ELSEVIER

Construction and Building Materials, Vol. 10, No. 8, pp. 583-588, 1996 © 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0950-0618/96 $15.00+0.00

PIh S0950-0618(96)00022-0

An investigation into the properties of micro-sphere insulating concrete

M i r o s l a w a Los iewicz , t Dav id P. Halsey,** S. J o h n Dews, * Paul O l o m a i y e * a n d Frank C. Harris*

t Department of Civil Engineering and Architecture, Technical University of Szczecin, Szczecin, Poland *School of Construction Engineering and Technology, University of Wolverhampton, Wulfruna Street, Wolverhampton WVl lSB, UK

Received 27 November 1995; revised 30 June 1996; accepted 31 July 1996

The aim of the investigation was to determine how the density of micro-sphere concrete influences other selected properties. A total of 672 specimens of different sizes were made from seven different concrete mixes. The ratio of cement to water mixes ranged 0.18-0.54. The mixes differed in cement content only, contents of micro-sphere and water being held constant. Due to the different cement contents, the specimens differed in porosity. The specimens were tested for compressive strength, thermal conductivity, vapour permeability, water capillary rise, water absorption and shrinkage, in accordance with the Polish standards. The total porosity of the concrete varied in the range 72.5%-78.5%, the micro-sphere structural porosity accounting for about 42% of the porosity. The density at 28 days ranged 760-870 kg m 3 and 480-615 kg m 3 for wet and oven dried concretes, respectively. When the cement content was varied in the range 15%-45%, the 28-day compressive strength ranged 0.5-3.0 MPa and the thermal conductivity of the oven-dried concrete varied in the range 0.10-0.16 W m 1K 1. Based on the analysis of all test data, it is concluded that the micro-sphere concrete may be a suitable substitute for cement based concretes, such as those made from expanded perlite and exfoliated vermiculite. © 1997 Elsevier Science Ltd. All rights reserved.

Keywords: micro-sphere; pulverised fuel ash; concrete

Introduction

For many years, micro-spheres (cenospheres, floaters) from pulverised fuel ash (PFA, called pulverised fly ash in some countries) have been substituting for manufactured glass micro-spheres 1-7. Low-density mineral aggregates, such as expanded perlite and exfoliated vermiculite are used for insulating concrete, fills and plasters 8-16. However, in suitable situations, PFA micro-spheres may act as a substitute 17. This paper examines the suitability of the use of the micro-spheres as an aggregate in insulating concrete.

Fly ash from Polish pulverised coal power stations contains 3%-52% spherical particles. The content of thin-walled hollow spheres (micro-spheres) has been estimated at 0.4%-8.6% by weight 3-5. PFA particles from Britain, which are predominantly spherical, also contain a similar extent (about 5% by weight) of hollow spheres 1,2-20.

According to data from the Power Station By-products Utilisation Enterprise, Katowice, Poland, the wall-thickness of tile micro-spheres accounts for 3%-7% of the total diameter 4. These micro-spheres decrepitate (crackle until they burst) on heating until 260°C is reached; their shells start to sinter at about 1100°C, collapse at about 1300°C,

*Correspondence to D. E Halsey

and melt at 1400°C 4. The chemical composition of the Polish micro-spheres is similar to that of the British cenospheresl~ and 19. In this research, micro-spheres from the surface of drained lagoons at one of the biggest regional power stations, Dolna Odra, situated near Szczecin, in northwest Poland, have been used. The chemical composi- tion is given in Table 1.

The thin walls of the micro-spheres are highly micro- porous (Figure 1) and their surfaces are generally smooth; however, on some surfaces, smaller aluminosilicate bobbles exist as excrescences (Figure 2).

Particle size analysis shows that particles of the size 0.10-0.32 mm are most frequent (Table 2). Micro- scopic observations suggest that among the micro-spheres of the size 0.063-0.40 mm (91.8%), only a very small number are shapeless (Figure 3), whereas, among particles of size equal to or greater than 0.63 mm (1.1%), no spherical particles are found. In this instance, spongy- structured sinters, oval in shape with melted surfaces, predominate. The smaller micro-spheres, i.e., those less than 0.056 mm (1.6%), contain mostly irregularly shaped particles and pieces of crushed micro-spheres. The physical properties of the micro-spheres are provided in Table 3. Due to the structure of the particle, there is little water absorption, but much water is needed to wet their surfaces.

