CA1063133A - Porous, lightweight, particulate aggregates and process of manufacture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A porous lightweight particulate aggregate for use in concretes, mortars, building blocks and other building elements, comprises expanded and vitrified low alkalinity particles of a material composed of an alkali metal silicate having an M2O/SiO2 ratio of from about 1:2 to 1:4 wherein M
is an alkali metal; silica in an amount of from about 0.1 to about 50% by weight of said alkali metal silicate; and an alkaline earth metal silicate in an amount of from 0.1 to 200% by weight of said alkali metal silicate, said particles having a density of not more than about 60 g/dm3 and a com-pression strength of up to about 275 kg/cm2 and having an expansion ratio of up to 15 times their original collapsed volume. Suitable fillers and additives may be added to said aggregate to provide specific and useful properties for a variety of uses thereof.
A process of manufacturing a porous lightweight particulate aggregate comprises kneading an aqueous slurry of an alkali metal silicate, finely divided silica and a finely divided alkaline earth metal silicate, in order to form a homogeneous mixture, reducing said slurry to parti-cles of a preselected size depending on the desired size of the finished aggregate, gradually raising the tempera-ture of said particles to from about 110 to about 700°C, and then firing until a temperature of from about 700 to about 1500°C is achieved and vitrification of the silicates is completed. Potassium nitrate may be added to provide for the formation of a large central pore to produce a hollow particle structure on heating the mixture.
The gradual increase in the temperature may be partially carried out in the presence of saturated steam in order to humidify and to therefore render the surfaces of the particles sufficiently plastic, whereby a hollow structure having a central large pore surrounded by a shell of highly porous material is obtained.

Description

DAD ~JROU TH NV~NT N
Thi~ invention refers to highly porousJ lightweight, particulate aggregates for u3e in concretes, morta~, building blocks and other building eleme~ts and? more Darticularly, it is related to aggregate~ having incxea~ed strength and reduced alkalinity and being highly compatible with binder~, as well :~
as to processes for the manufacture of said aggregate~.
It i8 a well known fact that aggregates have been used for lo~g in the building art~ and that in accordance wi~h the knowledge gained from the nature o~ 3aid aggregates, it has been always known that for high compreqsion strength, high temperature resistance and high adhesion of the concretes, the well recogni~ed construction codes call ~or the use of only heavy and strong aggregates which are not l:ightweight in their nature. The u3e of lightweight porous aggregates ha~

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1all63~L33 been generally restricted to elements that do not require a very high 3trength o~ the material, such a~ partitions,os~ e slabs and the like, but are generally excluded from struotural elements such as struts, girder~, beams and the like, particu~
larly in view of the fact that all the lighbweight aggregates .
available in accordance with the prior art9 were selected -from low stren~th aggregates suoh as pumice7 lava7 slag?
-fired clay, shale or cinder~ ~rom coal or coke, and the liket All these materials used for aggregates 9 as well as some other artificially prepared aggregates on the basis of expanded bentonites and expanded silicates, have left much to desire as to the strength of the material produced with the use thereof, in view o~ the ~act that all suoh aggregates, in the ~irst place, are available in very few par-tiole size~, and therefore do not lend themselves for a variety Qf USe~9 particularly in ~iew of the faot that all the prior art .
aggrega~es are o~ a small particle size ~hich required the provi~ion of larger amounts of water whereby the strength of the binders was reduced and crack~ were caused in the cured mass when dried. In the case of the prior art expanded silicate materials9 these materials were highly brittle, whereby the partiole size oould not be increased and, on the other hand, showed a high alkalinity, which oaused a chemioal reaction with the oommon binders used and therefore, the material was furtherly degraded with the oonsequent loss o~

