METHOD OF TREATING LIMESTONE AND LIKE CALCAREOUS MATERIALS

Abstract: A method of treating limestone comprising introducing into the voids and/or interstitial spaces within the limestone, a water excluding agent to render the limestone substantially impervious and to increase the interstitial bonding within the limestone. According to one form of the invention the water excluding agent comprises a calcium carbonate solution.

Description: TITLE: "Method of treating limestone and like calcareous materials" FIELD OF THE INVENTION The present invention relates to a method of treating ground and structures formed at least in part from porous or permeable calcareous rock material such as limestone and/or carbonate clays or sands. In particular, the present invention relates to such a method that prevents calcareous rock material porous or permeable calcareous rock material such limestone, carbonate clays or sands from being eroded by elements such as water, wind, acid rain and the like environmental influences including those which have been contaminated. Throughout the specification and claims the term limestone shall be taken as including all forms of calcareous rock material including limestone (which can be of variable composition), dolomite, dolostone, and oolite which are porous or permeable, carbonate clays, carbonate sands and like. BACKGROUND Throughout the world rock materials such as limestone are used in buildings both as foundations and as walls. In addition because of the prevalence of calcareous rock such as limestone in the geology of many localities, the buildings and associated structures of many cities, towns and villages are built upon such rock where the rock provides the ground support for the of buildings and structures. Limestone by its nature is very porous but the degree of porosity is extremely variable and will depend upon a large number of factors which include the material of which it was formed and its geological history. In addition the structure of limestone is not constant but will vary according to the prevailing environment. In fact it is the capacity of limestone to respond to variable climatic conditions that have created the famous cave sites throughout the world. The variable nature of limestone, its intergranular structure and its capacity to undergo change in its intergranular structure as a result of variations in the prevailing conditions results in it often being unreliable as a building material. In regard to its capacity to change its intergranular structure, it is a characteristic of limestone that variations in pH have the capacity to disturb and radically reduce the volume of the crystals which provide the cementation between the grains constituting the limestone and thus significantly affect the interstitial bonding between the grains. Generally the unreliability of limestone has not been a problem in circumstances where the prevailing climate and conditions are not the subject of change. This unreliability however does create a significant difficulty where limestone constitutes the basic ground support for a structure or has been utilised in a structure in forming the foundations, walls or roof of a structure and the prevailing environment for the structure changes. An example of such a change can include the incidence of "acid rain" which results in water having abnormal acidic properties being absorbed by the limestone and dissolving some of the constituents of the limestone and varying the crystalline characteristics of the limestone which results in the structural integrity of the limestone being severely compromised over a relatively short period compared to the usual geological influences. Another example of change can relate to a circumstance in which the agricultural or horticultural activities around a building intensify which result in the amount of water being applied to the ground increases and the application of pH altering materials such as manures and fertilisers to the ground. This can result in the water and dissolved agents being absorbed by the limestone forming the foundations and walls of a structure and the limestone of the ground formations supporting the building which can lead to a dissolution of at least some of the constituents of the limestone and/or a chemical reaction which in turn results in the structural integrity of the limestone being changed. Another change can comprise a circumstance whereby as a result of development of a site, a substantially impervious layer (such as the capstone which is commonly found overlaying a limestone deposit) is removed from a limestone ground structure which results in that structure being subjected to an increased flow of water which in turn results in the dissolution of some of the constituents of the limestone which in turn results in the structural integrity of the limestone being affected. It is because of the changes to prevailing conditions of the form referred to above, in many locations around the world, that buildings and structures which have stood for many years are suddenly undergoing rapid deterioration. Such deterioration can appear as rapid erosion of the exposed limestone or collapse of the supporting foundations. In the past, efforts to prevent the further deterioration of buildings has involved replacement of walls, application of concrete or cementitious compositions to the eroded areas, application of rock bolts, utilisation of buttresses and the like. However in many cases such remedies only serve to exacerbate the problem since they can introduce agents into the limestone