DISPERSED HYDRATED LIME: DEVELOPMENT AND PRODUCTION, TECHNIQUES AND APPLICATIONS

Transcription

1 DISPERSED HYDRATED LIME: DEVELOPMENT AND PRODUCTION, TECHNIQUES AND APPLICATIONS R Strotmann, Strotmann & Partners, Cologne, Germany 1. The development of dispersed hydrated lime The research on, and development of, dispersed hydrated lime originated with an effort to find an injection mortar with a stable sediment that was based on white lime. The injection mortar was to be composed in a fashion similar to a lime mortar, with binders added in a 1:4 ratio. One of the main problems we faced was making the solution of solid particles in the water solvent sufficiently stable to allow injection through a syringe. At the same time the proportion of water solvent was to be minimised in order to ensure a reasonable mechanical strength. As neither pit lime nor hydrated white lime was able to stabilise such systems, we tried several kinds of stabilising agents in order to achieve the required sediment stability. This technique led to an increased proportion of water solvent, however, as steric stabilising agents, such as acrylic acid or cellulose ether, thickened the mixture and enthalpic stabilising agents, such as pyrogene Silica, reacted with calcium hydroxide to form larger agglomerates. In 1992 Lehmkuhl first introduced the technique of dispersing mixtures of binders (aqueous silica binder), stabilising agents (pyrogene Silica) and aggregates in Germany. This technique, we realised, bore new opportunities for stabilising lime-based injection mortars. The dispersing process mechanically breaks up calcium hydroxide aggregates and distributes them evenly in the water solvent. The strong polarity and surface charge of the smaller aggregates now makes calcium hydroxide act as an enthalpic stabilising agent. Thus, the technique of dispersing hydrated lime produced stable fluid injection mortars with an incredibly low rate of shrinkage. At first we dispersed mixture of solid components of hydrated white lime and aggregates in water solvent, but today we produce the binder separately. This has advantages in the production process and improves and maintains constant the physical properties of the material. 407

2 2. The technique of dispersing For dispersing you need most of all a dissolver, that is a high performance stirrer/mixer with an adjustable speed range. A toothed dissolver disc is fixed to the stirrer shaft. The teeth on the edge of the disc are bent up and down in an alternating fashion. Rotation of the disc produces pressure on the front of the teeth and suction on their back. The alternating stress on the particles caused by pressure and suction, as well as the collision of the particles with the rotating disc, results in the break-up of larger agglomerates into smaller ones. The amount of shear produced in this manner depends on the size of the disc and on its rotation speed. Optimal transfer of shear energy to the dispersed mixture occurs when a certain flow pattern, the so-called doughnut effect, is achieved. The pattern of flow of a mixture in a dissolver depends on the speed of rotation of the mixer shaft and on the viscosity of the compound, where the latter again depends on the amount of solid particles in the mixture. In general you will find that the higher the shear, the higher the degree of break-up and dispersion of calcium hydroxide aggregates. 3. Dispersing agents and protective colloids The smaller the calcium hydroxide aggregates become, the larger their total effective surface area and their stabilising effect, but also the required amount of solvent. In order to minimise the proportion of water required, and also in order to reduce the stabilising effect of the small calcium hydroxide aggregates to the exact degree we set out to achieve, dispersing agents are added to the compound. The dispersing agents we use for this purpose originated in the ceramics and concrete industry and in the coatings and paint industry. Among the many dispersing agents tested, a sodium polyacrylate was found to be the most effective, where effectiveness was measured by the amount of dispersing agent required for reducing the viscosity of the compound to the degree required for as long as necessary. In the case of sodium polyacrylate, both the liquefying effect of the sodium and the protective-colloid effect of the polyacrylate counter-ion are utilised. Protective colloids effectively protect against re-agglomeration of calcium hydroxide aggregates. The separated aggregates tend towards building larger re-agglomerates on the basis of hydrogen and calcium-ion binding. As re-agglomeration spreads, the dispersed hydrated lime loses its outstanding physical properties, which makes the choice of a dispersing agent with a matching counter-colloid of utmost importance in the development of this technology. We also add a low-viscosity cellulose-ether as a second protective colloid. The proportion of organic materials in the compounds we use with dispersed hydrated lime is currently less than 0.1 weight-%. We are continually working on improvements to the materials we use and on reducing the proportion of organic admixtures. 408

3 4. Reactions A higher degree of break-up of calcium hydroxide aggregates and an accompanying increase in the specific surface area of these aggregates improve the reactivity of hydrated white lime. Studies have shown a significantly faster and more thorough carbonisation of dispersed versus conventional hydrated white lime. The qualitative progress over time of carbonisation in conventional hydrated white lime is compared with carbonation in dispersed hydrated white lime in two 3D diagrams of X-ray powder diffraction measurements (Figures 1 and 2). Figures 3 and 4 shows these differences even more clearly. Dispersed hydrated lime reaches virtually complete carbonisation after only 190 minutes, while non-dispersed hydrated lime shows a very slowly increasing share of carbonate with stagnation at a low level after 450 minutes. The diagrams show measurements over a period of 600 minutes of reaction time. It shows clearly a stagnation of carbonisation after 190 minutes with a final calcium carbonate share of 94%. After adding water, the process of carbonisation begins once again. In a comparative test with regular hydrated white lime the degree of carbonisation over the same period of reaction time only reaches 34% and cannot be increased to any significant degree by adding water. 5. Properties In injection mortars we have reached compressive strength of 3 up to 13 N/mm 2 and surface tensile strength of up to 0.55 N/mm 2. In stone repair mortar we measured a compressive strength of up to 9 N/mm 2 and a tensile strength of 0.25 N/mm 2. Furthermore, this high degree of strength does not adversely affect capillarity. As the porosity of the compounds increases, the water absorption is, depending on the aggregates, between 12 and 35 weight-%. One of the most outstanding properties of compounds based on dispersed hydrated lime is their resistance to freeze-thaw cycles and to salt-crystallisation. We developed shelter coatings, injection mortars and repair mortars with material loss of less than 1 weight-% after 25 freeze-thaw cycles and material loss of less than 10% after 10 salt-crystallisation cycles using 10% sodium sulphate. 6. Conclusion The fast reaction time of dispersed hydrated lime together with its outstanding weathering properties and adjustable strength makes it an ideal material in the presentation and conservation of art and monuments that had previously been inaccessible to other techniques. Lime compounds can now be used on exterior surfaces 409

4 to a larger extent than was possible before. Lime is therefore becoming an interesting alternative as a binder in stone conservation materials. Figure 1 3D diagram of X-ray powder diffraction: carbonisation of hydrated white lime (by P. Boos) Figure 2. 3D diagram of X-ray powder diffraction: carbonisation of dispersed hydrated lime (by P. Boos) 410

5 Figure 3:carbonisation of hydrated white lime (by P. Boos) Figure 4 carbonisation of dispersed hydrated lime (by P. Boos) 411