Abstract:
The invention is embodied in a workable binder which contains silica fume as the binding agent, and which uses alkali metal silicates, either hydrated or anhydrous or a combination thereof, as a hardening agent. The binder further includes a tempering or workability agent to facilitate handling. When mixed with aggregate, the binder can be used in a variety of refractory or construction applications to form a strong castable product having high strength without the use of calcareous components.

Description:
This is a continuation of application Ser. No. 001,573, filed Jan. 8, 1987 U.S. Pat. No. 4,780,142. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to hard setting refractory compositions and especially to rapid hard setting castable refractory compositions comprising fumed silica and a refractory material, such as one containing alumina, which when mixed with water will set to a hard mass at normal outdoor temperatures without any calcareous cementious material being present in the composition. 
     Calcareous cements that contain fumed silica and are rapid setting are known as shown in Miller et al Pat. No. 4,501,830. Also, Pat. No. 4,506,025 discloses a castable silica brick composition containing fumed silica, silica and portland cement. This composition is relatively slow setting and contains undesirable calcium oxide compounds. Japanese Patent No. 1981-78476 shows a similar castable composition based on the coagulation of superfine silica powder with the Ca ions of Portland cement. 
     U.S. Pat. No. 3,121,641 discloses a castable composition containing alumina and fumed silica and U.S. Pat. No. 3,067,050 discloses a gunning mixture containing alumina and fumed silica. However, these compositions do not set quickly to a hard set, and must be given a lengthy curing and then fired to obtain suitable strength and hardness. 
     SUMMARY OF THE INVENTION 
     The present invention is based on my discovery that alkali metal silicates with fumed silica and refractory material when mixed with water will set to a hard mass at normal atmospheric temperatures and even lower. The alkali metal silicates act as active reagents or accelerators to initiate and speed up the hardening action of the fumed silica in such compositions. They are effective in such small quantities as 0.2-5% of the mixture. 
     The preferred alkali metal silicates for use in casting compositions and other purposes that require more time than flash setting are the particulate alkali metal silicates that have a rate of solution in water less than that of hydrated sodium silicate having a molar ratio of SiO 2  :Na 2  O of 1:1. This is because the setting time depends upon the rate of solubility. The anhydrous alkali metal silicates dissolve with more difficulty than the hydrated silicates and the silicates containing more alkali metal in the molecule dissolve more readily. 
     Suitable alkali metal silicates that do not produce a very rapid or flash setting composition and that can be uniformly mixed before setting include the particulate anhydrous sodium silicates having a molar ratio of SiO 2  :Na 2  O of at least 1 to 1, preferably 2:1 to 4:1, particulate hydrated sodium silicates having a molar ratio of SiO 2  :Na 2  O of at least 3:1, preferably 3:1 to 4:1, and particulate anhydrous potassium silicate having a molar ratio of SiO 2  :K 2  O of at least 3:1. 
     Compositions containing fumed silica and liquid sodium silicates or hydrated sodium silicates having a molar ratio of SiO 2  :Na 2  O of 1:1 have a very rapid or flash set and are impractical for most uses since they cannot be uniformly mixed before setting, but they can be used for filling holes, patching and the like. 
     The binding mechanism of alkali metal silicates with fumed silica can be employed with a great variety of formulations, preferably those containing alumina such as bauxites, and high alumina materials. Other material used in refractory applications such as calcined clay, silicon carbide, spinel, and zircon may be used. This includes formulations for placement by gunning and shotcreting application as well as by casting. 
     One advantage of this invention is that it is not necessary to use calcareous cements or lime. 
     It is also an object of this invention to provide refractory cast products that can be installed with lesser concern for curing procedures and put into service soon after hardening. 
