CEMENT and CONCRETE RESEARCH. Vol. I I , pp. 307-322, 1981. Printed in the USA 0008-8846/81/030307-16502.00/0 Copyright (c) 1981 Pergamon Press, Ltd.

SUPERSULPHATED CEMENT FROM BLASTFURNACE SLAG AND CHEMICAL

GYPSUM AVAILABLE IN THE NETHERLANDS AND NEIGHBOURING COUNTRIES

J. Bijen, E. NI~I Intron Inst i tu te, Maastricht, The Netherlands

(Communicated by A.J. Majumdar) (Received Dec. 15, 1980)

ABSTRACT The present report covers research in to the mechanical st rength development and the surface hardness of supersulphated cement from b]ast furnace s lag, chemical gypsum and port land c l i n k e r . The tes ts were ca r r ied out on sand/cement mortar as wel l as on concrete. A co~lparison was.made wi th a type of supersulphated cement which was in the market un t i l recen t l y , and wi th port land and b las t furnace cement. A review has been made of the e f f ec t s of the hardening temperature, the r e l a t i v e humidi ty , the degree of gr ind ing of the cement, the use of a water-reducing a d d i t i v e , and treatment wi th a curing compound. The conclusion is drawn that one o f the cements produced, cons is t ing of 83~ m/m Dutch b las t furnace slag, 15~ m/m fluorogypsum (anhydr i te) and 2~ m/m port land c l i n k e r ground to the r e l a t i v e l y high spec i f i c surface of 500 m2/kg, is not i n f e r i o r to b last furnace cement as regards the proper t ies examined.

Yon Sul fath0t tenzement, he r g e s t e l l t aus Hochofenschlacke und Chemiegips und Port landzementk l inker i s t die mechanische St~rke- entwicklung und die Oberfl~chenh~rte untersucht worden. Die Unter- suchung i s t sowohl mit Sand/Zement- wie auch mit Betonn~rtel durch- gefOhrt worden. Es i s t e in Vergle ich mit einem bis vor kurzem auf dem Markt e r h ~ l t l i c h e n Sulfath0ttenzement und mit Port land und Hochofenzement gemacht worden. Der E in f luss yon Erh~rtungstemperatur, r e l a t i v e r Feucht igke i t , Mah l fe inhe i t des Zements, der Anwendung eines wasserreduzierenden Zusatzmi t te ls und der Nachbehandlung mit einem Cur ingml t te l auf diesen Eigenschaften ]s t nachgegangen. Schlussfolgerung i s t , das e iner der he rges te l l t en Zernent zusammen- g e s t e l l t aus 83 Gew. ~ n ieder l~nd ischer Hochofenschlacke, 15 Gew.~ Fluorogips (Anhydr i t ) und 2 Gew.~ Por t l andk l i nke r und bls zur r e l a t i v hohen spezi f ischen Oberfl~che von 500 m2/kg gemahlt was den untersuchten Eigenschaften angeht einem Hochofenzement n icht nachzustehen braucht.

307

308 Vol. I I , No. 3 J. B i jen , E. Ni~l

INTRODUCTION

The Dutch Ministry of Public Health and Environmental Protection, in the context or recommendations to be formulated on the subject of 'ffhe use of alternative materials and/or processes for production of cement, clinker or limestone, issued a commission for a preliminary study into the potential applications of supersulphated cement on the basis of blastfurnace slag and chemical gypsum available in the Netherlands or from neighbouring countries.

Port land c l i n ke r has a high calcium content (about 63% CaO), i t s main components being t r i ca l c i um s i l i c a t e , C3S , and dicalc ium s i l i c a t e , C^S. These Calcium s i l i c a t e s are p r a c t i c a l l y the only e x i s t i n g c r y s t a l l i n e z calcium s i l i c a t e s which react s u f f i c i e n t l y read i l y wi th water . The react ion products, CSH(I) and CSH (11) Which provide most of the mechanical s t rength of the cement rock thus formed, contain much less calcium, however. The remaining calcium is mainly present as calcium hydroxide, for the greater part c r y s t a l l i n e , and does not con t r ibu te to the mechanical s t rength . Moreover, in p rac t i ce a subs tan t ia l p ropor t ion (20 to 50%) of the cement remains unhydrated owing to the l im i ted rate of d i f f us ion of the react ing components. Of the calcium even tua l l y present in the cementstone, approximately 50 to 70% is not func t iona l as regards mechanical s t rength . Calcium s i l i c a t e s w i th less calcium do not react wi th water in t h e i r c r y s t a l l i n e form. In the form of g lass, however, they are reac t ive i f ac t i va ted by a h igh ly a l k a l i n e medium.

