POLITECHNIKA WARSZAWSKA
WYDZIAŁ INśYNIERII LĄDOWEJ
KATEDRA INśYNIERII
MATERIAŁÓW BUDOWLANYCH
WARSAW UNIVERSITY OF TECHNOLOGY
FACULTY OF CIVIL ENGINEERING
DIVISION OF BUILDING MATERIALS
ENGINEERING
al. Armii Ludowej 16, p. 551, 00-637 Warszawa, POLAND; tel.: (+48 22) 825-76-37, fax: (+48 22) 825-75-47, e-mail:[email protected]
Lech Czarnecki
Paweł Łukowski
Andrzej Garbacz
Bogumiła Chmielewska
Building chemistry – laboratory exercises
collaborative work under the chairmanship of Lech Czarnecki
9. GYPSUM BINDERS
Theoretical background
Practical task 1. Examination of influence of gypsum burning conditions on
structure and binding properties of gypsum binders
Practical task 2. Examination of influence of admixtures on setting time of gypsum
binders
Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
9. GYPSUM BINDERS
THEORETICAL BACKGROUND
Raw materials and production
Gypsum binders are classified as air binders. Gypsum has been widely used in
masonry, production of plaster, prefabricates, insulation elements, floor pavement, decorative
construction elements, forms and models. Natural raw materials for gypsum production are
sedimentary rocks of chemical background:
• gypsum rock (so called gypsum stone, raw gypsum), whose main component is CaSO4 ·
2H2O; depending on the burning conditions (dehydration) of raw gypsum (Fig. 9.1), the
binder obtained (building gypsum, plaster of Paris, ceramic plaster) contains calcium
sulphate hemihydrates CaSO4 ⋅ ½H2O of α and β type, or their blend (Fig. 9.2); above
800ºC, we obtain a binder called estrich gypsum comprising anhydrite II and about 3 –
5% of CaO, which is a result of partial decomposition of anhydrite II (Fig. 9.3)
• anhydrite rock, composed mainly of anhydrous calcium sulphate CaSO4; the binder is
received by milling the rock with suitable admixtures.
Another source of gypsum is production waste of phosphoric acid (used to make
chemical fertilizers, among others) by extraction method
Ca3(PO4)2 + 3H2SO4 + 6H2O = 2H3PO4 + 3CaSO4 · 2H2O
Raw materials here are apatites and phosphorites and a by-product – phosphogypsum.
Gypsum is also widely obtained in a desulphurizing of exhaust gases (Fig. 9.4).
Calcium oxide or calcium carbonate are applied as gas sorbents. Formation of calcium
sulphate in this process may be expressed as an equation
CaO + SO2 + ½O2 + 2H2O → CaSO4 · 2H2O
Setting and hardening of gypsum binders
The mechanism of gypsum binders setting process results from the differences in the
solubility of calcium sulphates of various degree of hydration and of different crystal
structure. Taking an example of building gypsum, the following setting stages may be
indicated:
1. Dissolution of calcium sulphate hemihydrate in water.
2. Hydration of calcium sulphate hemihydrate to calcium sulphate dihydrate, which is harder
to dissolve; formation of a solution supersaturated with respect to calcium sulphate
dihydrate.
3. Crystallizing of calcium sulphate dihydrate.
In the crystallizing process of CaSO4·2H2O, the following stages may be distinguished:
• growing of crystallization nuclei in the supersaturated solution; their stability is
conditioned by proper forming and size (too small nuclei are dissolved),
• growth of crystals, which produce a structure skeleton by crystal-to-crystal adhesion,
• crystal extension; adhesion contributes to fast setting and increase in mechanical
resistance.
Water evaporation involves further increase in stability. It is a reversible process
because re-dissolution of crystal adhesion layer, due to moisture, finally leads to worsening of
stability.
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
Natural gypsum – crystals of plate habit. Spatial structure of the
compound comprises double layers of CaSO4 divided by parallel double
layers of water particles. Ca2+ ions in CaSO4 layers are set alternately
with SO4 tetrahedrons. Crystalline water particles interact with Ca2+
ions and form hydrogen bonds with oxygen atoms of SO42– groups.
Weak reaction between water particles in the layers is the reason for
perfect cleavage of raw gypsum in this plane.
