Utilisation of Saudi sands for aerated concrete production

The/nternat,'onalJournat of Cement Composites and Lightweight Concrete, Volume 8, Number2

May 1986

Utilisation of Saudi sands for aerated concrete production Wajahat H. Mirza and Soliman I. AI-Noury*

Synopsis

This paper describes the utilisation of potential sources of abundant sands available in many parts of the Western Province of the Kingdom of Saudi Arabia. Suitable gradings of sands from five different sources have been used in making aerated concrete by mixing with cement, lime and a foaming agent. Basic properties like density and compressive strength have been determined. The use of some common types of surface treatments has been investigated as an effective measure to reduce moisture penetration and to improve resistance against sulphate attack. The behaviour of non-autoclaved and autoclaved aerated concrete has been observed to be different when subjected to sulphate attack or when exposed to very high tern perat u res.

Keywords Cellular concrete, autoclaving, compressive strength, density, fire resistance, chemical resistance, water absorption, protective coatings, concrete durability, aeration, concretes, sands, insulating concretes, lightweight concretes. INTRODUCTION Aerated concrete (foamed concrete) is a cellular material composed of cement-sand matrix containing a large quantity of fine pores. It is made by a chemical process during which gas, usually hydrogen, is introduced into a slurry, which contains mainly cement or lime and powdered silica sands. Among the various methods used for the formation of cellular foams is the introduction of aluminium powder into the slurry so as to achieve the gas evolution. This technique was originally developed in Europe but is now being used all over the world. The two basic characteristics of aerated concrete, viz., lighter weight and thermal insulation, make it particularly suited to the environment of the Kingdom of Saudi Arabia. Besides being utilised as structural concrete, its precast bricks and blocks can replace normal weight concrete and burned-clay masonry units commonly used as filler panels in concrete framed structures. Such a replacement can result in economic designs and more energyefficient air conditioning of the structures. At the same time vast reserves of valley and pit sands will become potential sources for local production of lightweight concrete. The following sections describe the use of natural sands available in abundance in the Western Province of Saudi Arabia in producing aerated concrete.

PROPERTIES OF AERATED CONCRETE

from different locations were collected. These sands were analysed for particle size distribution and their gradations are given in Table 1. Normally, high silica sands, finely ground down to the level of ordinary portland cement are used. In this study, the natural sands were used without consideration of the silica content but only the particles between US Sieves No. 16 and 100 were mixed in the slurry. The following properties of the aerated concrete were investigated: 1. Density and compressive strength 2. Moisture penetration and sulphate attack 3. Fire resistance The specimens for testing were prepared by mixing 60% sand, 30% ordinary portland cement and 10% lime. By performing a number of trial mixes, it was concluded that 0.6% of aluminium powder by weight of dry solids produced a stable foaming of the wet mixture. After thorough mixing and agitating, the mixture was poured into 100 mm steel moulds. Before demoulding the surface was trimmed level and then the specimens were autoclaved at a temperature of 295°C and a pressure of 16 atmospheres for 6 hours. The specimens were then kept in the laboratory air at 25°C and 50% relative humidity for 24 hours before testing. An average of five test results was taken for each reported value. Any result deviating by _+ 15% of the mean value was discarded. A brief description of the results obtained is given below.

For performing tests on lightweight concrete, five sands * Associate Professors, Civil Engineering Department, King Abdul Aziz University, Jeddah 21413, SaudrArabia ©LongmanGroupLtd19S6 0262-5075/86/08201081/$02.00

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Density and compressive strength values Table 2 contains the values of density and compressive strength for aerated concrete specimens produced by using the sands obtained from the different locations described earlier, and conforming with the test procedure outlined above. The range of densities obtained

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f;4 fza and &i ,~,Jo~..:;

Ut/lisat/on of Saudt sands for aerated concrete product~or',

lies between 910 kg/m 3 and 1310 kg/m ~. Predictably the compressive strength varied linearly with the density of aerated concrete. The compressive strength values ranged between 58kg/cm 2 and 120kg/cm 2 w i t h the lowest strength being yielded by the specimens having the minimum density. The converse for this was also found to be true. Sands with finer grading produced concrete of lighter densities as compared to coarsely graded sands. This may be attributed to the powdery effect produced by the presence of a bigger percentage of finely-graded particles.

