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Cement and Concrete Research 30 (2000) 831 832
Discussion
A discussion of the paper ``Durability of the hydrated limestonesilica

fume Portland cement mortars under sulphate attack'' by J. Zelic,
R. Krstulovic, E. Tkalcec and P. Krolo$
Edgardo F. Irassar*
Department of Civil Engineering, National University of Buenos Aires Center State, Av. Del Valle 5737, B7400JWI Olavarria, Argentina
Received 10 October 1999
A recent paper by Drs. J. Zelic, R. Krstulovic, E Tkalcec,

and P. Krolo [1] examines the effects of limestone filler and
silica fume on the sulfate resistance of mortar cements. I
agree with some parts of the paper, but I have some
commentaries that I would like to point out.
Form the experimental procedure, it is understood that
the authors have used mortar bars without gypsum added in
excess. Then, the test method that was used to determine the
potential sulfate resistance could not be the ASTM C 45268. This test method has been found effective in distinguishing the high C3A and the low C3A portland cements. It is
obvious that this method is not suitable for application in
blended portland cements, such as portland slag cements
and portland pozzolan cements [2,3]. For these types of
cement, test methods based on the external sulfate attack
with pH-controlled environment, such as the ASTM C
1012, are recommended. pH control is a decisive factor in
evaluating the attack created by gypsum formation, which
causes the reduction of strength and spalling of mortars.
The authors overlooked the important effect produced by
the addition of silica fume or limestone on the water/cement
ratio. They point out that the water/cement ratio varied from
0.50 to 0.67 to have the same flow consistency of the
mortar. However, they did not explain the changes observed
in the function of the increase of water/cement ratio.
The cement used is a commercially blended portland
cement containing blast furnace slag. However, the authors
failed to discuss the influence of slag or the interaction
between slag-silica fume and limestone filler. The slag
content was 30% in this commercial cement. The European
experience shows that, in order to obtain a good sulfateresistant cement, a minimum of 65% of the mass of slag is
needed (independent of Al2O3 content) [4]. On the other
$
Cem Concr Res 29 (l999) 819 826.

* Tel.: +54-2284-45-0628; fax: +54-2284-45-0628.
E-mail address: firassar@fio.unicen.edu.ar (E.F. Irassar).
hand, cements with low slag content (20% or 30%) are not
sulfate-resistant and sometimes, a low slag replacement level
could induce a reduction of sulfate resistance [5,6]. If the
cement used has no slag content, the sulfate resistance was
probably even worse due to the high content of C3S (70%) in
the clinker. The influence of C3S on the sulfate resistance of
portland cement has been discussed by several researches
[7 9] and the combination of high C3S, limestone filler and
pozzolans has been studied recently in our laboratory [10].
The synergic effect between the additions has been proven,
and should be taken into account. The authors cannot ignore
the composition of clinker and the addition of slag to judge
the effect of limestone filler and silica fume.
The authors also discussed the effect of blended cement
with silica fume (series P) and the effect of limestone filler
and silica fume (series PK). In the sulfate resistance of
mortar, the amount of CH and the location of crystal play a
decisive role. The precipitation of CH crystals at very early
stages causes massive deposits of oriented crystals on the
paste aggregate interface, which lead to the inevitable
increase of porosity in this region. Thereafter, in the sulfate
environment, several authors have observed a massive
gypsum formation around the aggregates. According to
Mehta [11], the presence of CH to form a gypsum environment is also needed for the expansive ettringite formation,
too. In this case, limestone filler increases the portland
cement hydration rate, leading to the precipitation of CH
located around the filler grains and the aggregate surfaces.
As the authors point out, the principal contribution of silica
fume is the CH consumption, while the limestone filler does
not contribute to the CH reduction. From the XRD pattern
presented for the PK series, the reduction of CH is the main
change when silica fume was added. From XRD in Fig. 5 of
Ref. [1], the reduction of the CH peak and the increase of
gypsum peak in P-0 mortar is the remarkable difference in
comparison to the P-8 mortarwhere the CH peak is
present, but not the gypsum peaks. The beneficial effect
0008-8846/00/$ see front matter D 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 0 8 - 8 8 4 6 ( 0 0 ) 0 0 2 4 6 - 5
832
E.F. Irassar / Cement and Concrete Research 30 (2000) 831832
of silica fume may be also computed in the pore size

