Keywords

1 Introduction

Nowadays, sustainable mining is increasingly considered as a more accessible challenge. To include mining waste in sustainable management practices is a global consensus.

Mining industry produces a large amount of waste rocks with different properties related to formation processes. Specifically, crystalline rocks are widely used as ornamental stone and as raw material in civil construction, mainly as concrete aggregate, due to their suitable properties.

ASTM defines aggregates as granular materials of mineral composition such as sand, gravel, shell, slag or crushed stone used with a cementing medium to form mortars or concrete (ASTM Standard 2016a). The use of aggregates for concrete is widely regularized by ASTM Standard (2009, 2011, 2013, 2014a, b, c, 2016b).

On the other hand, alkali-aggregate reactivity (AAR) is considered one of the most important pathologies in civil construction. Two types of Alkali-aggregate reactivity are commonly defined, the alkali-silica reaction (ASR), where there are two variants, depending on the velocity of the reaction and the alkali-carbonate reaction (ACR). Both are complex reactions of aggregates and alkali hydroxides in concrete that create expansion processes, and provoke, potential structural damages.

Although ASR pathologies in structures have been reported around the world related to crystalline aggregates (Ian Sims and Poole 2017) these materials are commonly used in civil construction, having obtained successful results.

Table 1 summarizes the main reactive minerals and the respective occurrence rocks. Clearly, potential reactive minerals occur in just about all the common rocks, which emphasizes the importance of accurate analysis in aggregates.

Table 1 Some potential reactive minerals
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The U.S. Department of Transportation (2012) defines the combination of petrographic examination, expansion testing and field performance as required actions to confirm the non-deleteriously-reactive nature of the aggregates.

Petrographic description of aggregates is a very well known method to identify its reactivity. It is due, basically, to the reliability and quickness that this technique provides. However, it is strongly recommended to confirm the results by other methods, specifically mechanical tests as the accelerated mortar test (ASTM Standard 1994).

Recently, Sanchez et al. (2017) have stood out microscopic techniques as important complementary tools in the AAR assessment. In addition, microscopic examination is highlighted as the appropriated measurement of deformation intensity (Murlidhar et al. 2016).

Specifically for rocks with quartz, the undulatory extinction in quartz is a criterion strongly used for identifying potentially alkali-reactive reactions (Dolar-Mantuani 1981). Consequently, rocks with micro and cryptocrystalline quartz moderately to strongly strained are considered as susceptible to AAR (ABNT 2008; CSA 1994).

The amount of strained and microcrystalline quartz needed to provoke potential reactivity reaction is variable. There are no defined values, consensually. Many factors can contribute to ASR-induced damage; consequently, cases should be studied separately. In this way, Brazilian norm suggested values of 5% of strained quartz, 3% of chalcedony, 1% of tridymite-cristobalite, 3% volcanic glass, 0.5% opal. Under these values, the aggregate is considered potentially innocuous. These values can be changed by petrographer considerations (ABNT 2008).

The aim of this paper is to evaluate the applicability as concrete aggregates of waste rocks by petrographic characterization I n the context of sustainable mining.

2 Materials and Methods

Locally, the study area is geologically inserted in the Structural Province of Borborema (Fig. 1), with composition and organization essentially Neoproterozoic (Brasiliano Domain). The stratigraphic units in this area are predominantly composed by formations inserted in the Seridó Group, associated to the Brasiliano/Pan-African Orogeny, including the Itaporanga intrusive suite, resulting of a Brasiliano plutonism (Angelim 2007).

Fig. 1
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Source Modified from Angelim et al. (2006)

Location and geological characterization of the study area.

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Tectonically, the Seridó Group experienced three deformational events causing compositional banding, geological folds and NNE-SSW foliation, respectively. The last one corresponding to the Brasiliano transcurrent Kinematic, with shear areas (Jardim de Sá 1984, 1987, 1994).

Samples of waste rocks from feldspar mining at Parelhas/RN, Brazil were collected. The samples are included in the intrusive suite Itaporanga. Geologically, the rock is classified as granite gneiss. Petrographic examination of samples was performed according to Brazilian norm (ABNT 2008) based in international standards (ASTM Standard 2012) with a Polarizing Microscope Nikon Eclipse E200MV POL. Table 2 shows the procedures followed.

