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Self-heating construction materials are of high potential within the design of de-icing and snow-melting systems used in constructions, roads, or pavements. In the past, mainly cementitious materials were subjected to the research of the design/optimization of self-heating mixtures using a wide range of electrically conductive admixtures, an appropriate determination of electrical properties, and electrode embedment/attachment. Taking into consideration the negative environmental impact of the production of cementitious materials, alkali-activated cementless composites are promising substituents that can be doped by electrically conductive admixtures to gain new functional properties, such as self-heating. The paper is focused on the design of alkali-activated metakaolin composite with 8 wt.% of graphite powder and experimental determination of basic physical and electrical properties to make a preliminary assessment of appropriateness for self-heating construction applications. The compressive strength, volt-ampere characteristic, DC electrical conductivity, as well as AC electrical characteristics were determined to assess the self-heating potential of the composite. The DC/AC electrical properties of the composite were promising in terms of self-heating function (σ = 3.01 × 10−1 S·m−1) at a reasonable level of mechanical properties for some construction applications (Rc = 10.5 MPa, Rf = 1.2 MPa). [ABSTRACT FROM AUTHOR]

This study deals with a utilization of super reactive alumina (RA) and boehmite powder (BP) as a partial replacement of calcium aluminate cement (CAC) in refractory composites. The reason for using these substitutes is their positive effect on the environmental performance of heat-resistant composites, specifically on reducing non-renewable resources consumption, CO2 emissions, and on the overall performance of the resulting composites. The basic physical and mechanical properties of five composite mixtures containing RA and BP were determined at room temperature and after exposure to 400 °C, 1000 °C, and 1400 °C. Both admixtures helped to reach better bending and compressive strength than the reference mixture without a replacement of CAC. It was proved that the replacement of CAC with 11.3% RA and 3.8% BP is a suitable combination which significantly improves the properties of the composite and can be recommended for further applications. [ABSTRACT FROM AUTHOR]

The design of sustainable construction materials is continuously gaining increasing importance in civil engineering. Geopolymers are attractive alternatives to cementitious materials in terms of environmental impact and specific material properties, such as durability, an initial increase in mechanical properties, or chemical and thermal resistance. Such favorable properties can be advantageously utilized within various applications involving the design of materials for heavily stressed industrial floors. The research presented in the paper was focused on the design of a geopolymer composite based on metashale MEFISTO L05 and waste metashale RON D460HR binders. The 1:4 raw/waste mix of binders activated by potassium hydroxide/silicate was supplemented by 0.11 wt.% of graphite fibers to optimize electrical properties and bestow on it some new material functions, such as self-heating. The further utilization of fine waste aggregate (crushed defective concrete products, waste concrete from auto-mixers) resulted in an ~85% utilization of input waste materials. An acceptable mechanical performance of the mortar for particular civil engineering applications was observed (28d: Rf ~ 2.5 MPa, Rc ~ 15 MPa), as well as favorable thermal and DC/AC electrical properties, predicting the self-heating potential. [ABSTRACT FROM AUTHOR]

The waste gypsum used in this study was a by-product of petroleum coke burning by petrochemical companies which has been treated with hydration. The waste gypsum has been stored in the atmosphere for over ten years and can be considered an inert filler. Its main chemical components were calcium sulfate dihydrate (47.90%), calcium hydroxide (21.64%) and calcium carbonate (14.80%). In this study, Portland cement and fly ash were used as cementitious materials, and waste gypsum of the particle size from 9.53 mm to 0.149 mm was selected as the fine aggregate to produce a controlled low-strength material (CLSM) and to verify the suitability of reusing waste gypsum. The water to binder ratio of 0.65 was used for the specimen. The test results showed that the CLSM specimen with a high amount of waste gypsum had air-hardening properties. The placement of the specimen in water caused abnormalities, such as cracking and disintegration of the specimens. The compressive strength of atmospherically maintained specimens increased with age, with 4.71 MPa and 6.08 MPa at 28 and 56 days, respectively. No significant changes in weight or volume were measured after the specimens had been left for 56 days and then immersed in seawater and water for 28 days. As specimens were immersed in seawater for up to 100 days, needle-shaped ettringite and C-S-H colloids filled the interface between the pores and the colloids. In accordance with the concept of eco-engineering, special consideration should be given to avoid long-term contact with water and to ensure the safety and durability of waste gypsum reuse through the design of multiple protective layers. [ABSTRACT FROM AUTHOR]

In this study, three industrial by-products (ultrafine fly ash, ground granulated blast-furnace slag (ggbs) and circulating fluidized bed co-fired fly ash) were used to produce ternary cementless composites without using alkali activators. The finenesses of ultrafine fly ash, ggbs and co-fired fly ash were 33,800, 5830 and 5130 cm2/g, respectively. The composite material was developed by mixing supplementary cementing materials of different particle sizes and exploiting the high-alkaline properties of the co-fired fly ash to develop a substantial hardening property like cement. The test specimens were made in the form of pastes and the water-to-cementitious-material ratio for the test was fixed at 0.55. The test results show that the flowability of the six different mixtures could be up to 120% and the setting time could be controlled within 24 h. At 60% of the ggbs proportion, the setting time could be held for 8 h. The compressive strength of each proportion reached 7 MPa at 7 days and 14 MPa at 28 days. The water-cured specimens exhibited better strength behavior than the air-cured specimens. Scanning electron microscopy found the main components of strength growth of the specimens to be hydrated reactants of C-A-S-H or ettringite. The results of the XRF analysis show that the specimens responded to higher compressive strengths as the Ca/Si and Ca/Al ratios increased. [ABSTRACT FROM AUTHOR]