Silica fume, a by-product from the production of silicon or ferro-silicon alloys, is commonly used as a cement and concrete additive to produce corrosion-resistant concrete, concrete having improved mechanical properties or water-tight concrete (see, for example, US 4,118,242, US 4,310,486 and US 5,472,501). However, its disadvantages are poor workability and relatively high surface water absorption due to the capillary action of fine capillary pores which result in high sorptivity of the concrete and therefore potential high chloride/sulfate build up at the splash and tidal zones of marine concrete structures. Furthermore, the high water absorption will result in a concrete which is water-tight but not damp-proof and will fail to meet the commonly used project specification of a water absorption of less than 1% of the concrete when it is tested in accordance with BS1881: Part 122.

Hydrophobic compound, such as calcium stearate, silicon, wax or bitumen mulsion are also commonly used as waterproofing admixtures of concrete, to impart a hydrophobic coating to the capillary surfaces as well as blocking some pores.

However such compound have the disadvantages of reduced compressive strength (typically 10-15% lower than the plain concrete) and reduced effectiveness under high hydrostatic head.

Therefore, for a durable waterproofing concrete both low permeability and low absorption to water are sought. One solution is to make separate additions of silica fume and hydrophobic compound during the production of the concrete. However, this requires additional mixing/dispersing and the hydrophobic compound is usually added as an mulsion in order to achieve a homogeneous mix. The pre-emulsifying of the hydrophobic compound and the complicated dosing procedure make such a procedure impractical and uneconomical for use on construction sites. It should also be noted that the mulsion can only have limited use in cold environments because it is generally not frost resistant. Furthermore, the emulsifier may reduce the hydrophobic action and cause excessive air entrainment and loss of strength.

Another approach is disclosed in US 4,762,867. This describes the use of an amphiphilic synthetic polymer dispersion to reduce water absorption in cement mortar and concrete. Synthetic polymer dispersion is very expensive and has an adverse effect on the compressive strength of the concrete. It is not practical and has only limited usage. It is therefore not generally used by the construction industry for large scale waterproofing concrete production.

According to the present invention from one aspect there is provided a waterproofing additive for cement and/or concrete, comprising at least one pozolanic material modifie with at least one hydrophobic material.

The pozzolanic material preferably comprises one or more pozzolanic components.

The pozzolanic material preferably comprises one or more of silica fume, microsilica and metakaolin. The silica fume may be densifie or undensified silica fume. The microsilica is preferably refined natural microsilica and is preferably amorphous. One particularly preferred composition for the pozzolanic material is a mixture of silica fume and microsilica, most preferably in approximately equal amounts by weight.

The hydrophobic material is suitably a hydrophobic compound, preferably an organic or organometallic compound, most preferably of a long chain paraffinic acid. The hydrophobic material may be a paraffinic or fatty acid ester, e. g. butyl stearate. The hydrophobic compound may be a soap, especially a metallic soap of a paraffinic acid, e. g. calcium stearate, magnesium stearate or aluminium stearate, or an organic soap of a paraffinic acid, e. g. an ammonium salt. The hydrophobic compound may be ofeic acid, a wax mulsion, siloxane or a silicon mulsion.

The or each pozzolanic constituent of the additive may be modifie with one or more hydrophobic materials. Preferred waterproofing additives are mixtures of at least one pozzolanic material (e. g. silica fume, microsilica or metakaolin) with at least one hydrophobic compound, or possibly two or more hydrophobic compound of different types. The combination of such materials has been observe to enable better mixing and a freer flowing powder nature of the finished product, as well as increased shelf life thereof.

The pozzolanic materia ! is preferably modified by spraying and/or blending with the hydrophobic material, suitably in a high speed/shear mixer. The mixing operation preferably results in an intimate, and preferably substantially fully mixed, mixture of the pozzolanic material and the hydrophobic material (s).

The total weight of the hydrophobic material (s) in the waterproofing additive is suitably in the range from 5 to 30% of the weight of the pozzolanic material, preferably 5 to 15% and most preferably around 10%.

