CA1067301A

CA1067301A - Method for dry spraying calcium sulfate hemi-hydrate

Info

Publication number: CA1067301A
Application number: CA283,159A
Authority: CA
Inventors: Walter D. Kobeski
Original assignee: United States Gypsum Co
Current assignee: United States Gypsum Co
Priority date: 1976-07-29
Filing date: 1977-07-20
Publication date: 1979-12-04
Also published as: AR214744A1; BR7704888A; ZA773879B; IT1079792B; DE2733540A1; GB1589994A; JPS5317501A; FR2359649A1; IN146244B; AU2643077A

Abstract

ABSTRACT
A method for dry spraying alpha or beta calcium sulfate hemi-hydrate (CaSO4.1/2H2O) which comprises blowing a stream of dry calcium sulfate hemi-hydrate particles through a hose to a spray nozzle, and prior to discharging the particles from the nozzle onto the surface to be sealed, introducing water into the stream of dry particles and intermixing the calcium sulfate hemi-hydrate particles and the water in the nozzle mixing chamber. In order to reduce dusting, it may be desirable to add additional water at the nozzle orifice.
The amount of water added to the dry calcium sulfate hemi-hydrate particles is an important factor in achieving a satisfactory composition for adhering to and sealing the surface, and generally the method is practiced by using from about 200 to about 100 parts by weight of water per 100 parts by weight of calcium sulfate hemi-hydrate. The calcium sulfate hemi-hydrate/water admixture sets very rapidly whereby it can be applied in relatively thin coatings and still provide adequate sealing and strength properties. In some applications, it may be advantageous to incorporate cut glass fibers into the calcium sulfate hemi-hydrate which is preferrably accomplished by introducing the cut glass fibers into the wetted hemi-hydrate particles after they are discharged from the end of the nozzle. The method of this invention is particularly useful in sealing roof and wall surface in underground mines and may also be used to form structural members in the mines.

Description

73~1 ,"
A METHOD FOR DRY SPRA~ING CALCIUM SULFATE HEr~ HYDRATE

BACKGROUND OF THE INVENTION
1. Field of the Inventlon~
This invention relates to a method for dry ;
spraying alpha or beta calcium sulfate hemi-nydrate or mixtures thereof. Specifically, the method comprises air~ conveying dry calcium sulfate hemi-hydrate (CaS04.~H20) particles through a hose to a spray nozzle where water is int~oduced into the stream of dry particles and intermixed therewith in the nozzle mixing chamber prlor to being sprayed.l In order to reduce dusting, it may be desirable to add additiona~ water at the nozzle orifice. This method is particularly useful in seallng surfaces in under- `
ground mines wlth a thin sealant coating to prevent air decrepitation or spalling~

2. Description of the Prior A~t'~
A severe problem encountered in underground mining ~.~: ;i, is commonly referred to as alr decrepitation of the mine roof and walls. Air penetration of these surfaces causes them to spall and crack creating safety hazards and unfit working conditions.
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-.. 3 Several products and systems have been developed ;~ to solve this problem. These systems may be classified as either structural or non structural. The non-struc- ~

tural system may ~oomprise mixes of cement and sand which ~ ~ ;

;~, are sprayed on the surface in thin layers as dlsclosed ln -~ U.S. Patent No. 3,224,203. These cement (concrete) coatings - have high strength and are effective, however, they are ^ very expensive and costly to apply, they are relatively - slow ln setting, and they require special pumping equip-~ ment in the mine. For example, U.S. Patent No. 2,255,189 . .
`~ discloses a special system for applying cement coatlngs ~ wherein the cement particles and water are admixed arter . . . , ~ ~, - discharge from : , .

73~1 the sprayin~ apparatus and as or ~ust prior to being deposite~ on the surface. U.S. Patent No. 39645,762 discloses a hydraulic cement mlx for spray application which is reported to be rapld setting, however, it re-quires on the order of 6 hours to set and it includes several special ingredients which add to the cost and com- ~ -plexity of the system.
The other non-structural systems may employ preml~ ;
xes of cement and ma~or portions of fillers such as vermiculite. U.S. Patent No. 3~055,434 illustrates a ;
non-strucb~ral system comprislng coating the mlne roof and walls with asphalt and/or asbestos flbers. This is a low strength system intended to keep the air of~ the surfaces, and lt is moree economical than the hydraulic cement systems. However, it does not have the strength -to hold in place loosened roof and wall material, and the a~bestos fibres present a health hazzard.
A recently developed method disclosed in U.S. Patent No. 3,892,442 employs a plastic foam or cement foam to coat the mine roof and walls, however, this system ls e~pensive -and requires special equipment in the mine. U.S. Patent No.

