Chemorheology of a Si/Al > 3 Alkali Activated Metakaolin Paste through Parallel Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA)

Abstract

Although geopolymers, as structural materials, should have superior engineering properties than traditional cementitious materials, they often need to improve their final characteristics’ reproducibility due to the need for more control of the complex silico-aluminate decomposition and polymerisation stages. Thermosetting of a reactive geopolymeric paste involves tetrahedral Silicate and Aluminate precursor condensation into polyfunctional oligomers of progressively higher molecular weight, transforming the initial liquid into a gel and a structural solid. Viscosity and gelation control become particularly critical when the geopolymer is processed with 3D printing additive technology. Its physical state modification kinetics should match the flow and setting characteristics required by the deposition process. The reaction kinetics and the elastic and viscous characteristics preceding gelation and hardening have been investigated for an alkali-activated Metakaolin/Sodium Silicate-Sodium Hydroxide paste with a Si/Al ratio > 3. A chemoreological approach has been extended to these inorganic polymerisable systems, as already utilised for organic thermosetting polymers. Differential scanning calorimetry and Oscillatory DMA were carried out to monitor the advancement of the polymerisation reaction and the associated variations of the rheological viscoelastic properties. Dynamic thermal scans were run at 1 °C/min and a frequency of 10 Hz for the dynamic mechanical tests. The observed kinetics of polymerisation and variations of the elastic and viscous components of the complex viscosities and shear moduli are described in terms of polycondensation of linear and branched chains of oligomeric macromolecules of increasing complexity and molecular weight up to gelation (Gel1) and cross-linking of the gelled macrostructure (Gel2) and final glassy state. Geopolymerization can be allocated into two main behavioural zones: a viscoelastic liquid paste below 32.5% of reaction advancement and a viscoelastic solid above. Initial complex viscosities range from 2.3 ± 0.9 × 10−5 MPa*s to 6.8 ± 0.9 × 10−2 in the liquid-like state and from 1.9 ± 0.1 MPa to 9.6 ± 2.1 × 102 MPa in the solid-like state.

Thermokinetic and Chemorheology of the Geopolymerization of an Alumina-Rich Alkaline-Activated Metakaolin in Isothermal and Dynamic Thermal Scans

“…The concurrent presence of activated silica and alumina units is mandatory for geopolymerization reactions to occur [ 16 ] since, although Al 3+ as a modifier ion cannot independently participate in the formation of a polymeric network, it can replace Si 4+ in the [SiO 4 ] 4− tetrahedron when presenting four-oxygen coordination [ 20 , 21 , 22 ]. In the forming polymeric network, the excess negative charge of Al in tetrahedron monomeric ions [AlO 4 ] 5− (compared to the [SiO 4 ] 4− ) is balanced by a positive alkali ion such as Na + and K + [ 4 , 18 ].…”

“…As depicted in the central portion of Figure 1 , sialate, sialate–siloxo, and sialate–di-siloxo molecules form linear polymer chains when behaving as difunctional (f = 2) until the Si/Al ratio is lower than two. However, when the Si/Al ratio is two or higher, cyclic sialate branching and macromolecular crosslinks are generated (lower right side of Figure 1 ), leading to the formation of macromolecules with functionalities higher than four (f > 4) oligomers [ 1 , 2 , 3 , 22 , 23 ] that rapidly increases the branching tendency of the growing macromolecule [ 26 ].…”

“…From the thermodynamic side of the neo-forming inorganic polymeric solution, the initially low Mw oligomeric chains are progressively hindered as polycondensation proceeds due to the modification of the solution’s dynamic (rheology) [ 22 ] and thermodynamic (solubility) properties [ 24 ]. The progressively weaker interaction of the solvating units with the growing macromolecules reduces their affinity with the solvent and increases their tendency to segregate from the solution.…”

“…As the polycondensation reaction continues, [Al(OH) 4 ] − concentration in the reacting medium reduces, increasing the Si/Al ratio above three, further favoring molecular branching and precipitation of Si-rich gel particles [ 27 , 28 ]. The alumina- and silica-rich gel particles then aggregate into larger particulate structures that drastically affect the material’s rheological [ 22 ] and ultimate properties [ 29 , 30 , 31 ].…”

“…The point where this event occurs marks the end of a liquid-like behavior and announces the transition into a solid elastic rubber identified in Figure 1 as “Gelation”. The still-containing reactive oligomers rubber network continues to react, increasing the crosslinking density and finally allowing geopolymer vitrification [ 22 ].…”