Perlite-based building materials a review of current applications Manjit Singh and Mridul Garg*
Abstract Perlite has been extensively used as a lightweight aggregate material in concrete or mortar, in expanded form perlite offers thermal insulation, fire resistance and other desirable properties when used in portland cementor gypsum.based plaster. The authors review current applications, with particular reference to lightweight fibrereinforced composites based on cement~gypsum plaster and perlite binder. The availability of local supplies of perlite has promoted interest in its use in India but progress has been hampered by a lack of local knowledge and Standards. A brief description is given of an ongoing research project examining the effect of litemix and feUite perlite aggregate on the properties of a plain g y p s u m plaster and a new water-resistant g y p s u m binder reinforced with glass f/bre.
Perlite has an important role in building and construction. It is mixed with cement, bitumen, lime, gypsum or other binders to form concrete or mortar with desired properties. in general, aggregates provide volume stability, resistance to wear or erosion and other desired physical properties to the finished products. There are two types of lightweight aggregates - natural and artifidai. Natural lightweight aggregates include diatomite, pumice, volcanic scoria, sawdust, volcanic cinders and tufts; artificial aggregates include expanded shale 1, slate 2, perlite 3, 4 and vermiculite ~. All are extensively used in mortar and concrete. These artificial lightweight aggregates are formed by heating, when the aggregate loses its water of crystallization and expands. Besides these aggregates, solidified blast furnace slag is also used for the manufacture of lightweight/reinforced concrete. Expanded perlite is extensively used in the construction, industrial, chemical, horticultural and petrochemical industries. The use of perlite in horticulture is well known in the USA because of its ability to improve aeration and drainage of soiP. Perlite retains moisture and nutrients on the surface which are made available to roots and cuttings. Standards have been laid down for various industrial applications, for example calcium silicate block and pipe thermal insulation, structural insulating board and asbestos-cement-perlite board. ~11 In India perlite is available to a limited extent and is being processed into lightweight aggregate by some industries. Its light weight, excellent thermal insulation and fire resistance properties make expanded perlite of interest for various products such as lightweight boards, paper pulp, glass etc. These products have previously not been produced in India and hence, no standards have been issued. Indian Standard specifications IS : 948812 and IS : 2547 (Part 11)13 on the use of perlite in gypsum are available. Adequate knowledge on perlite aggregate as a building material has not been available *Central Building Research Institute, Roorkee 247667, Uttar Pradesh, India. CONSTRUCTION & BUILDING MATERIALS Vol. 5 No. 2 JUNE 1 9 9 1
in India and work on the use of perlite aggregate in gypsum plaster and new binders has been carried out at the Central Building Research Institute, Roorkee. This article reviews the multifarious uses of perlite as a construction material. Recent investigations carried out on the use of perlite in gypsum plaster and composites are described and discussed. Perlite Perlite is a siliceous volcanic rock containing a small amount (2 to 6%) of combined water. The average chemical composition of the perlite ore is approximately 75% SiO2 with oxides of A1 (14.8%), K (4.8%), Na (2.9%), Ca (0.9%), Mg (0.1%), R203 (1.5%) and bound water (3.0%). When the presized crude perlite particles are heated to above 8 7 0 ° C (1600°F) they expand to form countless sealed glassy cells in a manner similar to popcorn. This accounts for the excellent insulating properties and light weight of expanded perlite. While the crude perlite rock may range from transparent through light grey to glossy black, the colour of expanded perlite ranges from snowy white to greyish white. Expanded perlite may be regarded as synthetic pumice, which has many advantages over natural pumice. Through the control of time and temperature of calcination, its properties may be regulated conforming to specifications of grain size and porosities. The chemical and physical properties of expanded perlite are shown in Tables 1 and 2 respectively.
