ROAD RESEARCH LABORATORY

Minis t ry of Transport

RRL REPORT LR84

- ~ . . ~

CALCINED BAUXITE AND OTHER ARTIFICIAL, POLISH-RESISTANT, ROADSTONES

by

J. G. James

CROWTHORNE

ROAD RESEARCH LABORATORY

1967

CONTENTS

Abs t r ac t

1. Introduction

2. Calcined bauxite

2 . 1 2 . 2 2 . 3

2 .4

The Colnbrook By-pass Exper iment The Blackbushe Exper iment The d i sadvan tages of calcined Demera ra

bauxite Other bauxi tes and c lays

3. P o l i s h - r e s i s t a n t stones: some theoret ical models

3.1 3 . 2 3, 3.

3 .4

3 .5

E x t r e m e l y hard ma te r i a l s Cong lomera t ions o f s m a l l hard pa r t i c l e s D i s p e r s i o n s of hard p a r t i c l e s in a sof te r

m a t r i x M a t e r i a l s which f rac tu re in an i r r egu la r

manner V e s i c u l a r m a t e r i a l s

4. Natura l roads tones

4.1 Poss ib l e improvement by heating

5. Ar t i f i c i a l roads tones

5.1 C e r a m i c s 5.2 G l a s s e s 5.3 Waste p roduc t s 5 .4 Soluble s i l i c a t e s 5 .5 Hydraul ic cemen t s 5 .6 P l a s t i c s

6. Conclus ions

7. R e f e r e n c e s

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12 12 14 14 14 15

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r a • c t CROWN COPYRIGHT 1967 Ext s f rom the text may be reproduced

provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CALCINED BAUXITE AND OTHER ARTIFICIAL, POLISH-RESISTANT, ROADSTONES

ABSTRACT

Calcined bauxite f rom Guyana has been found to have an exceptionally high r e s i s t ance to pol ishing under t r a f f i c . It is, however, expensive and only l imi ted suppl ies of road - stone s izes are avai lable . This Repor t s u m m a r i z e s the r e su l t s obtained f rom road t r i a l s with it and d i s c u s s e s methods whereby other aggrega te s might be p roduced with s imi l a r o r still be t t e r p rope r t i e s at l e s s cost .

1. INTRODUCTION

As traffic continues to inc rease in volume and speed it is e s s e n t i a l to ensure that roads a re adequately r e s i s t a n t to skidding. One development toward this end has been the product ion of specia l bituminous binders which a re designed to erode away in a cont ro l led manner ensuring that the aggregate r e m a i n s exposed thus mainta in ing a des i rable depth of surface texture. Neve r the l e s s , it is equal ly important to ensure that the aggregate (which m a k e s up the g r e a t e r par t of any surfacing) is of a type :which does not pol i sh unduly under the action of t raff ic .

The dependence of the r e s i s t ance to skidding of a road su r face on the polishin~ c h a r a c t e r i s t i c s of the roads tone was s tudied by Shergold et al i and appara tus was devised which enabled the degree to which a given stone would polish On the road to be a s s e s s e d in the labora tory 2. The resul tant 'Pol ished Stone Value ' (P. S. V. ) was determined for a very la rge number of roads tones and Fig , 1 reproduces the resul t s .

Simultaneously Giles, Sabey et al we re inves t iga t ing the relat ionship between skidding accidents and the s l i p p e r i n e s s of road surfaces and a r r ived at the conclusion that Certain l i m i t s were d e s i r a b l e for the 'Sideway' Force Coefficient ' (S. F. C. ) on roads if the r i s k of skidding was to be reduced to an acceptably low level 3, 4.

F r 0 m a c o r r e l a t i o n of t h e s e two l i ne s of r e s e a r c h the f ac t e m e r g e d t h a t few r o a d s t o n e s a r e a v a i l a b l e which will enab le the d e s i r a b l e S. F . C. s to be m a i n t a i n e d t h r o u g h o u t the y e a r at the m o s t d i f f icul t s i t e s i . e . o n s h a r p bends , s t e e p g r a d i e n t s , e tc . 5, 6.

M e a n w h i l e , it e m e r g e d tha t a c e r t a i n g r a d e of bauxi te , which h a d b e e n h e a t e d to a t e m p e r a t u r e of abou t 1 , 6 0 0 o c , and which w a s n o r m a l l y s o l d a s an a g g r e g a t e f o r r e f r a c t o r y f i r e b r i c k s , had a P. S .V . h i g h e r t h a n tha t of any n a t u r a l r o a d s t o n e ye t t e s t e d . It i s r e l a t i v e l y expens ive a n d s u p p l i e s a r e l i m i t e d but it i s now thought tha t new m a t e r i a l s wi th s i m i l a r o r even b e t t e r c h a r a c t e r i s t i c s migh t be m a d e in l a r g e q u a n t i t i e s a t l o w e r c o s t .

T h i s R e p o r t d e s c r i b e s the w o r k tha t h a s b e e n done with ca l c ined baux i t e , s u g g e s t s l i n e s w h i c h m i g h t be fo l lowed to p r o d u c e a l t e r n a t i v e s , a n d g i v e s the r e s u l t s of s o m e e x p l o r a t o r y work .

2. C A L C I N E D BAUXITE

2 . 1 The C o l n b r o o k B y - p a s s e x p e r i m e n t

In 1959 t r i a l a r e a s of s u r f a c e d r e s s i n g w e r e la id on the C o l n b r o o k B y - p a s s ( T r u n k Road A. 4) u s ing epoxy r e s i n a s the b i n d e r a n d v a r i o u s 1 / 8 in g r i t s a s the a g g r e g a t e . B e c a u s e the r e s i n had a g l a s s y t e x t u r e and w a s not e x p e c t e d to w e a t h e r away , it was c o n s i d e r e d d e s i r a b l e to u s e a g g r e g a t e s w h i c h w e r e a s p o l i s h - r e s i s t a n t a s p o s s i b l e . T w e l v e a g g r e g a t e s w e r e c o m p a r e d , s ix of which w e r e n a t u r a l r o a d s t o n e s and s i x w e r e m o r e expens iv e a r t i f i c i a l m a t e r i a l s s u c h a s s i l i c o n c a r b i d e a n d c o r u n d u m . One o f the a r t i f i c i a l a g g r e g a t e s s e l e c t e d f o r the t r i a l w a s D e m e r a r a baux i t e which had been c a l c i n e d to a h i g h t e m p e r a t u r e : it w a s only one q u a r t e r the p r i c e of c o r u n d u m but it w a s hoped tha t i t s h igh ly a l u m i n o u s cons t i t u t i on might give it s i m i l a r p r o p e r t i e s .

