ARCHIVES
of
ISSN (1897-3310)
Volume 10
Special Issue
4/2010
FOUNDRY ENGINEERING
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
55 – 60
11/4
The influence of composition of gypsum
plaster on its technological properties
M. Pawlak
Department of Materials Engineering and Production Systems, Technical University of Lodz
1/15 Stefanowskiego Str., 90-924 Łódź, Poland
*Corresponding author. E-mail address: [email protected]
Received 25.06.2010; accepted in revised form 28.07.2010
Abstract
Gypsum plasters used in art and precision foundry always are the composition of gypsum-silica-cristobalite. It is necessary considering the
specifity of plaster during heating stage. Plaster undergoes then, structural transformations causing significant variations of its volume
which are nonuniform and proceed with different intensity. The content of silica and cristobalite reduces dimensional variations of setted
gypsum plaster what increases dimensional accuracy and significant stresses reduction limiting the possibility of mould cracks occurrence
during heating.
The influence of cristobalite and silica addition on basic gypsum plaster properties like setting time, dimensional changes after setting,
bending strength and permeability in raw and heat treated state are presented in this paper. Experiments were done for mixes containing
30÷70% of the gypsum. It was proven that cristobalite has the biggest influence on the bounding time and expansion of the sandmix and
the strength and permeability do not depend on the type of additions and only on theirs total amount in the composition.
Keywords: Innovative foundry technologies and materials, Precision casting, Plaster mould, Technological properties of setted gypsum
plaster
1. Introduction
Gypsum plasters used in art and precision foundry always are
the compositions with materials allowing limitation of phase
transformation results of dihydrate α-CaSO4∙2H2O present during
heat treatment. Those additions, typically silica and cristobalite
decrease internal stresses arising during heat treatment of setted
plaster and therefore significantly reduce its dimensional changes
and risk of mould cracking.[1,2,4,5]
Disadvantages of application of such additions are the
changes of technological properties of gypsum plaster as: setting
time, dimensional changes and permeability of bounded sandmix.
These parameters significantly influence the mould technology.
Setting time, strictly speaking, the beginning of his process
decides of allowed time of gypsum plaster to be used in moulding.
The finish of the setting process points after what time the ready
mould can undergo further technological processes such as plaster
excess skimming or transport to drying station. Starting and
finishing setting time is counted from the moment of loose
mixture insertation into water. The dimensional change of the
setted plaster influences the dimensional accuracy of the mould.
Strength of the plaster is important in case of heat treatment, for
pure plaster it can be reduced by even 70%.[5]. Permeability is
also high temperature treatment dependent, what causes loosen of
gypsum plaster what in turn effects favourably on this parameter
[5].
Currently in the Department of Materials Technologies and
Production Systems of Technical University of Lodz studies on
the technology of plaster mould and plaster-bonded investment
casting using of vacuum are conducted within the frame of
Research Project No. N N508 3886 33 financed by Polish
Ministry of Science and Higher Education [2,4,5,6,7,8,9].
ARCHIVES of FOUNDRY ENGINEERING Vol ume 10, Special Issue 4/2010, 55-60
55
2. Methodology and scope of the
researches
The aim of the researches was to determine the influence of
the additions of cristobalite and silica to the plaster on:
The setting time tw of the gypsum plaster,
The dimensional changes of bounded gypsum plaster,
The bending strength of bounded sandmix Rgu and
permeability of bounded gypsum plaster in raw and heat
treated state.
2.1. Researches range
Following sandmixes were tested:
Two-component: plaster-silica of gypsum content 30,
40, 50, 60 and 70%
Two-component: plaster– cristobalite of gypsum
content 30, 50 and 70%
Ternary: plaster – silica – cristobalite of gypsum content
30, 40 and 50% (mass fractions of silica and cristobalite
were set to 40/30, 35/25, 30/20 respectively).
2.2. Tested materials
Following materials underwent investigations:
a) autoclaved gypsum α Hartform HF1made by Formula of
properties:
- water-gypsum ratio for liquefaction ø120mm - W/G=0,34
- setting time: start
- twp=10,5 min
finish
- twk=15,5 min
- bending strenght after 24h
-Rgu=10,55
MPa
b) silica SiO2 of following properties:
- SiO2 content
- granularity
- density
- 99,15%
- 0÷100 m
- =2610 kg/m3
c) cristobalite SiO2 of following properties:
- cristobalite SiO2
- tridymite SiO2
- silica SiO2
- granularity
- density
- 93,00%
- 4,00%
- 3,00%
- 0÷100 m
- 2250 kg/m3
d) distilled water.
2.2. Methodology
1. Plaster composition preparation
Plaster compositions for tests were put together in appropriate
mass fractions and were mixed in laboratory mixer LH during 1h.
Next they were dried in laboratory drier during 2h in temperature
40±1°C. Ready, loose compositions were stored in dried, hermetic
containers.
