CA2063395A1

CA2063395A1 - Process for rendering inert and/or immobilizing enviromentally harmful substances

Info

Publication number: CA2063395A1
Application number: CA 2063395
Authority: CA
Inventors: Fritz Jung
Original assignee: Individual
Current assignee: Techform Engineering AG
Priority date: 1989-07-14
Filing date: 1990-07-16
Publication date: 1991-01-15
Also published as: EP0408545B1; DE59000341D1; HU9200100D0; WO1991001280A1; NO920068L; HUT70357A; JPH05500325A; NO920068D0; EP0408545A1

Abstract

A process is useful for rendering inert and/or immobilizing inorganic and/or organic environmentally harmful substances in aqueous dispersion, suspensions, emulsions and/or solutions of waste materials. At least one inorganic binder, in particular based on portland cement clinker, clay cement klinker and/or hydraulic lime, is added to the dispersion, suspension, emulsion and/or solution, possibly before, during or after a treatment to precipitate and/or convert the heavy metals, sulphur compounds, other harmful substances and/or organic compounds in the dispersion, suspension, emulsion and/or solution. A homogeneous mixture of the harmful material(s) and the binder(s) in the dispersion, suspension, emulsion and/or solution is obtained, preferably by stirring. If necessary it undergoes gravity separation, after which its water content is reduced, preferably by compressing it in a press.

Description

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`.:.'; -1 -A PROCESS FOR RENDERING INERT AND/OR IMMOBILIZING ~I~RMFUL
SUBSTANC~ IN AQUEOUS DiSPERSlONS, SUSPENSIONS, EMULSIONS
AND/OR SOLUTIONS OF WASTE MATERIAI S

The purpose of the process is to use certain bindin~ agents in order ~o 5 treat environmentally harmful substances contained in aqueous dispersions, suspensions, emulsions and/or solutions of waste materials, and/or to treat organic compounds, in suGh a way as to bind them, render them inert and/or immobilize them.
The Filter cakes produced in filter presses or similar dewatering devices 10 from the waste water of flue gas scrubbing facilities or from sludges and similar aqueous dispersions, suspensions, emulsions or solutions, are by their nature mixtures of extremely fine-grained materials and this both favours and enhances the binding of harmful substances in the filter cakes. After they leavethe filter press, these fil~er cakes often still have a water content of 50 to 7~%
15 or higher. Nevertheless, it has proved extremely difficult to mix these fine-particled, often plastic filter cakes with cement or other binders in such a waythat the binder is homogenously distributed throughout the material of which the filter cakes are made up. ~ut, the particles of waste material which have not been broken down constitute weak points in the system because they 20 impair the binding of the harmful substances, and also as a rule they negatively - influence the hardening and the stability of the moulded or compacted productsmade from the material consolidated by the binding agent. In addition, the miscibillty of the waste materials with the binders is impaired by certain textures which form during the pressing process and which are similar to those 25 which come about during the deformation of plastic clays in the ceramics industry. These textures increase the need for water and negatively affect the miscibility of ~he materials with the binders. The only way in which to achiev0 a reasonably acceptable distribution of the binder in the material of which the filter cake is made up is to add large additional amounts of water while the 30 binder/filter cake mixture is being prepared. This reduces the strength and frost resistance of the T~oulded product formed, impairs its permeability and lowers its ability to bind the harmful materials.