An example of such a glass substance is granulated blastfurnace slag, consisting to 95% of glass and containing about 30-55% m/m CaO and 25-50% m/m SiO 2 •

In view of the prospect of a possible shortage of primary calcium (limestone) in the Netherlands it is logical that attention is being paid to this by-product with a lower calcium content, obtained as a by-product in ore processing. In the Netherlands, blastfurnace slag is already widely used for production of blastfurnace cement (54% of the Dutch cement market). However, supersulphated cement contains less portland clinker than blast- furnace cement (2% compared with 30~ m/m), which means it requires less primary calcium. Further, it contains rather much calciumsulphate (I0 to 15% m/m), which the Dutch fertilizerindustry already produces in large quantities (2 million t/y) and which in the future will become available on a massive scale as a waste product in desulphurization of flue gases of coal-fired power plants. In the past, supersulphated cement was only used in a limited range of applications in the Netherlands, e.g. for sewerage works and in the chemical industry. It used to be imported from Belgium under the trade-name "Sealithor (R)", but recently disappeared from the Dutch market altogether.

The principal technical reasons for the modest market share of super- sulphated cement in the Netherlands and in other countries are: - friability of the concrete skin, resulting in a dusty surface, - the slow hardening of the concrete, especially at low temperatures,

compared with portland cement, though not necessarily in comparison with blastfurnace cement.

With regard to o ther aspects, such as the energy costs of product ion,

Vol. I I , No. 3 309 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

the physical-mechanical p roper t ies and the chemical res is tance of mortar and concrete , supersulphated cement is c e r t a i n l y not i n f e r i o r to b las t fu rnace and por t land cement. This paper is concerned notably w i th the two negat ive aspects mentioned.

BACKGROUNDS

Reacting w i th water , supersulphated cement does not y i e l d the same products as b las t fu rnace cement. For both types of cement, however, a h igh ly a l ka l i ne medium is requ i red. In both cases por t land c l i n k e r is normally used to provide th is a l k a l i n i t y . In the h igh ly a l k a l i n e medium, d i f f u s i o n of ions can take place and d i s i n t e g r a t i o n of the s i l i c a t e - a l u m i n a t e s t ruc tu re begins. The i n i t i a l s t rength of supersuiphated cement is due to the format ion of e t t r i n g i t e , 3CaO.A1203.3CaSO4.32H20 wh i le in a l a t e r stage the format ion of calcium s i l i c a t e hydra tes , CSH ( I ) and CSH (11), is impo r tan t ( I ) .

The h y d r a u l i c i t y of b las t fu rnace slag depends on the glass s t r u c t u r e . The glass s t r uc tu re is determined by the chemical composit ion of the s lag , the tapping temperature and the v i s c o s i t y of the mel t , the g ranu la t ion temperature and the rate of g ranu la t l on (2 ,3 ) . These parameters are co r re la ted . Thus, the v i scos i t y depends on the chemical composit ions and the temperature. M ic roscop i ca l l y , b las t fu rnace slags have a heterogeneous chemical composit ion. Slags wi th the same chemical composit ion may d i f f e r in their microscopic structure. Most blastfurnace slags consists of so- called "mel ilite" glass (5), which is a mixture of akermanite (C2MS 2) and gehlenite (C2AS) . It appears that there is a close relation between the hydraulicity of the slag and the quantities of akermanite andgehlenite which are crystallized out when the slag is heated(3).

The optimum glass structure of the blastfurnace slag with a view to the strength development seems to be different for supersulphated cement and blastfurnace cement. However, research on this point is generally restricted to determination of the optimum (macroscopic) chemical composition(6,7). Other factors determining the glass structure are only marginally, if at all, considered in the studies carried out so far. The optimum chemical compositions for supersulphated cement stated in the literature should therefore be treated with some reserve.

The pulverization of the concrete skin is the result of a reaction with carbondioxide gas, CO 2. During the initial setting and further hardening, C0 2 can be taken up in the water of the cement paste, so that the ra ther small a l k a l i n e b u f f e r in the form of a few percent of c l i n k e r is soon e l im ina ted . The ground slag is no longer ac t i va ted then and remains unhydrated. The format ion of a f r i a b l e sk in in the course of cement hydrat ion can be prevented by b lock ing the penet ra t ion of ambient a i r fo r some t ime. This can be done by submerging the concrete or by leaving i t in the forms. The former is not always poss ib le , and the l a t t e r is o f ten ob jec t i onab le fo r economic reasons. On the basis of these cons idera t ions , i t was decided in the study under review to gr ind the supersulphated cement r e l a t i v e l y f i ne ( s p e c i f i c surface ca. 500 m2/kg).

