Alkali - Aggregate Reaction

Question 1

4 stages of alkali aggregate reaction

Answer 1

1) Aggregates react with alkalis in cement (in pore fluids)
2) Forms hydroscopic gel
3) Gel takes on water - expands
4) Cracks

Question 2

What do you need for alkali aggregate reaction?

Answer 2

Alkali in pore fluids
Reactive silica in aggregate
Water

Question 3

What happens if there is excess alkali?

Answer 3

Expansion is limited

Question 4

What happens is there is excess silica?

Answer 4

All hydroxyls mopped up by reactive silica limiting expansion

Question 5

Sources of alkali (5)

Answer 5

PC - sodium oxide
Pozzolanic - PFA GGBS
Chemical admixtures - super plasticisers, water reducers, retarders
Water
Aggregates unlikely unless from marine environment

Question 6

Sources of silica (3)

Answer 6

Aggregates
PC - consumed by hydration reaction
Pozzolanic - PFA, GGBS. Majority consumed by reaction with PC

Question 7

Types of aggregate that are sources of silica (4)

Answer 7

Low - limestone
Normal - flint
High - greywake
V High - calcined flint

Question 8

Features of alkali -aggregate reaction (4)

Answer 8

Surface cracks 2- 3 years
Extrudations - silica gel at surface
Surface pop outs - local
Disintegration

Question 9

Factors for alkali - aggregate reaction (4)

Answer 9

Alkali - PC
Silica - aggregate type
Moisture -fuels reaction
Temp

Question 10

Combating alkali - aggregate reaction (4)

Answer 10

Limit alkali
Pozzolanic eg PFA - dilution effect
Limit moisture
Limit silica - avoids types of aggregates

Question 11

Tests for alkali aggregate reaction

Answer 11

Mortar bar test - assesses aggregate alkali silica reactivity
BS812123 38 degree moist air 52 weeks - measures expansion