EP1042249A1

EP1042249A1 - Injection grout comprising fly ash

Info

Publication number: EP1042249A1
Application number: EP98925713A
Authority: EP
Inventors: Gérard DELEURENCE; François MIERSMAN
Original assignee: Surschiste SA
Current assignee: Surschiste SA
Priority date: 1997-12-29
Filing date: 1998-05-14
Publication date: 2000-10-11
Also published as: FR2773146A1; WO1999033762A1; FR2773146B1; PL341402A1; CZ20002451A3

Abstract

The invention concerns an injection grout containing: water, a cementing material, at least 100 kg/m<3> of fly ash derived from coal combustion in circulating fluidised bed.

Description

"Injection grout with fly ash"

The invention relates to an injection grout composition.

In general, injection grouts are intended, in the field of civil engineering, to improve or restore the characteristics of various terrains, whether from a mechanical point of view (resistance, cohesion, modulus of elasticity, ... ), from a hydraulic point of view (notably permeability) or from an environmental point of view (retention of pollutants most often). These lands may or may not be on an urban site; for example, it may be cracked rock masses to be reinforced or sealed, powdery soils to be consolidated, alluvium to be sealed, old masonry to be regenerated, bars or anchors with seals, natural or artificial cavities to be filled. , contaminated soil to be stabilized or land in which we want to put in place waterproofing sails. Compared with traditional consolidation or sealing techniques, injection grouts have the advantage of using relatively economical mixtures and of having a flexible and rapid implementation. In a simplified manner, these mixtures are sent with an appropriate pressure to the medium to be treated (possibly prepared by drilling injection holes).

In general, an injection grout contains water, cement and, depending on the desired performance, bentonite and / or fillers. It is a fluid suspension based on water in principle formed of grains the size of which does not exceed 0.3 mm; she must be sufficiently fluid to flow under pressure into the destination volume where it must, after hardening, have the desired properties.

Cement is, as we know, a mineral binder based on limestone and clay as main raw materials which, mixed with water, sets (we speak of hydraulic setting); various categories are known by designations such as CLK, ultrafine (except aluminum), CPA, etc.

The addition of fly ash has the effect of giving the mixture pozzolanic properties which promote hydraulic setting (there is formation of calcium silicate hydrates and calcium aluminates which improve the mechanical properties in particular).

The fillers (chemically inert pulverulent materials) can conventionally be natural sands, fly ash (most often from thermal power stations), fillers, natural pozzolans, sawdust, etc. They are the ones which give the grout its particular, rheological, mechanical, durability or fixing properties of certain pollutants.

There may also be additives to improve the fluidity of the mixture to be injected in particular: these are chemicals incorporated in small proportions (less than 5% in practice) to improve or supplement the characteristics of the mixture. For example, lignosulphates, resin soaps or synthetic detergents improve fluidity (or reduce the amount of water necessary to obtain a given fluidity), calcium or sodium chlorides accelerate setting hydraulic while carbohydrates or zinc oxide delay this setting. Bentonite is a mineral component of the family of smectitic clays (highly hydratable) which exhibits an apparent swelling with water of 3 to 18 times the initial volume. It exists in natural, permuted or activated form. It allows the formation of a homogeneous colloidal mixture, makes it possible to suppress the sedimentation of the cement during the implementation and increases the setting time and the resistance to washing out. This is why bentonite is generally considered to be a component essential (at a content typically greater than at least 3% of the dry extract of the grout) of any suspension for injection which we want to be stable (which is typically characterized by bleeding (rising water to the surface of the coulis) at most 3% in 2 to 3 h). Thus, in general, an injection grout typically has a composition respecting the following dosages:

* 50 to 300 kg / m ³ of cement;

^* 200 to 1200 kg / m ³ of load;

* up to 100 kg / m ³ of bentonite; with a possible addition of adjuvants (fluidizers most often).

In fact there are strong variations according to the needs and, for example, the notion of cement grout corresponds to the following narrower ranges (with a negligible quantity of fillers): + 200 to 450 kg / m ³ of cement; + up to 100 kg / m ³ of bentonite;

+ 600 to 950 l / m ³ of water; with 0.12 <C / E <2 where C / E represents the quantity of binder to water ratio.

