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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