Alkali-activated clay-based cement and mortar

Considerable research has been undertaken on metakaolin and alkali-activated waste binders for geopolymer concrete production. However, very little work has focussed on developing clay-based geopolymer concretes. Clay based geopolymer indicates that new clay based binder possess significant potential to be an alternative material to ordinary Portland cement (OPC) and existing geopolymer blends. Fly ash (FA) is the main source of binder material used to date for the production of geopolymer concrete, although other source materials are being investigated. A concern is that FA production will decrease in the near future as countries around the world will switch from coal to other renewable energies for electricity production. It has also been reported that not all types of FAs are suitable for geopolymer concrete production. Clay, which is a naturally occurring material, and abundant throughout the world, could be a suitable alternate source material to fulfil this requirement. A comprehensive analysis focusing on clay based geopolymer mortar and concrete has been performed in this research.<br><br> Geologically occuring natural clays from Ballarat and Nunawading areas of Victoria, Australia were selected as the source binder material. A mixture of liquid sodium silicate and sodium hydroxide was used as activator solution. Three curing temperatures: normal room temperature, 800C and 1200C were chosen to determine the optimum curing temperature. Four activator moduli ranging from 1.0 to 1.75 for each of two Sodium Oxide (Na2O) dosages of 10% and 15% were selected for determining the optimum mix deign. The source binder material was pre-treated up to 7500C for 5 hours to increase its reactivity. Some state-of-the art techniques such as X-Ray Computed Tomography (CT Tomography), monitoring of internal temperature profile during elevated temperature curing, thermo gravimetric analysis (TGA), zeta potential as well as other common techniques were applied for determining the mineralogical and microstructural characteristics and properties of source materials and their final products. The elevated temperature curing (at 1200C) was monitored for a duration of 100 hours. CT Tomography has been applied to determine the orientation of pore structure and its relation with strength. <br><br> The result of this research revealed that elevated curing temperature (1200C) is required for achieving the structural integrity of clay based geopolymer mortar. The setting time of clay based geopolymer is longer than that of FA based geopolymer mortar or normal OPC mortar. Clay based geopolymers generally showed an increase in strength up to 14 days. After this a slight increase in strength was observed for the specimens with a 10% Na2O dosage, while a decrease was observed for those with a 15% Na2O dosage, other than with an AM of 1.0. The compressive strength of specimens prepared with AM of 1.0 for both Na2O dosages of 10% and 15% exhibited superior strength compared with all other AM investigated. Pre-treatment at 1200C for 24 hours does not change the aluminium (Al) co-ordination of clay significantly but pre-treatment of clay at 7500C for 5 hours can change all the Al (VI) co-ordination into Al (IV) co-ordination. To understand the mechanism responsible for gaining strength of clay based geopolymer is critical. Results from this study clearly demonstrate that the more aluminium ions in Al (IV) co-ordination in the constituents of the clay powder as well as the greater dissolution of aluminium during geopolymerization play a significant role in determining the final product. <br><br> CT Tomography showed that large quantities of micro-pores as well as higher porosity made clay based geopolymer specimens loosely compacted. The curing temperature profile of a 300 mm3 clay based geopolymer concrete specimen showed three distinct phases. The first stage continued up to 14 hours while the duration of second and third stage varied along the depth of the specimen. In the elevated temperature curing, the strength gain phenomenon of smaller specimen is not similar to that of larger specimen. Longer curing duration is required for larger geopolymer specimen. Curing at 1200C for 24 hours is not sufficient for larger specimens to obtain full strength as the geopolymerization is not complete, even after 60 hours. Finally, a number of established correlation equations between the mechanical properties of both OPC and FA based geopolymer concrete do not match for clay based geopolymer concrete.<br><br>