583

584 Properties of micro-sphere insulating concrete: M. Losiewicz et al.

Table 1 Chemical analysis of micro-spheres from PFA (supplied by the Power Stations By-products Utilisation Enterprise)

Oxides Concentration (% by weight)

Silicon (SiO2) 53.22-54.82 Aluminium (A1203) 28.98-31.83 Iron (Fe203 total) 3.22-7.09 Calcium (CaO) 0.70-2.06 Magnessium (MgO) 0.90-2.02 Sodium (Na203) 0.66-0.98 Potassium (K20) 3.09-4.31 Water (H20) 0.164).39 Loss on ignition 0.22-1.35

Table 2 Sieve analysis of micro-spheres less than 1 mm diameter

Sieve size Sieve residue, range Sieve residue, mean (mm) (% by weight) (% by weight)

0.000 0.6-2.7 1.6 0.056 0.3-1.3 0.9 0.063 0.4-1.3 0.9 0.071 2.8-8.8 6.3 0.100 24.6-30.9 28.1 0.160 21.3-30.6 24.1 0.200 22.9-27.9 24.7 0.320 6.3-7.9 7.7 0.400 3.2-6.6 4.7 0.630 0.4-1.0 0.8 0.800 0.2-0.5 0.3

Figure 1 Micro-sphere wall thickness and porosity at 310 times magnification Figure 3 Micro-sphere 0.10 to 0.16 mm in size at 95 times magnifica-

tion.

Table 3 Physical properties of micro-spheres

Property Mean value Range

Dry bulk density (in bulk) Dry bulk density (compacted) Particle apparent density Shell specific weight Porosity Thermal conductivity (over-dried)

412 kg m - 3 395-425 kg m - 3

453 kg m 3 431--472 kg m 3 674 kg m 3

2 240 kg m -3 70%

0.092Wm 1K i

Figure 2 Excrescence of the micro-sphere about 0.12 mm in size at 310 times magnification

Methodology The main aim of the investigation was to determine the

influence of the density of micro-sphere concrete upon the

properties of the concrete. For this purpose, a total of seven

0.03 m 3 concrete mixes were made. The mixes differed in

cement content only, with micro-spheres and water contents held constant. Due to differing cement contents, the mixes

differed in porosity and the ratio of cement to water ranged

from 0.18-0.54; however, the consistency of all mixes

remained equal. A number of cylinders, prisms and plates

were cast to enable the properties of the concrete to be determined.

Concrete mixes

Technological parameters for the manufacture of the micro- sphere concrete were selected to get the least density and

maximum air entrapment, at a given cement content,

without using air-entraining agents. Ordinary Portland

Cement "35" (ASTM type 1), was used; its physical

properties are shown in Table 4. Oven-dried micro-spheres, obtained by sieving using a 1 mm mesh, were used as an

aggregate. In general, all concrete mixes were lean, the cement content ranged from 60-180 kg m -3 (approx.). The

lowest cement content was 15% by weight of the micro-

spheres. The cement contents increased by 5% increments

to 45% (Table 5). The micro-sphere to water ratio was held

constant at 1.2, thus the cement to water ratio ranged from

0.18-0.54.

Properties of micro-sphere insulating concrete: M. Losiewicz et al. 585

Table 4 Physical properties of Ordinary Portland Cement "35", ASTM Type 1 (manufacturer's data)

Specific surface area, Blaine 250 m 2 kg -1

Compressive strength of prisms made of 18.5 MPa standard mortar 1 : 3 : 0.5 at: 3 days

7 days 31.3 MPa 28 days 41.1 MPa

Setting time: initial set 110 min final sel 215 min

Table 5 Mix proportions of micro-sphere concrete (micro-sphere to water ratio was held constant at 1.2)

Mix number Mix proportions

Cement Cement to water ratio % of cement in (kg m 3) (by weigh t ) micro-sphere

(by weight)

1 60 0.18 15 2 80 0.24 20 3 100 0.30 25 4 120 0.36 30 5 140 0.42 35 6 160 0.48 40 7 180 0.54 45

The components were mixed in a paddle mixer of 15 dm 3 capacity in three stages: dry components were mixed, then two-thirds of the total water was added, followed by the remaining water. Total mixing time for each mix was 3-5 min. As a result of this procedure, it was possihle to obtain homogeneous concrete mixes with a stiff consistency, and a regular coating of cement grout on the aggre gate.