3,900,333 discloses a gypsum anhydrite (CaS04) partlculate composition including an accelerator whlch has been deve~
loped for application to mlne roof and wall surfaces.
Technical literature published by Karl Brieden & Co. dis- ;
closes that the gypsum anhydrite can be conveyed by air through a transport line and mixed with an aqueous stimu-lator (accelerator) solution in the nozzle mixing chamber and thereafter sprayed onto the roof or wall surface. Ho-wever, this system is a slow setting system and the coating must be applied in thick sections.

SUMMARY OF THE INVENTION
It is the general obJect of this invention to provlde a method for sealing surfaces by air conveying dry alpha or - ,' ' ~

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beta calcium sulfate hemi-hydrate particles of mixtures thereof through a hose to a spray nozzle where they are intermixed with water in the noz~le mixing chamber and then sprayed as a thin coating onto a surface to be sealed. ;~
Another object of the invention is to provide a method .
for dry spraying calcium sulfate hemi-hydrate wherein reinforcing fillers such as glass fibers are incorporated ~-into the hemi-hydrate prior to its application to the , surface to be sealed, which method is particularly useful when small pieces of material have to be secured in place.

A further object is to provide a method for dry spraying calcium sulfate hemi-hydrate to form a thin non-structural , ~
coating or a thick structural member. ~
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Broadly speaking, the above ob~ects are met by ~
the present invention which provides a method for spraying ~ ;
calcium sulfate hemi-hydrate which comprises (1) blowing an air stream of dry calcium sulfate hemi~hydrate (CaS04.1/2 ~
~2) particles through a hose to a spray nozzle having a mixing ~;
` chamber therein, (2) introducing water into the stream of dry hemi-hydrate particles, with the water content ranging from about 20 to about 100 parts by weight of water per 100 parts by weight of calcium sulfate hemi-hydrate, (3) ! intermixing the calcium sulfate hemi-hydrate particles and water in the nozzle mixing chamber, and (4) spraying the wetted calcium sulfate hemi-hydrate, the wetted calcium sulfate hemi-hydrate being sprayed onto a surface to be ~, coated, with the coating having a thickness ranging from about 1/8 inch to about 5 inches.
The method of this invention is especially designed for use in underground mines for it uses conventional : . , .
rock dusting equipment which is stand~rd mining equipment.

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In accordance with this method thin sealant coatings can be applied to the mine roof and walls providing a less costly and effective system. The resultant gypsum coating provldes a deslrable high reflectlvity, and the dry spray application requires less energy consump-tion than slurry systems and also less cleanup. It has been determined that good physical bonding to the surface of the mine can be achieved using commercially available alpha or beta calcium sulfate hemi-hydrate particulate material, including mixtures thereof, without incorporating special stimulators or accelerators.
These and other objec~s, features and advantages will becoms app~rent to those skilled in the art as completely disclosed in the following detailed description.
Whenever the term "calcium sulfate h~mi-hydrate" is used in the specification or the claims which follow, it includes alpha and beta calcium sulfate hemi-hydrate and mixtures thereof.

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The metho~ Or this invent.ion compri.ses blowi.ng or air conveyincJ a stream oE dry calcium suleate hemi-hydrate (CaSQ4.~112O) parti.cles thro~gil a hose to a spray nozzle, introducing water into the stream of dry particles, inter-mixing the calcium sulfate hemi-hydrate particles and water ; ;.
in the nozzle mixing chamber, and spraying the wetted calcium sulEate hetni-hydrate. The amount oE water added to the dry calcium sulEate hemi-hy~rate particles is an important Lactor in achievi.ng a satisfactory composition with respect ~ :
to two criteria. First, there should be sufficient ~ :
water admixed with the hemi-hyd.rate particles to assure adequate wetting for complete hydration upon di.scharge from the spray nozzle. In order to reduce dusting, it may be desirable to add additional water at the nozzle orifice. ~
Secondly, the water content must be selected to provide a ~:
mortar whereby the calcium sulfa-te hemi-hydrate is firmly adhered to and tightly seals the surface to which it is applied. ln general, the water content should range from .. ~.
about 20 to about 100 parts by weight of water per 100 parts by weight of calcium sulfate hemi-hydrate, with a particularly preferred range of 50 to 55 parts water per 100 parts hemi-hydrate. The optimum ratio Ol water to calcium sul~ate hemi-hydrate will depend on the type of hemi-hydrate and the particle size distribution., It has been found that dust particles may be present in the periphery of the spray. ~dditional water may be ;~
injected into the spray at the nozzle orifice by one or more water jets located on the end of the nozzle. .~ : ;
One of the advantages of this method of dry spraying .
calcium sulfate hemi-hydrate is that the wetted hemi~hydrate .:
particles set very fast and develop high early strength. . ~ .