Moisture penetration and sulphate attack Basically aerated concrete is considered to be more susceptible to external attack than normal concrete owing to its porous structure. In addition, it has comparatively low alkalinity (pH = 9 to 10.5) and therefore it does not provide protection to internal reinforcement as afforded by dense concrete. Thus if aerated concrete has to be used as either reinforced panels or as unreinforced lightweight masonry elements, a surface treatment may be deemed necessary to reduce the ingress of moisture or other chemical agents. This could help in prolonging the service life of the aerated concrete by slowing down its rate of deterioration. With this objective in view, 100 mm cubes of aerated concrete were cast following the same procedure as detailed in the previous section. For each test, five cubes were autoclaved and an equal

number was left non-autoclaved Three different types o; treatments were applied to the specimens, wz. cemen~ slurry, coating, sulphur mDregnat~on (by a pp~ng molten sulphur) and asphalt coating Besides determin ~ng the water adsorption cnaractenst]cs of these specimens resistance to sulphate attack ¢vas evaluated Oy placing the specimens under a VIgSO ~sotution of 5% concentration The amounts of water absorbed aS a percentage o: the total dry weight, after continuous ~mmerslon unaer water for 24 hours for plain and treated soec~mens are snown n Figure 1 From this figure. ~t s obvious that a treatment helps to reduce tne moisture penetration as the water absorption values of plain (untreated, concrete are nigher than those of the treated specimens Also autoclaving seems to imorove the penetration resistance. althougn slightly, possibly by blocking a tarter number of pores due to accelerated reaction of the cementinc compounds. Of the three types of treatments nvest~gated, asphalt an(] su DPur aroveo more effectwe than the cement slurry. However ~n the case of asphalt ~nitia! drying of the hghtwe/gnt concrete specimens seemed necessary as the asohalt coating tended to str D off The effectiveness of reduced molsture oenetrat,on due to surface treatments ~s tnen reflected n the resnstance to chemical attacK. The results of compresswe strength of ] 0 0 r a m cubes after 28 days aqu 90days under 5% MgSO,~ solution are given ~n Table 3 For

Table 1 Gradation analaysis of Saudi sands Sieve No./Size U.S.

B.S.

Sieve Opening Size mm or t~m

4 ¢,6in 4.75 8 7 2.36 16 14 1.18 30 25 600 50 52 300 100 100 150 Fineness Modulus Value

Makkah Sand 96.0 82.0 58.0 42.5 19.8 4.7 2.97

Percentage Passing .................... Madina Yanbu Ta~f Red Sea Beach Sand Sand Sand Sand 95.0 80.5 61.5 343 128 46

!00.0 98.5 92.0 70.5 39.6 i49

3.11

i 85

94.0 795 62.3 30.8 107 :i~ i 3 20

Table 2 Densities and compressive strength values of aerated concrete Source of Sand in Western Saudi Arabia Yanbu Makkah Madina Taif Red Sea Beach * 1 N/ram 2 = 9.807 Kg/cm 2

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Average Density kg/m 3

Average Compressive Strength kg/cm 2.

910 1260 1310 1200 1020

58.0 109.5 120 0 101.0 635

100.0 95.0 870 58.5 30.1 i2 7 2 17

Or~/sat/on o/Saud saf;ds for aerated concrete production

Ml/rza and A/-Noury

comparison purposes, 28 day strength of identical specimens without sulphate attack are also shown in this table. The non-autoclaved untreated concrete could not sustain the sulphate attack up to 28 days and disintegrated to such an extent that compression tests on such specimens were not possible. The autoclaved untreated concrete lost about 40% strength after 28 days and

about 60% strength after 90 days under the sulphate solution when compared with the 28 day strength of unattacked specimens. The surface treated lightweight concrete managed to show better resistance to sulphate attack, largely owing to reduced porosity. On an average, the non-autoclaved concrete lost between 20% and 40% strength while the autoclaved concrete lost 10% to 20% of its original strength. Of the three types of

50

Figure 1

,o[

Water absorption

Non Autoclaved

characteristics of lightweight concrete

Plain

Z 0

Autoclaved

30

m

I00 ¢o cI]

Slurry Coated

20

Sulfur Impregnated

w

I-I0

0

Table3 Results of sulphate attack on lightweight concrete Compressive Strength N,'mm 2 Water Absorption (%) Type of concrete Plain Coated with Cement Slurry Impregnated with Sulphur Treated with Asphalt

