1. CVL 2407 Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
Dr. Eng. Mustafa Maher Al-tayeb
2nd Semester 2013/2014
CVL 2407

2. Setting
This is the term used to describe the stiffening of the cement paste, although the definition of the stiffness of the paste which is considered set is somewhat arbitrary. Broadly speaking, setting refers to a change from a fluid to a rigid stage. Although, during setting, the paste acquires some strength, for practical purposes it is important to distinguish setting from hardening, which refers to the gain of strength of a set cement paste.

3. Setting
It seems that setting is caused by a selective hydration of cement compounds: the two first to react are C3A and C3S. The flash-setting properties of the former were mentioned in the preceding section but the addition of gypsum delays the formation of calcium aluminate hydrate, and it is thus C3S that sets first. Pure C3S mixed with water also exhibits an initial set but C2S stiffens in a more gradual manner.

4. Setting
It seems that setting is caused by a selective hydration of cement compounds: the two first to react are C3A and C3S. The flash-setting properties of the former were mentioned in the preceding section but the addition of gypsum delays the formation of calcium aluminate hydrate, and it is thus C3S that sets first. Pure C3S mixed with water also exhibits an initial set but C2S stiffens in a more gradual manner.

5. Setting
In a properly retarded cement, the framework of the hydrated cement paste is established by the calcium silicate hydrate, while, if C3A were allowed to set first, a rather porous calcium aluminate hydrate would form. The remaining cement compounds would then hydrate within this porous framework and the strength characteristics of the cement paste would be adversely affected.

6. Structure of hydrated cement
Many of the mechanical properties of hardened cement and concrete appear to depend not so much on the chemical composition of the hydrated cement as on the physical structure of the products of hydration, viewed at the level of colloidal products. For this reason it is important to have a good picture of the physical properties of the cement gel.

7. Structure of hydrated cement
Fresh cement paste is a plastic network of particles of cement in water but, once the paste has set, its apparent or gross volume remains approximately constant. At any stage of hydration, the hardened paste consists of crystallized hydrates of the various compounds, referred to collectively as gel (C-S-H), crystals of Ca(OH)2, some minor components, unhydrated cement, and the residue of the water-filled spaces in the fresh paste.

8. Structure of hydrated cement
The residue of the water-filled spaces (voids) in the fresh paste. These voids are called capillary pores but, within the gel itself, there exist interstitial voids, called gel pores. The nominal diameter of gel pores is about 3 nm while capillary pores are one or two orders of magnitude larger. There are thus, in hydrated paste, two distinct classes of pores represented diagrammatically in Figure.

9. Structure of hydrated cement
Solid dots represent gel particles; interstitial spaces are gel pores; spaces such as those  marked C are capillary pores.

10. Structure of hydrated cement
Because most of the products of hydration are colloidal (the ratio of calcium silicate hydrates to Ca(OH)2 being 7 : 2 by mass) during hydration the surface area of the solid phase increases enormously, and a large amount of free water becomes adsorbed on this surface.

11. Structure of hydrated cement
If no water movement to or from the cement paste is permitted, the reactions of hydration use up the water until too little is left to saturate the solid surfaces, and the relative humidity within the paste decreases. This is known as self-desiccation. Because gel can form only in water-filled space, self-desiccation leads to a lower hydration compared with a moist-cured paste. However, in self-desiccated pastes with water/cement ratios in excess of 0.5, the amount of mixing water is sufficient for hydration to proceed at the same rate as when moist-cured.

12. Volume of products of hydration
The gross space available for the products of hydration consists of the absolute volume of the dry cement together with the volume of water added to the mix. The small loss of water due to bleeding and the contraction of the paste while still plastic will be ignored at this stage. The water bound chemically by C3S and C2S was shown to be very approximately 24 and 21 percent of the mass of the two silicates, respectively. The corresponding amount for C3A and C4AF are 40 and 37 percent, respectively.

13. Volume of products of hydration
However this amounts are not accurate because our knowledge of stoichiometry of the products of hydration of cement is inadequate to state the amounts of water combined chemically. This water, determined under specified conditions is taken as 23 percent of the mass of anhydrous cement.

14. Volume of products of hydration
The specific gravity of the products of hydration of cement is such that they occupy a greater volume than the absolute volume of unhydrated cement but smaller than the sum of volumes of the dry cement and the non-evaporable water by approximately of the volume of the latter. An average value of specific gravity of the products of hydration (including pores in the densest structure possible) in a saturated state is 2.16.

