The current study presents research into the effect of graphene oxide (GO) with a carbon to oxygen ratio of 4:1 on the fluidity, hydration, microstructure, mechanical and physical properties of Portland cement pastes and mortars. The amounts of GO investigated were 0.02%, 0.04%, and 0.06% by weight of cement, while for mortars, an extra composition with 0.1% was also prepared. According to the results, the fluidity of cement paste and mortar increased and the hydration process was slightly retarded with the addition of GO. Despite this, improvements in compressive and flexural strength were established in the mortars containing GO. The maximum effects (~22% and ~6%, respectively) were obtained with the addition of 0.06% GO. The calculation of estimated strength proportional to samples of equal density showed that for mortars cured for 7 days the gain in strength was directly related to the gain in density. For mortar samples cured for 28 days, the estimated strength was found to be significantly higher than that of the reference sample, indicating that besides density there are other factors determining the improvement in strength of mortars modified with GO. The possible structure strengthening mechanisms are discussed.

The influence of aluminosilicate pozzolanic waste, specifically spent fluid catalytic cracking waste (FCCW) and metakaolin waste (MK) from the expanded glass industry, on the properties of hardened Portland cement paste were analysed. The study involved replacing part of cement with FCCW and MK and observing their impact on the hydration, microstructure, density, and compressive strength of hardened cement paste. Various analysis methods were employed, including X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM), to understand the changes in the structure of the hardened cement paste during hydration. The findings revealed that FCCW tends to accelerate the cement hydration process due to its high surface area and pozzolanic activity. Notably, the formation of portlandite crystals was observed on FCCW particle surfaces in a specific direction. These crystals appeared smaller and developed in different directions in compositions containing a composite binder with mixture of FCCW and MK in a ratio 1:1. This could be influenced by pozzolanic reactions activated by fine particles of MK and the formation of calcium silicate hydrates (C-S-H) and calcium alumino silicate hydrates (C-A-S-H) in the presence of portlandite. The XRD and TG results indicated that the specimens containing a composite binder exhibited the least amount of portlandite. The compressive strength of these specimens increased compared to the control specimens, although the amount of cement was 9% lower.

Refractory castables, representing a mixture of a refractory aggregate, calcium aluminate cement, ultra-fine particles, and deflocculants, have been created in the mid of past century ’80s for the metallurgy and petrochemical industries. The designed castable binder shows an excellent ability to preserve the material’s mechanical strength in the 600 °C to 1000 °C temperature range. Besides, the micro-scaled silica/alumina activation process increases the strength and sinterability temperature of the castables regarding traditional alternatives. However, a relatively high price (ten times the ordinary concrete cost) limits the structural application of these advanced materials. Therefore, the studies considered reinforcement aspects of these materials are rare. This investigation focuses on the bond performance of ordinary and stainless steel reinforcement in refractory castables. It is a part of the research project on developing structural concrete fire protection systems, employing refractory castables as a protective shell. This experimental program operates a conventional refractory concrete with a 50 MPa target compressive strength and 25wt% of aluminate cement. In addition, the alternative mixture was modified with a 2.5wt% microsilica, doubling the material strength. The pull-out tests of ribbed and smooth bars demonstrate the importance of the ribs’ mechanical interlock, ensuring reinforcement bonding with the refractory material. Moreover, the ordinary reinforcement demonstrates the bonding ability with refractory castables containing microsilica up to the 1000°C temperature, making the proposed fireprotecting system development possible.

The relatively low production cost and short lifespan of plastic products contribute significantly to the annual accumulation of plastic waste, raising serious environmental concerns. Conventional disposal methods like landfill and incineration not only waste valuable resources but also result in substantial secondary pollution. In response to the imperatives of sustainable development and environmental protection, in this work, different preparation methods (mechanical processing; heating and covering with milled sand and glass; covering plastic granules with polymers and then mineral materials such as microsilica and waste metakaolin; using other chemical additives) for plastic granules from waste and their influence on the properties of cementitious materials were studied. Lightweight concrete properties such as density, ultrasound pulse velocity, flexural and compressive strength, water absorption, and the interaction zone between the cement matrix and plastic granules were analyzed. It was determined that one-third by volume of natural aggregate can be replaced with specially prepared plastic granules from waste, obtaining a bending strength of the lightweight concrete of about 5 MPa, a compressive strength at 28 days of approximately 30 to 35 MPa, a density of about 1850 kg/m³, and an ultrasound pulse velocity of 3900 m/s.

This study examines the impact of the impregnation of fireclay-based conventional (CC) and medium-cement castables (MCCs) with liquid sodium silicate glass under vacuum conditions. The goal is to assess how this treatment affects the physical and mechanical properties and durability (alkali and thermal shock resistance) of these castables used in biomass combustion boilers, where they are exposed to temperatures up to 1100 °C. The research work employs standard test methods to evaluate the physical and mechanical properties. Additionally, advanced techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and specific tests for alkali resistance and thermal shock resistance are used. The research findings suggest that impregnation with sodium silicate glass under vacuum significantly enhances the alkali resistance of both CC and MCCs. This improvement is primarily due to the reduction in porosity and the increase in density. SEM images reveal that the impregnated samples are coated with a glassy layer and the pores are partially filled with sodium silicate. Tests for alkali resistance demonstrate the formation of a protective glassy layer (with a thickness of 0.9–1.5 mm) on the castable surfaces, thereby reducing the further penetration of alkali into deeper layers of the samples. However, it is important to mention that the impregnated refractory castables have reduced resistance to thermal shock cycles.

Alumina cement (AC) based refractory concretes are the primary building materials for steel, cement, glass, ceramics, and chemical industries. Those advanced materials are efficient at high operation temperatures (up to 1500 °C), conforming to thermal shock and mechanical and slag resistance requirements. For these purposes, fine-grade or ultrafine powders partially replace the cement. However, that increases the material density and, depending on the first heating cycle regime, the evaporated steam pressure can cause cracking and even explosive spalling of the concrete. The coarse aggregates and temperature-melting fibers can regulate the water evaporation process—the melted fibers form micro-channels, allowing excessive water removal from the heated material. However, the concrete curing conditions also affect the hydration products’ structure (density) and chemical composition, altering the water removal kinetics from the refractory material. This investigation focuses on the hardening conditions’ effect on the hydration process and explosive spalling resistance of the AC refractory concretes. The research object is the refractory concrete with a 130 MPa target compressive strength and 7wt% of AC. The alternative mixture contains polypropylene fibers (0.1wt%). The cylinder samples were cured at different temperatures, representing “normal” (20 °C), “summer” (35 °C), and “winter” (10 °C) conditions. The test results demonstrate the fiber efficiency in preventing the spalling failure characteristic of the samples produced without fibers. On the other hand, the curing temperature was the essential parameter, affecting the water removal kinetics of the refractory material with fibers—the “winter” concrete samples tend to explosive spalling due to the formed hydrates.