1~63iL33 ~trength in -the finai''prQd~ct, ;'~
I'herefore 9 in'accordance~With~'the prior art k~owledg~
of di~ferent type'~'o~ 'aggrégài'tes for''uise ~in ~onc~'ct'e con~t,r, tion, i-t wa~ con~i~iered'.impo~sible ~o ;obtain:mo~.~ar,~:o~
concretes having a' very ~iigh streng1i~ nd a ~ery.~ ..¢wei~ht becàuse the~~e ~wo properties o~ the~'concr~:e~ana-mo~.-k~
were exclusive of each other, whereby'~if:a high.~tP.~n~th ~a~i,.,~.,.
required, then heavyweight aggrégat~ ad:to be:llse~.in.Lorder.~,' to provide 3uch a high s-trength9 wi`thout ~n~t~.po~bi~y.o~..-obtaining a lightweight material. On the-other:.ha~d,~ h,.e.~,..
the weight of the material wa~ the domlnant bhar~-cter~ ti.~,,~, then the inclusion o~ lightweight aggregate~ o~ the above mentionea nature caused a con~iderable deorea~e i~ thQ -s-trengbh o~ the material produced9 whereby it wa~ practioall~
impoesible to obtain a concrete or a mortar having both characteristics in conjunotion~' `Also;in accordance ~ith'the prlor ar~9 certain lightwei~ht bu~lding blocki3 have been obtainedi through the u3e 0~ ligh~tweight aggregate~ o~ a lar!ge particle size, aoined ~ogether by mean~ of a suitable bin~ier, but in general terms Mhe~ lightweight buildi~g blocks are merely uoed for coverage purpos~saxld al~o as permanent ~al~ework materials, : . , and are excluded from use in ~trllct~ral elements suoh as .. :.
columns, girders9 beams9 supporting we~lls and the like, in . . .
view of the fact that they are extreme~r brittle and al~o ~0~313;~

present a very low strength to compression and to flexure and shear stres~es. In thi~ type of lightweigh-t building blocks, a problem alway~ extant in accordance wi-th the prior art i8 the ~a¢t that when expanded silicate material i~ used, this material presen-ts a high alkalinity which reaet~ with the binder~ co~monly used and decrea~es -the strength of the binding action o* such binder 9 whereby said blocks tend to ea~ily crumble through pressing or abrasive action.
~ herefore, ~or long it has been a need of the building arts to produce a lightweight aggregate which besidss it~ lightweight would present a very high strength -to compre~sion and a very low alkalini-ty which may prevent ~ide reaotion~
with the binders used to produce buildin~ block~ therewith, ln order to pre~ent orumbllng thereof.

BRIEF SUMMARY OF ~'HE INVENTION
Having in mind the defects of the prior art aggregates, it is an object of the present invention to provide a light_ weight aggregate which at the samç time may provide a very high strength to compression, ~lexure and shear stre~e~.
I~ is another object of the present in~ention to provide a lightweight aggregate ba~ed on an admixture of an alkali metal silicate and an alkaline earth mebal ~ilicate, which will show a very low density and yet a ver~ high strength and also a very low alkalinity and therefore which will be 63~33 highly compatible with all type~ of binders.
It is a further objeot of the in~tant i~vention ~o provide a lightweight aggregate of the above mentioned character, which de~pite it~ very high poro~ity~ will ~ot show brittlenes~ chaxacteris~ic~.
A more speGific obà~ o~ the p~e~ent invention i~
to provide a lightweight aggregate which will have a large particle ~ize with a hollow cen~er surrou~died by a ~hell of highly porous material A ~till further object of the in~ant invention is to provide a method of preparin~ a lightwei~ht aggreæate of the above mentioned charaoter, which method will be economic in ib~ performance and yet highly efficient in providing a poxoue, ~rong, lightweight and large ~i$e particulate material.
~ still further and more ~pecific object o~ the inetanb invention i~ to provide a method for preparing a lightweight aggregate of the above mentioned character, whi~h will provida for an expan~ion of ~he ~ilicate matèrials with a ratio of up to 15 tima~ the original oollap~ed volume9 wit~out thereby precluding it~ ~trength.
Obher obJe~ of b~q in~tant invention are to provide ~ab~ial~ ~d eleme~t~ obtained from bhe~e aggregate~ through bi~lng ~hq~eo~ by ~ul~ble ~i~ders.
The foregoing object~ and other ancillary thereto axe preferabl~ accompli~hed a~ follow~: an aqueou~ di~per~ion o~
an al~ali metal silicate, silica and an alkaline earth metal 1063~L33 silicate }g kneaded to form a slurry, the slurry i~ then reduced to particle~ having a aize graded in accordance with the pa.rticle si~e désired ~or the fini~hed aggrega-te, the -temperature is then slowly raised to f`rom about 110 to abou-t 700C and then firing is commenced until a temperature of from about 700 to about 1500a is ~chi.eved and vitri~ication of the silicate~ is completed~ When large size p~rticles.
having a hollow center are desired, the mixture is provided, before heating~ with a suitable amount o~ potas~ium nitrate which, on exploding when the heating is e~fected, provides the large central pore to produce a hollow particle st~ucture.
Thi~ hollow particle structure may also be produced when -the heating is carried out at.lea~t in p~rt in the presence o~
saturated ~team in order to humidify and to there~ore render the sur~aces of the particles ~ufficiently plastic, with or without the exi~tence of the potassium nitrate. ~ther suitable ~illers or additives may be added to the aggregate to provlde speci~ic and useful properties ~or a variety of uses thereo~. .
~ he novel features that are con~idered characteristic of the in~tant i~vention are set forth w~h particularity in the appended claims. ~he invention it~el~, however, both as -to it~ organization and its me-thod o~ operation, tagether with additional object~ and adva~tage~ -thereo~,. will be~t be undsretood from the following description o~ speci~ic embodime~t~ thereo~, which must be construe~ as illustrati~e ~L063~L33 ~
but no~ limita-tive of -the true ~cope and spirit of the invention.

