This invention relates to electrochemical stabilising treatments of mineral masses such as concrete, and also of rock or soil masses, and to arrangements for carrying out such electrochemical treatment processes. The invention further relates to mineral composites, especially for example to concrete composites, arranged with electrodes to enable such stabilising treatment processes to be carried out, and to stabilised mineral masses such as stabilised concrete resulting from the treatments, and in particular it relates for example to arrangements and processes for preventing or minimising the alkali-silica reaction of concrete and its consequences. The invention further relates to electrode arrangements suitable for carrying out such treatments.

Concrete, very often reinforced concrete, is widely used for the construction of massive works such as buildings, bridges and harbour walls and other structures, and also for making small, often precast, units, such as railway sleepers and lamp standards for street lighting. Concrete is made from a mixture of a highly basic cement powder and water together with small and large aggregates. The liquid mixture is mouldable until it sets to a durable solid of high compressive strength. The setting reaction between the water and the cement powder forms a hydrated paste that binds together the particles of aggregate.

After setting, concrete is left with pores containing highly alkaline material locally. Unfortunately this alkaline material can react with certain alkali-sensitive aggregates, e.g. sensitive cherts or opals, to form a hygroscopic gel. Such a gel often absorbs water and expands, creating local tensile stress in the concrete.

This phenomenon is well known as alkali silica reactivity or alkali silica reaction (asr). Tests are known for the purpose of identifying aggregates susceptible to asr, so that their use can be avoided. These tests do not always identify aggregates that give rise to a slow

asr. Tests are also known which can detect to some extent that asr has occurred in a specimen of hardened concrete under test. Unfortunately such tests have not always been applied, and aggregates highly susceptible to asr have sometimes been used for the construction even of structures such as concrete pavings of roads, as well as bridges and buildings. Accordingly, these structures have suffered from asr, which is not only unsightly but also liable to damage their structural integrity.

Electrochemical treatment methods are known for preventing the corrosion of steel reinforcing elements in concrete, e.g. elements in the form of steel rebars. Such corrosion can be due to chloride ingress or to carbonation.' Known electrochemical treatment methods include cathodic protection, chloride extraction (also known as chloride removal or desalination) and re-alkalisation.

In known processes a current is made to pass from an anode on the surface of the concrete to the reinforcing steel (cathode). A cathodic reaction takes place, of the form:

1/2 O + H 0 + 2 e = 2 OH 2 2

Positively charged sodium and potassium ions are also attracted to the cathodic reinforcing steel in such processes, which therefore tend to increase the alkalinity around the rebars or other reinforcing elements, and can thereby add to any tendency towards destructive asr.

Also known is the application of anode systems to the surface of a body of reinforced concrete for the purpose of cathodic protection, chloride extraction and re-alkalisation. It is further known to embed anodes into concrete in order to carry out cathodic protection.

Techniques are also known which have as their aim to prevent the alkali silica reaction. For example it is known that lithium hydroxide can stop asr when incorporated in concrete. A known treatment method uses lithium hydroxide for combatting asr in hardened

concrete and comprises introducing the lithium ion by vacuum, pressure injection or flooding.

The present inventor considers that one of the drawbacks encountered with the known electrochemical processes to be applied to reinforced concrete is that electrical charge is not passed beyond the network of rebars into the heart of the concrete. It is also considered that the known processes of vacuum treatment, pressure injection or flooding with lithium hydroxide are often inconvenient to apply in practice.

Aims of the present invention include providing effective treatments and arrangements for combatting corrosion and asr in concrete in a combined treatment, to enable effective penetration of protective treatments into the concrete, to reduce the disadvantageous effect of anti- corrosion measures which increase liability to asr, and to provide more convenient and/or effective anti-asr and realkalisation treatments. The treatments are applicable to cementitious materials such as concrete, with or without conductive reinforcing elements embedded therein, and to other mineral rock or soil masses which may be stabilised by similar treatments.

