The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

Differential scanning calorimetry study of ordinary Portland cement

On the hydration of water-entrained cement-silica systems: Combined SEM, XRD and thermal analysis in cement pastes

This paper aims to investigate the hydration and pozzolanic reactions in cement pastes with different levels of metakaolin replacement, using differential scanning calorimetry (DSC) and theoretical analysis based on reaction stoichiometry. It was found that the DSC technique could follow the hydration process quantitatively by measuring the peak temperature and enthalpy corresponding to decomposition of hydration products, as functions of age. The pozzolanic process can also be followed from the measurements of the changes in the amount and the nature of amorphous material in the paste and the change of the amount of calcium hydroxide. In addition, it was confirmed that a theoretical approach using reaction stoichiometry could give a good estimation of the concentration of calcium hydroxide in a metakaolin concrete.

Differential scanning calorimetry study of ordinary Portland cement paste containing metakaolin and theoretical approach of metakaolin activity

Internal curing of alkali-activated fly ash-slag pastes using superabsorbent polymer

Thermal methods are used extensively in investigating cements and effects of additives on their hydration behaviour Calcium chloride is the most effective and widely used accelerator for cement hydration, but the result is largely dependent on the rate at which it is added In this study the influence of calcium chloride dosage on the hydration behaviour of OPC pastes aged for different periods has been investigated by means of differential scanning calorimetry (DSC), thermogravimetry (TG) and X-ray diffraction (XRD) methods The results are discussed in relation to the relative amounts of calcium hydroxide and total reaction product formed Chloroaluminate was a product of hydration in the samples dosed with greater than 1% calcium chloride XRD was unable of itself to differentiate between monochloroaluminate and the calcium aluminate hydrate C4AH13 when present in small amounts Thermal methods proved effective in characterising the products of hydration in the presence of chloride

S. D. Main

Papers

Hydration studies of modified OPC pastes by differential scanning calorimetry and thermogravimetry