T1-4: Structure of Hardened Cement Paste Flashcards
Name three types of microscopy techniques, in the order from the smallest resolution to the biggest resolution (and from the smallest working range to the biggest working range)
HRTEM (high resolution transmission electron microscopy)
CTEM (conventional transmission electron microscopy)
SEM (scanning electron microscopy)
How are electron microscopy images formed?
Using any of the signals that result from the interaction of the primary electron beam (PE) of the microscope with the specimen
From a backscattered electron (BSE) image of a polished steel-reinforced concrete, what do the following colours represent:
White
Medium grey
Light grey
Black
White = steel rebar
Medium grey = aggregate
Light grey = CH
Black = ‘large’ pores
For this backscattered electron image of polished hardened cement paste, what can be identified?
- The light sections show unreacted cement
- The pale grey sections show Calcium Hydroxide /CH/ Ca(OH)_2
- The darker grey sections show ‘other products’
- The black sections show ‘large’ pores
What is hardened cement paste made up of (at any stage of hydration)?
Unhydrated cement
Hydration products
Voids (porosity) - three types:
- Capillary pores
- Gel pores
- Entrapped air voids
What two things determine the relative proportions of the hardened cement paste?
How do they relate to pore size distribution?
The degree of hydration
- greater = finer pore size distribution
The water:cement ratio
- lower = finer pore size distribution
What are typical pore size ranges for:
a) capillary pores
b) gel pores
a) 10 - 100 nm
b) 1 - 10 nm
[NAQ] image of C-S-H in a hardened C3S paste, hydrated for 8 years at 20C
NB. Ip = inner product, Op = outer product
[NAQ] structure of hardened cement paste
What name is given to voids that remain as a result of the originally water-filled spaces not being filled by hydration products?
Capillary pores
What does capillary porosity control?
As porosity increases, what impact does this have on strength?
Strength and permeability
As porosity increases, strength decreases
- higher W/C = lower strength
What happens are capillary pores are progressively filled by hydration products (three stages)?
- The volume of capillary pores reduces
- The volume of gel pores increases
- Strength increases, permeability reduces