The invention provides electrochemical processes and arrangements for carrying out stabilising treatment of mineral masses, for example of hardened concrete, by processes which comprise passing electric current around an electrochemical circuit which includes a mineral mass which is to be treated; the processes and arrangements being characterised in that the electrochemical circuit comprises:

(a) a first electrode located in a passageway or cavity in the body of the mineral mass to be treated;

(b) liquid electrolyte in electrochemical contact with the first electrode (a) and with the mineral mass to be treated, liquid electrolyte (b) also being at least partly located in the passageway or cavity in the body of the mineral mass to be treated;

(c) a treatment material for treating the mineral mass, present in solution in liquid (b), which treatment material has an ionic form suitable to stabilise the mineral mass; and

(d) a counterelectrode in electrochemical contact with the mineral mass to be treated and separated by mineral mass from the liquid electrolyte (b) containing the treatment material (c);

whereby mineral treatment material (c) in its ionic form migrates into the mineral mass to be treated during passage of the current around the electrochemical circuit.

The passageway or cavity can be formed in the body of the concrete or other mass to be treated, in any of a variety of ways, e.g. by drilling. The processes of the invention can utilise any of the features of the accompanying drawings.

Thus the invention provides a process for carrying out stabilising treatment of a mineral mass such as hardened concrete, which comprises passing current around an electrochemical circuit comprising a first electrode, a liquid electrolyte containing in solution a concrete treatment material having an ionic form suitable to stabilise the mineral mass, the electrolyte being in electrochemical contact with the first electrode and the mineral mass to be treated, and a counterelectrode which is in electrochemical contact with the mineral mass to be treated and separated by mineral mass from the liquid electrolyte containing the ionic mineral treatment material; and wherein the first electrode and electrolyte in contact therewith is located in a passageway or cavity formed in the body of the mineral mass to be treated; whereby mineral treatment material in its ionic form is made to migrate into the mineral mass to be treated.

In several examples the first electrode comprises at least one anode, the counterelectrode comprises a cathode, and the electrolyte liquid in electrochemical contact with the anode comprises a concrete treatment material in cationic form, such liquid being at least partly located within the passageway or cavity formed in the body of the

mineral mass to be treated. (The electrolyte liquid can for example in part form a pond on the surface of the mineral mass, and can extend continuously into the or each passageway or cavity containing the electrodes.)

Alternatively the polarity of the electrodes in use can be the reverse and the electrolyte liquid in electrochemical contact with the anode comprises a concrete treatment material in anionic form.

In certain embodiments of the invention particularly described below, the mineral mass to be treated is reinforced concrete having electrically conductive reinforcing elements located therein. This is of course very common, and most metal reinforcement is based on some form of steel, sometimes mild steel or stainless steel or galvanised steel rebars, or prestressing steel, which can either be embedded in the concrete or in metallic ducts. The electrochemical treatment can then use as a counterelectrode, e.g. cathode, at least one of such reinforcing elements, and in such a case, at least one anode and at least part of the electrolyte liquid in electrochemical contact with the anode, is preferably located in a passageway or cavity formed in the body of the concrete to be treated. (However, when epoxy coated steel reinforcement is used, the network can often be unsuitable for use as a cathode, and alternative cathodes may need to be provided, e.g. as described below.)

Usefully, the passageway or cavity (single or multiple) to accommodate the first electrode(s), e.g. anode(s), can extend or be located deeper in the concrete than the reinforcing element(s) located nearest the surface of the body of the concrete, and it can also be desirable to pass a recirculating current of the liquid electrolyte so that it flows through such a passageway or cavity formed in the body of the concrete under treatment.

The dimensions of such a passageway or cavity can be selected according to convenience, and drilled to produce minimal acceptable structural damage in the body of concrete under treatment. Diameters of a few centimetres e.g. often at least 5 cm, will often be found

sufficient, and depths corresponding to within about 10 cm - 1 metre of the depth of the mass of concrete to be treated.

Also provided by the invention is a hollow electrode system suitable for use as first electrode (a) in a process or arrangement as described above, and for insertion into a passageway or cavity in a mineral mass, for example hardened concrete, which is to be treated electrochemically, characterised in that the electrode system comprises:

an outer jacket, e.g. in the form of a tube, comprising insulating material and defining a passageway within said jacket, with an open end for insertion into said passageway of cavity in said mineral mass;

an inner longitudinal wall, extending within said outer jacket and dividing said passageway into passageways which are so arranged that when the electrode system is inserted into a passageway or cavity in a mineral mass to be treated, with said open end of said outer jacket disposed towards the deeper end of said passageway or cavity, then said passageways are in communication with each other so that they can form a path for circulating flow and return respectively of liquid electrolyte;

conductive electrode material located at or in the vicinity of said open end of said outer jacket, for contact with liquid electrolyte when such electrolyte circulates as aforesaid; and

means for providing an electrical connection between said conductive electrode material and an external circuit to be connected at the other end of the outer jacket of the electrode system.