Table 1 Chemical composition of perlite Constituents Si AI K Na Fe Ca Mg Oxygen (by difference)
Percentage 33.80 7.20 3.50 3.40 0.60 0.60 0.20 47.50
Q950-0618/91/020075-07 © 1991 Butterworth-Heinemann Ltd
75
Table 2 Physical properties of perlite Colour Refractive index
Wh ite 1.50
Free moisture, % max
0.50
Table 3 Mix proportions for perlite - portland cement plaster
pH of water slurry
0.50 - 8.0
Specific gravity
2.2 - 2.4
Coat
Volume cement
Max. ratio of perlite to cement (by volume)
Bulk density (loose), kg/m 3 Mesh size
50 - 400 4 - 8 mesh & finer
First
1
4
10 mm
48 Hr.
48 Hr.
Softening point, °C Fusion temperature, °C
870 - 1093 1260 - 1343
Second
1
4
0.04 - 0.06
1st and 48 Hr. 2nd coats, total 19 mm
7 days
Thermal conductivity at 24°C
Finish
1
3
1st and 2nd finish coats, 22m
w/m.k Solubility
* Soluble in hot conc. Alkaili and in HF • Moderately soluble in (4`100/0 in IN NaOH
Approx. minimum thickness
Minimum period moist curing
Minimum interval between coats
• Slightly soluble in (4.3%) in mineral acid (IN)
Specific heat, j/kgk
• Very slightly soluble (4.1o/o) in water or weak acids 837
U s e s of Perlite Some of the industrial uses of perlite are in construction (plaster and concrete), insulation (loose fill and underfloor insulation), the petrochemical and horticultural industries. A unique use for perlite is in cementing oil-gas, water and geothermal wells. Perlite plaster consists of a blend of expanded perlite aggregate and neat portland cement or gypsum mixed with water, for application by gun or trowel to wall or ceiling surfaces or to metal wire or gypsum lath for structural shed membrane fire proofings. Mixed with gypsum plaster or portland cement, it is used for lightweight insulating curtain wall construction and stair wall encasement. Construction applications Portland cement-perlite based plaster These plasters can be used on new or remodel work as a base coat for stucco finishes, ceramic tiles or masonry veneer. Perlite-portland cement plasters are r e c o m m e n d e d for as much as four-hour fire protection of structural steel columns. ~4 T h e y are ideal for weather exposed structural steel elements requiring fire protection. Sand aggregate can be added to enhance compressive strength. Where desirable, all plasters containing portland c e m e n t can be applied over metal or lath or other suitable surfaces prepared with an approved liquid bonding agent. Plastering over metal or wire lath is typically three coat work but some building codes permit two coat plastering under certain conditions. Two coat plastering is most typically used in repair and remodelling operations. Mix proportions for perlite-portland cement plasters r e c o m m e n d e d by the Perlite Institute, Inc. USA, are given in Tables 3 and 4. Gypsum perlite based plaster Perlite aggregate mixed with gypsum plaster provides an ideal base coat for internal wall and ceilings and for membrane fire proofing to the underside of floor and roof assemblies or structural steel members. 76
Table 4 Perlite portland cement plaster with lime Max. Vol Perlite per combined volume of Apprc~irn~e Cement & minimum Lime thickness
Minimum* period moist curing
Coat
Volume cement
Max. ratio Lime to cement (by volume)
First
1
1
4
10 mm
48 Hc
Second
1
1
41/2
First and second coats, total 19mm
48 H~
Finish
1
1
3
First & second finish coats, 22ram
* Minimum interval between coats 48 He 7 days
