Concrete constituents

Question 1

What are hydraulic cements?

Answer 1

Fine mineral powders that react with water (hydration) to form stable, durable compounds that increase in strength over time. Main example: Portland cement.

Question 2

What are the main oxides in Portland cement and their sources?

Answer 2

CaO (63–69%): Limestone

SiO₂ (19–24%): Clay and shale

Al₂O₃ (4–7%): Fluxing agents. Clay and Shale

Fe₂O₃ (1–6%): — Fluxing agents. Clay and Shale

Question 3

What are the four compounds present in cement clinker?

Answer 3

C₃S (Tricalcium silicate, 60–73%)

C₂S (Dicalcium silicate, 8–30%)

C₃A (Tricalcium aluminate, 5–12%)

C₄AF (Tetracalcium aluminoferrite, 8–16%)

Question 4

What are the products of Portland cement hydration?

Answer 4

C-S-H gel (Calcium silicate hydrate): Provides strength.

Ca(OH)₂ (Calcium hydroxide): Increases pH, protects reinforcement.

Question 5

How does water-cement ratio (w/c) affect concrete?

Answer 5

High w/c: More capillary pores, weaker structure.

Low w/c: Denser gel, higher strength.

Question 6

What are Supplementary Cementitious Materials (SCMs)?

Answer 6

Fine powders (e.g., GGBS, fly ash, silica fume) with little cementing properties alone but enhance Portland cement mixtures.

Question 7

Why use SCMs in concrete?

Answer 7

Cost reduction

Environmental benefits (lower CO₂, waste reuse)

Technical benefits (improved durability, microstructure)

Question 8

What are potential drawbacks of SCMs?

Answer 8

Slower strength development.

Risk of carbonation-induced corrosion in reinforced concrete.

Question 9

What is the key reaction for C₃S hydration?

Answer 9

C₃S + 6H → C₃S₂H₃ + 3Ca(OH)₂
(Forms C-S-H gel and portlandite)