Alabaster Plaster: What Is It? How Is It Prepared? What Are Its Applications?

Alabaster plaster  is one of the oldest construction materials, known to craftsmen for more than 5 thousand years. And it is still in demand, despite the emergence of numerous dry mortar mixtures and stricter requirements of the construction industry. Why?

Gypsum plaster = alabaster plaster?

People who are not involved in construction sometimes get confused: some people think that gypsum plaster and alabaster plaster are synonyms, and everyone just calls it the way they are used to, others consider alabaster plaster the best, high-quality variety of gypsum plaster.

Some people even believe that alabaster is a stone from which statues are sculpted, and when crushed, this material is used for finishing work, thus it must be harder and whiter than gypsum.

So, what is alabaster plaster?

Alabaster plaster is indeed a variety of gypsum plaster. Just like construction gypsum plaster, it is produced from a natural mineral — gypsum stone. Both materials are composed of calcium sulfate. The difference is that  gypsum is calcium sulfate dihydrate (CaSO4•2H2O) and  alabaster is calcium sulfate hemihydrate (CaSO4•0,5H2O).

The mineral is crushed and then calcined at a temperature of about 180°C.

The grinding of alabaster plaster is finer than that of construction gypsum plaster; therefore this material has less plasticity, but greater hardness.

Another unique feature of alabaster plaster is its drying time: on average, the alabaster plaster mortar sets in 5 minutes — that is, much faster than other mortar mixes.

Because of these properties, the scope of application of alabaster plaster is narrowed down to construction and finishing works, while gypsum plaster can be used in many industries, including medicine, jewelry, casting, art, etc.

Properties and types of alabaster plaster

The main average properties of alabaster are listed in the table below:

Compressive strength	4.0 MPa
Flexural strength	2.0 MPa
Binder grade	G5–G6 for dry mortar mixes, production of plasterboard, gypsum chipboard, and gypsum-fiber sheets G13–G25 for the production of high-strength elements
Water consumption (per 1 kg of dry mix, according to SNiP (Construction Norms and Regulations)	0.65–0.70 L
Setting time	5-30 min from the initial to final setting
Fire resistance	can be heated up to 700° without damage
Color	White, pearl gray, yellowish, light green, light pink (the color depends on the characteristics of the deposit and, according to experts, does not affect the strength properties of the finished surface)

Since one of the main features of the mortar is quick-setting strength, there are three types of alabaster plaster according to their hardening speed:

Type	Index	Initial setting, min	Final setting, min
Fast-hardening	А	2	15
Standard hardening	B	6	30
Slow-hardening	C	20	No standard

Advantages of alabaster plaster

• Quick and even application. Due to the speed of setting, the surface can be leveled in just minutes, and in an hour (or actually less), it is already ready for further treatment.
• The alabaster mortar is characterized by high adhesion, and it sticks perfectly to almost any prepared surface.
• Hardened alabaster plaster has excellent strength properties To this day, there are elements of palaces and churches made from this material that have been created more than 5 thousand years ago, and they have been preserved in excellent condition. The material does not shrink during setting and is resistant to cracks.
• Hardened alabaster plaster absorbs noise, so it can be used as an auxiliary material for sound insulation. 6 hours of exposure to open fire — this is the level of hazard alabaster plaster can withstand without significant deformation. The material is flameproof and prevents the spread of the flame.
• The material does not contain any chemical additives; it is environmentally friendly and can be safely used in bedrooms, nurseries, etc.
• It is reasonably priced  compared to dry mortar mixes with similar applications.

Application of alabaster plaster

Due to the properties of the material, the scope of application of alabaster plaster in construction is very wide. It is purchased by the producers of plasterboard and professional builders. Alabaster is also often used by private tradesmen for minor repairs.

1. Elimination of cosmetic defects of walls, ceilings, and GCB/GFS surfaces. Alabaster mortar is traditionally used to eliminate various chips, dents, cracks, etc.
2. Preparation of surfaces for finishing Alabaster mortar is excellent for puttying walls and partitions in rooms with normal level of humidity. It is used to prepare surfaces for the installation of all types of wallpaper, as well as for the application of decorative plaster. Some builders use alabaster plaster even in bathrooms (as a tile underlayment), but in this case the material should be completely covered with the facing material so that it does not come into contact with water. This principle is also applicable to kitchens, because alabaster plaster absorbs wet vapors.
3. Electrical works Alabaster plaster is an electrician’s best friend, as it is the most convenient material for quickly fixing the cable in the wall without the risk that it shifts while the mortar dries. In addition, many electricians use it when installing socket boxes. Even if the plug is pulled out of the socket roughly, due to the hardness of the alabaster, the underlying structure is guaranteed to remain in the wall, which many expensive modern dry mixes cannot always provide.
4. Interior design. When decorating rooms with stucco, a specific issue often arises: cast plaster elements have a considerable weight and therefore must be firmly fixed to the base. This is especially true for ceilings. Alabaster is the perfect option for such cases. It also comes in handy when you need to conceal and repair minor flaws in stucco molding, and is also indispensable for restoration works.

Preparation for work

When using alabaster plaster, half of the successful result depends on the quality of preparation, namely, on the selection of containers and tools.
To make the process easier, consider the following principles.