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2. Gypsum plaster and testing blocks preparation
Loose plaster composition was mixed with water in watergypsum ratio WG = 0,4 in accordance with following procedure
(PN-86/B-04360 points. 2.3.6.3):
pouring during 30 sec. weighted loose composition to
measured amount of water,
putting aside for 30 sec. to soak the plaster,
forceful mixing for 60 sec.,
air releasing on vibrating table for 30 sec.,
Ready liquid mixture of cream consistency was poured into
the testing moulds guiding the flow uniformly on whole surface.
After casting moulds were shaker 10 times to remove air bubbles
and better distribution of the plaster. After tarnish was observed,
as a first symptom of setting process, after 3 minutes the excess of
sandmix was removed and the surface was smoothed by steel
blade.
3. Determination of setting time, tw
Tests were done in accordance with procedure given in PN86/B-04360 with use of Vicat apparatus of moving parts weight
300±2 g (Fig. 1). Steel needle ø1,1±0,02 mm was used and
measuring ring of height 40±0,5 mm. Measuring ring was
fulfilled with plaster (point. 2.3.B). As a beginning of setting twp
was accepted the time, after which needle stopped at depth 2mm
from the steel plate being the base of measuring ring was taken.
As the end of setting twk the time after which needle stopped at
depth no more than 1mm. The time of start and finish of setting
was measured from the moment of pouring loose plaster into
water.
Fig.1. Test stand for determination of technological properties of
setted gypsum plaster.
a – Vicat apparatus with measuring ring
b – testing block 50 10 in the ring mould
c – mould for determination of linear dimensional change of
setted gypsum plaster
d – experimental mould with the lengthwise testing blocks
ARCHIVES of FOUNDRY ENGINEERING Volume 10, Special Issue 4/2010, 55 -60
4. Determination of linear dimensional change of setted
gypsum plaster, w
Test was done with use of mould of triangular cross section
25x25 mm and length 100 mm (Fig.. 1). Mould was fulfilled with
gypsum plaster and prepared for testing in accordance with
procedure given in point 2.3.B. the value of dimensional changes
were read off the dial indicator after 15, 20, 25, 30, 60, 90, 120,
240, 480 i 1440 min.
5. Determination of setted gypsum plaster bending strength,
Rgu
- absolute viscosity of the gas penetrating sample in
measurement temperature, Pa∙s;
h – sample height, m;
A – sample cross section surface area, m2;
p – pressure difference along sample height, Pa;
qv – penetrating gas flow velocity , m3/s;
kv – correction coefficient eliminating water vapors influence.
7. Thermal treatment of testing blocks
Dried testing blocks were placed in resistance furnace APE
800 and heated according to scheme presented in figure 3.
The typical lengthwise testing blocks were used to determine
the bending strength of the plaster in setted state Rgu of
dimensions 22,36x22,36x172 mm (according to PN-83/H-11070).
They were prepared in four cavities experimental mould. (fig. 1)
in accordance with procedure given in point. 2.3.B. The bending
strenght was measured in raw state (after 2 h) and after heat
treatment. Measurements were done with use of LRu apparatus.
6. Determination of gypsum plaster permeability in setted
state,
Test was made with use of testing blocks ø50x10 mm
prepared in ring moulds (Fig. 1) in accordance with procedure
given in point. 2.3.B. Determination of permeability was done in
accordance with standard PN-EN 993-4 applied for determination
of gas permeability of refractory materials. This standard was
chosen with regard to very low permeability of setted gypsum
plaster. For this reason the permeability of wet plaster was
measured after 24h not after 2h, and part of the blocks (in raw
state) were dried for 2 hours in 40±1°C and cooled down in the
exsiccator to the ambient temperature, the rest underwent heat
treatment. Measuring gauge for permeability measurements of the
moulding sand in hardened state was used. The scheme of testing
stand is presented in figure 2.
Fig. 2. The scheme of testing stand for permeability
measurements of setted gypsum plaster:1 – gas source,
2 – reducing valve, 3 – needle microvalve, 4 – tube pressure
gauge, 5 – grip of block 50 10, 6 – float flow-meter
Fig. 3. The scheme of testing blocks heat treatment
3. Discussion
3.1 Setting time, tw
Results of tests are presented in figure 4. The type and amount
of additions to the plaster composition have the decisive influence
on the setting time. Silica decreases his time and cristobalite
increases it visibly. The influence of these additions is mostly
noticeable at higher contents i.e. 70 % addition of cristobalite
increases starting time of setting tw by 75% in comparison to
plaster mix with analogous amount of silica. The same influence
can be observed in case of setting finish time twk. The amount and
type of additions influence also temperature range tw=twk-twp: at
70% addition of cristobalite this time is equal 4min., for silica
only, it is 2 min. The lower amount of additions the tw changes
and achieves at level 3 min for amount 30%.
Permeability of setted gypsum plaster in m2 was calculated
from the equation:
h 1
q
A p
kv
in which:
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At low amount (30%) plaster with cristobalite show bigger
linear change (0,315% after 2 h; 0,33% after 24 h) then plaster
with silica (0,265% after 2 h; 0,28% after 24 h). At total amount
of additions 40% linear changes for both plasters are the same.
