Development of alkaline activated binders using tungsten mine waste mud from Panasqueira mining (Portugal)

seccao geopolymers

 

The present research work deals with the development of alkali activated binders, aiming at the reuse of waste mud from the Panasqueira Mine. The mine waste mud is fully characterized and activated using adequate activators developed. The mine waste mud showed some reactivity only when activated with soluble silica. Thermal treatments were undertaken in order to enhance the reactivity of the waste mud using different treatment times and temperatures. The optimum thermal conditions were achieved using 120 minutes treatment at 950 oC. Although the reactivity increased significantly but X-ray diffraction and infrared spectroscopy tests showed that total destruction of the crystalline structure had not yet occurred.

The simultaneous effect of parameters that contribute to the early age mechanical behavior of the new binder, such as the percentage substitution of mine waste mud with calcium hydroxide, the concentration of the sodium hydroxide and the mass ratio of sodium silicate to sodium hydroxide were studied. Several mix designs were tested and the mix proportions leading to the maximum early age strength were identified. It is shown that water/sodium molar ratio has a significant effect on the early age strength while water/binder ratio has a rather modest effect. Parameters that affect the early age strength gain are also found to affect the long term strength gain. A loss of strength with curing time as been noticed for some mix designs and probable causes for this undesirable behaviour are discussed. 

The possible reactions of the aggregate with the new binder were also studied. Mechanical properties and microstructure of mixes using three different types of aggregates were studied. Results obtained indicate that aggregates react chemically with the binder and the interfacial transition zone (ITZ) paste/aggregate shows a dense and uniform zone. The traditional ITZ usually found in portland cement paste/aggregate was not detected.

The microstructure and the chemical composition of the hydration products were duly studied in order to identify the mechanism and the microstructure of the cementitious material formed. X-ray diffraction patterns point to the existence of highly crystalline reaction products, however phases containing alkali cations were not identified. Infrared spectroscopy spectra shows the existence of structural water. The mechanical behaviour of the binders apparently is affected by the presence of this water. Microstructure analysis indicates that the hydration products were calcium silicate hydrates and alkali aluminosilicate gel.

Furthermore, physical and mechanical properties of the new binder were studied and it is found that for conditions leading the optimum strength, workability was low and could not been measured using the common procedures and devices. The use of a superplasticizer led to denser mixtures but did not affect significantly the workability. Devices traditionally used for measuring the setting times of portland cement pastes were found not to be suitable for the new binders. The same seems to be valid for shrinkage evaluation, as the results obtained using the traditional test method reflects only part of the real shrinkage behaviour.

The new binders show high flexural strength although with a tendency for slight decrease at early ages. The modulus of elasticity of the new binders exhibits a higher value than that of other alkali activated binders, although a valid correlation with compressive strength have not been established. 

The adhesion of the new binder with OPC concretes was also studied. The results obtained show exceptionally high adhesion performance compared to that of commercially available mortars, albeit at a much lower cost. This has significant pratical implications, as the binder can become a serious competitor for the currently available products used for the restoration of OPC concrete structures. However, the new binder shows high efflorescences that need to be considered for certain applications.

A study of the durability of the new binder was also carried. The water absorption levels were found to be low and the abrasion and acid resistance is significantly higher than of comparable OPC concretes. The leaching tests performed to assess its capacity for fixing heavy metals, indicate good performance of the new binder and thus its positive environmental behaviour. 

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