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The use of inor0anic binders to treat was-te wa-ters is known, -for example, from the following pubications:
EP-A1 17741 - which stric-tly speaking has a very narrow field of application - d(3scribes a method o-f converting phosphoric acid extraction 5 residues into a dumpable product by treating them with ordinary hydrated lime or ordinary burnt lime. A neutraliza-tion reaction takes place in the filter cake which forms. The Ca-oxide or hydroxide used is not a hydraulic binder.
A similar problem and its solution is the subject of EP-A1 50370 according to which dilute acids are treated and likewise nelJtrallzed in practice 10 by means of brown coal ashes, which exhibit an alkaline reaction and possess only partially a latent hydraulic binding capacity, and even -that only in the presence of activating agents.
The consolidation of "dilute" liquid wastes described in LU-A 42343 requires the addition s)f quite specific natural or synthetic, organic 15 agglomerating agents. In orderto complement their effect, "high-gravity solids"
are added to the sludge which is formed, but the primary aim of doing this is not to bind the harmful substances by causing the mixture to set, instead the harmful substances are caused to adhere to particles with large surface areas and higher density, as is clearly shown by the use of inert substances such as 20 finely ground limestone and "tailings", as provided for in the patent. Cementand gypsum are suggested as equally good high-gravity solids, but their binding capability is not examined any greater detail. The process according to this LU-A depends on the use of expensive organic agglomerating agents whose anticipated long-term harmful effects after being dumped are not discussed in 25 that document.
The purpose of the present invention is to treat thin, aqueous waste products of widely varying origin, which as is known accumulate in very large amounts and often discontinuously in batches, and to convert them in an economically viable manner into fully dumpable solid products containing 30 wi~hin themselves the immobilized harmful substances which cannot leach outat a later point in time; and the large amounts of water which are separated out during the process must be practically free of harmful substances and suitabie for discharging into a receiving body of water.

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'`, , ~3-Th~3 subject of the present invention is thus a process for rendering inert and/or immobilizing inorganic ancl/or organic environmentally harmful substances contained in aqueous dispersions, suspensions, emulsions and/or solutions of waste materials by adding hydraulic binders be~ore separating a solids phase and an aqueous ph~se, and an agglomerating agent is also addecl;
the process is characterized by the fact that - without adding natural or synthetic organic agglomerating agents - at least one inorganic hydraulic binder is added to the aforementioned dispersions, suspensions, emulsions and/or solutions following treatment to precipitate and/or convert any heavy 10 nletals, sulphur compounds, or other harmful substances and/or organic compounds contained in them; the binder is based on Portland cement clinker, alumina cement clinker and/or hydraulic lime, and a homogeneous rnixture of the harmful substance(s) and the binder(s) is brought about in the dispersion, suspension, emulsion and/or solution preferably by stirring; and, possibly after15 gravity separation, the water content of the homogeneous mixture of harmful substance and binder thus obtained is reduced by compressing the mixture in a press. The dispersion, suspension, emulsion and/or solution of waste materials has a low pH value, preferably less than 5, or the pH is adjusted to this low value, and it is mixed, preferably at a high temperature, with an 20 inorganic material containing magnesium compounds, selected from the group of unburnt, semi-burnt or fully burnt dolomite or dolomitic lirnestone, magnesium chloride and magnesium sulphate, preferably containing more than 5%, in particular more than 10% and especially more than 25% magnesium compounds, calculated in each case as MgO, relative to the dry state of the 25 waste material~s) free of ignition losses, and/or it is also mixed with an inorganic material containing aluminium compounds, selec~ed from the group of blast furnace slags, pozzolanic materials, fly ashes, trasses, Portland cement clinker, alumina cement clinker and/or hydraulic (Al-Gontainin~) burnt lime, containing more than 5%, in particular more than 10% and especially more 30 than 2~% Al203 equivalent rela~ive to the dry s~ate of the waste material(s), free of ignition losses.
By adding the stated binders, after treating the waste waters, not only can one avoid the use of costly agglomerating agents, whose possible long-term harmful effects have not been investigated, but also the conditioned '~ ' " ' , . .

