For an initial period after mixing, the fluidity or consistence of a paste of cement and water appears to remain relatively constant. In fact, a small but gradual loss of fluidity occurs, which can be partially recovered on remixing. At a time called the initial set, normally between two and four hours after mixing at normal temperatures, the mix starts to stiffen at a much faster rate.
However, it still has little or no strength, and hardening, or strength gain, does not start until after the final set, which occurs some hours later. The rate of gain of strength is rapid for the next few days, and continues, but at a steadily decreasing rate, for at least a few months.
Hydration of Portland Cement
Setting times are measured by somewhat arbitrary but standardised methods that involve measuring the depth of penetration of needles or plungers into the setting paste.
They do not mark a sudden change in the physical or chemical nature of the cement paste, but the initial set defines the time limit for handling and placing the concrete (and thus cement standards set a minimum time for this) and the final set indicates the start of the development of mechanical strength (and so standards set a maximum time for this).
The cement paste also gets noticeably warm, particularly during the setting and early hardening periods. In other words, the hydration reactions are exothermic. The amount of heat released is sufficient to raise the temperature to 100°C or more in a day or so if the paste is kept in adiabatic (zero heat loss) conditions.
However, measurement of the rate of heat output at constant temperature is a more useful direct indication of the rate of reaction, and Fig. 1 shows a typical plot of rate of heat output with time after mixing. Immediately on mixing, there is a high but very short peak (A), lasting only a few minutes or less.
This quickly declines to a low constant value for the so-called dormant period, when the cement is relatively inactive; this may last for up to two or three hours. The rate then starts to increase rapidly, at a time corresponding roughly to the initial set, and reaches a broad peak (B), some time after the final set.
The reactions then gradually slow down, with sometimes a short spurt after one or two days giving a further narrow peak (C). The hydration reactions causing this behaviour involve all four main compounds simultaneously.
The physical and chemical processes that result in the formation of the solid products of the hardened cement paste are complex, but the following simplified description, starting by considering the chemical reactions of each of the compounds individually, is nevertheless valuable.
The main contribution to the short intense first peak (A) is rehydration of calcium sulphate hemihydrate, which arises from the decomposition of the gypsum in the grinding process. Gypsum is reformed:
2CŠ(0.5H) + 3H → 2CŠ.2H
[H = H2O in shorthand form]
Additional contributions to this peak come from the hydration of the free lime, the heat of wetting, heat of solution and the initial reactions of the aluminate phases. The behaviour of the aluminates is particularly important in the early stages of hydration.
In a pure form, C3A reacts very violently with water, resulting in an immediate stiffening of the paste or a flash set. This must be prevented, which is why gypsum is added to the clinker. The initial reaction of the gypsum and C3A is
C3A + 3CŠ.2H + 26H → C3A.3CŠ.32H
The product, calcium sulphoaluminate, is also known as ettringite. This is insoluble and forms a protective layer on the C3A, thus preventing rapid reaction.
Usually about 5 – 6% of gypsum by weight of the cement is added and, as this is consumed, the ettringite reacts with the remaining C3A to give to calcium monosulphoaluminate, which has a lower sulphate content:
C3A.3CŠ.32H + 2C3A + 4H → 3(C3A.CŠ.12H)
Eventually, if all the gypsum is consumed before all the C3A, the direct hydrate, C3A.6H, is formed. This causes the short third peak C, which can occur some 2 or 3 days after hydration starts.
Whether this peak occurs at all depends on the relative amounts of gypsum and C3A in the unhydrated cement, and it follows that it tends to be a feature of high C3A content cements.
The C4AF phase reaction is similar to that of the C3A, also involving gypsum, but it is somewhat slower. The products have an imprecise and variable composition, but include high- and lowsulphate forms approximating to C3(A.F).3CŠ.32H and C3(A.F).CŠ.16H, respectively, i.e. similar to the C3A products. The reactions or products contribute little of significance to the overall behaviour of the cement.
As we have seen, the two calcium silicates C3S and C2S form the bulk of unhydrated cement, and it is their hydration products that give hardened cement most of its significant engineering properties such as strength and stiffness; their reactions and reaction rates therefore dominate the properties of the hardened cement paste (HCP) (and concrete) and are extremely important.
The C3S (or, more accurately, the alite) is the faster to react, producing a calcium silicate hydrate with a Ca:Si ratio of between 1.5 and 2 and calcium hydroxide (deposited in a crystalline form often referred to by its mineral name portlandite).
A somewhat simplified but convenient form of the reaction is:
2C3S + 6H → C3S2.3H + 3CH
Most of the main peak B in the heat evolution curve (Fig. 1) results from this reaction, and it is the calcium silicate hydrate (often simply referred to as C-S-H) that is responsible for the strength of the HCP.
The C2S (or, strictly, the belite) reacts much more slowly, but produces identical products, the reaction in its simplified form being:
2C2S + 4H → C3S2.3H + CH
This reaction contributes little heat in the timescales of Fig. 1, but it does make an important contribution to the long-term strength of HCP. The cumulative amounts of individual products formed over timescales a few days longer than those of Fig. 1 are shown in Fig. 2.
The dominance of the C-S-H after a day or so is readily apparent; this is accompanied by an increase in the amount of calcium hydroxide, which, together with some of the minor oxides, results in the HCP being highly alkaline, with a pH between of 12.5 and 13 and this alkalinity has a significant influence on some aspects of the durability of concrete construction.