It has been found t~at about one hour after application, the , . ,, ,,, . , , , ~ : , - . ~

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~omf~osiLioll attaills abollt ~0~ o~ its fincll dry strencJth (compressive). T}lis pcrmits faster application and also thinner coatincJs ~hicll conserves energy and reduces expense.
Several problems may be encountered in performinq the metllod of this invention. Perhaps the most serious is dustiny at the spray nozzle orifiçe. The mine atmosphere is normally dusty and additional substantial dusting is very undesirable. Dusting may be afEected by several factors. In addition to the amoullt of water admixed with the hemi-hydra-te particles, the particle si~e distxibution is important.
moulding type hemi-hydrate (finer particles) generally requires more water than a gauging type (coarse partlcles) It is generally preferred that the particles range in size (stokes par-ticle siæe) from about 0.2 microns (moulding type) to about 725 microns (gauging type).
Another factor a~fecting dusting is the flow rate used to ,. ~ , transport the hemi-hydrate particles. In general, the ~ir stream flow rate should range from about 50 cfm to ~bout 140 cfm, and the line pressure should range from about 8 psi to about 22 psi.
In carrying out the method of this invention, a nozzle of the type used to spray gunnite (cement and sand) may be used, however, it may require modification flr optimum results. The calcium sulfate hemi-hydrate is fluidized with air and conveyec~ through a hose to a spray nozzle, and as it enters the nozzle, water is injected into the hemi-hydratç
particle stream through an annular water ring 6urrounding the nozzle. Forward from the annular water ring, the hemi~
hydrate particles and water are admixed in the mixin~ chamber o~ the s~ray nozzle. The turbulent stream of air, hemi-hydrate particles and water is activated by energy input resultin~ from the tnrbulence. ~Lter thorough mixing in the . .
nozzle, the wetted calcium sulfate hemi-hydrate is discharged _5-.~,.; .
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, ~ 673~)1 rrom ~he no%zle an(l applie~ to the surace t~ be sealed.
l`he l~emi-llydratc setF. very rapi~]]y, generally 15 to 20 millntes ~re re~luired to achieve substantial :trengtll, an~
ti-lerefore, it c~n ~e applied in very thin coats alld still c~ec~ively se~l Lll~ surfac~. In general, the sealant coatings may range Lrom about 1/~ inche to al~out 5 inches, with ~llicker coatillgs being present at surface inderltations. One of the advantages of this methocl is that the resultillg gypsum (CaSO4.2ll2O) coatincJ does not shrink when it sets, ancl in fact there is a slic3ht expansion which ~rovides better aclllerence ~Ind sealing properties.
Furthermoxe, thickne~sses greater than about 5 inche~ involve unnecessary e~pense and so they are not preferred;
In many undcrcJround extraction processes or other : . . .
mining activities, it may be necessary or desirable to incorporate a reillforcing material such as cut glas3 fibers -~
in the sealant co~ting which perEorm as an aid in securing looselled material present on the surface. This may be , . .
accomplis}led itl either of two ways. The cut glas~ fibers may be dry mixed wilh the calciulll sulfate hemi-hydrate particles and tllis admixture transported by air to the spray nozzle wllere it is mixed with water in the same manner as ; when the glass fibcrs are not present. In thi~ method, it as ~een found to be diicult to maintain a uniformly wet mixture possi;bly resulting in an unsatisfactory coating.
~ his difficulty may be due to plugging of the valves or '! nozzle parts, an~ tl~e ~ests indicated that fiber glass adversely afects tl-e wettili~ rate o the calcium sulfate hemi-llydrate. lhis may be alleviated by using di~ferent coatinys on tlle g]ass fibers. ~ more prererre~l method for ~ , , ,. - . . , incorporatins tlle cut glass fibers comprises introduci"g thcm into the ~tro.~rn o hemi-llydra~e particles aEter it has been wetted by lllc watQr. ll)is i6 aCCOlllpl..iS]led ~y inSta~ 3 . ~, .
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~t~73~1 a glass fiber cutter or chopper near the spray nozzle discharge orifice ~hereby t~e glass fibers are incor-, porated in the wetted hemi-hydrate particles ~ust after their dlscharge from the spray~nozzle.
A fiber glass rope is fed to the cutter or chopper and cut into short lengths generally from about ~ lnch to about 4 inches~ In general, the proportlon of glass fl-bers ranges f'rom about 0.5% by weight to about 6% by weight of the calcium sulfate hemi-hydrate. An alr hose is con- ;nected to the cutter or chopper and pressurized air is fed thereto to blow the cut glass fibers into the we~ted cal- ~, cium s`ulfate hemi-hydrate particles. It is preferred :
that the air pressure fed to the cutter or chopper range from about 16 psi to about 100 psi, however, this will vary according to the type of cutter or chopper. `
The effectiveness of the method of this invention is surprising and unexpected because it was generally believed that an intensive energy inputlis ! requiredi~order to ,~, set calcium sulfate hemi-hydrate. It has been found `-that sufficiént and rapid ~strength can be developed without the substantial energy input previously thought u necessary. Fv.rkhermore, establishment of an effective gypsum coatlng in an underground mine wa~ considered to be unfeasible because of hydrostatic pressure causing water seepage in mines. It has been found that the method of this lnvention can be used in most mines -whlch do not have a hydrostatic pressure problem.
The followlng examples illustrate the method of ~, ' dry spraying calcium suIfate hemi-hydrate particles.

