morphology of the cement-aggregate bond

The International Journal of Cement Composites and Lightweight Concrete, Volume 5, Number 2 May 1983

M o r p h o l o g y of the c e m e n t - aggregate bond • L. Struble f and S. Mindess $

* Paper presented at the International Conference on Bond in Concrete at Paisley College of Technology, Scotland, June 14-16, 1982. This paper was received too late for inclusion in the Proceedings but has been reviewed for inclusion in the Journal. tLeslie Struble is a scientist at Martin Marietta Laboratories. She works on projects relating to cement and concrete chemistry for both Martin Marietta Laboratories and outside contracts. SDr Sidney Mindess is a Professor in the Department of Civil Engineering at the University of British Columbia, Vancouver, Canada. He teaches and works on research projects dealing with chemistry and microstructure of inorganic building materials, especially concrete, and with materials testing.

@ Construction Press 1983

0262-5075/83/05210079/$02.00

SYNOPSIS The morphology of the interface between cement paste and rock has been studied by SEM at ages of 7 to 9 days. Several typical morphologies are shown. Adjacent to the rock surface, specimens generally exhibit a layer of apparently well-oriented calcium hydroxide crystals, but did not appear to constitute a duplex film.

KEYWORDS Cement-aggregate bond, interface morphology, bond strength, chemical properties, surface chemistry, concrete, portland cements, pastes, aggregates, crushed stone, limestone, adhesive bonding, microstructure, interfaces.

INTRODUCTION The bond that forms between cement paste and aggregate is an important consideration in concrete performance. As was summarized previously [1], the bond may influence concrete strength and probably influences concrete durability. However, the precise relationship between the nature of the bond and the performance of the concrete has yet to be determined. The nature of the bond itself is a matter of some controversy. It is not clear to what extent the bond results from mechanical interlocking of cement hydration products on the aggregate surface, from chemical reaction between cement paste and aggregate, or from a combination of both processes.

The present paper, describing part of an in-depth study of the paste-aggregate bond, highlights some preliminary investigations on the interface morphology. Subsequent studies will relate the interface morphology to tensile strength and permeability of the bond.

EXPERIMENTAL DETAILS Materials consisted of a fine-grained limestone rock and two cements that differ slightly in their calculated C3A contents. Chemical compositions of these materials are provided in Table 1.

Specimen preparation methods were based on:

1. duplicating the interface between cement paste and rock in concrete,

2. controlling certain experimental variables, and 3. providing the specific configuration needed for each

test.

In order to simulate the paste-rock interface in concrete, cement paste-rock composites are prepared by mixing paste and casting it against the rock surface. Pastes had a water-to-cement ratio of 0.35, which provided a reasonable consistency for casting. After one day, the specimens were demoulded and stored in saturated Ca(OH )2 solution until examined.

The rock surface was prepared so as to minimize the effects of surface roughness on the bond properties, especially its strength and permeability. In this way, varying the rock type would not affect the surface area available for bonding, but would only affect the chemical interaction, if any, between rock and paste. Rock surfaces shown in this study were ground to 600 grit, so

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Morphology of the cement-aggregate bond Struble and Mindess

Table 1 Chemical compositions of cements and rock

Cement 1 Cement 2 Limestone

Na20 0.27 0.40 <0.1 MgO 1.10 2.59 1.37 AI203 4.91 5.21 1.13 SiO2 21.67 20.12 2.79 P~O~ 0.18 0.27 0.06 SO3 2.59 3.37 0.15 K~O 0.75 0.97 0.37 CaO 63.05 61.71 51.77 TiO~ 0.16 0.22 0.05 Mn=O~ 0.12 0.10 0.03 Fe~O~ 3.77 3.05 0.21 LOI 0.88 0.90 Not determined Total 99.65 99.19 57.93

C3S ~: 46.2 49.2 C2S 27.3 20.5 C~A 6.6 8.7 C,AF 11.5 9.3

" B o g u e c a l c u l a t i o n s

that on a micron scale the roughness of each rock would be similar. It was subsequently found that neither this grinding nor polishing significantly smoothed the surface, and rock surfaces are now prepared by cutting wi th a low-speed diamond saw.

Specimens were cast in the form of 5-mm cubes, half rock and half paste. Their small size is convenient for electron-optical analysis. The fracture process, squeez- ing the specimen between two wedges, either along the interface or normal to it, is shown schematically in Figure 1.

A JSM U-3 scanning electron microscope, which does energy dispersive X-ray analysis (EDS), was used for the morphology investigations.

RESULTS

Fracture along the interface A ground limestone surface and a fractured paste surface are shown in Figure 2 and Figure 3, respectively. These two materials were not involved in paste-aggregate bonding and thus serve as controls. The rock surface is somewhat rough and individual crystals are evident. The bulk paste (prepared from cement 1 ) is fairly dense and few morphological features are apparent on a micron scale.

Paste

J

/ i

J

Rock

Figure 1 Specimen configurations for morphology examinations, showing fracture along the interface and normal to it

,11

I

J

J

J

f

J

.i,"1

f

j - . .

