This paper describes the design of cement-based coatings which have a high diffusion resistance to gases such as Carbon Dioxide and Oxygen, as well as Chloride Ions.

They are also impermeable to water molecules even under a 10 bar pressure, although they will allow the diffusion of vapour. By reinforcing with a variety of fibres and meshes, tough protective membranes can be formed with all the similarities to concrete.

The incorporation of Migratory Corrosion Inhibitors also enhances their abilities to retard the corrosion mechanisms of the steel reinforcement by providing a protective film having diffused through the pore system in the vapour phase.

• Resins (a) Polyester
  a. Epoxy

• Polymers (a) Styrene Butadiene Rubber
  a. Acrylic
  b. Modified Acrylic
  

• Pozzolans (a) Microsilica
  a. Pulverised Fuel Ash
  b. Ground Granulated Blast Furnace Slag
  c. Metakaolin

• Migrating Corrosion Inhibitors (a) Amino Carboxylates

• Combinations

The following factors will significantly affect the durability of cement based products:-

1. The mix design.

2. The cement content and type.

3. The water/cement ratio.

4. The degree and type of compaction.

5. The quality and type of aggregates.

6. Curing.

7. The use of chemical admixtures.

8. The use of polymers.

9. The use of pozzolanic materials.

For a cement matrix having a coherent pore system (porosity) to become permeable, there must be some form of interconnecting system of tubes or canals. Obviously, discrete sealed pores will not lead to either air or water permeability (1) (see Fig. 1). Also the size and width of pores have an influence on permeability, because the narrower the pores, the higher must be the pressure to force a liquid (generally water) through the pore system. It is important, therefore, to distinguish between air pores, capillary pores and gel pores.

Small air pores are usually artificially entrained into the matrix by chemical admixtures in order to increase workability and enhance resistance to the effects of frost. Large, irregular air pores are caused by poor placing and compaction of concrete.

Capillary and gel pores are as a result of the hydration process of cement and water.

In fact, the porosity and permeability of the hardened cement paste depends upon the water/cement ratio. There is, therefore, a relationship between water/cement ratio and compressive strength (2), capillary porosity and compressive strength (3), and permeability and water/cement ratio (4) (see Figs. 2, 3 and 4).

It is, however, important to note that, while strength and diffusion are affected by porosity, they are not uniquely inter-related.

Thus, the nature of the hardened cement can be summarised as follows:-

Immediately after mixing the cement and water together, an agglomeration of particles and water is formed. The water in the interconnecting cavities is termed "capillary water". Until it has been completely hydrated, Portland cement chemically binds water equivalent to approximately a quarter of its weight. Therefore, the water loses approximately a quarter of its volume. In addition to this chemically bound water, Portland cement loosely binds approximately 15% of its weight as "gel water".

Despite its loose chemical bond, gel water is not able to react with unhydrated cement and, in fact, evaporates in dry air or in an oven at 105°C. It is this gel water and the additional water required to "lubricate" the mix to form a workable material, that create more capillaries as they become evaporable water. Therefore, the greater the water/cement ratio, the greater the permeability (see Fig. 4).

The hydration by-products of the cement/water reaction form a coherent, homogeneous mass, the "cement gel". The cement gel is, however, made up of approximately 25% by volume of finely distributed pores, namely "gel pores". The total porosity (large capillary and, to a lesser degree, gel pores) of the hardened cement paste, therefore determines its strength (5) (see Fig. 5).

Fig. 1 Illustration of Permeability and Porosity

Fig. 2 Typical Relationship between Cube Strength between Cube Strength

Fig. 3 Typical Relationship and Water/Cement Ratio and Capillary Porosity

CEMENT CONSTITUENTS PROPERTIES AFFECTED

C₃S Tricalcium Silicate 55% Strength up to 28 days/setting time.

C₂S Dicalcium Silicate 20% Long term strength, i.e. after 28 days.

C₃A Tricalcium Aluminate 10% Setting time/24 hours strength.

C₄AF Tetracalcium Aluminoferrite 8% Very little effect.

C-S-H Calcium Silicate Hydrates : Form the "gel" structure.

C₃AH₆ Calcium Aluminate Hydrate.