CaSO4⋅2H2O
150°C–180°C
CaSO4⋅½H2O
Gypsum hemihydrate has two well-soluble variations: alpha and beta.
α-hemihydrate – obtained during calcination in steam atmosphere;
columnar crystals. β-hemihydrate – obtained in dry calcination; crystals
of no clear habit, plate-shaped aggregates of fibrous or scaly structure
are formed. Both forms have crystal lattice identical with that of
gypsum dihydrate. β variation, due to a larger specific surface, dissolves
more easily and thus binds faster, but its products have lower strength
compared to α-made ones.
170°C–250°C
CaSO4 III
350°C–600°C
Anhydrite III – occurrs in two variations, α and β, retaining the crystal
structure of gypsum hemihydrate. When in humid air, it absorbs water
from the environment; highest solubility in water among all described
here sulphates. Further heating of anhydrite III causes the
transformation of its crystal structure (the distance between lattice
points is reduced) into anhydrite II structure.
CaSO4 II
800°C–1000°C
CaSO4 II + ∼5%CaO
1200°C
CaSO4 I
1350°C
CaSO4 /CaO
blend melting
Anhydrite II – identical with natural anhydrite. Crystal structure of the
compound is similar to NaCl. SO42– ions are clustered and spaced
uniformly with Ca2+ ions in all directions. Such structure makes
anhydrite II the strongest among all gypsum types, of the highest
specific density and sparingly soluble. Crystals are prism-shaped (Fig.
9.3). Comparison of anhydrite II with gypsum dihydrate reveals a
weakening influence of crystalline water on gypsum crystal structure,
and thus, on mechanical properties of the material.
Estrich gypsum (anhydrous gypsum plaster) is formed in a partial
decomposition of CaSO4 II → CaO + SO2 · ½ H2O2
Fig. 9.1. Diagram of raw gypsum dehydration and burning
The speed of setting depends on the speed of each stage. The most essential factor is
water-solubility of the binder; the more soluble it is, the faster it sets. That is why binders
based on α and β calcium sulphate hemihydrate (building gypsum among others) are fastsetting binders – setting process starts after a few minutes whereas so-called anhydrous
binders, containing hard to dissolve anhydrite II, are slow setting (the process starts after a
few hours) and require activators. In anhydrous gypsum binder, the activator is CaO, which
accelerates hydration of anhydrite. Just as in case of hemihydrate gypsum, anhydrite II
undergoes hydration to calcium sulphate dihydrate during the setting process. This is a partial
process, however, mainly on the surface of anhydrite grains, causing their adhesion. Presence
of unhydrated anhydrite II (of compact and dense structure) gives the material higher strength
and abrasion resistance when compared to a material containing CaSO4 · 2H2O.
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
Modification of gypsum setting speed
Acceleration or retardation of the set is achieved by an application of admixtures,
which also affect crystal habit of CaSO4·2H2O formed (Fig.9.5), and in consequence,
technical properties of hardened material. The setting process may be accelerated by:
• proper comminution of the binder,
• paste stirring,
• increasing binder solubility by:
- some amount of well soluble anhydrite III in hemihydrate gypsum (burning of
gypsum rock in a temperature enabling formation of anhydrite III),
- addition of chlorides (e.g. NaCl), nitrates (NaNO3) or potassium hydroxide (KOH)
to calcium sulphate hemihydrate,
• application of anhydrite hydration activators, such as:
- Na2SO4, Al2(SO4)3, Fe2(SO4)3, alums (double salts of generalized formula
MeIMeIISO4 · 12H2O), these compounds produce double salts of MeSO4 · Ca2SO4
· nH2O type, which decompose easily releasing dihydrate gypsum,
- alkaline substances, such as CaO, cement,
• reducing the solubility of calcium sulphate dihydrate with respect to hemihydrate, by
adding into the solution co-ions of dissolved substance (Ca2+ or SO42–); the effect of coion causes a shift of balance of dissociation process in favour of constant phase; the effect
is greater in case of a substance of lower solubility product, like Ca2SO4 · H2O here,
• acceleration of crystallization by introducing the nuclei of hydrated calcium sulphate
crystalline phase.
