CEMENT and CONCRETE RESEARCH. Vol. 8, pp. 415-424, 1978. Pergamon Press, Inc Printed in the United States.

COMPARATIVE STUDY OF THE USE OF A QUARTZ POOR SAND AND A PURE

QUARTZ SAND FOR LIME SILICA BRICKS AND THE KINETICS OF THE

HYDROTHERMAL HARDENING MECHANISM

Tj. Peters

Min. Petr. Inst.,Univ. of Berne,Sahlistr.6,Schweiz

R. Iberg and Th.Mumenthaler

ZOrcher Ziegeleien,ZOrich,Schweiz

(Communicated by H.F.W. Taylor) (Received March 21, 1978)

ABSTRACT A series of test specimens were manufactured with a pure quartz sand (98% quartz) and quartz poor sand (50% quartz) under dif- ferent autoclaving conditions. The amount of dissolved quartz is the same for both sand types and it is not the determining factor for the formation of calcium-silicate hydrates (CSH). Semi- crystalline CSH-phases with a high Ca/Si-ratio are formed con- temporaneously with ii ~ tobermorite during the initial stages of autoclaving. After all Ca(OH) has been consumed the amount of ii ~ tobermorite increases, but ~he amount of the semicrystalline CSH-phases remains constant although the Ca/Si-ratio decreases. Under autoclaving conditions the hydrated silica species move toward the calcium source.

Mit einem quarzarmen Sand und einem reinen Quarzsand wurden ver- gleichende Untersuchungen unter verschiedenen Hartungsbedingungen durchgef~hrt. Die Menge des umgesetzten Quarzes ist unabh~ngig vom Quarzgehalt des Sandes. Am Anfang der HMrtung bilden slch gleichzeitlg semi-kristalline Calclumsilikathydrate mit relativ hohen C/S-Verh~itnissen und ii ~-Tobermorit. Nachdem das Ca(OH) 2 aufgebraucht ist, wird fur die weitere Bildung yon ii ~ Tohermorit Ca aus fr~her gebildeten semi-kristallinen CSH-Phasen aufgenom~en. Unter Autoklavbedingungen bewegt sich das hydratislerte Sil£cium in Richtung des Calcium Lieferanten.

415

415 Vol. 8, No. 4 Tj. Peters, ?,. [berg, Tn. Mumenthaler

Introduction

In many regions pure quartz sands do not occur in quantities sufficient for the fabrication of autoclaved building products. Instead the exploitation of the mineral resources available re- quires that sands with impurities such as felspars, carbonates and clay minerals must be used.

The huge deposits of arkosic fluvioglacial sands in the Swiss Molasse basin are an example. A comparative laboratory study was made with this sand which is used for autoclaved building pro-

ducts.

To estimate the influence of other components beside quartz on the quality of the autoclaved products, it is essential to under- stand the reaction mechanism during hardening. In the course of this investigation certain results did not conform with current ideas on the hardening mechanism as postulated by Budnikov et al (i), Butt et al (2) and Hochstetter (3). For this reason small- scale experiments were made to determine the relative mobilities of silica and lime.

Experimenta ~

The starting materials were: pure quartz sand with 98% quartz; a sand with 49.9% quartz, 12% orthoclase, 11% albite, 13% calcite and 0.5% dolomite; burned lime with 97.3% CaO, 1.7% MgO, 0.5%

SiO2, 0.2% FeO, 0.2% SO 3.

The grain size distribution of the quartz sand was made to match that of the impure sand. Both sands had grain sizes between O.i - 3.0 mm and a specific surface of 90 cm2/g (Mumenthaler and Peters

(4).

Mixtures of 15 kg sand, 1.2 kg lime and 1.5 kg water were left for 4.5 hrs in plastic bags (simulation of the reactor) before being pressed into moulds with 140 kg/cm 2. The test specimens were autoclaved at 8 bar (170°C) and 16 bar (2OO.5°C) for i, 3, 6, 12 and 18 hrs. Compressive strength and density were measured on 3 test specimens of iOxlOxlO cm, the elastic modulus on prisms of 12x12x36 cm, and the shrinkage on prisms of 4x4xl6 cm. The autoclaved samples were investigated by X-ray diffraction, differential thermal analyses (DTA) , scanning electron microscope (SEM) and chemical analyses. The amounts of quartz were deter- mined by the method of Talvite (5), the amount of soluble SiO~ by the method described by the Society of Chemical Industry (~) (hot HCI) as well as by the method of Florentin (7) (cold HCI).