At a coal mine inwhich a fire ln the mine had made it necessary to seal off a portion of the mine from the other working portions of the mine, bulkhead seals had been constructed with solid concrete blocks, The " , ., ' surface of the seal on the return air side was coated ;' ~ ' using a trowel grade compound and , ~ ' . .

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inspection oE the mine revea1ed that there were CJaS .leaks in the concrete blocJc seals. Prior tests usirly calciu sulfate hemi-hydrate as a sealant coatiny applied to the roof and wall areas in the mine to preven-t decrepitation had provell the feasibility o using the hemi-hydrate as a sealant coating. In this particular job, the calcium sul~ate hemi-hydrate was to be applied over the bulkhead seal sur~ace at an adequate thickness to seal of any gas leaks in the bulkllead seal.
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A total oE 53 one hundred pound bags of calcium sulfate hemi-hydrate were loaded into a pressure tank rock dust ; ;
distributor chambex (single tank -- approximate 3 ton capacity). ~ ~-Compressed air was then fed into the chamber causing the ;
calcium sulphate hemi-hydrate to behave like a liquid. The ~ -hemi-hydrate was conveyed (air blown) through approximately ;~
: ,.,: ' - - 650 fee-t of 2 inch I.D. hose to the discharge end having a -special spray nozzle containing an annular water ring. The tank pressure was about 22 psi and the line pressure was about 16 psi. The calcium sulfate hemi-hydrate was wetted and then sprayed onto the roof, walls and bulkhead seal -~
surfaces. Water was supplied to the spray nozzle via a 1 ;-inch hose from a 6 inch fire hose water line. The volume and pressure o the water waS ~ore than adequate. ' The calcium sulfate hemi-hydrate performed very well. ~ `
,, ~lthough at times there was'some surging of the dry hemi ; hydrate particles, the hemi-hydrate particles were wetted at ~ ~ ;
the spray nozzle reasonably well. The surging variation 1 resulted in some dusting, but this was not detrimental in ;~ any way to the application o the calcium sulEAte hemi hydrate.

f The calcium sulfate hemi-hydrate was also sprayed onto ~ ;
the mine roof and wail a~eas adjacent to the bulkheAd seal, extending for about six to seven feet into the mine entry.

