## 1 Introduction  
The cement industry is well known as being a driver of social and economic development in society, but at the same time it is associated with significant negative effects because of the release of greenhouse gases, principally CO\({}_{2}\) (800-850 kg/ton) [1] and due to high energetic production costs. One of the strategies to circumvent these negative effects covers the incorporation of active additions of a different nature [2; 3; 4; 5; 6], including different types of pozzolans, a great variety of which (slags, fly-ash, silica fume, etc.) are quite widely used.  
Additionally, clayey minerals are found among the pozzolans specified in the standards that are in force [7]. However, their use in the manufacture of commercial cements is very rare or non-existent, due principally to the environmental problems caused during their extraction and transport, and the need to protect natural areas. The scientific community is currently investigating clayey minerals, principally in relation to kaolinite (K), which needs a process of thermal activation for its complete transformation into metakaloinite (MK) [8; 9; 10; 11; 12; 13; 14]. Clay is in a very reactive state with regards to the thermal activation of the material because it produces the destruction of the structures beginning with the elimination of the hydroxyl groups on the laminar edges [15; 16]. The thermal transformations of kaolinite have been the subject of numerous investigations, showing that the parameters of thermal treatment such as temperature, heating rate, and time, as well as the cooling parameters at the end of the production cycle, significantly influence the dehydroxylation process [17; 18; 19].  
Over recent years, investigations have looked at obtaining recycled MK from kaolinite-based industrial waste [20], with the subsequent benefits that such a process entails (environmental and social). This line of investigation is within the European strategy on the Circular Economy, which prioritizes the recycling of industrial wastes as secondary raw materials for the manufacture of commercial cements that are both innovative and eco-efficient. Along these lines, this research is focused on two principal kaolinite-based wastes: (a) paper-making sludge from the paper industry that uses recycled paper as a raw material to obtain cellulose [21; 22; 23]; and (b) coal waste from coal mining [13]. In all cases, the pozzolanic behavior of this recycled MK is suitable. It is comparable with natural MK and silica fume (SF), and also shows excellent performance in both cement mixtures [24; 25; 26; 27], and blended cement matrices.  
However, little is known about the effects of these properties on the microstructures of the final C-S-H gels that are produced. Likewise, the question of whether the composition and morphology of C-S-H gels depends only on the initial composition of the sample is still a topic for further discussion.  
An in-depth study is presented in this paper on the effect that the initial composition of the activated products have on the mineralogy and the mechanical properties of binary cements. Accordingly, binary cements with substitutions of 10 and 20% of MK-based waste, thermically activated with different ratios of CaO/SiO\({}_{2}\) and SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\), were prepared, and their influence was studied on the hydrated phases and the mechanical properties at 28 days of hydration.