The physico-chemical and mineralogical characteristics of a MSWI fly ash have been studied. This fly ash has a coarser and wider size grading than that of typical cements. It contains a high soluble fraction made up mainly of chlorides and sulphates. The sulphates are principally present in the form of anhydrite. Chlorides are certainly present in the form of halite and sylvite, though the presence of calcium chloride is also a possibility. Two phases possibly possessing pozzolanic properties have also been identified: a calcium aluminodisilicate (CAS2) and an aluminosilicate (AS). This fly ash also contains high quantities of heavy metals (approximately 2 %), of which the most abundant are zinc and lead. This research has allowed us to propose a simplified quantitative mineralogical composition for this fly ash.
We have also studied hydration products formed in cement pastes in the presence of the MSWI fly ash. Analysis by X-ray diffraction has demonstrated the presence of Friedel's salt, ettringite and thenardite resulting from the presence of fly ash. DSC analyses have shown that cement pastes containing MSWI fly ash have a more concentrated interstitial solution than pure cement pastes. Tests carried out on samples in which the pore solution was replaced have allowed us to conclude that the C-S-H structure in cement pastes containing fly ash is very close to that of pure cement pastes.
The performance characteristics of mortars with increasing MSWI fly ash content (up to 20 % of the cement mass) have been determined. This study has shown that the MSWI fly ash increases mortar setting times. The retarding effect of zinc and lead is almost certainly the cause of the delayed setting times. Incorporating MSWI fly ash in mortars, up to 15 % (with an optimum for 10 %) in relation to the cement mass, increases their strength after (7, 28, and 90) d. The acceleration effect of chloride almost certainly explains this increase. Beyond 15 %, the fly ash causes a slight decrease in strength, which may be due partly to the weak bond between the cement paste and the MSWI fly ash. Strength reductions in the longer term (after 565 d) were observed in all the mortars containing MSWI fly ash. This decrease in strength, occurring under non-aggressive storage conditions, has not been explained.
This experimental study has allowed us to propose an approximate quantitative mineralogical composition for the MSWI fly ash to incorporate it into the CEMHYD3D cement hydration model and to take into consideration its interactions with cement components. Assumptions of the model and the results obtained using it will be presented in the second part of this article.