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Abstract:
Enhanced neutron radiation shielding capacity of protective structures can be achieved using cement-based composites with boron-containing aggregates. Experimental tests were performed to evaluate the effect of boron aggregates (colemanite, ulexite, borax, boron carbide) and nanosilica on the setting time and hydration heat of cement in mortars using isothermal calorimetry. Boron leaching test from mineral aggregates were performed in water and saturated Ca(OH)2 solution. Cement setting retardation effects were found qualitatively correlated with boron leaching from mineral aggregates. A linear dependence of compressive strength of borated mortars and heat released after 72 h of cement hydration was found. A maximum content of boron compounds in mortar, allowing for a systematical control of setting time, was evaluated

Keywords:
Boron minerals, Cement setting, Colemanite, Early strength, Heat of hydration, Isothermal calorimetry, Leaching, Nanosilica, Neutron shielding, Retardation, Ulexite

Abstract:
Multicomponent cement-based composites are known as versatile structural materials for enhanced radiation shielding. The use of selected elements, like boron, cadmium, or rare earth elements, provides an increased neutron shielding capacity. Because of profusion, reasonable costs and large cross-section for neutron capture, boron containing minerals are suggested as aggregates for radiation shielding concrete. Despite many advantages, boron additives may act as cement setting retarders. Uncontrolled setting and hardening is not acceptable in radiation shielding concrete technology. In this work we present results from isothermal calorimetry measurements on cement mortars with boron-containing aggregates. Four types of boron aggregates were used in the studies: colemanite, ulexite, borax and boron carbide. Based on calorimetric curves, the beginning of setting time was determined. Additionally early mortar strength was investigated and linear relationship between the heat generated in the isothermal calorimeter and the early compressive strength has been observed. The use of isothermal calorimetry allowed us to estimate the limits for the content of boron compounds to be used cement mortar.

Abstract:
This paper presents concrete mix design as well as the results of testing of concrete during production of precast foundations for wind power turbines which are to be located in the Baltic Sea. From 2002 to 2010 approximately 120 000 m3of concrete was cast to form 210 foundation blocks. Concrete production and casting was performed in Poland and the manufactured concrete blocks were transported to wind farms in Denmarkand Sweden. Massive reinforced concrete blocks were designed to be laid at the sea bottom about 7.5-12.5 m below the water level. For concrete mix design the following exposition classes were assumed: XC4, XF4, XS3 and XM3 according to EN 206-1. Exposure to exceptionally severe environmental conditions was assumed: chemical aggression and mechanical wear by seawater, freezing and thawing in saltwater. Because of thelarge size of blocks the danger of thermally induced cracking of concrete was carefully considered and computer simulation of stress build-up was used to select the optimal solution. Concrete mix C45/55 was designed using cement CEM III/A 32.5N HSR NA (containing about 60%GGBFS), silica fume, crushed aggregates up to 32 mm and water-reducing and air-entraining additives. High durability of concrete in the aggressive environment was predicted on the basis of microscopic testing of cement hydration products and quantitative evaluation of microstructure. The challenging issue was to maintain a stable air void system in concrete during the whole production process: air void characteristic was regularly monitored in fresh concrete using AVA and in hardened concrete using computer image analysis.Heat of hydration data and monitoring of temperature during concrete hardening provided necessary proof of a non-cracked structure

Keywords:
durability of concrete, air void system in concrete, sustainable production, heat of hydration