We have seen that, at any stage of hydration, the HCP consists of:
- a residue of unhydrated cement, at the centre of the original grains
- the hydrates (the gel), chiefly calcium silicates (C-S-H) but also some calcium aluminates, sulphoaluminates and ferrites, which have a complex fibrous form and contain the gel pores, which are between 0.5 and 5 nm wide
- crystals of portlandite (Ca(OH)2)
- the unfilled residues of the spaces between the cement grains – the capillary pores, between about 5 nm and 10 µmm wide.
We should add that the paste will also contain a varying number of larger air voids, from about 5 µmm upwards, which have become entrapped in the paste during mixing and have not subsequently been expelled during placing and compaction.The significant strength of HCP derives from van der Waals type bonds between the hydrate fibres.
Although each individual bond is relatively weak, the integrated effect over the enormous surface area is considerable. The unhydrated cement is in itself strong and its presence is not detrimental to overall strength, and it can even be beneficial since it is exposed if the paste or concrete is subsequently cracked or fractured and can therefore form new hydrates to seal the crack and restore some structural integrity provided, of course, some water is present.
No other common structural materials have this self-healing property. For any particular cement, the compressive strength of specimens stored at constant temperature and humidity increases with age and decreasing water:cement ratio; Fig. 1 shows typical behaviour.
The change with age reflects the progress in hydration reactions, i.e. the degree of hydration. At 28 days (a typical testing age when comparing cements) the reactions are about 90% complete for a typical Portland cement.
We should also note that the strength continues to increase at water:cement ratios below 0.38, even though there is an increasing volume of unhydrated cement in the ‘end state’.
This is direct evidence that unhydrated cement is not detrimental to strength – it is the quality of the hydrates that is the governing factor (there are, however, lower practical limits to the water:cement ratio.
We have seen that both the size and volume of the capillary pores are also influenced by age and water:cement ratio and it is therefore not surprising that the strength and porosity are closely linked.
In simple terms: less porosity (due to either increasing age or lower water:cement ratio or both), means higher strength. The relationship between the two was shown by Powers (1958) to be of the form
σ = k(1 – P)3
where k is a constant, σ = compressive strength and P = porosity = pore volume/total paste volume.
Note that in this expression the porosity is raised to power three, showing its great significance. Powers’ experiments were on ‘normally’ cured pastes, i.e. kept in water at ambient temperature and pressure, with variations in porosity obtained by varying the water:cement ratio.
This resulted in total (capillary plus gel) porosities ranging from about 25 to 50%. Porosities down to about 2% were obtained by Roy and Gouda (1975) by curing pastes with water:cement ratios down to 0.093 at higher temperatures (up to 250°C) and pressures (up to 350 MPa).
Figure 2 shows that at these very low porosities they achieved compressive strengths of more than 600 MPa, Powers’ results being consistent with their overall relationship of the form
σ = A log(P/Pcrit)
where A is a constant and Pcrit is a critical porosity giving zero strength, shown by Fig. 2 to be about 55%.
The size of the pores has also been shown to be an important factor. Birchall et al. (1981) reduced the volume of the larger pores (greater than about 15 µm diameter) by incorporating a polymer in pastes of water:cement ratios of about 0.2, and curing initially under pressure.
The resulting ‘macrodefect free’ (MDF) cement had compressive strengths of 200 MPa and above, with flexural strengths of 70 MPa, a much higher fraction of compressive strength than in ‘normal’ pastes or concrete.
Clearly, the extremes of low porosity and high strength cannot be achieved in concretes produced on a large scale by conventional civil engineering practice, but results such as those shown in Fig. 2 are useful per se in helping to understand the behaviour of HCP. Porosity is also a significant factor influencing the durability of concrete.
Water in Hardened Cement Paste and Drying Shrinkage
The large surface areas in the gel give the HCP a considerable affinity for water, and make its overall dimensions water-sensitive, i.e. loss of water results in shrinkage, which is largely recoverable on regain of water.
Now we will consider the various ways in which the water is contained in the paste and how its loss can lead to shrinkage. The possible sites of the water are illustrated in the diagram of the gel structure shown in Fig. 3, and given in the following list:
- Water vapour. The larger voids may be only partially filled with water, and the remaining space will contain water vapour at a pressure in equilibrium with the relative humidity and temperature of the surrounding environment.
- Capillary water. This is located in the capillary and larger gel pores (wider than about 5 nm). Water in the voids larger than about 50 nm can be considered as free water, as it is beyond the reach of any surface forces, and its removal does not result in any overall shrinkage; however, the water in pores smaller than about 50 nm is subject to capillary tension forces, and its removal at normal temperatures and humidities may result in some shrinkage.
- Adsorbed water. This is the water that is close to the solid surfaces, and under the influence of surface attractive forces. Up to five molecular layers of water can be held, giving a maximum total thickness of about 1.3 nm. A large proportion of this water can be lost on drying to 30% relative humidity, and this loss is the main contributing factor to drying shrinkage.
- Interlayer water. This is the water in gel pores narrower than about 2.6 nm; it follows from (3) that such water will be under the influence of attractive forces from two surfaces, and will therefore be more strongly held. It can be removed only by strong drying, for example, at elevated temperatures and/or relative humidities less than 10%, but its loss results in considerable shrinkage, the van der Waals forces being able to pull the solid surfaces closer together.
- Chemically combined water. This is the water that has combined with the fresh cement in the hydration reactions. This is not lost on drying, but is only evolved when the paste is decomposed by heating to high temperatures (in excess of 1000°C).
The above divisions should not be thought of as having distinct boundaries, but the removal of the water does become progressively more difficult as one proceeds down the list. An arbitrary but often useful division is sometimes made between evaporable and non-evaporable water.
There are a number of way of defining this, the simplest being that evaporable water is that lost on drying at 105°C. This encompasses all the water in (1) to (3) above, and some of (4).
The non-evaporable water includes the rest of (4) and all of (5); its amount expressed as a proportion of the total water content increases as hydration proceeds, and this can be used to assess the progress of the hydration reactions.