J

7

J

split along interface

split normal to interface

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Figure 2 SEM micrograph of ground limestone surface

Figure 3 Fracture surface of 7-day old paste

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Both paste and rock surfaces of a 7-day old specimen of cement 1 and limestone, fractured along its interface, are shown in Figure 4. Back-scatter was used to enhance the topography. Specimens generally fractured right at the interface, but it is evident that a large area of paste adhered to the rock.

Closer examination of the rock surface and its adhering paste shows interesting morphological features. The hydration products of the adhering paste (Figure 5) exhibit a better-developed morphology than the bulk paste of Figure 3. Figure 6 is the region right to the edge of this adhering paste. Away from the adhering

Figure 4 Micrographs (backscattered) of a 7-day old paste-rock composite fractured parallel to the interface; (a) paste surface, and (b) rock surface with adhering paste

Figure 5

(c)

Paste adhering to the rock surface of the fractured composite of Figure 4 (a and b), and diagram showing the region of the micrographs (c)

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paste (Figure 7), there are sparse regions of hydration product, but most of the rock appears similar to the unbonded rock surface of Figure 2.

The paste half of the fractured sample is shown in Figure 8. The hydration products are dense, though not as featureless as was the bulk paste of Figure 3.

However, the well-developed morphology of the paste that adhered to the rock during fracture, shown in Figure 5, is not evident in Figure 8.

Fracture normal to the interface Viewing a similar interface from the side (fracturing the specimen normal

Figure 6 Paste adhering to the rock surface of the fractured composite of Figure 4 (a and b), and diagram showing the region of the micrographs

A

Figure 7 Rock surface of the fractured composite of Figure 4(a), and diagram showing the region of the micrograph (b)

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Figure 8 Paste surface of the fractured composite of Figure 4.

Molar ratios

1. CaQ 11.7 3. CaO 2.0 SiO 2 1.0 SiO ~ 1.0

S03 0.2 2. CaO 3.9

Si02 1.0 AI203 0,3 SO3 0.2 Fe203 O. 1

4. AI203 1.0

5. CaO 1.0

Figure 9 Rock (right) and paste (left) of the fractured composites of Figure 9and Figure lO, with molar oxide ratios calculated from EDS analyses of the indicated areas

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to the interface) provides a somewhat different picture. This specimen (Figures 9-11 ), fractured when 9-days old, was prepared using cement 2. Figure 9 includes molar oxide ratios calculated from EDS analyses of several designated regions. The separations were clean, with no paste adhering to the rock.

Similar to the findings on fracture surfaces along the interface, the cement hydration products right at the interface (Figure 10c) are similar to those away from the interface (Figure lOb). The morphology in Figure 10c appears slightly more crystalline than in Figure lOb, but the effect is not as obvious as in Figure 5. Figure 10 and Figure 11 illustrate a feature that was not apparent in parallel fractures; a somewhat continuous layer, several microns thick, of apparently parallel calcium hydroxide crystals at the interface.

DISCUSSION These examples of the morphology of the interface between cement paste and aggregate provide too limited a sampling to warrant drawing conclusions about the nature of the bond. However, the authors wish to point out a few experimental observations that relate to other studies.

A significant feature of the paste-quartz interface has been a Ca(OH )2 duplex film - - plates of Ca(OH )2 oriented parallel to the surface and backed by C-S-H [2]. A similar duplex film was described for a paste-limestone interface [3]. However, others have found a localized increase in Ca(Ol-I)2 crystals, but no observed preferred orientation of Ca(OH)2 crystals [4]. The authors also did not observe any duplex film such as was shown in the first two cited studies. A definite layer of apparently

Figure 10 Micrographs of a 9-day old paste-rock composite fractured normal to the interface; (b) and (c) selected regions on the paste side of (a)

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Figure 11 Rock (left) and paste (right) of the fractured composite of Figure 9

well-oriented Ca(OH )2 crystals was, however, found at the interface. This layer was not apparent on the fractured surfaces, but was only observed when viewing a surface normal to the interface.

In one study, it was suggested that the paste at the interface has a higher water-to-cement ratio than the

bulk paste and is enriched in cement fines, which hydrated more rapidly [4]. The observation of cement paste near the interface with an especially well- developed morphology supports this suggestion.

A C K N O W L E D G E M E N T S Support by the US Army Research Office is gratefully acknowledged. The authors are indebted to M. Meyerhoff for the SEM micrographs presented here.

REFERENCES 1. Struble, L., Skalny, J. and Mindess, S., 'A review of

the cement-aggregate bond', Cement and Concrete Research, Vol. 10, No. 2, March 1980, pp. 277-86.

2. Barnes, B. D., Diamond, S. and Dolch, W. L., 'Micromorphology of the interfacial zone around aggregates in portland cement mortar', Journal, American Ceramic Society, Vol. 62, No. 2, 1979, pp. 21-24.

3. Langton, C. A. and Roy, D. M., 'Morphology and microstructure of cement paste/rock interfacial regions', 7th International Congress on the Chemis- try of Cement, Vol. III, 1980, pp. V11-127-VI1-132. Conjeaud, M., Lelong, B. and Cariou, B., 'Liaison p~te de ciment portland - granulets naturels', 7th International Congress on the Chemistry of Cement, Vol. Ill, 1980, pp. VI I -6-VI1-11.

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morphology of the cement-aggregate bond

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