Experiments on the relative mobility of silica and lime were carried out in 30-mm long and 2.5-mm wide gold tubes partly filled with H20. Quartz and Ca(OH) 2 were separated by gold sponge (dental quality). Fine grained (~ 2/4) as well as coarse grained (1-3 mm) starting materials were used. The gold tubes were held horizontally within hydrothermal bombs at 190 ° , 206 ° and 300 ° C.

Vol. 8, No. 4 417 AUTOCLAVING, S~O 2 MOBILITY, CSH, TOBERMORITE

Goldsponge

Ouartz Portlandi~ FIG. 1

Experimental arrangement for experiments on relative mobility of calcium and silica.

Results

Physical Properties

The data of the physical measurements on the test specimens from both sands are put together in FIG. 2. Compressive strength is generally higher for the specimens made of quartz poor sand. At 16 bar maximum strength for both sands is reached much sooner than at 8 bar. The elastic modulus is higher for the pure quartz sand specimens and after an initial increase tends to decrease with time. Shrinkage and density are higher for the bricks made from the quartz poor sand.

TABLE 1

Chemical analyses of lime silica bricks

P r e s - T ime ZgnJ t l o n CO 2 l u ~ e 1 O l l

4

b4~ h r s . t t t I t t

H20 I C a ( O H ) 2 d L s e o l v e d tquartZof I n

Q~artzsand

0 0 Z.+4 O+ql I . ~ ~l.i I 1 2.W, 0.15 Z.)I 0.5 441 I 3 Z.?O 0 . ~ 2.Z+, 5.Z I 0 Z.T2 0.45 ~.21 ] .3 I 12 2.71 0.54 ~.Z~ 1 . I 15.~ I 11 ~.l~ 0.74 Z.O8 0.6 1~9~

16 I 2.7Z 0.~1~ 1.11~ 1.0 55t 16 3 ?.$5 0.81 2 . ~ ".Z 16 6 Z.53 0.79 ~.76 0.15 8%6 16 12 2 . ~ 0.~, I . s~ 0 . ~ W~ lJ 11 2.~ 0.711 1.67 0 ~. IlK)

~ p u [ e San¢~

0 0 11 . t0 I.? 1.0~ t .~ l 1 11.;6 ?.95 ).+g t.,1 ~ ! | ] 11.10 ?.J ~.1¢ 5 . ] ?'tO I | 11 .gZ ?.5 6.42 ].& ~, | I IZ 11.5~ 7, | +I.+PO 0.5 S ~ I t l 10 .~ ?.0 ~.~1 : . ! I I Z

15 I 11.;9 | . t? 2.¢1~ ?.$ 445 16 ) I I . I ) | . ~ Z.?qJ 0.5 I%+ ~ 1£ 5 U.I~ l . ~ Z.~) 0.'+ 1 ~ 15 12 10.13 1.72 Z . l l 0.1 t~JS I+, I I 11 .?? I . ? l 3.0~ 0 1111

8 ~ 8 r t ~ n g

S o l u b l e $10 2 I n t O f s t a r t l n , m i x t u r e

Soc . o ! c h e m . mi x tuz ' e

Z.~ 1.0~ 4.11 ~.Oi 5.51 2.16 i . ~ 3.?Z 6.75 ~ .9,

1.14 6.r2

5 .~ S.*! 6.11 i .5?

5.1E 1.09 5.m Z.~ 6.~ 5.]) 5.44 4.~4

Z.g8 Z.! 521 5.? J.?5 T.O 1.1 7.55 ?.11 1.75

Ind. ( 1 9 6 4 ) r l o r e n ~ . 1n(~26}

0.2 0.33 1.)] Z.+4 ].08 3.1,2

).¢?

3.,~,

Z.lO

0.5Z 1.0J 1.1, ).5~ Z.34

1 .?5 3.~ 3.~ 3.25 ).P5

Chemical analyses

The decrease in absolute amounts of quartz after autoclaving is the same for pure quartz sand as for the quartz poor sand which initially contained only 50% quartz (TABLE 1 and FIG. 3, 4). With both sands most of the quartz is dissolved within the first

418 Vol. 8, No. Tj. Peters, R. ;berg, Th. Mumenthaler

hours of autoclaving. In figures 3 and 4 the amounts of soluble SiO 2 determined by the two methods and the Ca(OH) 2 content of the autoclaved samples are represented as a function of auto- claving time for the two pressures 8 bar and 16 bar.

Joo J

f " i / •

~o i f;" o!

l :t jlJ -¸- ,=! 4:"* ~" . . . . . . . . . . . . . :-:_:.