DETAI~ED DESCRIPTION
~ he porous, lightweight, particulate aggregate ln accordance with the present invention comprises graded ~ize particles o~ a material composed o~ an alkali metal silicate preferably having an M20/SiO2 ratio of from about 1:2 to about 1:4 with M being an alkali metal, pre~erably sodium7 from 0.1 to 50~0 of the alkali metal silicate o~ silica ~SiO2); and from 0.1 to 200% of the alkali metal silicate of an alkaline earth metal silicate9 preferably calcium ~ilicate ~aSiO3), said particles being expanded, highly porous particle~ of a density of no-t more than about 60 ~ dm3~
having a oompression strength of up to about 275 kg/cm2, being resistant to temperatures o~ the order of about 1250C. without any fusion or deformation a~d having an expansion ratio of up to 15 times their original collapsed volume and a considerable reduced alkalinity.
While applicant does not wish to be bound to any theoretical con~ideration, it is thought that the addition of sllica to the alkali metal silicate considerably reduces the alkalinity of the reaulting product, thus rendering the aggregate of the present invention highly useful for being applied, in conjunction with any commercial binder, to ~C~63133 produce mor-tar~, concretes or lightweight building bloc~7 without any unfavorable effect on ~aid binder through alkaline side reaction~ a~ wa~ the ca~e with mo~t of the prior art silicate type aggregate~ showing a relatively high alkalinity. The addition of suitable amount~ of an alkaline earth metal silicate to the aggregate of the pre~ent invention, on the other hand7 renders the thu~ obtained partic~es of the aggregate considerably harder and rigid and thu~ more resistant to compres~ion stresses, without thereby precluding the expaneion capacity of the particles to form a plurality of pores.
The aggregate of the pre~ent invention havi~g the above described basic CompQ~ition :i~ composed of lightweight expanded particles of eize~ that may be varied at will, of from about 1 micron to about 10 cm in their largest dimen~ion, having a very large compression ~trength and yet a very low den~ity and being al~o characterized by not being brittle and by ~howing a very low alkalinity, whereby its use may be broadened to fields of construction that were heretofore excluded for lightweight aggregate~ that were neces~arily of a very low strength.
The particulate a~regate of t~e preeent invention preferably contains7 in addition to the above de~cribed ba~ic component~, from 0..1 to 30~0 by weight of boric acid and the alkali metal te~r~borate formed by the reaction of boric acid and the alkali me-tal oxide or added as such, to provide ~or an improved expansion coefficient and more par-ticularly for a better vi-trification oft~ep~le upon heating thereo~.
Also, small amoun-ts of metal carbides may be included -through the addition of carbon -to the reacting mass prior to carr~ing ou-t the process that will be described in more detail hereinbelow9 in order -to provide a plurality of microcrystalline structures evenly diGtributed throughout the parti¢le and furnishing very high strength properties thereto as well as increasing the mel-ting point remarkably. The addition of carbon to the mass9 in view of the burning thereo~9 will also provide a very large plurality of small pores evenly distributed throughout the ~tructure of the particle. ~inally, the parti¢le may also oontain ~rom O.l to 30~ by weight o~
the alkali ~etal silioate of ~n alkali metal oxide, pre~0rably originating from the redox reaotion of previou~ly added alkali metal nitrate, and more particularl~ potassium oxide to in¢rease hardness and the melting point of the aggrega~e~
Other suitable fillers or additives may be added to the aggregate of the present invention in order to provide other useful and specific properties that may render the same highly suitable ~or certain specific purposes. Illustxative o~ su¢h fillers and additives are, inter alia, borax, diatomaceous earthsg ¢lays, expandable clays such as bentonite, kaolin, talc9 residual limestones9 asbestos, comminuted ~', ,.
_ 10 - ~

