Developments In Binders + Hydration

Question 1

State the range of resolution of a transmitted light microscope

Answer 1

0.5-1000 micrometers

Question 2

State the range of an SEM (scanning electron microscope)

Answer 2

300A-100 micrometers

Question 3

Rank these microscopy techniques in order of highest resolution:

1. SEM (scanning electron microscope)
2. HRTEM (high resolution transmission electron microscope)
3. Eye
4. Light Microscope
5. CTEM (conventional transmission electron microscope)

Answer 3

2,5,1,4,3

Question 4

How does a TEM produce an image of the chemical composition of C-S-H

Answer 4

A primary electron beam is fired at the specimen which causes x-rays to be produced, these can then be used to form an image.

Question 5

At any stage of hydration, HCP is made up of different proportions of…?

Answer 5

Unhydrated cement
Hydration products
Voids

Question 6

Name 3 types of voids in HCP

Answer 6

Capillary pores
Gel pores
Entrapped air voids

Question 7

State the typical pore size range of a gel pore

Answer 7

1-10nm

Question 8

State the typical pore size range of a capillary pore

Answer 8

10-100nm

Question 9

State the typical size of a cement particle

Answer 9

20000nm

Question 10

Define what capillary pores are and where they are found

Answer 10

The voids that remain as a result of originally water-filled pores not being filled with hydration products.
They are found in between the interlocking fibres of outer product C-S-H.

Question 11

State how capillary pore size is affected by w/c ratio and the degree of hydration

Answer 11

A lower w/c ratio means less original water filled pores, so a greater portion of the pores will be filled with hydration product.
The more hydration that takes place, the more outer hydration product is produced and fills the water pores.

Question 12

State how w/c ratio affects strength with regards to porosity

Answer 12

Higher w/c ratio, more capillary pores.
As porosity increases, strength decreases.
So higher w/c means lower strength

Question 13

State how continued hydration affects strength and permeability

Answer 13

Capillary pores are progressively filled by hydration products. Volume of capillary pores decreases, volume of gel pores increases.

Question 14

State the average Ca/Si ratio in C-S-H and how it varies with age

Answer 14

1.7-1.8, at all ages. (Use 1.7)

Question 15

Name the 4 most important CRM’s

Answer 15

Silica fume (sf), pulverised fuel ash (pfa), ground granulated blast-furnace slag (ggbs), metakaolin (mk)

Question 16

How is C-S-H produced from the reaction of the CRM structure and cement pore solution? What kind of reaction is it?

Answer 16

The SiO2 or SiO2-Al2O3 part of the CRM structure is attacked by OH- ions from the cement pore solution. The OH- ions attach to silicon or aluminium atoms, breaking the bonds between them and oxygen atoms. The Si-OH groups react with more OH- ions leaving negatively charged oxygens. This alkali-silica gel reacts with Ca2+ ions, from the Alite reaction, forming C-S-H.

Question 17

How is the content of CH affected by the addition of CRM’s

Answer 17

The CH content is reduced as it is consumed by the reaction of the CRM

Question 18

How is the chemical composition of C-S-H affected by CRM’s

Answer 18

As the levels of CRM’s increase, the Al/Ca or Si/Ca ratios increase as the levels of Ca reduce because it reacts with the alkali-silica gel.

Question 19

How is the nanostructure of C-S-H affected by the change in composition of the C-S-H. How can this lead to the problem of alkali-reactive aggregates.

Answer 19

Aluminium substitutes for silicon in the bridging tetrahedra only. The substitution of Al3+ for Si4+ ions result in an extra negative charge that must be balanced, this can be met by alkali ions (Na+ or K+)

Question 20

How does a change the change in nano structure of C-S-H, due to the addition of CRMs, affect the morphology of the C-S-H?

Answer 20

As the Al/Ca or Si/Ca ratios increase, the morphology goes from fibrillar to foil-like.

Question 21

How does the morphology of outer product (OP) C-S-H affect the durability of concrete.

Answer 21

Foil-like morphology, present in Op C-S-H with high Al/Ca or Si/Ca ratios, fills space with a finely distributed, poorly-interconnected porosity that accounts for the reduced chloride diffusion rates and reduced permeability, therefore enhancing the durability. (CRM must be allowed to react to affect the morphology so good curing needed)

Question 22

What is a CAC and what is the main cementing phase present in them?

Answer 22

Calcium Aluminate Cements are a cement made by fusing a mixture of limestone and bauxite. The main cementing phase is monocalcium aluminate (CA).

Question 23

How does the setting and strength of CAC’s compare to CEM1’s?

Answer 23

CAC sets in a similar time to CEM1 but develops strength more rapidly. The 1 day strength of a CAC is greater than the 28 day strength of a CEM1

Question 24

How do the hydrate phases that form on the hydration of CA vary with temperature and time? (Look at notes for full reactions, missing water etc)

Answer 24

Low temp(<10degrees)-only CAH10 and C-A-H formed
Slightly higher temp-C2AH8 is formed as well or instead of CAH10
Above 27 degrees-CAH10 and C2AH6 convert to C3AH6 and AH3
The products will convert to the other phases if the temp increases after initial reaction has happened.
The products will always convert regardless of temp due to time.

Question 25

How do the CAC conversions decrease strength. (Use conversion of CAH10 to C3AH6 example)

Answer 25

C3AH6 is more dense than CAH10 or C2AH8, so the volume of solids in the cement paste is halved. This increases porosity and permeability causing a loss in compressive strength.

Question 26

How do the CAC reactions increase strength after a long time?

Answer 26

The increased porosity caused by the conversion reactions initially decreases strength. But the new space (pores) can be filled with products from the hydration of the remaining CA leading to an increase in strength.