CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 15-22, 1992. Printed in the USA. 0008-8846/92 $5.00+.00. Copyright © 1991 Pergamon Press plc.

EFFECTS OF SILICA FUME ON ALKALI-SILICA EXPANSION IN MORTAR SPECIMENS

Karin Pettersson Swedish Cement and Concrete Research Institute

Stockholm, Sweden

(Communicated by G. Fagerlund) (Received May 22, 1990)

ABSTRACT

Alkali aggregate reactions have become a serious problem for concrete structures in recent years in Sweden. The major alkali-aggregate reaction products include siliceous gel. This gel can easily absorb water and increase considerably in volume. This will result in expansion and destruction of the concrete. The present paper reports some results of a study where granulated silica fume caused cracks in mortar in a similar way to alkali-silica expansion.

Since alkali-silica reaction (ASR) in concrete was recognized, it has been suggested that the incorporation of fine reactive pozzolans would be beneficial in reducing concrete expansion resulting from it. Their effectiviness varies widely, and much discussion of it is present in the literature.

The use of silica fume to control ASR in concrete has been reported in several studies. Diamond (I) demonstrated that reducing the alkali hydroxide concentration in pore solution to a critical level by the addition of pozzolans inhibits the alkali-silica reaction. Direct comparisons of effects of replacing part of cement with various pozzolans on alkalies and OH- ion concentration in pore solution and on the alkali- silica expansion have a possibility of revealing the differ- ences in their functions in reducing the expansion, (2)

During some experiments related to corrosion research it was noted that granulated silica fume caused cracking in mortar specimens when they were immersed in 1 M NaCl. Against this background we made some more mortar specimens where 10% of the cement content was replaced with silica fume, granulated and dispersed. The specimens were stored in a solution containing

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16 K. Pettersson Vol. 22, No. 1

IM NaCl in saturated Ca(OH)2. The purpose was to find out if the silica fume influenced £he ASR expansion because of the NaCl.

ExPerimental Droaramme

Ordinary portland cement from Sweden was used. The alkali content expressed as equivalent Na20 + 0.658 K20 was 1.1%. The chemical analysis is given in Table i.

TABLE i.

Chemical Analysis

Material CaO SiO 2 A1203 Fe203 SO 3 MgO Na20 K20 I g n i t i o n loss

OPC 63.0 19.7 4.6 2.3 3.2 3.2 0.3 1.2 2.2

Two types of silica fume were used, one was granulated and the other was dispersed in water 50:50 by weight.

FIG i. Mortar specimens including 10% of compacted silica fume. The specimens have been immersed in saturated Ca(OH)2 + IM NaCl for two months.

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FIG 2. Mortar specimens including 10% of dispersed silica fume, slurry and a reference mortar without silica fume. The specimens have been immersed in saturated Ca(OH) 2 + IM NaCl for two months.

Mortar specimens were cast with and, as a reference, without silica fume, binder:aggregate ratio 1:3 and a water:binder ratio of 0.6. The cement content was replaced by silica fume of about 10% by weight. Mortar specimens of 20 x 20 x 160 mm were cast and stored in 100% RH at +20°C for 28 days. Two series of tests, A and B, were made in this investigation. In series A, three specimens from each mix, compacted silica fume, dispersed silica fume and a reference specimen without any silica, were put in IM KCI solution where the pH was adjusted to 13.5 with NaOH and saturated with Ca(OH) In series B, three specimens from each mix were put in ~M NaCI solution, where the pH was adjusted to 13.5 with NaOH and saturated with Ca(OH)2.

Results and discussion

After storage of the mortar specimens in the respective solu- tions during two months, small cracks could be seen on the specimens containing granulated silica fume immersed in sodium chloride. After three months in the solution the cracking had a width of about 1 mm. Figs. 1-2 show some mortar specimens from series B. Fig.3 shows granulated silica fume. It can be seen that the the expansion of mortar with compacted silica fume was very large and that nothing had happened on the other specimens.