If on the other hand we are talking about sand concrete, the typical dosages are: - 1400 kg of sand, - 350 kg of cement,

- 160 kg of CFL, - 215 kg of water.

However, the implementation of bentonite has the drawback of requiring two stages in the preparation of a grout: first of all the preparation of a mother mud by mixing the bentonite and water and maturing in a primary tank or even in silo, then adding cement and fillers in a mixer coupled to the mixing tank. The ripening phase in the first stage requires having the primary tank and takes time, which has an obvious negative influence on the cost of the injection operation. The invention firstly aims to overcome this drawback with a grout composition of moderate cost and easy to use, giving a stable suspension without requiring the presence of bentonite or the like. Furthermore, the various ingredients of the pulverulent mass suspended in an injection grout, whether it is cement or fillers (in particular fines typically less than or equal to 0.1 mm in size), have a significant cost which makes them too expensive for certain applications for which moderate performance is required. And there is a need for a substitute product which is more economical while being able, in such cases, to replace cement or fines without loss of performance.

Finally and above all, the components of the injection grout have the drawback either of requiring for their preparation a significant consumption of electricity (cement in particular), or of involving the consumption of natural products (exploitation of quarries in particular). There is therefore also a need for a substitute product which can replace cement, sand or fines without having the disadvantages from an ecological point of view.

The invention therefore very generally relates to an injection grout composition having a moderate cost price thanks to the use of a pulverulent product which can, for a lower cost and / or without ecological disadvantage, be substituted without impair the required performance, bentonite, or even cement and / or fines.

To this end, it offers a composition of injection grout containing:

- some water ;

- cement or more generally a hydraulic binder;

- at least 100 kg / m3 of fly ash from the combustion of coal in a circulating fluidized bed. Another way of defining these ashes is to say that they are formed on average by at least 70% by mass of aluminum oxides, silicon and / or iron, have a total lime content of at most 10% by mass and the grains of fly ash have an irregular shape.

According to preferred characteristics of the invention, advantageously combined: - the quantity of fly ash is at least 270 kg / m3, preferably at least equal to 350 kg / m3;

- This composition contains a substantially zero amount of bentonite;

- the quantity of cement is at most equal to that of fly ash;

- the quantity of fly ash is substantially between 350 kg / m3 and 400 kg / m3;

- the quantity of cement is at least approximately equal to the quantity of fly ash; - the quantity of cement and fly ash is at least equal to

800 kg / m3;

- the quantity of water is at least equal to 700 kg / m3.

Such a grout has a moderate bleeding, which makes it particularly suitable for the production of diaphragm walls for example. According to other preferred characteristics of the invention, advantageously combined:

- The amount of cement is of the order of 2 to 3 times the amount of fly ash, and the composition contains a substantially zero amount of bentonite; - the quantity of fly ash is at most equal to around

300 kg / m3;

- the quantity of fly ash is at least equal to around 200 kg / m3;

- the quantity of water is at least equal to around 600 kg / m3. Such a grout is perfectly suitable for applications where a grout of low density and moderate bleeding is sought. According to other preferred characteristics, advantageously combined:

- The cement content is less than about 80 kg / m3 and the fly ash content is at least equal to about 600 kg / m3, and the composition contains a substantially zero amount of bentonite;

- the quantity of water is greater than around 700 kg / m3.

A filling slurry of low mechanical strength is thus obtained.

If, on the other hand, a filling grout of high mechanical strength is sought, according to other preferred characteristics:

- The amount of cement and fly ash is greater than 750 kg / m3 and the composition contains a substantially zero amount of bentonite;

- the quantity of cement and fly ash is at least equal to 1000 kg / m3; - the quantity of fly ash is at most equal to 250 kg / m3, preferably at most equal to 150 kg / m3;

- the total quantity of cement and ash is in the range of 1200 to 1300 kg / m3;

- the quantity of water is at least equal to 500 kg / m3. Finally, according to still other preferred characteristics:

- the load composition contains at least 1400 kg / m3 of sand;

- the quantity of charges is worth between double and triple the quantity of cement and fly ash; - the quantity of fly ash is at least equal to around

150 kg / m3;

- the amount of water is in the range of 200 to 250 kg / m3. This gives a completely effective sand concrete.