A particularly preferred composition is an approximately 1: 1 mixture (by weight) of densifie silica fume and microsilica modifie w th approximately 10% (by weight of the combine pozzolanic constituents) of butyl stearate. Another particularly preferred composition is metakaolin modifie with approximately 10% (by weight) of butyl stearate.

According to a second aspect of the present invention there is provided a cement mortar or concrete comprising a waterproofing additive according to the first aspect of the invention. The cement mortar or concrete preferably comprises an amont of the waterproofing additive such that the pozzolanic material of the additive is present in the cement mortar or concrete in an amount in the range from 5 to 15% of the weight of cement, and most preferably from around 8 to 10%. The cement mortar or concrete preferably has improved water impermeability (suitably less than 10 mm after 28 days when measured in accordance with DIN 1048) andlor reduced water absorption (suitabfy less than 1 % when measured in accordance with BS1881: Part 122), preferably whilst retaining acceptable compressive strength. Thus, the waterproofing additive is preferably capable of acting to improve the waterproofing of cement mortar and/or concrete.

According to a third aspect of the present invention there is provided a method of preparing a waterproofing additive for cement and/or concrete, comprising modifying a pozzolanic material by spraying and/or blending with a hydrophobic material. The step of spraying and/or blending is performed using a high speed/shear mixer.

According to a fourth aspect of the present invention there is provided a method of preparing cement mortar or concrete comprising mixing cement, sand and water with a waterproofing additive according to the first aspect of the invention and/or manufactured in accordance with the third aspect of the invention.

In ASTM C618, a pozzolan is defined as a siliceous and aluminous material which, in itseK, possesses Irttle or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compound possessing cementitious properties.

The present invention will now be described by way of example only, with reference to the following embodiments which are illustrative and not limiting.

EXAMPLE 1-Waterproofing for Cement Mortars The mixture proportions of the cement mortars in this example are based on the following composition: cement (OPC): water: sand: waterproofing additive: liquid superplasticizer in the ratio of 1 : 0.38: 2.61: 0-0.08: 0.01-0.02 (by weight) respectively. The waterproofing additive comprises a pozzolanic material (e. g. silica fume, refined natural microsilica or metakaolin) modifie with a hydrophobic material (e. g. an organic or organometallic compound such as butyl stearate, calcium stearate, another metallic soap of a paraffinic aåd, oleic acid, wax mulsion, siloxane or a silicon mulsion). Thus, in the waterproofing additive the hydrophobic compound (s) are provided integrally with the pozzolanic material.

The silica fume that was used was a commercially available product having a typical composition as shown in table 1 (with constituents indicated by weight in %): Table 1: Typical Composition of Silica Fume Composition Si02 S03 c Fe203 CaO mgo Na20 K20 Usual range 85-95 0.1-0.5 1.0-6.0 0.1=0.4 0.1-0.5 0.1-1.0 0.1-1.0 0.1-1.0 Typical 90-94 1.0 max 3.0 max 0.3 0.3 0.4 1.0 max 1.0 max Both densifie silica fume (as supplie by Scancem Materais Pty Ltd, Australia) and undensified silica fume (as supplie by Elkem Materials, Norway) are suitable. The typical density range for the densifie silica fume is 500-650 kg/M3 and for undensified silica fume is 250400 kg/m3.

The microsilica that was used was a refined natural amorphous silica (as supplie by Microsilica New Zealand Limite, New Zealand). The typical composition is given in table 2 (with constituents indicated by weight in %): Table 2: Typical Composition of Refined Natural Microsilica SO3ClLOIAlkalicontentBulkDensityCompositionSiO2 Typical 90.8 0.15 0.001 2.2 0.012 700kg1m The metakaolin that was used was a commercially available product produced by high temperature treatment of kaolin (as supplie by ECC International, UK or Engelhard Corporation, USA). The typical composition is given in table 3 (with constituents indicated by weight in %): Table 3: Typical Composition of Metakaolin Composition Si02 AI203 Fe203 CaO MgO K20 Na20 Typical 52-55 40-42 0.6-4.6 0-0.1 0.2-0.4 0. S2.4 <0.1 The selected pozzolanic materials were processed by spraying and blending with one or more hydrophobic compound in a high speed, high shear mixer to produce a surface modWied waterproohng additive for cement and concret. The preferred hydrophobic compound are butyl stearate, calcium stearate or other metallic soaps of paraffinic amds which are commonly used as waterproofing additives in the cement and concrete. Other suitable hydrophobic compound include oleic acid, wax mulsion, siloxane and silicon mulsions, but with reduced effectiveness.

