## 1 Introduction  
Aging mainly consists of a hardening process, which comes from the physical-chemical transformations due to the loss of volatiles and partial oxidation of the bitumen component. The related research is focused on different aspects, such as (i) the mechanism and chemistry of the involved reactions; (ii) the effect on performance; and (iii) the strategies to enhance the binder resistance to aging. The first one has been the subject of a huge amount of work and the interested reader may find comprehensive review papers on the topic in the literature, such as those by [PERSON] [1] and [PERSON] et al. [2]. With regard to performance, rheology is nowadays the prevalent technique to characterize bituminous binders. It is used to evaluate the performance grade and many other properties, such as the workability or the fatigue and rutting resistance. A comparison of these properties before and after a period of mechanical/oxidative stress, either artificially induced or due to service life, allows quantifying the effects of aging. Due to the high number of available rheological tests, the choice of the aging index is quite subjective and basically any rheological function and/or operating condition can be used to compare the unaged and aged performances [3]. [PERSON] et al., underlined that even if some rheological indexes have good correlation with specific properties, the preferable choice is the development and analysis of full master curves [4]. According to this suggestion, and in order to better clarify the possible choices, we will discuss the available aging indexes that can be directly or indirectly derived from the master curves of the linear viscoelastic functions, such as the phase angle and the magnitude of the complex modulus. Therefore, the analysis starts from the development of viscoelastic master curves, and then continues with the application of the \(\delta\)-method [5] to obtain the apparent molecular weight distributions (AMWD) and the derivation of the relaxation spectra \(H(\overline{\tau})\)[6]. The use of these three different representations, derived from the same experimental data, allows evaluating aging under several points of view. This permits comparing aging indexes with different physical meaning and also to qualitatively evaluate the changes in the colloidal structure of the binder.  
Experimental data were derived for a bituminous binder subjected to several degrees of artificial aging both with and without the loading of an organo-modified layered silicate (LS) as anti-ageing additive. Organo-modified LS were first introduced in the field of polymeric materials [7] and the term \"organo-modified\" indicates that the native clay has been subjected to an ionic exchange reaction, to introduce organic cations into the interlayer spacing. This reduces the hydrophilicity of the clay, thus enhancing its compatibility with bitumens, whose molecules may be driven into the clay galleries. At a high compatibility level, a complete delamination of the clay platelets can be obtained, thus resulting in a dispersion of bi-dimensional platelets with a dimension of a few hundred nanometres and thickness in the order of one nanometre. The high aspect ratio guarantees an extraordinary interfacial interaction between the silicate and bitumen, even at low clay loadings. One of the main consequences related to such interactions is a beneficial effect for the bituminous binders in terms of aging resistance. This has been confirmed by a consistent number of papers and reviews published in the last years [8; 9]. Therefore, the use of an organo-modified LS as additive has two advantages in the scope of this work. First, it allows evaluating the sensitivity of the indexes to describe its anti-aging effect with respect to the unmodified binder. Second, the high degree of LS/bitumen interactions may affect the bitumen structure, and this is another aspect that can be evidenced by using the same rheological data and indexes.