Gypsum is a widely used soft sulfate mineral, primarily composed of calcium sulfate dihydrate, known for its applications in construction, agriculture, and art. Its properties include rapid setting time and versatility in use, such as in drywall and plaster, although it has some limitations like cost and moisture retention. The document also details the mineral's history, chemical composition, and various forms like selenite and alabaster.

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Gypsum
Prepared by
Noor Adib
Darun Amjad
Ali rizgar
Supervisor:
Ali Rizgar
Salahaddin University – Erbil
College of Engineering
Department of Civil Engineering
Construction Materials

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What is the gypsum?
Gypsum: is a soft sulfate mineral composed of calcium sulfate dehydrate,
with the chemical formula CaSO4.2H2O. It is widely mined and is used as a
fertilizer and as the main constituent in many forms of plaster, blackboard
chalk and wallboard. A massive fine-grained white or lightly tinted variety of
gypsum, called alabaster, has been used for sculpture by many cultures
including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire
and the Nottingham alabasters of Medieval England. Gypsum also
crystallizes as beautiful translucent crystals of selenite (mineral). It also
forms as an evaporate mineral and as a hydration product of anhydrite.
Gypsum Types
• Type I - Plaster used as an impression material
• Type II - Plaster used as a model material
• Type III - Stone used as a mounting material and a model material
• Type IV - Stone used as a die material
• Type V - Stone used as a die material when one wants a more expanding die
material (e.g. during fabrication of base metal alloy castings)

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Etymology and history
Gypsum was known in Old English as spærstān, "spear stone", referring to its
crystalline projections. (Thus, the word spar in mineralogy is by way of comparison
to gypsum, referring to any non-ore mineral or crystal that forms in spear like
projections). In the mid-18th century, the German clergyman and
agriculturalist Johann Frederic Mayer investigated and publicized gypsum's use as
a fertilizer. Gypsum may act as a source of sulfur for plant growth, and in the early
19th century, it was regarded as an almost miraculous fertilizer. American farmers
were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved,
resulting in the so-called "Plaster War" of 1820. In the 19th century, it was also
known as lime sulfate or sulfate of lime.
Chemistry
The setting reaction of gypsum consists of water reacting with the hemihydrate
(CaSO4 1/2H2O) to form the dehydrate of (CaSO4 2H2O). That reaction is written
as: (CaSO4 1/2H2O + 1 1/2 H2O -> CaSO4 2H2O) the setting reaction is
exothermic, which means that the gypsum has set and can be removed from the
impression after the heat has cooled down to room temperature. From the above
reaction we can draw certain conclusions. The first one relates to the minimal
amount of water that is needed to allow all CaSO4 1/2H2O to react, the second one
to the final theoretical volume, and the third one to the effect of using different
temperatures.

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Physical properties
Gypsum crystals are plastic enough to bend under pressure of the hand. Sample on
display at Muse cantonal de geology de Lausanne, and gypsum is moderately
water-soluble (~2.0–2.5 g/l at 25 °C) and, in contrast to most other salts, it exhibits
retrograde solubility, becoming less soluble at higher temperatures. When gypsum
is heated in air it loses water and converts first to calcium sulfate hemihydrate,
(basanite, often simply called "plaster") and, if heated further, to
anhydrous calcium sulfate(anhydrite). As for anhydrite, its solubility in saline
solutions and in brines is also strongly dependent on Na Cl (common table salt)
concentration. Gypsum crystals are found to contain anion water and hydrogen
bonding.

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Physical properties of gypsum
Chemical Classification Sulfate
color Clear, colorless, white, gray, yellow, red, brown
streak White
luster Vitreous, silky, sugary
Diaphaneity Transparent to translucent
cleavage Perfect
Mohs Hardness 2
Specific Gravity 2.3
Diagnostic Properties Cleavage, specific gravity, low hardness
Chemical Composition Hydrous calcium sulfate, CaSO4 2H2O
Crystal system Monoclinic

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Mechanical properties
ANSI/ADA Specification No. 25 - 2000
The properties of gypsum before and after setting affects the outcome of the
process gypsum is used for. For example, the fines of the gypsum affect the ability
of the gypsum to adapt to fine details of the impression material. The setting time
affects both working time as the time one has to wait before one can use the
poured impression. The setting reaction is also associated with a certain level of
expansion. During impression removal it is important that the impression gypsum
has sufficient strength to avoid fracturing the cast during this process. Detail
reproduction, surface hardness and abrasive resistance are important properties
to consider when one work on the cast.
Crystal varieties
Gypsum occurs in nature as flattened and often twinned crystals, and transparent,
cleavable masses called selenite. Selenite contains no significant selenium; rather,
both substances were named for the ancient Greek word for the Moon. Selenite
may also occur in a silky, fibrous form, in which case it is commonly called "satin
spar". Finally, it may also be granular or quite compact. In hand-sized samples, it
can be anywhere from transparent to opaque. A very fine-grained white or lightly
tinted variety of gypsum, called alabaster, is prized for ornamental work of various
sorts. In arid areas, gypsum can occur in a flower-like form, typically opaque, with
embedded sand grains called desert rose. It also forms some of the largest crystals
found in nature, up to 12 m (39 ft) long, in the form of selenite.

