Petrographic analysis of carbonate rocks for alkali-aggregate reactivity

Content Provider	Semantic Scholar
Author	Paudel, Lalu Prasad
Copyright Year	2011
Abstract	Quality of aggregate is of extreme concern when it is to be used for infrastructures. Besides, many physical and mechanical properties of the aggregate, presence or absence of deleterious constituents and alkali-silica reactivity are especially important when aggregates are to be used in concrete structures. High potential of alkali-silica reactivity or alkali-carbonate reactivity and presence of deleterious constituents may impair the infrastructures. A ledge rock sample from the heap to be taken for crushing was petrographically analysed for alkali-silica reactivity. In overall, two rock clans (dolosparstone and dolomicrosparstone) with three sub clans (rock type X, Y and Z) from the sample 2 are identified. Rock type X (dolosparstone) constitutes 82.94% of the whole sample, and shows notable amount of quartz and calcite veins, and carbonaceous material and hematite on the mosaic of dolospars. Rock types Y (dolosparstone) and Z (dolomicrosparstone) contain trace amount of microquartz, mega quartz (>15 mm) and carbonaceous opaques. The rock type Z is dominantly composed of dolomicrospars. Major portions of all the rock types are characterised by mosaics of dolomite in association with variable amounts of muscovite, quartz, and calcite. Calcite often replaces the mosaics of dolomite and bands of quartz, forming a vein networks in rock types X and Y. Silica is represented by a low-temperature mega quartz either in ground or in veins, a trace amount of microquartz in rock types Y and Z. There is no other reactive silica components, thus showing a low potential to alkali-silica reactivity. However, the sample shows potential of alkali-carbonate reactivity as significant proportion of rock type having dolomicrospars are found. INTRODUCTION Quality of aggregate is of great concern to infrastructural projects which utilise aggregates for various purposes, e.g., building, dams, bridges, pavements, etc. The durability of structures partly rely on the quality of construction material used in the structures. The physical properties and proportion of deleterious materials which bear potential reactivity with the cement are crucial in any projects which uses cement to bind the aggregate for making concrete structures. Physical properties are greatly influenced by composition and texture of aggregates, and proportion of deleterious constituents are influenced by composition of constituent rock types in aggregates and degree of weathering of the constituents. Petrographic examination of aggregates is the most crucial part of characterising aggregates during the study of aggregates. In many instances, petrographic results are decisive in accepting or rejecting the aggregates, and are helpful in deciding whether the aggregate samples are to be further analysed for chemical and physical tests. Therefore, requirement of further chemical and physical tests is governed by petrographic results because the latter supplements other expensive and time consuming tests. There are little number of earlier studies on petrography of rocks for aggregates in the Nepalese context (Maharjan and Tamrakar, 2004; Maharjan and Tamrakar, 2008; Khanal and Tamrakar, 2009). Few of these studies carried out physical and mechanical tests and indicated suitability of rocks or gravels for concrete or road aggregates. The present study is mainly targeted to petrographic examination for alkali-silica reactivity of a rock sample for concrete aggregates.
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