Characterisation of the reaction products

The general description of the microstructure of reaction products as observed by SEM has been mentioned already. Since these were the first reported cases of ASR in India, detailed examination of the reaction products was undertaken in order to compare them with the features reported in the literature6715.

A composite gel sample was made in both cases by carefully scooping out the gel from various locations; this was used for the chemical analyses, X-ray diffraction and optical microscopy. The chemical analysis of the gels is presented in Table 11.1. The alkali contents were determined by flame photometry. The compositions were similar to the ranges indicated by others as representative of alkali-silica gel15.

11.2.3.1 Petrography. The composite gel was petrographically examined under a polarising microscope in immersion liquids. In the case of quartzite aggregates containing secondary silica minerals, the material showed the following distinct composition: (i) amorphous gel-type matter of irregular shape with a refractive

SL

no.

Constituents

Quantity (%)

Quartzite Granite aggregates aggregates

і

Loss on ignition

14.04

16.70

2

Si02

43.31

49.36

3

CaO

21.76

15.94

4

ai2o3

2.78

1.77

5

Fe203

0.66

0.49

6

MgO

0.83

0.49

7

Alkalis

(a) Na20

3.74

3.88

(b) k2o

12.88

11.71

index of 1.48-1.50 which compared favourably with 1.455-1.502 reported by Mather15; (ii) distinct small grains of chert or chalcedony with a refractive index of 1.50-1.52 (Figure 11.10); (iii) crystalline material with patches of opaque mineral; and (iv) crystals with slight anisotropy and no birefringence, presumably of the crystalline white deposits with a refractive index of 1.42-1.4816. The white material also showed occasional grains of aragonite and calcite with a refractive index of 1.65-1.66. In the case of granite aggregates containing strained quartz, similar features were noted, except for the presence of secondary silica such as chert or chalcedony.

image214

Figure 11.10 Grains of partially crystalline opaline silica in the reaction products (polarised

light, x25).

11.2.3.2 X-ray diffraction analysis. Typical X-ray diffractograms of the alkali – silica gel obtained in both cases are given in Figure 11.11. The nature of the gel was predominantly amorphous, and in addition to the typical cement hydration products, some new crystalline products, believed to be due to ASR, were identified16. In the case of quartzite pebbles, the peaks at 2? degrees = 5.5, 16.7 and 25.7 (Cu-Ka) were assigned to a crystalline alkali-silicate hydrate of composition NaSi17O13(OH)3’3H2O, and other prominent peaks at 2? degrees = 31, 29.8 and 29.4 to a composition K2Ca(SO4)2- H2O. In contrast, in the case of granitic aggregates, the prominent peaks at 2? degrees = 6.9, 13.6, 30.5 and

53.3 were ascribed to a composition of crystalline potassium-sodium-calcium – silicate hydrate (K2Na2Ca)16Si32O80- 2H2O. These compositions are different from these reported earlier17.

11.2.3.3 Infra-red spectroscopy (IR). The use of IR to study ASR has been reported before, when certain absorption bands in the 650-1600 wave number region were taken as characteristic of silica gel and calcium carbonates18. Composite samples of the reaction products in both the cases discussed in this chapter were studied by recording the IR spectra in the range 4000-200/cm, and compared with a synthetic silica gel. The samples were prepared by grinding and passing through a 45-p m sieve before drying in an oven at about 110°C for a few hours. The fine powder was made into a pellet with KBr.

The results are presented in Figure 11.12. In the case of the reaction products (alkali-silica gel) obtained in the two cases, the broad band observed in the region 3000-3600/cm is due to the O-H stretching vibrations. The

image215

image216

Figure 11.12 IR spectra of products of ASR and synthetic silica gels.

bands at 1400 and 1140/cm are assigned to carbonates and feldspars respectively. While the band at 1100/cm is due to monosulphates, the bands at 680 and 590/cm are assigned to S04 group. The band at 1000/cm indicates the presence of hydrated calcium silicates. Lastly, the peaks at 1030, 960, 860, 760, 460 and 440/cm are assigned to different modes of SiO4 vibrations.

Similarly, the IR spectrum from the synthetic silica gel contains a broad band in the range 3000-3600/cm. Most of the bands characteristic of different modes of Si04 vibrations, such as 1230, 1150, 1050, 940, 790 and 450/cm are observed. The occurrence of a ‘shoulder’ at 590/cm, which is assigned to SO| , is also observed. The absence of bands at 1400, 1100, 1000 and 680 described earlier is quite understandable as the alkali-silica gel was extracted from the concrete cores. In all other respects, the IR spectra of the two materials were similar.

In summary, the microstructure of the ASR products was predominantly of the amorphous gel type, with occasional crystals. In the case of metastable silica minerals, distinct reaction products seemed to be formed, whereas in the case of strained quartz the result of ASR was alteration in and of the aggregates16.

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