refinement, which blocks the sulfate ingress into the mortar
structure. The XRD pattern shows the presence of CH, but
the sulfate ions after 120 days cannot ingress and convert
the CH to gypsum. Some long crystals of ettringite are
shown as evidence of sulfate reaction, but they could not
induce expansion because crystal growth into the pores was
not restricted and the surrounding environments are dominated by CH, as indicated in the XRD analysis. The
expansive cracking occurs when the internal stress overcomes the tensile strength and the mortar presents a dominant gypsum environment due to the continuous removal of
CH. Under this condition, Mehta [11] suggests that the CSH
starts to lose strength and stiffness, and the ettringite crystals
become expansive. This is the scene in the P-0 mortar.
The positive effect of the limestone filler addition (PK
series) on sulfate resistance is demonstrated by the extended
failure timefrom 60 (P-0) to 150 days (PK-0). The authors
affirm that the sulfate attack is centered on the hydrated
calcium aluminate, and its transformation into more stable
compounds (such as monocarboaluminate) during the hydration of cement contribute to the better stability of the PK
series in sulfate solution. However, for PK-0, a strong peak
of ettringite was detected, which confirmed my dissident
opinion that monocarboaluminate is an unstable compound
in sulfate environment.
For the PK series, the authors omitted to discuss the
dilution effect of the harmful compounds of portland clinker
in sulfate the environment (C3A and C3S), which has been
indicated to contribute to the improvement of the sulfate
performance of portland cements [12,13]. In this experiment, C3A dilution has significance because the cement
used has a 6% C3A content. On the other hand, the dilution
of C3S can be analyzed in association with the increase of
the C3S hydration rate induced by the addition of limestone
filler [14].
I would agree with the authors that during cement
hydration, carbonate ions coming from limestone filler
modify the hydration products. Reaction products from
C3A phase of portland cement in the presence of limestone
filler include: monocarboaluminate, monosulfoaluminate,
and ettringite. Vernet and Noworyta [15] report that monocarboaluminate stability is better than those of the monosulfoaluminate and aluminate hydrates, and these authors
also inferred a relative greater resistance of limestone filler
cements exposed to sulfate attack. In our experimental
studies [14,16], monocarboaluminate formation was detected by XRD after 3 days of water curing, and the
transformation of monosulfoaluminate into monocarboaluminate and ettringite was detected in 28 days. But, the XRD
results (see Fig. 5 of Ref. [14]) of mortars with limestone
filler immersed in sulfate solution denote that the ettringite
formation originated by the transformation of monocarboaluminate in the sulfate environment. Their conclusions
overlook the fact that ettringite is more stable than monocarboaluminate in the sulfate environment as described in
the solubility product and stability study (Ksp (monosulfoaluminate) = 1.7 1028, Ksp (monocarboaluminate) = 1.4
1030, Ksp (ettringite) = 1.1 1040 [17]). The only
contribution that I can glean from this point of view is
related to the part of aluminates that remain as ettringite (a
stable compound in sulfate medium) due to the monosulfoaluminate decomposition by carbonate ions in the limestone filler cement. However, monocarboaluminate is an
unstable compound in the presence of a new external source
of sulfate ions, as shown by the XRD analysis of limestone
filler cement studied in this paper and in our previous paper.
References
[1] J. Zelic, R. Krstulovic, E. Tkalcec, P. Krolo, Durability of the hydrated
limestone silica fume Portland cement mortars under sulphate attack,
Cem Concr Res 29 (1999) 819 826.
[2] ASTM C 452-68, Standard Method of Test Potential Expansion of
Portland Cement Mortars Exposed to Sulfate, Annual Book of Standards, ASTM, Philadelphia, 1971 Part 9.
[3] P.K. Mehta, O.E. Gjorv, A new test for sulfate resistance of cement,
J Test Eval 2 (1974) 510 514.
[4] J. Geisler, H. Kollo, E. Lang, Influence of blast furnace cements on
the durability of concrete structures, ACI Mater J 92 (1995) 252 257.
[5] ACI Committee, ACI 226.1R-87: Ground granulated blast furnace
slag as a cementitious constituent in concrete and mortar, ACI Mater
J 92 (1995) 321 322.
[6] J. Frearson, Sulfate resistance of combinations of portland cement and
ground granulated blast furnace slag, Fly Ash, Silica Fume, Slag, and
Natural Pozzolans in Concrete, SP-91, American Concrete Institute,
Detroit, MI, 1986, pp. 1495 1524.
[7] D. Dimic, S. Droljc, The influence of alite contents on the sulfate
resistance of portland cement, Porc. 8th. Int. Congress on the Chemistry of Cement, Rio de Janeiro, Brazil, 1986, V, (1986) 195 199.
[8] Rasheeduzzafar, Influence of cement composition on concrete durability, ACI Mater J 89 (1992) 574 586.
[9] M. Gonzalez, E.F. Irassar, Ettringite formation in low C3A portland
cement exposed to sodium sulfate solution, Cem Concr Res 27 (1997)
1061 1072.
[10] E.F. Irassar, M. Gonzalez, V. Rahhal, Sulphate resistance of type V
cements with limestone filler and natural pozzolan, Cem Concr Comp
submitted.
[11] P.K. Mehta, Mechanism of sulfate attack on portland cement concrete another look, Cem Concr Res 13 (1983) 401 406.
[12] I. Soroka, Effect of calcareous filler on sulfate resistance of portland
cement, Ceram Bull 55 (1976) 594 595.
[13] R.D. Hooton, Effects of carbonate additions on heat of hydration and
sulfate resistance of portland cements, in: R.D. Hooton (Ed.), Carbonate Additions to Cement, ASTM STP 1064, Philadelphia, 1990, pp.
73 81.
[14] M. Gonzalez, E.F. Irassar, Effect of limestone filler on the sulfate
resistance of low C3A portland cement, Cem Concr Res 28 (1998)
1655 1667.
[15] C. Vernet, Noworyta, Mechanism of limestone fillers reactions in the
system (C3A CSH2 CH CC H), Proceedings of 9th International
Congress on the Chemistry of Cement, New Delhi, India, (1992)
430 436.
[16] V. Bonavetti, V. Rahhall, E.F. Irassar, Studies on the carboaluminate
formation in limestone filler blended cements, Cem Concr Res
submitted.
[17] F. Zhang, Z. Zhou, Z. Lou, Solubility product and stability of ettringite, Proceedings of 7th International Congress of the Chemistry of
Cement, Paris, France II, (1980) 88 93.

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Cement and Concrete Research 30 (2000) 831 832

A discussion of the paper ``Durability of the hydrated limestonesilica

A recent paper by Drs. J. Zelic, R. Krstulovic, E Tkalcec,

Cem Concr Res 29 (l999) 819 826.

E.F. Irassar / Cement and Concrete Research 30 (2000) 831832

of silica fume may be also computed in the pore size

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