Fig. 9.2. Building gypsum,
CaSO4 · ½ H2O – powdered, 1000x
Fig. 9.3. Crystals of CaSO4, formed by
isothermal heating of CaSO4·2H2O at
800ºC. Morphology of sulphate dihydrate
crystals is kept unchanged, 4000 x
Fig. 9.4. Synthetic gypsum, 1000x
Fig. 9.5. Hardened gypsum paste. Visible
CaSO4·2 H2O crystals and relics of unhydrated
CaSO4·½ H2O, 4000x
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
•
•
•
Retardation of the setting process may be achieved by:
reducing the speed of hemihydrate dissolution by increasing the viscosity of mixing
liquid, e.g. adding glycerine,
inhibition of crystal growth by formation of coat on the surface of crystallizing salt, e.g.
addition of colloidal substances, such as glue, caseine (milk protein), sulphite liquors
(waste product in cellulose manufacturing, containing mostly lignosulphonate acids),
retardation of nuclei growth of the new crystalline phase due to adsorption of admixtures,
such as:
- organic polar substances, e.g. tartaric or citric acid, ethanol; their activity may be
illustrated as follows: a face of gypsum crystal with Ca2+ ions of strong electric field
adsorbs polar particles on its surface; the adsorbed layer inhibits the growth of crystal
face; subsequent Ca2+ and SO42– ions are incorporated in the remaining faces and the
crystal achieves a habit of hexagonal plate; cations of high electric charges, e.g. Fe3+,
Al3+, contribute to a clear increase in the number of plate crystals,
- sodium phosphates or hydrogen phosphates (e.g. Na3PO4, Na2HPO4) added to calcium
sulphate hemihydrate react with CaSO4 ⋅ ½ H2O producing calcium phosphates, which
inhibit the growth of crystal nuclei by precipitating on them.
PRACTICAL TASK 1. EXAMINATION OF INFLUENCE OF GYPSUM BURNING
CONDITIONS ON STRUCTURE AND BINDING PROPERTIES OF GYPSUM
BINDERS
The equipment necessary to perform the task:
mortar,
scales,
microscope,
stop watch,
micro slides and cover slips,
porcelain and quartz crucibles
Reagents and materials used:
gypsum dihydrate (dried),
glycerine,
dryer,
burner with ceramic triangle
polystyrene sheet,
a pair of metal tongs,
desiccator,
distilled water.
Task performance
The aim of the task is determination of loss of gypsum dihydrate crystallizing water
in dehydration temperature (160ºC) and above 350ºC, as well as determination of setting time
and microscope observation of a shape of product grains.
Determination of loss of gypsum dihydrate crystallizing water in burning process at
160ºC
Powder an initially dried sample of gypsum dihydrate to be tested (about 10 g) in a
mortar and then:
• weigh a ceramic crucible,
• weigh 2.0 g of previously powdered gypsum in the crucible,
• put the crucible with the sample into a dryer for 40 min.,
• take the crucible out and cool it in a desiccator,
• determine the mass of the crucible with the sample of burnt gypsum,
• calculate the percent loss of sample mass,
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
•
leave the sample of burnt gypsum for setting time determination and microscope
observation.
Determination of loss of gypsum dihydrate crystallizing water at over 350ºC
Simultaneously with the task described above, the other sample may be used for the
determination of loss of gypsum dihydrate crystallizing water at over 350ºC. To do this, you
should:
• determine the mass of a quartz crucible,
• weigh 2.0 g of gypsum, previously dried and powdered, in the crucible,
• place the quartz crucible with the sample over a burner using a ceramic triangle; heat for
40 minutes, shaking light,
• remove the crucible from the burner and cool in the desiccator,
• determine the mass of crucible with the sample,
• leave the sample for further determination.
Determination of setting time of gypsum binders obtained from gypsum dihydrate at
160ºC and over 350ºC
To determine the setting time, follow the procedure below:
• pour 5 drops of water into a sample of binder and stir with a glass rod; switch the stop
watch at the moment you add water,
• spread the paste on the surface of a circle drawn on polystyrene sheet,
• measure the time from the moment of adding water till the paste starts losing plasticity.