which only serve to accelerate the deterioration, This can be the case with the application of some cementitious materials, which may introduce additional agents to the site which may in the long term become detrimental and/or increase the exposure of the supporting limestone to the influence of water flow. In addition the use of rock bolts can result in the destruction of the surrounding rock and create additional flow paths for water. The present invention seeks to provide a means which is able to reduce the vulnerability of limestone in situ or present in structures to deterioration from the prevailing conditions,. DISCLOSURE OF THE INVENTION Accordingly the present invention resides in a method of treating limestone comprising introducing into the voids and/or interstitial spaces within the limestone, a water excluding agent to render the limestone substantially impervious. According to a preferred form of the present invention, the water excluding agent is introduced into the limestone as a liquid wherein said water excluding agent is capable of solidifying or generating a precipitate when introduced into the interstitial spaces in the limestone to substantially seal and/or fill the interstices. According to a preferred feature of the invention the water excluding agent comprises an aqueous solution of calcium carbonate. Throughout the specification and claims any reference to a calcium carbonate solution shall be taken as a solution resulting from the dissolution of calcium carbonate bearing materials which can comprise limestone using an aqueous acid solution irrespective of whether the solution comprises calcium carbonate or calcium bicarbonate solution or a calcium carbonate or bicarbonate complex or the like. According to a preferred feature of the previous feature of the invention the solution is applied to a body of limestone by application to the exposed surface of the body and maintaining the application of the solution until the body has become substantially impervious. According to an alternative or supplementary feature of the previous feature the solution is applied to a body of limestone by application to existing flow paths existing in the body and maintaining the application of the solution until the body has become substantially impervious. According to an alternative or supplementary feature of the two previous features the method further comprises the formation of at least one bore hole or passage into the body, the application of the solution into the bore hole or passage and maintaining the application of the solution until the body has become substantially impervious. According to a preferred feature of the invention the application of solution to the body is monitored by sensing the presence of the solution in the body at a position spaced from the location of application of the solution. According to one particular embodiment the method comprises forming at least one second bore hole or passage in the body terminating at said position and locating sensors capable of sensing the presence of said solution in the at least one second bore hole or passage. According to a preferred feature of the invention the aqueous solution of calcium carbonate is formed by dissolving calcium carbonate bearing material in an aqueous acid solution. According to a particular example of the invention, the calcium carbonate bearing material is dissolved in sulfuric acid. According to another particular example of the invention, the calcium carbonate bearing material is dissolved in carbonic acid. According to a preferred feature of the invention, the calcium carbonate bearing material comprises limestone. In one preferred embodiment of the invention the limestone is of a high grade form. According to a particular example of the invention the high grade limestone has a calcium carbonate content of the order of 98 percent. The present invention will now be described , by way of several particular embodiments and examples to illustrate the method of the present invention BEST MODE FOR CARRYING OUT THE INVENTION The embodiments relate to the treatment of the ground structure for a building where the ground structure is formed principally from limestone. This is a relatively common situation in many localities around the world and one example comprises the city of Fremantle in Western Australia, Australia where the ground structure comprises significant limestone outcrops and many of the city's historic structures and buildings are built upon that limestone. It has been found that as a result of the development of many sites the overlaying indurated caprock which is substantially impervious to water has been at least partially removed which has exposed the underlaying more fragile and porous limestone to water flow. In addition with the establishment of gardens and lawn around the buildings the amount of water flowing into the limestone has increased significantly and with the application of manures and fertilisers to the overlaying soil the acidity of the water has been affected. Each of these influences has lead to the circumstance where the structural integrity of the underlying ground structure of many buildings in the city is being affected. In addition many buildings in the city are formed at least in part from limestone and as a result water as well as water containing dissolved carbon dioxide (ie weak carbonic acid) and acid rain is being absorbed by that limestone both aerially and from the ground which is causing the limestone of the buildings to deteriorate as a result of the presence of