, . t o l

N...o!

;

J T~ !

i

I . . . .

~,..___..~::.: . . . . . . . . . , / f / "

6 ' t ~1 ~ l l 6 ,Z 41 ~"T

i

I :~re q ~ l ~ Z SO.el o - - - - - - ~ l N f ~. QU(]TYZ ~ ~ C I • • I E l ~ l O O 16iNrJ

FIG. 2 Physical properties of specimens made from both sands at 8 and 16 bar as a function of hardening time.

In the 16-bar experiments the quantity of SiO^ bound in calcium- silicate hydrates (CSH) corresponds to the to~al amount of dis- solved quartz. In the specimens autoclaved at 8 bar the amount of dissolved quartz is much higher than the quantity of SiO 2 bound in CSH-phases. This difference consists of silica which zs so- luble in the aqueous solution under autoclaving conditions but precipitates during cooling as amorphous silica, generally in the form of small spheres. This amorphous silica is extractable by neither cold nor hot HCI (the methods of Florentin (7) and Soc. Chem. Ind. (6), respectively). This portion of silica decreases with autoclaving time.

At 16 bar the Ca(OH) 2 content of the specimens decreases rapidly during the first 3 hrs of autoclaving. A small fraction, more coarse-grained Ca(OH) 2, is used up more slowly. Likewise the amount of silica soluDle in cold HCI (Florentin method) increases

Vol. 8, No. 4 419 AUTOCLAVING, SiO 2 MOBILITY, CSH, TOBERMORITE

rapidly during the first 3 hrs and then remains constant. At 8 bar the process is much slower, but after 12 hrs autoclaving time the same situation is reached as after 3 hrs at 16 bar.

9['~1~)2 IQUARTZ POOR SAND I

S~O 2 I~ \ = ~ soluble in hot HCI I I ~ ~ ..~ --==dissolved quartz °/°/' o., ~ .:,oar

. . . . . . . . .

/ ' //,,,,/ , . . " - ,,dissolved quartz s t, /// ', ." "-.8~a~ /. ! / / • s~uble ~n hot HCl

/ I / l / ~_ ~ / •

0 / ~ 16bar / ~/ , o _ . , ~ _ _ _ _ _ _ . _ . . .~o,ob,~io~o,e He, / I \ //" ~

If./ - ,~ 8~r - l ~ r / / / \ " *sc~ubie incold I-El

" ) l l l I / / ! "

Ifl'/\ -- 1[~/~ '~" ~O . . . . -O- . 16_bQ.Lr \ \ - o , . - . 14 , - -- ~ -- --~ = . _ _ , i i

1 3 6 12 18 hrs

FIG. 3 Amounts of dissolved quartz, of Ca(OH)? left over, of silica so- luble from CSH in cold HCI and of siliSa soluble from CSH in hot HCI in silica bricks made with quartz poor sands.

9 Ca(OH) 2 IPUR E OUARTZSAND I \o

SiO 2 \

% \ ,,dissolved quartz 7! \ 16bar / , s o l u b l e in hot HCI

' \ , , ~ - -'dLssoNed quartz ! \ ~iCC_. :y-'~-8~Qr

\ ~ 1 / 1 6 bar

5!I ° \ ~ / - ~ : / i =soluble ~n hot Ha

I Z , / / \x _ - " " 8 ~ _.solub,e ~ c o,d ~,

I / / " ...8 bar i i /

1 / / " -

, -o - 16b~ ~o -- -- 9 . . . . . . . . . . . a . . . . . . --T r __ ~

1 3 6 12 18 hrs

FIG. 4

As FIG. 3 but in specimens made with pure quartz sand.

420 Vol. 8, No. 4 Tj. Peters, R. Iberg, Th. Mumenthaler

Assuming that at the stage where practically all the Ca(OH)~ has been used up the same two types of CSH-phases have been for~ed, it is possible to calculate the Ca/Si-ratio in these two types of calcium-silicate hydrates with the equation

in which a gives the moles of SiO 2 soluble in cold HCI, b gives the moles of SiO 2 soluble in hot HCI, and c gives the moles of Ca(OH) 2, The results from 16-bar/3 hrs and 8-bar/12 hrs give 2 simultaneous equations with the 2 unknowns x and y. This calcu- lation gives a Ca/Si-ratio of 0.82 for the phase with silica so- luble in hot HCI and a Ca/Si-ratio of 1.65 for the phase with SiO 2 soluble in cold HCI. The Ca/Si-ratio 0.82 corresponds to the composition of ii ~ tobermorite C5S6.H and Ca/Si = 1.65 lies in the range for CSH-II.

