Die Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege International e.V. (WTA-INT) verleiht jährlich den WTA-Young Professional Award für herausragende Leistungen auf den Gebieten der Forschung und Praxis der Bauwerkserhaltung und Denkmalpflege.

Der diesjährige WTA Young Professionals Award in der Kategorie "Scientific" wurde

                              Herrn Ameya Kamat Ph.D. für seine Dissertation
                "Improving salt weathering resistance of hydraulic mortars with
                                 an encapsulated crystallisation inhibitor" 

verliehen.

In einer Kurzpräsentation stellte Herr Kamat die Schwerpunkte seiner Arbeit vor. Die gesamte Arbeit kann auf der Homepage der TU Delft eingesehen werden.
 

Summary

Repeated crystallisation of salts in the pores of building materials is a common cause of damage in buildings. Sodium chloride (NaCl) is one of the most common salts, responsible for weathering in the built environment. Mortars, especially when used as plasters and renders on the surface of walls, experience fast degradation as they are exposed to conditions perfectly conducive to salt weathering. As a consequence, their service life is often compromised, requiring frequent replacements. Costs associated with replacement interventions have a high economic and a social impact. Over the last two decades, the use of crystallisation inhibitors as an additive to prevent/mitigate salt crystallisation damage in building materials has shown promising results. Sodium  ferrocyanide (NaFeCN), a crystallisation inhibitor of NaCl is particularly effective in preventing damage by inhibiting/delaying NaCl nucleation and altering NaCl’s crystal habit. When mixed-in air lime-based mortars, NaFeCN has been shown to considerably improve the salt weathering resistance.
The present work aims to extend the application of NaFeCN to hydraulic mortars with a final goal to improve the durability of renovation mortars with respect to salt damage. As a first step, the state of the art was reviewed, focusing on the applications of the crystallisation inhibitor as an additive for mitigation of salt damage in porous building materials, including mortar. Based on the outcome of the literature review, two issues were identified that needed to be addressed in order to improve the performance of mortars with mixed-in inhibitors. The first issue concerned with the possible interactions that could arise on mixing the inhibitor and the hydraulic binders. These interactions could negatively affect the properties of mortar. The second issue was related to the loss of the inhibitor due to its susceptibility to leach out of mortar. A high rate of leaching can reduce the effectiveness of the inhibitor in the long run, and shorten the service life of mortars. An experiment-based research plan was formulated to investigate and address these issues.
The experimental campaign was initiated by assessing the effect of the inhibitor on the fresh and hardened properties of hydraulic mortars. Two commonly used hydraulic binders, natural hydraulic lime (NHL) and Ordinary Portland Cement (OPC) were investigated at the level of binder-paste and mortar. It was experimentally demonstrated that the addition of the inhibitor up to a concentration of 1% of the binder weight does not have any negative effects on the fresh and hardened properties of the studied hydraulic mortars. These conclusions meant that the inhibitor can be added to the hydraulic mortars without compromising on the latter’s functionality.
In the next step, an experimental leaching test protocol was developed to quantify the rate of leaching of the inhibitor in mortar. This step was essential as the literature lacked data on the leaching behaviour of the inhibitor. Specimens made of NHL mortar with mixed-in inhibitor were prepared and subjected to leaching tests taking into account diffusion- and advection-driven transport. In the diffusiondriven test, a high diffusion coefficient of the leached NaFeCN ([Fe(CN)6]4 – ) ions was measured, marginally lower than the diffusion coefficient of salt ions (Cl – ). In the advection-driven test, most of the inhibitor leached out of the mortar as efflorescence after a single wetting-drying cycle. The experiments concluded that the rapid depletion of the inhibitor in the mortar due to leaching will decrease the long-term effectiveness of the inhibitor against future salt load.