Such a hollow e.g. tubular electrode system can be suitable for insertion into a passageway or cavity in a mineral mass such as hardened concrete to be treated electrochemically, the electrode system comprising an outer jacket, e.g. in the form of a tube, comprising insulating material and defining a passageway within said jacket, an inner longitudinal wall extending within said outer jacket,

e.g. in the form of an inner tube, said inner wall dividing said passageway into passageways which are in communication at an open end of said jacket which is arranged to be inserted into the deeper end of said passageway or cavity, and which are arranged to be able to form a path for circulating flow and return respectively of liquid electrolyte when the electrode system is inserted into a passageway or cavity in a mineral mass to be treated, the electrode system having conductive electrode material located at or in the vicinity of the said open end of the jacket for contact with liquid electrolyte circulating as aforesaid, and means for providing an electrical connection between the electrode material and an external circuit to be connected at the other end of the electrode system.

It is often convenient to have the outer jacket in the form of an outer tube, and the inner longitudinal wall can be provided by an inner tube extending within the outer tube, to define a first passageway within the inner tube and a second passageway outside the inner tube and within the outer tube.

The inner tube or other longitudinal dividing wall can for example be itself of conductive electrode material and provide the connection between an external electrical circuit and the end of the electrode system which is deepest in the passageway in the mineral mass to be treated, where most of the electrochemical current flux is intended to pass between electrode and mineral. Alternatively, both tubes can be of insulating material and a suitable electrode material, e.g platinum wire or carbon, may be constituted by or connected to the end of a wire or other connection passed down the inner tube. Many alternative arrangements and details are practical, and suitable example arrangements are mentioned below in connection with Figure 1 of the drawings hereof.

In an example of the invention described below, mineral treatment material is used which can stabilise the concrete or other mineral mass by combatting the alkali silica reaction. The preferred treatment material for this purpose comprises lithium, most often and preferably as lithium hydroxide in aqueous solution. Alternatively a

lithiu salt can be used, e.g. lithium chloride, bromide, carbonate, borate, nitrate, or nitrite, alone or with the preferred hydroxide. Usable lithium concentrations are not critical, they can for example but without limitation be in the range 0.1 to 5 molar, e.g. about 0.3 to 2 molar, e.g. about 1 molar. The electrolyte liquids can conveniently include additives such as surfactants to improve wetting.

Alternatively or additionally, the concrete treatment material can comprise calcium (e.g. as calcium hydroxide) for chloride extraction, and/or sodium ions (e.g. as sodium carbonate or sodium borate) for re- alkalisation of concrete, especially concrete which has become carbonated and lost significant alkalinity. These materials are applied in alkaline aqueous solution. Usable concentrations of the treatment materials can again for example but without limitation be in a range of about 0.1 to 5 molar, e.g. about 0.3 to 1 molar.

Electrolyte pH can usefully be maintained at for example about pH 11 to 12, and can be raised again if it tends to fall during treatment with periodic additions of lithium hydroxide solution or other alkaline treatment liquid.

The technique is normally carried out at ambient temperature, but it is allowable for the temperature to rise somewhat by the heating effect of the current that passes, so long as thermal expansion stresses are minimised and kept within tolerable limits. The heating effect may also be advantageous during examples of the application of this invention to the removal of chloride, where the heat encourages release of chloride from a chemically bound state.

Usable current densities and treatment times, or charge densities representing the product current * time, can be for example as follows. When the major part of the current passes through the surface of the concrete under treatment, suitable treatment is often accomplished at current densities of about 0.5 - 1 ampere per sq metre of steel surface area or of concrete surface area, and overall charge densities of about 30 - 30000 ampere hours per sq metre. Approximately corresponding currents and charge rates can be used based on the

surface area of the passageways and cavities having anodes located therein. Treatment may last for any suitable period ranging from several days to some months. The practical but not critical upper limits of current density are such as to avoid chemical or thermal damage to the concrete: the lower limits, again not critical, are such as to avoid excessively long treatment times.