~63:~L33 fiber glae8, rockwool, ochre, volcanic ashe~, garbage cinders, tripoli and the like~
The hi~hly porous particle of the aggregate o~ the preæent invention may be composed o~ a very large plurality o~ small pores evenly distributed throughout -the ma~s o~ the material or it may have a relatively large central pore surrounded by a shall of the highly porous material, depending on -the type o~ -treatment used for preparing the aggregate.
~o~h ~tructl~es, however, are e~ually resi~tant to compres~ion and high temperatures and both provide an absorbe~t outer surface which render~ the per~ormance o~ binders highly efficient, ina~much as any binder added will be absorbed into the pores and on c~ri~g will provide a s-truct~re having a vexy high adhesion which will preve~t any tendenoy to crumbling of the material.
The aggregate o~ the presen-t invention may be prepared by -the process which includes the step~ of: :
a) dissolving the alkali metal silicate in an amoun-t of water sufficient to form a solution containing from about 35% to about 50% of total solids, and adding an amount of from 0.1 to 50% by weight o~ the alkali meta} ~ilicate of ~inely divided silica and an amount of ~rom 0.1 to 200% by weight of the alkali metal silicate o~ a finely divided alkaline earth metal silicate, with a vigorous kneading action aftex each addition until a homogeneous slurry is obtained;

63~
b) reducing the ~hus ob-talned slurry -to particles of a pre~elected size depe~ding on the desirea size of the fi-nished aggregate;
c) hea-ting the thu~ obtained par-ticles to a tempera-ture of from about 110C to about 700C~ for a period of time from about 0~1 ~econds -to about 5 minutes in oraer to produce the necessary e~pansion of the particles through the blowing action of the water vapor released, with the ~ormation of a large plurality of pores within each par~
ticle;
d) firing the expanded and relatively dried particles to a temperature of from about 700 to about 1500C fox a period of time of from about 5 seconds to about 1 minute to complete the expansion of the material and prcduce vitrifica-tion thereo~; and e) slowly cooling said fully expanded and vitri~ied particles to room temperature whereby a particulate lightweight highly prrou~ aggregate material is obtained.
~ he propertias of the aggregate obtained by means of the above described process may be modified by the addi~on, to the starting slurry, of certain other additives that p~uce additional and improved properties to said finished aggregate.
In accordance with certain preferred embodiments of the process of the present invention, b~ic acid may be kneaded -~s into the slurry formed in a) above, in an amount of from about 0.1 to about 30% by weight of the alkali metal silicate _ 12 63~ 3 which boric a~id, on partially reacting with the al~ali metal oxide moiety of the silicate material9 produces alkali metal tetraborate and on being heated produce~ metaboric acid and water vapor9 thus forming a silica-te-tetraborate glass which aids to obtain a highly resist~nt vi-trification of the material and provides for a further expansion thereof during the vitri~ication operation described :in d) above.
Another preferred embodiment of the invention comprises the additionl to the kneaded~r~ obtained from a) above, either with the previous addition of the boric acid or without such addition9 of an amount of ~rom about 0,1 to about 400% by weight of the alkali metal silicate o~
finely divided carbon, to provide for an inorea~ed thermal oxpan~ion and vitri~ication9 as well as ~or an increase in the melting point of the aggregate. ~he addition of carbon to the slurry prior to heating, produces the formation o~
small amounts of metal carbides on reacting with the impurifying metals of the silica and silicates of themixture when the la W er i9 being ~ired to high temperatures, thus producing a plurality of microc~stalline structures that considerably increa~e the hardness of the aggregate and improve the melting point thereo~. Also, the addition o~ carbon aids to the obtention of an extremely lightweight structure, in view of its ~atural porosity and its homogeneou~ di~tribution throughout the mix-ture. The addition o~ carbon al~o produces .