     It is a further object of this invention to provide more predictable and reliable properties than is often the case with complex calcium aluminate cement systems. 
     A preferred composition to harden both cast and gunning materials comprises fumed silica in from 3-30% of the total refractory composition, powdered anhydrous sodium silicate with a silica to soda ratio of 1:1 to 4:1 in the range of 0.2-5% of the total mix, and a major amount of refractory aggregate comprising alumina. When the anhydrous sodium silicate is replaced with hydrated sodium silicate the molar ratio of SiO 2  :Na 2  O for similar results should be 3:1 to 4:1. With anhydrous potassium silicate the molar ratio of SiO 2  :K 2  O should be at least 3:1. With poured or vibrated castables 0.1 to 1.0% by weight sodium hexametaphosphate is helpful. 
     Castables and gunning formulations may be made without the inclusion of calcarious cementitious components using fumed silica as the sole binder. This is made feasible by my discovery that the addition of alkali metal silicate reagents to the fumed silica promotes a hardening reaction even without CaO similar to that common to Portland cement products. For casting, the dry mix is tempered with clean water and can be poured and vibrated into place, or troweled, in the normal manner. However, since these are relatively dense due to the low water requirement, they must be mixed a few minutes longer and are somewhat stickier on pouring. When placed they should not be covered, misted with water, sealed, or in any way prevented from drying out. Gunning requires no special procedures. After the forms are stripped the cast or gunned products can be allowed to dry out, even by the application of mild heat, and allowed to set. Shortly after setting these products, unlike Portland cement products, can be fired or otherwise uniformly exposed to high heat without harm to the products. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Several examples of formulations which embody this invention are given in the examples to follow: 
     Example 1 
     A refractory castable was formulated as follows: 
     35.4% Mulcoa 60-4 mesh 
     26.6% Bauxite-10 mesh 
     17.7% Fused alumina-325 mesh 
     8.9% Calcined alumina-325 mesh 
     11.1% Fumed silica (spherical particles average 0.15 microns) 
     0.3% Sodium Silicate-200 mesh (Portil N) 
     0.1% Sodium hexametaphosphate 
     Mulcoa 60 is a calcined kaolin product of the C. E. Minerals Company containing about 60% alumina. The bauxite may be South American, Chinese, or a blend of calcined bauxites. 
     The sodium silicate of the above example (Portil N) was in the weight ratio of SiO 2  to Na 2  O of 3.22:1, and is a hydrated sodium silicate of about 14% water of hydration. Henkel KGaA in Germany markets Portil N. The 0.3% level of sodium silicate allows adequate working time to cast the material. Larger grain size reagents having this ratio are also available. Philadelphia Quartz Company&#39;s SS-20 and SS-65 are anhydrous sodium silicates of SiO 2  :Na 2  O ratio of 3.2:1 that can be used in higher levels of sodium silicate because of their lower reactivity. In combination or singly the range of silicate powders can preferably span from 0.2% to 1.5% of castables and up to 5% of gun mixes. The limitation on the latter is to avoid a high level of alkali inclusion and a rapidity of set which can complicate clean up procedures. 
     To mix, 4 to 41/2% water can be put into the mixer first or added to the dry castable. A minimum of 5 minutes mixing time is used to develop a casting consistency. Internal and external vibrators are both useful for good placement. However, the high densities obtained with these low water requirements make external vibration more effective when practical. Shortly after the material sets, normally within an hour, forms can usually be removed. At this point it can be gently heated or placed in an oven for drying and then uniformly heated at high temperatures and even fired. Strong direct flames should be avoided until considerable drying has taken place, or until 48 hours have elapsed if no other source of heat has been used. 
     Example 2 
     A series of castable were formulated as follows: 
     