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Occurrence
Gypsum is a common mineral, with thick and extensive evaporate beds in
association with sedimentary rocks. Deposits are known to occur in strata from as
far back as the Archaean eon.[10]Gypsum is deposited from lake and sea water, as
well as in hot springs, from volcanic vapors, and sulfate solutions
in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by
groundwater in near-surface exposures. It is often associated with the
minerals halite and sulfur. Gypsum is the most common sulfate mineral. Pure
gypsum is white, but other substances found as impurities may give a wide range
of colors to local deposits. Because gypsum dissolves over time in water, gypsum is
rarely found in the form of sand. However, the unique conditions of the White
Sands National Monument in the US state of New Mexico have created a
710 km2
(270 sq. mi) expanse of white gypsum sand, enough to supply the
construction industry with drywall for 1,000 years. Commercial exploitation of the
area, strongly opposed by area residents, was permanently prevented in 1933
when president Herbert Hoover declared the gypsum dunes a protected national
monument.
Gypsum is also formed as a by-product of sulfide oxidation, amongst others
by pyrite oxidation, when the sulfuric acid generated reacts with calcium
carbonate. Its presence indicates oxidizing conditions. Under reducing conditions,
the sulfates it contains can be reduced back to sulfide by sulfate-reducing bacteria.
Electric power stations burning coal with flue gas desulfurization produce large
quantities of gypsum as a byproduct from the scrubbers.
▪
Crystals in the Cave of the Crystals in Mexico. Note person (lower right) for scale.

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▪
Crystals that formed as the water evaporated in Lake Lucero, White Sands
National Monument.
▪
Veins in the silts/marls of the Tea Green and Grey Marls, Blue
Anchor, Somerset, United Kingdom.
Setting behavior
❖ Setting time
The setting time should be such that the operator can finish pouring the
impression material. After completed pouring, the cast should set within a short
time period.
The setting reaction starts as soon as the powder contacts water. As soon as
calcium sulfate dehydrate crystals form, the viscosity starts increasing. The time
one has until the gypsum can no longer be poured is called the working time,

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while the time it takes until the reaction has reached completion is called final
setting time.
The initial setting time is measured with a penetration test. Such a test is the Vick
at apparatus, which consists of a road with a 1 mm diameter having a mass of 300
g. A container with a fixed amount of gypsum is placed under the needle and the
needle is allowed to penetrate the gypsum. When the needle fails to penetrate
to the bottom, one has reached the initial setting time.
The setting time can be controlled in several ways. The easiest way is to add an
accelerator. An efficient accelerator is potassium sulfate. By mixing the gypsum
with a 2% potassium solution the setting time can be reduced from ~ 10 min to 4
min. If one wants to.
Setting expansion
Theoretically, gypsum should shrink, while in reality it expands. The reason is
that during setting, the gypsum crystals push each other apart.
Uses
.
Gypsum is used in a wide variety of applications:
• Gypsum board is primarily used as a finish for walls and ceilings, and is
known in construction as drywall, wallboard, sheetrock or plasterboard.
• Gypsum blocks are used like concrete blocks in building construction.
• Gypsum mortar is an ancient mortar used in building construction.
• Plaster ingredients are used in surgical splints, casting moulids and
modeling.
• A binder in fast-dry tennis court clay.

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• As alabaster, a material for sculpture, it was used especially in the ancient
world before steel was developed, when its relative softness made it much
easier to carve.
• A wood substitute in the ancient world: For example, when wood became
scarce due to deforestation on Bronze Age Crete, gypsum was employed in
building construction at locations where wood was previously used.
• A tofu (soy bean curd) coagulant, making it ultimately a major source of
dietary calcium, especially in Asian cultures which traditionally use
few dairy products.
• Adding hardness to water used for brewing.
• Used in baking as a dough conditioner, reducing stickiness, and as a baked-
goods source of dietary calcium. The primary component of mineral yeast
food.
• A component of Portland cement used to prevent flash setting of concrete.
• Soil/water potential monitoring (soil moisture).
• A common ingredient in making mead.
• Used in mushroom cultivation to stop grains from clumping together.
Disadvantages of Gypsum Plaster:
• Gypsum plaster cannot be used for outside walls since they retain
dampness. Also, gypsum plastering cannot be done in areas which are
continuously damp such as bathroom etc.
• Gypsum plaster is costlier than traditional cement motor plaster (cement
and sand) for same thickness of plaster. But in areas where natural/river
sand is not available for construction, cement mortar plastering would
require a 6 mm gypsum layer to finish it making cement plaster costlier.

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Advantages of Gypsum Plaster:
Contractors and Builders started preferring gypsum plaster instead of due to its
superior finish and time saving attributes. Some of the advantages of gypsum
plaster are:
1. Ease of Application (Workability): Gypsum can be directly applied over
brick/block work without separate finishing. It is also very easy to apply and level
gypsum plaster.
2. No Shrinkage Cracks: Gypsum reaction produces less heat as compared to
cement reaction with water. So there are fewer Shrinkage cracks in gypsum
plaster as compared to traditional cement plaster
3. Quick Setting Time: Gypsum sets quickly (i.e., within 25-30 min). So painting
could be started 72 hours after application of gypsum plaster. Plaster has to be
dried up before painting.
4. No curing time required: Unlike Traditional Cement Plaster, Gypsum plaster
doesn’t need any curing saving water and time during construction
5. High Productivity: Reduces time considerably when compared to conventional
cement plaster
6. High Performance: Excellent high strength after drying, Durable and Light
weight (Reduces dead load on structure)
7. Smooth Finish: Perfectly lined, levelled, smooth walls and perfect right-angled
corners
8. Reduced Supervision: Careful quality checking is required for cement plaster as
cement and sand has to be properly proportioned. In contrast, gypsum plaster
doesn’t require same amount of quality checks for application thus reducing
supervision efforts.
9. Readily available raw materials: Gypsum is a readily available material. Natural
Sand, which is a raw material used in Traditional cement plaster, is hard to
obtain. It is also banned in multiple states in India
10.Fire resistant: Gypsum plaster is highly resistant to fire
11.Low thermal conductivity: Gypsum has low thermal conductivity. This saves
electrical cost for heating and cooling rooms in a building.
12.Decorative application: It can be easily applied to decorative purposes also
and can be mold into different shapes.