that water. The embodiment has been developed for the purpose of reducing the opportunity for water and the various deleterious contaminants dissolved therein to be able to flow into and through the limestone and to reduce the degree of residence of any water in the structure of the limestone. This is effected by significantly reducing the porosity of the limestone and substantially filling or at least sealing the interstitial spaces and voids which are an inherent characteristic of the limestone. In the case of structures formed of limestone blocks it is common for the cementitious mortars which have been used to bond the blocks together, to fret away. One of the reasons the cementitious mortars fret away from the limestone host is that the surface grains (skeletal silica and others) are weakly held together by a minimum of needle shaped crystals. Therefore if the degree of cementation between the grains can be increased the bond generated between the cementitious mortar and the limestone can also be increased to reduce the likelihood of fretting The method of the embodiment comprises formation of a calcium carbonate or bicarbonate solution and introducing the solution into a body of limestone to allow it to be resident in the voids of the limestone which will result in the calcium carbonate precipitating from the solution as calcite whereby with the continued application of the solution the interstitial spaces and voids within the limestone are filled and/or sealed. As a result any water which is incident on the body of limestone at a later date will flow around the body rather than into it to be resident for a period of time and causing the deterioration of the internal structure of the body of limestone. The solution of calcium carbonate is formed by dissolving the calcium carbonate in an acid solution. According to one specific example the solution is formed from a high grade limestone having a calcium carbonate content of the order of 98%. The limestone is crushed and/or pulverised using a suitable milling means and is mixed with an acid solution comprising sulphuric acid. Once the solution has been created the pH of the solution is controlled such that it is slightly acidic if not substantially neutralised and a suitable buffering solution is added to stabilise the pH of the solution. According to another particular example of the embodiment the acid used to dissolve the limestone comprises carbonic acid. As an alternative to limestone it is possible to utilise any form of naturally occurring calcareous material which has a high concentration of calcium carbonate. ln use the solution of calcium carbonate is introduced into a limestone body in order to provide some protection against deterioration of the limestone body. This can be effected by applying the solution of calcium carbonate to an external face of the limestone by suitable means to enable the solution to be absorbed by the limestone and to percolate through the limestone. The rate of delivery should be such that the solution is absorbed by the body of limestone rather than being allowed to run off. In addition if it is apparent that a flow path for water has already been created within the limestone as a result of weathering or by some physical damage to the body of limestone the solution can be introduced into that site at a rate whereby the solution will be absorbed by the body rather than merely passing through the body. The above form of application is appropriate where the body of limestone and the region to be treated is accessible as in the case of build foundations or walls or parapet walls or the like. In circumstances where the region to be treated is not accessible, such as the ground structure below a building or the central region of a thick wall or the like, it may be appropriate to drill one or more holes into the body where the holes terminate at locations which are to be rendered substantially impervious. The solution of calcium carbonate is introduced into the holes in a controlled manner to enable the absorption of the solution. In some circumstances it can be appropriate the solution be introduced by way of a suitable pumping means to generate a hydrostatic pressure which is sufficient to enable the solution to be adequately distributed into the injection site. It is understood that the application of the solution of calcium carbonate to the limestone results in it becoming resident, at least for a period of time in the voids and interstitial spaces which exist in the granular structure of the limestone and while present in the voids or spaces the calcium carbonate will precipitate from solution in the form of calcite crystals which with continued application of the solution, will substantially seal, or fill or at least significantly reduce the volume of the voids and the interstitial spaces within the limestone. The presence of the crystallised calcite within the voids and interstitial spaces within the limestone acts to exclude water from the interstitial spaces of the limestone by reducing the presence of potential flow paths and residence sites in the limestone. In addition the calcite crystals serve to increase the degree of cementation between the grains within the limestone which results in a significant increase in the structural integrity of the limestone. Therefore since calcite is a more stable form of calcium carbonate then this property serves to increase the inherent chemical stability of the treated limestone which together with the increased cementation provided by the calcite leads to the treated