The s k i d - r e s i s t a n c e of the t r i a l a r e a s w a s m e a s u r e d r e g u l a r l y wi th the p e n d u l u m a p p a r a t u s and an i n t e r i m r e p o r t on the r e s u l t s i s s u e d in 1960 7. The e x p e r i m e n t cont inued f o r 4 y e a r s (unt i l t he r o a d w a s r e s u r f a c e d by the County a s p a r t of i t s r e g u l a r ~ m a i n t e n a n c e p r o g r a m m e ) and s o m e of the r e s u l t s ob ta ined o v e r t h i s p e r i o d a r e shown g r a p h i c a l l y in F i g . 2a . The c a l c i n e d baux i te w a s c o n s i d e r a b l y b e t t e r t h a n the b e s t n a t u r a l r o a d s t o n e u s e d in the t r i a l d e s p i t e the f a c t t h a t t h e l a t t e r had b e e n d e l i b e r a t e l y chosen to be r e p r e s e n t a t i v e of t he b e s t a v a i l a b l e . The P. S .V . of c a l c ined baux i t e

2

P . _ J

was m e a s u r e d and found to be 75*, h i g h e r than tha t Of any n a t u r a l l y o c c u r r i n g s tone yet t e s t ed .

It was dec ided to obtain c o n f i r m a t o r y e v i d e n c e of i t s r o a d p e r f o r m a n c e in m o r e convent iona l t y p e s of s u r f a c i n g .

2 .2 The B lackbushe E x p e r i m e n t

In 1962 an e x p e r i m e n t was begun on T r u n k Road A. 30 a t B lackbushe , H a m p s h i r e 8 to c o m p a r e the p e r f o r m a n c e of a n u m b e r of r oads tone§ in b i t uminous s u r f a c i n g m i x t u r e s and in t a r a n d b i t u m e n s u r f a c e d r e s s i n g s : c a l c ined bauxi te w a s inc luded . The t e s t s e c t i o n s in th i s e x p e r i m e n t w e r e d e l i b e r a t e l y m a d e long enough to p e r m i t S . F . C . m e a s u r e m e n t s to be m a d e a s wel l a s p e n d u l u m m e a s u r e m e n t s . The r e s u l t s of the S. F . C . m e a s u r e m e n t s a r e shown in F i g s . 2b and 2c. Again the ca l c ined bauxi te h a s p roved to be c o n s i d e r a b l y b e t t e r t han the bes t n a t u r a l r o a d s t o n e .

2 .3 The d i s a d v a n t a g e s of D e m e r a r a c a l c i n e d b a u x i t e

The c r u s h i n g s t r e n g t h of the D e m e r a r a c a l c i n e d b a u x i t e = used in t h e s e e x p e r i m e n t s is r e l a t i v e l y low. It w a s f e a r e d at f i r s t tha t th i s migh t c a u s e it to w e a r away p r e m a t u r e l y but f r o m the e x p e r i e n c e ga ined so f a r th i s would not s e e m to be so. It h a s a v e r y good A g g r e g a t e A b r a s i o n Value and t h i s a p p a r e n t l y c o m p e n s a t e s f o r • the low c r u s h i n g s t r e n g t h to s o m e ex ten t in r e s i s t i n g t r a f f i c w e a r . However , it does have two d i s a d v a n t a g e s wh ich l i m i t i t s adop t i on g e n e r a l l y ; t he se a r e i t s high cos t r e l a t i v e to tha t of n a t u r a l s tone a n d i ts l imi t ed ava i l ab i l i t y .

The r a w bauxi te , a f o r m of c l ay , i s m i n e d and c a l c i n e d in Guyana: i t is then shipped a c r o s s the A t l a n t i c and i s a v a i l a b l e a t the d o c k s he re , a t about £15 p e r ton,, a s a m i x t u r e o f s m a l l l u m p s a n d d u s t , mainly the l a t t e r . A f t e r g r a d i n g by i n t e r m e d i a t e a g e n t s the p r i c e of s i n g l e - s i z e d m a t e r i a l r i s e s to about £30 p e r ton. The r e a s o n s f o r th is a r e p a r t l y the handl ing c o s t s and p a r t l y t he v e r y s m a l l y i e ld of m a t e r i a l in the s i z e s r e q u i r e d f o r r o a d w o r k i . e . 3 / 8 in and l a r g e r .

Mos t of the P. S .V . m e a s u r e m e n t s in t h i s w o r k w e r e m a d e b e f o r e 1965. In tha t y e a r the me thod of t e s t w a s a m e n d e d 2 w i th the r e s u l t tha t c u r r e n t v a l u e s a r e about 10 p e r c e n t l o w e r t h a n b e f o r e . Throughout th i s R e p o r t the v a l u e s h a v e b e e n a d j u s t e d a n d the r e v i s e d 1965 equ iva l en t s q u o t e d

3

2 . 4 ~ O t h e r b a u x i t e s and h igh ly a l u m i n o u s c l a y s

The D e m e r a r a baux i t e , c a l c i n e d to about 1, 600°C, is p r o d u c e d a s an a g g r e g a t e f o r r e f r a c t o r y work and f o r tha t p u r p o s e the v e r y h igh a l u m i n a con ten t and the low a m o u n t s of d e l e t e r i o u s c o n s t i t u e n t s a r e c a r e f u l l y c o n t r o l l e d . F o r r o a d s t o n e p u r p o s e s it i s p o s s i b l e t h a t l e s s p u r e f o r m s of b a u x i t e might be adequa t e . Al though t h e r e a r e m a n y c h e a p e r b a u x i t e s and f i r e c l a y s on the m a r k e t , none of t h e m is a v a i l a b l e in a h ighly c a l c i n e d f o r m su i t ab le fo r i m m e d i a t e t e s t i n g a s r o a d s t o n e . Some of t h e s e g r a d e s , i m p o r t e d into the Uni ted K i n g d o m f o r a l u m i n i u m e x t r a c t i o n , h a v e high i ron c o n t e n t s and o t h e r s h a v e h igh s i l i c a c o n t e n t s ( see T a b l e 1). They cos t b e t w e e n £6 and £10 p e r ton.