The invention stems from the observation, which had nothing obvious a priori, that fly ash obtained by burning coal in a circulating fluidized bed can have chemical, mineralogical properties, Morphological and pozzolanic such that these ashes can be effectively substituted, within a wide variety of injection grouts, for bentonite, but also for cement and / or fines.

Such ash has a cost price close to, if not lower than that of bentonite, sand and / or fines.

In addition, coal ash is considered inert and non-dangerous and its preparation does not involve any specific industrial structure, large energy consumption, or depletion of quarries or natural sites. On the contrary, the exploitation of the ash deposits deposited contributes to the reconquest of the sites.

Given its lightness and density in place, the coal ash also contributes to transport savings. Finally, bringing durability to the structures, it often allows a general saving over time. Such fly ash is to be distinguished from fly ash from the combustion of lignite.

In fact, one can distinguish several types of fly ash, according to whether they result from the combustion of coal or lignite, according to whether this combustion takes place in a boiler (with loading of a mass to be burnt, and unloading of ashes ) or in a circulating fluidized bed, and depending on whether this combustion is associated (or not) with the elimination of the sulfur pollutants upstream or downstream of the combustion site.

Circulating fluidized bed boilers (LFC for short) operate at temperatures of around 850 ° C maximum, so that fly ash (often in the form of illite-micas) have irregular shapes, sometimes flaky, often porous, unlike the fly ash of conventional boilers (which operate at a much higher temperature, above 1000 ° C, typically around 1350 ° C, in which case the ash particles melt) which are generally vitrified and generally spherical in shape. LFC fly ash conventionally have a BET surface (for example greater than 10 m ² / g) as well as a fineness (more than 10,000 cm ² / g) much higher than that of conventional boiler ashes (respectively of the order of 1 to 2 m ² / g and 3000 cm ² / g). It is recalled here that the BET surface and the fineness are complementary parameters respectively characterizing the particle size of the ash and the cumulative surface of all the grains of ash.

LFC boilers are in principle combined with upstream pollution control (this is what conditions the moderate level of temperature), and this by limestone filler which will become lime in part during combustion.

However, CFL ash obtained by burning coal contains less lime than CFL ash obtained by burning lignite since the lignite already contains limestone. Conversely, LFC ash from coal contains more alumina, iron oxides and silica than LFC ash from lignite.

This is why fly ash obtained by combustion of coal in a circulating fluidized bed can be defined by the following conditions:

* content of alumina, silica and iron oxides of at least 70% (preferably at least 75%);

* irregular shape;

* total lime content less than 10% (in practice there is less than 4% in free lime, preferably of the order of one percent).

Various tests of injection grout compositions were carried out using, for example, LFC fly ash from coal obtained at the EMILE HUCHET IV power station in Carling in the Moselle and characterized as follows:. Si0 ₂ 44.5%;

. Al ₂ 0 ₃ 22.6%;

. Fe ₂ 0 ₃ 8.3%;

. CaO 5.9% including 1.3% free lime;

. MgO 3.4%; . Na ₂ 0 0.4%;

. K ₂ 0 4.0%; . 0.1% MnO;

. Ti0 ₂ 0.9%;

• P ₂ 0 ₅ 0.1%;

. Cr ₂ 0 ₃ <0.01%; . S0 ₃ 3.9%;

. Loss on ignition 6.0;

. grains in the form of platelets, mostly illicit; . fineness: 0/315 microns of which 80% pass through a mesh of 45 microns; . absolute density: 2.5;

. BET specific surface (measured by Krypton) 12.4 m ² / g; . activity index: 83 to 92% at 28 days, 92 to 98% at 90 days. The various tests were carried out in the laboratory on various samples prepared according to standard NF-P18-357:. density measurement by weighing a known volume of grout;

. measurement of exudation or sweating according to standard NF-P18-359, using a 200 ml graduated cylinder; the penetrant corresponds to the ratio of the height of the supernatant water to the total height of the grout in the test piece; . fluidity measurement according to standard NF-P18-358, by measuring the flow time at the MARSCH cone (nozzle of 4.75 mm or 10 mm depending on the case) of a given volume of grout; DPU measurement (in some cases), i.e. measurement of the practical duration of use; . measurement of the end of setting time according to standard NF-P18-356; it corresponds to the instant from which the measuring needle sinks no more than 0.5 mm into the grout;

. measurement of mechanical resistance in bending or in compression according to standard NF-P18-360 (with 4 x 4 x 16 cm prisms) after storage in water for 7, 28 or 90 days; . measurement of resistance to sulphated media (in certain cases) according to standard NF-P18-837;

. 28 day withdrawal measurement (in some cases) according to standard NF-P18-433. EXAMPLE 1:

Grout samples for diaphragm walls were prepared under the references T1, T2, A2 to A4, A8 and M.

