Structure of Hardened Cement-Fly Ash Systems and Their Related Properties

Doktorsavhandling, 1992

The influence of fly ash on the pore structure and microstructure of cement paste and mortar has been investigated. The methods applied include water vapour sorption isotherms, scanning electron microscopy, X-ray diffraction analysis and thermogravity/differential thermal analysis. The desorption isotherm test shows that, with low to medium cement replacement (F/(C+F) up to 0.30), fly ash does not significantly influence the pore structure as far as the ability to hold moisture is concerned. The same conclusion is applicable to the sorption isotherm, for which the specific surface area is a dominating factor. However, high volume fly ash addition results in a coarser pore structure, which leads to more moisture loss per paste at high relative humidity conditions, e.g. at 85% RH. Correspondingly, the sorption isotherm reveals a lower specific surface area value for the high volume fly ash mortar. The microstructural analysis shows that more calcium hydroxide per cement is produced in the paste with fly ash, which in turn reacts with fly ash to form calcium silicate hydrates and calcium aluminate hydrates similar to those produced by the cement hydration. However, the amount of calcium hydroxide per cement in the paste is lower only for the high volume fly ash specimens at late ages, i.e. later than 28 days. The analysis of the amount of cement at each age shows that the hydration of cement is approximately the same for the paste with and without fly ash. In conjunction with this, the hydration products in the paste with fly ash are usually a compound of calcium-aluminate-silicate with a low calcium oxide to silicon-alumina ratio. Since fly ash hydration is a slow process, the effect of calcium sulphate hydrate (as an activator) on the hydration of fly ash was studied. It has been shown that an appropriate addition of gypsum (CaSO4.2H2O) enhances the decomposition and reaction of fly ash. The reaction products include ettringite ({Ca6[Al(OH)6]2.24H2O}[(SO4)3.1.5H2O]), as well as calcium aluminate hydrate, e.g. C3AH6. It was observed that X-ray energy dispersive analysis of the ettringite-like phase reveals, more often than not, the inclusion of Si in the crystal lattice. The pore-filling effect of fly ash has been shown to be about the same as that of cement. However, this effect depends totally on the reactive phase of fly ash, which is about 50% for the fly ash studied in this work. The other factors influencing the pore structure, such as carbonation and ageing of the hydration products, were also tested. It is shown that the former results in coarser pore structure, and the latter affects some change in the C-S-H gel structure. In addition to the experimental work, the author has theoretically treated the effect of fly has, the desorption and adsorption isotherms, and the relation between shrinkage and the desorption isotherm.