WO1990014322A1

WO1990014322A1 - Method and plant for the production of injectable cement mixtures

Info

Publication number: WO1990014322A1
Application number: PCT/EP1990/000810
Authority: WO
Inventors: Agostino Balducci; Bruno Bosco; Claudio Rebuscini
Original assignee: Ing Giovanni Rodio & C Impresa Costruzioni Speciali SpA
Current assignee: Ing Giovanni Rodio & C Impresa Costruzioni Speciali SpA
Priority date: 1989-05-22
Filing date: 1990-05-18
Publication date: 1990-11-29
Anticipated expiration: 1991-11-22
Also published as: EP0473628A1; IT1230847B; IT8920581A0; JPH04505442A

Abstract

In a method for the production of a cement mixture for injection into fine gravel soils, porous or microfractured rocks and/or materials a commercial cement is reduced to fine grain size by milling to a required granulometry an aqueous suspension having a cement versus water ratio within the range from 0.25 to 2.5 by weight; before, during or after said milling step, at least one fluidifying additive is added to said suspension. The obtained milled mixture is finally injected into said soils or rocks.

Description

Method and plant for the production of injectable cement mixtures

The present invention concerns a method for the production of cement mixtures suitable to be used, thanks to their characteristics of permeability at low viscosities, in consolidation injections and/or in sealing fine grain soils or rocks and/or porous or microfractured materials.

The present invention further concerns a plant for carrying-out the above mentioned method.

The known methods at present used for the consolidation of sandy soils generally involve the injection of mixtures based on sodium silicates. Such techniques, while foreseeing a setting time compatible with the duration of the operation and while giving good sealing properties and mechanical cohesion to the treated soils, have the drawback of creating a possible source of pollution of the groundwater due to the release of water-soluble chemical substances.

The necessity has therefore been identified to obtain injectable mixtures based on insoluble products which are suitable to give consistency to the soil, avoiding any risk of groundwater pollution.

Cement is an ideal product for this purpose, because it allows to obtain excellent consolidation results and, being insoluble, it does not affect the groundwater.

However, it must be injected as a suspension, and the use of cement mixtures for injections is thus limited by the particle size of the commercially available cement, which, in addition to the aggregation phenomena of the cement particles dispersed in water, prevents the penetration into fine alluvional soils and microfractureri rocks or materials. Due to this grain size and aggregation phenomena, soil fracture problems arise during injections, resulting in formation of localised panels of cement, called "claquage", in said fractures, without obtaining the reqLiired uniform and diffuse permeation of the soil by the cement mixture.

If a cement mixture with very much smaller particle siz e were available, .it could be possible to inject the same into the soil with good penetration properties, even in fine sands, without the abovementioned soil fracture effects.

A known method for obtaining cement mixtures with sufficiently fine particle size to be injected into the ground (e.g. particles passing a 10 micron filter) provides for decanting a suspension of a commercially available cement into water to separate by settling the heaviest fraction, which is about 80% of the total cement used, from the supernatant suspeneion, which contains, the finest particles and can be uniformly injected into the soil.

However this method is a very expensive one, because the actually used portion is only 20% of the whole cement mixture and it takes a long time to be carried-out, due to the time necessary for sufficient settling out to take place, and because it is a "batch" process.

GB-A-494856, in the name of H. Ridley, discloses the preparation of a cement mortar by milling an aqueous mixtυre of cement and sand or other solids to a gel consistency.

The obtained mixture is then sprayed on a surface (e.g. the walls of a tunnel), on which it adheres because of its gelly consistency. This mixture is clearly unsuitable for injecting purposes.

Accordingly, an object of the present invention is to overcome the aboyementioned problems by providing a method which supplies an injectable cement mixture into fine granular soils and porous or microf ractured rods or materials using commercially available cement, quickly and without waste.

A further object of the present invention is to provide a plant for the production of said cement mixture which can be injected within a short time into fine-gram soils and rocks end/or porous or microf ractured materials.

More specifically, the present invention concerns a method for the production of mixtures of cement or similar hydraulic binders to be injected into fine-grain soils and into porous or microfractured rocks or materials, characterised in milling to a required granulometry an aqueous suspension having a ratio of cement versus water within the range from 0,25 to 2,5 by weight; and un adding to said suspension, before, during or after said milling step, at least one fluidifying additive.

