Tone Anita Østnor

Feasibility of Calcined Marl as an Alternative Pozzolanic Material

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 Mossbauer 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.

Reactivity and Microstructure of Calcined Marl as Supplementary Cementitious Material

The reactivity and microstructure of cement paste and mortar where cement is replaced by up to 65 vol.% calcined marl are discussed. It was found that the compressive strength evolution of mortar is following the same linear relation with amount of hydrate water at early ages for different cement replacements (35-65 vol.%), but that this deviates and give higher strength than predicted by the bound water at higher ages. Strength increases on a long term in spite of depleted calcium hydroxide at earlier ages and are discussed in terms of changes in CSH and CAH. XRD does not reveal any unusual crystalline products, but ettringite and hemi-/mono-carboaluminate hydrate. SEM with WDS in a 2 year old mortar found pure CAH in a pore with atomic Ca/Al = 1.6 and some Si that might be a hydrogarnet.

Promising Early Age Evaluations of Fly Ash - Calcined Marl - OPC Ternary Cement

The search for new supplementary cementing materials (SCM) creates opportunities to using new materials such as calcined clays and marl. However, such changes require a transition period, where introduction of new SCMs are most successful when they can be coupled and employed with the existing materials.

Alternative Binders Based on Lime and Calcined Clay

Alternative binders are in this study defined as binders without Portland cement. In this particular project it will be based on calcined clay or fly ash as a source of reactive silica and alumina in combination with lime and calcium sulphate (e.g. gypsum) and/or calcium carbonate to stabilize special calcium aluminate phases like ettringite and/or calcium monocarboaluminate hydrate, respectively. Gaining strength is all about maximizing the transformation of liquid water to hydrates with hydraulic properties as fast as possible. Hence, admixtures speeding up the reaction kinetics can be part of the formulations as well. Two synergy principles have been described for making improved binders based on slaked lime and pozzolanic SCMs. One showing how neutral salts may accelerate by forming strong alkaline solutions in situ and another one how calcium carbonate can play a role when alumina containing SCMs are used by leading to an even higher conversion of liquid water into solid hydrates leading to lower porosity and higher strength. Calcined marl may substitute for calcined clay, but then the calcium carbonate content should be reduced in accordance with the remaining calcium carbonate content after calcination.