A s a r e s u l t of s o m e e n q u i r i e s a few s a m p l e s of c a l c i n e d c l a y s w e r e o b t a i n e d and t e s t e d but , a s c a n be seen f r o m the r e s u l t s g iven in T a b l e 2, no th ing of v a l u e e m e r g e d . F u r t h e r w o r k c o v e r i n g a w ide r r a n g e of m a t e r i a l s , t e m p e r a t u r e s and t i m e s of hea t ing i s n e e d e d to e s t a b l i s h w h e t h e r good p o l i s h - r e s i s t a n t s t ones of adequa t e s t r e n g t h c a n be o b t a i n e d f r o m o t h e r b a u x i t e s o r c l a y s b y s i m p l e hea t ing .

4

TABLE 1

Typical ana lyses of bauxite and o ther c l ay s

Type

Bauxite Bauxite Bauxite Bauxite Bauxite Bauxitic f i rec lay

Chinaclay F i rec lay F i rec lay F i rec lay F i r ec l ay F i r ec l ay Marl Ganis ter Ganis te r Dinas Quartzi te

Dis t r i c t of or igin

Guyana (Demerara)

Alumina

67

Constituents

Silica

3

I ron oxide

1.5

W a t e r

27 .5

O t h e r s

F r a n c e ( l ) Ghana N. I re land France(2)

Glenboig(1) St. Austel l Glenboig(2) Derby Ches te r Leeds Dowlais Staffordshire Sheffield La i s t e rdyke Glamorgan U.S.A.

60 57 52 52

40 40 34 36 28 20 21

16-30

1.5 1 1

.

1.5 i0 27

40 46 47 52 59 71 67

63-76 89 97 98 98

24 13

8 0 . 5

3 m

2 2 3 1 .5 1 .5 3 3 1 1 1

12 27 25 17.5

13 14 15 10 I0 6 7

5-I0

I 1.5 5 3

4

2

1.5 3 . 5 1-3 4 0 .5

Note: This Table was compiled f rom in format ion given by

Fox 9, Knibbs 10, Searle 11 and individua ! ;Suppliers

/ "

5

\

TABLE 2

R e s u l t s of po l i sh ing t e s t s on some ca lc ined c l ays

• Type

Bauxi te (Guyana)

Bauxi te (Guyana)

Bauxi te (Ghana)

C h i n a c l a y (St. Aus te l l )

Baux i t i c f i r e c l a y (Glenboig)

F i r e c l a y (Glenboig)

A l u m i n i u m r e f i n e r y was t e

Stated t e m p e r a t u r e

of c a l c i n a t i o n oc

1, 600

1, 300

1, 550

Not known

1, 400

1, 320

1, 200

Stated typ ica l composi t ion

A lumina

88

88

75

43

40

35

8

Si l ica

7

7

2

54

50

57

Iron Oxide

2

2

20

4

3

21

Others

3

3

3

2

6

5

68 (Mainly CaO)

Polished stone value

70-78*

Too weak to test

Too weak to tes t

41

51

44

55

* Note: It i s p o s s i b l e to so r t D e m e r a r a Calcined bauxite into f r ac t ions with n o t i c e a b l y d i f f e r en t appea rance . Tes t ing these d i f ferent f r a c t i o n s p roduced the range of r e s u l t s quoted. The P. S.V. of the m i x e d m a t e r i a l a s suppl ied i s about 75

3. POLISH-RESISTANT STONES : SOME THEORETICAL MODELS

The d i s c o v e r y that an agg rega t e p roduced for the r e f r a c t o r y indus t ry is of va lue as a n o n - p o l i s h i n g m a t e r i a l in ano ther indus t ry unde r l ines the r e c u r r e n t ques t ion , "What a r e the f a c t o r s which cause some m a t e r i a l s to r e m a i n n o n - s k i d ? " . If they could be i so la ted then it should be p o s s i b l e to p r o d u c e d e l i b e r a t e l y s tones with the d e s i r e d c h a r a c t e r i s t i c s .

F r o m c e r t a i n known e x a m p l e s it i s p o s s i b l e to outline some idea l ized t y p e s of m a t e r i a l which would be l i k e l y to p e r f o r m well. F ive of these a r e i l l u s t r a t e d in F ig . 3.

6

3.1 E x t r e m e l y h a r d m a t e r i a l s ( F i g . 3 . 1 )

The s i m p l e s t concep t of a n o n - p o l i s h i n g s tone i s a n a n g u l a r shape with a s h a r p point u p p e r m o s t c o n s i s t i n g of a u n i f o r m m a t e r i a l of e x t r e m e h a r d n e s s . In th i s con tex t the w o r d h a r d i s u s e d in the s e n s e e x p r e s s e d by M o h ' s Sca le : it m u s t not be c o n f u s e d with h igh r e s i s t a n c e to c r u s h i n g which i s an unconnec ted p r o p e r t y . F o r i n s t a n c e , m a t e r i a l s such a s fused a l u m i n a and s i l i con c a r b i d e c o n s i s t of a m a s s of c r y s t a l s which a r e indiv idual ly v e r y h a r d but a r e w e a k l y bonded t o g e t h e r and thus have a l ow c r u s h i n g s t r e n g t h . On the o t h e r hand s o m e s t o n e s wi th high c r u s h i n g s t r eng th , such a s h o r n f e l s , m a y p o l i s h r e a d i l y on the road .

Table 3 l i s t s a n u m b e r of m i n e r a l s and t h e i r h a r d n e s s on Moh ' s Sca le t o g e t h e r with t h e i r Knoop H a r d n e s s . The Knoop T e s t invo lves indenta t ion by a s p e c i a l l y cut d i a m o n d and e n a b l e s m o r e a c c u r a t e g r a d u a t i o n to be obta ined .