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harmful substances are encased in SUCII a way that, together with the long-term consolidation of the binders and -the simultaneous lelimination of]
secondary reactions, the harmful subs-tances are particularly well rendered iner-t and immobilized, while -the dumpable product-thus obtained has a hiyh degree 5 of mechanical strength.
The attainment of these effects can be additionally simplified by waiting with the dewatering, and in particular the fiiter press stage, until a certain amount of time has elapsed, depending on the harmful substances and the binder involved, but before the binder sets. In this way, an optimally 10 homogeneous mixture of harmful substances and binder is obtained and as a result the harmful substances are uniformly, durably and reliably bound, rendered inert and immobilized throughout the mixture without any weak points occurring. Given the fact that the dispersion, suspension, emulsion and/or solution of waste materials is highly diluted with water, the harrnful 1~ substances are very rapidly bound in particularly large amounts by chemical or adsorptive processes to the calciun-l ions of the binder or its hydration products with their large surface areas, and this process is further promoted by the hightemperatures which are often present.
A subsequent compressing stage, which is carried out for e~ample in a 20 filter press, vacuum press, ram press, roller press, briquetting press or another similar water-reducing device, is greatly simplified and accelerated by the factthat the binder has already been mixed with the dispersion, suspension, emulsion and/or solution of waste material; the formation of structures is prevented and the dry matter con~ent of the filter cake thus formed is 25 increased. Another possible advantage resulting frorn the greater reduction of the water content is the associated reduction in the chloride content of the filter cake, thereby improving the strength of the moulded product, consolidated with cement, obtained from the press. The large surface area of the hardening products and also the denser structure resulting from the low 30 water content both considerably reduce the diffusion of ions in the product thus obtained and further improve the immobilization of the environmentally harm~ul compounds.

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sj ~- - 5-The unexpectedly high degr~e to which the harmful subs~ances are bound can be achieved in the process according -to -the invention by adding the inorganic materials to precipitate and convert the harmful susbtances.
Typical harmful substances which can be particularly ef-fectively 5 rendered inert by the process according to the invention are lead, zinc, cadmiurn, thal!ium, arsenic, and chromium compounds, as well as fluorine compounds, organic acids such as tartaric acid, oxalic acid, etc. It is appropriate to wait, usually for a few rninutes to a few hours, depending on the type of harmful substance involved, on the binder used, and also on the 10 prevailing temperature, in order to achieve the maximum possible degree of binding. As regards the amount of binder needed, binding eFfects can be expected starting from amounts of binder in the ~ to 5% range relative to the dry mass of the waste material and the binder.
Typically, the amount of binder added coincides with the experience 15 gained in soil stabilization practice. A significant binding effect is normally obtained from 7% binder onwards. The quantitative ratios mentioned in Claim 2 are especially preferred.
In order to convert into harmless ettringite any sulphur compounds contained in the aqueous dispersion, suspension, emulsion and/or solution of 20 waste material, it has proved particularly efficient to choose a method as described in Claim 3. ~y forcing ~he formation of ettringite in this way before the water content is subsequently reduced, e.g. in a filter press, the binder isprevented from expanding at a later point in time, and also it improves the filtering capability and dry substance content of the filter cake. Finally, by 25 precipitating out the sulphur compounds, a very low sulphate content is achieved in the water expressed during the subsequent filtering step. This is especially important if the expressed water has ~o be discharged into a public sewer or a receiving body of water which are normally governed by strict limits on the amounts of sulphate they may contain, and if these limits could not 30 otherwise be met.
As a rule, heavy metals and other harmful substances are well bound in all the cements. However, a far better than average binding efFect is achieved by cements or hydraulic limes which form large amounts of ettringite, i.e.
which contain large quantities of aluminate or alumina~e ferrite. Therefore, : ' -' .