         ______________________________________       1      2        3       4______________________________________Mulcoa 47     9 lbs    9 lbs    9 lbs 9 lbs-10 meshFused alumina 4 lbs    4 lbs    4 lbs 4 lbs-325 meshFumed silica  2 lbs    2 lbs    2 lbs 2 lbsCalcined alumina         3 lbs    3 lbs    3 lbs 3 lbs-325 meshRaw kyanite   2 lbs    2 lbs    2 lbs 2 lbs-35 meshSodium silicate         1 oz     1 oz     1 oz  1 oz-200 meshSodium hexameta-         1/4 oz   1/2 oz   1 oz  2 ozphosphateWater         19 oz    19 oz    19 oz 19 ozPhysical data 1000° F.:Bulk Density  150      153      146   147lbs/cu ftCold crush strength         3500     3800     2800  3400psi______________________________________ 
    
     This data shows the effect of varying sodium hexametaphosphate (SHMP). The level of Test 2 was optimal, about 0.2%. The sodium silicate in all the above tests was in powder form and had a weight ratio of SiO 2  to Na 2  O of approximately 3.22 to 1. 
     Example 3 
     A blend was made of 9 lbs. of calcined Missouri flint clay, -1/16 inch, 4 lbs. of -325 mesh fused alumina fines, 2 lbs. of fumed silica, 2 lbs. of -35 mesh raw kyanite, and 1/2 oz. SHMP. To this was added several levels of -200 mesh hydrated sodium silicate (Portil N), of a weight ratio of Sio 2  :Na 2  O of 3.22:1 (approximate molar rate of 3:1)). All were cast with 19-20 oz. water (about 6%). Results were as follows: 
     
         ______________________________________Wt Portil N       Work Time    1000° F. Cold Crushoz        %       min          Strength psi______________________________________0         0       No set       --1/2       0.16    Skin, no hard set                          --3/4       0.23    130          --1         0.31    18           38002         0.62    8            64003         0.94    7            74004         1.25    Flash set    --______________________________________ 
    
     The flash set can be avoided by adding most of the Portil N at the end of the mix just prior to casting. Coarser sized silicates such as Philadelphia Quartz&#39; SS-20 and SS-65 can be used initially but preferably not above 1.5% in most cases or added just prior to casting. Levels above 0.5% of any of the powders are not recommended as initial blends with formulations of 85% alumina and above. 
     Formulations containing graphite, silicon carbide, chromium oxide, zirconia, and zircon have all been made and are usable with this method of bonding. Silicon metal additions are useful with graphite and silicon carbide to promote higher strengths and oxidation resistance. However, the use of powdered metals and even iron ball milled raw materials can produce hydrogen gas in amounts sufficient to be of concern. 
     Example 4 
     The following formulation is an example of a gunning mix using this novel bonding system. 
     15 parts Calcined Bauxite 1/4 by 1/16 inch 
     65 parts Calcined Bauxite-1/16 inch 
     5 parts Fumed silica 
     10 parts Raw kyanite-100 mesh 
     5 parts Ball clay 
     1 part Sodium silicate (Portil N) 
     This product gunned with little rebound or dusting and required less water than conventional gunning materials. A higher level of silicate accelerator is used than with ordinary castables to promote fast hardening to prevent any tendency to slump. The practical limit is at about 5% to avoid excessive sodium content (a flux). SHMP is not needed in the gunning mixes. Many other aggregates can be used, such as calcined clays, fused and tabular aluminas, etc. Additives such as carbons, silicon carbide, chrome oxide, zirconia and zircon can be advantageous in this system. Metal powders such as silicon are beneficial to properties but may produce hydrogen. 
     The level of fumed silica was also investigated. A practical lower limit of 5% produces good strengths and reasonably low water requirements, and is preferred for high alumina castables, and in gun mixes (where higher levels are dust producing). The 10% level has been found overall to be adequate and optimal in most cases for castables. Levels of 15-25% impart an undesirable stickiness and water requirements climb. Silica contents may be rather high at the higher levels. However, the entire range from about 3% to 30% fumed silica is viable for this bonding mechanism. 
     Fine powdered sodium silicates most useful have roughly 63% Sio 2 , 19% Na 2  O, and 18% H 2  O corresponding to a molar ratio of SiO 2  :Na 2  O of approximately 3:1. The two coarser silicates, namely Philadelphia Quartz Company&#39;s SS-20 and SS-65, are essentially anhydrous, containing roughly 75% SiO 2  and 23% Na 2  O. As stated, these also work but preferably at higher concentration levels. 
     Compositions were tested with potassium silicate molar ratio of SiO 2  :K 2  O of about 3:1 (Portil Potassium) from Henkel KG 2  A containing 56% SiO 2 , 28% K 2  O, and 16% H 2  O). In this hydrated form this reagent is too readily soluble and caused an almost instant setting reaction. This material was dehydrated by calcined at about 1300° F. for about 18 hours and grinding to -65 mesh. This material was substantially free of all water. This treatment produced a successful result, as described in the following example. 
     Example 5 
     A blend of 12 lbs. of -1/4 inch and down of Mulcoa 47 brand calcined clay, 2 lbs. of fumed silica, 4 lbs. of -325 alumina fines, 2 lbs. of -100 mesh kyanite, 1/2 oz. of SHMP, and 11/4 oz. of dehydrated potassium silicate, -65 mesh, was mixed with 1 lb. 4 oz. water. This was a good working castable. The working time prior to an initial set was about 8 minutes, somewhat less than that obtained with the sodium compound. Dried cold crush strengths after about 20 hours at 220° F. averaged 3800 psi. 
     Anhydrous potassium silicate of a weight ratio of SiO 2  to K 2  O of 2:1 (molar ratio of 3:1) is manufactured by the Philadelphia Quartz Company as Kasil SS Powder, and gives the same results as the above dehydrated material. 
     The following shows the results of studies on the effect of reagent ratios (and corresponding molar ratios) of SiO 2  :Na 2  O and SiO 2  :K 2  O with and without hydration of the silicates on the working time for mixing with water to obtain a set. 
     Calcined clay (MULCOA 47) 3×8 mesh 7 lbs. 
     Calcined clay (MULCOA 47) -8 mesh 5 lbs. 
     Fumed Silica 2 lbs. 
     Fused alumina fines-325 mesh 4 lbs. 
     Raw kyanite-100 mesh 2 lbs. 
     Sodium hexametaphosphate 1/2 oz. 
     To this was added several alkali silicate additions as shown in the following table with differing SiO 2  :Na 2  O and K 2  O ratios both hydrated and anhydrous. The results are indicated. 
     
