7. Análisis y conclusiones
7.3. Análisis fase 3:
The chemical composition and the physical characteristics of Portland cement (the influence of mineral additives and of the use of blended cements will be discussed in another section of this chapter) have an influence on the hydration process and, therefore, on the durability of concrete exposed to freezing and thawing cycles. The influence of the cement is mainly related to the porosity of the cement paste, and involves two different mechanisms. The chemical composition and the physical characteristics of the cement influence the rate of hydration (and thus the total porosity for a given hydration period), and also the distribution of the hydration products in the capillary pores (and thus the size distribution of these pores). As explained in Chapters 2 and 3, the amount of freezable water in a cement paste depends directly on the total capillary porosity as well as on the size distribution of the capillary pores.
Although, in terms of ultimate (long term) strength, a slow rate of hydration is better, a rapid rate of hydration is often considered to be a very positive characteristic, because a good quality cement paste is then obtained in a relatively short period of time. This is particularly helpful when concrete is subjected to freezing and thawing cycles only a few days after casting, or after poor curing. The rate of hydration is mainly influenced by the chemical composition of the cement, and especially the relative proportions of tricalcium silicate (C3S) and dicalcium silicate (C2S), since C3S reacts more rapidly than C2S. The
fineness of cement also has an influence because, in pastes made with high fineness cements, the surface of the cement grains directly in contact with the mixing water is increased, which in turn increases the rate of formation of hydration products.
The influence of cement on the size distribution of capillary pores is related to the fineness of the cement. Cements with a higher fineness have a larger number of particles per unit mass, which results in the formation of a larger number of smaller capillary pores. Furthermore, the finer cement particles tend to subdivide the space between the larger particles into smaller pores. This purely physical phenomenon leads to a finer pore-size distribution, although the total pore volume is not significantly affected. A finer distribution yields a lower amount of freezable water, since the amount of ice that can form at a given temperature decreases with the size of the pores (Chapter 2). The permeability of the paste is also reduced, and this reduction has both a positive effect (since the ingress of moisture into the concrete from outside is more difficult), and a negative one (since the higher resistance to the internal flow of water increases the internal disruptive pressures caused by the forced movement of water from the capillaries to the air voids during the freezing process).
For properly air-entrained concretes of the type commonly used in practice (with water/ cement ratios ranging from approximately 0.40 to 0.50), it has not been possible, from the point of view of frost resistance, to establish any significant distinction between Portland cements of different types (according to ASTM Standard C-150) or cements of the same type produced by different companies (Tyler et al., 1951). This is not necessarily true in the case of de-icer salt scaling resistance, but the laboratory data that are available indicate
that the influence of the fineness and composition of Portland cements is generally not very significant (Rose et al., 1989; Fagerlund, 1986). For high-strength concretes made with very low water/cement ratios, however, the type of cement was found in certain investigations to have an influence on frost resistance. This topic will be discussed in a later section of this chapter.
In the last decade, the use of limestone fillers as partial replacement for clinker in the manufacture of Portand cement has increased significantly. These fillers, generally used in small quantities (less than 5% of the total cement mass), are mainly composed of particles with a mean diameter similar to or slightly smaller than that of the cement grains. These particles can be considered chemically inert, and their action is mainly physical: they act as nucleation sites for the hydration products and a more uniform microstructure with a slightly refined pore structure is obtained. It is generally considered that limestone fillers, when they are used in small amounts, do not significantly influence the frost resistance of concrete, even if this particular aspect has never been rigorously investigated. When they are used in higher amounts, although the frost resistance of properly air-entrained concrete of sufficient strength does not seem to be affected seriously, the de-icer salt scaling resistance could be significantly reduced.
The production and the stability of the air-void system in fresh concrete can also be influenced by the composition and the characteristics of the cement. Soluble alkalies in the cement, for instance, can help to stabilize the air voids produced during the mixing process. On the other hand, compatibility problems between the cement and the admixtures can occur. These phenomena and others related to the influence of cement on air-void production and stability are discussed in Chapter 6.
Water
Apart from the influence that it can have on air entrainment in certain cases (Chapter 6), it is generally assumed that drinking water has no significant influence on the frost durability of concrete. The use of salt water is not allowed by most national standards but if, for special reasons, this type of water is to be used to make concrete exposed to a severe freezing and thawing environment, tests should be carried out to ensure that the dissolved salts will not have any deleterious effect on frost resistance.
Coarse and Fine Aggregates
Coarse aggregates, as previously mentioned, can have a very large influence on the frost resistance of concrete. Fine aggregates, however, are not normally frost susceptible, except for a few rare cases where they contain substances with an exceptionally poor frost behaviour. The good durability of fine aggregates is simply due to their small size which is almost always lower than the critical size beyond which the internal pressures caused by freezing water exceed the tensile strength of the aggregate (this phenomenon is described in detail in section 5.3.1).
The mechanisms of air entrainment are not normally affected by the coarse aggregate. Although in most cases the mineral composition of the fine aggregate has no great importance, its size grading can sometimes influence the mechanisms of air entrainment (this feature is
discussed in Chapter 6). In certain rare cases of improperly designed concrete mixtures, a poor choice of gradings (for both the fine and the coarse aggregates), or an inadequate ratio between the fine and the coarse aggregates, can indirectly have a detrimental effect on the frost resistance of concrete by causing segregation or excessive bleeding.