Performance of plain and slag-blended cements and mortars exposed to combined chloride-sulphate solution

The durability of reinforced concrete structures exposed to aggressive environments remains a challenge to both researchers and the construction industry. This study investigates the hydration, mechanical properties and durability characteristics of ground granulated blast-furnace slag (GGBS) - blended cements and mortars exposed to a combined sodium chloride - sulphate environment, at temperatures of 20°C and 38°C. The conditions were chosen so as to assess the performance of slag blends under typical temperate and warm tropical marine climatic conditions. Slags, having CaO/SiO2 ratios of 1.05 and 0.94, were blended with CEM I 52.5R at 30% replacement level to study the influence of slag composition and temperature. Parallel control tests were carried out with CEM I 42.5R. Pastes and mortar samples were cast using 0.5 water to binder ratio, pre-cured for 7 days in water before exposure. Flexural strengths were determined once the samples were 7, 28 or 90 days old. Hydration was followed using x-ray diffraction (XRD), thermal analysis, and calorimetry. Also, sorptivity, gas permeability and chloride diffusion tests were carried out on mortar samples to measure transport and durability characteristics. The results show improved mechanical and transport properties for slag blended cements exposed to environments rich in sodium chloride and sulphate

Effects of metakaolin on nanomechanical properties of cement paste

Metakaolin (MK) is a pozzolanic material, which is a dehydroxylated form of the clay mineral kaolinite. It is obtained by calcination of kaolinite clay at a temperature between 500 °C and 800 °C. In cement matrix, MK reacts with Ca(OH)2, to produce calcium silicate hydrate (CSH) gel. MK also contains alumina that reacts with Ca(OH)2 to produce additional alumina-containing phases, including C4AH13, C2ASH8 and C3AH6. This research aims to provide a better understanding of the effects of MK on the nanomechanical properties of the main phases present within the cement paste. Two different mixes were prepared, one control mix and the other one with 10 % MK (by cement weight). A constant water-binder ratio of 0.4 was used for both the mixes. Fraction volumes determined from nanoindentation testing show an increase in the amounts of high-density CSH at the cost of low-density CSH gel in cement pastes containing 10 % MK

Transport Properties of Carbon-Nanotube/Cement Composites

This paper preliminarily investigates the general transport properties (i.e., water sorptivity, water permeability, and gas permeability) of carbon-nanotube/cement composites. Carboxyl multi-walled carbon nanotubes (MWNTs) are dispersed into cement mortar to fabricate the carbon nanotubes (CNTs) reinforced cement-based composites by applying ultrasonic energy in combination with the use of surfactants (sodium dodecylbenzene sulfonate and sodium dodecyl sulfate). Experimental results indicate that even at a very small dosage the addition of MWNTs can help decrease water sorptivity coefficient, water permeability coefficient, and gas permeability coefficient of cement mortar, which suggests that CNTs can effectively improve the durability properties of cement-based composites.Structural EngineeringCivil Engineering and Geoscience