Cements, in a general sense, are adhesive materials which are capable of bonding together fragments or particles of solid matter into a compact whole. This definition covers a wide variety of materials, but for engineering purposes it is generally restricted to ‘calcareous cements’, i.e. those which contain compounds of lime as their principal constituent or, sometimes, certain allied compounds of magnesium.
Mixing cement with water produces a plastic workable paste. For some time these characteristics of the paste remain virtually unchanged, and this period of time is known as the ‘dormant period’. At a certain stage, however, the paste begins to stiffen to such a degree that, although still soft, it becomes unworkable. This is known as the ‘initial set’ and the time required for the paste to reach this stage as the ‘initial setting time’. The ‘setting’ period follows, in which the paste continues to stiffen until a stage is reached when it may be regarded as a rigid solid. This is known as the ‘final set’ and the time required for the paste to reach this stage as the ‘final setting time’. The resulting solid is known as the ‘hardened cement paste’ or, sometimes, as the ‘cement stone’. With time the hardened paste continues to harden and gain strength, a process known as ‘hardening’. The various stages of setting and hardening are shown in Figure 2.1.
Microscopic examination of the hardened cement paste reveals the CSH gel to be an undifferentiated amorphous mass in which unhydrated cement grains, hexagonal crystals of calcium hydroxide, and occasionally small numbers of hexagonal or cubic crystals of aluminate and of sulphoaluminate are embedded. Pores, either filled With water or empty, are also detectable in the mass. As already mentioned, this amorphous mass is a rigid gel, i.e. a solid made up of small particles of colloidal dimensions. The particles are mainly hydrates of calcium silicate with some aluminates and ferrites. The hydrates of the calcium silicate are poorly crystallised and their structure is characterised by a high degree of disorder. Accordingly, these hydrates are sometimes described as ‘crystallites’ or ‘quasi-crystallites’.1,2
In discussing the hydration process it was stated that setting and hardening of the cement paste is brought about by the formation of a CSH gel. The gel fills the space between the cement grains, bridges between them, and thereby causes stiffening of the paste and its subsequent hardening. The continued formation of the gel gradually fills the capillary pores, the porosity of the paste decreases, and its strength is increased. However, the mechanical strength of the paste and its stability in water require further discussion with respect to the nature of the bond between gel particles. Generally speaking, the strength of the paste may be attributed to cohesion forces (van der Waals forces) acting between the gel particles (secondary bonds), or to the intergrowing of the crystallites and the formation of chemical bonds (primary bonds) at their points of contact. In this context it is generally meant that a chemical bond is a solid to solid contact similar to that existing at the grain boundary of a polycrystalline material, where some of the atoms approach the spacing of the atoms in the crystals, and a close fit exists between the lattices of the neighbouring crystals. Such bonds could be formed during a crystallisation process accompanying a chemical reaction when the mobility of the atoms allows for a regular arrangement. The strength of a material characterised by such bonds (e.g. gypsum) is determined by the number of bonds per unit volume, the bond strength, and the strength of the crystals themselves.
It was stated earlier that the volume of the hydration products is smaller than the combined volumes of the reacting cement and water by approximately 25% of the water volume. Under normal conditions this reduction in the volume of the cement-water system increases the porosity of the paste and is not reflected in the bulk dimensions. A change in bulk dimensions may be caused by the presence of excessive free lime or magnesia in the cement or, as will be seen later (Chapter 6), due to chemical attack of aggressive solutions, etc. These types of volume change take place only under special conditions and involve chemical changes in the cement paste. The following discussion, however, is limited mostly to volume changes caused by physical factors such as external loading and changes in moisture content and temperature, and involve no chemical changes. An exception is carbonation shrinkage which is included in this discussion.
Hardened cement paste may be attacked either by a process of dissolution or by chemical transformation or by both at the same time. The intensity of the attack depends on the specific properties of the aggressive agent, its concentration, the presence of other ions in the solution, etc. Ambient conditions such as temperature and pressure, as well as the length of time and the nature of the contact (i.e. continuous or periodic) between the paste and the aggressive agent, also affect the intensity of the attack. Also, regardless of the specific nature of the agent, the intensity of the attack is determined to a considerable extent by the porosity of the paste. In a dense paste the attack is essentially limited to the surface proceeding with time to the inside. A porous paste, on the other hand, allows the aggressive solutions to penetrate it, and the attack takes place throughout the mass. Such an attack is, therefore, more intensive.
As explained in Chapter 1, Portland cement is a heterogeneous material comprising mainly alite, belite, tricalcium aluminate, and celite, which make up about 90% of the whole. It is to be expected, therefore, that the properties of the cement will be determined by the properties of the individual constituents and their relative content in the cement. The properties of the individual constituents were summarised earlier in Table 1.4. In this chapter, the effect of these properties on the properties of the cement is considered, and the use of this effect to produce different types of Portland cement is discussed. The effects of minor constituents and of the cement fineness, are also included in the discussion.
The application of Griffith’s theory to explain failure in hardened cement paste is discussed in Chapter 4. Failure due to external loading is brought about by the spontaneous growth of the critical crack when the strain energy release rate equals the maximum rate of energy requirement. At this point the rate of energy release is known as the ‘critical strain energy release rate’, Gc, and is, apparently, a characteristic property of the material.
Concrete may be regarded as a composite two-phase material in which the discontinuous phase is the aggregate particle and the matrix phase is the hardened cement paste. Volume changes in the hardened paste were discussed in some detail in Chapter 5 and the following deals with the corresponding changes in concrete. These changes are discussed by considering the effect of the added aggregate and its properties on volume changes of the paste.
The ability of concrete to withstand the damaging effects of the environment and of its service conditions without deterioration for a long period of time is referred to as its ‘durability’. Clearly the durability of concrete is of prime importance in engineering applications, and it is not surprising therefore that this subject has been widely discussed in a number of publications.1-5
In previous chapters the properties of concrete were discussed with respect to composition and the properties of the individual constituents. The production of concrete, however, involves various stages, and the treatment of the concrete at each stage affects its properties and determines its quality. The effect of steam-curing and autoclave treatment, for example, was discussed in some detail. Nevertheless, the discussion was incomplete because the effects of the specific technical factors involved were not considered. In fact, none of the technical factors associated with concrete production have yet been discussed in relation to their possible effects on properties of concrete. Such a discussion is presented here. It is assumed that the reader is familiar with these factors and concrete production is, therefore, not treated here. In any case, relevant information can be found elsewhere.1,2