Table 1 shows the compositions, Table 2 shows various behavioral properties and Table 3 shows the mechanical strength performance.

The T1 and T2 grouts are control grouts with bentonite but without LFC ash. No adjuvant was necessary to achieve satisfactory fluidity.

Grout A2 to A4 and A8 are grout without bentonite but with LFC ash. And the grout M is a "mixed" grout containing bentonite and ash.

The E / C and E / L ratios represent the relationships between WATER / cement and water / binder.

The quantity of LFC ash in the slurries A2 to A4 and A8 is much higher (in a ratio of the order of 10 to 1) than the quantity of bentonite in the control grouts T1 and T2.

For these various grouts, containing 150 to 370 kg / m ³ of cement, the preparation procedure was as follows:

^* preparation and hydration of bentonite mud (if necessary); * incorporation of water (and adjuvant if necessary);

* incorporation of dry materials;

* homogenization for 4 minutes;

^* making test pieces and measuring the properties of fresh grout. We observe that the density of the grouts A2 to A4, A8 and M is higher than that of the grouts T1 and T2, which is explained by the fact that the bentonite has been replaced by larger amounts of ash although the amount of water has decreased.

The bleeding of the A2, A3 and A8 grouts of the order of 2 to 4% is much lower than that of the control samples (8%). Likewise, the setting time of these samples is shorter than that of the control samples (of the order of 60 h).

The A4 sample, lightly loaded with LFC ashes, on the other hand has a high bleeding (10%) and a long setting time (48 h).

Finally, sample M has very low bleeding and a very moderate setting time.

We deduce that, to obtain a low bleeding (typically less than 5%), a minimum of about 350 kg / m ³ of LFC ash is required (with possibly a moderate amount of bentonite - see the grouts T2 and M). The bleeding is then much better than with bentonite alone. In the aforementioned grouts, it can be noted that the quantity of LFC ash is at least of the order of the quantity of cement (it can go up to at least a 3/1 ratio).

The setting time is shorter than the intermediate quantities of LFC ash; the optimum is found for A3 and M. For this parameter, it seems preferable that the quantity of CFL ash is of the order of 350 to 400 kg / m ³ (preferably equal to that of cement), or quantities LFC upper ash provided that the quantity of cement is lowered twice as much (the total being around 600 kg / m ³ ).

From the point of view of mechanical strength, the presence of CFL ash in the absence of bentonite, leads to a significant increase from an amount of about 270 kg / m ³ . The optimum is reached with the A3 grout, followed by the A8 grout, which leads to the conclusion that close quantities of CFL ash and cement, above 350 kg / m ³ , as well as a total quantity greater than 800 kg / m ³ are favorable for obtaining very good mechanical performance. The ease (MARSCH cone with 4.75 mm nozzle except for A8 (10 mm nozzle)) is better when the quantity of LFC ash is between 450 and 500 kg / m ³ .

EXAMPLE 2: Various lightened grout samples were prepared (for example intended for the cementing of petroleum casings) under the references B2 to B5.

Table 4 gives the compositions in kg / m ³ , Table 5 gives various properties (fluidity measured with nozzle of 10 mm for B2, B3, B4, and 4.75 mm for B5) and Table 6 in gives mechanical properties.

The cement used was of the CPA type; compared to CLK cement, CPA cement is characterized by the fact that it consists of at least 95% ground clinker and does not contain slag.

Grout B2 is a control grout containing a filler formed to 80% of classic fly ash of the aluminous silico type of coal-fired thermal power plants and to 20% of diatoms (these are commonly used to lower the density of the grout) .

Grouts B3, B4 and B5 contain more and more water, less and less cement and CFL ash and have increasingly low densities. Grout B3 has an amount of LFC ash equal to the amount of fly ash and diatoms of grout B2. The increase in the quantity of water induces a drop in density and an increase in the bleeding and the setting time.

