The increasing use of geopolymer materials in the construction and civil engineering sectors generates a large amount of non-biodegradable waste that will end up in landfills. It is therefore necessary to anticipate solutions for the proper management of this waste. In this work, new geopolymer materials were fabricated by partially replacing the reactive raw minerals (fly ash, FA, or metakaolin, MK) with used geopolymers (fully fly ash-based, FAref, or metakaolin-based, MKref), in order to develop a strategy to reuse geopolymer waste. Their workability and setting behavior were studied in the fresh state, and the geopolymerization process was investigated by calorimetry and by electrochemistry. Mechanical properties and the ability for coating mineral aggregates were assessed, and the resulting adhesion properties were analyzed using matrix/sand mortars. It appears that the new geopolymer materials as well as the mortars are endowed with good performances. The compressive strengths are above 50 MPa and therefore meet the requirements of different construction materials. This demonstrates the recyclability of geopolymer materials. Moreover, an analysis of the influence of the substitution of recycled geopolymers on the setting and on the mechanical performances of mortars makes it possible to propose a binder-recycled geopolymer interaction model for the formation of new binding matrices.

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Recycling geopolymer waste, by reusing it as a raw material for manufacturing new geopolymer binding matrices, is an interesting asset that can add to the many technical, technological and environmental advantages of this family of materials in the construction field. This can promote them as promising alternatives to traditional materials, such as Portland cements, which are not so environmentally friendly. Recent studies have shown that the partial replacement of reactive aluminosilicates (metakaolin and fly ash) up to a mass rate of 50% by geopolymer waste does not significantly affect the compressive strength of the new product. In line with these findings, this paper investigates the effects of aggressive environments, i.e., high temperatures (up to 1000 °C) and acid attacks (pH = 2), on the characteristics of these new matrices. Different techniques were used to understand these evolutions: mineralogical analysis by X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TGA-DTA), mechanical characterization and scanning electron Microscopy (SEM) observations. The results are very satisfactory: in the exposure temperature range explored, the new matrices containing geopolymer waste suffered losses in compressive strength similar to those of the matrices without waste (considered as materials reference). On the other hand, the new matrices exhibited good chemical stability in acid media. These results confirm that the reuse of geopolymer waste is a promising recycling solution in the construction sector.

Phosphogypsum results from the attack of phosphate ores with sulphuric acid with the aim of producing phosphoric acid. Large quantities of this industrial waste are produced annually, even though it is very harmful because the ores often have a high content of heavy metals. It is therefore necessary to undertake studies in order to reduce its production or to valorise it in the elds of building and civil engineering, in particular. Recent works propose a new way of valorisation in the technology of geopolymers: the addition of phosphogypsum in the mixtures improves the re resistance of the products, which constitutes a great potential of use in many applications. Our study aims to highlight the effects of the addition of phosphogypsum on certain characteristics of geopolymer matrices based on metakaolin and y ash, in the fresh state and in the hardened state. Monitoring the workability of the geopolymer matrices indicates that the addition of phosphogypsum increases the viscosity, but impairs the owability and the shaping. Thus, the phosphogypsum addition rate is limited to 15% for metakaolin-based matrices and 12% for those based on y ash, with a setting retarding effect in both cases. Moreover, the introduction of phosphogypsum to these matrices up to a mass rate of 6% has no signi cant effect on the mechanical resistance. Beyond that rate, the mechanical resistance deteriorates signi cantly.

In this work, we are interested in describing the evolution of the rheological behavior of geopolymer materials since their elaboration until the start of their kinetics of setting. We considered the case where these materials are obtained from the dissolution of a quantity S of metakaolin in a quantity L of alkaline activation solution (sodium-based). In this context and for all the values of the structural identification parameters P = (% Na2O) and R3 = (L/S) in our experiments, it has been established that the rheological behavior is described by the viscoplastic Herschel-Bulkley model. In particular, we have been able to determine how rheological parameters of the model are affected by variations of P and / or R3. For the considered formulations, this analysis is of importance insofar as it was at the base of the identification of critical values of P and R3. Moreover, we verified that for these values, the setting time is maximum. This result is retained to define the optimized formulation of our materials.