Relative linear change increases for plaster with silica for total
amount of additions in it 70%,increases by 18,9% after 2 h and
14,3% after 24 h being w=0,315% and 0,33% respectively.
Analogous addition of cristobalite affects in reverse and much
more intensively changing values by 50,7% after 2 h and by
49,0% after 24 h being w=0,165% and 0,175% respectively.
In case of ternary mixture (gypsum-silica-cristobalite) of high
additions content , values are in medium positions and for most
often applied content 60% they are w=0,265% after 2 h and
0,275% after 24 h.
3.3. Bending strength in bounded state, Rgu
Fig. 4. The dependence between plaster mix setting time
and gypsum HF1 content
The twp and twk times are in medium position in comparison to
above discussed two-component mixtures in comparison to
ternary mixtures (gypsum-silica-cristobalite) of total additions
amount equal, also it is characteristic that difference tw increases
as the amount of additions decreases.
Tests results are presented in fig. 6. On this base it can be
stated that:
type of addition, so silica, cristobalite or its mixture has in
fact no influence on setted gypsum plaster bending strength,
both in raw and heat treated state,
3.2. Relative linear dimensional change
of setted gypsum plaster, w
The results of tests are presented in figure 5. The type and
amount of additions to the plaster composition have the decisive
influence on the relative linear dimensional changes.
Fig. 6. The dependence between bending strength Rgu of setted
gypsum plaster and gypsum HF1 content
Fig. 5. The dependence between relative linear dimensional
changes of setted gypsum plaster and gypsum HF1 content
and time
58
the mineralogical composition has the decisive influence on
the bending strength of setted plaster Rgu, or in other words
the fraction of plaster in basic gypsum plaster. The change of
gypsum fraction in the plaster from 30% to 70% increases
the value of Rgu by 3,5 times for raw plaster and by 4 times
after heat treatment,
thermal treatment causes significant changes in bending
strength, in case of mix of 70% of gypsum fraction bending
strength decreases by 68%, and for plaster of 30% of gypsum
fraction, the Rgu decreases even by 72%.. The reason for
such high strength change after heat treatment is a structure
refinement due to phase transformation connected with
redistribution of lattice water [5]. Plasters containing about
60% of additions are mostly applied in technique so they
contain about 40% of gypsum in basic composition. In case
of tested plaster of such composition (40% gypsum, 60%
additions) the bending strength Rgu is equal, independently
ARCHIVES of FOUNDRY ENGINEERING Volume 10, Special Issue 4/2010, 55 -60
on the additions, 1MPa, what is about 22% of the strength
in raw state.
Analyzing permeability characteristics it can be stated that the
most advantageous plaster should contain 30% of gypsum.
However it has low strength and thus the plasters containing 40%
of gypsum are used.
3.4. Permeability in setted state,
Results of tests are presented in fig. 7. This base it can be
stated that:
Type of addition, so silica, cristobalite or its mixture has in
fact no influence on bonded gypsum plaster permeability,
both in raw and heat treated state.,
4. Conclusions
The analysis of test results allows to formulate following
conclusions:
1. Setting time of setted gypsum plaster depends on its
mineralogical composition. The higher silica content shorten
setting time, cristobalite acts in reverse.
2. The type and amount of additions to the plaster composition
influences on the relative on linear dimensional changes of
setted gypsum plaster. Silica increases and cristobalite
decreases the dimensional changes.
3. Setting time and dimensional change can be controlled by
changing the amount and proportion of additions to gypsum
plaster.
4. The type of additions does not influence on the bending
strength Rgu and permeability of setted gypsum plaster but
theirs total ratio to gypsum amount.
5. Heat treatment of setted gypsum plaster decreases the bending
strength Rgu by about 70% and increases permeability by
about 25%.
Acknowledgements
Fig. 7. The dependence between permeability of setted gypsum
plaster and gypsum HF1 content
the mineralogical composition has the decisive influence on
the permeability of setted plaster , or in other words the
fraction of gypsum in basic gypsum plaster. The change of
gypsum fraction in the plaster from 30% to 70% increases
the value by 3,5 times for raw plaster and by 4 times after
heat treatment,
at high gypsum content ( over 45%) in tested gypsum plaster,
its permeability changes slightly (about 7%) in raw state and
after heat treatment remains unchanged,
more intense changes in permeability occur in sandmix
containing relatively low amount of gypsum (30÷40%) – in
this range permeability changes of about 25%, both in raw
state and after heat treatment,
heat treatment causes significant increment of permeability
of gypsum plaster of about 25%, and this change is
independent on the gypsum fraction in the mixture.
The change of permeability after heat treatment occurs, as in
case of strength, due to refinement of the structure during heating.
Structure, at first consists of quite big, needle-like crystals and
with temperature increment changes into smaller ones what
causes decrement of micro pores in setted gypsum plaster.
The work was made as a part of the research project No. N N508
3886 33 financed by founds for science in the years 2007-2010 by
the Polish Ministry of Science and Higher Education.
References
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[6] Pawlak M., Niedźwiedzki Z.: Computer aided process of
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