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rapid-setting cemen-ts containing 1 2CaO.7AI203 and in particular 11 CaO.7AI203.CaF3 are particularly suitable, and -the amounts of these substances in the cements should correspond -to those given in Claim ~.
Compared with the customary cemen-ts, these cemen-ts, in particular those 5 c:ontaining quantities of 11 CaO.7AI203.CaF2, are characterized not only by anabove-average ability to bind harmful substances, in particular those which are difficult to render inert, such as chromium compounds, but also the~/ are less susceptible to harmful substances such as zinc compounds, phosphate cornpounds or organic compounds contained in the dispersions, suspensions, 10 emulsions and/or solutions of waste material, which tend to interfere with the setting process. In addition, because of its acicular structure and its ability to bind large quantities of water, the et-tringite which forms along with other hydration products during the setting process provides -for exc~llent filterability and a correspondingly high content of clry matter in the filter cake.
In light of these facts, ettringite may advantageously be directly added to the aqueous dispersion, suspension, emulsion and/or solution of waste material, as provided for in Claim 5, .
In this context, in order to avoid swelling reactions in dispersions, suspensions, emulsions and/or solutions containing reactive sulphur 20 compounds, it is possible, as provided for according to Claim 6, to add cements and/or hydraulic limes having the low contents of calcium aluminates mentioned there.
Also, the formation of ettringite can be prornoted by using Gement and/or lime with high contents of fly ash or blast furnace slag. In this case, the 25 aluminates or the aluminate ferrites come from these additives. Combinations of these substances with appropriate cements, such as PSZ 400, have proved particularly suitable.
In this context, it is also particularly suitable to proceed according to Claim 7 in order to further boost the abili~y to bind harmful substances, 30 especiallv heavy metals and/or organic compounds and/or disruptive ammonium compounds. The added amounts mentioned there result from the fact that the binding capabili~y of these materials depends among other things on their chemical nature and is, for example, very high in the case of bentonites and zeolites, but lower for blast furnace slags.

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ln all these cases -the -formation o-f e-ttringite can be controlled, as provided for accordin~ to Claim 8, by lowering the calcium ion concentration of the liquid phase by adding alkali compounds, especially alkali carbonates, preferably in an amount in the range between 0.1 and 5%.
In order to bring about further improvement of the -filtration characteristics of the aqueous dispersion, suspension, emulsion and/or solution of waste material, the filtra-tion aids provided for in Claim 9 may advan~ageously be added in the process according to the invention.
Easily soluble substances, e.g. alkalis, calcium hydroxide or calcium sulphate, contained in the binder or released at the start o-f the hydration process can enter into solution while being mixed with the aqueous dispersions, suspensions, emulsions and/or solu-tions of waste materials and may be remove(i with the expressed water in the filtering or pressing stage which follows mixing, and this can have a deleterious effect on the setting process and on the strength of the moulded or compacted products which are subsequently produced. Since this is the case, it may be advantageous to add additional amounts of these easily soluble substances before the binder sets in order to compensate for the above-mentioned losses and/or to use binders which contain either small quantities, or none at all, of these easily soluble substances. For this purpose, particular preference should be given to a variantof the process according to Claim 10. in this context, it is also advantageous for the filter water or the expressed water, including the easily soluble componen~s of the binder, and possibly also of the waste materials, which are contained therein, to be operated in a closed cycle and to be added to new dispersions, suspensions, emulsions and/or solutions of waste materials who~e water content has not yet been reduced.
In order to compensate for the losses of easily soluble cornponents, it is advantageous to proceed according to Claim 11.
~iince the solubility of harmful substances depends very rnuch on the pH
value - at low pH values the solubili~y is frequently several powers of ten higher - it is a matter of particular interest to ensure the long-term maintenance of a pH in the mixture of waste material and binder which undergoes water reduction. ~he long-term maintenance of a high pH value can be achieved by adding an alkali depot in the manner advantageously provided for in Claim 12.