According to a preferred method, 0,5 to 7% by weight of fluidifying additives are added to a cement mixture having a ratio of cement/water of 1.5 before the milling step. Moreover, the present invention concerns a plant for the production of injectable mixture of cement or similar hydraulic binders, characterized in that it comprises a ball mill or similar milling means, for milling an aqueous suspension of commercially available cement; means, for adding to said suspension at least one fluidifying additive, before, during or after said milling step; and means for feeding said milling suspension to injection means.

The invention vail now be described in more detail with reference to the attached drawings which are given for illustrative and non-limitative purposes, and wherein: Fig 1 is a flow chart of a plant according to the invention:

Fig 2 is a schematic diagram of a particular embodiment of a plant according to the invention, provided with a hydraulic particle separator/classifier;

Fig 3 is a graph comparing the particle size range of a commercially available cement before and after milling according to the invention;

Fig 4 is a graph of the parti cl e size range in a medium- fine sand.

According to present invention, a cement mixture suitable to be injected into fine-grain soils and porous or microfractured materials and/or rocks in order to obtain consolidation thereof is prepared by reducing the particle size of the commercial cement, i.e. cement normally available on the market, to a sufficiently low value by milling to the required granulometry an aqueous suspension of said commercial cement having a ratio of cement versus water within the range from 0.25 to 2.5 by weight.

A ratio of cement/water, by weight, of 2.5 resulted as the maximum ratio of a millable mixture, and mixtures having a cement/water ratio of less then 0.25 proved to be too diluted and brought to a self-grinding of the mill components.

Mixtures having a high ratio of cement versus water, i.e. a C/W ratio higher than 0.5, are further diluted to the required concentration before the injecting step.

Before, during or after said milling step at least one fluidifying additive i s added to the suspension, in order to make the suspension injectable or, according to the preselected cement/water ratio, suitable to undergo the milling step too. Actually it was found that while suspensions with a cement/water ratio up to 1.0 (w/w) may be successfully milled without addition of fluidifving agents, suspensions having a higher ratio of cement vs water can be milled only if a suitable fluidifying additive is added to the mixture before or during the milling step, to avoid the mill blocking. Mixtures, having a ratio higher than 1.5 are preferably treated with said additives before the milling step.

Said additives are preferably added to the cement mixture in an amount within the range from 0,5 to 7% by weight of the cement, according to the mixture characteristics and the particle average size. Any suitable agent can be used which will reduce flpeculation of the cement and involve "sol vatation" thereof by any extension of the setting time, allowing the separation of the fine from the coarse particles and the subsequent injection of tbe fines into the soil. For instance such additives could be chosen from the following groups of compounds: polynaphthalensulphonic acid, pelyalkylsulphonic acid, polymethylmetacrvlic acid, polyphosphoric acid, their salts, such as sodium salts, melamine resins and mixtures thereof. A preferred additive is sodium polynaphtalensulphonate. The method of the invention is preferably carried out batchwise, by milling almost all the cement mixture that entered the mill to the desired granulometry and feeding it to the injection means. More preferably, a classification/separation step is carried out prior to the milling step to enhance the process efficiency.

However it is possible to carry out further classification steps as disclosed hereinafter with reference to Fig 1, that shows a flow chart of a possibie plant for the production of such injectable cement mixtures according to the method of the present invention.

It should be noted that by "cement mixtures" is meant here both a simple suspension of cement or any eimilar hydraulic binders in water, and a more complex mixture in which, besides the binder, further mineral fillers, e.g. silicates or carbonates, are present.

In the pref erred embodiment of Fig 1 the plant comprises a tank 1 or similar means for the preparation of a suspension of commercially available cement in water, preferably provided with a stirrer to keep the cement in suspension, connected by a feeder line 2 to classification/separation means 3. Two run-off lines 4 and 5 exit from said ciassification/separation means 3. Line 4 takes the fine particle fraction (i.e. that with fine grai size) to the injecting means (not shown). Line 5 takes the coarse particle fraction (i.e. that with coarse grain size) through suitable feeding means, e.g. a pump 6, to milling means 7. A line 8 is provided at the outlet of the milling means 7 to connect the same to the classification/separation means 3. Preferably, the milling means 7 consists of a ball mill, with a plate agitator. An exit line 17 is connected with line 8 upstream a valve 19 to feed the milled mixture to the injecting means; a further valve 18 is provided to control with valve 19 the mixture flow through line 17 to the said injecting means. A valve 13 or similar controlling means is incorporated into the feeder line 2 at the outlet from tank 1, to regulate the flow of fresh mixture which enters the classifier/separator 3, and a similar valve 14 is located on the fine particle fraction outlet line 4.

