Sulfate Attack on Concrete - Chapter 2 pot

2 Chemistry and physics of

cement paste

2.1 CONCRETE COMPONENTS

Concrete is an inorganic composite material formed, in its simplest form,

from a simple reactive binder, an inert filler, and water. In reality, modern

concrete is a complex material typically made of a form of hydraulic cement,

fine and course aggregate, mineral and chemical admixtures, and mix water.

The structural properties of plain concrete depend primarily on the chemical

reactions between the cement, water and other mix constituents, as well as

on the spatial distribution and homogeneity of the concrete components.

The chemistry, structure, and mechanical performance of the products of

the hydration reactions in concrete are, in turn, influenced by the production

processes and the environmental conditions prevailing during the pro￾duction of concrete. Thus, in designing concrete for service in a specific

environment, not only the concrete materials per se, but also the processing

techniques and environments of use have to be taken into account. This fact

is sometimes neglected in engineering practice.

2.1.1 Hydraulic cements

Modern hydraulic cements, cements capable of developing and maintaining

their properties in moist environment, are based either on calcium alumin￾ates (calcium aluminate or high-alumina cements) or on calcium silicates

(Portland-clinker based cements). In this work, focus will be entirely on

Portland cements and their modifications.

Portland cements and other Portland clinker-based hydraulic cements are

produced by inter-grinding Portland cement clinker with limited amount of

calcium sulfate (gypsum, hemihydrate, anhydrite; industrial by-products)

and, often, with one or several mineral components such as granulated blast

furnace slag, natural or artificial pozzolan, and/or limestone. Cement clinker

is a precursor produced by heat treatment of a raw meal typically containing

sources of lime, silica, alumina and ferrite. The main reactive components of

© 2002 Jan Skalny, Jacques Marchand and Ivan Odler

cement clinker are calcium silicates, aluminates and ferrites, plus minor

components such as free oxides lime and periclase, and various alkali sulfates.

Table 2.1 summarizes some primary clinker components and their chemical

abbreviations. Note that the actual chemical compositions of many of the

listed compounds are much more complex (Taylor 1997).

Reaction of individual clinker minerals and other cement components

with mix water proceeds under given environmental conditions as a com￾plex set of interdependent reactions. It is not only the chemical com￾position of the anhydrous compounds present, but also their “reactivity”

and the composition of the liquid phase (pore solution) at any given

moment, that control the direction and kinetics of the concrete setting and

hardening. This “reactivity” depends, among other factors, on the crystal

structure of the individual compounds (concentration and form of crystal

defects) and on the temperature of hydration. Presence of chemical admix￾tures and reactivity of “inert” aggregate play an additional role. Typical

compositions of Portland cement, fly ash, slag, and microsilica are given in

Table 2.2.

2.1.2 Aggregates

Aggregate is the most voluminous component of concrete. Depending on

the desired concrete properties, primarily strength but also durability and

other properties, the mass of aggregate in concrete represents about 3.5 (for

Table 2.1 Clinker components: chemical and mineralogical names, oversimplified

chemical formulas#, and abbreviations*.

# For more accurate and detailed information, see Taylor (1997)

*Cement chemical abbreviations: C – CaO, S – SiO2, A – Al2O3, F – F2O3, M – MgO, K – K2O,

N – Na2O, S – SO3, C – CO2

Compound Chemical formula Abbreviations

Alite, tricalcium silicate Ca3SiO5 C3S

Belite, dicalcium silicate Ca2SiO4 β-C2S

Tricalcium aluminate Ca3Al2O6 C3A

Tetracalcium alumino-ferrite or

ferrite solid solution

Ca2(AlxFe1− x)2O5 C4AF, Fss

Free lime CaO C

Periclase, free magnesia MgO M

Arcanite K2SO4 KS

Thenardite Na2SO4 NS

Aphthitalite K3Na(SO4)4 K3NS4

Calcium langbeinite K2Ca2(SO4)3 KC2S3

Gypsum CaSO4·2H2O CSH2

Hemihydrate CaSO4 · 0.5H2O CSH0.5

Anhydrite CaSO4 CS

© 2002 Jan Skalny, Jacques Marchand and Ivan Odler