Class 6 cement Flashcards
what is lime, how is it produced and the chem reaction
Lime is CaO
produced by Calcining(heating) natural calcium carbonate CaCO3 at 700-900c
CaCO3(s) → CaO (s) + CO2 (g)
how is portlandite/hydrated or slaked lime formed + chem reaction
Quicklime is mixed with water
CaO (s) + H2O (l) → Ca(OH)2 (s)
what happens to slacked lime over time when exposed to air -chem reaction
it hardens slowly reacting with CO2
Ca(OH)2 (s) + CO2 (g) → CaCO3 (s) + H2O (l)
slow reaction due to low CO2 in air
what is the difference between hydraulic cements and non hydraulic cements
Hydraulic Cements – These can set and harden in the presence of water
Non-Hydraulic Cements – These do not harden underwater and require air to set.
what are the 3 main non-hydraulic cements
lime and gypsum cements and plasters
What is the hydration reaction of C₃S (Tricalcium Silicate)?
2C3S+6H→C3S2H3+3CH
Forms C-S-H (Calcium Silicate Hydrate) and Calcium Hydroxide (CH).
What is the hydration reaction of C₂S (Dicalcium Silicate)?
2C2S+4H→3.3(CaO).(SiO2).3.3(H2O)+0.7CH Also forms C-S-H but with less calcium hydroxide (CH).
What is the hydration reaction of C₃A (Tricalcium Aluminate) without sulfur addition?
C3A+6H→C3AH6
Forms Hydrogarnet (C₃AH₆).
What is the hydration reaction of C₄AF (Tetracalcium Aluminoferrite) without sulfur addition?
C4AF+2CH+10H→C3AH6 +C3FH6
Forms Hydrogarnet (C₃AH₆) and Ferrous Hydrogarnet (C₃FH₆).
What do the abbreviations C, S, A, F, Ŝ, H stand for in cement chemistry?
C = CaO (Calcium Oxide)
S = SiO₂ (Silicon Dioxide)
A = Al₂O₃ (Aluminium Oxide)
F = Fe₂O₃ (Iron Oxide)
Ŝ = SO₃ (Sulfur Trioxide, from gypsum)
H = H₂O (Water)
what does the relative humidity in pores of cement need to be for hydration to take place
80%+ but bellow 90% its slow
what is the affect of hydration products being less dense than clinker
when cement undergoes hydration, the solid products formed occupy a larger volume than the original dry cement particles. Specifically, 1 unit volume of cement produces approximately 2.2 unit volumes of hydration products
order the rate of hydration of C4AF,C3A,C3S and C2S from fastest to slowest and say roughly how long it takes
C4AF:
Hydrates almost completely from the start.
Reaches ~100% hydration early and stays constant.
Does not significantly contribute to strength.
C3:
Starts at ~80% hydration and continues hydrating slowly.
Reacts very quickly (even faster with gypsum).
Generates a lot of heat, influencing early setting.
C3S:
Starts at ~40% hydration and steadily increases.
Contributes most to early strength development in cement.
Reacts faster than C₂S.
C2S:
Hydrates very slowly in the first days.
Significant hydration after 100 days.
Contributes to long-term strength rather than early strength.
what is a capillary pore, size, what do they influence
the space not filled by hydration products
Typically 0.05 – 10 μm in ‘diameter’ (non-circular)
Influence most properties including strength and durability
They are especially important for durability since they determine the transport
properties in cement paste
what are the 4 main types of pores and their sizes and what do they affect
Gel Pores (~1-2 nm): Smallest and found within the C-S-H phases high porosity and low permeability
.
Capillary Pores (~10 nm - 10 µm): Larger pores that affect permeability.
Entrained Air Voids (~10 µm - 100 µm): Created intentionally using air-entraining admixtures to improve freeze-thaw resistance.
Entrapped Air Voids (~100 µm - 1 mm): Result from inadequate compaction, reducing strength.
what is the overall significance of pores
The pore structure in concrete determines its strength, durability, and permeability.
Smaller pores (gel pores) improve strength, while larger pores (capillary and air voids) can weaken the material if not properly controlled.
what is cement made of
Cement is the constituent of mortar and concrete binds the
aggregates together and makes the material set
