Materials Lecture 9

What are the four columns in a sieve analysis table?

Sieve Size, percentage of individual fraction retained, cumulative percentage retained, percentage passing

Nominal maximum aggregate size (NMAS)

One sieve greater than the first sieve to retain cumulatively more than 10% of aggregates

Maximum aggregate size (MAS)

One sieve greater than NMAS

Fineness modulus

obtained by adding the cumulative percentages by mass of a sample aggregare retained on each of a specific series of sieves and dividing the sum by 100

What is the range for a typical fineness modulus?

2.3-3.1

p_i

percent passing a sieve of size d_i

d_i

sieve size

D

maximum size of aggregate

n

=0.45, recommended by the Federal Highway Administration

What does cement do?

When hydrated, cement acts as a glue that binds aggregates

Raw materials of Portland cement

• calcareous materials

• argillaceous materials

Example of a calcareous material

CaO (lime)

Examples of argillaceous materials

• SiO₂ (silica)

• Al₂O₃ (alumina)

• Fe₂O₃ (iron oxide)

Production process of Portland Cement

• Heated to 2500~3500 F

• Clinker

• Cooled down

• grinded

• a small amount of gypsum in added to adjust setting time

• The role of alumina and iron oxide is to reduce production temperature

Traditional chemist: Al₂O₃

A

Traditional chemist: SiO₂

S

Traditional chemist: CaO₂

C

Traditional chemist: SO₃

S_

Traditional chemist: Fe₂O₃

F

Traditional chemist: H₂O

H

Tricalcium silicate

• Chemical formula: (3CaO)(SiO₂)

• Short formula: C₃S

• Weight Percentage: 45~60%

Dicalcium silicate

• Chemical formula: (2CaO)(SiO₂)

• Short formula: C₂S

• Weight Percentage: 15~30%

Tricalcium aluminate

• Chemical formula: (3CaO)(Al₂O₃)

• Short formula: C₃A

• Weight Percentage: 6~12%

Tetracalcium aluminoferrite

• Chemical formula: (4CaO)(Al₂O₃)(Fe₂O₃)

• Short formula: C₄AF

• Weight Percentage: 6~8%

Gypsum

• Chemical formula: (CaO)(SO₃)(2H₂O)

• Short formula: CS_H₂

• Weight Percentage: 5%

What are the most important ingredients of cement?

C₃S and C₂S

What is the key product that makes concrete strong?

C-S-H

Stage 1 of hydration reaction

• initial hydrolysis

• rapid release of heat when C₃S meets water

• lasts about 15 minutes

• Ca²⁺ and OH^- ions are quickly released into solution

• pH value rises to over 12 with a few mins

Stage 2 of hydration reaction

• induction period

• relative inactivity

• lasts about 2-4 hours

• Ca²⁺ and OH^- ions continue to be released into solution, until their concentrations reach critical values for next step (crystallization)

Stage 3 of hydration reaction

• acceleration

• hydrate reaction accelerate

• reaches maximum around 4~8 hours after mix

• C-S-H starts forming on the surface of C₃S particles

Vicat apparatus

• 1 mm needle is allowed to penetrate cement paste for 30 seconds

• repeated for every 15 mins

• the time for 1 in penetration is defined as initial set time

• final set time is defined when no penetration is visible

Stage 4 of hydration reaction

• deceleration

• hydrate reaction continues

• reaction slows down until it reaches a steady state within 18~36 hours after the mix

• Thickness of C-S-H increases, which forms a barrier for water to diffuse through

Stage 5 of hydration reaction

• steady state

• hydrate reaction continues

• reaction becomes even slower as the thickness of C-S-H increases

• hydration reaction approaches 100% asymptotically

• thickness of C-S-H increases, which forms a barrier for water to diffuse through

Ranking hydration speed

• C₃A> C₃A >C₄AF > C₂S

• fast hydration reaction of C₃A will lead to “flash setting” which can be avoided by adding Gypsum

Abram’s Law

• strength of concrete monotonically decreases with water-cement ratio

• most important discovery of concrete

What is the normal range for water/concrete ration for concrete

0.4~0.5, however for completion of hydration reaction is 0.22~0.25

Why is extra water needed for concrete?

• to ensure workability of concrete

• extra water will inevitability lower the strength (quality) of concrete

Why is stage 2 in hydration reaction relevant to concrete properities?

• this is why concrete remains in a plastic state for a few hours

• crucial for workability

• the end of stage 2 determines initial set

Why is stage 3 in hydration reaction relevant to concrete properities?

the beginning of stage 3 determines initial set time, while the end determines the final set time

Why is stage 4 in hydration reaction relevant to concrete properities?

this stage determines the rate of early strength gain

Why is stage 5 in hydration reaction relevant to concrete properities?

this stage determines the rate of later strength gain