Cement Flashcards
What’s the technical reason for caring about cement?
Its the key binding agent in mortar and concrete
What is the environmental reason for caring about cement?
Cement is responsible for >60% of the CO2 emissions arising from the production of modern concrete mixes
What is the socio-economic reason for caring about cement?
We depend on concrete and thus cement to provide essential building and infrastructure services
How was cement used in 5600BC?
- Concrete floors of huts in Lepenski Vir, Yugoslavia
- Material uncertain but probably lime
How was cement used in 2500 BC?
- Great Pyramid, Giza, Egypt
- Cementing material lime (from limestone, CaCO3) or burnt gypsum
How did greeks create mortars?
Used lime and sand
How is lime produced?
By calcining (heating) natural calcium carbonate
(limestone, chalk, marble, etc.) at approximately 700-900°C
How is portlandite formed?
Using quicklime and water
What is slaked lime?
Hydrated lime also called slaked lime is produced when water is added to calcium oxide -> raise pH
Why does slaked lime not harden in water?
Water is a product
How did romans create stronger mortar and concretes?
Mixed lime with fine volcanic ash containing silica and alumina
When was Portland cement patented and who by?
1824 - Joseph Aspdin
What is ‘Portland cement’ made from?
Limestone + fine clay heated to 1200°C
What’s the dominate phase in Portland cement clinker?
alite (C3S)
What’s the difference between alite and belite?
Alite is more reactive
What are the two types of inorganic calcareous cements?
Hydraulic and Non-hydraulic
What are some hydraulic cements?
- PC
- Natural cements
- Calcium aluminate cements
- Pozzolanic cement
What are types of Portland cement?
- Ordinary PC
- Rapid hardening PC
- Low heat PC
- White PC
What are non-hydraulic cements?
- Lime
- Gypsum cements and plasters
How are the different Portland cements differentiated?
Using British/European Standard designation
What are cements currently standardised in terms of?
Raw materials
What are the typical ratios for Portland cement?
- 75% limestone/chalk
- 25% clay or shale
- 4% gypsum
What temp is rotary kiln usually at?
1450 degrees
What happens during calcination?
Calcium carbonate is converted into calcium oxide, which reacts with raw material constituents to form clinker minerals.
What’s an example of a cement plant?
Dunbar in Scotland
Why are raw materials crushed and ground prior to being fed into a cement kiln?
- To homogenise the feedstock
- To provide more surface area for reactions in the kiln
What are the different types of packing of cement?
- Ready-mix - onsite
- Precast - off site
- Retail
- Other/ not specified
What are SCMs?
SCMs are materials used as a partial replacement of portland cement to improve both fresh and hardened concrete properties
What are some common SCMs?
- Coal fly ash
- Calcined clay
- Limestone (+gypsum)
- Granulated blast furnace slag
How long does it take for the phases along a heat exchanger to form in a rotary kiln?
30 min
What is a phase?
A phase is a form of matter that is uniform throughout in chemical composition and physical state
How can phase composition of Portland cement clinker be estimated?
From the ‘Bogue composition’
Why is seawater not good for concrete?
Seawater which contains a high chloride concentration (not good for chloride- induced steel corrosion)
How does hydration occur?
Continues in the presence of sufficient amounts of water
How are hydration products 1 unit volume of cement?
They are less dense than clinker
What are the process of hydration products?
Setting and hardening
What is PC like in the initial setting?
Plastic workable paste
What is the PC like in the final set?
Stiff unworkable
What is the PC like during hardening?
Rigid solid gaining strength with time
What is the hydration of cement?
Hydration is the process by which Portland cement becomes a firm, hardened mass after the addition of water
What does unhydrated cement look like?
White
What is C-S-H?
Calcium-silicate hydrate (C-S-H) is the main binder in cement and concrete. It starts forming from the early stages of cement hydration and it progressively densifies as cement sets.
What are capillary pores?
- Space not filled by hydration products
What size are capillary pores usually?
0.05-10 micrometers
What are features of C-S-H?
- Enormous specific surface
- High porosity
- Low permeability
What is the Interfacial transition
zone?
- The interfacial transition zone (ITZ), which is known as the most important interface in concrete, is located between cement paste and aggregate in traditional concrete
- Weakness zone
What does it mean if the w/c = 0.3?
High performance
What if the w/c = 0.45
Structural purposes
How does porosity change?
With increasing degree of cement hydration
What are some binder types?
- Portland cement binder (CEM I)
- Reactive belite Portland cement binder (RBPC)
- Calcium sulfoaluminate cement binder (CSA(B))
- Blended (Portland) cement binders (CEM II-V, LC3)
What are pros of alkali -activated materials?
- Heat and fire resistant
- Superior thermal expansion properties
- Much faster setting process and strength development
What are cons of alkali -activated materials?
- May be prone to shrinkage and cracking
- Steel corrosion chemistry is different
- Health and safety (handling high pH solutions)
- Activator adds expense and environmental impacts