The h a r d e s t known m a t e r i a l is d i a m o n d . Undoubtedl .y, d i a m o n d s with su i t ab ly cut f a c e t s c o r r e c t l y o r i e n t a t e d would m a k e v e r y good r o a d s t o n e s if they w e r e r e a d i l y a v a i l a b l e in l a r g e s i z e s . The next h a r d e s t m a t e r i a l s in c o m m o n u s e a r e the a r t i f i c i a l a b r a s i v e s m a d e in an e l e c t r i c f u r n a c e , s i l i con c a r b i d e and a l u m i n i u m oxide (corundum) . C o m p a r e d with d i amond they a r e r e l a t i v e l y c h e a p a t about £100 p e r ton but they a r e s t i l l too e x p e n s i v e f o r r o a d u s e . T h e y have a n o t h e r d i s a d v a n t a g e , in c o m m o n wi th d i a m o n d , t ha t t h e y a r e only n o r m a l l y a v a i l a b l e in g r a i n s i z e s up to abou t 1 / 8 in.

F u r t h e r down the s c a l e c o m e the c h e a p e r a b r a s i v e s such a s e m e r y (a f o r m of c o r u n d u m con ta in ing i ron) and g a r n e t w h i c h o c c u r n a t u r a l l y in some p a r t s of the wor ld . T h e i r c o s t i s s t i l l high, h o w e v e r , c o m p a r e d to that of r o a d s t o n e s and they a r e a l s o only a v a i l a b l e i n s m a l l g r a i n s i ze s .

F ina l ly we c o m e to the m i n e r a l s of wh ich m o s t n a t u r a l r o a d s t o n e s a r e c o m p o s e d . T h e s e a r e not g e n e r a l l y h a r d enough to m a i n t a i n s h a r p e d g e s u n d e r the po l i sh ing wh ich o c c u r s on the r o a d .

7

TABLE 3

H a r d n e s s of v a r i o u s a b r a s i v e s and m i n e r a l s

M i n e r a l

D i a m o n d Boron c a r b i d e S i l i con c a r b i d e A l u m i n i u m oxide ( co rundum) T u n g s t e n c a r b i d e Topaz S i l l i m a n i t e C o r d i e r i t e Q u a r t z G a r n e t g roup O l i v i n e g roup Ep ido t e g roup C h a l c e d o n y F e l d s p a r ( o r t h o c l a s e ) P y r o x e n e group A m p h i b o l e g roup Apa t i t e Ze01 i t e s F l u o r i t e D o l o m i t e C a l c i t e G y p s u m T a l c

H a r d n e s s

M o h ' s Scale

10

9

8 7 .5

7 - 7 . 5 7

6 . 5 - 7 . 5 6 . 5 - 7

6-7 6

6 - 6 . 5 5-7 4 - 6 . 5

5 3 . 5 - 5 . 5

4 3 . 5 - 4

3 2 1

I~mop n u m b e r

7, 000 2 ,900 2 ,500 2 ,000 1, 880

1, 400-1, 500

800-900

600-700

500

200

130 5O 2O

Note: T h i s T a b l e was c o m p i l e d f r o m i n f o r m a t i o n g iven by T h i b a u l t and N y q u i s t 12, K n i l l l 3 and Gi l l and Spence 14

3 . 2 C o n g l o m e r a t i o n s of s m a l l h a r d p a r t i c l e s (Fig. 3 .2 )

R e f e r e n c e h a s a l r e a d y been m a d e to two m i n e r a l s of t h i s type , f u s e d a l u m i n a and s i l i c o n c a r b i d e . As ob ta ined f r o m the e l e c t r i c f u r n a c e t h e y c o n s i s t of m a s s e s of s m a l l c r y s t a l s but they a r e i n s u f f i c i e n t l y c o h e r e n t fo r u s e a s r o a d s t o n e s . When u s e d a s a b r a s i v e s t h e y a r e b r o k e n down into s m a l l p a r t i c l e s which a r e bonded toge the r w i th s y n t h e t i c r e s i n s to m a k e g r i n d s t o n e s . Subs tan t ia l q u a n t i t i e s of b r o k e n g r i n d s t o n e s a r e a v a i l a b l e and w e r e t r i e d a s an a g g r e g a t e in the C o l n b r o o k B y - p a s s e x p e r i m e n t . However , the r e s i n o u s bond was too weak and the m a t e r i a l r a p i d l y w o r e away .

8

N a t u r a l l y o c c u r r i n g e x a m p l e s which m a y be c i t e d a r e s a n d s t o n e s and g r i t s t o n e s . In t h e s e the ind iv idua l p a r t i c l e s c o n s i s t of m i n e r a l s of i n t e r m e d i a t e h a r d n e s s , such a s q u a r t z , which a r e bonded t o g e t h e r by a so f t e r s i l i c eous o r c a l c a r e o u s m a t e r i a l . Q u a r t z i s not a s h a r d a s the a b r a s i v e s , but p r o v i d i n g the bond i s weak enough to a l low s low a t t r i t i on to o c c u r u n d e r t r a f f i c , such s t o n e s c a n m a i n t a i n a h a r s h t e x t u r e b e c a u s e of the cont inua l e x p o s u r e of f r e s h p a r t i c l e s . C l e a r l y , howeve r , the bonding m e d i u m m u s t be n e i t h e r too w e a k (or the s tone wil l a b r a d e too r ap id ly ) no r too s t r o n g (or the s tone wil l po l i sh ) . As is to be expec ted , s a n d s t o n e s and g r i t s t o n e s a r e found to r a n g e f r o m one e x t r e m e to the o t h e r 15. In the m i d d l e i s a g r o u p of good p o l i s h - r e s i s t a n t s t ones su i t ab l e fo r r o a d u s e but the n u m b e r of t y p e s i s s m a l l .

3 .3 D i s p e r s i o n s of h a r d p a r t i c l e s in a s o f t e r m a t r i x (F ig . 3 . 3 )

In th i s type the binding m a t e r i a l p r e d o m i n a t e s o v e r t he a b r a s i v e p a r t i c l e s . Idea l ly the p a r t i c l e s would be v e r y h a r d but , p rov id ing tha t the m o r t a r e roded away at a s low r a t e e x p o s i n g f r e s h p a r t i c l e s occas iona l l y , the g r i t would not need to be of e x t r e m e h a r d n e s s . I m p e r f e c t e x a m p l e s of th i s c l a s s a r e the r a g s t o n e s o r g r i t t y l i m e s t o n e s : The m a j o r i t y of l i m e s t o n e s po l i sh r e a d i l y but t h e r e a r e s o m e wh ich conta in a s m a l l p e r c e n t a g e of s i l i c e o u s g r i t (up to about 15 p e r cent ) and t h e s e have m a r k e d l y b e t t e r P. S. V. s ( see F ig . 1). On the o t h e r hand, in c o m m o n with m o s t l i m e s t o n e s they tend to have low a b r a s i o n r e s i s t a n c e and a r e not r e a l l y su i t ab le f o r h e a v y duty .