It can be noted that the quantity of cement is between 2 and 3 times the quantity of CFL ash, which is at most equal to around 300 kg / m ³ .

The mechanical characteristics of the B3 grout are of the same order as those of the control grout; they fall naturally when the quantities of LFC cement and ash decrease.

LFC ash therefore effectively replaces fly ash and diatoms in light grouts. EXAMPLE 3:

Samples of filling slurry with low mechanical resistance (typically usable for filling large cavities) were prepared under the references C1 to C3. Table 7 gives the formulations in kg / m ³ , Table 8 gives various properties and Table 9 shows the shrinkage after 28 days.

These grouts contain very small amounts of cement

(less than 80 kg / m ³ ) but significant quantities of CFL ash (more than 600 kg / m ³ ), approximately of the order of the quantity of water less than

20% near (this quantity of water is in the order of 700 to 750 kg / m ³ ).

The fluidity was measured with a 10 mm nozzle.

The bleeding is weak (less than 2%) as is the setting time (12 hours). The mechanical resistance in compression is low (1 to 2 Mpa) substantially proportional to the quantity of cement. The mechanical resistance in bending was lower than the precision threshold of the measuring device used (0.35 Mpa).

The shrinkage after storage for 28 days in a room at 20 ° C and 50% ± 5 %% relative humidity was all the more important as the amount of cement increased and that of LFC ash decreased.

EXAMPLE 4:

Samples of filling grout with high mechanical resistance (in principle at least equal to 40 MPa) were prepared under the references D1 to D5.

Table 10 specifies the compositions, table 11 shows various properties, table 12 shows mechanical performance and table 13 shows the dimensional variations in sulfated medium (in microns per meter). An adjuvant (1 to 2% of the weight of cement and ash) was added to the grouts D3 to D5. These grouts have an W / C ratio of 0.5; the control grout D1 contains 1280 kg / m ³ of cement, and the grouts D2 to D5 contain more and more LFC ash and less and less cement, with a total binder (cement + ash) slightly decreasing from 1280 to 1075 kg / m ³ . The quantity of ash varies between 10% (see D2) and 65% (see D5) of the quantity of cement; in other words, the quantities of LFC ash in D2, D3, D4 and D5 represent 10%, 20%, 30% and 40% of the quantity of cement in D1.

The bleeding is greatly reduced while the fluidity (with 10 mm nozzle) is increased. On the other hand, the setting time is moderate (20 h).

The practical duration of use decreases when the quantity of ash

LFC increases but is still acceptable for D2 and D3 (40 mm and 30 mm).

Mechanical performance also decreases when the amount of ash increases, but this decrease is very moderate with D2. It should be noted that this drop in mechanical performance can be partly explained by the drop in the total amount of binder.

To measure the water resistance with a high sulphate content, 4 x 4 x 16 cm test tubes were immersed in a solution of 50 g / l of magnesium sulphate (MgS0 ₄ ). Samples D2 and D3 showed less swelling than that of D1; on the other hand, the swelling of D4 was very significant.

In addition, the external faces (4 x 4 cm) of similar test pieces were subjected for one month to the penetration of a saline solution (NaCl) at 150 g / l in the case of the grouts D1 and D2. Measurements of free and total chlorides were then carried out at different depths. This made it possible to assess the ability of the grout to bind the chlorides.

The penetration depth of the chlorine ions was the same for D1 and D2 (of the order of 12.5 mm, even though D2 had poorer mechanical strength and higher porosity: there is therefore in D2 a improved ability to bind chlorides; it has even been found that the amount which can be bound by D2 is about double that which can be bound by D1. In summary, LFC ash can effectively replace around 10% (or even 20%) of the amount of cement in a high-strength filler grout. In other words, such a grout advantageously contains up to 150 or even 250 kg / m ³ of CFL ash in combination with cement, with a total of cement and CFL ash of the order of 1200 to 1300 kg / m ³ .

Finally, it is established that the presence of ash at at least about 100 kg / m ³ allows a very significant increase in the capacity to bind chlorides.

EXAMPLE 5: Grout samples were prepared under the references E1 to

E3.

Table 14 gives the formulations while Table 15 shows various properties.

It can be noted that the E1 grout is identical to D1, the E2 grout is similar to D3 (replacement of 20% of the cement by LFC ash) but the E3 grout corresponds to a total binder of nearly 750 kg / m ³ formed at more than 90% LFC ash (see grout C2).

