Effect of alkalies on portland cement hydration II. Alkalies present in form of sulphates

CEMENT and CONCRETE RESEARCH. Vol. 13, pp. 771-777, 1983. Printed in the USA. 0008-8846/83 $3.00 + 00. Copyright (c) 1983 Pergamon Press, Ltd.

EFFECT OF ALKALIES ON PORTLAND CEMENT HYDRATION

I I . ALKALIES PRESENT IN FORM OF SULPHATES

I . Odler and R. Wonnemann Technical Un ivers i ty Calusthal

3392 C laus tha l -Ze l l e r fe ld Federal Republic of Germany

(Communicated by M. Regourd) (Received Jan. l l ; in f inal form July 15, 1983)

ABSTRACT Alkalies present in cement in the form of Na?SO 4 and/or K2SO 4 do not a l ter the progress of C3S and C3A hydration. The setting time becomes accelerated especially with K2SO 4 due to the formation of syngenite CaSO4.KpSO4"HpO. The compressive strength is lowered s ign i f icant ly . = -

Introduction

In the f i r s t paper of this series (1), data on the effect of alkal i oxides incorporated in the crysta l l ine la t t i ce of cl inker minerals on hydration and properties of the resultant cement were reported. The subject of the present paper is similar. Data, however, were obtained on cements in which alkalies were present in the form of water soluble sulphates.

Experimental

For the preparation of the experimental cements, a laboratory made clinker containing only the four main cl inker oxides was used. The composition of this cl inker and the details of i ts preparation are given in the f i r s t paper of this series. The cements were prepared by grinding the cl inker - in addition to gypsum - with amounts of sodium and potassium sulphate corresponding to 0.72 and 1.26 percent Na20 and 0.88 and 1.48 percent K20. These amounts were selec- ted to obtain a lka l i contents identical with those incorporated into the crysta l l i ne la t t i ce of cl inker minerals. These cements were designated 0.72 NS, 1.26 NS, 0.88 KS and 1.48 KS. In addition to that, a cement with no alkal i sulphates added, was prepared to be used as a control. All cements were ground to a specific surface area of 300 m2/kg (Blaine) with amounts of gypsum to obtain an overall S03-content of 3.0 percent.

Pastes made from these cements (w/c = 0.5) were hydrated for various times at 20°C. The hydration process was stopped by grinding the paste with acetone, f i l t e r i n g the resultant suspension, washing the residue with acetone and ethyl ether and drying in air .

The tests performed are described in the f i r s t paper of this series. In

771

772 Vol. 13, No. 6 I . Odler and R. Wonnemann

addi t ion to that, data on the progress of hydrat ion heat l i be ra t i on - as measured by heat f low calor imetry - and on the morphology of the hydrat ion products formed

as determined by scanning electron microscopy - are reported.

Results and Discussion

Figure 1 shows the e f fec t of a l ka l i sulphates on the combined water content. Due to the presence of Na2SO 4 and K2SO 4, th is value increased, but only s l i g h t l y and only a f te r longer hydrat ion times.

25

r 20

o ~ o 15

E 10 o u

O---O 0,?2 NS J ~

O--O 1,26 NS ~.~ ," / ,,,- -A 0,88 K~

D- -D 1,48 K~.; -- con t ro l

i , i i

1/4 1/2 1 3 6 9 12 18 24 48 72 Time [ h ]

FIG. l Effect of a lka l ies on combined water content of cement pastes

28d

Figure 2 shows the progress of C3S hydrat ion. The hydrat ion of th is phase was not a l tered not iceably due to the presence of Na2SO 4 or K2SO 4 in the cement. Ear l ie r reports on an en- hanced C3S hydrat ion due to a l ka l i su l - phates (2,3,4) were not confirmed in our experiments.

I 100 \

g 80 o

~ 6o

} 4o

u

20

0 - - 0 0,72 N5

0 ~ 0 1,26 N~

A---A 0,88 KS

Y [3--CI 1,48 KS

. . . . . . c o n t r o l

1/4 112 1 2 3 6 9 12 1824 48 72 28d

T i m e [ h ] FIG. 2

Effect of a lka l ies on the progress of C3S hydrat ion

From Figure 3, i t appears that the progress of C3A hydrat ion was not a l tered by the presence of a l k a l i su l - phates in cements e i the r . Data of Os- beck and Joens (4), who reported an accelerated C3A hydrat ion a f te r 1 and 3 days and a ~low-down a f te r 38 days, were not confirmed in our experiments.

