BDCS Notes - Hydration

Hydration: chemical reaction between cement particles and water.

Hydration process occurs through two mechanisms:

1. Through-solution

2. Topochemical

Through-solution process involves:

1. Dissolution of anhydrous compounds into constituents

2. Formation of hydrates in solution

3. Precipitation of hydrates from the supersaturated solution

The through-solution process dominates the early part of hydration.

Topochemical hydration is a solid-state chemical reaction occurring at the surface of the cement particles.

The aluminates hydrate much faster than the silicates. Gypsum is used to slow down the rate of aluminate hydration. The balance of aluminate to sulfate determines the rate of setting.

The solidifying of paste begins 2 to 4 hours after the water to the cement. If there is an excess of both aluminate and sulfate ions, the workability stage may only last for 10 minutes and setting may occur in 1 to 2 hours. If the availability of aluminate ions is high, and sulfates low, either a quick set (10 to 45 minutes) or flash set (less than 10 minutes) can occur.

Chemical Reaction of Hydration:

Calcium silicates combine with water to form calcium-silicate-hydrate, C-S-H. The crystals begin to form a few hours after the water and cement are mixed. The calcium-to silicate ratio varies between 1.5 and 2.0.

Structure Development in Cement Paste

The sequential development of the structure in a cement paste is summarized by:

1. Initial C-S-H phase

2. Forming of gels

3. Initial set-development of weak skeleton

4. Final set-development of rigid skeleton

5. Hardening

In less than 10 minutes, the water becomes highly alkaline. As the cement particles hydrate, the volume of the cement particle reduces, increasing the space between the particles. During the early stages of hydration, weak bonds form. Further hydration stiffens the mix and begins locking the structure of the material in place. Final set occurs when the C-S-H phase has developed a rigid structure, all components of the paste lock into place and the spacing between grains increases as the grains are consumed by hydration. The cement paste continues hardening and gains strength as hydration continues. Hydration occurs as long as unhydrated cement particles and free water exists. The rate of hardening decreases with time.

Evaluation of Hydration Progress

Measure the following properties:

1. The heat of hydration

2. The amount of calcium hydroxide in the paste developed due to hydration

3. The specific gravity of the paste

4. The amount of chemically combined water

5. The amount of unhydrated cement paste using X-ray quantitative analysis

6. The strength of the hydrated paste, an indirect measurement

Voids in Hydrated Cement

Due to random growth in various crystals, voids are left in the paste structure (as cement hydrates). Concrete strength, durability, and volume stability are greatly influenced by voids.

Two types of voids are formed during hydration:

1. The interlayer hydration space

2. Capillary voids

Interlayer Hydration Space: Occurs between layers in the C-S-H. The space thickness is between 0.5 nm and 2.5 nm. This is too small to affect strength. It can affect the porosity of the paste. Water in the interparticle space is strongly held by hydrogen bonds, but can be removed when humidity is less than 11%, resulting in shrinkage.

Capillary Voids: The result of the hydrated cement paste having a lower bulk specific gravity than the cement particles. The amount and size of capillary voids depends on the initial separation of the cement particles, which is controlled by the ratio of water to cement paste. Voids should range from 10 nm to 50 nm. Anything greater than this will decrease strength and increase permeability.

Properties of Hydrated Cement

Evaluated with either cement paste (water and cement) or mortar (paste and sand).

Setting: Refers to stiffening of the cement paste or the change from a plastic state to a solid state. With setting comes strength, although this is not the same as hardening, which refers to the strength gain in a set cement paste.

Two levels describe setting: initial set and final set.

Vicat Test: Requires that a sample of cement paste be prepared, using the amount of water required for normal consistency according to a specified procedure. A needle is allowed to penetrate the paste for 30 seconds and the amount of pentration is measured. This process is repeated every 15 minutes until a penetration of 25 mm or less is obtained.

Gillmore Test: Requires a sample of cement paste of normal consistency be prepared. A Pat with a flat top is molded and the initial Gillmore needle is applied lightly to its surface. The process is repeated until the pat bears the force of the needle without appreciable indentation, and the elapsed time is recorded as the initial set time.

If the cement is exposed to humidity during storage, a false set might occur, in which the cement stiffens within a few minutes of being mixed. To solve this problem, the cement paste can be vigorously remixed without adding water in order to restore plasticity of the paste without losing strength.

Soundness: Refers to the ability of cement paste to retain its volume after setting.

Compressive Strength: The compressive strength of mortar is measured by preparing 50 mm cubes and subjecting them to compression. Mortar is prepared with cement, water, and standard sand. ASTM specifies minimum compressive strength standards.