CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 177-186, 1993. Printed in the USA. 0008-8846/93. $6.00+.00. Copyright © 1993 Pergamon Press Ltd.

ALKALI METAL SALTS AS SET ACCELERATORS FOR HIGH ALUMINA CEMENT

T.Matusinovic and N.Vrbos University of Zagreb

Department of Chemical Engineering and Technology Marulicev trg 20, 41000 Zagreb, Croatia

(Sef=eed) (Received Feb. 4; in fnud form March 4, 1992)

ABSTRACT The effect of alkali metal salts on the setting time of high aluminia cement (HAC) has been studied. The influence of concentration of the salts, chemical nature of anion and the type of alkali metal cation have been investigated. The results of the research indicate that alkali metal salts are set accelerators of HAC. The lithium cation has more effect on the setting time than other alkali cations because of its ability to form tetrahedral symmetry while the others will form the octahedral type. Lithium salts remove the nucleation barrier, caused by an initially fast precipitation. The influence of pH in mixing water is not important. The effect of hydroxyl group is greater than effect of other investigated anions due to the replacement leading to a further centre for oxobridge formation. The lithium salts cause decreasing of flexural and compressive strength of HAC mortars but also cause the strength development at early ages.

Introduction

Lithium salts have been reported as accelerating setting agents for alumina concretes in the patent literature(l-3). Parker reported that the setting time of HAC pastes could be influenced by addition of small amounts of many materials. Double(4) and Currell(5) studied the chemistry of hydration of HAC in the presence of accelerating and retarding admixtures. Novinson et ai.(7,8) investigated the effects of lithium salts on refractory mortars. In chemical terms, the mode of action of these admixtures is not well understood. In this paper alkali metal salts as set accelerators of HAC have been studied. The influence of concentration of the salts, chemical nature of anions and the type

177

178 T. Matusinovic and N. Vrbos Vol. 23. No. 1

of alkali metal cations have been investigated. The research has

been carried out to develop rapid setting and hardening materials

for the repair of concrete. We have been conducting experiments to

determine the optimal setting time and the strength of the

materials made with lithium salt admixtures.

Experimental Methods

The high alumina cement used was a normal production of "Giulio

Revelante", Pula, Croatia. The general analysis of such cement was:

CaO, 40.2%, A1203, 39.0%, FeO, 4.3%, Fe203, 11.7%. The admixtures

used were commercial Analar grade reagents dissolved in

demineralized water prior to mixing with HAC. LizS and RbOH were

prepared in our laboratory (9-11). The setting time of HAC in

dependence of the pH of lithium salts in mixing water has been

measured. The pH of each solution was measured with a standard

glass electrode. The electrode was calibrated with buffer solution

at 25°C at pH 4 and 9. The salt solution was poured into the bowl

and the HAC were added to the water. These were mixed together by

means of mechanic stirrer for prescribed time intervals. All

experiments used a water/cement (w/c) ratio of 0.24. The setting

time was determined using a modification of the JUS ~ method

B.C8.023. In our modification the penetration of the needle into

hardening paste was measured every 5 seconds due to extremely rapid

setting time for lithium salt modified. The experiments were

repeated three times to obtain reliable standard deviations and

statistical means. For determination of compressive and flexural

strength the specimens (40 x 40 x 160 mm) were prepared according

to JUS B.C8.022 at w/c ratio of 0.5. The specimens were tested at

the age of 1,3,7,28, and 90 days. Three specimens were tested for

each age.

Results

The measurement of the influence of different mass fraction,(w), of

lithium nitrate on the setting time of HAC has been done to choose

the fraction of the salt which can give a setting time convenient

for the research. The results of the measurement are shown in Table

i.

The measurements of the influence of different alkali metal salts

on the setting time has been made. The results of the research are

shown in Table 2.

*JUS - Yugoslav standard

Vol. 23, No. I SET ACCEI22ATORS, ALKALI METAL SALTS, ALUMINATE CEMENT 179

TABLE 1

Lithium nitrate: Comparison of setting times at different mass

fractions.

Lithium salt

LINO 3

w(LiNO~)/%

0

0.0005

0.001

0.005

0.01

0.05

0.i

Setting time/s

16500

7330

1650

710

440

F

M

F - setting of HAC with lithium nitrate during the mixing time

M - setting of HAC immediately by adding of the lithium nitrate

TABLE 2

Alkali metal salts: Comparison of setting times at 0.01% mass

fraction.

Anions

Cations

OH- S = C03 = Br" CI- NO3 = SO Z

Setting time/s

Li

Na

K

Rb

Cs

300

5140

7200

9140

iiii0

325

5620

7400

370

6300

7980

9260

11990

305

8940

10120

10930

11200

360

9720

10200

10680

11400

440

6480

7920

9360

11480

560

7800

8100

9300

11630

- dash llne indicates that the chemical was not available

The different lithium salts have been tested and the results are

shown in the Table 3.

Discussion

The results of the research indicate that alkali metal salts are

set accelerators of HAC. The data in Table 1 demonstrate that LiNO 3

accelerates the setting time of HAC even at the mass fraction of

180 T. Mamsinovic and N. Vrbos VoL 23. No. 1

TABLE 3 Lithium salts: Comparison of setting times and pH of 0.01%

lithium salts in mixing water.

