**1. Introduction**

The recycling of industrial by-products is an ever rising issue in the sustainable waste management field. Indeed, many studies have focused on the conversion, after appropriate procedures, of these secondary raw materials into a new resource that would otherwise be landfilled. In this respect, over the last years, industrial waste recycling and reuse have become important environmental challenges that many countries are facing in order to reduce overall costs and negative environmental impacts [1–10].

Waste glass and tyre rubber are among the most recycled secondary raw materials from industrial and municipal activities, accordingly, various investigations, mainly in sustainable construction technology, have been carried out with the aim to face the problems relative to the disposal space limitations of these by-products associated with the increasing costs [8,11–17].

In recent years, the amount of waste glass has gradually increased due to the widespread development of urban areas and industries. After the forming process, different types of glass products can be obtained, specifically container glass, flat glass, light bulbs, fluorescent and cathode ray tubes. The biopersistence and chemical inertia of these products lead to long-term accumulation. Accordingly, in the ambit of environmentally sustainable management policies, glass needs to be reused and/or recycled. This material can be indefinitely recycled by preserving the original properties [18,19], and accordingly, the products collected during the sorting operations can be used for the production of abrasives, reflective paints for highways, cullets in glass production, lubricants, additives, fractionators, in road beds and fiberglass production [19–23].

In the last 20 years, recycled glass has been widely used in construction materials, specifically in bricks, normal concrete, pavement materials and asphalt concrete [19,24–28], which is a practice that reduces landfill operations and consumption of natural resources, and also minimizes greenhouse emissions [18,29]. In this respect, it is worth saying that in the case of building materials, recycled glass as aggregate in structural concrete is still not widely used.

The growing amount and disposal of waste tyre rubber has become an environmental issue in many countries. Every year, millions of end-of-life tyres are discarded all over the world and stockpiled tyres represent a threat to human health and the environment through air, water and soil pollution, with its associated economic and social risks [15,30]. The volume of waste, which is globally produced, makes management of the accumulated rubber very hard, with potential fire risks. Tyre burning, although easy and cheap, represents an extremely dangerous method of disposal because fires are difficult to be extinguished and uncontrolled emission of hazardous compounds and potentially toxic gases are released in air. This is very dangerous to humans, animals and plants, and causes ground and surface water contamination generated by the oils and residue ashes left after burning [31,32]. Tyre rubber may be also used as fuel after carbon black production but this solution is not economically advantageous because this material has lower quality and higher costs as compared to conventional fuels [15].

Due to the biopersistence and chemical inertia of waste rubber, recycling operations are rising issues in the sustainable waste management field, as an alternative to landfilling, along with the awareness that new products can be produced with different properties with respect to the original materials.

Tyre rubber can be used for applications in civil and non-civil engineering, for example in erosion control, earthquake shock-wave absorption, road construction as a modifier in asphalt paving mixtures, in breakwaters, in crash and sound barriers, in reefs, playground equipment, as a fuel in cement kilns or for electricity production after incineration [14,16,33–35].

Over the last few years, waste tyre rubber incorporation into cement concrete has been considered one of the most effective, cheap and eco-friendly recycling solutions because it contributes to reducing the cost of some natural aggregates, the great volume of tyre waste, and the emission of toxic compounds and carbon dioxide by preventing tyre fires [36–39].

The main purpose of the present research was to prepare and characterize, by physico-mechanical procedures, eco-friendly non-structural cement composites based on inorganic and organic by-products of recycled waste porous glass (PG) and end-of-life tyre rubber (TR). The secondary raw materials were added as partial and/or total replacement of the conventional sand aggregate, which was made on a volume basis rather than on a weight basis due to the low specific weight of both waste materials. The specimens were characterized by rheological, mechanical, thermal, porosimetric, microstructural and wettability tests. The aim was to obtain lightweight thermo-insulating composites specifically for indoor applications in perfect agreement with the current policies of environmental sustainability. They are also cost-effective because they are prepared through a cheap process where the renewable aggregates are not pre-treated (no addition of chemicals to improve adhesion to the cement paste) and the mixture preparation does not require complex manufacturing processes or expensive procedures.