In operation. a suspension of commercially available cement in water is first prepared in tank 1. During this preparation or later in the course of feeding it through the feeder line 2 up to the classification/separation means 3, one or more deflocculant additives are added to the cement in water suspension to assist the following separation process into fine and coarse particle fractions and to extend the setting time of the cement.

Suite obviously, if the suspension is milled without classification the fluidifying additive may be added also during or after the milling step, as above di scl osed .

The mixture containing commercially available cement from tank 1 i s sent to the classification/separation means 3 where a continuous separation is carried-out into a fraction rich of fine particles (the fines) and another one rich of large particles (the coarses).

At this point the fines mixture can be at least partially tapped and sent through line 4 to the injection means (not shown), while the coarse particle mixture is taken and fed through line 5 to the milling means 7. Means 6 are shown in fig 1 for circulating the mixture through the plant, e.g. i n the form of a pump.

The coarse mixture entering the milling means 7 is milled along its path through said means 7; at the outlet there is a mixture whose percentage of fines depends on the size of the mill 7, the dwell time etc. Such a percentage is naturally the highest possible, but it is not necessary that all the coarse mixture is reduced to a fine particle mixture, in so far as the milled mixture leaving means 7 is recycled along the line 8 up to the classifier/separator 3, where it is once again separated and classified together with the commercial cement suspension entering from tank 1.

As previously mentioned, the supply of suspension from tank 1 to the classifier/separator 3, and the removal of the fine particle mixture from the latter along the line 4 are controlled by means of valve means 13 and 14 on the lines 2 and 4 respectively. This allows a choice of either a batch or stepwise milling.

In the case of the former, after a sufficient quantity of suspension is prepared in the tank 1, this is sent to the classifier/separator 3. Operating the mill feeding pump 6, the coarse particles are taken off and sent to be completely milled to the required size and the milled mixture is sent to the injecting means.

In a stepwise milling, the coarse particles are not immediately and completely milled as required and the milled mixture is sent to the classifier/separator 3 for further separation. In this way the particle size is progressively reduced until there are only fine particles. In both cases it is possible to carry out a continuous milling; its operation is similar to those already described, but the flow along the line 2 is regulated in such a way as to constantly feed fresh suspension to the tank 1, and at the same time tapping off the fines from line 4 and/or the milled mixture through line 17.

The preferred milling means for the continuous process is a ball mill, agitated by revolving plates, in which the mixture to be milled enters at the lower end and passes up tnrouqh the mill. The balls, which fill ail the free spaces, grind the particles of cement which are carri ed along by the flow of suspension as obtained by the action of the feeding pump 6.

The classification/separation means 3 could be any suitable device for the separation the fines from the coarses in suspensions; e.g. a hydrocycione or a centrifuge could be used. In the preierred embodiment shown in Fig 2, a hydraulic classifier/separator 9 is used, where an ascending current having a predetermined flow rate allows to separate the fine particles from the larger ones, by making the fine particles to raise up toward the upper part of the classifier/separator 3, while the coarse particles remain on the floor of the classifier/separator 9.

As it can be seen from Fig 2, the diagram of the plant is similar to that previously described with reference to Fig 1, and similar components have been re f erred to with the same reference numbers i n both figures. As already described, the classifier/separator of this plant consists of a cylindrical container or tank 9 where the suspension coming from tank 1 along line 2 and the mixture returning from mill 7 along line 8 are fed together along the common line 12 into the lower part of the classifier/separator 9, and create, due to the controlled feeding of the mixture from tank 1, an ascending current separating the fine particles by running the same toward the outlet line 4. As a result, the mixture portion in the lower part of the classifier/separator 9 is enriched of coarse particles; line 5 i s positioned to remove this larger particle fraction. Line 4, for tapping fine particles is positioned at a pre-selected higher level. corresponding to the suspension portion where fine particles only are present.

To ensure uniform supply of coarse particles, the lower part of classifier/separator is provided with slow stirrer means 10.

In the lower part of the cylindric container 9 there is a run-off line 16, fitted with a valve 15, used for emptying the plant or for running off a batch operation.

The feeding of the commercial cement suspension along line 2 results in the fluid, and the particles entrained by it, rising at a linear speed through the classifier/separator 9. By suitably regulating the feed rate by means of valve 13, a rising speed may be chosen such as not to raiss the coarse particles beyond a safety level, and in any case not to the tapping line 4.