Treatment of certain structures, e.g. those comprising prestressing steel embedded in the concrete or in metallic ducts, by processes according to the invention can suitably include protection against hydrogen embrittlement, e.g. by monitoring the electrochemical potential with a half cell and keeping the potential of the steel below the hydrogen evolution potential.

The amount of treatment or charge density required can be estimated in any of several suitable ways, e.g. by measuring depletion of the treatment cation in the treatment electrolyte in known manner, and comparing this with a desired rate of incorporation of the cation in the mass of concrete or region of such a mass which is to be treated, e.g. up to about 0.1% by weight of lithium. Where chloride is to be extracted, the appearance of chloride in the electrolyte can be followed analytically by known methods and compared with the amount of chloride expected on the basis of sampling the concrete and estimating the effective mass under treatment. More than 50% of the chloride may be extracted, e.g. in some cases up to 80-90%.

Lithium treatment can be carried out simultaneously with chloride extraction or cathodic protection or realkalisation if desired. Thus for example a combined treatment can be applied, for preventing corrosion of reinforcing steel in reinforced concrete, as well as for stopping or reducing the asr of which there may be a risk in any case or as a result of the intensified alkalinity resulting from the anti- corrosion treatment.

Treatments according to examples of the invention can result in effective application of material that stabilises the concrete, e.g. by inhibiting asr, by extracting chloride, or by realkalisation, to

structures which are suffering from deterioration.

The treatments can for example lead to useful build-up of lithium within the concrete, and especially at the surface of reinforcing metal embedded in concrete, e.g. the surface of reinforcing steel rods or bars or gridwork, so as to mitigate the asr process there or elsewhere.

Alternative concrete treatment materials having positively charged ionic form can also be applied by processes according to examples of the invention, either to stop deterioration, or to stabilise the concrete by improving or changing its properties in a desired direction; e.g. positively charged inhibitors, and/or monomers that can be polymerised in situ to make the concrete impermeable or less permeable or to increase its strength, and/or relatively inert electrolytes e.g. to assist in chloride removal. Cationic inhibitors of the quaternary ammonium and phosphonium type, e.g. tetraethyl phosphonium cations, are examples of cationic concrete treatment material that may be used to protect embedded steel from corrosion. Pyrol and its derivatives are examples of charged monomers which can be electrochemically diffused into the concrete and allowed to polymerise in situ to coat the reinforcing elements.

In the absence of a reinforcing network that can effectively conduct electricity and provide a cathode for the process, alternative cathodes can be installed. Occasionally, concrete structures lack an electrically conductive reinforcing network suitable for use as a cathode connection. For example, the reinforcement may consist of epoxy-coated steel, or it may be based on plastics material, or it may be absent altogether. In the absence of a suitable network of reinforcing metal elements to provide a cathode for the treatment, it is within the scope of the invention to provide one or more electrodes, e.g. a network of electrodes, preferably with a suitable cathodic electrolyte, which can be placed on the surface or in holes or passages drilled in the concrete under treatment and used as cathodes. Anionic concrete treatment materials can be applied from electrodes arranged as described herein and used as cathodes, e.g.

nitrite, against corrosion in concrete.

In this way the invention can be applied for example to plain concrete highway pavement where there is no reinforcement to act as a cathode, and in such a case anodes and/or cathodes can if desired be provided in the form of electrodes as described herein.

It can be seen that the invention also provides mineral mass such as hardened concrete containing a ionic mineral-treatment material suitable to stabilise the concrete, applied by processes as described herein. In particular, the invention provides steel-reinforced concrete containing lithium ion which has been applied by a process as described herein.

An example of the invention is described herein and illustrated with reference to the accompanying drawings, in which:

Figure 1 shows, in diagrammatic section, transverse to the surface, a mass of steel- reinforced concrete to which a process according to the invention is applied, and which together with the features described constitutes an example of a composite according to the invention; and

Figure 2 shows, in diagrammatic section, a reinforced concrete beam having embedded therein an anode to allow treatment, by a process according to an embodiment of the invention, of multiple depths of reinforcement in the beam.

