portlandite), Ca(OH)2
(b) Calcium Silicate hydrate, C-S-H
(b) C3A reaction products
Which natural mineral is used as a structural model to describe the C-S-H structure?
Tobermorite
Describe the chemistry of the pozzolanic reaction in an approximate chemical reaction, using cement chemistry nomenclature.
Blend cement with other sources of SiO2 (often waste). These react with CH to form more C-S-H. Results in filling pores at longer age and improves strength and durability.
Define the terms: paste, mortar, fine aggregate, coarse aggregate
Paste: Cement + water
Mortar: Paste + sand (fine aggregate)
Fine aggregate: <5mm
Course aggregate: 15-20mm
Give three reasons why aggregate is added to concrete
Improve strength, durability, fire resistance, control shrinkage and prevent cracking
Give three reasons why adding excessive water to concrete can be undesirable
Required in cement hydration and makes concrete flow. Too much results in:
-form extra pores, reducing durability and resistance
-reduce strength
-delay hardening
causes bleeding, segregation, plastic settlement
-increased drying shrinkage
Describe the process of plastic settlement
Solid particles can sink through the paste leaving water pockets under the aggregate and reinforcing bars causing cracks on surface.
Describe the process of segregation
water separates from the aggregate (non adhesive mix), fine aggregates help to avoid this.
Describe the process of plastic shrinkage cracking
water evaporation from the surface makes the paste shrink. Aggregate particles stay in place and restrain the shrinkage.
Describe the characteristic engineering properties of two non-Portland cements
Calcium Aluminate cement: -high early strength, used in prestressed components -very sensitive to water content Alkali-activated cements: -60-90% less CO2 emissions than portland -need alkaline solution