The influence of composition of gypsum plaster on its technological properties

Transcription

1 A R C H I V E S of F O U N D R Y E N G I N E E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 10 Special Issue 4/ /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., Łódź, Poland *Corresponding author. address: marek.pawlak@p.lodz.pl Received ; accepted in revised form 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-caso 4 2H 2 O 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 N financed by Polish Ministry of Science and Higher Education [2,4,5,6,7,8,9]. A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0, S p e c i a l I s s u e 4 / ,

2 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 t w of the gypsum plaster, The dimensional changes of bounded gypsum plaster, The bending strength of bounded sandmix Rg u and permeability of bounded gypsum plaster in raw and heat treated state 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) 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 finish - bending strenght after 24h -Rg u =10,55 MPa - t wp =10,5 min - t wk =15,5 min 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 points ): 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, t w Tests were done in accordance with procedure given in PN- 86/B 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 t wp 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 t wk 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. b) silica SiO 2 of following properties: - SiO 2 content - 99,15% - granularity m - density - =2610 kg/m 3 c) cristobalite SiO 2 of following properties: - cristobalite SiO 2-93,00% - tridymite SiO 2-4,00% - silica SiO 2-3,00% - granularity m - density kg/m 3 d) distilled water 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. Fig.1. Test stand for determination of technological properties of setted gypsum plaster. a Vicat apparatus with measuring ring b testing block in the ring mould c mould for determination of linear dimensional change of setted gypsum plaster d experimental mould with the lengthwise testing blocks 56 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0, S p e c i a l I s s u e 4 / ,

3 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, Rg u - absolute viscosity of the gas penetrating sample in measurement temperature, Pa s; h sample height, m; A sample cross section surface area, m 2 ; p pressure difference along sample height, Pa; qv penetrating gas flow velocity, m 3 /s; k v 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 Rg u 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 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, t w 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 t w by 75% in comparison to plaster mix with analogous amount of silica. The same influence can be observed in case of setting finish time t wk. The amount and type of additions influence also temperature range t w =t wk -t wp : at 70% addition of cristobalite this time is equal 4min., for silica only, it is 2 min. The lower amount of additions the t w changes and achieves at level 3 min for amount 30%. Permeability of setted gypsum plaster in m 2 was calculated from the equation: in which: h 1 A p q k v A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0, S p e c i a l I s s u e 4 / ,

4 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. Fig. 4. The dependence between plaster mix setting time and gypsum HF1 content The t wp and t wk 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 t w increases as the amount of additions decreases Bending strength in bounded state, Rg u 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 Rg u 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 the mineralogical composition has the decisive influence on the bending strength of setted plaster Rg u, 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 Rg u 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 Rg u 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 Rg u is equal, independently 58 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0, S p e c i a l I s s u e 4 / ,

5 on the additions, 1MPa, what is about 22% of the strength in raw state 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., 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. 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 Rg u and permeability of setted gypsum plaster but theirs total ratio to gypsum amount. 5. Heat treatment of setted gypsum plaster decreases the bending strength Rg u 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 N financed by founds for science in the years by the Polish Ministry of Science and Higher Education. References [1] Akerman K.: Gips i anhydryt (Gypsum and anhydrite), PWN, Warszawa [2] M. Pawlak, Z. Niedźwiedzki: Dilatometric studies of gypsum plaster in raw and heat treated state, Archives of Foundry Engineering, Vol. 8, Issue 3, July-September 2008, p [3] F. Nadachowski: Zarys technologii materiałów ogniotrwałych (Outline of engineering of refractory materials), Wyd. Śląsk, Katowice [4] M. Wiktorski, M. Pawlak, Z. Niedźwiedzki: Wpływ składu masy gipsowej na termiczne zmiany wymiarowe formy stosowanej w odlewnictwie precyzyjnym (Effect of plaster moulds compositions on thermal dilatation plaster moulds used in precision casting), Archiwum odlewnictwa, 2005, rocznik 4, nr 17, s [5] M. Pawlak: Wpływ temperatury i czasu wygrzewania na właściwości związanej gipsowej masy formierskiej, (Influence of temperature and time of firing on the properties of gypsum), Polska Metalurgia w latach , PAN, Akapit, Kraków 2006, s A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0, S p e c i a l I s s u e 4 / ,

6 [6] Pawlak M., Niedźwiedzki Z.: Computer aided process of dimensional distortion determination of bounded plaster plaster, Part II (Komputerowe wspomaganie procesu określania zmiany wymiarowej związanej gipsowej masy formierskiej, cz. II). Archives of Foundry Enginerring, Vol. 10, Issue 1, January March 2010, p [7] Pawlak M., Niedźwiedzki Z.: Computer aided process of dimensional distortion determination of bounded plaster plaster, (Komputerowe wspomaganie procesu określania zmiany wymiarowej związanej gipsowej masy formierskiej) Archives of Foundry Engineering, Vol. 9, Issue 3, July September 2009, p [8] Pawlak M., Niedźwiedzki Z.: Dilatometric studies of plaster plaster in raw and heat treated state (Badania dylatometryczne mas gipsowych w stanie surowym i po obróbce cieplnej). Archives of Foundry Engineering, Vol. 8, Issue 3, July September 2008, p [9] M. Wiktorski, M. Pawlak, Z. Niedźwiedzki: Badania derywatograficzne kompozycji gipsowych stosowanych na formy gipsowe w odlewnictwie precyzyjnym (Derivatograph investigation of plaster moulds composition for plaster mould used in precision casting), Archiwum Odlewnictwa, PAN, 2004, rocznik 4, nr 13, s [10] PN-86/B Spoiwa gipsowe. Metody badań. Oznaczanie cech fizycznych (Binder based on calcium sulfate. Methods of tests for physical properties). 60 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 1 0, S p e c i a l I s s u e 4 / ,