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~3:133 the ~eggregation o:f h3110w and solid volumes throughout the mass, in ~iew of ~he release of carbon dioxide that does not diffuse as quic~ly a~ other lighter ga~les, -there~ore improvi~g the insoluble charac-teri~tic~ of the par-ticles. It also aids in the expansion of the material through the release of carbon monoxide which forms very ~mall pore~ when the material is being fired in ~-tep d) above.
A ~truc~ure of the aggregate partioles that ha~
been found highly u~e~ul for certain applioatio~9 ~uch as ~or the manu~acture of the so called ~'m~neral foam ~uilding block~", is one in which each particle of the aggregate is ~ormed as a cen-trally hollow sphere-like particle having a very hard ~hell o~ highly porou~ ~ilica-te material on the ~urface o~ the sphere-like particle alld a very lar~e oen-tral ~pheroidal pore surrounded by said material.
~ he above described hollow particles m~y be obtained in accordance with ~he process of the present invention, by the ~urther addition, to the ~lurry obtained in acc~rdance with a~y one of the above de~cribed embodiments, o* a blQwing ~ .
agent which i~ ~ufficiently volatile to produce a violent `~
explosion on heating thereo~ during -the heat expansion etep o) de~oribed above.
While any kind o~ known sulfur type blowing agent as well as any highly volatile solid finely divided or~a~ic material ~uoh as wheat flour~ cornstarch, wood powder~ wood 1~63133 pulp9 cork, ~awdust and the like may be conveniently used to produce the large central pore in the hollow particles, a highly preferred embodiment of the present invention com~rises adding an amount o-f from about 0.1 to about 30% by weight o~
the alkali metal silicate of the po-tassium nitrate which in the presence of a reduclng environment~ viole~tly releases nitrogen which forms the central pore, and at -the sAme time forms potassium oxide which increases the hardness and the melting point of the finished aggregate.
The above mentioned hollow structure o~ the par-ticles of the aggregate of this ~mbodiment of the present Lnvention may al~o be obtained, with or without the addition of a blowing agent having explosive characteristics, by a mere slight modi~ication of the expansion step described in c) above. ~or this purpose, the heating operation i~ carried out in t~o stages, namely, by firstly heating the par-ticles to a temperature of ~rom about 110 to about 300C
in a moist environment such as a stream of saturated steam in close contact with -the particles, in order to reincorporate water molecules to the surface~ thereof, whereby said surfaces will remain sufficiently plastic to per~it the particles to be inflated in a globe-like ~ashion during the heat expansion s-tep, and then continuing the heating operation under dry conditions until a temperature of from about 300 to about 700C. is reached a~ before. f J
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- 15 _ ., `' :' , 1al63~33 The gr~i~ing of the slurry ~ormed by kneading toge-ther the i~redients de~cribed above; may be achieved by any known mean~. It i9 preferred in accoxdance wi-th the presen-t i~vention, however~ to carry out such operation, when the slurry has a liquid consistency, by spraying the fluid -through a suitable nozzle into a heated environment to thereby produce pre_dried particles. When the ~lurry has a thick paste-like consistency, then the pre~e~red method i~ to entrain the slurry in a ~a~t moving gas or vapor stre~m and ejecting the same through a suitable nazzle. Finally, when the slurry has been permi-tted to dry and form a solid, then the method includes the use o~ a grinder for solids, which permit~ the obtention o~ particles of any de~ired ~izea The preoent inventio~ will be more ~ully undcr~tood by havi~g rs~erence to the ~ollowing illustrative and non limitative examples, E~,P,I~ 1 A kneader was charged with 10 kg o~ an aqueou~
~odium silicate syrup containing 5 kg o sodium ~l~ioate of the formula Na20.2SiO2 and 2 kg of ~inely divided silica (SiO2) were thoroughly incorporated to said syrup, ~ollowed by 10 kg o~ calcium silicate (CaS:L03). ~he kneader was operated for about 2 minutes until a -thick paste o~ silicate material was obtained. ~he paste was permi-tted to dry at a temperature o~ 90C. and wa~ divided into coarse particles :