Microscope observation of a shape of binder crystals
Preparation of material for microscope observation:
• place bits of binders obtained on two micro slides,
• put a drop of glycerine on coverslips and use them to cover the binder samples on the
slides,
• spread the preparation obtained on the slides to receive a thin, hardly visible layer of
binder in glycerine,
• place the preparation in the observation field of a polarizing microscope, first, conduct
observation at 40x magnification and then increase the magnifying power,
• note the difference in crystal habit.
Present the results of experiments according to a pattern given in Table 9.1.
Table 9.1
Structural and physical properties of tested gypsum binders
Roasting
temperature
160ºC
> 350ºC
Loss of water
%
Crystal habit
Kind of binder
received
Setting time in
minutes
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
PRACTICAL TASK 2. EXAMINATION OF INFLUENCE OF ADMIXTURES ON
SETTING TIME OF GYPSUM BINDERS
The equipment necessary to perform the task:
Vicat apparatus,
micro slides and coverslips,
polarizing microscope,
200cm3 measuring cylinder,
crystallizer.
Reagents and materials used:
building gypsum,
citric acid,
glycerine,
distilled water,
NaCl,
Na2HPO4.
The aim of the exercise is to measure the setting time of building gypsum in
comparison to the setting time of gypsum containing modifiers, and measuring the influence
of modifiers on the habit of crystals formed. The setting time is determined by Vicat
apparatus with an automatic needle plunge. In this method, gypsum paste is placed in a ring
which is put on a glass plate and the apparatus stand. Determination consists in plunging
(dropping) the steel needle in the paste (every 30 s) and determining the start and end times of
the setting process.
The start is considered to be a period from the moment of mixing gypsum with water
to the moment when the needle plunged in the paste stops at 3 – 4 mm from the surface of the
glass plate. The end is considered to be the moment when the needle stops at 1 mm below the
surface of the sample.
Preparation of gypsum paste for testing - Composition No. 1 (without admixture)
• determine the mass of crystallizer on the balance
• weigh 200 g of building gypsum in the crystallizer with an accuracy of ±1.0 g
• measure 130 cm3 of distilled water in a measuring cylinder and pour it into an evaporator
prepared for the paste,
• move the whole amount of gypsum from the crystallizer to the evaporator with water
while stirring, so that a total time of mixing gypsum with water is not longer than 1
minute,
• switch on the stop watch at the moment you put gypsum into the water in order to
determine the set time.
Compositions No. 2 and 3 (with admixtures)
No. 2 gypsum paste with NaCl and No. 3 with citric acid or sodium orthophosphate hydrate
Na3PO4 are made in the way given for Recipe No. 1 with a difference that admixtures
(weighed with an accuracy of ±0.1 g) are added to working water. Required amounts of
admixtures are given on their containers.
Determination of setting time by the Vicat method
• fill the apparatus ring placed on the glass plate with the paste prepared,
• remove excess paste with a knife and put the whole on the apparatus stand,
• fix the steel needle just over the surface and then, plunge it in various points of the sample
every 30 s,
• note the time (from the moment of mixing with water) until the needle stops 3 – 4 mm
from the surface of the glass plate,
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Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT
note the time (from the moment of mixing with water) until the needle drops not more
than 1mm.
Record the test results in Table 9.2 and present as a bar graph with the start and end
times of setting marked. Give the results in minutes.
•
Table 9.2
Gypsum paste test results
Tested paste No.
w/g
Kind of
admixture
Amount of
admixture
Setting time, min
start
end
1
2
3
Determination of the influence of admixtures on gypsum crystal habits
Preparing samples for microscope testing:
• put a small amount of gypsum on a micro slide,
• pour a drop of distilled water on a coverslip and then use the slip to cover the gypsum
powder on the slide,
• crush the paste between the slides to make almost a transparent thin layer,
• after 10 minutes place the preparation on a microscope table and conduct observation.
The test should be repeated with NaCl and Na3PO4 or citric acid solutions as
dispersants. Pay attention to the habit of crystals, i.e. crystal shape characteristics, e.g. needlelike, plate, columnar, blade-like, etc. Present the results of observation according to a pattern
given in Table 9.3.
Table 9.3
The influence of admixtures on the gypsum dihydrate crystals habit
Sample
1
2
3
Admixture used
Crystal habits formed
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