Period of Sulphate Attack = 28 days

Period of Sulphate Attack = 90 days

28-day Compressive Strength in N/ram 2 (without Chemical attack)

Non-autoclaved

Autoclaved

Non-autoclaved

Autoclaved

Non-autoclaved

Autodaved

Non-autoclaved

Autoclaved

39.5

36.7

--

3.54

--

2.18

2.05

5.87

24.8

22.9

2 12

5.68

1.96

5.50

3.16

6.90

16.5

15.6

3.01

6.94

2.82

6.65

3.45

7.36

14.3

13.5

2.67

5.13

2.18

4.83

3.21

6.35

83

UtJhsat~on of Saudi sands for aerated concrete production

A~!~rzaa, ,o i4/-f~ouq

treatments, sulphur impregnated specimens provided more effective resistance to sulphate attack than the specimens coated with other two types of treatments.

Fire r e s i s t a n c e o f a e r a t e d

compression was aetermlnea aria compared w~m me compressive strengths of corresponding specimens cured and tested at norma~ temperature. The comparison of SUCh strengths (as strength ratios) is presented in Figure 2. It appears ma[ the aerated concrete shows a gain in compresswe strength ranging from 20% to 40% for an exposure temperature of up to 400°C. tne maximum increase of around 40% being observed for autoclaved specimens made with powdered sand and the least increase of 17% for me non-autoclaved specimens. This gain n streng[n may ~)oss~blv De due ~o the release of adsorbed moisture and the hydration ,.~.[ any unhydrated cemem particles. Once th~s phase {s over, any further nse En tempera[ure results in a reduction in compressive strength of all soec~mens the rate of fall being quite steep. The non-autoclaved specimens crumbled and disintegrateG a: 800°C The other types of specimens showed some residual strength even after exposure to 1000°C.

concrete

In order to examine the behaviour of aerated concrete when subjected to very high temperatures, 100mm cubes were cast for three different conditions of casting as given below: 1. Cement-lime-sand mixture, autoclaved 2. Cement-lime-sand mixture, non-autoclaved

3. Cement-lime-powdered sand mixture, autoclaved These specimens were exposed to elevated temperatures ranging from 100°C to 1000°C, the duration of exposure after each 100°C temperature rise being 2 hours. After this exposure, the specimens were cooled down to room temperature and their residual strength in

+ 50

1

I

Figure 2 Effect of heat on compressive strength of

]

40

aerated concrete

30

g

2o 10 o

k-

o3 uJ

-20-

J

ce O3 LIJ

-50

-

a.

-40

-

0 o

-50-

=C

N S--Natural

Send

P S - - P o w d e r e d Sand

Z -60

-

Z

-70

-

-r ('>

-80

-

-90

-

uJ

N,~

(.9

Non

I

_ I00 0

200

I

400 TEMPE

84

RATURE

I,,

600

,~,

800

('C)

v

I000

Utl/,,sat/op of Saudi sands for aerated concrete production

CONCLUSIONS 1. From the experimental investigation described in this paper, it can be concluded that the abundant supplies of natural sands available in Western Saudi Arabia can be utilised in producing lightweight concrete. This form of concrete is quite suited to the hot, arid environment of the region. 2. The resistance of aerated concrete to moisture penetration or ingress of chemically aggressive agents can be successfully improved by surface treatment, particularly by molten sulphur which is also a by-product of the local petroleum industry. 3. The fire resistance of aerated concrete has been evaluated. It has been found that autoclaved aerated concrete made with powdered sand offers maximum resistance to high temperatures. The non-autoclaved concrete offers the least resistance, a similar trend as

Mirza and A/-Noury

was observed for resistance of lightweight concrete to chemical attack.

REFERENCES 1. Building Research Establishment, Autoclaved Aerated Concrete, Digest No. 178, London, H.M.S.O., June 1975, pp. 4 2. Comite Europeen du Beton, Autoclaved Aerated Concrete, Construction Press, Lancaster/New York, 1978, pp. 90. 3. Hoff, G. C., 'Porosity-strength considerations for cellular concrete', Cement and Concrete Research, Vol. 1, No. 1, January 1972, pp. 91-100. 4. Legatski, M., 'Cellular Concrete', Special Technical Publication, 169B, American Society for Testing and Materials, 1978, pp. 836-51

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