310 Vol. I I , No. 3 J. B i jen , E. Ni~l

Enlargement of the specific surface will raise the rate of the cement-water reaction. This again will reduce the negative effect of absorption of CO 2 in the water.

COMPOSITION OF BASIC MATERIALS AND CEMENTS

The blastfurnace slags examined came from blastfurnace works in the Netherlands and neighbouring countries. The chemical composition and origin are stated in table I.

The samples of Dutch blastfurnace slags are different in that one sample consisted of granulated wet slag, while the other was a mixture of granulated dried slags (dry) of a composition averaged over a certain period. Table I also shows the chemical composition of the portland clinker used and the supersulphated reference cement (Sealithor R).

Fluorogypsum(anhydrite), CaS04, and phosphogypsum (calciumsulphate di-hydrate), CaSO4.2H20 , were used as calciumsulphate source. Fluorogypsumis a by-product obtained in HF production, and phosphogypsum is obtained in the production of phosphoric acid. Composition and origin are stated in table 2.

Chemical Composition Portland Clinker and

TABLE I .

and Or ig in of Granulated Blast furnace Slags, SuperSulphated Reference Cement

Analysis Foreign Dutch Portland Seali- clinker thor

A B C D E F (R)

Insoluble in acid % 0,74 D,46 0,62 0,79 0,28 0,60 0,66 0,48 Loss on ignition % -2,10 -2,03 -1,46 -1,35 -I,30 -0,76 1,15 -0,90

CaO % 43,20 43,61 40,97 40,18 35,49 35,44 63,31 40,55 SiO2 % 35,23 34,17 40,21 40,15 31,95 31,67 25,18 29,04

Al203 % 12,50 11,05 8,12 7,04 14,21 16,27 4,67 11,05 MgO % 3,50 5,86 7,88 9,90 II,33 11,08 1,26 4,75

Fe203 % 0,50 0,99 0,60 0,59 1,8 0,69 1,39 1,44 TiO 2 % 0,78 0,18 0,0 0,0 0,72 1,20 0,0 0,83

K20 % 0,98 0,56 0,58 0,70 0,79 0,77 0,81 0,97 Na20 % 0,66 0,69 0,51 0,41 0,54 0,61 0,25 0,69

SO 3 % 0,22 0,10 0,057 0,13 0,15 0,12 0,225 7,36 S as SO 3 % 0,06 0,21 0,12 0,10 0,02 0,01 n.d. 0,13

Total excl. loss on ignition % 97,53 97,42 99,67 99,20 96,38 97,86 97,10 96,81

Vol. I I , No. 3 311 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

TA~E 2.

Composition of Chemical Gypsum

F I uorogy ps um (anhydr i re)

Phosphogypsum

Free moisture (45oc) % m/m

Bound water (45-215oc)

rel. to total weight % m/m

rel . to dry weight % m/m

Sulphate (SO 3)

r e l . to t o t a l weight % m/m

rel. to dry weight % m/m

pH

0,10 30,50

0,36 13,62

0,36 19,60

57,40 31,59

57,67 56,53

13,7 3,4

Composition acc. to ASTM C 471:

Free moisture % m/m 0,1 30,50

CaSO4.2H20 1,7 65,10

CaS04 96,2 2,3

Because i t was found that the use of phosphogypsum led to a very considerable retardation of the cement hardening process, the gypsum was sp l i t into a coarse and a fine fraction in a hydrocyclone. Most of the phosphate and the f luorides, which cause this delay are present in the fine fraction. The coarse fraction amounted to 70% of the gypsum, the fine fraction to 30%. In the coarse fraction, 86% m/m of the particles was larger than 75 ~m. The total phosphate i .e. P205 content of this fraction, relative to the dry weight was 0.50% m/m, the proportion of phosphate soluble in saturated gypsum water was 0.19% m/m; the total f luorine content (as F) was 0.203% m/m, and the proportion of the fluorine soluble in saturated gypsum water was 0.018% m/m.

After the separation in the hydrocyclone, the coarse fraction of the acid phosphogypsum had a pH of 5.1. With about 0.5% m/m Ca(OH)2, relative to the dry gypsum weight the coarse fraction was given the same pH as the fluorogypsum (pH=13.7). When the slags, gypsum and portland clinker had been ground, another quantity of 0.1% m/m Ca(OH) 2 was added, so that the pH of this cement became the same as that of the cement activated with anhydrite (pH=12.8). The phosphogypsum probably contained incorporated acid components.

