Influence of calcium carbonate on the durability of mortar exposed to sea water solution

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Ramachandran, V. S.; Feldman, R. F.; Beaudoin, J. J.

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Influence of calcium carbonate on

the durability of mortar exposed

to sea water solution

Ramachandran, V.S.; Feldman, R.F.; Beaudoin, J.J.

NRCC-36893

A version of this document is published in / Une version de ce document se trouve dans:

Le Premier Colloque Canadien sur le Ciment et le Béton : The First Canadian Symposium on Cement and Concrete (Quebec City, Que., Canada, August-09-89), pp. 1-9, 89

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INFLUENCE OF CALCIUM CARBONATE ON THE

DURABILITY OF MORTAR EXPOSED TO SEA WATER SOLUTION

Vangi S. Ramachandran Rolf F. Feldman James J. Beaudoin

ABSTRACT

Normal portland cement mortar discs containing precipitated or ground limestone at dosages of 0, 2.5, 5 and 15% were made at water/cement ratios of 0.42 or 0.6. They were hydrated in lime water or sea water solution for periods up to 1 year. The length and modulus of elasticity changes were monitored periodically.

Mortars containing CaC03 (both coarse and fine) exhibited more than 3 times the expansion of the reference in sea water. The expansions were greater in samples containing fine CaC03. The modulus of elasticity were low especially in samples containing precipitated CaC03 and exposed to sea

water. ···

INTRODUCTION

. Various types of byproducts and waste materials such as fly ash, slag, silica fume and rice husk are used in concrete as additives with success. Limestone dust poses disposal and environmental problems and has been suggested for use as an additive to portland cement. The Canadian Standard CSA CAN3-A5.M83 allows the addition of up to 5% carbonate for Type 10 . normal portland cement and this has been extended to Type 30 high early

portland cement.

Although considerable amount of work has been carried out on the physical and mechanical properties of concrete containing limestone dust there are some conflicting results and also not much data is available on the

durability of concrete containing calcium carbonate. There is concern about the long term durability of concrete containing lime dust [1].

Feldman and Ramachandran have developed a non-destructive method of evaluating' the properties of mortar discs exposed to various solutions [2]. This method was adopted to determine the modulus and expansion of mortar bars containing 0-15% CaC03 of two finenesses and exposed to sea water up to a year.

Materials Section, Institute for Research in Construction, National Research Council Canada, Ottawa, KlA OR6

EXPERIMENTAL

Materials

Cement: Type 10 normal portland cement was used having the following

chemical composition: Si02 = 19.83%, Al203 = 4.18%, Fe203 = 3.20%, CaO = 60.06%,

MgO = 4.09%, Na20 = 0.45%, K20 = 0.89%, S03 = 3.93%, free lime= 1.15%, and loss on ignition = 1.53%. The mineralogical composition was as follows: C3S = 49.93%, C2S = 19.18%, C3A = 5.66%, C4AF = 9.74%. _{_;.}

,,,

ｾＺ＠ A laboratory concrete sand was used. The fineness modulus was 2.38. Sieve analysis according to sieve size (mm) and weight percentage passing in brackets was as follows: 5 (1 00); 2.5 (93.9); 1.25 (79.4); 0.63 (67.5); 0.32 (17.15); 0.1.4 (3.37).

Ca!cjQm CaCbonate: Two types of calcium carbonate were used. One type was . . .. . a precipitated carbonate supplied by Anachemia It had a surface area of 6.88

m2tg determined by the nitrogen adsorption method. The second type was obtained by grinding limestone to particle sizes finer than 15 IJ.m. Nitrogen surface area was 2.25 m2tg. Particle size analysis was performed using a Sedigraph particle size analyzer. The particle size range (diameter) for the precipitated material was 1·5 IJ.m. The ground material had particles in the range 1-40 IJ.m.

Test SolutjCms: Two solutions were used for sample exposure. One was a saturated lime solution. The other was a laboratory-prepared sea water containing 2. 7% NaCI, 0.32% MgCI2, 0.22% MgS04 and 0.13% CaS04. Specimen Preparation

Mortar mixes were prepared at water-cement ratios of 0.42 and 0.60. The cement:sand ratio for all mixes was 1:2.75. Calcium carbonate additions (ground and precipitated) were made in amounts of 0, 2.5, 5 and 15% by weight of cement. Superplasticizer (melamine type) was used for mixes at w/c = 0.42 containing 5, 15% calcium carbonate. The amounts were 0.4 and 1% by weight of cement respectively. Mortar cylinders (0 76.2 x 152.4 mm) were cast for each mix and cured for 7 days at 100% RH. Cylinders from each mix were then sliced to produce 6 mm thick discs. The discs were cured at 100% RH for an additional 7 days prior to immersion in the salt solution. Stainless steel Demec gauge reference points were positioned 50.8 mm apart on the specimen

surface for length change measurements. The reference points were inserted in pre-drilled holes and secured with epoxy adhesive. In addition to the disc specimens, mortar cubes (50.8 mm) were cast for compressive strength determinations at 14 days.

