Effect of micropore structure on cellular concrete shrinkage

CEMENT and CONCRETE RESEARCH. Printed in the United States.

Vol. 7, pp. 323-332, 1977.

PergatnonPress, Inc

EFFECT OF MICRCPORE STRUCTURE ON CELLULAR CONCRETE SHRINKAGE

Halina Ziembicka Research and Development Centre "CEBET" of Concrete Industry, Warsaw, Poland

(Refereed) (Received June 9, 1975; in final form March 11, 1977)

ABSTRACT This paper presents the results of studies on the effect of micropore structure on the shrinkage of autoclaved cellular concrete with sand aggregate. It has been found that the shrinkage of cellular concrete is the function of vol me and specific surface of micropores of radii 75 < r < 625 1 . On the base of mathematical analysis the requirements concerning the micropore structure of cellular concrete have been proposed to ensure the proper shrinkage characteristic of this material.

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323

324

Vol. 7, No. 3 H. Ziembicka

General Volume changes (shrinkage - swelling) resulting from moisture content variations are particularly significant in regards to cellular concrete because of it s high tota~ porosity (65-85%) and specific surface of the pores in the range of about 30 m /g. A number of investigations (1) have shown that shrinkage is not dependent on maximum water saturation being characteristic for a given cellular concrete, but is a function of desorption properties. Various cellular concretes stored in the same temperature and relative humidity show different drying rates and achieve different desorption humidity. Essential differences in the behavior of these materials are observed when their moisture content is less than 20 per cent of the mass; i.e. in the range of capillary action. Different desorption processes are then accompanied by various shrinkage characteristics of given cellular concrete. Those phenomena can be explained by the action of capillary pressure (2), (3) in porous materials not f u l l y saturated with water. The course of these phenomena has not been f u l l y explained so far (4). Capillary pressure, qh' is determined according to 26 w RT = In(h) qhrh MVf where: 6w = water surface tension; r. = so called constant; T=absolute temperature; M=molecu~ar weight of equal to the volume of pores f u l l y saturated with water

La Place or Kelvin law: [I]

Kelvin's radius; R=gas water; V#=water volume, and, h=r~lative humidity.

I t appears from equation [ l ] that the capillary pressure is inversely proportional to the capillary radius and is closely related to sorption properties of porous materials. I t is therefore possible to determine volumetric changes in concrete on the basis of known total volume and size of the pore radii. Considerable research work is being done at present to determine the correlation of shrinkage and porosity of mortars and concretes. The works by Hansen (3), Mackenzie(5), Hashin and Fagerlund(6) are worth mentioning here. They investigated the correlation of the shrinkage of mortars and ordinary concretes with the structure of porosity characteristic for the adsorption and desorption properties of these materials. Because of the complex character of the phenomena, the formulas proposed by those authors are based on various simplifying assumptions and seem therefore to ~ somewhat controversial - f i r s t of all because the considerations are limited to a single pore size only (the assumption is made that all pores saturated with water have the same r a d i i ) . Regarding the significant diversification of the pores radii (7) i t seems necessary to determine the effect of s t a t i s t i c a l distribution of the micropores radii on the course and values of volumetric deformations of cellular concrete. Facing this problem, the a r t i c l e deals with the results of investigations carried out to determine the relationship between the structure of microporosity and shrinkage of cellular concrete. As noted later in this a r t i c l e , we have assumed that micropores are pores whose radii range from 75 to IOOOA. To settle the problem of terminology i t would be necessary to add that the concept of macro- and microporosity used in the literature is not e x p l i c i t ; there are considerable differences in this respect (8), (9), depending on the phenomena described and research methods used. In this case: -the minimum value of the micropores range (r = 75A) was imposed by the test method applied, -the maximum value of the micropores range ~r = IOOOA) results from the observation that the pores with radii up to lOOOA are decisive for the specific sur-

