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.

  • The principle products are: 1) calcium silicate hydrate, poorly crystalline material of extremely small particle size and 2) calcium hydroxide, a crystalline material. The reaction can be measured by the rate of heat generation.
  • Stage 1 - Rapid heat generation (15 min.) -- on mixing with water, calcium and hydroxide ions are released from the surface of the C3S; pH rises to a very alkaline solution. When the calcium and hydroxide reach critical concentrations, crystallization of CH and C-S-H begins. Early chemical reactions are temperature dependent.
  • Stage 2 - Dormant period - causes cement to remain plastic (2-4 hours). The reaction slows. CH crystallizes from the solution, C-S-H develops on the surface of the C3S and forms a coating. As the thickness increases, the time it takes water to penetrate the coating increases, thus the rate of reaction becomes diffusion controlled. C<small><small>2</small></small>S hydrates at a slower rate because it is a less reactive compound.
  • Stage 3 - Acceleration period - Critical concentration of ions is reached and silicate hydrates rapidly, maximum rate occurs at this stage. Final set has passed and early hardening begins (4-8 hours).
  • Stage 4 - Deceleration - rate of reaction slows; completely diffusion dependent reaction.
  • Stage 5 - Steady state - constant rate of reaction (12-24 hours). Temperature has little effect on hydration at this point.

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.

  • If the supply of sulfate from the gypsum is exhausted before the C3A is completely hydrated, a second reaction can occur. The product of this reaction is monosulfoaluminate. This reaction may occur before the formation of the ettringite if the reaction of C3A and the sulfate ions is faster than the gypsum will allow.
  • The ettringite decreases the reaction by forming a diffusion coating around the C3A similar to the reaction of C3S. The coating can be broken down by the conversion to monosulfoaluminate.
  • If the monosulfoaluminate is exposed to another source of sulfate ions, the a new reaction will occur forming more ettringite. This new formation causes volume to increase and leads to tensile cracking. This tendency is the basis for sulfate attack of Portland cements.
  • In the absence of sulfates, C3A reacts with water to form two unstable calcium hydrates which later convert to hydrogarnet. This is the same process found in HAC. A pure C3A paste will not develop significant strength.

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.

  • Experience has shown cements low in C3A and high in C4AF are sulfate resistant. The conversion from ettringite to monosulfoaluminate is inhabited by the presence of the iron component.
  • ** The rate of hydration is on the order of C3A > C3S > C4AF > C2S. Reactions for even identical compounds may vary due to: 1) fineness, 2) rate of cooling of clinker, and 3) impurities.

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:

  1. Powers-Brunauer -- a chemical structure model based on the layered claylike configuration. The sheets are randomly arranged and contain absorbed water on their surfaces. Water can penetrate the interlayer, micropore region. Interlayer water can be permanently removed with strong drying.
  2. Feldman and Sereda -- a model composed of a completely random array of single layers forming irregular interlayer space. Water can move in and out of the interlayer space even after drying.
  3. Munich -- a physical model where C-S-H is considered a three-dimensional arrangement of colloidal particles. The chemical nature of the model is secondary. Van der Walls' forces bind surfaces together, but strong covalent bonds are more common. Water is attracted to the solid particles resulting in disjoining pressures which reduce particle interaction.

*** In all these models, water affects the structure of the C-S-H. As water is removed from capillaries, a resultant compressive stress is induced. Loss of interlayer water affects particle or sheet bonding.

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:

  1. Capillary pore -- space formed between hydrating gains.
  2. Gel pores -- very small spaces in the C-S-H coating. Constitutes the bulk of porosity in a cement paste.

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.

  • Calcium Silicates -- hydration of these materials is not accompanied by an increase in volume. Recall, these crystals will only occupy free space. If this space is filled, the growth or hydration will stop.

  • Calcium Aluminate -- The hydration product of this material (ettringite) will continue to form even when a solid surface is encountered. Since there is no volume in which the crystal can grow, internal pressures develop.

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.

wn = 0.24a g/g of original cement

where a = degree of hydration and wn = nonevaporable water

wg = 0.18a g/g of original cement

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.

wmin = ( wn + wg ) g/g of original cement

(w/c)min = 0.42a

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.

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