R The S e a l i t h o r cement conta ined on ly a n h y d r i t e . The s l a g s , gypsum and c l i n k e r were mixed as s ta ted in tab le 3, and ground in a b a l l m i l l .

312 Vol. I I , No. 3 J. Bi jen, E. Niel

Composition of Various Cements

TABLE 3.

Cement Type of slag Slag content symbol

% m/m

Gypsum content Clinker Specific content surface

(Blaine) % m/m % m/m m2/kg

A

B

C

D

E

F

FK 5

Fphos ph o

foreign

Dutch

F

F

S Sealithor R 79 (commer- cially available cement)

Sfine Sealithor R 79 fine ground

83 15 fluoro- 2 530 gypsum

83 15 " 2 513

83 15 " 2 509

83 15 " 2 538

83 15 " 2 519

83 15 " 2 530

80 15 " 5 522

83 15 phospho- 2 540 gypsum coarse fraction (dry) 14 anhydrite 7 407

14 " 7 522

The cements used for reference are portland cement A (PoA) and blastfurnace cement A (HoA). These cements meet the requirements of the Dutch standard NEN 3550.

MECHANICAL STRENGTH DEVELOPMENT

Exper imenta l The start and the end of the cement binding process (Vicat) and the normal consistency were determined in accordance with the Dutch standard N 493.

The mechanical strength tests were performed in accordance with the Dutch standard NEN 3072. Prisms of sand-cement mortar (3:1), 4 x 4 x 16 cm, were stored under plastic film for one day at 20°C. Then the forms were removed and the prisms were stored under water at a temperature of 20oc. In order to ascertain the effect of the temperature, tests were also carried out at 5°C.

To determine the effect of water reduction, 3% m/m of Melment L 10 (A 20% aqueous solution of a sulphonated melamine formaldehyde resin) was added to cement F (see table 3); the water-cement factor was reduced and the workability was kept equal to that of the cement without a water reductor.

V01. I I , No. 3 313 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

Wi th s u p e r s u l p h a t e d cement F and w i t h c o m m e r c i a l l y a v a i l a b l e c e m e n t s , c o n c r e t e was p r e p a r e d f o r e x a m i n a t i o n o f t h e c o m o r e s s i v e s t r e n g t h d e v e l o p m e n t . The c o n c r e t e was made w i t h 320 kg/m3 c e m e n t , and had a w a t e r - c e m e n t f a c t o r o f 0 . 4 8 . The fo rms w e r e removed f r o m t h e c o n c r e t e cubes a f t e r 3 d a y s . The cubes w e r e s t o r e d a t 20°C and R.H. o f 100%.

Resvlts The start and the end of the binding process, the normal consistency and the dimensional s t a b i l i t y are stated in table 4. The mechanical strength development is represented in the figures I-7. Table 5 shows the results of the concrete tests.

TABLE 4.

Binding Time, Normal Consistency and Dimensional Stab i l i ty of Cement Pastes

A B C D E F FK5 Fphos - S Sfine PoA pho

HoA

.,u,~D:n~:n- E E E E E E E E E E i.~ E i ~ ~ ~ iJ~ LP~ ~ co Lt%

start ~ -- ~ -- ~r~ N ~ • -- ~ ~

............ T ........ T ....... = - ' - : .... : ............. = ..... ~ ...... Z ...... ---

t e r m i - E • E E E E E E E E I

n a t i o n ~ E r.-. co la%

• . = • . ~

Normal

c o n s i s ~ t ency

w a t e r - ~ ~ ~ ~ ~ ~ ~ N o~ ~ ~ r.. cement ~ ~. N ~ N. ~. N N. ~. ~. ~. N

r a t

Dimen- sional stabi- l i t y Chate- I ier mm

TABLE 5.

Compressive Strength of Concrete (MPa)

Compressive strength after

Concrete wi th Supersul phated cement S

C o n c r e t e w i t h S u p e r s u l p h a t e d cement F

3 days 9,8 14,9 7 days 35,8 38,0

27 days 55,6 53,4 57 days 57,2 58,2

314 Vol. I I , No. 3 J. Bi jen, E. Ni~l

4.3. Discussion Cements A-F and S Cement A and cement F on the basis of Dutch slags show the fastest initial strength development and also have, apart from the commercially available reference sunersulphated cements, the highest compressive strength after 28 and 56 days (fig.'s I and 2).

compressive strength [MPa]

80. I

0 B , . t ,-J C /

6o i I -"

* s / / . . . 4

1 3 7 14 28 56

The bending strength after 28 and 56 days is about the same for all the cements. The loss of bending stress strength is probably connected with the conversion of 3CaO.A1203.3CaSO4.321120(ettringite) into EaO.A1203 . CaSO 4. 12H20.

