Chapter 2
EARLY-AGE CONCRETE:
CHEMO-PORO-MECHANICS
When concrete sets, it turns from a slurry into a solid. The material thus develops its strength, stiffness and fracture properties. The time it takes for concrete to harden may take some 28-90 days; which is why a great deal of attention has to be given to nurture well the early-age behavior of this material. For what is at stake is the longevity of the material and the structure built with it. During the early minutes, hours and days that follow the setting, an impressive number of chemical reactions take place that all affect the development of mechanical properties. Many of the reactions that take place are exothermic, meaning that heat is released which -if it cannot escape- can entail temperature rises in massive concrete structures. Finally. the chemical reactions entail precipitation processes of solid matter in the water-filled space, which in turn may lead to eigenstress development in the material. The branch of materials science that deals with this early age behavior of concrete is chemo-poro-mechanics; a blend of physical chemistry and mechanics of materials.
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2.1
Hydration Mass and Volume Balance: Re- actants, Products and Hydration Degree
2.1.1 Hydration Reactants and Products: Stoichiom- etry
When cement and water react, hydration products are formed. Like all chemical reactions, they are described by some stoichiometric relations of the form:
nRi+ n2R2...+nNRN mP+m2P2+...+mMPM
(2.1)
where Ri=1,N and Pi=1,M stand for the reactants and the products respec- tively; and where ni=i,N and mj=1,M represent the stoichiometric coefficients of reactants and products. More specifically, if we let v; = -n; < 0 for reactants (i.e. those compounds which are consumed in the chemical reac tions), and ; = m; > 0 for products, the very meaning of the stoichiometric coefficients comes from the mass balance equation of the chemical reactions:
N+M
(2.2)
where M stand for the molar mass of compound k = 1,N+M (of dimen- sion [M] = M.N-1, where N stands for the base-dimension of amount of substance, associated with the unit mol, which is another way of number- ing things in units of 6.02214129 x 1023). The molar masses are listed in the Table of elements. The most important ones for concrete are the molar masses of the substances the most abundant on Earth: Water (MH,o = 2MH + Mo = 18.015 g/mol), Calcium (Mca = 40.078 g/mol), Silicon (Msi = 20.085 g/mol), and Aluminum (MAi = 26.982 g/mol). Because of this restriction to just a few --yet abundant-- elements, it is common practice in cement chemistry to regroup elements in functional units, which makes it easier to the complex chemical reactions. Beside water denoted by H - H2O, the two most important ones are:
C = CaO Calcium oxide, also known as quicklime or burnt lime, of mo
composition of limestone containing carbonate CaCO3 by heating the
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blended dry feed
precalcined feed
clinkering
900'Capprox.
Figure 2.1: Most Portland cement is made in a rotary kiln. Basically, this is a long cylinder rotating abou