A cement board is a combination of cement

Cement

In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

 and reinforcing fibers formed into 4 foot by 8 foot sheets (or 3 foot by 5 foot sheets), 1/4 to 1/2 inch thick that are typically used as a tile

Tile

A tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...

 backing board. Cement board can be nailed or screwed to wood or steel studs to create a substrate for vertical tile and attached horizontally to plywood

Plywood

Plywood is a type of manufactured timber made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, re-usable, and can usually be locally manufactured...

 for tile floors, kitchen counters and backsplashes. It can be used on the exterior of buildings as a base for exterior plaster (stucco

Stucco

Stucco or render is a material made of an aggregate, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as decorative coating for walls and ceilings and as a sculptural and artistic material in architecture...

) systems and sometimes as the finish system itself.

Cement board adds impact resistance and strength to the wall surface as compared to water resistant gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

 boards. Cement board is also fabricated in thin sheets with polymer modified cements to allow bending for curved surfaces.

Composition

Cement boards are mainly cement bonded particle boards and cement fibre

Fiber cement siding

Fiber cement siding is a building material used to cover the exterior of a building in both commercial and domestic applications.-Usage:...

. Cement bonded particle boards have treated wood flakes as reinforcement, whereas in cement fibre boards have cellulose fibre, which is a plant extract as reinforcement. Cement acts as binder in both the cases. The fire resistance properties of cement bonded blue particle boards and cement fibre boards are the same.
In terms of load-bearing capacity, cement-bonded particle boards have higher capacity than cement fibre boards. Cement particle boards can be manufactured from 6 mm to 40 mm thickness making it extremely suitable for high load bearing applications.
These boards are made of a homogeneous mixture and hence are formed as single layer for any thickness.
Cement fibre boards are more used in decorative applications and can be manufactured from 3 mm to 20 mm thickness. Fibre boards are made in layers of very thin thickness, making it extremely difficult to manufacture high thickness boards. Many manufacturers use additives like mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

, aluminium stearate and cenosphere

Cenosphere

A cenosphere is a lightweight, inert, hollow sphere filled with inert air or gas, typically produced as a byproduct of coal combustion at thermal power plants. The color of cenospheres varies from gray to almost white and their density is about 0.4–0.8 g/cm³, which gives them a great buoyancy...

s in order to achieve certain board qualities.

Advantages

As a tile backing board, cement board has better long-term performance than paper-faced gypsum core products because it will not mold, mildew or physically break down in the continued presence of moisture or leaks. Cement board is not actually waterproof, but it is highly resistant to absorbing moisture and has excellent drying properties. In areas continually exposed to water spray (i.e. showers) a waterproofing barrier is usually recommended behind the boards or as a trowel

Trowel

A trowel is one of several similar hand tools used for digging, smoothing, or otherwise moving around small amounts of viscous or particulate material.-Hand tools:...

-applied product to the face of the boards behind the finish system.

Disadvantages

One major disadvantage of cement board is the weight per square foot. It is approximately twice that of gypsum board, making handling by one person difficult. Cutting of cement board must also be done with carbide-tipped tools and saw blades. Due to its hardness, pre-drilling of fasteners is often recommended. Finally, cement board is initially more expensive than water resistant gypsum board but may provide better long term value.

Installation

Cement board is hung with corrosion resistant screws or ring-shank nails. Cement board has very little movement under thermal stress, but the boards are usually installed with a slight gap at joints in shower pans, bathtubs, and each other. These joints are then filled with silicone

Silicone

Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

 sealant

Sealant

A sealant may be viscous material that has little or no flow characteristics and stay where they are applied or thin and runny so as to allow it to penetrate the substrate by means of capillary reaction...

 or the manufacturer's taping compounds before applying a finish. The filled joints are taped like conventional gypsum board, but with fiberglass tapes that provide additional water resistance. Combined with a water impermeable finish, cement board is a stable, durable backing board.

Water resistance

The category of construction material known as cement board includes both water resistant and waterproof board. Each has its own best use.

Typically water resistant cement board is composed of a treated gypsum core with a non organic fiber reinforced covering, either on one or both faces. This type of board requires fastidious sealing of all cut edges and penetrations to maintain the manufacturer's warranty for wet area installations. Gypsum core "cement" board panels are ideal for moist but not truly wet installations of tile and/or stone walls.

There is a class of cement board strictly constructed of a Portland cement

Portland cement

Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

 based core with glass fiber matt reinforcing at both faces. This type board is truly waterproof. These panels can be immersed in water without any degradation (excluding freeze thaw cycles). These panels do not require the sealing of edges and penetrations to maintain their structural integrity. These Portland cement based products are smaller in size compared with the gypsum core based products. Typically they range in size from 30" x 48" to 36" x 60". They are, as one would expect, considerably heavier than the gypsum core type panels.

Portland cement based panels are ideal for truly wet locations like shower surrounds and for locations where a Portland cement based thin-set material is used for bonding tile and stone surfaces to a substrate. They are also ideal for floor tile and stone installations over a structural subfloor.