Vol. 7, No. 3

325 CELLULAR CONCRETE, SHRINKAGE, MICROPORESTRUCTURE

face and thus they also determine the sorption properties of cellular concrete; the specific surface of pores with radii up to lO00A is over 90 per cent of the total specific surface of pores in the cellular concrete (lO). Test Procedure and Results Autoclaved cellular concrete with a density of about 700 kg/m3 was tested. This material (with sand aggregate) Wasoproduced according to the Polish technology "Unipol" ( l l ) and contained ll,3A tobermorite and CSH (1) phase (12). The tests covered over 60 samples of cellular concrete taken from the current production of prefabrication plants. The cellular concrete shrinkage was determined on small beams 40 x 40 x 160 mm by measuring the changes of their length in the moisture interval from maximum water saturation (w ~ 60% of the mass) to desorption moisture content at relative humidity ~ = 40-45 per cent and temperature t ~ 20°C. Changes in the beams' length were examined in five successive cycles of wetting and drying in the above mentioned moisture interval. Simultaneously with shrinkage examination, the tests on structure of porosit y were performed by means of Carlo Erba mercury porosimeter. These measurements determined exactly the distribution of volume and specific surface of pores versus their radii on the basis of well-known behavior of non-wetting liquid in the capillaries. The porosimeter used measured the pores with radii of r > 75A. The pore radii were determined on the bases of the following relationship: r = 28 cos 0 [2] P where: r = pore radius; ~ = mercury surface tension; 0 = mercury contact angle; and p = absolute pressure exerted. When using mercury (taking approximately: a = 480 dynes/cm, 0 = 141,3°) and assuming that all pores are spherical, the following relationship is used in practice: r - 75.000 [3] P ° p = absolute pressure, kG/cm2" The results of the where: r = pore radius, A; tests are shown in Tables I-2 and Fig. I-2. Fig. l presents the results of testing the shrinkage of cellular concrete during successive cycles of wetting and drying. Three possible cases of total shrinkage changes due to successive cycles of wetting and drying are i l l u s t r a t e d , namely: a) decreasing of total shrinkage b) no changes c) increasing of total shrinkage. Table l contains the results of shrinkage examination for typical representative samples. The following data have been given in this Table:

a) b) c)

results of shrinkage - swelling tests of cellular concrete samples in successive cycles of wetting and drying shrinkage changes that occur after the successive cycles of wetting and drying total shrinkage after f i f t h desorption.

Table 2 and Fig. 2 show the results of testing microporosity characteristics for cellular concrete samples when starting the shrinkage examination. According to the assumption made earlier, micropores are the pores whose radii range

326

Vol. 7, No. 3 H. Ziembicka

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FIG. 1 Shrinkage c h a r a c t e r i s t i c s of c e l l u l a r concrete during f i v e succeeding cycles of wetting and drying.

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FIG. 2 D i s t r i b u t i o n of the predominating micropores (example).

"0,30 -0,23 -0,21 -0,21 -0, 21

-0,33 -0,28 -0~24 -0,24 -0,24

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1 2 5 4 5

1 2 3 4 5

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Shrinkage

Cycle No.

No.

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4

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-0,02 -0,03 -0,05 -0,03 -

..,

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Permanent shrAnkage

-0,10 -0,02 -,,0,01 0,02

-0,10 -0,12 - 0 , 13 -0, 15

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-0,09

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Changes of reversible shrinkage A 6",. [mm/m]

Table 1 Test results of cellular concrete shrinkage during successive cycles of wetting-drying

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Number o f t h e s a m p l e s t e s t e d

Total

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0,110 0,215 0,270 0,330

16

0,30

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m

13,30 23,10 26, 40 28,70

0,085 0,220 0,285 0,340

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0,31-0,40

22,50 26,80 28,20

16,30

0,335

0,295

0,110 0,220

12

0,41-0,50

i

17,20 28,70 31,50 32,80

,J,

0,115 0,270 0,335 0,390

20

0,51-0,60

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Characteristic of shrinkage and structure of microporosity of the cellular concrete samples

Table 2

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Vol. 7, No. 3

329 CELLULAR CONCRETE, SHRINKAGE, MICROPORESTRUCTURE

from 75 to IO00A. This interval of structure of porosity has been separated from the total pore volume of c e l l u l a r concrete. Table 2 shows average d i s t r i b u t i o n of micropore volumes and specific surface for c e l l u l a r concrete samples of d i f f e r e n t shrinkage. For c e l l u l a r concrete samples, shrinkage of which is shown in Table I , the d i s t r i b u t i o n of predominating micropores has also been determined (Fig. 2). Evaluation of the Test Results The results given in Table 1 and Fig. 1 indicate that shrinkage of c e l l u l a r concrete should be analysed with respect to permanent ~ and reversible ~r parts. P ' The largest permanent shrinkage in c e l l u l a r concrete occurs a f t e r the f i r s t desorption. With an increase of the number of successive cycles of wetting and drying the increment of permanent shrinkage tends to approach zero and the reversible shrinkage becomes s t a b i l i z e d ( ~r = const). Occurence of permanent shrinkage is accompanied by the effect of decreasing shrinkage amplitude after the successive desorption (Cl
330

