Alkali activation of Iranian natural pozzolans for producing geopolymer cement and concrete.

The challenge for the civil engineering community in the near future will be to realize the building of structures in harmony with the concept of sustainable development, through the use of high performance materials which have low environmental impact and can be produced at reasonable cost. Geopolymers are novel binder materials that could provide a route towards this objective. Although research on geopolymer has advanced, most of the previous research conducted on geopolymers has dealt with pastes and concentrated on the material's chemistry and microstructure. There is little information available concerning the engineering and durability properties of geopolymer concrete and none considering the use of natural pozzolans for production of geopolymer concrete. This investigation has studied the potential of using five natural pozzolans from Iran as geopolymer precursors. Most of the raw materials contain zeolites and clay minerals and have a high loss on ignition. Therefore, trials were made where samples were calcined at 700, 800 and 900°C. The solubility of both the raw and calcined materials in an alkaline solution was used as an indicator for pozzolanic activity. Improvements in pozzolanic properties due to heat treatment and elevated curing temperatures (20, 40, 60, and 80°C) were studied by using alkali solubility, XRD and compressive strength tests. It has been found that geopolymer binders can be synthesized by activating natural pozzolans and condensing them with sodium silicate in a highly alkaline environment. A new model is presented which allows the prediction of the alkali activated pozzolan strength from information on their crystallinity, chemical compositions and alkali solubility. Two types of Iranian natural pozzolans, namely Taftan which can be activated without calcination and Shahindej which was calcined were selected for further activation to study the effect of the alkaline medium on the strength of the alkali-activated natural pozzolan. The effect of the type, form, and concentration (molarities =2.5, 5.0, 7.5, 10.0 M) of the alkaline hydroxide, the modulus of sodium silicate (Si02INa20 ratio =2.1, 2.4, 3.1) and different curing conditions on the geopolymerisation of the above two natural pozzolans were studied. The optimum range and contributions for each factor is suggested based on their effect on compressive strength. An optimum paste formulation has been developed for concrete mixing together with the procedure of addition of the raw materials to the reaction mixture and suitable curing methods for producing the geopolymer concrete derived from them. The properties of this geopolymer concrete in both the fresh and hardened states have been investigated in terms of setting time, workability, air content, compressive strength, splitting tensile strength, static modulus of elasticity, ultrasonic pulse velocity, and drying shrinkage. Studies related to durability such as gas permeability, chloride ion penetration, and sulphate resistance have been undertaken and compared to these for typical OPC concretes. Some problems were encountered in applying the standard concrete durability tests. In this study attempts have been made to determine the relationships between the different properties of geopolymer concrete with its compressive strength and compared to results for ope concrete, to help to explain the differences between alkali-activated natural pozzolan concrete and ope concrete. In the countries which have large resources of natural pozzolan, geopolymer concrete based on alkali activation of these resources can help decrease the energy consumption and environmental impacts involved in using traditional cement pastes.