Durability of concrete and cement composites

Chemical degradation of concrete

Chemical attack on concrete is a rather complicated subject since the chemistry of concrete itself is complex and the material is used in a very wide variety of environments. Of the types of degradation that are associated primarily with chemical changes occurring within the hydrated cement matrix, sulfate attack in its various guises is probably the most widespread threat to concrete durability, but acid attack can also take place in concrete sewers, silos, dairies, etc. and bacteria can mediate sulfate and acid attack in various environments. This chapter presents a brief overview of these important forms of degradation.
Acid attack on Portland cement concrete is not unexpected, since all of the phases in cement paste are basic, and many of them (for example, calcium hydroxide) are readily dissolved by acids. Typically, acid attack leads to loss of binder and strength in concrete and eventually to loss of section. Unlike acid attack, sulfate attack commonly involves expansive reactions which fracture the concrete leading to ongoing degradation, loss of strength and function. Several different processes may be involved and the literature on this subject has grown remarkably in recent years, prompting one reviewer to describe the current situation as more than a little confused (Neville, 2004). Notwithstanding the confusion, the topic is of evident interest and, while recognising that overlaps exist and that other classifications might be preferred by some workers in this diverse field, we shall attempt to provide a simplified account here of the following three categories:

· sulfate attack involving expansive ettringite formation along with other reactions, in which the sulfate is introduced principally from the external environment
· thaumasite sulfate attack (TSA), a combined attack requiring both sulfate and carbonate sources
· delayed ettringite formation (DEF), sulfate attack arising from vigorous heat curing, in which the source of sulfate is internal to the concrete.

Apart from TSA, the usual mechanisms of sulfate attack have generally been thought to involve changes in the chemistry of the hydrated aluminate phases in the concrete (accompanied by other processes that are dependent on the nature of the cations introduced with the sulfate anions). TSA is the exception in attacking the calcium silicate hydrate (C-S-H). Thus many instances of sulfate attack might be mitigated in principle by the use of a siliceous cement (based on alite and belite) in which aluminate phases are not present. Such a cement, however, would be very expensive to manufacture since the aluminates and ferrites act as fluxes in the kiln, significantly lowering the cement burning temperature and time. Aluminates are also naturally present in most large scale silicate sources. Sulfate Resisting Portland Cements (SRPC) (BS 4027, 1996) with low aluminate levels are obtainable (for a small price premium) and resist sulfate attack on the hydrated aluminate phases, although they remain vulnerable to TSA and may be subject to attack by salts such as magnesium sulfate which can interact deleteriously not only with aluminates but also with other hydration products such as CH and C-S-H. Since the sulfate exposure conditions in different countries differ, selection of SRPC is a trade-off between performance and cost, and specifications remain as national standards, having not yet been incorporated into the common European cement standard (BS EN 197-1, 2000).

For a general introduction to cement chemistry the reader is referred to standard textbooks on concrete (Neville, 1995; Mindess et al., 2002) or cement (Bensted and Barnes, 2002; Bye, 1999; Hewlett, 1998; Taylor, 1997; Lea, 1970). Only the barest outline is given here. Cement clinker coming out of the kiln consists of:

· calcium silicates
 alite (impure C3S)
 belite (impure C2S)
· calcium aluminates
 impure C3A
 various ferrite phases approximated by C4AF but with a wide range of compositions and with extensive substitution.

All of these phases react with water at varying rates to form a range of hydrated silicates and aluminates along with calcium hydroxide. The C3A, when present at appreciably high levels, tends to cause flash set (i.e., rapid heat evolution and initial stiffening but suppression of subsequent hardening) unless low levels of sulfate are added to the cement to modify its hydration so avoiding this potential problem. Traditionally gypsum was added, which often partially dehydrated during grinding, and its role in set regulation of Portland cement-based systems has been considered elsewhere (Bensted, 2002a); now iron (II) sulfate is commonly used instead to reduce Cr(VI) in order to comply with EU directive 2003/53/EC (British Cement Association, 2005).

Table 4.1 describes the shorthand system often used to represent the oxides in cement chemistry and lists the various phases discussed in this chapter. For the purposes of discussion of sulfate attack, and assuming gypsum to be the sulfate source, we can simplify the hydration of the aluminate phase as follows at early ages when ettringite, C6As3H32, is formed:

To some extent the aluminium in ettringite is partially substituted by iron and the impure form is referred to as AFt, the `A’ and `F’ denoting the presence of aluminium and iron and the `t’ indicating `trisulfate’. At later stages of cement hydration, the gypsum runs out, and additional aluminate reacts with the ettringite to form C4AsH12 or AFm phases in which `m’ denotes `monosulfate’ (reaction 4.2). In addition to sulfate-bearing AFm, hydroxy (and carbonate) AFm phases such as C4(A,F)H13, hydrocalumite, may also be formed depending on the available anions. These can enter into solid solutions with the sulfate AFm phases.

C2S reacts similarly to C3S but less calcium hydroxide is formed. Many types of cement now also contain blended pozzolanic or latently hydraulic compounds such as fly ash, ground granulated blast furnace slag, microsilica or metakaolin, which hydrate alongside the cement phases to give a modified chemistry and microstructure. Most provide additional silica, reducing the amounts of free calcium hydroxide formed, and some also provide additional reactive alumina. In some cases (provided they have had sufficient curing to react) such blending agents can confer additional chemical durability, and this is discussed below for the different forms of chemical attack on concrete.

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