Microstructure and Rheology of Lime Putty

Abstract

The rheology of lime binders, which is critical in the final performance of lime mortars and plasters, is poorly understood, particularly in its relationship with the microstructure and colloidal characteristics of slaked lime (Ca(OH)(2)) suspensions (i.e., lime putties). Here, the contrasting flow behavior of lime putties obtained upon slaking (hydration) of soft and hard burnt quicklimes (CaO) is compared and discussed in terms of the differences found in particle size, morphology, degree of aggregation, and fractal nature of aggregates as well as their evolution with aging time. We show that lime putties behave as non-Newtonian fluids, with thixotropic and rheopectic behavior observed for hard and soft burnt limes, respectively. Aggregation of portlandite nanoparticles in the aqueous suspension controls the time evolution of the rheological properties of lime putty, which is also influenced by the dominant slaking mechanism, that is, liquid versus vapor slaking in hard and soft burnt quicklimes, respectively. These results may be of relevance in the selection of optimal procedures and conditions for the preparation of lime mortars used in the conservation of historical buildings.

Diferencias en las propiedades reológicas de suspensiones de cal calcítica y dolomítica: influencia de las características de las partículas e implicaciones prácticas en la fabricación de morteros de cal

“…However, this value is much lower than that found by Ruiz Agudo and Rodríguez Navarro (22) for plate-like portlandite crystals in lime putty (r p =7.5). This discrepancy is due to the thickness of the plate-like crystals, which is higher in dry hydrated calcitic lime (CL) (l -=34.8 nm) than in calcitic lime putty (l -=11.5 nm) (22), while the crystal length is almost the same (is 87 nm and 85 nm in CL and lime putty (22), respectively). Los valores de movilidad electroforética (µ e ) y de potencial ζ de las dos cales son significativos para la estimación de la estabilidad cinética de las suspensiones en agua (Tabla 2).…”

“…La subestimación que se produce al medir la densidad de empaquetamiento de materiales secos en polvo se debe a la tendencia a la aglomeración de partículas secas de tamaño inferior a 100 µm (26), que da lugar a los agregados polidispersos descritos anteriormente. Sin embargo, durante la preparación de las suspensiones de cal los agregados se dispersan debido a las fuerzas electrostáticas entre las cargas presentes en la superficie de los cristales de portlandita y las moléculas polares del agua (22,(27)(28) Diferencias en las propiedades reológicas de suspensiones de cal calcítica y dolomítica: influencia de las características de las partículas e implicaciones… Differences in the rheological properties of calcitic and dolomitic lime slurries: influence of particle characteristics and practical implications… adsorption typical of non-porous adsorbents (25). Moreover, the hysteresis loop, both for the CL and the DL isotherm, is of type H3 (inset in Figure 6a), which is typical of aggregates of plate-like particles (9).…”

“…This underestimation of the packing density in dry powders is due to the tendency to agglomerate of dry particles of less than 100 µm (26), which gives rise to the polydisperse agglomerates described above. However, during the preparation of the lime suspensions in a polar medium such as water, aggregates are dispersed because of the electrostatic forces between the charges present on the surface of portlandite crystals (22,(27)(28).…”

“…disposición de plaquetas hexagonales angulares y polidispersas de cal calcítica (22) en suspensiones en reposo (28). Al aumentar la velocidad de cizalla se destruye esta estructura porque las fuerzas hidrodinámicas vencen las interacciones electrostáticas entre cargas opuestas presentes en los planos basales (de carga positiva) y laterales (de carga negativa) de plaquetas de portlandita (22). Conforme continúa el cizallamiento, las plaquetas se alinean espontáneamente según la dirección del flujo (29).…”

Mechanical activation of frozen lime putties

“…Both these effects most likely cause the loss of plasticity and the rapid sedimentation of particles in suspension. This is probably related to changes in surface charge (analogous effect is known for organic material dispersions) along with the aggregation of portlandite crystals [12,14,15,16].…”