Preparation, casting and testing of specimens All mixes were compacted on a vibrating table, at a frequency of 8000rpm, within 15 s. After casting, all moulded specimens were kept at an air temperature of about 20°C in moist-curing boxes. These boxes were water tight with water in the base, and the concrete samples placed on a grate above the water level. After 2 or 3 days, the specimens were de-moulded and replaced in the boxes, until required for testing.

Testing of the concrete was conducted in compliance with ':he Polish standards, relevant to l ightweight aggregate and cellular concrete, as well as fine mortars (PN-75/B- 06263, PN-80/B-06258, PN-80/B-04300) . For ty-e igh t 80 mm by 80 mm cylinders from each mix, were tested for density, porosity, compressive strength and water

absorption at 28 days. Al l specimens were oven-dried at 105°C, until a constant weight was obtained, before testing.

Six 50 mm by 50 m m by 250 mm prisms, from each mix, were tested for capil lary absorption. The macro- porosity of the hardened concrete, defined as the ratio of volume of pores of 1-3 mm diameter to the total volume of concrete was estimated from polished microscopic sections, using a microscope equipped with an Eltinor 4 Integrating Unit (Germany).

Twenty-four 50 mm by 50 mm by 250 mm prisms, from each mix, were tested for shrinkage in a climatic chamber, at a relative humidity of 65% and an air temperature of

20°C. Half of the specimens were put into the climatic chamber for a 3-day moist curing, and the other half for a 28-day moist curing. The length of all specimens was measured using a dial indicator having an accuracy of __.0.005 mm.

Six 4 mm by 55 m m cylinders, from each mix, were used to test the coefficient of thermal expansion using a Direct Dilatometer DO-105 (Poland). Six 2 5 0 m m by 250 mm by 50 mm oven-dried plates, from each mix, were tested for thermal conductivity at a stationary heat flow using the Bock apparatus (Germany). The Bock apparatus involves the sample being placed between a heated and cooled plate. The power of the heater (in W) and the temperature difference between the hot and cold sides of the sample were used to calculate the thermal conductivity. Six 100 mm by 20 mm cylinders, from each mix, were used for testing concrete vapour permeabili ty at a stationary vapour flow, using the wet method.

Results and discuss ion

A total of 420 cylinders, 210 prisms and 42 plates were tested. A summary of the porosity, density, compressive strength, thermal conductivity and vapour permeabil i ty of the specimens is shown in Table 6. The total porosity of

micro-sphere concrete with 15% to 45% cement content varies between 78.4% and 72.4%, respectively. The apparent density of the wet concrete at 28 days ranges from 760-867 kg m -3, when it is oven-dried (Table 6). The relationship between the cement content and dry density of the concrete is linear (Figure 4(a)).

The total porosity, determined from the density and specific gravity of the specimens, is composed of the structural porosity of the micro-spheres and cement paste, air:voids, and incidental air bubbles. Air bubbles, 1-3 mm

Table 6 Summary of micro-sphere concrete properties at 28 days

Mix namber Total porosity Apparent density, Apparent density, Compressive aThennal aVapour (%) wet oven-dried strength conductivity ~aerrneability 1

(kgm 3) (kgm -3) (MPa) (Wm 1K 1) (10 gm-lh-lpa )

1 78.37 760 481 0.55 0.111 116.5 2 77.20 779 507 0.84 0.118 105.6 3 76.16 788 530 1.03 0.124 89.1 4 75.30 815 550 1.46 0.132 75.0 5 74.47 817 568 1.68 0.142 67.7 6 73.07 851 599 2.29 0.149 64.2 7 72.44 867 613 2.88 0.153 52.8

aFor these tests, the temperature was 20°C, and the samples were over-dried; the relative humidity during vapour permeability testing was 100% on one side of the sample and 45% on the other side

586 Properties of micro-sphere insulating concrete: M. Losiewicz et al.

W/InK

700 1 0.16 600 . ~ O.V.