Figures 4a-c show the DTA curves of the cements a f te r d i f f e r e n t hydration times. I t appears that the gypsum in the cements containing a l k a l i su l - phates was used up fas ter than that in

O'--O 0.72 N~ , . ~ ' ~

,4, ' O--..O 1,26 N~

z~--~ 0,88 KS . ~

D--D 1,48 KS con t ro l ~~~"~i

'" / "

i , I 6 9 12 2/* 48 72 28d

Time [ h ]

FIG. 3

Effect of a lka l ies on the progress of C3A hydrat ion

Vol. 13, No. 6 773 ALKALIES, SULFATES, SYNGENITE FORMATION, CEMENT HYDRATION

\

\ E

100

f

S /

r /

/ / / /

/ V

N

/

/

/ . , ,

2~ a~ ~0

temperature (°C)

FIG. 4a

DTA curves of cement pastes a f ter d i f fe rent hydration times - control cement with no alkal ies

V

k ~

~V f - - - - . . . . n h ..V,~- t h

"'" ~ 61, f / " I ;~

/ / .... 7~ h

, , - 'V f 2,~

/3/

c

100 200 4O0 5OO temperature (°C)

FIG. 4b

DTA curves of cement pastes a f te r d i f fe rent hydration times - cement with 1.26% Na20 added as Na2SO 4

• c,r----- Z ~ Oh 1/2h

- - 4 , " f h

" 6 h

~ 12h

\~ " "-8h i

~h

28d

100 200 400 500

temperature (°C)

FIG. 4c

DTA curves of cement pastes after d i f ferent hydration times - cement with 1.48% K20 added as K2SO 4

(E = e t t r i n g i t e ; M = monosulphate; G = gypsum; C = port landite)

in the control cement. Along with i t , the conversion of e t t r i n g i t e to monosulphate was moderately accelerated. This may be due to the lower original gypsum content in the cements, with a lka l i sulphates added.

Figure 5 shows the ef fect of a lkal i sulphates on the progress of hydration heat evolution. In the control cement and that containing Na2SO 4, a sharp heat evolution peak shortly a f ter mixing with water (due to both C3A and C3S hydration) and a period of renewed intensive heat evolution start ing a few hours la ter (due to a renewed intensive C3S-hydration), were reg- istered. In the cement containing potassium sulphate, an endothermic ef fect prior to the development of the f i r s t hydration heat evolution peak was observed. This thermo-

FIG. 5

Effect of a lkal ies on the rate of hydration heat evolution

2

#2

- - 1 , 2 6 N~, . . . . 1,48 K~ ....... control

k...: .{.. ." ""

i i i J

5 10 15 20

Time ~h ]


e f fec t was apparently due to the d isso lu t ion of potassium sulphate in the mixing water r i gh t a f te r mixing. The absence of a s im i la r thermoeffect in the cement doped with sodium rather than potassium sulphate was due to the ex i s t i ng d i f f e r - ences in the heat of d i sso lu t ion of these two sulphates (Na2S04: + 1.93 kJ/mol, K2S04:-26.8 kJ/mol).

Table 1 Sett ing and Strength Properties of the Experimental Cements

Sett ing Test Compr. Strength H20 beg. end (N/mm 2)

% min min 1 d 3 d 7 d 28 d

Control 25 180 210 20.0 41.5 61.8 74.2

0.72% Na20 as Na2SO 4 23 85 160 18.8 29.6 39.8 56.5

1.26% Na20 as Na2SO 4 25 85 140 9.3 22.4 32.5 42.6

0.88% K20 as K2SO 4 24 60 145 16.1 33.7 43.7 66.3

1.48% K20 as K2SO 4 25 15 20 8.5 23.1 35.5 48.8

Table 1 shows the ef fect of a l k a l i sulphates on se t t ing and strength development. The se t t ing time decreased s i g n i f i c a n t l y due to the presence of a l ka l i sulphates in cement, the K2SO 4 being more e f fec t i ve than Na2SO 4 in th i s respect. In supplementary studies the microstructure of the cement pastes employed in the set t ing tes t was studied. Figure 6 shows the s t ruc ture of the paste made out of the con t ro l , a l k a l i - f r e e cement at the end of the se t t ing . Only hydrates of low c r y s t a l l i n i t y , i den t i f i ed by electron microprobe analysis as CSH-phases, were present. C rys ta l l i ne phases, such as crys ta ls of e t t r i n g i t e , were not detectable. On the other hand, the s t ruc ture of the pastes made out of cements con- ta in ing K2SO 4 was d i f f e ren t , as i t contained - in addi t ion to the i l l - c r y s t a l - l ized phases - also well developed c rys ta ls i d e n t i f i e d by electron microprobe analysis as syngenit (CaSOa'K2SO4"HgO) and in a lesser extent as calcium su l - phate (Fig. 7). This f ind ing is i n - l i n e wi th s im i la r resu l ts published e a r l i e r (5,6). I t may be assumed that the very fast se t t ing of th is cement is related to the formation of the above c r y s t a l l i n e phases. Unlike in cement pastes made out of cements containing K2SO 4, no new c r y s t a l l i n e phases were detectable in those made out of cements w~th Na2SO ~. Thus the o r ig in of accelerated set t ing in the l a t t e r cements is not obvious~