With cements on the basis of blastfurnace slags and calciumsulphate, this loss of strength is not unusual, and either a recovery sets in after 6 to 12 months, or the strength stabilizes(9).

FIG. I. Compressive strength development of mortars with super sulphated cements A-F and S as a function of time.

18-

1 6 ¸

14

12'

1o

8

6

bending strength [MPa]

f

/ A I .f i ., ; ii7 i .,

'~P ~.--t,.. [dsys]

FIG. 2. Bending strength development of mortar with super sulphated cements A-F and S as a function of time.

Vol. I I , No. 3 315 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

Fluoro~ynsum - ohosphogypsum Phosphogypsum causes a retardation of the binding process compared with fluorogypsum (anhydrite). The start of the binding process is retarded by about 20 hours compared with the fluorogypsum (see table 4). The re ta rda t ion which a lso appears from f i g . 3 and f i g . 4,

FIG. 3. Compressive strength development of mortar with cements F and reference cements as a function of time.

is caused by

6O

2 0

Compces l l ve i t r e n g t h [ M P I ]

@...:y~.@ • F . . ' " , /

~ FksF 3 % m l l t ." ." . . . " ~ " /

G F fosfo . •" " . s .~" / ~

.- / . . / _ . , . . < ~ ---,,- . . . f

..'" / . .A ' " / ,~" . . . . . ----" "" . - ' / / i , , ; - ;~- ' ' - -

• /;< :~ / /

3 7 14 28 56

the f l uo r i des and wate r -so lub le phosphates which are present :n the phosphogypsum. Although these substances are not present in large q u a n t i t i e s , they s t i l l cause a considerable re ta rda t ion owing to the hiqh gypsum and low c l i n k e r content compared wi th por t land cement. A h i ~ e r propor t ion of phosphate and f l u o r i n e w i l l have to be ex t rac ted from the phosphogypsum to make i t su i t ab le for use in supersulphGted cement. Another cause of the re ta rda t ion may be that the gTD;um has to be appl ied in the form of anhydr i te .

1 8

16

14

1 2 -

10- .

8 .

6 .

4 .

2'

0

bending strength [MPa}

...o~.. . . . . . .o " " . . .

/...#.." .p'~- o ~ "

/ v • ~... r;" 2- - 7---".-'~. ---'~ I.." , / / . . - ~ . . ~

l.'" / / I ..,~:.::~" • ,

l : / # D . ' / ~ , , - ,o • / " ' • " / ~ PoA

t! .~" " ~ , , ~ t i m [a,y,]

FIG. 4. Bending st rength development of mortar w i th cements F and reference cements as a funct ion of t ime.

316 Vol. I I , No. 3 J. Bijen, E. Ni~l

Clinker content An increase in clinker content from 2 to 5% m/m (F K ) has a negative effect on the initial strength development (fig.'s 53 and 4). This is in agreement with the observations of Tanaka et al (6).

Comparison with reference cement Cements E and F on the basis of Dutch slags have a good overall strength development at 20°C, at least as good as that of the reference cements portland cement A and blastfurnace cement A. The commercially available supersulphated cement S has a Ibwer initial strength development, but reaches a higher compressive strength after 56 days. Cements E and F come in class B (cement with a high initial strength), in accordance both with the old Dutch standard N 491 and the new standard NEN 3550.

Grinding fineness A higher grinding fineness enhances the rate of initial strength development and reduces the setting time. This appears from the measurement data for cement S and the finer cement Sfine; see fig.'s 5 and 6, and table 4.

Water reducer With identical workability, addition of 3% m/m of Me lment L 10 relative to the cement weight, results in a 30% water reduction. Although a retardation of the setting process was observed, both the initial strength and the mechanical strength after 28 and 56 days are higher than when no water reducer is used; see fig.'s 3-6, cements F and F 3% Melment.

5 0 - o

46"

42" o

3 8 - o

3 4 -

3 0 .