In the 16-bar experiments the Ca/Si-ratio of this semicrystal- line CSH-phase continuously decreases to a value of 1.22 after 18-hrs autoclaving time assuming a constant Ca/Si-ratio for the ii i tobermorite phase. The SiO 2 for the tobermorite phase comes from dissolved quartz and the calcium must originate from the semicrystalline calcium-silicate hydrate, because practically no Ca(OH) 2 is left at this stage.

At 2OOOC (16 bar) the amount of Ca(OH)? decreases much faster than at 170°C (8 bar) although at 17OOC the solubility of Ca(OH)

2 is higher, indicating that the decrease is determined by the rate of formation of the calcium-silicate hydrates.

X-ray analysis

X-ray identification of the phases in the binding mass poses pro- blems of overlap of reflections, especially in the products made from the impure sand. In the specimens autoclaved at 16 bar semi- crystalline CSH-I and ii ~ tobermorite are abundant, whereby the amount of ii ~ tobermorite increases with autoclaving time con- firming the results of the chemical analyses. In the products autoclaved at 8 bar the mineralogical composition after short autoclaving times is more heterogenous with~C2SH , and xonotlite besides semicrystalline CSH-I and ii ~ tobermorite.

Textures

Under the scanning electron microscope (SEM) the specimens con- sist of sand grains enveloped in a thin layer of binding mass in the form of loosely-packed aggregates with large pores. At first the vermicular calcium-silicate hydrates and unreacted Ca(OH) 2 form a network on the sand grains and, with increasing autocla- ving time the size and the perfection of the outline of the crys- tals in the binding mass increase. Between the strongly etched quartz crystals and the binding mass a gap forms that becomes more pronounced with increasing time and autoclaving temperature. Starting from the binding mass well-crystallized calcium-silica- te hydrates grow into the pores. These observations confirm Moor- head and McCartney's (8) hypothesis, whereby silica dissolved from

Vol. 8, No. 4 421 AUTOCLAVING, SiO 2 MOBILITY, CSH, TOBERMORITE

the quartz grains diffuses through the product layer.

FIG. 5:

SEM from lime silica brick showing gap between strongly

etched quartz and the binding mass from. which CSH crystals grow into the pore space. (800 x)

Nr.

TABLE 2 Results of representative trials with gold tubes

T~mp. Time Startinz material Prcducts

K 6a 190°C 16 hrs.

K 6b 190°C 16 hrs.

fine-grained Ca(OH) 2 sheafs of CSH on the edges of portlandite crystals

flne-~rained quartz etzhe! quartz

K 9a 3OO°C iO days

9b 3OO°C iO days

fine-grained quartz etched quartz

fine-grained Ca(OH) 2 verm~cular and fibrous :SH- phases, Ca(OH)., and sgheres of SiO~ - gel

K lla 206°C 32 days

llb 206°C 32 days

fine-grained quartz smocth quartz grains

fine-grained Ca(OH) 2 sheafs of CSH cn the edges of Ca(OH) 2 crystals

K 12a 206°C 32 days

12b 206°C 32 days

fine-grained quartz

coarse-grained Ca(OH)

2

smooth and etched quartz- grains

fei~ like network of CSH- phases on portlandite. CEH-ghases generally ver- micular, and seldom lath shaped

K 13a 206 °~ 32 days

13h 206°C 32 days

fine-grained Ca(OH)

2-3 mm quartz crystals

sheafs of CSH on portlani=te

stron{ly etched quartz surfaces and traces of CSH

In the experiments using the small gold tubes (TABLE 2) forma-

tion of calcium-silicate hydrates only occured on the Ca(OH)26 side of the gold sponge. Generally a mat of vermicular (FIG. ) and platelike (FIG 6) CSH-phases grow on ~n_ portlandite crys- tals. On the other side of the gold sponge, however, no

~22 Vol. 3, No. Tj. Peters, ~. [berg, Th. Mumenthaler

formation of CSH-phases was observed; only quartz grains with strongly etched surfaces were present. This demonstrates the re- lative mobility of silica compared to calcium under normal auto- claving conditions. According to Moorhead and McCartney (8) only above the critical temperature of water do CSH-phases form ad- jacent to the supply of lime.

