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

Abstract

A system of alkali-activated fly ash (FA)/slag (AAFS) mixtures as a clinkerless cement was investigated with different dosages of Na 2 CO 3 , 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 Na 2 CO 3 activated FA/slag mixtures generally increased with time and the Na 2 CO 3 dosage. The hydration products were mainly C-(N)-AS -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.

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

“…Gaylussite (Na 2 Ca(CO 3 ) 2 Á5H 2 O) is a secondary reaction product of sodium carbonate activation and will be entirely transferred to more stable products (Eq. 1) [4,28]. As shown in Table 4, the amounts of calcite of all samples after a long period of curing are remaining at the same levels, indicating that the released CO 3 2-anions from gaylussite are transferred to other phases such as hydrotalcite-like phases.…”

“…It is reasonable to assume that the formation of natron is mainly caused by the lack of sufficient ions from slag particles such as Mg 2? and Al 3? to react with the released CO 3 2-anions from the conversion of gaylussite. Abdalqader et al [28] characterized the reaction products of sodium carbonate activated slag-fly ash blenders and reported that no new phase was formed with up to 25% addition of fly ash or difficult to be identified. While, in the present study, the presence of LP in the mixture is observed to be involved in the chemical reaction.…”

“…In general, corresponding with the peak at approximate 1640 cm -1 , the broad peak centred at around 3333 cm -1 due to the scissor bending of molecular water can be considered as the H-O-H stretching vibrations, i.e. the existence of free water in the reaction products [49,50] respectively, which can be attributed to the vibrations of activators used, initially precipitated calcium carbonate or generated carbonate salts such as gaylussite and hydrotalcite [28,51,52]. In general, the intensities of the peaks at around 873 cm -1 , the vibration of carbonate anions, are proportional to the content of LP in the designed mixtures.…”

“…As reported by Bernal et al [4] and our previous study [26], CO 3 2-anions concentration in the pore solution and the initially precipitated calcium carbonate significantly control the sodium carbonate activation process. According to the previous researches [4,23,[28][29][30] The present research aims to study the effect of LP on the reaction kinetics, reaction products and strength development of sodium carbonate activated slag. The heat evolution of mixtures with different LP incorporation levels was measured, up to 7 days.…”

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

“…Depending on the activators applied, CO 3 2-and SO 4 2-anions are also identified in the samples as shown in the spectra. The peaks centred at about 1410 and 1493 cm -1 are assigned to be the vibrations of v 3 [CO 3 2-], which can be calcium carbonate or hydrotalcite [40][41][42]. A small peak at this position was also observed in the mixture activated by pure sodium sulfate, possibly due to a slight carbonation or originated from slags.…”