fJ~ t) This alkali depot is added to the mixture o-f waste material and binder preferably before the water content is reduced. Dependin~3 on the type and ef-fect o-F the all<ali depot, it may be necessary to add very clifferent amounts o-f such material, so tha~ the alkali depot is added advantageously in amounts 5 ranging from 1 to about 75% relative to the combined amount of dry waste material and binder.
When the alkali depot is made available in -the form of a rnixture of at least two substances acting as "neutrali~ation barriers", the second substance with the low pH reactivity represents an "inert neutralization depot" as long as10 a high pH exists. This depot is activated only if the pl I value drops below 7 and its reaction prevents a further decline in the p~l value and thus an increase in the solubility of the harmfut substances. In general, -these limits have proved to be suitable for completely rendering inert or irnmobilizing harm-ful substances by means of the process according to the invention. In special cases, other pH
15 neutralization barriers can be selected by carefully choosing the materials which make up the alkaline depot.
By applying the process according to the invention, it is possible to reduce the water content of the waste material/binder mixture to such an extent that this mixture can be converted into moulded products after the 20 compressing stage. It is therefore possible, immediately after the pressing stage, to process the filter cakes into moulded products in another press or extruder, or similar, by applying pressure and/or heat. Mechanical pressure of more than 3 or, in more demanding applications, of more than 15 or more than 40 bar, and temperatures of about 5() to 70C have proved appropriate. The 25 higher temperature considerably accelerates the setting of the binder. It mayalso be possible to produce the moulded products simply in the course of the compressing stage, without the need for any separate, addi~ional press.
It has furthermore proved advantageous to neutralize the alkaline-reacting compounds, especially calcium hydroxide, contained in the surface 30 layer of these moulded products by means of liquids, preferably carbonic acid, organic acids, solutions of carbona~es or bicarbonates and similar, and/or gases, e.g. carbon dioxide, C02-containing exhaust gases, etc.; and/or ~o coa the rnoulded products with an iner~ protective layer consisting of mineral materials, e.g. water glass solution or organic su~stances, e.g. based on ' ~ .
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t; ~ 3 ~i acrylate, styrene bu-tadiene, vinyl copolymer or mixtures, for example of plastic-modified hydraulic masses.
The mixtures produced by -the process accorcling to the invention may also advantageously formed into finished parts, building bricks, granula-tes, 5 aggregates, concrete aggregates, and similar, possibly with the ~urther addition of binders, additives or water.
The fil~er cakes formed in the press may also, however, be allowed to harden without any further processing and -they may then be dumped ~ ithou-t any harm to the environment.
The advanta0eous proper-ties of rnoulded products obtained from the process according to the invention, in particular the strength values which can be achieved, are explained in more detail in the following example, which is notin any way intended to restrict the scope of the invention:

Example:
A waste water from a flue gas scrubber fitted on a garbage incinerating plant was treated in the usual manner by adding hydrated lime, iron chloride solution and reagents to precipitate the heavy metal; the sludge formed had a dry substance content of 3.9% and was pressed on a filter press. The resulting filter cake had a dry substance content of 35% and was then mixed in a ratio of 1:2, relative to the dry state, with Portland cement PZ ~5 ~H) in a positive-drive mixer. In order to achieve an adequate distribution of the cement and the filter cake, it was necessary to add a further 50% water. Using state-of-the-artmethods, concrete cubes were produced from the filter cake/cement/water mixture obtained in this way.
In a second series of tests, the same amount of Portland cement was added to the sludge in the manner according to the invention before the filter press stage was reached. Because of the high water content of the sludge at this stage in the process, it was possible to rnix the waste material and cementsimply by stirrin!g the sludge for 3 minutes. The mixture was then fed to ~he filter press. The Filter cake thus obtained had a dry matter content of 55/0 and it was tamped into concrete cube moulds without any further addition of water or cement.

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o-The strength values obtained in the concrete cubes manufactured in -this way are listed in Table 1. It can be seen that the addition of cement according to the invention be-fore the water content is reduced on the filter press decisively improves the development o-f the strength properties. It was also 5 remarkable that the filtering time in the filter press dropped to a quarter of the usual value when the binder was added prior to the filtering stage, and also that, as mentioned, the dry matter con-ter-t of the filter cake increased frorn 35 to 55%
In further tests, the mixture ratio of the "filter cake waste material" to 10 cement was varied. It was found that the amount of additive needed to bring about hardening could be signifiantly lowered by ad~ling the binder, in the manner according to the inven-tion, ahead of the filter press.
Even better results can be obtained when alumina cement is used as the binder.