The strengths at any hydrat ion time were lowered s i g n i f i c a n t l y in the presence.of e i the r Na2S04 or K~S04 in the system. The o r ig in of th is e f fec t of a l ka l i sulphates is not obvious. I t is l i k e l y that the observed decl ine of strength is re lated to, c loser not spec i f ied, changes of the s t ruc ture and in- t r i n s i c propert ies of the hydrates formed under the above condi t ions. These changes may be due to the al tered composition of the l i q u i d phase brought about by the d isso lu t ion of a l ka l i sulphates in the mixing water. Data on the e f fec t of a l ka l ies on cement strength are not uniform as both pos i t i ve and negative ef fects were reported (3 ,4 ,7 ,8) . In a recent study Osbaeck (8) found, in l i ne with our data, a negative e f fec t of soluble a lka l ies on 28 day st rength, whi le , at the same time, nonsoluble a lka l ies exhib i ted no e f fec t on strength development.

The resu l ts obtained in our work ind icate s i g n i f i c a n t di f ferences as to the e f fec t of sodium and potassium oxide on port land cement hydrat ion and i t s proper-

Vol. 13, No. 6 ALKALIES, SULFATES, SYNGENITE FORMATION, CEMENT HYDRATION

775

FIG. 6

SEM-photographs of the paste made out of the control a l k a l i - f r e e cement (w/c - 0.5) r i g h t a f te r se t t ing (215 min.)

t i es . Moreover, i t appears that the e f fec t of these oxides depends also on whether they are incorporated in to the c r y s t a l l i n e l a t t i c e s of c l i nke r minerals or present in the form of sulphates. The l a t t e r fact may be helpfu l in control. l i ng the propert ies of the resu l tan t cement by varying the content of SO 3 in c l i nke r and thus the f rac t ion of a l ka l ies bound in the form of sulphates.

Conclusions

I . A lka l ies present in cement in the form of Na2S04 or K2S04 do not a l t e r . s i g - n i f i c a n t l y the progress of hydrat ion - that of the C3S-and C3A phases in pa r t i cu l a r .


FIG. 7

SEM-photographs of the paste made of the cement containing 1.48% K20 as K~SO 4 r ight af ter set t ing (20 mln.)

2. The set t ing of cement is accelerated due to the presence of Na2SO 4 and es- pecia l ly of K2SO 4, In the la t te r case, acicular crystals of syngenit CaSO 4 K2SO4-H20 are formed as one of the reaction products.

3. The compressive strength is decreased s ign i f i can t l y due to the presence of both Na2SO 4 and K2SO 4 in cement.

Acknowledgements

The f inancial support of th is work by the Deutsche Forschungsgemeinschaft is g ra te fu l l y acknowledged.

Vol. 13, No. 6 ALKALIES, SULFATES, SYNGENITE FORMATION, CEMENT HYDRATION

777

References

I. I. Odler and R. Wonnemann. Cem. Concr. Res. 13, 477-482 (1983). 2. D. Kn~fel. Silikattechnik 22, 262-265 (1971)__ 3. I. Jawed and J. Skalny. Cem. Concr. Res. 8, 37-52 (1978). 4. B. Osbaeck and E.S. Joens. Proc. 7th In t l . Cong. Chem. Cem. Paris 1980,

Vol. I , pp. I I - 135-140. 5. W. Richartz. Zement-Kalk-Gips 26 (1), 67-74 (1973). 6. S. Sprung. Zement-Kalk-Gips 27~5), 259-267 (1974). 7. J. Skalny and J.F. Young. Proc. 7th In t l . Cong. Chem. Cem. Paris 1980, Vol

I , pp. I I - I/3-45. 8. B. Osbaeck. Zement-Kalk-Gips 32 (1), 72-77 (1979).

Documents

Effect of alkalies on portland cement hydration II. Alkalies present in form of sulphates