Gypsum is extensively used in many countries. Premixed lightweight plasters essentially consist of gypsum plaster and lightweight aggregate (ie expanded perlite) and are characterised by low density, high thermal insulation and sound absorption properties, These plasters are batch mixed with water and applied in one coat to all types of concrete and masonry. With the exception of the necessary additives for good workability, only water needs to be added and the plasters can work without error independently of weather conditions. In some countries, particularly Britain, the two or three coat method of plastering is still employed. Browning plaster is used as an under coat; it is hemihydrate gypsum plaster with factory-added retarder which is mixed with aggregate (eg sand or exposed perlite) either in the factory or at the building site. The setting time is several hours and is subtantially longer than that of single coat plasters. The next day a smooth finishing coat of plaster of Paris and hydrated white lime is applied. Applications of peflite - gypsum base coat plaster and finish coats should be in accordance with the American National Standard Institute (ANSI) Standard Specification for gypsum plastering. All metal lath surfaces and gypsum lath ceiling attached by resilient clips should be three coat work. Unit masonry and gypsum lath may be either three coat or two coat work. Table 5 provides proportions for perlite gypsum mixes. CONSTRUCTION & BUILDING MATERIALS VoI. 5 No. 2 JUNE 1991
Table 5 Recommended mac~mum proportion.Perlite per 45.35 kg of gypsum plaster Two coat work Double-up
Three coat work ScraC-h Brown
coat
coat
57 litres
57 litres 57 litres 85 litres
** 57 litres ** 57 litres 85 litres
plwming
Gypsum Lath Masonry * Metal Lath
-
-
85 litres
• Except monolithic concrete • * Where plaster is 25 mm or more in total thickness the proportion for the second coat may be increased to 85 litres
Concrete Perliteconcretes find applicationin insulation,where the concrete has an oven dry density of 400 to 480 kgkn 3 and a compressive strength of 7 to 10.5 koJcrn2;as light weight insulatingstructuralconcrete, with densitiesof 480 to 800 kg/m 3 and compressive strength of 14 to 42 kg/cm2; and as lightweight structural concrete, with densities ] 120 to 1600 kg/m 3 and compressive strength of 84 to 154 kg/cm ~. The insulating concrete is impervious to water,rot and vermin. Lightweightinsulating structuralconcrete islightand easy to support during the forming stage. Figure 1 shows typical curves for concretes having a compressive strength of 14 kg/cm 2 and suggested uses as a lightweight insulating structural concrete. The structural perlite concrete (densities 1120 to 1600 k_g/m3) consists primarily of the same mixes as those for lightweight insulating structural concretes except that half of the perlite aggregate has been replaced with sand. This type of concrete is recommended for use where the surface is protected from the elements of weather and severe wear.
25C
:>OC
%
150
eJ
I00
50
I00
F/g 1
200
300
400
500 600 Strain
700
800
900
I000
Stress-stra/n relationship for perlite concrete (28 days cured,. 15 x 30cm cylinders)
Lightweight tile mortar Lightweight perlite aggregate can be used in tile mortars as a sand replacement. In addition to cost savings made through the reduction of labour fatigue, better installations can be achieved. There are several accepted mix proportions in use by tile setters in the USA, but the most accepted mix proportion is one bag of portland cement (94 lbs), one bag of perlite (4 cubic feet) and half a bag of hydrated lime by weight or by half parts by volume. CONSTRUCTION & BUILDING MATERIALS Vol. 5 No. 2 JUNE 1991
The mix can be used for the scratch coat, levelling coats and settling bed, permitting 48 hours for the scratch coat to set up. 15 These tile mortars are light, heat and cold insulating, crack resistant, vermin proof, resilient, of uniform quality, fireproof, sound insulating, moisture proof and easy to use. To ensure a satisfactory bond between lightweight perlite aggregate mortar bed and the tile, a thin coat of neat portland cement is troweled or brushed over the wet tile before it is installed on the mortar. Bathroom and kitchen ~e ceilings can also be installed with perlite mortar. Lightweight periite tile mortar is eminently suitable for remodelling. The lower weight of the finished installation places a minimum of stress on old walls and building structures.
