## 1 Introduction  
### Background  
Concrete and RCSs occupy one of the main roles in modern construction around the world, the growth in the pace and volume of which is inevitably associated with an increase in the need for raw materials and construction materials for structures and buildings [1,2]. The traditional mineral components of the concrete mixture are non-renewable resources, which, despite their large number, raise the question of the relevance of their local replacement with natural and renewable counterparts. An increase in the production of RCSs makes relevant the search for ways to reduce cost, which can be quite high [3, 4, 5, 6]. At the same time, the growing volume of production processes within the agricultural industry entail an increase in the amount of waste, most of which, despite potentially interesting properties, is not employed, which opens up the question of finding promising methods of use. All of the above indicates the need to develop environmentally friendly concrete, in which traditional mineral components are locally or completely replaced by natural analogues, including agricultural waste. This approach can make it possible to achieve environmentally friendly concrete with a reduced cost, while having improved strength features compared to conventional concrete [7,8]. Many wastes from the agricultural industry have strength features comparable to the mineral components of concrete and can be successfully used in the construction industry. One of the promising natural materials is rubber seed shells (RSSs). The RT (_Hevea brasiliensis_) is cultivated mainly in Indonesia, Thailand, India, Vietnam, Nigeria, Myanmar, Sri Lanka, Cambodia, Congo, Malaysia, Liberia and Brazil [9,10]. It is of interest due to the high content of rubber in the milky sap of this plant (about 40-50%), which makes it an essential raw material for the global manufacturing of natural rubber. In addition, the whey produced in the process of coagulation and separation of rubber from latex contains proteins in an amount of 0.6%, which makes it suitable for animal feeding [11, 12, 13, 14, 15]. The seeds of the plant are employed for cultivation but also contain about 35-37% drying oil. The fruit of the RT is a hard, three-leaved pod (shell) containing three hard-shelled, void seeds (Figure 1). The fruits look like chestnuts; they are dark brown, round, three-leaved boxes with a smooth shell, 2-3 cm in size. The seeds are small and ovoid, about 3 mm in diameter, with juicy cotyledons containing up to 40% fatty oil. Ripe leaves open, fall into the soil, and under favorable weather conditions begin to germinate in a few weeks. The shell of the RT, despite its outstanding strength features, is the only unemployed and non-utilized part of the plant, which lies on the ground in large quantities and decomposes [16, 17, 18, 19, 20, 21, 22]. The water absorption of the RT shell is 20-30%. The relatively high renewability and low susceptibility to decay of this raw material creates an urgent need for its disposal in the agricultural industry. Combustion of such material is impractical due to high CO\({}_{2}\) emissions into the atmosphere [23, 24, 25, 26]. The foregoing confirms the significant potential of using RT seed shells as a partial replacement for CA both for construction, due to its outstanding strength features, and for the agricultural industry due to the lack of well-studied and environmentally friendly technologies for its disposal [27, 28, 29, 30, 31, 32]. In addition, local replacement of CA in concrete with RT seed shells can significantly reduce the cost  
Figure 1: The assembly of the fruit of the RT. and weight of such a composition, which makes the presented technology attractive for the needs of modern construction [33; 34; 35; 36; 37; 38; 39]. However, despite this, studies considering the local or complete replacement of CA in concrete with the shells of RT seeds are extremely few due to the low prevalence of this plant and the lack of stable links between the rubber and construction industries [40; 41].