Applicability of the standard specifications of ASTM C618 for evaluation of natural pozzolans

Abstract

Nowadays, the production of binary cements, containing pozzolans (including silica fume, fly ash, natural pozzolans, etc.), is a global practice. Many countries have ample resources of natural pozzolans, capable of being used in binary cements, which reduce environmental impacts while reaping greater economies of scale. The ASTM C618 standard provides one of the most applicable methods for evaluating natural pozzolans. Some research results show contradictions between performance of pozzolans in concrete and the specifications of ASTM C618, as pozzolans having a high pozzolanic activity according to this method do not always exhibit suitable performance in concrete, however, in the other researches ASTM C618 showed compatibility. This treatment analyses the chemical and physical properties of different natural pozzolans by means of ASTM C618 and complementary tests, viz. methods of EN 196-5, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) studies, insoluble residue content and thermo-gravimetric investigations. Measurements of mechanical and transport properties, for concretes containing various fractions of the pozzolans, were performed for further verification. The results illustrate that the pozzolanic properties, determined via the ASTM C618 standard, show some disparities with the performance of concretes, whereas the EN 196-5 standard agrees well with performance.

Introduction

In recent decades, environmental considerations and energy efficiency requirements have motivated researchers to search for sustainable solutions in development. One of the most polluting industries is cement production, since the production of 1 metric-ton of cement results in the emission of about 1 metric-ton of CO₂ [1], [2]. Therefore, the application of blended cements, in lieu of ordinary Portland cement, is rapidly increasing [3].

Contemporary surveys reveal that the majority of European cement production is allocated to blended cements [4]. EN 197-1 [5] designates 27 different cement types, from which 26 are categorized as blended cements. Blended cements consist of different supplementary cementitious materials (SCM), such as fly ash, silica fume, blast furnace slag, limestone and natural pozzolans. Natural pozzolans, owing to their abundance and relatively low costs, present considerable potential for employment in the cement and concrete industries. Additionally, their application generally results in decreases in pollutant emissions and increases in concrete durability properties [6], [7], [8], [9], [10], [11]. For a long time the use of natural pozzolans has been mostly restricted to Italy, where considerable reserves of natural pozzolans are found [12]. Nowadays, ample resources of natural pozzolans capable of being used in binary cements can be found in countries such as Italy, China, USA, Chile, Greece, Cameroon, Algeria, France, Turkey, Iran, Saudi Arabia, and Honduras [12], [13], [14], [15], [16], [17], [18], [19], [20].

Natural pozzolans are characterized by their pozzolanic activity and performance. Hitherto, several methods for assessment and classification of natural pozzolans have been presented, and debated [21], [22], [23]. From these methods, the requirements of ASTM C618-03 [22], for selecting pozzolans for use in concrete, and the methodology of EN 196-5 [23] for measuring pozzolanic activity of pozzolanic cements, have gained ample application in evaluating pozzolanic activity properties.

The outcomes of some research projects have exhibited some anomalies between these different approaches [24], [25], [26]. The authors investigated and found that ASTM C618 can have significant problems in characterizing natural pozzolans [26]. So, it was decided to make a comprehensive study on pozzolanic activity of different types of natural pozzolans with the different approaches that existed, such as ASTM C618 and the EN 196-5 method, X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM) studies, determination of insoluble residue content, and thermo-gravimetric investigations. Finally, since the performance of pozzolans in concrete mixtures (i.e. their effects on concrete properties), is ultimately the most representative judgment strategy for pozzolanic activity evaluation, the results were cross-checked with measurements of mechanical and transport properties of concretes with the same pozzolans.