To ensure a long-term positive effect of the inhibitor, its rate of leaching in mortar had to be slowed down. Hence in the next phase, encapsulation of the inhibitor to control its release and consequently, reduce its leaching in mortar was explored. On the outcome of the literature review encompassing fields such as medicine and polymer chemistry, two bio-based hydrogels, chitosan and alginate were selected as potential capsule materials. The selection was based on the hydrogel’s diffusion-based release and a pH-tunable response. The capsules were produced in a two-step process. First, the inhibitor was encapsulated in calcium alginate capsules (CA) using extrusion dripping and ionic gelation. Next, the obtained capsules were complexed with chitosan to obtain chitosan-calcium alginate polyelectrolyte capsules containing the inhibitor (CsCA). Since the release behaviour of CsCA capsules in an alkaline environment, was not known in the literature, the release of the inhibitor from capsules was investigated in solutions over a pH range of 7-13. In the studied pH range, the CsCA capsules released lower amount of inhibitor compared to CA capsules. Moreover, the magnitude of release was found to be dependent on the capsules’ chitosan to alginate ratio. The results suggested that at a certain pH, the release of the inhibitor from CsCA capsules can be controlled by modifying the chitosan: alginate ratio. On the outcome of these positive results, the release of the inhibitor from CsCA capsules was tested in mortar pore solution and in hardened mortar. The outcome of these experiments showed a slower leaching (release) of the inhibitor from mortars containing CsCA capsules as compared to mortars with CA capsules and mortars with mixed-in inhibitor. The slower release of the inhibitor was attributed to chitosan’s role in reducing the permeability of CsCA capsules and its electrostatic attraction to the negatively charged [Fe(CN)6]4 – ions, thereby hindering outward transport of the inhibitor.
The final phase of this research involved the assessment of the performance of the hydraulic mortars with inhibitor in both encapsulated and mixed-in form. The performance was assessed on three aspects (a) impact on fresh and hardened properties of mortar (b) salt-weathering resistance and (c) leaching of the inhibitor. Two types of hydraulic mortars, NHL-based and cement-based two-layer plaster system, were investigated. For comparison, reference mortars without inhibitor (and capsules) were also assessed. The impact on the fresh and hardened properties of mortar was studied using various characterisation and complementary experimental techniques. The addition of the capsules (equivalent to 1% inhibitor by binder weight) did not have any negative effects on the properties of hydraulic mortars.
The salt-weathering resistance of the mortars was assessed by performing an accelerated salt-weathering test (RILEM 271-ASC). At the end of the test, the reference NHL mortars suffered severe damage in terms of material loss. Differently, NHL mortars with inhibitors, both in encapsulated and mixed-in form showed negligible material loss, strongly demonstrating the role of the inhibitors in reducing the damage. In case of cement-based two-layer plaster system, no damage was observed in any of the specimens including the reference specimens. Therefore, no concrete conclusions on the inhibitor’s effect could be drawn in the two layer-cement based plasters. Finally, at the end of the weathering test, the leached inhibitor from each type of mortar was quantified using ICP-OES. The mortars with encapsulated inhibitor showed lower leaching than the mortars with mixed-in inhibitor. These results confirm that encapsulating the inhibitor is beneficial in reducing leaching of the inhibitor. A lower leaching is expected to extend the effectiveness of the inhibitor over long time and thereby, increase the service life of mortars.
The results from the above experimental studies definitively conclude that incorporation of the crystallisation inhibitor (sodium ferrocyanide) improves the saltweathering resistance of hydraulic mortars without any negative effects on the properties of mortar. Moreover, mortars containing encapsulated inhibitor, thanks to controlled-release are expected to have a longer service-life than mortars with mixed-in inhibitor. In addition to that, incorporating capsules in mortar do not have a negative impact on the fresh and hardened properties of mortars, making their application feasible in practice.
This research marks the first-ever application of using CsCA capsules towards controlled-release of additives in the field of building materials. The results suggest new opportunities in expanding the use of bio-based hydrogels in the field of building and construction. In future, additional research is needed to optimise the capsule composition to tune a case-specific release response. Additionally, research addressing some fundamental issues, such as the effect of the inhibitor on crystallisation pressure and salt mixtures, will help in developing this technology further.