L/ghtwe/ght cornpos/tes based on cement~gypsum plaster and perlite binder A coating has been formulated by blending together 30% /3.hemihydrate plaster, 35% expanded perlite aggregate and 5% polypmpylene fibre, a small quantity of retarder and water. It was used to enhance durability, heat and sound insulation and water-proofness of bamboo wall 1°. A patent has been claimed in Japa n17 which includes production of cement clinker of high strength (compressive strength 46.0 N/ram 2 at 28 days). The cement clinker was produced by calcining a mixture of lime stone, loam, perlite and calcium chloride at relatively low temperature ( 1050 - 1150 oC). The utilization of the clinker is being examined. A crack free floor foundation material has been developed by blending 20% expanded perlite aggregates with the calcined gypsum in conjunction with a small quantity of naphthalene (0.6%) and water18. Stucco mortar of good fire resistance, thermal insulation and sound absorption properties was prepared by intergrinding the structural gypsum, expanded perlite, sand and asbestos wastes 19. Jakal 2o prepared insulative, lightweight, nailable, fireproof and water.proof boards by blending together portland cement, perlite, cellulose pulB glassfibre and additives. Products having smooth surfaces have been manufactured by moulding a Paste.like mixture containing white portland cement, perlite foam together with small quantities of methyl cellulose, butadiene copolymer latex and water2L The product has great utility as a building material. High.strength cement products of 4 x 16 x 1 cm having a bending strength of 32.0 N/rnm2 have been manufactured by Ohtono and Kasguwanyra 22 by reinforcing with alkali resistant glass fibre a mixture of cement-perlite (1:3) and water (0.55 parts by weight). Water repellent cement products have been produced by pouring a moist mixture of cement, perlite, gypsum, glass fibre, silane and water repellent compound into the mould, having a glass fibre cloth at the bottom. After covering with another glass fibre cloth, the moulded product was hardened for 24 hours and heated under applied pressure23. F'~e resistant, stable and high strength asbestos cement sheets (density 1.05 g/cm3) have been manufactured by compaction and autoclaving of mixtures containing expanded perlite (18 parts by weight), asbestos (185), mineral wool (25), portland cement (90) coupled with additives24. These sheets had tensile and bending 77
strengths of 21.0 and 18.0 Nknm 2 respectively. An insulative composite of density 206.4 kg/rn3 and thermal conductivity 0.43 Btu/h/ft 2 at 3 0 0 ° F was produced by Baer25 by blending 100 parts by weight of the expanded perlite with alumina cement, clay and wollastonite, followed by mixing with polyester fibre, surfactants and curing accelerators at a pressure of 0.245 N/Mm2. Glass fibre reinforced lightweight cement products have been produced by mixing perlite (50% by weight) with cement, asbestos, glass fibre and water, followed by moulding autoclaving at 180°C and a pressure of 1 N/mm2 for a period of 12 hours and curing for one day at room temperature 2~. The cement products thus produced have specific gravity of 0.20 and bending strength of 1.52 N/mm 2. Evert et a127 manufactured high-strength, fire-resistant fibre reinforced cement composites having bulk density of 690 kg/m3 and bending strength of 12.2 Nknm 2 by blending together cement, red mud, perlite, sodium silicate and water. KatsumF8 produced building boards with good nailability, weathering resistance, water resistance, light weight (0.9 g/cm3) and high strength (compressive strength 6.5 N/rnm2) by mixing portland cement (100 parts by weight) water, granulated blast furnace slag (70), asbestos (20), kraft pulp (30), perlite (25) and fly ash (25) followed by moulding and curing. Lightweight boards having a specific gravity of 0.85 with improved heat resistance were produced by mixing cement, asbestos, expanded perlite and water followed by moulding and curing for 4 weeks 29. Lightweight, homogeneous insulating composites with improved thermal conductivity and compressive strength have been manufactured by stirring expanded perlite with ethoxylated alkyl phenols and then mixed with portland cement and water followed by moulding for 12 hours at 15°C or above for 8 to 10 days3°. The composites are characterised by: density, 450 kg/m3; thermal conductivity, 0.08 w/mk at 20°C; and compressive strength, 150 N~:m3. Lightweight building boards can be manufactured by moulding a slurry containing portland slag cement (16 parts by weight), monocalcium-alumino cement - 3 CaO. AI203. CaSO4. 