~63~l33 ~ ~
1 by means of a slow rotatory hammermill, whereby pre-dried par- ;
ticles of silicate material were produced. The particles ~ ~.
were heated by a stream of air until a temperature of 650C . :
was obtained and the temperature was maintained for a period of about 10 seccnds more, whereby particles expanded about 10 times its original volume and containing a large plurality ; i~
of evenly distributed pores were obtained. The expanded par- :-ticles were fired to rapidly achieve a temperature of 1250C .
(about 1 minute) and the vitreous highly expanded particles (13 times their original volume) were slowly permitted to cool down to room temperature. The coarse particulate aggregate thus obtained was formed of porous particles of about 3 cm ::
and showed a density of about 250 g/dm and a compression ~ .
strength of about 210 kg/cm and didnot melt nor deform at . .; :
temperatures below about 1200C. .. ~
EXAMPLE 2 . : .
A kneader was charged with 10 kg of a 35% sodium . ~
silicate aqueous syrup of the formula Na~0.2.8SiO2. Then 1 kg - .
of finely divided silica, 5 kg of calcium silicate and 1 kg ~ :-of boric acid were successively kneaded into the syrup to thereby form a liquid slurry of the silicate material. The slurry was spray dried into an atmosphere of air maintained ..
at a temperature of about 500C, whereby small expanded par- :
ticles of about 1 mm were obtained. The particles were then fired at a temperature of 1500C for a ~ :.

.: . - . . : .

;3~33 few seconda -to produce a virtually in~tantaneoua vitri~ication thereof, and were then permi-tted to cool to room temperature~
The fine particula-te aggregate thu~ obtained had a den~i~y of about 60 g/dm3 and a compression stri3ngth o~ about 272 kg/cm2 and did not melt nor de~orm at te~peraturea below about 1250C.
~.~
A mixture o~ 10 kg of sodium silicate (Na20.3SiO2) syrup at a 40% concentxation, 2 kg o~ ~inely divided silioa, 8 kg of finely divided calcium silicate, 0.5 kg o~ boric acid and 10 kg o~ finely divided coke was aubjected to the procesa described in ~xample 1.
A coarae aggregate ahow.in~ a densi-ty of about 50 ~ dm3 and a oompreasion ~tren~th o~ about 22~ k~ cm2 was produced, and the aggregate did not melt nor deform at -temperaturea below about 1250C.
EXAMP~ 4 ~ he addition o~ 1.0 kg o~ potaasium nitrate to the mixture of Example 3 and following the process o~ Example 2 produced an aggregate compoaed o~ relatively fine (3 mm), hollow particles ahowing a large hollow interior surrounded by a ahell o~ highly poroua silicate material. ~he denait~
waa about 40 g/dm3 and the compreseion strength was abou-t 250 kg/cm2 and the aggregate did not melt or deform at temperatures below about 1200C.