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e calcium sulf.1~c 11emi-11y(lrate cOatinr~J was appliecl in ~11icknesses rangi11g ~rom ~ inch ~o a few inches at surace in~entations with r1o dislodc3ement oE the coating. During Lhis applicatior~ to the mine roof ancl wall areas, the air Elow rate used to transport the hemi-hydrate particles was l1cld at about l~0 cfm (tank pres~ure was about 22 p9i) and the line p~essure was held at about 16 psi.
The approx;mate size of the bulkhead seal was 20 feet in width and 7-1/2 feet in height. It required a~proximately four to five minutes to spray each bulkhead seal. It required at least 500 pounds of calcium sulEate hemi-hydrate material to coat one bulkhead seal consisting of approximately 600 square feet of wall, roof and seal area.
~ Inspection of the sprayed bulkhead seals showed that s the calcium sulfate hemi-hydrate coating was efPective in ; .
~ sealing the bulkheads.
.` E:X~MPLE 2 ~ ~ .
~n additional spray test was conducted to evaluate the method employing the addition oE cut glass fibers in the A calcium sulfate hemi-hydrate coating. ~ glass cutter was ~ mounted at the discharge end of the spray noz~le, and air . ~ . .
was supplied from an air compressor. l'he air was supplied to the cutter at 60 psi. TI1P glass fibers were blown into ;~
the stream of wetted hemi-hydrate particles just aFter their discharge ~rom the nozzle.
The procedure for handling and air blowing the calcium sulfate hemi-hydrate was the same a5 that employed in Example l. The dry spraying of t1ie hemi-hydrate particles remained the same, with the only change comprising cutting and blowing one inch lengths of ~iberglass illtO the wettecl calcium sulfate hemi-hydrate. The wetted hemi-hydrate containir1g the cut gla5s fibers was sprayed onto the roof and wall areas in the mine entry. lhe application oE the _g_ :

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hemi-hydrate compositlon containlng the one lnch glass fibers was feasible. In this application, the glass fiber content in the sealant coating was approximately -

4% by weight. The sealant coating containing the cut glass fibers had a dry density of 75.2 lbs./ft.3 and a dry compressive strength of 2289 psi. It has improved resistance to cracking when exposed to physlcal abuse (bumping) from mine equipment.
EX

i A verticle pressure tank rock dust distri~utor was used to spray dry particles of calcium sulfate hemi-hydrate. The tank was charged with 1,000 pounds of calcium sul~ate hemi-hydrate contalning ~ inch lengths of chopped fiberglass at a ratio of 10 pounds of fiber~
glass per ton of hemi_hydrate~ equivalent to 0.5% by weight. After charging, the tank was pressurized and ~ ~
the calcium sulfate hemi hy~rate was air blown through ; ;
150 feet o~ 2 inch I.D. heavy duty transfer hos'e to a .~j ~ - , .
special spray nozzle where it was wetted and sprayed. `~ '~
The line pressure was held steady at 11 psi, and the hemi-hydrate particles containing the cut fiberglass '!:'' ' '~' ''' :'`I '' ,..: ' :
were conveyed at a flow rate df approximately 80 pounds per minute. There appeared to be good, uniform oonveyance of the dry material.
However, there did appear ko be excessive dusting at the spray nozzle discharge orifice O Periodically, -there was poor wetting of the hemi-hydrate particles which resulted in inconsistent application of the coating.
Water rate during this application was limited to approximately three gallons per minute. It was decided to perform additional tests using increased w ter flow.
EXAMPLE 4~
The vertical pressure tank rock dust distributor was --10-- , ' :: ': , : :' , ' 7J3t)~L