limestone having a greater resistance to deterioration as a result of the prevailing conditions of the location. According to another embodiment of the invention a body of limestone, which may comprise a ground structure, is treated in the manner described above but during the application procedure the region which is to be treated is monitored to ensure that there is an adequate delivery of solution and to ensure that the treatment is effective. This is effected by drilling at least one first bore into the body of limestone to enable the solution of calcium carbonate to be introduced into the body of limestone and drilling at least one second bore into the body to terminate at location spaced from the at least one first bore. A sensor means for detecting the presence of the solution is inserted into the at least one second bore to determine whether the aqueous solution of calcium carbonate is flowing into the second bore from the first bore and to monitor the continued flow into the second bore. When the sensor means does not detect the presence of the solution in the second bore it is believed that this can be taken as an indication that the calcite which has precipitated from the solution has filled and/or sealed the interstitial spaces or voids of the limestone in the region between the first and second bores at least and that the region is substantially impervious to water. As a result of the embodiments described above a body of limestone is able to be treated to render the limestone more resistant to erosion and the influences of water which is incident on the site. In addition the treatment of the limestone according to the embodiments does not require the introduction of exotic materials which may suffer deterioration over time and/or produce agents or influences which may contribute to the deterioration of the limestone. Furthermore the process of the embodiment involves the placement of a material into the structure of the limestone which is compatible with the limestone since it is formed of the same compound as the principal compound of the limestone it is introduced into the site of actual or potential deterioration Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It should be appreciated that the invention need not be limited to the particular scope of the embodiments and examples described above.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS 1. A method of treating limestone comprising introducing into the voids and/or interstitial spaces within the limestone, a water excluding agent to render the limestone substantially impervious and to increase the interstitial bonding within the limestone. 2. A method of treating limestone as claimed at claim 1 wherein, the water excluding agent is introduced into the limestone as a liquid wherein said water excluding agent is capable of solidifying or generating a precipitate when introduced into the interstitial spaces in the limestone to substantially seal and/or fill the interstices. 3. A method of treating limestone as claimed at claim 1 or 2 wherein, the water excluding agent comprises an aqueous solution of calcium carbonate. 4. A method of treating limestone as claimed at claim 3 wherein, the aqueous solution of calcium carbonate is formed by dissolving a calcium carbonate bearing material in an aqueous acid solution. 5. A method of treating limestone as claimed at claim 4 wherein, the calcium carbonate bearing material is dissolved in sulfuric acid. 6. A method of treating limestone as claimed at claim 4 wherein, the calcium carbonate bearing material is dissolved in carbonic acid. 7. A method of treating limestone as claimed at claim 4, 5 or 6 wherein, the calcium carbonate bearing material is limestone. 8. A method of treating limestone as claimed at claim 7 wherein, the limestone is of a high grade form. 9. A method of treating limestone as claimed at claim 8 wherein, the high grade limestone has a calcium carbonate content of the order of 98 percent. 10.A method as claimed at any one of claims 3 to 9 wherein the solution is applied to a body of limestone by application to the exposed surface of the body and maintaining the application of the solution until the body has become substantially impervious. 11.A method as claimed at any one of claims 3 to 9 wherein the solution is applied to a body of limestone by application to existing flow paths existing in the body and maintaining the application of the solution until the body has become substantially impervious. 12. A method as claimed at any one of claims 3 to 9 comprising the formation of at least one bore hole or passage into the body and the application of the solution into the bore hole or passage and maintaining the application of the solution until the body has become substantially impervious. 13.A method as claimed at any one of claims 10 to 12 wherein the application of solution to the body is monitored by sensing the presence of the solution in the body at a position spaced from the location of application of the solution. 14.A method of treating limestone as claimed at claim 13 wherein, the method comprises forming at least one second bore hole or passage in the body terminating at said position and locating sensors capable of sensing the presence of said solution in the at least one second bore hole or passage. 15.A method of treating limestone substantially as herein described.

Inventors: CUTLER BRIAN, CUTLER, BRIAN
Applicants: CUTLER BRIAN, NOTWHEAT PTY LTD, CUTLER, BRIAN, NOTWHEAT PTY. LTD.
Application year: 1998
Main classification: C04B41
IPC codes: C04B41, E02D3