Effect of temperature on structural quality of the cement paste and high-strength concrete with silica fume

Experimental investigation conducted to study the thermo-mechanical properties of concrete at Temelin (Czech Republic), Mochovce (Slovakia), and Penly (France) nuclear power plants reveals structural integrity degradation between 100 and 200 °C due to both a loss of water bound in hydrated cement minerals and subsequently air void formation. Test results indicate changes in strength, average pore radius and calculated permeability coefficients for Mochovce specimens exposed to temperatures up to 400 °C. It demonstrates that the permeability coefficient measured on the basis of pore sizes using mercury intrusion porosimetry is suitable technique for the evaluation of concrete quality. It confirms that strength and permeability coefficient are equivalent structural quality variables of concrete. At 400 °C gel-like hydration products are decomposed, at 600 °C Ca(OH)2 is dehydroxylated, and CaCO3 dissociation to CaO and CO2 accompanied with the re-crystallisation of non-binding phases from hydrated cement under re-combustion are dominant processes between 600 and 800 °C. This stage of concrete is characterised by the collapse of its structural integrity, revealing residual compressive strength.This paper reports high-strength concrete behaviour subjected to temperatures up to 200 °C. In accordance with previous results, research studies of structure-property relation show the changes in strength, dynamic modulus of elasticity, strain–stress behaviour, and shrinkage-induced deformations influenced by a hydrate phase decomposition. Volume reduction of the hydrate phase due to the loss of bound water mass is the cause of air void formation, and pore structure coarsening. The main attention is herein devoted to the evaluation of utility property decrease of high-strength concrete and microstructure degradation of the cement paste with the same composition than that in concrete when attacked by elevated temperatures.