Lithium salt

LiOH

Li~S

Li2CO ~

LiBO 2

Li2SO 4

LiNO~

LiCI

LiBr

pH values

12.3

11.8

11.2

10.4

6.4

5.9

5.6

5.4

Setting time/s

300

325

370

390

560

440

360

305

TABLE 4 Flexural strengths of HAC mortars and HAC mortars made with 0.01% alkali metal salts.

Time/

days HAC + LiNO I

1 7.32

3 7.92

7 8.99

28 9.65

90 10.68

Flexural strength/MPa

HAC + NaNO~

7.44

8.30

9.88

10.12

10.77

HAC + KNO~

7.68

8.15

10.12

10.51

10.91

HAC + RbNO~

7.92

8.35

10.30

10.72

11.03

HAC + CsNO~

8.14

8.72

10.67

10.94

11.25

HAC

8.62

8.95

11.10

11.20

11.36

TABLE 5 Compressive strengths of HAC mortars and HAC mortars made with 0.01% alkali metal salts.

Time/

days HAC + LiNO~

1 49.18

3 61.87

7 70.10

28 76.25

90 82.08

Compressive strength/MPa

HAC + NaNO~

52.09

62.12

73.71

79.85

85.73

HAC + KNO~

53.62

63.78

75.05

82.29

88.75

HAC + RbNO~

55.42

64.95

79.55

86.90

91.15

HAC + CsNO~

56.45

66.35

HAC

58.70

67.97

82.66 84.50

88.75 92.60

93.12 96.25

Vol. 23, No. 1 SET ACCELERATORS, AI.,KALI METAL SALTS, ALUMINATE CEMENT 181

TABLE 6 Compressive and flexural strengths of HAC mortars and HAC mortars made with 0.05% Li2CO3.

Time/h Compressive strength/MPa Flexural strength/MPA

HAC HAC+ Li~CO 3 HAC HAC+ Li2CO 3

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

6.0

i0.0

15.0

20.0

X

X

X

X

X

X

X

2.65

8.80

24.58

36.65

43.83

46.77

48.12

1.82

3.86

4.89

5.93

8.96

12.60

13.12

18.33

20.02

21.25

23.87

30.62

32.16

36.56

X

X

X

X

X

X

X

X

2.19

2.64

3.34

4.61

6.95

7.22

X

X

X

1.42

2.44

3.11

3.93

4.21

4.37

4.56

4.96

6.48

6.88

7.15

x - test could not be performed because specimens were too soft to be removed from the mold

5xi0-4%. With increasing the fraction of LiNO~, the setting time decreases. With the fraction of 0.1% setting occurs immediately by adding of LiNO~.

The principal hydraulic constituent in HAC is CaAl204 (CA). The hydration process of CA is generally believed to occur through initial dissolution, formation of a metastable gel and subsequent

precipitation principally CaAI204xlOH20 (CAHI0), but also Ca2Al2OsxSH20 (C2AHB), and their conversion to Ca3A1206x6H20 (C3AH6) (12).

The composition of the hydration products shows a time-temperature dependency: the low-temperature hydration products (CAHI0) is thermodynamically unstable especially in warm and humid storage condition when a more stable compound, C~AH6, is formed. Laboratory and field experience with HAC concretes show that on prolonged storage the hexagonal CAHI0 and C2AH 8 phases tend to convert to the cubic C3AH 6 (13). After dissociation of CA, the formed metastable gel will acquire stability by condensation of monocoordinated OH groups linked to A1 to form oxobridges between

182 T. Matusinovic imdN. Vrbos Vol. 23, No. 1

two A1 centers leading to the crystalline CAHI0 (6).

To produce such an oxobridges condensation structure it is

necessary to bring the OH group in a position that a lone pair on

the oxygen can overlap with a d orbital of AI, resulting in the

formation of an oxobridge and a molecule of water which would

remain hydrogen bonded to the oxygen atom.

Solid-state NMR data indicate that aluminium in CA is entirely 4-

coordinated but the principal hydration products of CA, CAHI0, C2AHs,

AH~ and C3AH 6 contain 6-coordinated aluminium. The 27AI NMR work

shows that the hydration of calcium aluminate cements proceeds as

a conversion of 4- to 6- fold-coordinated aluminium (14).

In situ 2VAl NMR studies of the hydration process were carried out

on cement samples hydrated with demineralized water and with a

solution of Li2CO 3 in demineralized water (8). Without any additive,

the A1 conversion starts only after approximately 3-4 h induction

period and appears to end after 18-20 h. At a w/c ratio of 0.5 the

fraction of 4-coordinated aluminium in the final product amounts to

approximately 30% to 40% of the entire A1 content. The data

obtained with the additive are in sharp contrast to the hydration

with demineralized water, the conversion of AI(4) to AI(6) starts

immediately after mixing, if fraction of 0.5% aqueous lithium

carbonate solution is used as the hydration medium. However, the

rate constant of the aluminium conversion process is within

experimental error, identical to that found in the absence of the

lithium additive. Thus, the action of lithium-containing setting

accelerators is based on a shortening of the induction period while

having no effect on the rate of the phase transformation process.

