Fly ash-based cements: the GEOASH project
Fly ash-based geopolymer cements reduce CO2 emissions by 90% when compared to Portland cement. The GEOASH (2004–2007) project was carried out with a financial grant from the Research Fund for Coal and Steel of the European Community. The GEOASH project is known under the contract number RFC-CR-04005. It involves: Antennuci D., ISSeP, Liège, Belgium; Nugteren H.and Butselaar-Orthlieb V., Delft University of Technology, Delft, The Netherlands; Davidovits J., Cordi-Géopolymère Sarl, Saint-Quentin, France; Fernández-Pereira C. and Luna Y., University of Seville, School of Industrial Engineering, Sevilla, Spain; Izquierdo M. and Querol X., CSIC, Institute of Earth Sciences “Jaume Almera”, Barcelona, Spain.
Seventeen samples of (co-)combustion European fly ashes were tested on their suitability for geopolymeric cements. Normally, curing of fly ash-based matrices is done at temperatures between 60 and 90°C. In this project, since the idea is to use the geopolymer as a cement, the curing is taking place at ambient temperature, with a modified (Ca,K)-based geopolymeric system. The Final Technical and Scientific Report was presented mid 2008. Detailed information may be found in the updated 2nd edition of Davidovits’ book Geopolymer Chemistry & Applications, Chapter 12.
Two methods were used and compared with. One, called the classical or conventional method, relies on alkali-activation and pure NaOH (8M, 12M), i.e. User-hostile conditions. The second is based on geopolymerization with (Ca,K,Na) geopolymeric systems, i.e. User-friendly conditions. The geopolymeric method was developed for the implementation of all kind of geological materials, eg. Rock-based geopolymers. The (K,Na,Ca)-poly(sialate-siloxo) process is based on the system fly ash / slag / Ksil / H2O reacting at room temperature. The ashes, 60-80% by weight of the mix, were mixed with the geopolymeric slurry containing alkali-silicate solution (molar SiO2:M2O > 1.40), blast furnace slag and water, and cured at room temperature.
The investigations by Palomo and his team (Fernández-Jiménez and Palomo, 2003), are often taken in the literature as a reference. They claim that the pure NaOH based zeolitic system should be considered as the reference in the determination of the chemical parameters leading to a material with optimal binding properties. According to these standard criteria, any fly ash with a mullite content higher than 5% is not suitable and may not be used. Six fly ashes were selected and submitted to this criteria. The results are shown in the Figure below. Only two fly ashes, CSIC-4 and CSIC-5 have mullite content lower than 5% and might work with alkali-activation. The Figure displays the results of the (Ca,K)-based geopolymeric method. It shows the 28 day compressive strength obtained in relation with the mullite content. All values are higher than 50 MPa, the majority reaching strengths higher than 70 MPa. It is therefore important to notice that practically all class F fly ash types, i.e. those with low free CaO, can be used with this user-friendly system.
It has also been measured that for a given fly ash, the conventional alkali-activation (zeolitic method) provides lower compressive strength than the (Ca,K,Na)-based geopolymeric procedure. The geopolymeric method yields higher strengths as well as lower costs (no thermal activation needed) and safer and easier handling treatment, i.e. user-friendly.
Compressive strength at 28 days in relation with mullite content, room temperature curing (GEOASH).