Referring to Figure 1, this shows, in section transverse to a surface, a matrix of concrete with a surface 1, which is to be stabilised by treatment with a liquid concrete treatment electrolyte as described herein. In the example illustrate in the drawing, the concrete treatment electrolyte liquid can comprise for example lithium hydroxide solution of about 1 M concentration. The concrete is reinforced concrete, and reinforcing steel bars and/or rods or grillwork are shown partially at 2 and are located embedded in the concrete substantially parallel to surface 1 and at some depth relative to surface 1.

A pond of concrete treatment electrolyte 3 having an anode connection (not shown in Figure 1) is in electrochemical contact with surface 1 of the concrete. If more convenient, the pond may be substituted or supplemented with a saturated porous mat and/or a recycling system applying liquid to the surface (not shown in Figure 1). This anode at the surface is not essential to the invention and in certain alternative examples it may be dispensed with as practically unnecessary.

Formed in the concrete is a substantially cylindrical hole 4 in the form of a blind passageway. A jacketed hollow anode system comprising a tight-fitting open-ended tube 5 with a diameter of about 5 cm, of nonconducting material, is inserted in hole 4. Tube 5 may be of any suitable plastics material, such as perspex, polyethylene, or ABS. If there is space left between the inner surface of hole 4 and tube 5, it is usually provided with a temporary hydrophobic sealant such as petroleum jelly to stop escape of liquid from the deeper end of tube 5 as far as possible during treatment. Such a sealant is desirable in order to limit as far as possible the electrolyte-concrete interface to the bottom of hole 4, and thus tend towards maximum contribution made by current flowing at deeper depths within the concrete, to the total electrochemical current passing through the jacketed anode system. If desired, a setting sealant can be used, such as acrylic resin. This may require to be drilled out again after the concrete treatment, so that the hole may be refilled with concrete or cement mix. (Alternatively, the anode may be left in place against the possible need for a repetition of the treatment in the future.) Hole 4, produced e.g. by drilling, extends to a depth (relative to surface 1) which is deeper than the depth at which reinforcing grillwork 2 is embedded in the concrete.

Fitted coaxially within tube 5 is a smaller-diameter open-ended tubular anode 6 of non-corroding conductive material, such as comrnercially available platinised titanium tubing. Other commercially available anodic materials may be used, such as the metal oxide coated titanium electrodes as previously used in the cathodic protection of steel in concrete, or carbon anodes. Tubular anode 6 forms a

recycling pathway for liquid electrolyte within tube 5, this pathway consisting of a central passageway within anode 6 and an outer passageway around anode 6. If desired, tubular anode 6 may be substituted by a combination of conducting and non-conducting material. For example, the metal tubular anode 6 illustrated may be replaced by a nonconducting tube 6 fitted with an anodic section at its deeper end, which can take the form of a metal tube section or an anodically active wire section located at, or extending to, the deeper end of hole 4. If desired, such an anodic wire or a connection to anodically active metal or carbon in any suitable form can be insulated for the greater part of its depth and have only an exposed end at the deeper end of hole 4, but it also seems to be quite acceptable for the metal anode connection to be in electrochemical contact with electrolyte liquid over the entire depth of hole 4.

When the treatment is carried out, the space within the jacketed hollow anode system is at least partly filled with the concrete treatment electrolyte.

Figure 1 shows only a single jacketed hollow anode system, but in practice it will often be convenient to provide a plural number of such anode systems, spaced apart by distances of the order of 10 cm to 1 m, and connected electrically in parallel. Even though examples of the anodes described herein have non-conducting jackets or outer tubes, it can be quite important to ensure that as far as possible the holes and passages which are made in the concrete to accommodate the anode systems are clear of the network of rebars or other reinforcing elements: this precaution is intended to avoid current shunting during the electrochemical treatment. Rebar locators, e.g. commercially available locators which rely on magnetic detection, should preferably be used to select suitable drilling sites for placement of the anodes.

To carry out treatment according to an example of the invention, a voltage of up to 50 volts d.c. is generated by a transformer-rectifier powered from the public a.c. mains electricity supply and is applied between the anodes and the steel reinforcement (cathodes), to give a

current density up to about 1 amp per square meter of reinforcement steel or anodic surface. Any convenient a.c. or d.c. generator may of course be used instead of a mains supply as original power source.