12H20 (11 parts), gypsum (55), perlite < 1 0 0 mesh) (12.5%) and paper pulp and glass fibre (5). The lightweight composites gave bending strengths of 11 - 12.5 N/ram 2 and 7.0 - 7.5 N/mm 2 before and after dipping in water (22 hours) respectively. High strength and water resistant cement building boards have been manufactured by mixing cement (64.5 parts by weight), perlite (30), pulp (5), vinyon (0.5) and water (23). Hideo32,33 has produced sintered articles of high bending strength (36.0 N/mm2) resistant to acid and erosion by mixing 60 weight percent perlite with 307O cement and 107O flyash followed by moulding, curing and sintering at 1100-1800 oC. Fibre reinforced lightweight building composites of bending strength 11.0 N/mm 2 have been produced by blending perlite (45 parts by weight) with cement (50), pulp fibre (5) and water (10) followed by moulding and curing34. Lightweight fibre reinforced cement sheets 35 have been produced by mixing cement-sand (1:1), perlite (10 weight percent) and pulp (7) with 1.57o surfactant followed by moulding and curing for 14 days. The sheets had a dry density of 1.0 g/cm3 and bending strength of 78
11.0 Nknm 2. Kyoichiro36 manufactured high-strength, good sound absorptive, nailable and lightweight fibre reinforced composites by mixing wood fibres, cement slurry, perlite, elastic chips and certain additives followed by pressing and moulding. Cement products having density of 0.98 g/cm3 were produced by blending and pressing a mixture containing cement (76 parts by weight), pulp (9), perlite (15) and fibre at a pressure of 8.0 MPa followed by curing37. Fire resistant boards having excellent humidificationdehumidification properties38 have been produced by mixing cement (100 parts by weight),/~-wollastonite (15), perlite (15), methyl cellulose (2) and water followed by moulding, pressing and curing at room temperature for 7 days and finally drying at 60°C for 8 hours. Gypsum boards of good sound absorbing properties for use as ceiling panels were produced by mixing calcined gypsum (CaSO 4. 1/2 H20), perlite, glass fibre, 10% polyvinyl alcohol solution, retarder and water for 10 minutes followed by vibration at 60 Hz for 50 seconds and hardening39. The hardened product was dried at 80°C for 4 hours followed by bumishing and coating with an alkyd resin paint and drying. The sound absorption was 137o and 58% before and after burnishing respectively. Watanabe and Nishihara 4° produced lightweight (specific gravity 0.64), nonflammable gypsum plates by mixing /3-hemihydrate plaster (65 parts), perlite (20), asbestos (8), glass fibre (4), pulp (3) and 2 7o solution of starch and water. The slurry was shaped into a plate and followed by pressing and cudng. The plate had a bending strength of 7.2 N/mm 2. High-strength, water- and weather-resistant lightweight gypsum composites have been manufactured by blending perlite, asbestos, pulp, non.ionic surfactant and water in a high speed mixer for 10 minutes, followed by mixing with 100 parts of calcined gypsum then moulding and cudng at 50°C for 10 hours 41. The composite has a specific gravity of 0.43 and bending strength of 3.0 N/mm2. High-strength, lightweight, load bearing insulating and internally coated blocks were produced by homogenizing moist expanding perlite with gypsum42. The blocks have compressive strength more than 30 Nlmm2 and density 1.5-8 kg/cm3. Glass fibre reinforced gypsum composites with density 570-830 kg/m3 were prepared using a semi-dry method by mixing /3-hemihydrate plaster, expanded perlite and glass fibre together and compacting at pressures of 345-1380 kN/m 2. Although these composites have good insulating and fire resistant properties they are weak (modulus of rupture 0.5 - 2.5 MN/m2). They are suitable for indoor applications eg ceiling, tiles. This process of manufacturing composites requires less capital investment and labour than the traditional spray suction method42. Lightweight, high-strength fibre reinforced gypsum composites have been manufactured by mixing granulated slag (307O), gypsum (30%), cement (5%), fine SiO2 (5%), fibres (207O) and perlite (107O) followed by moulding and curing in steam at 80°C for a period of 12 hours and then curing in humid air at 25°C for 14 days to obtain a building board 44. The board has density of 1.1 g/cm3 and bending strength of 17.0 N/mm 2
CONSTRUCTION & BUILDING MATERIALS Vol, 5 No. 2 JUNE 1991
0.14 260°C ~
308
~
•~ 0.09 149°C 9.05
~
304
0
0 03 Fig 4
0.02
0.01
0
Fig 2
I
I
I
25
50
75
I
I
I00 125 Density, kg/m 3
I
I
150
200
Relationship between thermal conductivity and density at various mean temperatures I I
/
.•0.5
~
0.4
....
48 Kg/m 3 128 Kg/m 3 .......... 175 Kg/m 3
/
///
""
.'""
..,"
/ f
.~0.3
.:
If ....'" s~ ,°,.""
g Z 02 .~.,~ .°...'"
