Strength and drying shrinkage of reactive MgO modified alkali-activated slag paste

Conventional alkali-activated slag (AAS) cements suffer from significant drying shrinkage which hinders their widespread application. This paper investigates the potential of using commercial reactive MgO to reduce the drying shrinkage of AAS. Two different reactive MgOs were added at a content of 2.5-7.5 wt% of the slag, which was activated by sodium hydroxide and water-glass. The strength and the drying shrinkage of those reactive MgO modified AAS (MAAS) pastes were measured up to 90 days. It is found that MgO with high reactivity accelerated the early hydration of AAS, while MgO with medium reactivity had little effect. The drying shrinkage was significantly reduced by highly reactive MgO but it also generated severe cracking under the dry condition. On the other hand, medium-reactive MgO only showed observable shrinkage-reducing effect after one month, but the cement soundness was improved. The hydration products, analysed by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy techniques, showed that Mg was mainly incorporated in the hydrotalcite-like phases. It is concluded that the curing conditions and the time of hydrotalcite-like phases formation and their quantity are crucial to the developed strength and shrinkage reduction properties of MAAS, which are highly dependent on the reactivity and content of reactive MgO. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

Modeling of a coal-fired slagging combustor: Development of a slag submodel

In a slagging combustor or furnace, the high combustion temperature makes the molten slag layer cover the wall and capture the particles. If these particles contain combustible matter, they will continue to burn on the running slag. As a result, the total amount of ash deposition will be much greater than that in dry-wall combustors and the total heat flux through the deposition surface will change greatly. Considering the limitations of existing simulation methods for slagging combustion, this paper introduces a new wall burning model and slag flow model from the analysis; of particle deposition phenomena. Combined with a conventional combustion simulation program, the total computational frame is introduced. From comparisons of simulation results from several kinds of methods with experimental data, the conclusion is drawn that the conventional simulation methods are not very suitable for slagging combustion and the wall burning mechanism should be considered more thoroughly.

Chemical and mechanical stability of sodium sulfate activated slag after exposure to elevated temperature

The chemical and mechanical stability of slag activated with two different concentrations of sodium sulfate (Na₂SO₄) after exposure to elevated temperatures ranging from 200 to 800 °C with an increment of 200 °C has been examined. Compressive strengths and pH of the hardened pastes before and after the exposure were determined. The various decomposition phases formed were identified using X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. The results indicated that Na₂SO₄ activated slag has a better resistance to the degradation caused by exposure to elevated temperature up to 600 °C than Portland cement system as its relative strengths are superior. The finer slag and higher Na₂SO₄ concentration gave better temperature resistance. Whilst the pH of the hardened pastes decreased with an increase in temperature, it still maintained a sufficiently high pH for the protection of reinforcing bar against corrosion.

Fast-track construction with slag cement concrete: Adiabatic strength development and strength prediction

The early-age strength development of concrete containing slag cement has been investigated to give guidance for its use in fast-track construction. Measurements of temperature rise under adiabatic conditions have shown that high levels of slag cement-for example, 70% of the total binder-are required to obtain a significant reduction in the peak temperature rise. Despite these temperature rises being lower than those for portland cement mixtures, however the early-age strength under adiabatic conditions of slag cement concrete can be as high as 250% of the strength of companion cubes cured at 20 degrees C (68 degrees F). The maturity and, hence, strength development were calculated from the adiabatic temperature histories based on several Maturity functions available in the literature. The predicted strength development with age was compared with the experimental results. Maturity functions that take into account the lower ultimate strengths obtained at elevated curing temperatures were found to be better at predicting the strength development.

Strength development of mortars containing ground granulated blast-furnace slag: Effect of curing temperature and determination of apparent activation energies

The strength development of mortars containing ground granulated blast-furnace slag (ggbs) and portland cement was investigated. Variables were the level of ggbs in the binder, water-binder ratio and curing temperature. All mortars gain strength more rapidly at higher temperatures and have a lower calculated ultimate strength. The early age strength is much more sensitive to temperature for higher levels of ground granulated blast-furnace slag. The calculated ultimate strength is affected to a similar degree for all ggbs levels and water-binder ratios, with only the curing temperature having a significant effect. Apparent activation energies were determined according to ASTM C1074 and were found to vary approximately linearly with ggbs level from 34 kJ/mol for portland cement mortars to around 60 kJ/mol for mortars containing 70% ggbs. The water-binder ratio appears to have little or no effect oil the apparent activation energy. (c) 2005 Elsevier Ltd. All rights reserved.