Table 2 Procedures in thin sections
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3 Results and Discussion

Figures 2, 3, 4, 5 and 6 show thin sections of the rock samples. The minerals identified are quartz, k-feldspar, plagioclase, biotite, opaques, apatite, and zircon. Mostly, the rock presents a medium-grained texture, inequigranular, with the largest crystals of quartz, without a clear out mineral orientation, although biotite crystals show slightly preferential orientation (Fig. 2).

Fig. 2
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General overview. Biotite crystals show, slightly, preferential orientation. Crossed polarized light (upper photograph), plane-polarized light (lower photograph).

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Fig. 3
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Xenomorphic crystals of quartz, with undulatory extinction and micaceous material filling a fracture, with bright birefringence colours (crossed polarized light). Qz: quartz; Mcr: microcline; Pl: plagioclase

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Fig. 4
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Xenomorphic-hypidiomorphic microcline crystals showing cross-hatched twinning (crossed polarized light). Bt: biotite; Mcr: microcline; Pl: plagioclase

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Fig. 5
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Large alkali feldspars with string perthite (crossed polarized light). Per: perthite; qz: quartz; Mcr: microcline

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Fig. 6
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Crystals of plagioclase with slight alteration and triple junctions (crossed polarized light)

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The predominant mineral is quartz (Table 3) appearing essentially, as xenomorphic crystals, clear and colourless, with undulatory extinction and with fractures, which are occasionally filled by fine micaceous material, with bright birefringence colours (Fig. 3).

Table 3 Modal percentage of minerals
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The second most abundant mineral, K-feldspar, appears as xenomorphic-hypidiomorphic microcline crystals, with characteristic cross-hatched twinning. The crystals are of varied size, generally larger than 0.3 mm, some crystals reaching 1.0–1.5 mm (Fig. 4). Further, some K-feldspar crystals show some characteristic textures as string perthite (Fig. 5).

Plagioclase arises generally as hypidiomorphic crystals, usually with twin lamellae, polysynthetic, showing a low degree of alteration and, commonly composing aggregates with triple junction contacts (Fig. 6).

The mafic mineral in the rock is, predominantly, biotite (Fig. 2) which occurs in general as hypidiomorphic crystals, with pallet shapes showing, discreetly, preferential orientation. Its colour is yellow with pleochroism in brown tones. Biotite shows low degree of secondary alteration. In addition, biotite crystals contain apatite, zircon and opaques inclusions.

Opaques occur as small crystals (<0.3 mm) with dark colours, usually idiomorphic, in quadratic sections, suggestive of being magnetite.

Apatite and Zircon are present in lower amounts (1%) and small crystals (<0.1 mm) essentially idiomorphic and included in biotite. Apatite is colourless and high relief with rounded hexagonal crystals.

Zircon is slightly colourful, quadratic to prismatic elongated, with strong birefringence colours.

4 AAR Reactivity

Two types of deleterious minerals have been found: strained quartz, as shown in Fig. 3 and microgranular quartz. The maximum undulatory extinction angle was 17°. The percentage found for strained quartz was 15%, which transcends the recommended values in Brazilian standards of 5% (ABNT 2008). Regarding to microcrystalline quartz, the percentage found was 1%.

Perthitic texture is considered potentially reactive for AAR in the specific case of flame Perthite, as it indicates deformation processes. In all the samples studied, there is no evidence of that specific texture. However, string perthite—not suggestive of AAR—is commonly found (Fig. 5).

The textural and mineralogical characteristics of the protolith—probably a syenogranite—due to the high amount of K-feldspar and low percentage of mafics—have certainly influenced on the potential alkali-aggregate reactivity of the samples studied. In this respect, petrographic description indicates stability of the protolith.

The rock is classified as potentially reactive due to the percentage of strained quartz and the undulatory extinction angle, with low degree by Brazilian Standards (ABNT 2008). However, these results must be confirmed by standard mechanical test methods.

5 Conclusions

The rock studied shows characteristics that make it susceptible to be a reactive aggregate mainly, strained quartz, undulatory extinction angle and microcrystalline quartz. Furthermore, the materials show slight alteration processes, which also have to be considered.

In addition, the rock formation processes and the protolith characteristics have influenced, considerably, the low potential reaction degree of the rock.

Finally, as established in international standard norms, these results must be confirmed by standard mechanical tests (ASTM Standard 1994).