again charged with 1~000 pouncls of calclum sulfate hemi-hydrate contalnlng ~ inch lengths of chopped glass rlbers.
The tank was pressurlzed and the line pressure was held at 11 psi. The dry hemi-hydrate/glass fiber material flow ~;
rate was approxlmately 80 per minute. There was good, even conveyance of the dry hemi-hydrate particles.
In this Example, water was fed to the spray nozzle at a rate of five gallons per mlnute. At the spray nozzle -orifice there was periodic dusting. A uniform wet spray -could be maintained at times, however, thia~was not cons~
istent. When the hemi-hydrate particles were adequately `
wetted, they sprayed and applied to the surface very well.
The ~ inch pleces of glass fiber appeared to disburse in ~ ~
the material quite well. ~ ~, The next test was run using the pressure tank rock dust distributor in a horizontal position instead of a vertical position. This tank and set up are identical to that used in the mines for rock dusting. The tank was charged with 1,000 pounds of calcium sulfate hemi~
hydrate containing ~ inch lengths of glass fiber. Pres-surizing and air conveying of the hemi-hydrate particles -was identical to that used ln Example 4. I~t was deter- -~
mined that the rate of water flow was erratic~ varying .
from 3-1/2 to 5 gallons per minute. The dry hemi-hydrate/glass fiber material was air blown at 80 pounds ~
per minute. It was calculated that the water available ~ -for wetting the hemi-hydrate particles ranged from 29 to 42 pounds per minute for 80 pounds o~ hemi-hydrate.
The skandard consistency for thlsppa~ticular hemi-hydrate material would be 52 pounds o~ water for 80 pounds o~ hemi-hydrate. It appeared that the water was su~ficient for -wetting the hemi-hydrate even though its rate varied.
Again, there was poor wetting of the hemi-hydrate and there ~t;7301 was ~usting at tlle spray nozzle oriflce. It was concluded that the fiberglass was impeding the wetting of the hemi-hydrate.

The same pressure tank rock dust distrlbutor, in the horizontàl position, was charged with 1,000 pounds of calcium sulfate hemi-hydrate containing 1/2 inch lengths of glass fiber. The glass fiber content was approximately 10 pounds per ton of hemi-hydrate. A
change was made in the water line wherein a 2 inch I.D.
hose was installed which was reduced to a 1/2 inch entry ~'t .
at the spray nozzle. It was felt that thls would supply a more uniform rate of water to the nozzle. The water -~
rate after attaching the nozzle was determined to be

5 gallons per minute. , .~ .. . .
The tank was pressurized and the line pressure for the dry material was held at 11 psi, ylelding a dry material flow rate of 80 pounds per mlnute. Again there was good, even oonveyance of the dry material.
: Spraying performance was again inconslstent. When , the calcium sulfate hemi-hydrate was adequately wetted, ': :
it sprayed and applied well. The 1/2 inch lengths of glass fiber appeared to disburse well in the wetted hemi-hydrate.
The molds whlch were sprayed in Examples 3-6 were inspected after the coating material had set. The molds were peeled and the appearance of the coating was quite acceptable. The results obtained indicated that thls type of dry calcium sulfate hemi-hydrate spraying is a feasible method for applying a sealant coating.
EXAMPLE 7 ~-~, The pressure tank rock dust distributor was charged with 1,000 pounds Or calcium sulfate hemi-hydrate. The tank was pressurized with the line pressure held at 11 psi and the dry material was sprayed at the rate of 80 pounds per minute. ~12-;-: . . .