3 . 4 M a t e r i a l s which f r a c t u r e in an i r r e g u l a r m a n n e r (F ig . 3 . 4 )

Some s tones tend to f lake in such a m a n n e r t h a t t h e i r edges round off and they b e c o m e smoo th . F l i n t s , f o r i n s t a n c e , h a v e low P . S . V s . When f l in t s have been c a l c i n e d to a b o u t 1, 000°C, h o w e v e r , t h e i r cons t i tuen t s i l i ca u n d e r g o e s a c h a n g e f r o m a type w h i c h i s f o r p r a c t i c a l p u r p o s e s a m o r p h o u s to a c r y s t a l l i n e type and t h e y b e c o m e r idd led with i r r e g u l a r c r a c k s and weak p l a n e s . The P. S . V . r i s e s c o n s i d e r a b l y in the p r o c e s s (see F ig . 1). T h i s m a y be due in p a r t to a change in the h a r d n e s s of the s i l i c a but it i s p r o b a b l y a l s o due l a r g e l y to the new mode of f r a c t u r e which it adop t s , the f o r m e r conchoida l f lak ing giving way to i r r e g u l a r c r y s t a l l i n e b r e a k s . The c r u s h i n g s t r e n g t h is a d v e r s e l y a f f ec t ed but not s e v e r e l y enough to p r e v e n t i t s use as a r o a d s t o n e .

3 . 5 V e s i c u l a r m a t e r i a l s (Fig. 3 .5 )

V e s i c u l a r r o c k s a r e t h o s e c o n t a i n i n g n u m e r o u s s m a l l c a v i t i e s and m a y be exempl i f i ed by c e r t a i n b l a s t - f u r n a c e s l a g s . The p r e s e n c e of a high p r o p o r t i o n of v e s i c u l a r s l ag in a b a t c h o f t en i m p r o v e s i t s P . S . V . but r e d u c e s i t s c r u s h i n g s t r e n g t h . In p r a c t i c e , the l a t t e r d i s a d v a n t a g e ou twe ighs the i m p r o v e m e n t and t h e r e h a s b e e n a t endency for v e s i c u l a r s l a g s to d i s a p p e a r f r o m the r o a d s t o n e f i e ld in r e c e n t y e a r s .

9

4. NATURAL ROADSTONE S

Most n a t u r a l l y o c c u r r i n g roads tones conta in a mix tu re of s eve ra l m i n e r a l s and m a y be c l a s s i f i e d as compounds of types 3.2 and 3 .3 . Unfo r tuna te ly the m i n e r a l s of which they a re composed tend to be of too s i m i l a r h a r d n e s s to each other for d i f f e ren t i a l wear to take place , o r of too f ine a g r a i n s ize for such wea r to produce a h a r s h texture , o r of i n s u f f i c i e n t h a r d n e s s to r e s i s t po l i sh ing by road dust and t y r e s . A de t a i l ed p e t r o g r a p h i c a l examina t ion of na tu ra l rocks with high P. S. Vs. was m a d e by Kni l l 13.

Befo re t u r n i n g to a r t i f i c i a l s tones it i s worth cons ide r ing one way in w h i c h n a t u r a l s tones might be m o d i f i e d to improve the i r p o l i s h r e s i s t a n c e .

4.1 P o s s i b l e i m p r o v e m e n t by heat ing

The change obta ined in f l in t by ca lc in ing has a l r eady been noted. The P. S .V. r i s e s f r o m about 35 to 55 although the c rush ing s t r eng th d rops . It i s p robab le that mos t rocks will undergo a s i m i l a r change with hea t al though g e n e r a l l y to a l e s s e r extent.

An inves t i ga t i on into the effect of heat on the c rush ing s t r e n g t h of r o c k s was made at the L a b o r a t o r y some y e a r s ago as pa r t of a study on a g g r e g a t e - d r y i n g p r o c e s s e s 16. Table 4 r ep roduces some of the r e s u l t s and it can be seen that a l l of the rocks tes ted were a f f ec t ed by hea t ing to 1 ,000°C for two minu tes .

It i s p roposed to c a r r y out f u r t h e r work to see what the c o r r e s p o n d i n g ef fec t is on the P. S. V.

10

TABLE 4

Resu l t s of c rush ing and impact t e s t s on a g g r e g a t e s before and a f te r hea t ing to 1 ,000°C for 2 m i n u t e s

Aggregate type

C r u s h i n g : t e s t r esu l t ~

Imp'act - t es t re sult*

Before Af te r Before Af te r

6 .7 37 .4 19..6 16.1 16.5

Hornfels 5.9 Granite 23.3 Fl int g rave l 7 .4 Slag 19.1 Quar tz -do le r i t e 13.5

10. 12.

7. 9.

11. 10.

7.

37. 15. 21. 19.

1 4. 4 21. 9 9. 7 13. 0 I I .

6 7 7 2 0

11.3 14.2

1 8 . 6 19 .0 17.5 15.2

Basalt Sandstone Granophyre Ol iv ine-do le r i t e L imes tone Quartzi te

4 11. 0 13. 9 18. 2 20. 7 23. 9 14.