Cement mortar mixtures in which a range of hydrophobically modified pozzolanic materials with cement, sand, water and superplasticizer were prepared in a Hobart mixer. The liquid superplasticizer was added to adjust the mix to a constant workability (flow). The mortars'compressive strengths were tested according to ASTM C109 and the water absorption was tested by a modifie method according to BS 1881: Part 122 at 7 days. The results oIncluding results for a control sample with none of the hydrophobic material) are summarized for each of the chosen pozolanic materials in tables 4,5,6 and 7.

Table 4: Test Results of Cement Mortar with Modifie Densifie Silica Fume Hydrophobic Butyl Calcium Oleic Wax Silicon Compound None Stearate Stearate Acid Emulsion Siloxane Emulsion Compressive Strength (MPa) @ 1 day 36.0 35.0 31.0 24.0 24.5 17.5 7.0 * 7 days 63.5 63.0 54.0 52.0 49.0 52.5 46.0 * 28 days 68.5 70.5 61.5 70.0 64.5 71.0 74.0 Water absor tion @ 7 days (%) 2.20 0. 58 0.68 1.94 1.56 1.55 2.09 Table 5: Test Results of Cement Mortar with Modifie Undensified Silica Fume Hydrophobic Butyl Calcium Oleic Wax Si (acon Compound None Stearate Stearate Add Emuísion Siloxane Emulsion Compressive Strength (MPa) @ 1 day 36.0 34.5 31.0 30.0 35.0 7.0 3.0 63.568.560.066.566.062.066.0@7days * 28 days 68.5 72.0 66.0 70.0: 69.5 75.0 74.0 <BR> <BR> <BR> <BR> <BR> Water absorp-<BR> <BR> <BR> <BR> <BR> <BR> <BR> tion @ 7 days (%) 2.20 0.65 0.80 1.41 1.46 1.43 1.69 Table 6: Test Results of Cement Mortar with Modifie Microsilica Hydrophobic Butyl Calcium Oleic Wax Si (acon Compound None Stearate Stearate Acid Emulsion Sitoxare Emulsion Compressive Strength (MPa) * 1 day 38.0 36.0 35.5 29.5 33.0 1.5 12.0 @ 7 days 66.5 63.0 59.5 54.5 62.0 43.5 54.0 @ 28 days 76.0 78.0 73.0 79.0 71.0 73.5 76.5 Water absorp tion @ 7 days (%) 2.12 0.63 0.79 1.61 1.70 1.43 2.04 Table 7: Test Results of Cement Mortar with Modifie Metakaolin Hydrophobic Butyl Calcium Oleic Wax Silicon Compound None Stearate Stearate Acid Emulsion Siloxane Emulsion Compressive Strength (MPa) @ 1 day 33.0 32.0 30.5 33.0 33.0 1.0 2.5 @ 7 days 62.5 63.0 64.0 65.0 64.0 60.5 56.0 @ 28 days 65.0 67.5 64.0 71.0 68.0 71.0 72.0 Water absorp- tion @ 7 days (%) 2.30 0.58 0.63 1.29 1.42 1.53 1.66 The above study shows that the waterproofing additive can effectively redue the water absorption of the cement while generally no or little adverse effect on the 28 days compressive strength was observe. In fact, in many cases increased compressive strength was observe. The fluctuation in the strength data of the plain mortars was due to the different sources of cerment available at the different times of testing. Therefore comparison should be restricted to within each group of tests. Pozzolans modifie with butyl stearate showed the best combine performance in term of strength development and water absorption.