500 ,.~/ 400 0.10

T ~ f i t i r 2O 3° 40 c , % t 21o ' 3~ I t.; re,/°

3.0

120 2.5

I00

2.0 80

1.5 60 i.O a;O .

i t I i i ~ r

20 30 40 c, % 20 30 Z.O c. %

Figure 4 Relationship between cement and (a) dry density, (b) thermal conductivity, (c) compressive strength and (d) vapour permeability, of the concrete Figure 6 Polished section of concrete with a cement content of 15% at

62 times magnification

W/inK MPa

2.5 ." "2~"" 4

0,16 20 . " ": :'" .' ". 0.1 t

o.12 1.s ~ ..'. ". ". '". 0.10 ,.0 • . .; ~.':" ..

i i p 0.5 b kg/mr" I r T I

1()6 g/mhPa 500 550 600 kg/m 3

120

100

8 0

6O

,;o ,;o ,'5o ,;o,°,~ Figure 7 Influence of the density upon thermal conductivity (a), compressive strength (b) and vapour permeability (c) of micro-sphere concrete

(a)

(b)

Figure 5 Polished sections of micro-sphere concrete with a cement content of (a) 15% and (b) 45%, at 13 times magnification

in diameter are regularly distributed in the concrete and occupy a mean volume of about 4%. The pore structure of the hardened concrete is shown from the cut sections in Figures 5 and 6.

With cement content varying between 15% and 45%, the mean 28-day compressive strength varies between 0.6 MPa and about 2.9 MPa, respectively. The standard deviation

varies between 0.1 and 0.4, respectively, indicating reliable values despite low compressive strengths. The curvilinear relationship between the compressive strength and cement content or concrete density is shown in Figures 4(c) and 7(b). At the density 450-550kg m -3, the compressive strength of micro-sphere concrete is rather lower than that of the perlite concrete (1.2-2.0 MPa t2) and vermiculite concrete (1.2-1.3 MPaa2). This may be due to the lower cement content and the lack of modifying agents.

The results of water absorption show that the concrete volume absorption amounts to about 42% and the moisture content at the end of 1 hour of water saturation is 24% by volume, and only 25% after 24 hours.

From the moisture transfer findings, it appears that the capillary absorption for the micro-sphere concrete is high. As shown in Table 7, the rate of the moisture transfer increases with reduction of concrete density. Capillary absorption of the micro-sphere concrete, having a cement content of 35%-45%, is very similar to ceramic brick capillarity 2].

Shrinkage of the micro-sphere concrete increases as the cement content increases. At the end of the 3-day moist curing, the final shrinkage of the concrete amounts to about 0.3-0.7 mm m -1, whereas, at the end of the 28-day moist curing, the final shrinkage of the concrete is almost double and amounts to 0.7-1.3 mm m - t (approx.) (Table 8). The shrinkage stabilizes within 100-130 days in the case of the

Properties of micro-sphere insulating concrete: M. Losiewicz et al.

Table 7 Rate of moisture transfer by capillary absorption during first 3 h

587

Mix number Capillary rise of mositure (cm) after:

10 min 20 min 40 min 60 min 120 min 180 min

1 10.0 14.0 void void void void 2 7.3 10.2 13.4 15.3 18.3 20.0 3 6.0 8.3 11.3 13.3 16.5 18.4 4 4.5 6.3 9.0 10.9 14.0 16.4 5 3.3 4.6 6.5 8.1 11.0 12.5 6 2.7 3.7 5.3 6.35 8.7 10.5 7 2.0 3.0 4.3 6.5 7.8 9.5

Table 8 Shrinkage of micro-sphere concrete (specimens were 4 cm by 4 cm by 16 cm, temperature was 20°C and relative humidity was 65%)

Mix no. Moisture content (% by weight) Capillary shrinkage (ram m -1) after:

Initial Final 7 days 28 days 56 days 100 days 130 days 160 days 190 days

1 57.8 1.1 0.165 0.225 0.255 0.280 0.295 0.295 0.295 2 52.8 1.2 0.215 0.245 0.275 0.290 0.305 0.305 0.305