Fig 3 shows in a graph the distribution of particle sizes in a commercially available cement (Portland 525) (20) and in a mixture after milling according to the method of the invention (21), to be compared with the granulometry of the sample sand in fig. 4.

Further particulars .of the method according to the invention will now be given with reference to the foilowing examples.

EXAMPLE 1

A plant as. disclosed Fig 2, is comprising:

- a tank of fresh mixture 1 with a volume of 2 litres fitted with a regulation valve 13 on line 2,

- a classifier/separator 9 with a volume of 0.65 litres, and an inner diameter of 0.046 m, fitted with a valve 14 on the outlet line 4,

- a pump 6,

- a microsphere and plate mill 7 having a volume of 0.50 litres.

Two litres of a 15% suspension of cement 525 ptl, to which sodium polynaphtalensuiphonate was added as previously disclosed were fed into the above plant.

The suspension was fed from said tank 1 to the classifier/separator 9 along line 2 at a rate of 1.51/h.

The stirrer 10 in the bottom of the classifier/separator

9, the pump 6 and mill 7 were switched on.

Because of the flow-rate and the free cross-section of the classifier/separator, the fluid rising speed was of 1.67 cm/minute and 36 minutes after feeding began, the fine particle suspension started to flow through line 4. The level in tank 1 was continuously topped up with fresh mixture and the run-off of line 4 was collected in a suitable container under stirring.

In the course of the experiment, which ran for 8 hours,

12 litres of suspension containing 2.00 kg of dry cement were fed into the system.

At the end of the experiment, all the fraction tapped from head were collected together and homogenized.

One part of εaid suspension was fed at a pressure of 0.5 atmospheres into a sample of fine sand with particle size of less than 0.500 mm (Fig 45, suitably compacted in a rigid container.

After 28 days cure said sample of consolidated sand was compression tested.

The average breaking load for the samples was 9.3 kg/cm³.

Another part of said suspension was suitably dried and sent to a granulometry analysis in a mercury porosi meter (POROSIMETER SERIES 2000, CARLO ERBA STRUMENTAZIONE). The particle size distribution of the cement is shown in Table

EXAMPLE 2

A plant of the type disclosed in Fig 2, comprises:

- a tank of fresh suspension 1 with a volume of 2 litres fitted with a regulation valve 13 on line 2,

- a classifier/separator 9 with a volume of 1.00 litres and an inner diameter of 0.063 m, fitted with a regulation valve 14 on line 4,

- a pump 6,

- a rotating drum mill 7 having a volume of 10.00 litres, containing 5.00 litres of zirconium oxide microspheres. Two litres of a 15% suspension of cement 525 ptl, added with sodium polynaphtalensulphonate as above disclosed were fed to tank 1 in said plant.

Said suspension was fed from said tank 1 along line 2 and the inner duct 12 to the classifier/separator 9 at a rate of 3.0 1/h.

The stirrer 10 in the lower part of the classifier/separator 9, the pump 6 and mill 7 were switched on.

Considering the flow-rate and the free cross-section of the classifier/separator 9, the fluid rising speed was 1.67 cm/minute, and 60 minutes after feeding began the finished product started to flow through line 4.

The level in tank 1 was continuously topped up with fresh- mixture and the run-off of line 4 waε collected in a suitable container under stirring.

In the course of the experiment, which ran for 8 hours. 24 litres of suspension containing 4.00 kg of dry cement were fed into the system.

At the end of the experiment, all the fractions run off were collected together and homogenized.

One part of sai d suepensior was injected at a pressure of

0.5 atmospheres into a sample of fine sand (suitably compacted in a rigid container), whose particle size distribution cur ve is shown i n Fig 4.

After 23 days cure said sample of compacted and consolidated sand was compression tested. The average breakage load of the sampies was 8.9 kg/cm².

Another part of said suspension was suitably, dried and sent to granulometry analysis with a mercury porosimeter

(POROSIMETER SERIES 2000, CARLO ERBA STRUMENTAZIONE). The particle size distribution of the obtained cement is shown in Table 1.

EXAMPLE 3

A plant of the type illustrated in Fig 2, comcrrises:

- a tank of fresh mixture 1 with a volume of 2 litres, fitted with a regulation valve 13 on line 2,

- a classifier/separator 9 with a volume of 1^.75 litres. en inner diameter of 0.063 m, fitted with a valve 14 on the outlet line 4,

- a pump 6.