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Mixture ratio Compressive strength BiDder added binder:waste (relative (N/mm2) after D days . _to dry mass in each case) _ I -ll a*er filter press bef. filter press 1 7 28 ¦ 120 . . = ~ ~ = =
x 2: 1 n.e 0.8 1.5 2.9 x 2: 1 2.2 3.8 6.3 10.
x . ._ 09 1 _ 1.0 2.1 3.3 4.5 x 0.2: 1 0.2 1.0 1.6 2.7 .....
x 0.4: 1 (Alurni~la) 3.4 3.9 5.0 . ~ __ __ _ _ .... _ 20 n.e. = cannot be removed from the moid . ' ~ .

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Claims

1. A process for rendering inert and/or immobilizing inorganic and/or organic, environmentally harmful substances contained in aqueous dispersions, suspensions, emulsions and/or solutions of waste materials by adding hydraulic binders before the solids phase and aqueous phase are separated, characterized in that - without adding any natural or synthetic organic agglomerating agents -at least one inorganic binder, especially a binder based on Portland cement clinker, alumina cement clinker and/or hydraulic lime is added to these dispersions, suspensions, emulsions and/or solutions, after they are treated to precipitate and/or convert the heavy metals, sulphur compounds, other harmful substances and/or organic compounds contained in them, and a homogeneous mixture of the harmful substance(s) and the binder(s) is brought about in the dispersion, suspension, emulsion and/or solution, preferably by stirring and, possibly after gravity separation, the water content of the homogeneous mixture of harmful substance and binder is reduced, preferably by compressing the mixture in a filter press, and the dispersion, suspension, emulsion and/or solution of waste material has a low pH value, preferably less than 5, or can be adjusted to such a value, and that it is mixed, preferably at high temperature, with an inorganic material containing magnesium compounds, selected from a group comprising unburnt, semi-burnt or fully burnt dolomite or dolomitic limestone, magnesium chloride and magnesium sulphate, containing more than 5%, in particular more than 10% and especially more than 25% magnesium compounds, calculated as MgO, relative to the dry state of the waste material(s) without ignition loss; and/or the dispersion, suspension, emulsion, and/or solution of waste material is mixed with an inorganic material containing aluminium compounds, selected from the group comprising blast furnace slags, pozzolanic materials, fly ash, trass, Portland cement clinker, alumina cement clinker and/or hydraulical (Al-containing) burnt lime, preferably containing more than 5%, in particular more than 10% and especially more than 25% Al2O3 equivalent, relative to the dry state of the waste material, without ignition loss.

2. A process according to Claim 1, characterized in that the binder(s) is (are) used in an amount ranging from 10 to 90%, in particular from 20 to 60%, and especially from 30 to 50%, in each case relative to the combined total of dry mass of the waste material(s) and binder(s).

3. A process according to one of the Claims 1 to 2, characterized in that in order to precipitate in the form of non-disruptive, insoluble ettringite any sulphur compounds contained in the dispersion, suspension, emulsion and/or solution of waste material, metallic aluminium powder and/or aluminium compounds, preferably in the form of sodium aluminate and/or alumina cement, if necessary combined with anti-foaming retarders, are added before, together with or after the addition of the binder, in amounts up to 35%, in particular 1 to 10%, in each case relative to the mass of the dry waste material(s).

4. A process according to one of the Claims 1 to 3, characterized in that fast-setting cement containing more than 5%, preferably more than 10% and especially more than 15% 12CaO.7Al2O3 or 11CaO.7Al2O3.CaF2, relative to the dry mass of the binder, is used as the binder.

5. A process according to one of the Claims 1 to 4, characterized in that ettringite is added to the aqueous dispersion, suspension, emulsion and/or solution of waste material.

6. A process according to one of the Claims 1 to 5, characterized in that when reactive sulphur compounds which impair the setting or long-term stability of customary binders are present in the dispersion, suspension, emulsion and/or solution of waste material, cements and/or hydraulic limes containing less than 3.5%, and especially 0%, calcium aluminates calculated according to Bogue are used as the binders.

