Feasibility of Calcined Marl as an Alternative Pozzolanic Material

Abstract

Calcareous clay rich in smectite was calcined at temperatures of 600–1000 °C using a pilot- and industrial scale rotary kiln. Compressive strength of mortars was tested between 1–365 days, when 20–65 % of OPC was replaced by calcined clay at equal w/c-ratios. With respect to reactivity as a pozzolan, the optimum calcination temperature was around 800 °C. With a replacement level of 50 % the 1-day strength was reduced but high enough for demoulding concrete infield practice, while after 28 days almost the same strength as with no replacement could be obtained. The raw and reactive calcined state of the clay was characterised using different methods like XRD, TG/DTG, SEM, FTIR, Al²⁷-NMR and Mössbauer Spectroscopy. At the optimum calcination temperature calcium carbonate from the clay is only partly decomposed. The main calcium carbonate source is coccoliths which enabled the formation of a reactive Ca enriched glass phase together with the decomposing clay minerals. Oxidation of Fe²⁺ to Fe³⁺ resulted in a structural disordering increasing the reactivity of the calcined clay. Pozzolanic activity was tested in pastes of calcined clay and calcium hydroxide.