Characterisation of reactive magnesia and sodium carbonate-activated fly ash/slag paste blends

A system of alkali-activated fly ash (FA)/slag (AAFS) mixtures as a clinkerless cement was investigated with different dosages of Na2CO3, as a sustainable activator. The effect of incorporating various proportions of reactive magnesia (MgO) was also examined. Mechanical, mineralogical, and microstructural characterisation of the cement pastes was carried out using the unconfined compressive strength, X-ray diffraction, thermogravimetric analysis, infrared spectroscopy and scanning electron microscopy. It was found that the strength of Na2CO3 activated FA/slag mixtures generally increased with time and the Na2CO3 dosage. The hydration products were mainly C–(N)–A–S–H gel of low-crystallinity, which is rich in Al and may have included Na in its structure, and hydrotalcite-like phases. Adding reactive MgO in the mixes showed an accelerating effect on the hydration rate as suggested by the isothermal calorimetry data. Additionally, findings revealed variations on the strength of the pastes and the chemical compositions of the hydration products by introducing reactive MgO into the mixtures.The financial support of the PhD scholarship for the first author from the Yousef Jameel Foundation and Cambridge Overseas Trust are gratefully acknowledged.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.conbuildmat.2015.06.01

Assessing the chemical involvement of limestone powder in sodium carbonate activated slag

This study aims to investigate the effect of limestone powder (LP) on the reaction of sodium carbonate activated slag. The results show that the incorporated LP up to 30% improves the strength development, especially at advanced curing ages. A slightly accelerated reaction is observed for samples containing low amount of LP (≤5%), while mixture with 10% LP shows the optimized results with respect to the heat release and strength development. Chemical effect of incorporating LP is observed at high replacement levels (≥15%), indicated by the formation of a new phase, natron (Na2CO3·10H2O). Besides, relatively high contents of hydrotalcite-like phases are generated when increasing the dosage of limestone powder. The chemical changes, including the volume changes of generating natron and the transformation of natron to calcite, is potentially responsible for the enhanced mechanical properties

Characterisation of reactive magnesia and sodium carbonate-activated fly ash/slag paste blends

A system of alkali-activated fly ash (FA)/slag (AAFS) mixtures as a clinkerless cement was investigated with different dosages of Na2CO3, as a sustainable activator. The effect of incorporating various proportions of reactive magnesia (MgO) was also examined. Mechanical, mineralogical, and microstructural characterisation of the cement pastes was carried out using the unconfined compressive strength, X-ray diffraction, thermogravimetric analysis, infrared spectroscopy and scanning electron microscopy. It was found that the strength of Na2CO3 activated FA/slag mixtures generally increased with time and the Na2CO3 dosage. The hydration products were mainly C-(N)-A-S-H gel of low-crystallinity, which is rich in Al and may have included Na in its structure, and hydrotalcite-like phases. Adding reactive MgO in the mixes showed an accelerating effect on the hydration rate as suggested by the isothermal calorimetry data. Additionally, findings revealed variations on the strength of the pastes and the chemical compositions of the hydration products by introducing reactive MgO into the mixtures

Development of greener alkali-activated cement: Utilisation of sodium carbonate for activating slag and fly ash mixtures

Alkali activated fly ash/slag (AAFS), a newly evolved type of alkali-activated cements (AACs), is here studied with the aim of developing a more sustainable alternative to Portland cement (PC), known for its adverse environmental impact. In this study, sodium carbonate (Na2CO3) was used as the alkali activator for the fly ash (FA)/slag blends. The effects of different factors on the strength, reaction rate, hydration products and microstructure were examined; these factors include the activator dosage, FA/slag ratio, and curing regime. It was found that increasing the Na2CO3 dosage significantly increased the compressive strength. The inclusion of up to 25 wt% fly ash marginally decreased the compressive strength up to 28 days while the inclusion beyond 25 wt% can lead to a remarkable reduction in strength, particularly for water-cured specimens. Sealed curing in general was found to be beneficial to the strength development of AAFS paste especially at a 50/50 ratio of FA/slag. Both the activator dosage and FA/slag ratio were found to have notable influences on reaction rate and reaction products and microstructure. Increasing the activator dosage accelerates the kinetics of the reaction while increasing the FA/slag ratio slows the reaction rates. The main binding phase is C-(N)-A-S-H with varying Ca/Si ratios ranging between 0.6 and 1.0 depending mainly on FA/slag ratio. The results indicated the possibility of production of greener cementing materials by utilizing appropriate ratios of FA/slag, dosage of sodium carbonate, and curing regimes

Phase modification induced drying shrinkage reduction on Na2CO3 activated slag by incorporating Na2SO4

This paper aims to study the phase modification, reaction kinetics, mechanical properties and drying shrinkage of sodium carbonate activated slag by incorporating sodium sulfate in the activator. The results show that the reaction process is firstly controlled by CO32− anions, and later runs similar to that of sodium sulfate activation. Besides, the relatively unstable phase gaylussite, commonly found in the sodium carbonate activation, is not observed in the reaction products upon hybrid activation, and monosulfoaluminate rather than ettringite is identified, probably caused by the reduced aluminate-to-sulfate ratio and increased pH value. The drying shrinkage is considerably reduced by up to 41% when replacing 50 wt% sodium carbonate by sodium sulfate, most possibly attributed to the induced phase modification. Furthermore, the relationships between the phase modification and drying shrinkage, and the potentially involved chemical reaction are discussed