Time Evolution of the Modulus of Elasticity of Metakaolin-Based Geopolymer

Abstract

The objective of the research is to develop a new family of geopolymeric materials and to use an experimental methodology to characterize the mechanical behavior of the materials obtained by alkaline activation of metakaolin using a compound activator. The researchers also intend to study the unknown time evolution of the modulus of elasticity and the influence of the composition of the aggregates on the strength of the material. Like the material’s strength, the results have a direct influence on structural safety evaluations. For the analysis of the mechanical properties of the mixtures, different types of tests were carried out: Flexural and compression tests on parallelepipeds and compression tests on cylinders were performed to assess the main strength characteristics of metakaolin-based geopolymers. Regarding the aggregate composition, the results show that the correction of the aggregate particle size line did not improve the mechanical properties. From about 400 h of curing, at ambient temperatures, the mechanical properties of the geopolymeric material are almost invariable. The highest value of the elastic modulus of elasticity occurs around 420 h, at about 18 GPa. The modulus of elasticity is independent of test load rate as per standards, and 1.7‰ strain was observed during maximum compressive stresses in the rupture tests. Also, the secant modulus values at 60% and 80% of maximum stress are within 12% of the value at 40% of maximum stress.

Influence of Curing Temperature on the Strength of a Metakaolin-Based Geopolymer

“…Indeed, the maximum strength was probably reached before that point. Considering the deformation of the material, which was equivalent to that attained on the 28th day of curing for an OPC-based concrete, Lopes et al [69] have shown that metakaolin-based geopolymers reach maturity around day 15 or 16 of curing. Furthermore, and as stated above, increasing the curing temperature by a limited amount helps to reduce this time interval without reducing the final strength.…”

“…The remaining diagrams were derived (only) from the point in gray, assuming a given value of the modulus of elasticity (MOE). In this regard, it is worth adding that the presented MOE values have a scientific basis: (i) for the MKW geopolymer, MOE = 12 GPa, as determined by Lopes [71]; (ii) for the MKB geopolymer, MOE = 18 GPa, as determined by Lopes [69]; (iii) the EC2 [39] was used to estimate the expected value for the OPC mortar given the compressive strength (MOE = 41 GPa).…”