Part 4. Hydration of Portland Cement
Chemistry of Hydration - It is assumed that each compound hydrates independently of others in Portland cement. This is not completely true because interaction between hydrating compounds will affect the mix.
Calcium Silicates - The hydration reaction of the two calcium silicates, which make up the largest percent of Portland cement, are similar.
Tricalcium Aluminate - Hydration of C3A occurs with sulfate ions supplied by dissolved gypsum. The result of the reaction is calcium sulfoaluminate hydrate, called "ettringite" after a naturally occurring mineral.
Ferrite Phase (C4AF) forms the same hydration products as C3A, with or without gypsum. The reaction is slow and is decreased further by gypsum. If the iron oxide content is increased, the reaction is slower.
Properties of the Hydration Products
Some general comments on the properties of hydration products affecting the overall behavior of the cement.
C-S-H, calcium silicate hydrate -- very poor crystallinity; the exact chemical compound is variable. The ratio of C/S varies between 1.5 and 2.0 and depends on many factors; temperature, w/c ratio, impurities, etc. Likewise, measures of the water content vary considerably.
Because of the poor crystallinity, C-S-H develops very small irregular particles and consequently a very high surface area. In general, the surface area of the hydrated cement is about 1000 times larger than the unhdyrated cement. Therefore, the increase in surface area greatly influences physical properties of the C-S-H hydrate.
Considerable work has been done in modeling the structural components of C-S-H, with much disagreement among scientists. C-S-H is considered a layer structure composed of calcium silicate sheets randomly connected by strong ionic-covalent bonds. The remainder of the interlayer space is classified as: 1) capillary pores, relatively large openings where water can form menisci; 2) micropore, smaller spaces where water cannot form menisci. The water forces the layers apart by exerting a disjoining pressure. This pressure decreases with lower water content; 3) interlayer space, layers are close enough that the trapped water bonds the sheets together by van der Waal forces.
There are three accepted models for the C-S-H structure:
Calcium Hydroxide -- a well understood hexagonal crystalline material. Crystals are much larger than C-S-H particles and are sometimes visible to the naked eye.
Calcium Sulfoaluminate (ettringite) -- These hexagonally-shaped prism crystals are considerably longer than CH crystals. Large clusters of ettringite needles may be visible in concrete affected by sulfate attack. Monosulfoaluminate tends to form very thin, hexagonal plates.
Microstructure of Hydrated Cement Pastes
The development of cement microstructure relates to the five chemical stages described earlier in this chapter.
C-S-H -- the largest component of the cement paste (50-70%) and is the most important component in the hydration process. The amount of C-S-H coating on a C3S grain is very small during stage 2 of hydration and increases rapidly in stage 3. The spines of the forming C-S-H radiate outward from each grain with the bulk of the material below the spines. As the C-S-H hydrates further, the coating thickness grows forcing the outward spines of adjacent particles to interlock to form solid bonds. As hydration continues the intermeshed spines contribute to an increase in the undercoating of C-S-H growth. The effect is to bond the cement grains together with the C-S-H coating.
CH -- constitutes 20-25% of the cement volume. In the acceleration stage, CH grows in the capillary pore space. CH will only grow in free space; on encountering another CH crystal it will grow in another direction; also it will grow completely around a hydrating cement grain. The latter effect gives the CH a larger apparent volume in cement pastes than it would have as a pure crystal.
Calcium Sulfoaluminate -- a small component of cement pastes (10-15%) having little effect on microstructure. Young spiny ettringite crystals grow into capillary space and later convert to flat monosulfoaluminate crystals. There will be unhydrated residues in the cement paste, mainly caused by calcium hydroxide, even in very matured hydrated pastes.
Porosity -- a major component of microstructure which will influence paste properties. Pore size distribution is difficult to measure. Many tests require drying, which affects the pore structure. There are two classifications of pore sizes:
Properties of Hydrated Cement Pastes
Hydration products have lower specific gravities and larger specific volumes than their parent cement compounds. Therefore, every hydration reaction is accompanied by an increase in solid volume.
Volume change is directly related to porosity. It is possible to calculate pore space by measuring the loss of evaporable water and nonevaporable water. The evaporable water describes water held in capillary and gel pores. This amount can be determined by oven drying a sample. Nonevaporable water is a measure involving the microstructure of the hydration product and is obtained from a paste heated to very high temperatures (1000 C0). T.C. Powers developed several empirical relationships for degree of hydration based on the amount the two types of water described above.
where a = degree of hydration and wn = nonevaporable water
where wg = gel water or evaporable water
Other relationships for volume of hydration products and porosity are available (see p. 105). Based on these, a minimum water/cement ratio relationship for complete hydration can be formed.
Therefore, for complete hydration, the w/c ratio should not fall below 0.42. However, complete hydration is not required for high ultimate strength. This means that paste with low w/c ratios will self-desiccate unless external water is added. Generally, this is not a problem in the field.
This web site was originally developed by
Charles Camp for his
CIVL 1101 class.