         ______________________________________                    Hy-  Working                                Cold CrushSilicate Wt. Ratio Addition drate                         Time   (after 230° F.)______________________________________Sodium 3.2:1     1 ounce  Yes  30 min.                                3800 psi 3.2:1     3 ounces No   60 min.                                7000 psi 2:1       1 ounce  Yes  Flash set                                -- 2:1       1 ounce  No   90 min.                                6300 psi 2:1       2 ounces No   12 min.                                7700 psi 1:1       1 ounce  Yes  Flash set                                -- 1:1       2 ounces No   15 min.                                2200 PsiPotas- 2:1 -150  32 grams Yes  Flash set                                --sium  mesh 2:1 -40   1 ounce  Yes  4 min. 3650 psi mesh 2:1       34 grams No   8 min. 3700 psi 2.5:1     1 ounce  No   90 min.                                3700 psi 2.5:1     2 ounces No   20 min.                                6700 psi 2.5:1     4 ounces No   5 min. 6200 psi______________________________________ 
    
     All ratios tested can work. In the hydrated form the lower ratios materials would set too rapidly for casting. They could all be used for gunning. All of the castables were tested with about 6% water. The silicates were all in powder form. 
     The percent amounts given in the specification and claims are all weight percents. 
     The mesh amounts are Tyler screen sizes. 
     The ratios given in the specification and claims are molar ratios except where specifically indicated as weight ratios. The weight and molar ratios for the sodium silicate are substantially the same and the 2:1 weight ratio for the potassium silicate is approximately a 3:1 molar ratio. 
     The term fumed silica is also known in the trade as volatilized silica, arc silica and silica fume. A typical analysis including particle size of silica fume is shown in U.S. Pat. No. 4,310,486.