Test Methods

Mortar discs were placed in separate baths containing either lime

solution or sea water. Deflection and length change measurements were made

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at several time intervals up to 1 year. Deflection measurements were made on discs loaded at the midpoint and supported along the edge at points 120° apart. Loads applied using an lnstron testing machine did not exceed 35% of the failure loads. The modulus of elasticity was calculated using tho deflection data and elastic plate theory [3]. Length change measurements were obtained with a Demeo mechanical strain gauge. The smallest division on the gauge

corresponds to a strain of 2 x 1 o-3% .

Compressive strength measurements of mortar cubes were performed according to ASTM C190 .

RESULTS AND DISCUSSION

Modulus of Elasticity

Figures 1 and 2 present the results of the changes in moduli for mortars containing 0-15% CaC03 ground or precipitated respectively, and prepared at 0.42 and 0.60 w/c. Values are plotted for intervals up to one year.

All samples prepared at w/c = 0.42 and containing ground CaC03 show similar or slightly higher moduli with respect to the reference containing no CaC03 (Fig. 1 ). Addition of 15% precipitated CaC03 drastically reduces the modulus (Fig. 2).

All samples increase in modulus with time of exposure in lime water. On exposure to sea water, generally, all the samples containing ground or precipitated CaC03 show lower moduli than the references (Fig. 1 and Fig. 2). All samples containing precipitated CaC03 showed a significant reduction in modulus with respect to the reference.

At w/c of 0.6 all samples, both reference and those exposed to sea water, generally showed lower moduli (Figs. 1 and 2) than those fabricated at w/c =

0.42. Exposure to sea water of mortars (w/c

=

0.6) containing precipitated and ground CaC03 results in even more drastic reduction in moduli than was observed for samples prepared at w/c

=

0.42.

Length Change

The results of the length change measurements for the same samples for which moduli measurements have been carried out are presented in Figure 3 and Figure 4. At w/c

=

0.42 addition of ground CaC03 reduces the expansion values with respect to the reference whereas samples containing precipitated CaC03 showed enhanced expansion values.

On exposure to sea water, all samples show rapid expansion. The samples containing precipitated CaC03 and 15% ground CaC03 show much higher expansions than the corresponding reference specimens.

At w/c

=

0.6 addition of both precipitated and ground CaC03 reduces the expansion relative to the reference sample.

On exposure to sea water the expansions for samples containing both ground and precipitated CaC03 are significantly higher than the references. Generally, samples prepared at w/c of 0.6containing CaC03 expand more rapidly when exposed to sea water than those prepared at w/c = 0.42 .

lime water. These values were higher than was expected. Experiments were done to determine expansion values when pastes and mortars containing 0 and 5% CaC03 were autoclaved to 216°C, after precuring for 14 days. The

expansion values were found to be in the range of 0.06 and 0.08%. This confirms that the expansion observed during exposure to sea water is not the result of MgO or CaO hydration. The expansion of most of the specimens containing CaC03· when exposed to sea water, was greater than the reference samples. This may be related to the formation of carboaluminate and the

enhanced formation of thaumasite.

Compressive Strength

The behaviour of mortars exposed to sea water could depend on their strength before exposure. Compressive strength results for all samples moist cured for 14 days are presented in Figure 5.

All samples prepared at w/c = 0.42 exhibited higher strength than those prepared atw/c.= 0.60, In general, strengths are not affected by the addition of 2.5 to 150fc, ground CaC03 but the addition of 15% precipitated CaC03 results in a decrease of approximately 50%.

Although the compressive strengths (Fig. 5) are similar, in many cases, the samples containing both types of CaC03 exhibit much higher expansion than the reference samples when exposed to salt water (Figs. 3 and 4).

CONCLUSIONS

1. Use of small size mortar discs permits examination of the influence of a large number of test variables on their properties.

2. By using this method, the potential durability of a material such as cement with carbonate additions exposed to sea water, can be predicted.

3. Mortars containing CaC03 (both coarse and fine) have higher expansion values than the reference specimens when exposed to sea water.

However, those containing coarse CaC03 show relatively lower expansions than those containing fine CaC03.

4. Higher compressive strength does not necessarily correlate with better durability to sea water.

REFERENCES

1 . Mayfield,

L.L.,

"Limestone Additions to Portland Cement - An Old Controversy Revisited", Cement Concrete Aggregates, Vol. 10, 1988, pp. ＳｾＸＮ＠

2. Feldman, R.F. and Ramachandran, V.S., "New Accelerated Methods for Predicting Durability of Cementitious Materials", Proc. I International Conference Durability of Building Materials and Components, STP 691, American Society for Testing, 1980, pp. 313-325.

3. Timoshenko, S., "Theory of Plates and Shells", McGraw and Hill, New York and london, 1940, p. 295. I I .-.. ｾｲ＠

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