- a microsphere and olate mill 7 having a volume of 0,50 litres,

A mixture volume equal to the system dead volume, two litres of 15% suspension of cement 525 ptl, to which was added sodium polynaphtalensulphonate as above dislosed, was fed into tank 1 of said plant.

Said suspension was fed from said tank 1 along line 2 and the inner pipe 12 up to the classifier/separator 9 at a rate of 3.00 1/h.

The slow stirrer 10 in the lower part of the classifier/separator 9, the pump 6 and mill 7 were switched on.

Consi deri ng the f l ow-rate and the free cross-secti on of the cl assi f i er/separator 9, the f l ui d ri si ng speed was of

1.67 cm/mi nute.

Considering the volumes of the classifier/separator 9, the mill 7 and the connecting pipework, the supply was shut off 40 minutes after feeding began and the process stooped.

The mixture thus obtained was run off through the drain valve 15, and collected in a suitable container.

One part of said suspension was fed at a pressure 0.5 atmospheres into a sample of fine sand suitably compacted in a rigid container, whose particle size distribution curve is shown in Fig 4.

After a 28 days cure, said sample of compacted sand was compression tested. The average breaking load of the samples was 9.7 kg/cm².

Another part of said suspension was suitably dried and sent to particle size analysis with a mercury porosimster

(POROSIMETER SERIES 2000, CARLO ERBA STRUMENTAZIONE). The particle sizs distribution of the obtained cement is shown in Table 1.

EXAMPLE 4

A plant according to fig.2 embodiment, comprises:

- a fresh mixture tank 1 having a volume of 250 1 provided with a control valve 13 on line 2;

- a secondary classifying tank 9 having a volume of 6001; provided with a stirrer and a tapping line 4 having a control valve 14

- a pump 6

- a microsprhere and plate mill 7 having a volume of 10 1. 250 1 of 525 ptl cement 66% suspension, with a ratio cement versus water C/W = 2, are prepared in tank 1 and treated with sodium poiynaphtalenesuiphonate as previously disclosed. This mixture is fed to secondary tank 9 through line 2 and successively through line 5 and pump 6 to mill 7 at a flow rate of 60 1/h.

Mill 7 is started. Because of the free inner volume of mill 7 and of the flow rate of pump 6, three minutes after the feeding began, the milled suspension began to flow from the mill exit. The valve 18 is open and valve 19 is ciosed to feed the mixture directly to the end use, where it may be used as such or diluted prior to its injection. In the meanwhile, fresh suspension is continuously fed to tank 3 and operation of the plant is continued. Globally, during the test which lasted 8 hours. 480 1 of suspension containing 534.13 kg of dry cement were fed to the plant.

Every hour, from the starting of the test, a sample of the cement flowing from the mill was taken and partially used to fill a steel cylinder with a 38.5 mm i.d. and 100 mm high. Another portion of said sample was diluted with water to a cement percent of 25% and was injected at a pressure of 0.5 atm into a sample of sand having granulometry of less than 0.5 mm (fig.4), suitably compacted in a rigid container.

A further portion of the suspension was dried and sent to granulometric analysis as previously disclosed. The particle size distribution is shown in Table 1.

After 28 days cure the samples of suspension and of injected sand underwent a compression test. The average breaking load value was 580 kg/cm² for the mixture sample and 30.3 kg/cm² for the injected sand sample.

EXAMPLE 5

In a plant as disclosed in Example 4, 250 1 of a 50% suspension of 525 ptl cement (C/W ratio = 1) were prepared in tank 1 and added with a fluidifying agent as previously disclosed.

The plant was operated as disci osed in Example 4, and during the test, which lasted 8 hours, 480 1 of suspension containing 363.12 kg of cement were fed to the plant.

Samples were collected and treated as disclosed in Example 4.

The average granulometry values of the resulting dried suspension are shown in Table 1.

After 28 days cure the injected sand and mixture samples underwent a compression test. The average breaking load value was 180 kg/cm² for the mixture samples and 27.3 kg/cm² for the i njected sand samples.

EXAMPLE 6

In a plant as disclosed in Example 4, 250 1 of a 33% suspension of 525 ptl cement (C/W ratio = 0.5) were prepared in tank 1 and treated with a fluidifying additive as previously di sclosed.

The suspension is fed to the classifier 9 at a flow rate of 200 1/h. The stirrer 17, pump 6 and mill 7 are started.