Ca(OH)₂ Calcium Hydroxide : Gives the cement paste its high alkalinity

HEAT

Permeability and porosity can, therefore, be influenced in several ways, i.e.:-

1. The use of chemical admixtures

1. To entrain small, discrete air pores to enhance resistance to freeze/thaw cycles in vulnerable areas;
2. To modify the calcium silicate hydrate crystal formation during the hydration induction phase and, therefore, the gel pore formation.

2. Curing

A grain of cement will only hydrate when it is totally surrounded by a membrane of water. If this membrane is ruptured by evaporation, then the hydration process is adversely affected. The effects of poor curing manifest themselves in durability of the concrete in terms of possibly surface drying, causing plastic shrinkage cracking and, also, unhydrated cement, which may cause further disruption when re-hydrolysed.

Fig. 5 Hydration of Cement

"POROSITY" is defined as the total volume fraction of hollow spaces (i.e. pores) in a material. A fluid cannot pass through the material if the pores are discrete.

"PERMEABILITY" is defined as the rate of flow of a fluid under a pressure differential through a material with inter-connected pores.

Porosity Permeability

* Microsilica * Fibres

* Soluble Silicates * Long chain Polymers

* Pulverised Fuel Ash * Low water/cement ratio

* Polymers * Polymers

* Ground Granulated Blast

Furnace Slag

* Metakaolin

Fig. 6 Influence of Microsilica on Porosity of Cement Pastes - Total Pore Volume

Fig. 7 Influence of Microsilica on Porosity of Cement Pastes - Pore Size Distribution

Fig. 8 Hydration of Portland Cement and Portland Cement

Modified with Pozzolanic Materials

As stated earlier, pozzolanic materials have been used for construction since ancient times. Natural pozzolans, or in some cases calcined earths, were blended with lime (calcium hydroxide) and water to make strong cementitious materials (8).

A pozzolan may be defined as a siliceous or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties.

Microsilica is a particularly interesting material, which can pore block (it has a mean particle size of 0.1m m) in its own right but, more importantly, by reacting with the dissolved calcium hydroxide liberated by the cement hydration, it forms stable calcium silicate hydrates (gel) which further reduce and refine the pore capillary system (see Figs. 6 and 7). It is the by-product of the manufacture of silicon and ferro silicon alloys. These processes require large quantities of electrical energy for the furnaces and, therefore, tend to be situated in countries with cheaper, hydro-generated power, i.e. Norway, Iceland and Canada. High purity quartz is introduced into the furnace and, at these very high temperatures (2000¡ C), vaporises to form silicon monoxide. This then oxidises above the furnace and condenses to form microspheres of amorphous silica. This almost pure (85-92%) silicon dioxide, in the form of a fume, is then collected in bag filters.

Pulverised fuel ash is collected in a different way to that of microsilica and is a waste ash in the flue gases of coal burning power stations. The flue gases enter very large concrete precipitators horizontally, at a temperature of approximately 160¡ C. The hot gases then pass across large, vertical steel plates, which have a high voltage alternately switched across them. The charge attracts ash from the gas and the gas exits the precipitator at the other side, at a temperature of approximately 60¡ C. Once the charge has been switched off, the ash falls into a holding hopper below the precipitator. This is then usually air graded to give the acceptable particle sizes. Pulverised Fuel Ash has a similar action in cement pastes to that of microsilica, in that it produces stable calcium silicate and aluminium hydrates (see Fig. 8).

However, materials produced from calcined china clays are more reactive than the above (see Fig. 9).

These materials are manufactured from selected kaolins which are refined and calcined under conditions designed to give optimum pozzolanic reactivity, and are subject to strict quality control procedures. By contrast, as stated above, most other pozzolanic materials used in concrete are by-products from other industrial processes, and are well known to be highly variable in chemical composition.

Fig.9 Reactivity of Pozzolanic Materials (Chappelle Test)

Calcined china clays react rapidly with calcium hydroxide, converting portlandite to stable cementitious compounds. At the same time they significantly reduce the porosity and permeability of the cement hydrates and the concentration of alkali metal ions (K⁺ and Na⁺) in the pore water.

As can be seen from the pore size distribution from mercury intrusion porosimetry in Fig. 10, the total pore volume decreases significantly when 10% of cement is replaced by a calcined china clay based on metakaolin.

Fig. 10 Effect of Metakaolin on Pore Size Distribution of a Mortar Bar

Fig. 11 Physical Data for Cementitious Materials

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