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In thi~ te3~, a gla~ cutt~r wa~ hel~ above ~he ~p~ay noz~le arld chopped cJla~3 fiber w~ fed into the wetted hem.l- :
hydrate while ~pray.ing the rnold3. The chopped gla~ iber varying in length rrom 1~2 to 1 inch appeared to lay down well in a mat form itl the mold~. :
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: X~MP1E 8 Te~t~ were conducted to determine the compre~ive ~trength~ of various calcium ~ulfate hemi-hyd~ate composition~
and the xate at which the ~trength i~ developad. rh~ hemi~
hydrate compo~ition wa~ wetted in a manner to duplic~te as clo~e a~ po~sible what occur~ in a ~pray nozzle. Com~re~siYe .
: strength~ were mea~ured uaing molded cylinders ha~ing a 2 inch diameter and a 4 inch height. Set time wa~ determined by a ; ~:
300 gram V~cate ~et te~t. The te~t procedure employed a ` :
~tandard Vicate needle ha~ing a weight o~ 300 gram~ wlth a point 1 ~m. in diameter per ASTM Standard C472. ~he wetted hemi-hydrate compo~ition ~a~ formed into ~ampleB having a `
thickness of about 1 inch. When the composition begin~ to ~tif fen and 105e glos~, the needle i~ gently placed on t~e surface and ~elea~ed to penetrate freely of it~ own w~ight.
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The set i8 measured from the time o~ mixing wlth water until when the needle no 1onge~ penetrate~ to th~ bottom a~ t~e ma~s, : :
generally when the needle will not penet~ate through half the height o~ the sample.
In all of the composi~;ons tested, the ~a1cium ~ulfate . hemi-hydrate was a rapid ~etting material (}IYDROCAL Wh~te).
The fo110wing compositions were tested, yielding the indicated `~ results~
C~mposition Formulation ; ~ 100% HYDROCAL White Inaterial : ~ 1 part HYDROCAL/k part Rock Dust C 1 pa~t ~l~DROCAI,/1 part ~ock Du~t D 1 part ll~DROCAL/1.5 pa~t~ nock Du~t ; -13--: . , - ~ .
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1 Hr. wet den~ity (lb3.~ft.3~114.2116.1 116.1 115.2 1 llr. wet strength (psi)29601970122~ 755 Dry den3ity tl~./ft. 99.~ 97.394.4 91.
Dry stxength (ps~)6875S000 34202060 set time (min.) 29 25 25 2B ~ -These results indicate that the calcium sulf~te hemi-hydr~te ~ . - .
compositionY did achieve approximately 40~ oP thelr ultlm~te ~ `~
compre~ive ~trength after 1 hour set. ~ -,, ,: . .
EX~MPLE 9 ?
A test wa~ carried out to evaluate air conveylng and qpraying calcium sulfat~ hemi-hydrate particle3 u~ing a ~antam rockdu~ter pump. Dusing th~ test~, 400 lb~. o~ No. 1 Moulding Pla~ter hemi-hydrate, 300 lb~ of ~IYDROCAL Whlte hemi-hydrate, and 100 lbs of TUF-ART Pln~ter hemi-hydrate were sprayed. The pump provided an air flow rate of 140 cfm and a line ~2 inch hose) pre~sure ranglng irom 8 to 20 p8i. T11e hemi-hydrate material was dumped into a one bag hopper, feed ~crew transferred the material into the air Y~lve box, -and the d~y hemi-hydrate particles were moved with air through the ho~e to the ~pr~y nozzle. In th~ ~pray nozzle, w~t~r was -injected into the hsmi-hydrate particle~ through ~ ~peclally designed wate~ ring collar. The wetted heml-hydrate wa~
~prayed through the noz%le orifice by ~ir pres~ure without undue pul~ation.
The den~itie~ and compre~3ive 3trength~ of the ~arlou~
mixes wns determined. The compres8iv~ ~trengths of th~ dry gyp~um (CaSo4.2ll2O~ was mea~ured using a molded cylinder ~ ;
having ~ 2 inch dinmeter ~nd 4 inch height.
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No. 1 Moldlng No. 1 Molding HYDROCAL TUF-hR]
Pla~ter Plaster WhltePlaster Water/llemi-hydra~e (pts. by wt./100 pts.) 50.6 50.2 29.9 61.2 1. Ilr. wet density ~lbs./ft.3) 102.6 102.4120.3 92.4 Dry density (lbs./ft.3)83.6 84.0 109.474.4 Dry cnmpressive strength ~psi) 2530 2069 4456 557 , ,.

At the time of measuring the compressive strength, the TUF-ART Plaster material was ~till wet when broken whlch explains the poor compressive strength. Tl-e compressive strength3 of the other materials illustrates the feasibility Oe the dry ~pray method of thi3 invention.
: EXAMPL~ 10 The method of this invention was evaluated to determine -' its utility in forming structural members for use ln underground mines. The purpose of the test was to determine whether i- ;
bags to be used in constructing pack walls could be fllled by air conveying dry calcium sulfate hemi-hydrate partlcles which are wetted in a spray nozzle and discharged into the ~ ~
bags. ~he bag to be filled was an open weave, synthetic ~ -I strand having dimensions of about 4' xS' x7'. It was 3upported ~,! at its top by a pipe rame work and had a 4" x4" mesh concrete . reinforcing wire liner.
A Reed gunnite machine,was used to pump the calcium sulfate hemi-hydrate particles ~commerclally available under the IIY~OC~L Gray trademark). There was some dusting at the .. , nozzle oriflce, however, no dust escaped from the biag which acted like n du~t collector. ~`
~ ho IIYDROCAL Gray heml-hydrate developed a compresslv~
strength o about 2200 psi approximately 30 mlnute3 after set. The wet denslty of the wetted heml-hydrate was 115 lbs/ft.3. The ~praying technique was able to ~111 the bag Almost entlrely to its top.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for spraying calcium sulfate hemi-hydrate which comprises:
(1) blowing an air stream of dry calcium sulfate hemi-hydrate (CaSO4.1/2 H2O) particles through a hose to a spray nozzle having a mixing chamber therein, (2) introducing water into the stream of dry hemi-hydrate particles, with the water content ranging from about 20 to about 100 parts by weight of water per 100 parts by weight of calcium sulfate hemi-hydrate, (3) intermixing the calcium sulfate hemi-hydrate particles and water in the nozzle mixing chamber and (4) spraying the wetted calcium sulfate hemi-hydrate, the wetted calcium sulfate hemi-hydrate being sprayed onto a surface to be coated with the coating having a thickness ranging from about 1/8 inch to about 5 inches.