One theory for this cracking could be that not all of the com- pacted silica fume has reacted with the cement component and

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FIG 3. Granulated silica fume.

therefore it is possible for some part of it to react with other components and create silica gel, with expansion and destruction of the mortar specimens.

FIG 4. The cracking in mortar specimens with 10% granulated silica fume, after two months in saturated Ca(OH)2 + IM NaOH.

Vol. 22, No. 1 SILICA FUME, ALKALI-SR/CA REACTION, EXPANSION 19

FIG 5. The cracking in a higher magnification.

Analysis with SEM/EDAX has been made of broken mortar speci- mens with granulated silica fume which had been immersed for three months in IM NaCl. Figs. 4 and 5 show small black spots consisting of granules from compacted silica which still have not reacted with the cement components. The cracking structure is also visible. A lot of the cracks have their origin in the silica granule. Fig. 6a shows, to a larger scale, the granulated silica fume in mortar specimens after two months in saturated IM NaCI.

In the cement paste, the silica gel produced by the reaction is found in pores, and/or fractures. In the pores, the gel is often in the form of small opalescent scales having a smooth surface. These are formed by desiccation of the silica gel, (3), (Fig. 7c). This kind of structure has been seen in these experiments, (Fig. 7a and b).

In the solution with KCI nothing had happened after 4 months. No effects were seen in the reference specimens.

The results of the experiments indicated that granulated and dispersed silica fume behaved differently in their effect on mortar specimens when they were immersed in IM NaCl solution including saturated Ca(OH)2. For the specimens with granulated silica fume cracks were observed after about two month in the solution, while nothing happened with the specimens containing dispersed silica fume and the specimens without silica fume.

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FIG 6a. Granulated silica fume in mortar specimens after two months in saturated Ca(0H)2 + IM NaCl.

S I K ~

II K

- - " 1 ~ " I " " ' . . . . : u , a . , , a . p

1. O0 ~.00 B. OO 4. OO 5. OO

I 5 7 c N T 3 . 3 1 K E V IO~V/c4 A E O A

FIG 6b. Spectrum of an EDAX analysis of Fig 6a. (Si=silicon,

Ca=calcium).

Vol. 22, No. 1 SILICA FUME, ALKALI-SILICA REACTION, EXPANSION 21

FIG 7a. Cement paste in mortar specimens with 10% granulated silica fume after two months in saturated Ca(OH)2 + IM NaCl.

An explanation of the cracking on the mortar specimens could be that not all of the granulated silica fume has reacted with the cement components and therefore it has been possible for

FIG 7b. Cement paste in mortar specimens with 10% granulated silica fume after two months in saturated Ca(OH)2 + IM NaCI.

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M ~ A

1 . O 0

7 I:::: N T

:1. K ( x

C~K~

2. OO 3. OO 4. OO :~ . 4 4 K E V l O = , V / c h A E 1:3 A X

FIG 7c. Spectrum of an EDAX analysis of Fig 7a.

the remaining part to react with other ions in the solution and create expansive gel.

These results from mortar specimens may not have direct relevance to concrete since in concrete when mixing most granules are disintegrated due to the grinding effect of the aggregate. However, it is known that disintegration may not be complete since fragments of granules are often detectable by thin section analysis. The non-relevance of the reported effect for any concrete is therefore not completely settled. Further investigations have to be done on this.

The reported results might be of importance for the salt- frost-scaling properties and for the corresponding testing method(s) of silica fume containing concrete~mortar~paste.

References

i. Diamond S, Alkali Reactions in Concrete-Pore Solution Effect, Proc. 6th Int. Conf. Alkalies in Concrete, Copenhagen, 1983, pp. 155-166. 2. Kawamura and Takemoto, Effects of Pozzolans and A Blast Furnace Slag on Alkali Hydroxides Concentrations in Pore Solutions and Alkali-Silica Expansion. Cement Ass. of Japan, 1986. pp 262-265. 3. Durand and Berard, Use of gel composition as a criterion for diagnosis of alkali-aggregate reactivity in concrete containing siliceous limestone aggregate. Materials and Structures/Mat~riaux et Construction, 1987, 20, pp 39-43.