Because of the free cross section of classifier and the flow rate through it the fluid rising speed was 1.67 cm/min. Because of the volumes of the classifier, the mill and the feeding lines, 65 minutes after starting the plant the end product flows from output line 4.

The level in tank 1 was continuously topped up with fresh mixture and tha fractions flowing at 4 were collected.

During the test, which lasted 2 hours, 400 1 of suspension containing 174.00 kg of dry cement were fed to the plant. At the end of the experiment samples of the collected and homogeneized fractions were injected and dried as disclosed in Example 4; the resulting granulometry of the dried samples is shown in Table 1.

After 28 day cure the compacted sand samples were subjected to a compression test. The average breaking load value was 35.9 kg/cm².

It should be noted that, as disclosed in previous examples, mixtures obtained according to the invention are injectable in a sample of compacted and pressed sand, having the granulometry shown in fig. 4, at very low pressures, namely at 1,5 atm or less (e.g. 0.5 atm).

The cited sand granulometry is characterizable as follows: max. diameter 0. 5 mm

60% " 0.37 mm

10% " 0.16 mm

nominal " 0.32 5

uniformity coefficient U=2.28

permeability " less than 5 x 10^-3 cm/sec

Claims

1. A method for the production of mixtures of cement or similar hydraulic binders to be injected into fine-gram soils and/or into porous or microfractured rocks or materials, characterized in milling to a desired granulometry an aqueous suspension having a ratio of cement versus water within the range from 0.25 to 2.5 by weight; end in adding to said suspension, before, during or after said milling step, at least one fluidifying additive.

2. A method according to claim 1, characterized in adding to said mixture an amount of fluidifying additives within the range from 0.5 to 7% of the cement weight.

3. A method according to claim 2, characterized in that said additive is chosen amongst the foil owing compounds: polynaphthaienesulphonic acid, polyalkylnapthalensulphonic acid, polymethylmetacrylic acid, polyphosphoric acid and their salts, melamme resins, and mixtures thereof.

4. A method according to claim 2, wnerein said additives are added to a cement mixture having a cement/water ratio higher than 1.5 before the cited milling step.

5. A method according to claim 2, characterized in adding said additives to a suspension having a cement/water ratio lower than 1.0 after the milling step,

6. A method according to claim 1, wherein milled suspensions having a cement/water ratio higher then 0.5 are futher diluted before injection.

7. A method according to any claim from 1 to 7. wherein said injectable cement mi xture is at least partially classified prior to the milling step and at least a part of the r esuiting suspension of fine particles is tapped to injection means.

8. A method according to Claim 7 , characterized in that it further comprisee the steps of:

feeding said cement suspension in a controlled way to a classification/separation means to separate a suspension fraction containing fine particles from a suspension fraction containing coarse particles;

feeding said suspension fraction containing coarse particles to milling means in order to reduce at least in part their particle size;

returning the suspension exiting said milling means to said classification/separation means;

and repeating the above mentioned operations.

9. A method according to claim 7 or 8 wherein said seperation between fine particle mixture and cosrse particle mixture is carried-out by means of a hydraulic classifier/separator.

10. A plant for the production of an injectable mi xture of cement or similar hydraulic binders, characteri zed in that it comprises: a ball mill or similar means for milling an aqueous suspension of commercially available cement; means for adding to said suspension at least one fluidifying additive, before, during or after sai d milling step; and means for feeding said milled suspension to injection means.

11. A plant according to Claim 8, characterized i n that it further comprises:

one or more tanks for the preparation of a suspension of commercial ly available cement in water;

means for feeding said suspension in a controlled way to classification/separation means to separate a suspension fraction containing fine particles from a suspension fraction containing coarse particles;

means for feeding sai d coarse particle fraction to said milling means:

means for returning the milled mixture exiting said milling means to said classification/separation means.

12. A plant according to Claim 11 characterized in that it further comprises means for tapping at least a part of said fine particles suspension and sending it to injection means.

13. A method for consolidating fine-grain soils and/or porous or microfractured rocks or materials, characterized in injecting a cement mixture as obtainable according to any claim from 1 to 9.

14. An injectable mixture as obtained according to any claim from 1 to 9.

15. An injectable mixture according to claim 14, characterized in that it is injectable at a pressure of

1.5 atm or less into a sample of compacted sand having a granulometry characterizable as follows:

max. diameter 0.5 mm

60% " 0.37 mm

10% " 0.16 mm

nominal " 0.325

uniformity coefficient U=2.23

permeability " less than 5 x 10^-3 cm/sec