## 1 Introduction  
The current concept of the stabilization and improvement of construction materials is closely linked to the idea of mixing \"marginal\" soils with lime, or to a greater degree with cement in such a way that through hydration and pozzolanic reactions, the characteristics of the soils are improved. The use of industrial by-products in this type of work is not new, especially with fly ash and blast furnace ash; in contrast, the use of mineral-type ash from thermal power stations has not been widely studied. Within the concept of \"thermal power station ashes\", there are two types of ash, on the one hand, fly ash and on the other, bottom ash. The pozzolanic character of thermal power station ash makes it especially suitable for mixing with additives containing high percentages of CO in their composition, such as lime or cement. Silicious-aluminous ash, such as those used in this study, can combine with the calcium oxide in lime or cement forming layers with high load-bearing capacity. However, the majority of studies focus on the use of cement and not on lime. This is mainly due to the number of concrete road pavements in the USA whose bases and sub-grades take advantage of the use of layers with the high stiffness modulus provided by stabilization with cement. The use of lime is focused primarily on highly plastic soils or for the construction of bases and sub-grades of flexible and semi-rigid roads, as is the case in Europe, and more specifically in Spain. There are several studies on the addition of fly ash from thermoelectric power stations and lime to soils and gravels. [PERSON] demonstrated that the addition of lime to the soil/fly ash mixture increases the optimal water content of the mixture and reduces the maximum density obtained. Moreover, the load-bearing capacity of a soil sample depends on its lime content, with a clear increase being observed in its load-bearing capacity as the added lime content is augmented [1]. The use of ash from blast furnaces as an additive in granular layers for the construction of base layers is contemplated by important official bodies, such as the American Coal Ash Association (ACAA), which recommends the application of F-type fly ash along with a catalyst material such as lime or cement for treated granular layers. The dosage is between 10% and 15% of fly ash along with 2 to 8% of lime [2]. [PERSON] and [PERSON] concluded that the simple compression strength (SCS) in soil-ash and gravel-ash mixtures with added lime decreases with content greater than 4% of lime [3]. The Dept of Transport at the University Aristotle (Thessalonica) analyzed the application of fly ash from thermoelectric power stations in granular layers of sand and gravel and in the stabilization of soils. They carried out numerous characterization limit tests (Atterberg, Proctor and SCS) on samples with different contents of ash (5, 10 and 20% by weight) and for different curing times (7, 28 and 90 days). It was observed that the SCS values increased significantly as the ash proportion and curing periods increased, but less than was observed for stabilized soils. This was due to the higher number of gaps present in the granular layer. For this reason, they recommend a dosage of 10% given that the increased strength does not justify a larger proportion of material [4]. In the study of the evolution of simple compression strength over time for different percentages of fly ash and/or lime, a delay in the acquisition of strength is observed [5]. Another interesting study confirms this phenomenon, as well as analyzing how the strength always increases as the lime content does. However, this is not true for fly ash whose optimum value is between 10 and 15% for these soils. This is due to the decrease in the density of the mix, and thus, to the decrease in load-bearing capacity and SCS, with the addition of ash above a threshold value, which varies with the particle size distributions and plasticity of the type of soil [6].