FIG. 6 SEM of trial K 12b. CS}{- phases grow on portlandite flake. (15OO x)

FIG. 7 SEM of trial K llb. CSH- phases on edges of port- landite. (3900 x)

Discussion

The results of this investigation show that the rate of solution of quartz (Assarson, (9) , Hochstetter, (3) does not primarily determine the rate of formation of calcium-silicate hydrates. In the reactor before autoclaving and during the heating up period at low temperatures (about 80°C) part of the Ca(OH) 2 is dissolved, whereby the pH is increased to above 10.7. Due to high pH and increased temperature during the initial stages of autoclaving large amounts of SiO 2 go into solution. Under normal autoclaving conditions semi-crys[alline calcium-silicate hy- drates with a relatively high Ca/Si-ratio together with well- crystallized ii ~ tobermorite are formed. The formation ii i =o- bermorite is more pronounced in the impure sand as is to be ex- pected from earlier investigations (Sa,~man, (!0) , Mitsu!a and Taylor, (ii).

After an initial period in which all the Ca(OH) is cons'mmed, calcium is taken out of the semicrystalline cal~ium-silicate hydrates to form ii ~ tobermorite. The Ca/Si-ratio of the earlier formed semicrystalline calcium hydrates is hereby de- creased. Only this second mechanism is in accord with the har- dening mechanism postulated by Budnikov et al (i) , and ="~t~_~ et al (12).

The following reaction scheme can be sketched out for the har- dening process in lime silica bricks made with impure quartz sands:

Vol. 8, No. 4 423 AUTOCLAVING, SiO 2 MOBILITY, CSH, TOBERMORITE

in reactor heating up in autoclave

Sandl--~Ca(OH)2÷sand÷H20 ---~quartz into solutlon_

:a01 ! i | ~-Ca(OH)_ into solution .......... J

H20 J 2 pll increases

autoclaving

m ~

aeml-cryst.CSH - *semi-cryst.CSH wlth

~ / S > ~ . 5 ~ d e c r e a s l n g C / S - r a t i o

I~ tobermorlte - - - ~ Lncrease Of amount C/S - 0.83 of iI~ tob~rmorltr

With pure quartz sand often xonotlite instead of ii ~ tobermo- rite is formed.

Under autoclaving conditions the hydrated silica species move towards the source of calcium. Besides nucleation the diffusion of silica is the determining factor for the rate of formation of calcium-silicate hydrate. The reaction rate can be increased by higher autoclaving temperatures, larger surface area of the re- actants and by additives (Butt and Raskovic (12).

For practical applications this study has shown that the amount of quartz in the sand is not critical - the amount of dissolved quartz was even higher for the impure sand with 50% quartz than for the pure quartz sand. Physical properties such as com- pressive strength are determined by the grain size distribution, grain form, moulding pressure, moulding humidity, poresize dis- tribution and the autoclaving cycle.

Acknowledgment

The "Kommission zur f~rderung der wissenschaftlichen Forschung" supported this project financially.

References

i. P.P. Budnikov, L.A. Kroitschuk and B.N. Vinogradow, Proc. 2nd. Int. Symp. Autocl. Calcium Silicate Bldg. Prod., (Hannover) , iO, (1969).

2. J.M. Butt, U.J. Palm, L.L. Schmidt, Z.M. Larionova, O.S. Wolkov, V.R. Garaschin and G.F. Gruner, Proc. 2nd. Int. Symp. Autocl. Calcium Silicate Bldg. Prod. (Hannover) , 2, (1969).

3. R. Hochstetter, Tonind.-Ztg., 91, 450-453 (1967).

4. Th. Mumenthaler and Tj. Peters, Beitr. z. Geol. d. Schweiz, Geotechn. Serie, Lfg. 56, K~mmerly und Frey, Bern (1977).

5. N.A. Talvite, Analyt. Chem., 23-626, (1951).

6. Society of Chemical Industry, Monograph 18, (1964).

7. D. Florentin, Ann. chimie analyt, appliq. 8, ii, 321 (1926)

8. D.R. Moorhead and E.R. McCartney, Proc. ist. Int. Symp. Autocl. Calcium Silicate Bldg. Prod. (London), 86-91, (1967).

9. G.O. Assarsso~ J. Phys. Chem. 64, 328 (1960).

¢2~ Vol. 8, No. 4 Tj. Peters, R. Iberg, Th. Mumenthaler

iO. Z. Saumann, Proc. 2nd. Int. Symp. Autocl. Calcium Silicate Bldg. Prod. (Hannover) , 73, (1969).

ii. T. Mitsuda and H.F.W. Taylor, Cement and Concrete Res. 5, 203, (1975).

12. J.M. Butt and L.N.Rashkovlch, Strojizdat Moscow, (1965).