4 6 9 8 2 1

8 . 3 12.7

8.3 8 .8

10.3 13 .0

The tes t s were c a r r i e d out in a c c o r d a n c e with the me thods desc r ibed in B.S. 812. The v a l u e s r e p r e s e n t the p e r c e n t a g e

1 of f ines formed when a s tandard sample of ~ inch ch ipp ings is c rushed or impac ted under s t andard condi t ions : thus the h igher the value the weaker the stone

5. ARTIFICIAL ROADSTONES

In setting out to manufac tu re a non-pol i sh ing stone the mos t p romis ing s ta r t ing-poin t would appear to l ie be tween Types 3 .2 and 3.3. For the g r a n u l a r p a r t i c l e s in e i t he r type t h e r e i s a var ie ty of ab ras ive g r i t s ava i lab le the most a t t r a c t i v e of which i s probably the compara t ive ly plentiful , s m a l l - s i z e d , c a l c ined bauxi te . Because the gri t is l ike ly to be more expens ive than the m a t r i x or bonding medium, Type 3.3 (in which the l a t t e r p r e d o m i n a t e s ) i s p r e f e r r e d . The r e m a i n i n g dec is ion i s the choice of the bes t m a t r i x . It must be cheap, ea s i l y compounded with the gr i t , and have a s trength such that it w e a r s away u n d e r t ra f f ic at the op t imum ra te . Several poss ib le m a t e r i a l s a re d i s c u s s e d below and e x a m i n a t i o n of some of them has a l r e ady begun.

11

5.1 C e r a m i c s

C e r a m i c p r o c e s s e s a r e one obvious choice. Clay is p l en t i fu l and cheap and, by va ry ing the heating p r o c e s s and the added f luxes , it i s p o s s i b l e to p roduce m a t e r i a l s ranging f r o m weak porous b r i c k s to dense p o r c e l a i n s with a c ru sh ing s t rength of 50, 000 l b s / sq. in. By us ing c h i n a c l a y s white aggrega te s can be produced which have addi t ional advan tages in some s i tua t ions .

At the beg inning of th is work it was r e a l i s e d that f i r e b r i c k s have a somewhat s i m i l a r compos i t ion to the theore t i ca l ideal: they a r e a m i x t u r e of v a r i a b l e p a r t i c l e s and f requent ly contain calc ined baux i te . Samples were obtained f rom va r ious sou rces and tested with the r e s u l t s shown in Table 5.

As one would expect with b r i ck s , the c rush ing s t rengths tend to be low, i but some v e r y high P. S. Vs. were found which conf i rm the va lue of t h i s l ine of r e s e a r c h . No fu r the r work is being c a r r i e d out on f i r e b r i c k s because of the high p r i c e of the bes t examples (up to £40 p e r ton) and the r e l a t i v e l y low s t rengths , but r e s e a r c h is p lanned on spec i a l l y compounded m i x t u r e s .

5 .2 G l a s s e s

F o r a n u m b e r of y e a r s now an a r t i f i c i a l aggregate cons is t ing of a f o r m of d e v i t r i f i e d g l a s s has been marke ted . Th i s ma te r i a l , "Synopal" , i s p roduced p r i m a r i l y as a white aggregate for use where l igh t colour i s r equ i r ed . Samples tes ted at the Labora to ry have had an ind i f f e r en t P. S.V. (40 to 49) but it i s p robab le that by adding a sui table a b r a s i v e gr i t to it du r ing manufac tu re th i s could be r emed ied . The m a n u f a c t u r i n g p r o c e s s is fu l ly d e s c r i b e d in the i n v e n t o r ' s patent 17.

Synopal i s of g rea t i n t e r e s t a s it has demons t ra t ed that the produc t ion of a r e a s o n a b l y p r i c e d roads tone i s a c o m m e r c i a l p r a c t i c a l i t y . Although the manu fac tu r e involves a double heating p r o c e s s and the p lant cos t s £500, 0 0 0 1 8 , s eve r a l p l an t s have been bu i l t and the aggrega te i s sold e x - w o r k s at about £6 p e r ton.

12

TABLE 5

Resu l t s of t es t s for pol ishing, c rush ing s trength, and abras ion r e s i s t ance on some c rushed f i r e b r i c k s

Type of br ick

t t , t

, , , !

t , , t

, t , !

Mossi te br ick

Silica br ick (unidentified) Scottish f i r ec l ay br ick ("Nettle 1")

" " " ("Dougall") Sil ica br ick (Pearson Kaolith 1500) Silica br ick (Morgan HT Chinaclay) Scottish f i r ec l ay br ick ("Nettle 2") Bauxite br ick (Stein 70)

" " (Stein 73) (Stein 84P) (Alumantine 55) (Alumantine 63) (Alumantine 72)

Calcined D e m e r a r a bauxite for compar i son

Po l i shed Stone Value

72 67 63 60 58 58 83 78 87 84 88 87 72

75

C rush ing Strength

( l b s / s q . in)

8 ,800 10, 500 2 ,500 8,500 5, 500

26,100 10, 000 13,000

1 1 , 0 0 0

Aggrega te Abrasion:

Value (lowest bes t )

21 5

19 12

8 12 12

6 8

36 38

5 14

4

Note: For compar i son of these va lues with those of na tu ra l roads tones see Fig. 1 and R. R . L . Road Note No. 24 15

13

5 .3 W a s t e P r o d u c t s

T h e r e a r e m a n y w a s t e m a t e r i a l s or cheap b y - p r o d u c t s f r o m min ing and o the r i n d u s t r i a l p r o c e s s e s which migh t be t r e a t e d to m a k e su i t ab l e m a t r i c e s .

M a t e r i a l s of ou t s tand ing po ten t i a l a r e the s l a g s ob ta ined f r o m m e t a l - e x t r a c t i o n p r o c e s s e s which a r e p r o d u c e d in an a l r e a d y m o l t e n f o r m and would t hus r e q u i r e no f u r t h e r heat ing .

L iqu id s l a g s a r e not ob ta ined f r o m the e x t r a c t i o n of a l u m i n i u m , but m i l l i o n s of tons of w a s t e in the f o r m of a c a l c a r e o u s mud a r e ob ta ined each y e a r and a r e at p r e s e n t dumped. Until r e c e n t l y a s i m i l a r s i tua t ion ex i s t ed with the fly a s h obta ined f r o m l a r g e m o d e r n f u r n a c e s , but s e v e r a l u s e s f o r th is have now been found, one of which is in the m a n u f a c t u r e of l igh tweight a g g r e g a t e s . Mos t m a t e r i a l s of th i s type a r e c a p a b l e of being s i n t e r e d to f o r m a g g r e g a t e s and, given su i t ab le t r e a t m e n t migh t wel l y i e l d s u c c e s s f u l r o a d s t o n e s .