~0.1 I
200
Fig 3
I
I
400 600 Meen temperoture,°C
I
800
Relationship between mean temperature and thermal conductivity at different densities
compared to 1.5 gk:m 3 and 11.5 Nknm 2 for that made without SiO2 dust and perlite. Gypsum panels having a specific gravity of 0.75 and bending strength of 0.75 N/mm 2 were produced by mixing o~-plaster with perlite, asbestos, polyvinyl alcohol and silane coupling agent, followed by moulding and hardening for one week 45. Kuper and Kalnajs 46 manufactured fibre reinforced lightweight stiff decorative ceiling tiles which had a density of about 3.25 g/in 3 by mixing gypsum plaster, methyl cellulose, chopped glass fibre (length 1.2 - 2.5 cm, perlite and mica. Composites based on high strength gypsum plaster and vermiculite have also been manufactured for use as partitions 47.
Insulation applications Loose f'#l insulation Perlite loose fill insulation conforming to ASTM specification C549-7348 is an inert volcanic glass CONSTRUCTION & BUILDING MATERIALS Vol. 5 No. 2 JUNE 1991
41 °C
0.04
I 25
I
50
I
I
I
75 IO0 125 Density, kg/m 3
I
I
150
200
thermal Relationship between density and therr conductivity at different mean temperatures
expanded by special heat process and treated with a nonflammable silicone. The resultant lightweight product is a white granular material which pours easily. It provides a quick, inexpensive method for permanently insulating masonry walls. Figure 2 indicates the relationship between thermal conductivity and density at various mean temperatures. The recommended density49 for loose fill insulation is 32 to 176 kg/rn 3 and Figure 3 shows thermal conductivity as related to increasing mean temperature for expanded perlite at densities of 48, 128 and 175 kg/m 3. Expanded perlite is an inexpensive insulation for low temperature and cryogenic applications 5°. It is widely used because of its low thermal conductivity, cost, ease of handling and low moisture retention sl in insulated storage tanks for liquid gases such as oxygen, liquid natural gas and nitrogen, at a temperature as low as - 240oC. Expanded perlite is used as an insulating cover on the surface of molten metal to prevent excessive heat loss during delay in pouring, to top off ingots to reduce piping and decrease lamination, to produce refractory blocks and bricks, and in many important foundry applications. The data in Figures 3 and 4 cover a range of per]ite densities at mean temperatures as high as 980°C. Although thermal conductivity increases appreciably above 980 °C (1800°F), expanded perlite has been used at service temperatures as high as 1093°C. Under floor insulation Water-repellent, dust-suppressed perlite for under floor insulation is produced in accordance with ASTM : C 549-8152. This type of plaster is characterised by a density of 70 to 100 kg/m 3, thermal conductivity of 0.039 to 0.047 w/m.k. Perlite under floor insulation may be used under floating concrete floors, asphalt floors and floating board floors (thickness 6 to 10 cm). It is especially useful when used to level floors and greatly reduces sound transmission from floor to floor, from floors to walls and from under floor piping systems. Because of its neutral pH, the product does not foster corrosion in piping and electrical wiring that may be in the under floor area. 79
Table 6 Sieve analysis of expanded perlite aggregate Sieve size
Grading requirement: percent retained on each sieve (cumulative)
IS: 9488-1980 requirements
(IS sieve)
Litemix
Max
4.75 mm 2.36 mm
Fellite
Min
0 0,4
0 0,4
0 5
0
1.18 mm 600 micron
28.4 45.4
0.9 13.4
60 95
5 45
300 micron 150 micron
73.9 88.0
65.4 86.4
98 1O0
75 85
Table 7 Bulk density of expanded perlite aggregate Expanded
Bulk Density (kg/m 3)
perlite
Loose
Compacted
Litemix
160.0
260.0
55.0
120.0
Fellite
Table 8 Properties of lightweight plaster produced using litemix perlite aggregate Litemix content O/o
Bulk density (kg/m 3
Compressive strength (N/mm 2)
Thermal conductivity (K Callmlhl°C)
5
1254
14.4
--
10 15
1238 1245
9.3 9,3
---
20
1120
9,0
--
25 30 35
1004 950 837
7.8 6.6 4.8
0.14 -0.08
40
789
4.7
0.078
Development of premixed lightweight insulativegypsum plasters Studies have been carried out at CBRI, Roorkee, into the effect of lightweight perlite aggregates, such as litemix and fellite, on the properties of plain gypsum plaster and a newly.developed water resistant gypsum binder reinforced with glass fibre. The sieve analysis of the expanded perlite aggregate was made to the method laid down in IS:9488-198012. The results are given in Table 6, from which it can be seen that both perlite aggregate samples conformed to the grading requirements of the above standard. Bulk density of the perlite aggregates was determined. The results are reported in Table 7. Data show that the fellite perlite aggregate is much lighter than the iitemix perlite aggregate.