3 3 47.0 1.3 0.250 0.345 0.390 0.405 0.405 0.405 0.405 days 4 46.6 1.4 0.295 0.375 0.415 0.470 0.470 0.470 0.470 moist 5 38.9 1.7 0.445 0.530 0.595 0.655 0.655 0.655 0.655 curing; 6 36.4 1.8 0.470 0.565 0.655 0.685 0.685 0.685 0.685

7 35.6 1.9 0.470 0.610 0.675 0.690 0.705 0.705 0.705

1 56.5 1.5 0.385 0.480 0.530 0.545 0.560 0.565 0.570 2 54.6 1.7 0.430 0.575 0.615 0.645 0.650 0.655 0.655

28 3 43.0 1.8 0.515 0.655 0.710 0.740 0.765 0.770 0.770 day 4 38.4 2.0 0.575 0.765 0.845 0.890 0.920 0.930 0.965 moist 5 37.0 2.1 0.605 0.905 0.985 1.030 1.050 1.050 1.060 curing 6 38.5 2.3 0.690 0.970 1.045 1.110 1.140 1.140 1.170

7 35.0 2.5 0.690 0.995 1.090 1.180 1.200 1.235 1.250

3-day moist curing, before the air drying process, and 130- 180 days in the case of the 28-day moist curing. Air- shrinkage of the micro-sphere concrete is lower than the shriiLkage of vermiculite- or perlite- concrete of the same density. This may be due to a lower cement content, but drying-shrinkage is also affected by volume fraction, stiffuess of the aggregates and water content.

The coefficient of thermal expansion of micro-sphere concrete within the temperature range -20-100°C, varies in the range 3.1-4.2x10 6°C-1. This is lower than that of perlite and vermiculite concretes, which range from 7.6- 11 x 10-6°C 1 between -22°C and 5 6 ° C 14.

Thermal conductivity of the micro-sphere concrete ranges from 0 . 1 1 - 0 . 1 5 W m - I K - t depending on the cement content (Table 6). Figures 4(b) and 7(a) show the linear character of the relationship between thermal conductivity and cement content and thermal conductivity and dry density. There is a high similarity in the insulating value of the micro-sphere and perlite/vermiculite concrete. At the dry density of 400-560kg m -3, the thermal conductivity of the perlite concrete ranges from 0.097- 0.131 W m- lK ill (approx.) and that of the vermiculite ranges from 0.09-0.16 W m- lK - u r .

Vapour permeability of the oven-dried concrete de- creases as the cement content increases, and ranges from 53-117× 10-6g m- th 1pa-1 (approx.) (Table 6). The cur- vilirlear character of the relationship between permeability and cement content, and concrete density is shown in Figures 4(d) and 7(c).

Proportions of micro-sphere concrete mixes influence den~dty, porosity, compressive strength, water absorption, shrinkage absorption, thermal conductivity and expansion,

55O

500

450

"~ 400

Figure 8

0.16 0.14 0.12 Thermal conductivity W/mK

E..~

15 20 25 30 35 40 45 Cement % of micro-sphere

General pattern of relationship between thermal conductivity (A), dry density (() of micro-sphere concrete and proportions of concrete mixes>

and vapour permeability. A summary of the influence of the concrete mix properties upon density and thermal con- ductivity is provided in Figure 8. This may be useful for the preliminary design of micro-sphere concrete mixes.

Conclusion

The properties of micro-sphere concrete containing 60- 180kgm -3 of cement are sufficiently favourable that micro-sphere concrete may act as a suitable substitute for cement based concretes, such as those made from expanded perlite and exfoliated vermiculite, used in the construction

588 Properties of micro-sphere insulating concrete: M. Losiewicz et al.

industry. A model (Figure 8), that may be useful for preliminary design of micro-sphere concrete mixes, is provided.

A c k n o w l e d g e m e n t s

We gratefully acknowledge members of staff at the Technical University of Szczecin for their help in materials testing. This paper was written while M. Losiewicz was a Visiting Research Fellow at the University of Wolverhamp- ton. Two anonymous referees provided many useful comments upon an earlier draft of this paper.

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