26"

22"

18'

14.

lO

6

compressive strength [ MPa] .ss.:' / 0

I " / "" f

.~" / /

.."""//

°.°'°

...'" ~ ./'/

/

4

P

d

/ '

• A

• F ~ ) F 3% m l l m l n t • S 0 S f l |n i~ PoA (~ MoA

f ,

J

FIG. 5. I n i t i a l s t reng th development o f mortar w i th cements A and F and re ference cements.

Vol. I I , No. 3 317 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

FIG. 6. I n i t i a l bending strength development of mortar with cements A and F and reference cements.

1@

16 ¸

14

12

8 .

4 -

2 -

bending strength [MP l ]

• A •

• F . . " Ir 3% mek~eRt . • S . ' "

0 S flirt

S J . . ' " ~ 7 _ . s ~ " "

-"" i / ~ . < ' ~ . . " "

- .~y..- - - , . - t , m [d.~]

Effect of temperature Fig. 7 is a graphic representat ion of the compressive strength development of cement F and the reference cements, port land A and blastfurnace A, hardened under water at 5 ° and at 20°C. The loss of compressive strength of mortar hardened at 5oc compared with mortar hardened at 20oc is re l a t i ve l y largest for supersu]phated cement F.

FIG. 7. Compressive strength development of mortar wlth supersulphated cement F, b las t - furnace cement and port land A cement, hardened at 20 and 50°C as a funct ion of time.

70- c o m p r e s s i v e s t r e n g t h [ M P a ]

6 0 -

SO-

40"

30"

20'

10'

• F ~r PoA

HoA = 200C

~ S°C

p

f /

r 7

/

/

/

~ / ~ / / , , / - / / /

/ /

.O

. . , . [d,,,]

; , 2'8 s'6

318 Vol. I I , No. i~ J. Bi jen, E. Ni~l

In an absolute sense, however, the hardening process at 5°C is not slower than that of blastfurnace cement at the same temperature. Portland cement A shows the fastest strength development at 5°C.

Compressive strength development of concrete The compressive strength development of concrete (table 5) is analogous to that of the mortar (fig. I)

FRIABLE SKIN

Experimental The thickness of the soft surface layer was measured with application of a method developed by Tanaka et al (6). In this method a small steel drill, as sketched in fig. 8, is placed on the concrete surface and rotated 30 times with a pressure of 300 g. The penetration depth is a measure of the degree of friability. The accuracy is * 0.05 mm.

The test specimens were prepared as described in the paragraph on mechanical strength development. In order to approximate the practice conditions as much as possible, the specimens were taken out of the forms after 2 days and stored in the air at 20°C or 5°C and a R.H. of 100 or 60%. The penetration depth was measured after 7 days and after 2 months. The compressive strength and the bending strength were also measured after 7 days.

In order to limit the range of the test, a number of cements were selected on the basis of mechanical strength data. Besides the reference cements, cement F was selected, because it had the best strength development, and cement A because it had the best strength development of the cements with no Dutch slags. The effect of the clinker content was examined by comparison of cement F (2% m/m of clinker) with cement FK5 (5% m/m of clinker). Melment L 10 was used as water reducer and a commonly used curing compound on a paraffin basis was added in order to ascertain their effect.

Surface hardness tests were performed on concrete made with cement F. The concrete obtained 320 kg/m3 cement, and the water-cement factor was 0.48. After removal of the forms (after 3 days), parlor the test specimens were stored in an air-conditioned room at 20°C and a R.H. of 100%, the rest in the open air. During the testing period, the outdoor temperature varied from -3 to + 15oc, and the weather was alternately dry and wet.

Results The results are shown in table 6. No essential differences were found between the penetration depths after 7 days and after 2 months.

Discussion Cements A and F, and reference cements at 20°C Cement A gives a larger penetration depth than cement F after 7 days, which is in agreement with the results of the mechanical strength tests. The penetration depth with cement F is about equal to that with portland cement A, while with blastfurnace cement the depth

Vol. I I , No. 3 319 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

TABLE 6.