7. A process according to one of the Claims 1 to 6, characterized in that in addition to the binder(s), materials are added to the dispersion, suspension,emulsion and/or solution of waste material before, together with or after the addition of the binder(s), but before the water content is reduced, for the purpose of enhancing the ability to bind harmful substances, in particular heavymetals and/or organic substances such as hydrocarbons and/or disruptive ammonium compounds; such materials are selected from at least one of the groups comprising, fly ash, blast furnace slag, trass and similar (in each case they are added in amounts of 3 to 60%, in particular 5 to 45% and especially 10 to 30%, in each case relative to the combined dry matter of the waste material(s), binder(s) and additive), and structured materials such as zeolites,in particular clinoptilolites, clays, bentonites, fine-grained silica, for example residues from the silicon industry or precipitated silica (in each case they areadded in amounts of more than 0.5%, in particular more than 1.5% and especially more than 3%, the upper limit being 30%, in each case relative to the combined dry matter of the waste material(s), binder(s) and additive(s)).

8. A process according to one of the Claims 1 to 7, characterized in that alkali compounds, in particular alkali carbonates, are added preferably in an amount between 0.1 and 5% relative to the mass of the binder, in order to reduce the calcium ion concentration of the liquid phase.

9. A process according to one of the Claims 1 to 8, characterized in that in addition to the binder(s), at least one filtration aid, chosen from the groups comprising anionic, cationic or neutral inorganic flocculating agents and minerals such as sands, and/or fibres, are added to the dispersion, suspension, emulsion and/or solution of waste material, before the water content is reduced .

10. A process according to Claims 1 to 9, characterized in that in order to prevent losses of easily soluble components from the binder during the subsequent water reduction stage, when they are removed for example, in the expressed water, cements and/or hydraulic limes containing anhydrite as the sulphatic setting regulator, and/or crushed Portland cement clinker and/or alumina cement clinker are added as binders.

11. A process according to one of the Claims 1 to 10, characterized in that in order to compensate for losses of easily soluble components from the binder during the subsequent water reduction stage, when they are removed, for example, in the expressed water, these easily soluble substances, in particular alkalis, lime hydrate and/or setting time regulators are added to themixture of waste material/binder before the binder sets.

12. A process according to one of the Claims 1 to 11, characterized in that in order to maintain a high pH value of more than 8 on a long-term basis in the mixture of waste material and binder which undergoes water reduction, an alkali depot is added, preferably before the water reduction stage, in particular in the form of alkali and alkaline-earth compounds, especially coarse-grained Portland cement clinker, hardened cement stone, comminuted recycling concrete, gas concrete, asbestos cement wastes, hard-burnt lime or dolomite or half-burnt dolomite, blast furnace slag, fly ash and/or alkali hydroxide, possibly in the form of a mixture of at least two substances acting as neutralization barriers, one of the substances being most strongly reactive at a pH of more than 8 while the other substance is chiefly reactive at a lower pH
in the range between 5 and 7, and a possible third substance is reactive in the range just below pH 5.

13. A process according to one of the Claims 1 to 12, characterized in that the water content of the waste material/binder mixture is reduced to such an extent during the compression stage, preferably in the press, that the waste material/binder mixture can be converted into moulded products, possibly with the application of pressure and heat.

14. A process according to Claim 13, characterized in that the alkaline-reacting compounds present in the surface layer of these moulded products, especially calcium hydroxide, are converted into insoluble, neutral-reacting compounds by the action of at least one fluid chosen from the groups comprising carbonate, hydrogen carbonate and carbonic acid solutions, and organic acids, and/or the action of gases, e.g. carbon dioxide and CO2-containing waste gases, and/or further characterized by the fact that the moulded products are provided with an inert protective coating consisting of mineral substances, for example water glass solution, or organic substances, for example based on acrylate, styrene butadiene, vinyl copolymer or mixtures, for example plastic-modified hydraulic masses.