Similar mortars could be prepared using a mixture of more than one waterproofing additive. More than one pozzolanic material could be used, and the or each pozzolanic materia (cou (d be modified with the same or different hydrophobic materials or with more than one hydrophobic material.

EXAMPLE 2-Waterproofing for Concrete The effectiveness of the waterproofing additive on concrete is demonstrated by the following study.

The modWied pozzolanic material employed in this study was densifie silica fume (as described in detail in relation to example 1) modified with 10% butyl stearate. Other pozzolanic materials modified With butyl stearate are expected to perform similar or better as shown in the cement mortar studies. Two typical water to cement (w/c) ratios of 0.36 and 0.45 were used in this study and the dosage levels of the modifie silica fume (MSF) were at 5% and 10% by weight of the total binder (cement + silica fume) content. The concrete mixture proportions and the test results are shown in tables 8 and 9, one table for each of the water: cement ratios.

The concrets were prepared in a concrete drum mixer. The cement (OPC), densifie silica fume and the liquid superplasticizer (high range water reducing agent: HRWRA) were commercially available materials. The fresh properties and strength of the concrete were tested according to the guidelines set in BS 1881. The water absorption was tested at 7 and 28 days in accordance with BS 1881: Part 122. The water penetration was tested at 28 days according to DIN 1048 and the rapid chloride penetration test was carried out at 28 days according to ASTM 1202.

Table 8: Test Results of Concrete (water: cement=0.36) MSF-1MSF-2MaterialsPlain (no MSF) at 5% at 10% Mixture Proportion OPC (kg) 420 399 378 Sand 786786786 Stone (kg) 1063 1063 1063 Water (kg) 151 151 151 Mod-Mled Silica Fume (kg) - 21 42 HRWRA (L/100 kg Binder) 1.2 1.3 2.0 Results Initial Slump (mm) 155 200 200 Initial Setting Time (hors: min) 7: 45 Not tested 11: 00 Final Setting Time (hrs: min) 9: 35 Not tested 13: 15 (MPa)CompressiveStrength @ 1 day 47.0 35.5 30.0 <BR> <BR> <BR> <BR> * 3 days 64.0 58.0 53.0 @ 7 days 72. 5 70.0 68.5 @ 28 days 76. 0 79. 0 78.0 @ 56 days 76. 5 84.0 82.0 Water absorption (%) @ 7 days 1.83 0.86 0.65 <BR> <BR> <BR> <BR> days1.800.760.59@28 Water penetration @ 28 days (mm) 15.0 3.0 1. 0 Rapid Chloride Penetration @ 28 days (Coulombs) 2889 1130 479 Table 9: Test Results of Concrete (water: cement = 0.45) MSF-1MSF-2MaterialsPlain (no MSF) at 5% at 10% Mixture Proportion OPC (kg) 380 361 342 Sand (kg) 782 782 782 Stone (k) 1057 1057 1057 Water (kg) 171 171 171 Modifie Silica Fume (kg)-19 38 HRWRA (L/100 kg Binder) 0.9 1.2 1.4 Results Initial Slump (mm) 130 140 105 Initial Setting Time (hrs: min) 5: 30 5: 25 6: 00 Final Setting Time (hrs: min) 6: 45 7: 00 7: 25 Compressive Strength (MPa) @1 day 32.5 31.5 27.0 @ 3 days 49.5 47.0 44.0 cl 7 days 57.0 57.5 56.5 <BR> <BR> <BR> <BR> * 28 days 65.0 68.0 64.5 @} 56 days 65.5 70.0 69.5 Water absorption (%) <BR> <BR> <BR> <BR> * 7 days 2.28 1.02 0.76 @ 28 days 2. 20 0.99 0.70 Water penetration @ 28 days (mm) 25.0 6.0 1.5 Rapid Penetration @ 28 days (Coulombs) 2641 1763 712 The results show that the compressive strength of the concrete modifie with 5-10% of the particular waterproofing additive Gan be maintained or increased by up to 4% and 10% at 28 days and 56 days respectively. The water permeability, water absorption and chloride pemmeability are drastically reduced. Even at a water/cement ratio of 0.45 and an addition of 5% modifie silica fume, the water penetration was only 6 mm, which is far below the 20 mm (average) penetration as specified in ENV206 for water impermeable concret. The water absorption can also be controlled to below 1%.