2. The method of claim 1 in which the water content ranges from about 50 to 55 parts by weight of water per 100 parts by weight of calcium sulfate hemi-hydrate.

3. The method of claim 1 in which the dry calcium sulfate hemi-hydrate particles range in size from about 0.2 microns to about 725 microns.

4. The method of claim 1 in which additional water is added to the wetted calcium sulfate hemi-hydrate after it is discharged from the nozzle orifice in step (4), with the total water content being within the range of about 20 to 100 parts by weight of calcium sulfate hemi-hydrate.

5. The method of claim 1 in which the air flow rate used to transport the dry hemi-hydrate particles in step (1) ranges from about 50 cfm to about 140 cfm and the line pressure ranges from about 8 psi to about 22 psi.

6. A method for spraying calcium sulfate hemi-hydrate which comprises:
1) mixing dry calcium sulfate hemi-hydrate (CaSO4.1/2 H2O) particles and a reinforcing filler, (2) blowing an air stream of the dry calcium sulfate hemi-hydrate particles/reinforcing filler mixture through a hose to a spray nozzle having a mixing chamber therein, (3) introducing water into the stream of dry hemi-hydrate particles/reinforcing filler mixture, with the water content ranging from about 20 to about 100 parts by weight per 100 parts by weight of calcium sulfate hemi-hydrate, (4) intermixing the calcium sulfate hemi-hydrate particles, reinforcing filler and water in the nozzle mixing chamber, and (5) spraying the wetted calcium sulfate hemi-hydrate/reinforcing filler mixture, the reinforcing filler comprising cut glass fibers having a length ranging from about 1/4 inch to about 4 inches, and the wetted mixture being sprayed onto a surface to be coated, with the coating having a thickness ranging from about 1/8 inch to about 5 inches.

7. The method of claim 6 in which the amount of cut glass fibers ranges from about 0.5% by weight to about 6% by weight of the calcium sulfate hemi-hydrate, and the hemi-hydrate particles range in size from about 0.2 microns to about 725 microns.

8. The method of claim 6 in which the air stream flow rate in step (2) ranges from about 50 cfm to about 140 cfm and the line pressure ranges from about 8 psi to about 22 psi.

9. A method for spraying calcium sulfate hemi-hydrate which comprises:
(1) blowing an air stream of dry calcium sulfate hemi-hydrate (CaSO4.1/2 H2O) particles through a hose to a spray nozzle having a mixing chamber therein, (2) introducing water into the stream of dry hemi-hydrate particles, with the water content ranging from about 20 to about 100 parts by weight per 100 parts by weight of calcium sulfate hemi-hydrate, (3) intermixing the calcium sulfate hemi-hydrate particles and water in the nozzle mixing chamber, (4) spraying the wetted calcium sulfate hemi-hydrate particles from the spray nozzle, and (5) introducing a reinforcing filler into the stream of wetted calcium sulfate hemi-hydrate particles just after their discharge from the spray nozzle orifice, the reinforcing filler comprising cut glass fibers having a length ranging from about 1/4 inch to about 4 inches, and the wetted hemi-hydrate particles being sprayed onto a surface to be coated, with the coating having a thickness ranging from about 1/8 inch to about 5 inches.

10. The method of claim 9 in which the amount of cut glass fibers ranges from about 0.5% by weight to about 6% by weight of the calcium sulfate hemi-hydrate, and the hemi-hydrate particles range in size from about 0.2 microns to about 725 microns.

11. The method of claim 9 in which the air stream flow rate in step (1) ranges from about 50 cfm to about 140 cfm and the line pressure ranges from about 8 psi to about 22 psi.

12. The method of claim 9 in which the glass fibers are cut by a glass cutter mounted near the dis-charge orifice of the nozzle, and an air stream passes through the cutter and blows the cut glass fibers into the wetted hemi-hydrate particles under pressure.