-~63~33 The starting mixture u3ed in Example 1 was kneaded and divided into particles and was thlen subjected to a ~irst 810w heating s~ep in the pre~ence o~ saturated steam until a temperature of about 300C. was reached~ Thereafter the process described in Example 1 was followed by ~ir3too~eting the hea-ting step under dry hea-t to a temperature o:f about 700C. and ~iring until a temperature of about 1500C. was reached. ~he aggregate was compri~ed of very large particles ,~
(about 5 cm) having a hollow ce~ter surrounded by a shell o~
highly porou~ silicate matexial, which ~howed a density o.
about 35 ~/dm3 and a compre~sion strength o~ about 195 kg/cm The coarse aggregate did not melt or de~orm below about ~aO
It can thus be seen that a novel lightweight aggregate has been obtained9 which may be applied to a vast variety o~
u~es in view of its very low density as compared with sol,id, heavy,aggregates of a conventional nature9 without appreciab~fe ', di~erences in the compression strength and temperature resistance thereo~. There~ore, the aggr'egates of the ~re,s,e~nt invention can be used a~ lightweight aggregates ~or con~crete~
and,mortars based on Portland cement~ lime and gyp~um ~or use in the manu~acture o~ various products use~ul in the building ar-ts, also in view o~ the ~act that a broad range o~ partiQle .
sizes can be obtained (~rom about 0.001 cm to about 10 cm) and because the particles of the aggregate o~ the pre~ent in~ention have a very large number o~ pores tha-t permit a very soIia ,, f, 10&3~33 anchorage of the binders without the need of u~ing large amount~ of water as was the case o~ the prior art products, and also because the significantly reduced alkalinity o~ the aggregate particles preve~ts undesired side reactions with the binder~ tha-t tend to produce weak spot~ and cracks in the ~inished and cured material, particularly when the ambient temperature varie~.
Another advantage of the aggregates of the pre3en~
invention is that, being Q~ a vitreous and completely insoluble character~ they permit the use of steam to cure the concretes and mortara without an~ deletereous effect on said aggregate as was the case with the prior art aggregates, partioularly the lightweight prior art aggregates.
A~ the ~trength of the aggregates o~ ~he pre~e~t order ~,~ vJ~:
invention i~ o~ a ~imilar ~dcr of ma~ b~ as prior art ~olid aggregates (from about 200 to 250 kg/cm2), this enables for the first time the use of lightweight aggregates for structural concretes, therefore permitting the obtentio~ of Portland cement conoretes of a density as low as about 250 ~ dm3 without materially affecting the strength thexeof.
Thi~ characteristic~ of course, opens an entirely new field to lightweight aggregate material~, and ~uoh aggregates may be also used in concretes for prestressed members and prefa~ricated members for low cost houeing.
As the aggregates of the present invention have a ..

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1~63~3~
highly porous structure, they permit the obtention o~
concre-tes and mortars of very efficient acoustic properties for use as partitions, ce~lings and the like and the building blocks obtained wi-th these aggregates are admirably suitable ~or very }ow cost lightweight acouatic and thermally insulating partitions, supporting walls, alab and girder closuresj and the like.
Differont types of building and orname~tal element~
can alao be obtained b~ molding mixtures of the above describ;ed aggregate~ and suitable binders such as fast curi~g silica ~olution~, resins9 organic gums, rubber_typs adhesivea and the like, and then curing the bindera.
~ he aggregates of the preaent invention can al~o be uaed for epray coating of building element~ ~or therm~1 and acouatic inaulation purpose~7 as well aa low cost lightweight fillers for pla~tics and the like.
Although certain ~pecific embodiment~ of the prese~t invention have been ahown and described, it ia to be under~tood that many modifications thereof are pos~ible. ~he inven~ion~:
there~ore, is not to be restric-ted except in~ofar as i~
neces~itated by the prior art and by the ~piritOf~ the append~d claims.

Claims (19)

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

1. A porous lightweight particulate aggregate com-prising expanded and vitrified low alkalinity particles having a density of not more than about 60 g/dm3, a compres-sion strength of from about 195 to about 275 kg/cm2, a tempera-ture resistance without undue fusion or deformation of from about 1200 to about 1250°C and an expansion ratio of up to about 15 times the original collapsed volume of said particles, said expanded and vitrified low alkalinity particles having been obtained by heating and firing dried particles of a com-position comprising an alkali metal silicate having an M2O/SiO2 ratio of from about 1:2 to about 1:4 wherein M is an alkali metal, from about 0.1 to about 50% by weight of said alkali metal silicate of silica, from about 0.1 to about 200% by weight of said alkali metal silicate of an alkaline earth metal silicate, and from about 0.1 to about 30% by weight of said alkali metal silicate of boric acid which upon firing produces a silicate-tetraborate glass.