When the process is applied, treatment electrolyte is pumped through the central axial passage within anode tube 6 and returns by the outer passageway in contact with, and within, tube 5, and contacts the concrete surface of hole 4 substantially only at the open end of tube 5 farthest from the outer surface 1, i.e. at the bottom of hole 4. This is normally the preferred direction of flow although it may sometimes be convenient to have the liquid flow in the reverse direction.

Pumping and recycling details are not shown in Figure 1. The rate of pumping is believed not to be critical and can usefully be dictated by criteria of mechanical convenience, such as the need to avoid deposit of any adventitious sediment and consequent blockage, and the type of pumps that may be to hand. Normally it may be convenient to arrange for a recirculation rate of about 1 litre per hour per jacketed anode system.

Recirculation of the electrolyte can allow measurement of the amount of concrete treatment material, e.g. the lithium ion, entering the system. Once a suitable amount has entered, the treatment can be stopped. Treatment times can be of the order of a few days to over three months. When the treatment is essentially chloride extraction, it is usual to sample the concrete to measure its chloride content and in that case treatment can often be continued usefully until over 50%, e.g. up to 75%, or even 80-90% of the expected chloride has been removed. Under these conditions it is an advantage of electrochemical chloride extraction that in the region of the cathodic reinforcing bars the residual chloride levels can be expected to be less than the average residual level in the concrete mass as a whole.

Referring to Figure 2, this shows, in diagrammatic section, a reinforced concrete beam 7 having embedded therein an anode 8, of which details are not shown in Figure 2, but which is of generally

similar tubular construction to the anode system comprising tubes 5 and 6 as illustrated in Figure 1. Anode 8 allows treatment, by a process according to an embodiment of the invention, of multiple depths of reinforcement in the beam, and particularly in the arrangement illustrated, it allows effective delivery of electro¬ chemical current flux via deeper levels 9 and 10 of reinforcing elements in beam 7, as opposed to shallower level 11 which would be the preferred conductor of current if the electrochemical treatment were carried out with a superficial anode only, and without anode 8. Lines of current flux (fluid flow) are shown schematically at 12. This example also utilises a surface anode 13. The corresponding arrangements for treatment of the reinforcing steel 14 at the other side of the beam are omitted from the diagram.

Using the arrangements described herein, it is possible to achieve a usefully high degree of penetration of concrete treatment cations to depths within the mass of concrete under treatment, which are deeper than the network of metal reinforcing elements. In many constructions, the reinforcing network is not far from the surface of the concrete, and leaves a large mass of concrete without further reinforcement at deeper depths, often not sufficiently treated by conventional techniques.

On the other hand, the present technique can achieve usefully and selectively high concentrations of treatment cation, e.g. lithium ion, to inhibit asr in the vicinity of steel reinforcing elements including any located at some depth that would be electrically shielded by metal at shallower depth from the effect of electrochemical application of cations but for the use of electrodes and electrolytes in passageways formed at depth as described herein. This can be the situation, for example, with structures having multiple layers of reinforcing steel, such as structural beams of columns. Normal treatment from the surface will treat the top layer of steel (which may get for example about 60-70% of the current, with lesser current to the lower steel). Arrangements according to the present invention can be made to treat the bottom layers of steel, with, if desired, lesser treatment to upper steel layers.

Variants of the example described, using for example calcium hydroxide electrolyte or sodium carbonate electrolyte, can achieve useful chloride removal and/or realkalisation in masses of concrete located at such depths beneath a reinforcing network, or in the absence of a reinforcing network that can effectively conduct electricity and provide a cathode for the process, by the use of alternative cathodes as described herein.

In further embodiments of the invention, the electrodes arranged as described above in connection with the drawings and examples can alternatively be used as cathodes rather than anodes. When they are used as cathodes, then there is no need to use non-corroding electrode materials as described above and selected to withstand anodic reactions; alternative conductive materials such as mild steel can be used instead. The electrolyte used in connection with the cathodes can include an anionic concrete treatment material such as nitrite ion.

The invention extends, as will be apparent to the skilled reader, to modifications, variations, combinations and subcombinations of the features described and illustrated herein.