5 . 4 Soluble s i l i c a t e s

One of the f i r s t u s e s f o r sod ium s i l i ca t e ( " w a t e r g l a s s " ) when it w a s c o m m e r c i a l l y deve loped in the e a r l y n ine teen th c e n t u r y w a s in the m a n u f a c t u r e of a r t i f i c i a l s a n d s t o n e s fo r bui lding p u r p o s e s . The a r t c o n s i s t e d of m ix ing su i t ab le f ine a g g r e g a t e s and f i l l e r s with ~he s i l i c a t e and then c o n v e r t i n g the l a t t e r to a w a t e r insoluble m a t e r i a l by the u s e of v a r i o u s a g e n t s a n d / o r heat . Soluble s i l i c a t e s w e r e l a t e r o u s t e d a s b i n d e r s fo r a r t i f i c i a l bui ld ing s t ones by P o r t l a n d c e m e n t which w a s m o r e r e l i a b l e . Sodium s i l i ca te , howeve r , is a c h e a p m a t e r i a l a n d m a y j u s t i f y r e - e x a m i n a t i o n a s it p r o v i d e s a p r o c e s s not involv ing h igh t e m p e r a t u r e s ; t h i s m e a n s tha t if co lou red a g g r e g a t e s w e r e r e q u i r e d c h e a p e r p i g m e n t s than those r e q u i r e d fo r c e r a m i c p r o c e s s e s cot/ld be u s e d . F u r t h e r m o r e , with p r o c e s s e s which can be c a r r i e d out at t e m p e r a t u r e s be low the m e l t i n g point of g l a s s ,

~ t m i g h t be p o s s i b l e to i n c o r p o r a t e ba l lo t in i in the m i x t u r e and thus p r o d u c e a r e t r o r e f l e c t i v e a g g r e g a t e .

5 . 5 H y d r a u l i c c e m e n t s

A r t i f i c i a l bu i ld ing s t o n e s a r e s t i l l m a n u f a c t u r e d with P o r t l a n d c e m e n t a s the b inding m e d i u m . It is doubtful , however , a r o a d s t o n e could be p r o d u c e d by th i s m e a n s which would have a d e q u a t e s t r e n g t h and a b r a s i o n r e s i s t a n c e .

if

14

5.6 Plastics

Pigmented p l a s t i c s have been used to make g r a n u l e s fo r rol l ing into the sur face of asphal t (19). The cost of p l a s t i c s (£100- £500 pe r ton) is too high to make them the b a s i s of r o a d s t o n e s fo r more widespread use. However, if in the fu ture l a rge quan t i t i e s of was te p l a s t i c s become ava i l ab le at lower p r i c e s , they would have to be recons idered .

6. CONCLUSIONS

D e m e r a r a bauxite ca lc ined to about 1 ,600oc has been found to have a P. S.V. be t te r than that of any n a t u r a l l y - o c c u r r i n g roads tone so f a r tested. However, it i s r e l a t i ve ly expensive , p a r t i c l e s l a r g e r than 1 i n a re in short supply, and it has a r e l a t i v e l y low c r u s h i n g strength.

Cons idera t ion of the f ac to r s which affect the P. S.V. l e a d s to the conclusion that it should be pos s ib l e to manu fac tu r e a g g r e g a t e s with s t i l l h igher P. S. Vs . , with h i g h e r s t r eng th s if n e c e s s a r y , probably at a lower p r i c e . The most p r o m i s i n g app roach i s to incorporate a suitable h a r d gr i t in a somewhat so f te r m a t r i x . Work has a l r eady begun to inves t iga te the i nco rpo ra t ion of f ine c a l c i n e d - bauxite gr i t into va r ious m a t r i c e s such a s b l a s t - f u r n a c e s lag and c e r a m i c s .

It i s a l so poss ib le that the P. S.V. of na tu ra l r o a d s t o n e s m i g h t be improved by heat t r e a t m e n t . Work ha s been s t a r t ed to see if a s ignif icant improvemen t can be ach ieved without p roduc ing any harmfu l s ide -e f fec t s such as grea t r educ t ion in c r u s h i n g s t rength .

15

.

.

3.(a)

(b)

.

.

.

.

.

,

10.

7. REFERENCES

MACLEAN D. J . and F. A. SHERGOLD. The polishing of r0adstone in re la t ion to the res i s tance to skidding of bituminous road surfacings;~ Road Research Labora tory Technical Paper No. 43, H . M . S . O . , London 1958

BRITISH STANDARDS INSTITUTION. Bri t ish Standard 812:1960. Methods for sampling and test ing of mineral aggregates ; and Amendment No. 3 Revised determinat ion of the Polished Stone Value; London, 1965, Bri t i sh Standards Institution.

ROAD RESEARCH LABORATORY. Research on road safety (Chapter 14, The road surface); H . M . S . O . , London, 1963

GILES, C. G. The skidding res i s tance of roads and the r equ i r emen t s of modern t raff ic; Proc. Inst. Civ. Engrs, 6, 216-249

1957,

SHERGOLD, F. A. Summary of information on the S. F. C. va lues achieved with s tones of known P. S. V. , Road Research Labora to ry Technical Note No. 22, !965, unpublished.

HOPKINS, L. C. The p rob lems of providing aggregates for road surfacing m a t e r i a l s which have good non-skid proper t ies ; Journ. Inst. Highw. Engrs , 1959, 6, 203-212.

HOSKING, J. R. An es t imat ion of the requi red and available tonnages of roadstone for Category A and B sites; Road Research Labora to ry Technical Note No. 31, 1965 (unpublished).

JAMES, J . G. A s m a l l - s c a l e road experime nt to compare va r ious aggrega te s for use in r e s in -based surface dress ings : Road Resea rch Labora to ry Resea rch Note No. RN/3843, 1960 (unpublished).

HOSKING, J. R. An exper iment comparing the per formance of roads tones in d i f ferent bi tuminous surfacings; A. 30 Blackbushe, Hampshi re (An In te r im Report); Road Research Laboratory Report in the p r e s s .

FOX, C. Bauxite; Crosby Lockwood, London 1927.

KNIBBS, N. V. S. Indust r ia l u s e s of bauxite: Benn & Sons Ltd. , London, 1928.

16

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12.

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14.

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16.

17 ̧"

18.