2. The aggregate according to claim 1 wherein said alkali metal is sodium and said alkaline earth metal is calcium.

3. The aggregate according to claim 2 said composition further comprising one or more additives selected from the group consisting of carbon, metal carbides and potassium oxide.

4. The aggregate according to claim 1, wherein said expanded and vitrified low alkalinity particles contain a large plurality of small pores evenly distributed throughout the body thereof.

5. The aggregate according to claim 1 wherein said expanded and vitrified low alkalinity particles are globe-like particles having a relatively large pore in the center thereof surrounded by a shell of material containing a large plurality of small pores evenly distributed throughout said shell.

6. A process for the manufacture of a porous lightweight particulate aggregate comprising expanded and vitrified low alkalinity particles having a density of not more than about 60 g/dm3, a compression strength of from about 195 to about 275 kg/cm2, a temperature resistance without undue fusion or deformation of from about 1200 to about 1250°C and an expansion ratio of up to about 15 times the original collapsed volume of said particles, comprising:
a) dissolving an alkali metal silicate in water to form a solution of from about 35% to about 50% by weight of total solids, and kneading into said solution from about 0.1 to about 50% by weight of the alkali metal silicate of finely divided silica, from about 0.1 to about 200% by weight of the alkali metal silicate of a finely divided alkaline earth metal silicate and from about 0.1 to about 30% by weight of said alkali metal silicate of boric acid which upon firing produces a silicate-tetraborate glass, in order to obtain a homogenous slurry;
b) partially drying the thus obtained slurry to produce a paste-like or solid mixture:
c) reducing said mixture to particles of a pre-selected size depending on the desired size of the finished aggregate;
d) heating said particles to a temperature of from about 110°C to about 700°C for a period of time of from about 0.1 second to about 5 minutes to remove the remaining water and expand the particles with the formation of a large plurality of pores therein;
e) firing said expanded particles at a temperature of from about 700°C to about 1500°C for a period of time of from about 5 seconds to about 1 minute to complete the expansion of the material and produce vitrification thereof; and f) slowly cooling said expanded and vitrified par-ticles to room temperature.

7. The process according to claim 6 wherein said other additives added to the slurry further comprising from about 0.1 to about 400% by weight of carbon, based on the weight of said alkali metal silicate.

8. The process according to claim 7 wherein a blowing agent in an amount of from about 0.1 to about 30% by weight of said alkali metal silicate is added, whereby hollow particles having a large central pore surrounded by a hard porous shell are obtained.

9. The process according to claim 8 wherein said blowing agent is an organic blowing agent.

10. The process according to claim 9 wherein said organic blowing agent comprises wheatflour.

11. The process according to claim 8 wherein said blowing agent is potassium nitrate.

12. The process according to claim 6 wherein the heating of the said particles is effected in two stages, namely, a first stage under moist environmental conditions in which the temperature is raised to from about 110 to about 300°C and a second stage under dry conditions until a temperature of from about 300 to about 700°C is reached, whereby a hollow particle having a relatively large central pore surrounded by a hard porous shell is obtained.

13. The process according to claim 12 wherein the moist environmental conditions are produced by contacting the parti-cles with saturated steam.

14. The aggregate according to claim 3 wherein said carbon is present in an amount of from about 0.1 to about 400% by weight of said alkali metal silicate of finely divi-ded carbon.

15. The aggregate according to claim 1 said composition further comprising at least one blowing agent selected from the group consisting of a sulfur type blowing agent, an organic volatile solid finely divided blowing agent, and potassium nitrate.

16. The aggregate according to claim 1, said composition to be heated and fired further comprises about 0.1 to about 30% by weight of said alkali metal silicate of potassium nitrate as a blowing agent.

17. The aggregate according to claim 1, consisting essen-tially of said particles.

18. The aggregate according to claim 1, said composition consisting essentially of said alkali metal silicate, said silica, said alkaline earth metal silicate and said boric acid.

19. The process according to claim 6, said slurry con-sisting essentially of said water, said alkali metal silicate, said silica, said alkaline earth metal silicate and said boric acid.