The presence of LFC ash leads to very low bleeding and a high proportion of LFC ash leads to a moderate setting time, but on the other hand to very weak mechanical properties. EXAMPLE N ° 6:

Two samples of sand concrete were prepared under the references F1 and F2. Table 16 gives the compositions while Table 17 gives mechanical properties. Grout F2 is distinguished from grout F1 by a quantity of

160 kg / m ³ of CFL ash replacing an equal amount of fine limestone; the total of binder is of the order of 500 kg / m ³ while the sand represents a load of 1400 kg / m ³ , or between double and triple the amount of binder.

Grout F2 has much better mechanical resistance than grout F1.

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

Table 11

Table 12

Table 13

Table 14

Table 15

Table 16

Table 17

Claims

1. Composition of injection grout containing:

- some water ; - a hydraulic binder such as cement;

- at least 100 kg / m3 of fly ash from the combustion of coal in a circulating fluidized bed.

2. Composition according to claim 1, characterized in that the fly ash is formed at least 70% by mass of aluminum, silicon and / or iron oxides, have a total lime content of at most 10 % by mass and the grains of fly ash have an irregular non-spherical shape.

3. Composition according to claim 1 or claim 2, characterized in that the amount of fly ash is at least 270 kg / m3.

4. Composition according to claim 3, characterized in that the quantity of fly ash is at least equal to 350 kg / m3.

5. Composition according to claim 3 or claim 4, characterized in that this composition contains a substantially zero amount of bentonite.

6. Composition according to any one of claims 3 to 5, characterized in that the quantity of cement is at most equal to that of the fly ash.

7. Composition according to any one of claims 3 to 5, characterized in that the amount of fly ash is substantially between 350 kg / m3 and 400 kg / m3.

8. Composition according to any one of claims 3 to 7, characterized in that the amount of cement is at least approximately equal to the amount of fly ash.

9. Composition according to any one of claims 3 to 8, characterized in that the quantity of cement and fly ash is at least equal to 800 kg / m3.

10. Composition according to any one of claims 3 to 9, characterized in that the amount of water is at least equal to 700 kg / m3.

11. Composition according to claim 1 or claim 2, characterized in that the amount of cement is of the order of 2 to 3 times the amount of fly ash, and the composition contains a substantially zero amount of bentonite.

12. Composition according to claim 11, characterized in that the quantity of fly ash is at most equal to around 300 kg / m3.

13. Composition according to claim 11 or claim 12, characterized in that the amount of fly ash is at least equal to around 200 kg / m3.

14. Composition according to any one of claims 11 to 13, characterized in that the amount of water is at least equal to around

600 kg / m3.

15. Composition according to claim 1 or claim 2, characterized in that the cement content is less than about 80 kg / m3 and the fly ash content is at least equal to about 600 kg / m3, and the composition contains a substantially zero amount of bentonite.

16. Composition according to claim 15, characterized in that the amount of water is greater than around 700 kg / m3.

17. Composition according to claim 1 or claim 2, characterized in that the amount of cement and fly ash is greater than 750 kg / m3 and the composition contains a substantially zero amount of bentonite.

18. Composition according to claim 17, characterized in that the quantity of cement and fly ash is at least equal to 1000 kg / m3.

19. Composition according to claim 17 or claim 18, characterized in that the quantity of fly ash is at most equal to 250 kg / m3.

20. Composition according to claims 17 to 19, characterized in that the quantity of fly ash is at most equal to 150 kg / m3.

21. Composition according to claims 17 to 20, characterized in that the total amount of cement and ash is of the order of 1200 to 1300 kg / m3.

22. Composition according to claims 17 to 21, characterized in that the amount of water is at least equal to 500 kg / m3.

23. Composition according to claim 1 or claim 2, characterized in that the composition of fillers contains at least 1400 kg / m3 of sand.

24. Composition according to claim 22 or claim 23, characterized in that the quantity of fillers is between double and triple the quantity of cement and fly ash.

25. Composition according to claims 22 to 24, characterized in that the quantity of fly ash is at least equal to around 150 kg / m3.

26. Composition according to claims 22 to 25, characterized in that the amount of water is of the order of 200 to 250 kg / m3.