Results of Skin Pulverization Test

Cement

Setting conditions after form removal

Curing Penetra t ion 7 days 7 days Temperature Re la t ive Compound d e ~ t h a f t e r compressive bending

Humidity 7 days s t rength s t ress s t rength

°C % mm MPa MPa

A 20 100 - 0,55 33,8 10,6 A 3% Me lment 20 100 - 0,20 37,3 7,1 A 20 100 yes 0,60 36,1 10,5 A 20 60 yes 1,00 27,3 5,3

F 20 I00 - 0,20 44,9 8,8 F 3% Me lment 20 100 - 0,15 64,0 13,7 F 20 100 yes 0,50 45,4 11 ,6 F 20 60 yes 0,55 31,6 5,7 F 20 60 - 0,25 40,8 6,4 F 5 I00 - 0,80 18,9 4,5 FK5 20 I00 - 0,75 25,2 8,4

S 20 100 - 0,60 27,7 7,5 S 5 100 - I ,30 3,2 0,6 S fine 20 100 - 0,30 37,1 7,7 S fine 5 100 - 0,80 9,7 1,4

. . . . . . . . . . . . . . . . . I . . . . I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Po-A 20 100 - 0,25 31,1 7,3 Po-A 20 100 yes 0,50 23,7 6,1 Po-A 20 60 yes 0,30 23,4 4,4 Po-A 5 I00 - 0,35 11,7 2,8

Ho-A 20 I00 - 0,60 26,0 7,1 Ho-A 20 100 yes 0,75 21,4 9,1 Ho-A 20 60 yes 0,85 20,2 4,1 Ho-A 5 I00 0,95 7,1 2,4

is g rea te r and about the same as the depth measured fo r supersulphated cement A. A lower r e l a t i v e humidi ty only resu l ts in a minor increase in the penet ra t ion depth fo r cement F. The negat ive e f f e c t of lowering the r e l a t i v e humidi ty is more pronounced in the reduced compressive s t rength and bending s t rength a f t e r 7 days. The same is observed in tes ts w i t h cur ing compound.

C l inker content Cement w i th 5% por t land c l i n k e r appears to g ive a g rea te r penet ra t ion depth than cement with 2~ portland cl inker. This is in contrast with observations of Tanaka et al (6), who conclude that a higher clinker content gives a somewhat higher surface hardness. Evidently, the

320 Vol. I I , No. 3 J. B i jen , E. Niel

higher initial mechanical strength (fig.'s 3 and 4) and the short period prior to the start of the setting process (table 4) are decisive. Another cause might be that the grinding fineness of the slag is lower with 5% clinker than with 2%. Blastfurnace slag is less easily ground than portland clinker, and the two substances were ground in combination.

Grinding fineness The effect of a higher grinding fineness appears from a comparison of the degree of skin friability of concrete made with the commercially available supersulphated cement S, and of concrete made with cement of a higher grinding fineness, Sfine. The finer cement gives a lower degree of skin friability.

Water reducer As expected, lowering of the water-cement fac tor has a favourable e f f ec t on the degree of skin f r i a b i l i t y . Cement A wi th 30% water reduction gives the same d r i l l penetrat ion depth as cement F as such.

Curing compound The curing compound used had in all cases a negative effect on the concrete surface hardness; apparently, the paraffin, interferes with the water-cement reaction. In the case of cement F, when hardened at 20oc and a R.H. of 60%, it could moreover be concluded that the compressive strength and the bending strength are affected, while the contrary would be expected.

Effect of the temperature Lowering of the hardening temperature from 20 to 5oc results in a relatively high increase in the degree of skin pulverization in the case of cement F. The penetration depth after 7 days is greater than that for portland cement A (0.80 versus 0.35 mm), but a little smaller than in the case of blastfurnace cement A (0.95 mm). Portland cement A is rather insensitive to temperature decreases as regards the degree of skin pulverization. In the case of blastfurnace cement, the low compressive and bending strengths after 7 days at 20oc are remarkable; these values are more than two times lower than for the concrete hardened under water. The cause of this is not understood.

Concrete surface hardness The penetration depth in cement F concrete (see table 7) stored in the open air is somewhat greater than for the same concrete stored indoors at 20oc and a R.H. of 1O0%. The difference is small, though. Neither the measurements, nor visual inspection reveal much difference with the mortar tests (table 6). To obtain a well-founded answer to the question whether the surface hardness might also be affected as a result of carbonation of cement hydration products, notably ettingite (2), an additional study will be carried out.

CONCLUSIONS

Raw materials As a raw material for production of supersulphated cement, the Dutch blastfurnace slags came out best in our tests, both in terms of mechanical strength development and in terms of skin friability. The Dutch phosphogypsum caused a considerable retardation of the

Vol. I I , No. 3 321 SUPERSULPHATED CEMENT, BLAST FURNACE SLAG, GYPSUM, STRENGTH

TABLE 7.