19.

SEARLE, A. B. Re f r ac to ry m a t e r i a l s ; Griff in & Co. L t d . , London, 1950 (3rd edition).

THIBAULT, N. W. and HELEN L. NYQUIST. The m e a s u r e d Knoop Hardness of hard subs tances ; T r a n s . Amer . Soc. Meta ls , 1947, 38, 271-330.

KNILL, DIANE C. Pe t rog raph i ca l a s p e c t s of the pol i sh ing of na tura l roads tones ; J . appl C h e m , 1960, 10 (pt. 1), 28-35

GILL, R. M. and G. SPENCE. Wear r e s i s t a n t c e r a m i c s ; New Scient is t , 1962, 29 I , 574-577

HOSKING, J. R. Roadstone t e s t data in t abu la r fo rm; Road R e s e a r c h Labora to ry Road Note No. 24, H . M . S . O . London 1959.

SHERGOLD, F. A. The effect of high t e m p e r a t u r e on the s t rength of roadmaking aggrega tes ; Rds. and Rd. Cons t r . 1953, 31 (366), 161-163.

KROYER, K. K. Br i t i sh Patent No.795787, Pa t en t Office, London, 1958.

JORGENSON, S. F. Syntet iske v e j m a t e r i a l e r (Synthetic r o a d ma te r i a l s ) ; Ingeniorens Ugeblad, 1962, 32 (August 11), 1 6 - 1 7 .

FARBWERKE HOECHST AG., B r i t i sh Pa ten t No. 975223, Patent Office, London, 1964.

17

5

0 10

5

0

F (Stag) .(Calcined bauxite) I ~ Artificial group

F I i i i Basalt group

5 F (Normal f l i n t )

0

(Calcined f l int)

I I ~ I ' ' Flint group

sl-.. 0 I i r / / / / i A , I Gabbro group

~ \ \ \ \ \ \ ~ l l l l l / J k \ \ \ \ \ ~ I

I i I

1 Granite group

J Gritstone group

Ul I I e 0 o.

E 10 f "6 9

~ I I ,~ 0 E

z s F 0 I . . . . . . . "~'/"}'}'}~---'~',,'i I I I I I I Hornfels group

15 [- . (Normal l imestones)

( Gr i t ty l imestones) V / f / f / A ~ J f / f i 4

I I I

1of 5

0

1 Limestone group

I Porphyry group

0 i , r . . / / /H ,~ . . . . . . . . . . , FY / / / / } ~ \ \ \ ' ~E / / / / / / J I i i Quartzite group 25 30 35 40 45 50 55 60 65 70 ?5

Polished stone value

Fig1 DISTRIBUTION OF POLISHED STONE VALUES IN DIFFERENT GROUPS OF ROCK

Note: this table is largely based on Fig. 14 of Technical Paper 43 published in 1958. Since 1%5, however, the method of determining the P.S.V. has been modified and values obtained by the new method are I0 per cent lower than formerly.

as def in i t ive.

o .

(/)

(/3

90, L I I I I I

7 0 1 ~ ~ -- ~ . , . . , . t , , . ~ - " i Calcined bauxite

5ol1 . ' ( ~ ~ ~ ' ~ 7 ~ ' ~ ] 6 Nat ura l roads tones , F V ~ X ~ ~ (/Gritstone best,

: , , J \Li:estone:orstJ

0 1 2 3 4 5 6 7 (June t959)

A g e ( y e a r s ) (a)Epoxy-resin surface dressings with variousl/s-in~'aggregates

0-9 O) u

"~ 0.7 U

u

o 0.5 o

~ 0.3

Trunk Road A6 Colnbrook By-pass .... . ~ . .

I I I .I I %

"11,~ ~. : - C a l c i n e d bauxite

NoturQ roo ,on.s (Gr i tst0ne best, \G ranophy re wors t , /

I I I I I I 0 1 2 3- 4 5 6

(sept 1962) Age (years)

(b)Tar surface dressings with various 3/8-in. aggrega.tes " Trunk RoadA30 at Blackbushe

C

u

0 u

u

b ~ , 0.5 o

" 0

b~ 0-3

0.9~ I ( ( ( . i

0.? .

6 Natural roadstones Gr i ts tone best, '~ Granophyre worst, , /

I I I I I I 0 1 2 3 z, S 6 ?

(Sept 1962) . A g e ( y e a r s ) i

(c)Ro[[ed asphalts with vorious-t/2-in.aggregates ro[led into the s u r f a c e

Trunk Rood A30 at B[ackbushe

Fig.2. SKID-RESISTANCE AND SIDEWAY FORCE COEFFICIENT MEASUREMENTS ON VARIOUS SURFACINGS MADE WITH

CALCINED BAUXITE AND OTHER AGGREGATES

i i i !iii i!i! i i i i i ! i~

3.1 Very hard m a t e r i a l s

3.2 C o n g l o m e r a t i o n s of s m a l l ha rd p a r t i c l e s .

. . - ~

• (~..-~!~.. • . . ~ , ~ . ~ . ' - - . . . . . . . . ~ : . ~

. . . . . o~'-'-" ~': , 1..,,,,)" • .F. ":. "..... C~C~..:..~. .~...: ='. :':i~:~

~i~ ..... ~ ............................. ~..: : .~ ".,-'.~ , ; :- ~ " ~ , ~ - , , . L : ' : : , ~ : ~ : - ' : ~

. . . . . . . . . : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : , : , . ~,~. ~ % . ~ ' / ~ . .~:"1 ' : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : . . . . . . . . . . . . . . . . . . . . . . . . . . ::::::::::::::::::::::::::: - - " " ~ ' ~ ' ~ . : ' . : : . . . 1 " " ' ~ 1 : : " : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : . ~ [ ° • . . ~ , . . . • ~, - . ===========================================

3.3 D ispers ions of hard pa r t i c l es in a s o f t e r m a t r i x .

3.4 M a t e r i a l s w h i c h f r ac tu re in an i r r e g u l a r a n g u l a r manner•

iijiiiii iiiiU iiji iii!i iiii i iiiiiiiiiiiiiiii ............ i i! 3.5 Vesicular materials.

Fig.3 . SOME THEORETICAL TYPES OF POLISH-RESISTANT ROAOSTONE

L

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