Effect of thermal treatment on phase composition and pozzolanic reactivity of a natrolite-rich rock

The Fohberg phonolite, Kaiserstuhl Volcanic Complex (SW Germany), is a zeolite-bearing subvolcanic rock used as raw material for blended cement manufacturing. Rock-forming zeolites are of the natrolite group (natrolite, Ntr, and gonnardite, Gon) which amount to 45-50 wt.% of the bulk rock. The natrolite/gonnardite ratio, determined by powder X-ray diffraction (pXRD) and expressed as x_Gon = Gon/(Ntr+Gon), varies between 0.1 and 0.2 but may reach > 0.3 in exceptional cases. During industrial processing, the crushed rock is tempered in a rotary kiln at about 400 °C to enhance pozzolanic reactivity.

In-situ pXRD experiments (Bruker D6 Phaser equipped with Anton Paar BTS 500 heating stage) up to 460 °C were conducted to unravel mineralogical changes of the material during processing, and to detect structural changes of the zeolite constituents. Natrolite starts to convert to metanatrolite during heating at 380 °C; the phase transition is fully reversible. Gonnardite shows a strong, gradual decrease of the a-lattice parameter during heating, and becomes irreversibly unstable at 380 °C. The disappearance of gonnardite is accompanied by an increase of amorphous content of the sample. Structural changes are reflected by a distinct mass loss between 330 and 400 °C, related to zeolite dehydration. The other phonolite phases remain unaffected.

To test the pozzolanic reactivity, untempered and tempered phonolite samples with variable x_Gon were blended with a commercial CEM I 42,5 R reference cement. Indicators of reactivity are compressive strength and Ca(OH)₂ consumption (Frattini’s test). Results of the tests are shown and related to the pXRD experiments.