The timescales and contributions of the reactions of the individual compounds to the development of the cement’s strength are shown in Fig. 3. This further emphasises the long-term nature of the strength-giving reactions of the calcium silicates, particularly of the C2S (or, more correctly, the belite).
In fact the reactions can never be regarded as complete, and the extent of their completeness is called the degree of hydration. In common with most chemical processes, increasing temperature accelerates all of the above reactions. With decreasing temperature, hydration will continue even below 0°C, but stops completely at about -10°C.
The physical processes occurring during hydration and the resulting microstructure of the hardened cement paste are equally, if not more, important than the chemical reactions, and numerous studies have been made of these by scanning, transmission and analytical electron microscopy.
Fig. 4 illustrates schematically the hydration of a single grain of cement in a large volume of water. The important features are:
- The processes take place at the solid–liquid interface, with solid products being deposited in the region around the diminishing core of unhydrated cement in each cement grain.
- The very early products form a surface layer on the cement grain, which acts a barrier to further reactions during the dormant period.
- The dormant period ends when this layer is broken down by either a build-up of internal pressure by osmosis, or by portlandite (Ca(OH)2), or both, enabling hydration to proceed more rapidly.
- The hydration products (known as the gel) consist of:
- needle-like crystals of ettringite, deposited early in the hydration
- an amorphous mass, mainly C-S-H, of small, irregular fibrous particles, some solid, some hollow and some flattened, typically 0.5 – 2 mm long and less than 0.2 mm diameter, with very high surface area estimated to be of the order of 200,000 m2 /kg, i.e. approaching a thousand times greater than the fresh cement grains from which it has been formed
- large hexagonal crystals of portlandite interspersed in the fibrous matrix.
- The gel contains many small gel pores, typically between 0.5 and 5 nm wide, in between the fibrous particles, and as hydration continues, new product is deposited within the existing matrix, decreasing the gel porosity.
- There is some difference in density and structure between the hydrates deposited within the original surface of the cement grain, known as inner product, and the less dense hydrates deposited in the original water-filled space, which contain more crystals of portlandite and aluminoferrite and are known as outer product.
- The rate of hydration reduces over a long period after peak B owing to the increased difficulty of diffusion of water through the hydration products to the unhydrated cement. It has been estimated that, for this reason, complete hydration is not possible for cement grains of more than 50 mm in diameter – even after many years there is a residual core of unhydrated cement.
- At complete hydration:
- the gel porosity reaches a lower limit of about 28%
- the volume of the products of hydration is little more than twice that of the unhydrated cement, but about two-thirds of the combined initial volume of the unhydrated cement and the water which it consumes.
In reality of course, hydration is occurring simultaneously in a mass of cement grains in the mix water, and so the hydration productions interact and compete for the same space.
An important and vital feature of hydration is that it occurs at a (nearly) constant overall volume, i.e. the mixture does not swell or contract and the HCP or concrete is the same size and shape when hardened as the mould in which was placed after mixing. Using this fact, and the measured properties of the fresh and hydrated materials it can be shown that:
- At a water:cement ratio of about 0.43, there is just sufficient mix water to hydrate all the cement and fill all of the resulting gel pores. Therefore at water:cement ratios lower than this, full hydration can never occur unless there is an available external source of water, for example if the cement or concrete is immersed in water. This is the condition of insufficient water, and the paste is subject to self-desiccation. In practice, in a sealed specimen the hydration will cease somewhat before all of the available water is consumed, and an initial water:cement ratio of about 0.5 is required for full hydration. Self-desiccation can also have other effects.
- At a water:cement ratio of about 0.38, the volume of hydration products, i.e. the gel, exactly matches that of the fresh cement and water. At values lower than this, hydration will be stopped before completion, even if an external source of water is available. This is called the condition of insufficient volume. At water:cement ratios higher than this there is an increasing amount of unfilled space between the original grains in the form of capillary pores, between about 5 nm and 10 mm wide, and so on average they are about a hundred times larger than the gel pores within the gel itself. Calculations give the relative volumes of unhydrated cement, gel and capillary pores at complete hydration shown in Fig. 5. In reality, for the reasons discussed above, hydration is never complete and therefore the volumes in Fig. 5 are never achieved, but they may be approached. However, at any stage of hydration, the volume of capillary pores will increase with the water:cement ratio.
The diagrams in Fig. 6 provide a visual illustration of this. These show idealised diagrams of the structure of two cement pastes with high and low water:cement ratios, say of the order of 0.8 and 0.4 respectively, on mixing and when mature, say after several months.
In the high water:cement ratio paste the grains are initially fairly widely dispersed in the mix water and, when mature, there is still a significant capillary pore volume. On the other hand, in the low water:cement ratio paste, the grains are initially much more closely packed, and the hydrates occupy a greater volume of the mature paste, which therefore has a greater volume of capillary pores (but which, if the water:cement ratio is low enough, may eventually disappear altogether).
Although it is important to distinguish between capillary and gel pores, in practice there is a near continuous distribution of pore sizes. Figure 7 shows typical measurements that illustrate this, and also provides direct evidence of the substantial reduction in both overall pore volume and pore size with reducing water:cement ratio for pastes of similar age, in this case 28 days.
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