The induction period during the precipitation of CAHI0 and C2AH 8 from

a supersaturated solution is a reflection of the nucleation barrier

to the formation of these compounds. Double et al. (5) attribute

the accelerating effect of lithium salts to a removal of this

nucleation barrier, caused by an initially fast precipitation of

lithium hydro meta aluminate. This compound is then thought to act

as a heterogeneous nucleation substrate, thus eliminating the

induction period. This view is entirely compatible with NMR results

presented by Novinson (8), which indicate that the accelerator

affects only the induction but not the rate of phase conversion

once the nucleation barrier has been broken. The NMR results suggest further that the measurements of setting time appears to be most closely related to the end of the induction period, at which point the AI(4)- to -AI(6) conversion is about to begin. The results of such a measurement in the present study prove it. The oxobridges condensation structures will be affected by alkali metal cations forming co-ordination linkage with the hydroxyl groups. Of the ions studied, Li* should be different in behaviour from the other cations because of the ability to form tetrahedral symmetry with OH groups, while Na ÷, K ÷, Rb ÷, and Cs ÷ will form the octahedral type (15). This is proved by our experimental results

Vol. 23, No. 1 SET ACCELERATORS, AIJfAIJ METAL SALTS, ALUMINATE CEMENT 183

(Tables 2 and 3), lithium having a drastic effect, but differences between the other cations are not great and exhibit a definitive trend.

The setting time of HAC with the same salts of alkali metal decreases in the following order:

Cs > Rb > K > Na >> Li

The sequence follows the trend of crystal radii, hydration numbers and enthalpies of hydration.

TABLE 7 Hydration data on the alkali metal cations (16).

Parameter Li Na K Rb Cs

Crystal radii/nm 0.068 0.098 0.133 0.148 0.167

Hydrated radii/nm 0.340 0.276 0.232 0.228 0.228

Hydration number 25 16 10 - 10

-AHh/kJ mol -l 530 420 340 315 280

The compressive strength of HAC mortars could not be measured within 4 hours because the specimens were too soft to be removed from the mold. After 4 h, when the period of induction time ended the specimens had the minimal compressive strength of 2.65 MPa. It can be seen from table 6 that there is a sudden increase in strength for HAC mortars up to the age of 4 h. After 20 h HAC mortar had a compressive strength of 48.12 MPa, approximately 50% values of the infinitive compressive strength. HAC mortars with lithium carbonate showed after 30 min a compressive strength of 1.82 MPa and increases rapidly with aging. These results support the results of research made by Double (5) as well as Novinson (8). The results of the measurements of compressive and flexural strength of alkali metal nitrates (Tables 4 and 5) show that alkali metal salts decrease the strength of HAC.

It is apparent that the strength increases with an increase in age of HAC mortars as well as HAC mortars with alkali metal salts. The investigated lithium salts can be classified in two groups, alkaline lithium salts, from lithium hydroxide and very weak acids (K^ ~ i0 -7 mol dm -3) and acid lithium salts derived from strong and very strong acids (K A ~ 10 -2 mol dm -3) (17). The data were plotted as setting times v_ss. pH of the solution.

Figs. 1 and 2 illustrate the relationship between setting time and pH for alkaline salts and acid salts.

Alkaline lithium salts, salts of weak and very weak acids (hydroxide, carbonate, sulfide and metaborate) have a linear

184 T. Matusinovlc and N. Vrbos VoL 23, No. 1

E

r-

400

I00

0

u3

I;3t) 0

v

SETTING TIME 1 -O.059pH + 3.2116

t=lO

I I I

I0 I I 12

pilaf 0.001°/o tithium salt in mixing water

FIG. 1 Setting times of paste v_ss. pH for alkaline lithium salts.

u')

QU,

E .,,.,

or) r-.

-$ u~

600

4O0

200

A

E} L ) Lt}

133 0

s< _ j

cC J Br~ - ~ ' ~ ' ' / r SETTING TIME

L t= 100"2599pH+ 1,0935

l I I

55 6 6.5

pilaf 0.001°/o [ithium sort in mixing water

FIG.2 Setting times of paste v__ss, pH of acid lithium salts.

Vol. 23, No. 1 SET ACCELERATORS, ALKALI METAL SALTS. ALUMINATE CEMENT 185

expression with a negative slope and follow the equation

t = 10 -°'°sgPs + 3.2116

where t is the setting time.

The acids salts (bromide, chloride, nitrate, sulphate) follow a linear plot wlth a positive slope and seem to obey the equation

t = 1 0 0"2599pH + 1.0935

The effect of hydroxyl group is greater than the effect of the other investigated anions due to the replacement of molecules H20 by hydroxyl groups in the A1 environment leading to a further centre for oxobridge formation. The other investigated anions have a lesser effect while they substitute OH groups in the coordination sphere of AI, which leads to the removal of a hydroxyl group necessary In the process of oxolation.

Acknowledqment

The authors acknowledge financial support from the Ministry of science, technology and informatics of Croatia.

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