Different waterproofing additives, or mixtures of additives, as described for example 1 could be used in preparing concrete as for this example.

EXAMPLE 3-Water Sorptivity of Concrete One of the additional benefits of the novel waterproofing additives described above is the reduced water sorptivity of the modifie concret. The sorptivity test is in fact similar to the surface water absorption test. The only difference is that the specimen in this case is not fully immersed in water, instead on (y the bottom face of the test specimen is placed in contact with water. In this way capillary suction can be directly visually monitored by observing the height increase of the wetted areas, or by measurement of the weight gain. The following example demonstrates the sorptivity theconcreteundercapillaryforces.behaviourof The concrete mixture proportions used for the test were the same as given in example 2 with 10% addition of modifie silica fume. Additionally, a concrete with 10% unmodified silica fume was prepared for comparison. The concrete was cured for 28 days and core samples having dimensions of 75 mm in diameter and 75 mm in height were taken. The samples were coated with an epoxy resin over the length of the longitudinal faces and were dried in an oven at 105°C for 3 days and cooled in a desiccator for 1 day before the test. The test specimen was placed in contact with water at the bottom surface of the cylindrer and the weight gain of the specimen was measured after 30 and 60 minutes and 24 hours of water contact. The specimens were split into two halves after 24 hours and the height of the wetted areas were measured. The results are shown in table 10.

Table 10: Comparaison of Water Sorptivity of Plain Concrete, Silica Fume Concrete and Concrete with Modifie Silica Fume Concrete Plain SF-Concrete MSF-Concrete W/C Ratio 0.36 0.45 0.36 0.45 0.36 0.45 Weight gain (g) @ 30 min 2.5 4.2 1.9 3.9 0.4 1.0 @ 60 min 4.0 7.5 3.2 5.9 0.9 1.9 @ 24 hours 13.8 22.8 10.2 17.6 3.0 5.1 Height of wetted area @ 24 houz (mm) 51 63 38 48 4 10 It is evident that the sorptivity of silica fume concrete is still very high, due to the presence of fine capillary pores. The hydrophobically modifie silica fume is highly effective in reducing the capillary suction of the concret, which is highly beneficial for a durable concrete so as to prevent rising damp and formation of efflorescence, reduce the build up of chloride and sulfate concentrations at the tidal and splash zones of marine concrete, minimize freeze-thaw damage and reduce algal growth on the concret's surface.

In summary, the improved waterproofing additives described above have been observe to provide an improved and economical solution to produce highly durable water impermeable and damp-proof concrete which is able to overcome the disadvantages of the relatively high water absorption of waterproofed concrete containing silica fume and the reduced compressive strength, relatively high water permeability of waterproofed concrete containing hydrophobic compound. It was found that waterproofing concrete can be produced with the addition of a modified pozzolanic composition to meet the water impermeability of concrete as specified in ENV 206 and the water absorption of less than 1 % tested in accordance with BS1881 : Part 122.

As described in the examples above, pozzolanic materials are used as carriers for the hydrophobic compound and therefore a pre-emulsifying is not necessary, providing a substantial avantage over the prior art. The pozzolan appears to act as a reactive pore fille by reacting with the hydration products of the cement to form insoluble precipitants which reduce the porosity and the pore size of the cement paste. At the same time the absorbe hydrophobic compound is believed to migrate toward the air/water interface of the concrete and form a hydrophobic layer at the concrete surface and the walls of the capillary pores which form an effective barder against capillar, v suction of water. The novel waterproofing additives described above have been found to provide a highly advantageous solution for providing concrete with both water impermeable and damp-proof characteristics. The use of such waterproofing additives is very straightforward and does not differ from the normal use of silica fume in concrete production. In fact, the workability of fresh concrete containing the said novel waterproofing additive has been found to be better than the normal silica fume concrete, as the hydrophobic compound act as an internal lubricant for the cement particules.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.