• Get rid of metal containers. Alabaster plaster will firmly stick to the iron walls, which means loss of material, inconvenience, and a damaged container. Plastic containers are a better option, but rubber is the best one: the mortar does not stick to the elastic walls, and dried residue can be easily shaken out by squeezing the mold several times and then turning it over. In addition, you can purchase special buckets for gypsum/alabaster plaster at hardware stores.
• Do not use containers with mortar remnants. The dried mortar accelerates the hardening of the new batch to be mixed.
• As for the trowel, we recommend using modern tools made of plastic or rubber. They are very convenient, and the mortar does not stick to them. A traditional steel trowel (preferably a new one) is also relatively suitable: according to the observations of some craftsmen, rust accelerates the setting of the already rapidly hardening mortar.

For small mortar batches

Small batches of alabaster plaster should be prepared in nylon buckets or compact rubber containers . For this, master builders often use halves of ordinary children's toy balls of suitable diameter.

A trowel is an optimal tool for stirring in small doses of alabaster mortal.

For large batches

Line a plastic or a rubber bucket with a single piece of cellophane (dense and without holes). Fix the “loose ends” to the edges of the container so that the polyethylene film does not move during stirring. After use, remove the film from the bucket and throw it away.

To stir the mortar, use a construction mixer or a drill with a mixing paddle.

Mortar mixing and handling

It is important to determine three aspects: the proportion for mixing, the method of mixing, and nuances that should be taken into account when mixing.

The proportions of the mixture depend on the purposes for which the mortar is planned to be used. Construction Norms and Regulations (SNiP) offer the following recommendations:

	Dry mix amount	Water amount
For mixing alabaster plaster and putty mortar	1 kg	0.65 L
For mounting and repair mortar	1 kg	0.5 L
For liquid putty mortar	1 kg	1 L

To avoid ruining the material and prepare a quality mortar, strictly adhere to the technology.

• Remember that the mix is added to the water, but never the other way around!
• Pour the powder in gradually, like flour into pancake dough, and thoroughly mix the mass until homogeneous.
• Properly mixed putty mortar has the consistency of mousse or yogurt.
• If the mortar starts to harden and you didn’t have time to use it up or do what you planned, just throw it away without trying to “resuscitate” it by adding water. Once it has “come to a sticky end”, hardened alabaster mortar is 200% no longer suitable for anything.
• When applying the mortar, take into account the fact that when the material dries, it slightly increases in volume.

Best practices:

1. Theoretical foundations and SniPs are all well and good, but in practice, the dry mixes can behave differently depending on the brand and even the batch. For this reason, before mixing the entire required volume, perform a test with only 100 grams of the material.

2. Use cold water for mixing.  

3. When mixing the mortar, you should definitely strive for homogeneity. However, make sure not to overdo it: some experts point out that overly long and thorough mixing, especially using a mixer, breaks the structure of alabaster plaster, thus decreasing its strength.

How to extend the mortar working life?

When working with alabaster, novice craftsmen, as well as private tradesmen without extensive experience and building skills, often wonder if it is possible to slow down a little the hardening of the mortar.

There are indeed several ways to achieve this. Some of them are quite scientific, some belong to the category of “folk” methods that nevertheless have proven themselves well in practice.

Method 1. Bone glue.

Good old method, approved by more than one generation of builders. Any bone-based glue can be added when mixing, whether it be slightly diluted painter’s or carpenter’s glue. Dosage is the key: 2% of the total weight of the mixture.

Method 2. Citric acid.

The recipe is as follows: for each 0.5 cup of alabaster plaster, take 4-5 grains of citric acid and throw them into cold water while mixing. However, the craftsmen note that, despite its apparent simplicity, this method is not universal, because  the amount of acid must be adapted for a specific alabaster mixture; that is, tests and experiments are required.

Method 3. PVA glue

Regarding this method, there are two groups of builders. The first group claims that 3% of PVA glue per mixture mass produce an excellent result — that is, this additive prolongs the mortar working life by almost several times and does not affect its strength in any way in the future. The other group objects that the PVA film formed on the surface during drying can block the evaporation of water thus causing recrystallization and coarsening of sodium sulfate hydrate crystals. This may lead to a deterioration in strength characteristics.    

How to choose and where to buy?

It may seem that alabaster plaster is a simple mix without any chemical additives or plasticizers, so you can just buy the first available bag of any brand and use it, because it is difficult to ruin this material. However, there are still nuances to be considered.

1. Since alabaster plaster is extremely susceptible to moisture, it should be stored in dry rooms. Thus, it is advisable to purchase the mix in stores, and not in markets where the containers are often not sealed properly, and especially not at open-air sites.
2. Check the integrity of the packaging carefully as the material can partially or completely lose its properties as a result of a minor defect.
3. No matter how simple the alabaster mix is, choose not only by price, but also by manufacturer. Large manufacturers should be given a priority, because they have established and regulated production that ensures stable quality. For mixes sold by no-name brands, quality can vary a lot from batch to batch.
4. Give preference to brands that indicate the proportions for mixing the mortar on the packaging, because, despite the SNiP regulations, recommendations may vary.

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