Results of Skin Pulverization Test on Concrete made with Cement F; Penetration Depth in ram

Age storage condi t ions

20°C 100% R.H. outdoors -3 vs.+15 °C

7 days 0,20 0,25 28 days 0,15 0,20 56 days 0,15 0,35

FIG. 8. D r i l l used for t,~sting o f skin p u l v e r i z a t ion

O.Scm 5.5¢m 1~4¢m

' ' i l ~ c m 7 4 ¢ m

se t t i ng process compared wi th fluorogypsum (anhydr i te ) . The phosphate and f l u o r i d e content o f the phosphogypsum w i l l probably have to be reduced to make i t a su i t ab le component for supersulphated cement. Mechanical s t rength Supersulphated cement cons is t i ng of a mixture of 83% m/m of Dutch b las t fu rnace slag, 15% m/m of fluorogypsum (anhydr i te) and 2% m/m of por t land c l i n k e r , which mixture is ground to a s p e c i f i c surface o f 500 m2/kg, meets the requirements fo r c lass B (cements w i th a high i n i t i a l s t rength) in accordance wi th the Dutch standard NEN 3550. At a low temperature (5oc) the i n i t i a l s t rength o f th is supersulphated cement corresponds w i th that o f b las t fu rnace cement A. Increase o f the g r ind ing f ineness enhances the i n i t i a l s t rength .

The behaviour o f concrete appears to be analogous to that of mortars. In supersulphated cement made w i th Dutch b las t fu rnace slag, an increase of the c l i n k e r content to 5~ m/m has a negat ive e f f e c t on the mechanical s t rength . Water reduct ion by means of a so -ca l led s u p e r p l a s t i c i z e r considerably improves the mechanical s t rength . Surface hardness The surface hardenss o f supersulphated cement made wi th Dutch slags and o f the above-mentioned composit ion, hardened in the open a l r at 20oc and a R.H. o f 60 or 100~, is h igher than that o f b las t fu rnace cement and about equal to that o f por t land cement A. At 5°C and aR.H.of 100%, the sur face hardness is lower than that o f por t land cement A, and s l i g h t l y h igher than that of b las t fu rnace cement A. Increasing o f the s p e c i f i c sur face improves the surface hardness. Measured over a per iod o f 2 months, no s i g n i f i c a n t d i f f e rence between mortar and concrete is found. The surface hardness of supersuiphated cement made w i th Dutch b las t fu rnace slags and 5% m/m por t land c l i n k e r is lower than when 2% m/m por t land c l i n k e r is used. Water reduct ion improves the surface hardness. Continued research

322 Vol. I I , No. J. Bijen, E. Ni~l

Further studies on supersu]phated cement are performed on: - optimization of the composition - the effect of separate or combined grinding of slags, clinker and

anhydrite - the effect of carbonation of cementhydration products on the strength - corrosion of reinforcement steel.

ACKNOWLEDGEMENT

We thank the Dutch Minister of Public Health and Environmental Protection for sponsoring this study.

REFERENCES

I. R.W. Nurse, Slag cements, The chemistry of cements, vol. 2 ed. H.F.W. Taylor, Academic Press, London (1964) pp 36-66.

2. F. Schr~der, Slags and slag cement, Proceedings of the V-International Symposium on Cement Chemistry, Tokyo (1968) vol IV pp 140-199.

3. V.I. Satarin, Slag portland cement, Proceedings of the VI- International Symposium on Cement Chemistry, Moscow (1974). Principal Paper, pp 1-51.

4. H. Busch, A. Petzold, Proper t ies and behaviour of b last furnace slags in r e l a t i o n to t h e i r heat t reatment , Cement Wapno Gips, 27 (1972) pp 9-12, T rans la t ion B.R.E. L ibrary no 1718.

5. S. Solacolu, Die Bedeutung der thermischen Gleichgewichte des Systems MgO - CaO - Al20 ~ - SiO^ flir das Schmelzen und Granulieren der Hochofenschlacke, Zei~went- KaZlk - Gips 4 (1958) pp 125-137.

6. T. Tanaka, T. Sakai, I. Yamane, Zusammensetzung Japanischer Hochofenschlacken f~r Sul fath~t tenzemente, Zement - Kalk , Gips 2 (1958) pp 50-55.

7. F. Keil, F.W. Locher, Hydraulischer Eigenschaften yon Gl~sern, Zement - Kalk - Gips 6 (1958) PF) 245-253.

8. F.M. Lea, The chemistry of cement and concrete, ed. 3, Edward Arnold Ltd., London (1970) pp 418-485.

9. M. Raverdy, Etude d'un liant ~ base phosphogypse et de laitier, Bull. Lias. Labo. Ponts et Chauss. Nov. (1978) pp 69-77.