This study addresses the issue of the resistance to high temperatures of lightweight concrete lightweighted with sintered fly ash aggregate. Lightweight concretes with different amounts of lightweighting and their properties after loading temperatures of 600, 800 and 1000 °C were investigated. In particular, the effect of high temperature on the mechanical properties of the concrete was determined on the test specimens, and the effect on the microstructure was investigated by X-ray diffraction analysis and scanning electron microscopy. It was found that there is an increase in compressive strength between 0 and 21% up to 800 °C, where the increase in strength decreases with increasing degree of lightening. At 1000 °C, the internal structure of the lightweight concrete destabilized, and the compressive strength decreased in the range of 51–65%. After loading at 1000 °C, the scanning electron microscope showed the formation of spherical-shaped neoplasms, which significantly reduced the internal integrity of the cement matrix in the lightweight concrete due to the increase in their volume. It was found that the lightweight concretes with higher lightweighting showed significantly less degradation due to higher temperature.

The circular economy is currently a highly discussed topic in many areas of production, due to the potential scarcity of natural resources. In the construction sector, it is often associated in the first place with the high shortages of natural aggregates for the production of concrete. To produce concretes up to strength classes C 30/37, the partial or complete replacement of natural aggregates by concrete or brick crushed recyclates in the 4/22 mm fraction can be used very efficiently and safely. This paper will show the current legislative limitations for the use of recyclates as aggregates for concrete production, the possibilities of their use when limiting the degrees of environmental influence or physico-mechanical parameters. In both cases of strength classes C 16/10 and C 30/37, a decrease in compressive strength of 16% was observed with replacement of coarse natural aggregate. The results of physico-mechanical properties of different formulations experimentally and practically verified will be shown and the areas of their safe use will be discussed.

The growing production of construction and demolition waste has been stimulating the need to look for sustainable alternatives to natural aggregate used in the production of cement composites. This study evaluates the potential use of recycled brick material as a substitute for natural coarse aggregate in terms of environmental safety, mechanical properties, the solidification effect of the cement matrix, and life cycle assessment (LCA). Recycled brick materials sourced from four recycling yards were characterized using physical, chemical, and leaching tests. Cement composites containing 50%, 70%, 80%, and 100% replacement of natural coarse aggregate with recycled brick material were subsequently designed. The results demonstrated significant variability in the environmental properties of the recycled materials depending on their origin. Compressive strength after 28 days ranged from 31.8 to 36.2 MPa, and all mixtures met the requirements for strength classes C20/25 to C25/30 according to ČSN EN 206+A2. At the same time, a significant solidification effect of the cement matrix was confirmed, leading to a reduction in the concentrations of dissolved substances by 68.0–80.1% and sulfates by 97.8–99.2% compared to the original recycled material. A life-cycle assessment demonstrated a gradual reduction in the environmental impacts as the proportion of recycled brick material increased, with the best results achieved by a mixture with 100% replacement of natural aggregate. The results confirm that recycled brick material can be safely used as a full-fledged substitute for natural coarse aggregate while maintaining the required mechanical properties and reducing environmental impacts.

The high-strength concrete is a cement composite reaching high compressive strength, namely, pursuant to the legislation, higher than 60 MPa in the terms of cube compressive strength. The development of high-strength concretes exceeding 100 MPa is still an up-to-date issue and the production of these concretes is still limited only to a prefabrication. Contemporary construction industry and projecting activity have begun to focus on a construction of statically demanding buildings, which can include e.g. high-rise buildings. Such projecting often requires using of the state-of-the-art materials like cement composites with high mechanical parameters for construction of more subtle buildings. Within this article, the procedure of ready-mixed concretes development with the compressive strength around 100 MPa designed according to a project documentation for actual construction of high-rise building with the height up to 160 meters and 46 floors is described, together with the influence of the aggregate on the resulting composite strength.

The study examines the use of stone sludge generated during the processing of natural stone. The aim is to verify its benefits in the production of concrete at stonemasonry workshops, particularly with regard to reducing the volume of by-products produced and the consumption of natural raw materials. The experiment focuses on analysing the material parameters of selected types of stone sludge, and on the production of mortars and concretes with varying proportions of sludge. The mortars were evaluated using stone sludge as a partial replacement for cement at 5, 10 and 15%, and for fine natural aggregate at 20, 35 and 50%. The study also aims to assess the effect of further processing by grinding stone sludge and the subsequent impact on the workability of fresh mortars and the resulting flexural and compressive strength. The next stage involved verifying the key properties of the concretes and assessing the potential for reducing the use of natural raw materials and cement whilst still achieving the required mechanical properties. The results obtained evaluate the potential for using stone sludge in the construction industry, particularly its impact on the properties of cement composites, the conservation of natural resources and the reduction in waste sent to landfill.