The effect of expanded perlite aggregate on the properties of plain plaster of Paris (a) Effect of litemix aggregate The effects of litemix perlite aggregate on the properties of plain plaster of Paris are given in Table 8. The results indicate that with increase in perlite aggregate content, the bulk density and strength of plasters are reduced. Beyond 30% of perlite, the fall in the level of strength 80
Table 9 Properties of lightweight plaster produced using feUite perlite aggregate Fellite content (%)
Bulk densit~ (kg/m °
Compressive strength (N/ram 2)
Thermal conductivity (K
Cal/mlhl°C)
5
1189
10.38
-
10 15
1031 874
7.85 5.12
0.12 --
20 25
777 685
4.90 3.30
0.09 --
is more or less the same, while the reduction in density is not appreciable. At 25% perlite addition, the dry set density of the lightweight plaster obtained is less than the 1040 kg/m 3 density specified in IS : 2547-1976 for the metal lathing plaster 13. Whereas at 35% and 40% perlite addition, the attainment of density is lower than the specified density for the browning plaster. However, the strength is higher than the minimum specified value of 0.93 - 1.0 N/mm 2 for all types of plaster. Although IS : 2547 (Part I1) - 1976 does not specify the thermal conductivity of the premixed lightweight plaster, the thermal conductivity of gypsum plaster containing 25%, 35% and 40% perlite aggregate was determined. The K-value is much lower than the K-value of conventional building materials like bricks, foam concrete, lightweight concrete.
(b) Effect of fellite aggregate The effects of fellite perlite aggregate on the properties of plain gypsum plaster are given in Table 9. It can be seen that similar densities, strength results and K-value can be obtained using 10% and 20% fellite perlite aggregate contents as with 25%, 35% and 40% of litemix perlite aggregate (cf Table 8), for use in metal lathing and browning plasters. These lightweight premixed gypsum-perlite plasters are recommended for internal uses only.
Lightweight glass fibre reinforced gypsum binder composites Investigations have been conducted using both litemix and fellite expanded perlight aggregates as filler in a newly developed water-resistant gypsum binder, to impart low density and thermal insulation to the composites for structural purposes. The thermal conductivity of glass reinforced gypsum binder composites containing 10 to 15 % of perlite has been found to be 0.086 to 0.09 K Callm/h/°C The studies are in progress.
Conclusions (1) Lightweight, insulative and acoustic composite materials based on perlite act as wood substitute as these products possess wood-like properties. (2) Perlite products, being lightweight and fire resistant, may place a minimum of stress on old walls and building structures. (3) Perlite aggregate plaster affords fire proofing properties and adds crack resistance to the structure. CONSTRUCTION & BUILDING MATERIALS Vol. 5 No. 2 JUNE 1991
Recommendations (1) IS: 2547(Part II).1978 clearly describes the requirements for density and compressive strength of gypsum based lightweight plasters 13. Gypsum plaster on mixing with the perlite aggregate yields low thermal conductivity values, which is of paramount significance in designing certain buildings, cold storages, refrigeration, heated buildings etc). This should be incorporated in the standard. (2) Little information is available on the interaction of perlite with gypsum, cement and other building materials. There is scope for detailed investigation of the lightweight plaster. (3) In the USA, Canada and Japan standards have been formulated on perlite for various applications such as building boards and blocks. There is a need to introduce standards elsewhere, particularly in India where local perlite is now available and the production of building materials can begin, particularly boards based on perlite and cement.
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