Vol. 7, No. 3 H. Ziembicka V..~s; ice/gl

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micropore volume is most d i s t i n c t in this microporosity range. The correct shrinkage characteristic is conditioned also by proper distribution of pore radii - proper specific surface of micropores. Relationship between the shrinkage and volume of micropores in the range of pore radii 75-625A has been proved through mathematical analysis by means of the least squares method (Fig. 3). This relationship can be expressed by the following equation: V75_625 = 0,229~t + 0,204 [5] 0

where: V - micropore volume in the range of radii 75-625 A; and ~t - total shrinkag~ -625 This dependence is characterized by a linear correlation coefficient r = 0,786 and standard deviation 6, = 0,0165 cm3/g. Thus, in the limiting conditions, when total shrinkage of the cellular concrete is equal to ~+ = 0.50 mm/m, the micropores volume ought to be contained with the probabTlity p ~0--~0 in the reliance interval:

n.n'~h_7 20 m

I0

?

\

FIG. 4 Distribution histogram of micropores specific surface, when Eta0,50 mm/m

Vol. 7, No. 3

331 CELLULAR CONCRETE, SHRINKAGE, MICROPORESTRUCTURE

0,30 < V75_625 <0,34 [cm3/g] [6] Another parameter of micropore structure important for the proper characteristics of c e l l u l a r concrete shrinkage is the specific surface of the micropores which is the function of the micropore radii d i s t r i b u t i o n . For c e l l u l a r concrete the requirementsoare ~,< 0.50 mm/m - the specific micropore surface in the range of radii 75-625 A is a~proximately S ~ 26 m /g. The d~stribution of this variable has been verified with the use of the c r i t e r i o n × and shows that i t may be approximated by normal d i s t r i b u t i o n (Fig. 4). I t has been found, on the basis of the test results, that from the viewpoint of the proper characteristics of c e l l u l a r concrete shrinkage (~, ~ O.50mm/m), the specific surface of the micropores in the range of radii 75-625 ALdetermined by means of a porosimeter, cannot be greater than S=30.5 m2/g. Correctness of the assumed interpretation seems to be confirmed by the tests results carried out by the author on c e l l u l a r concrete with sand aggregate produced by NIPISilikatobeton I n s t i t u t e in T a l l i n (Soviet Union). Conclusion Test results described in this paper showed a functional relationship between the shrinkage and micropore structure of c e l l u l a r concrete. They also determined the requirements to be met by the micropore structure in the range of pore radii 75-625 A from the viewpoint of proper shrinkage process in c e l l u l a r concrete (ct~0.50 mm/m). Bibliography I.

La~ M., Ziembicka H.: Technical Journal. Krakow. Z. 9-B (160) 12-19, 1972.

2.

Hansen T.C.:

Technical University of

Materiaux et constructions. 7.

7-9, 1969.

Table 3 Results of tests carried out on c e l l u l a r concrete samples from NIPISilikatobeton of Tallin

No.

Microporosity structure

Shrinkage x/

,i

i

i

Specific surface

Volume

V -825 Ecm3/g] 1

-0,10

O, 170 i

2

-0,34 ,L

3

-0,42

4

-0,57

1 1

,

14,55 ,

,

0,233

,

i

23,19 J

0,241

26,98 35,36

X/According to the information given by the NIPISilikatobeton Institute - shrinkage after the first desorptlon.

332

Vol. 7, No. 3 H. Ziembicka

3.

Hashin Z.:

J.App.Mech. 29, 143-150, 1962.

4.

AdamsonA.W.: 1960.

5.

Mackenzie J.K.:

6.

Fagerlund G.: Sympozium RILEM/JUPAC, Pore Structure and Materials Properties, Praha, 1973.

7.

Ziembicka H.: Polish-American Symposium: Concrete Today and Tomorrow in Housing. Polish Contribution, Warsaw 1973, 79-96.

8.

Macmillan M.: Symposium RILEM/JUPAC, Pore Structure and Materials Properties, Praha, 1973.

9.

Ziembicka H., Tabak R.: New Test Method and Evaluation Criterion of Shrinkage in Autoclaved Cellular Concrete. "CEBET," Warszawa, 1972/73 (in Polish).

Physical Chemistry of Surfaces.

Los Angeles, California,

Proc.Phys. Soc. 683, 2-11, 1950.

I0.

Ziembicka H.: Fourth Scientific-Technical Conference of Concrete Industry, 1973, Jadwisin, 49-61.

II.

Jatymowicz H., Siejko J., Zapotoczna-Sytek G.: Cellular Concrete. Arkady, Warszawa, 1975.

12.

Ziembicka H., Tabak R.: New Test Method and Evaluation Criterion of Shrinkage in Autoclaved Cellular Concrete. "CEBET," Warszawa, 1975 (in Polish).

Technology of Autoclaved