15. Volume of products of hydration
Example
Let us consider the hydration of 100 g of cement. Taking the specific gravity of dry cement as 3.15, the absolute volume of unhydrated cement is 100/3.15 = 31.8 ml. The non-evaporable water is, as we have said, about 23 percent of the mass of cement, i.e. 23 ml. The solid products of hydration occupy a volume equal to the sum of volumes of anhydrous cement and water less of the volume of non-evaporable water, i.e.

16. Volume of products of hydration
× 100(1 – 0.254) = 48.9 ml.
Because the paste in this condition has a characteristic porosity of about 28 percent, the volume of gel water, wg, is given by
whence wg = 19.0 ml, and the volume of hydrated cement is = 67.9 ml.

17. Volume of products of hydration
Summarizing, we have:

18. Volume of products of hydration
It should be noted that the
hydration was assumed to take
place in a sealed test tube with
no water movement to or from
the system. The volumetric
changes are shown in Figures.
The ‘decrease in volume’ of 5.9
ml represents the empty capillary
space distributed throughout the
hydrated cement paste.

19. Volume of products of hydration
If the total amount of water been lower than about 42 ml, it would have been inadequate for full hydration as gel can form only when sufficient water is available both for the chemical reactions and for the filling of the gel pores being formed. The gel water, because it is held firmly, cannot move into the capillaries so that it is not available for hydration of the still unhydrated cement.

20. Volume of products of hydration
Thus, when hydration in a sealed specimen has progressed to a stage when the combined water has become about one-half of the original water content, no further hydration will take place. It follows also that full hydration in a sealed specimen is possible only when the mixing water is at least twice the water required for chemical reaction, i.e. the mix has a water/cement ratio of about 0.5 by mass.

21. Volume of products of hydration
If the actual water/cement ratio of the mix, allowing for bleeding, is less than about 0.42 by mass, complete hydration is not possible as the volume available is insufficient to accommodate all the products of hydration. It will be recalled that hydration can take place only in water within the capillaries.
For example , if we have a mix of 100 g of cement (31.8 ml) and 30 g of water, the water would suffice to hydrate x g of cement, given by the following calculations. Contraction in volume on hydration is:

22. Volume of products of hydration
0.23x × = x.
Volume occupied by solid products of hydration is:
Porosity is:
and total water is 0.23x + wg = 30. Hence, x = 71.5 g = 22.7 ml and wg = 13.5 ml. Thus, the volume of hydrated cement is
0.489 × = 48.5 ml.

23. Volume of products of hydration
The volume of unhydrated cement is 31.8 – 22.7 = 9.1 ml and the volume of empty capillaries is ( ) – ( ) = 4.2 ml.
If water is available from outside, some further cement can hydrate, its quantity being such that the products of hydration occupy 4.2 ml more than the volume of dry cement.
0.489x – x/3.15 = 0.171x
wg =0.19x
0.171x + wg = x x = 4.2
x = g =3.7 ml

24. Volume of products of hydration
In contrast to these compacts which had an extremely low water/cement ratio, if the water/cement ratio is higher than about 0.42 by mass, all the cement can hydrate but capillary pores will also be present. Some of the capillaries will contain excess water from the mix, others will fill by imbibing water from outside. Figures shows the relative volumes of unhydrated cement, products of hydration, and capillaries for mixes with different water/cement ratios.

25. Volume of products of hydration

26. Volume of products of hydration
As a more specific example, let us consider the hydration of a paste with a water/cement ratio of 0.475, sealed in a tube. Let the mass of dry cement be 126 g, which corresponds to 40 ml. The volume of water is then × 126 = 60 ml. These mix proportions are shown in the left-hand diagram of Figer, but in reality the cement and water are of course intermixed, the water forming a capillary system between the unhydrated cement particles.

27. Volume of products of hydration

28. Volume of products of hydration
Let us now consider the situation when the cement has hydrated fully. The non-evaporable water is 0.23 × 126 = 29.0 ml and the gel water is wg such that
whence the volume of gel water is 24.0 ml, and the volume of hydrated cement is 85.6 ml. There are thus 60 – ( ) = 7.0 ml of water left as capillary water in the paste. In addition, 100 – ( ) = 7.4 ml form empty capillaries.

29. Volume of products of hydration
If the cement paste had access to water during curing these capillaries would fill with imbibed water. This then is the situation at 100 percent hydration when the gel/space ratio is 0.856, as shown in the right-hand diagram of previous Figure. As a further illustration, the centre diagram shows the volumes of different components when only half the cement has hydrated. The gel/space ratio is then