Chemistry for sustainable building materials

Highly alkaline hydrogels are becoming increasingly important in building materials research. We have been able to show repeatedly that cationic alkaline hydrogels based on diallyldimethylammonium hydroxide (DADMAOH) in particular can be used effectively to seal water-bearing cracks or as coupling media for electrochemical chloride extraction. Upscaling is a challenge, as commercial cation-selective membranes have proven to be less effective here. On the other hand, neutral gels and gel particles can be easily prepared from diallyldimethylammonium chloride (DADMAC) in large quantities and with a wide range of compositions. Here it is shown that these neutral gel particles can be converted into DADMAOH particles with residual halide contents of <0.3 % by means of inverse static anion exchange in NaOH solution. The physical and chemical properties of the alkaline particles correspond to those of the alkaline gels and fulfill all necessary requirements for building material applications, but are much easier to produce.

T. B. Mrohs, O.Weichold
A Simple Preparation of Crosslinked, Highly Alkaline Diallyldimethylammonium Hydroxide Hydrogel Particles via Inverse Static Anion Exchange
Gels 2024, 10, 743. https://doi.org/10.3390/gels10110743 (open access)

As part of our work on highly alkaline diallyldimethylammonium hydroxide (DADMAOH) hydrogels for the repair of mineral building materials, T. Mrohs and O. Weichold analysed the resistance of the four crosslinkers shown above. Classic amides such as N,N′-methylenebisacrylamide (BIS) decompose by slow hydrolysis, which leads to liquefaction of the gels. N,N,N′,N′-tetraallylpiperazinium dibromide (1b) decomposes surprisingly quickly due to chemical instabilities. In contrast, hydrogels with tetraallylammonium bromide (1a) or N,N,N′,N′,N′-tetraallyltrimethylene dipiperidinedibromide (1c) show no traces of degradation products even after 28 days at 60 °C. This corresponds to a shelf life of over 15 months at room temperature – sufficient for applications such as realkalisation or chloride extraction. These results show that such innovative materials are ideal for long-lasting construction applications.

T. B. Mrohs, O. Weichold
Hydrolytic Stability of Crosslinked, Highly Alkaline Diallyldimethylammonium Hydroxide Hydrogels
Gels 2022, 8, 669. https://doi.org/10.3390/gels8100669

Alkaline hydrogels based on diallyldimethylammonium hydroxide (DADMAOH) can fulfil 3 important functions of repair materials for cracked concrete: Sealing of the crack to prevent water leakage, re-alkalisation of carbonated cement paste, which restores the alkali buffer, and re-passivation of steel reinforcement, which prevents corrosion of the reinforcement. In a proof-of-concept experiment, for example, the sealing properties of the gel were successfully tested on a cracked test specimen, where no leakage was observed under a water pressure of 5 metres for 28 days (WTA test W2.1-E).

A. Jung, O. Weichold
A 3-in-1 alkaline gel for the crack injection in cement-based materials with simultaneous corrosion protection and re-passivation of crack-crossing steel rebars
Construction Building Mater. 2022, 344, 128092. https://doi.org/10.1016/j.conbuildmat.2022.128092

N,N’-methylene bisacrylamide (BIS) is a very popular crosslinker for radical polymerisation in water. It is highly reactive but tends to undergo alkaline hydrolysis and suffers from low solubility. This study shows that BIS forms only inhomogeneous networks with slowly polymerising systems such as N,N-diallyldimethylammonium chloride (DADMAC). As a result, gels with very low cross-linking densities, i.e. high swelling capacities, disintegrate during the swelling test. Coherent, i.e. highly cross-linked gels are not accessible due to the solubility limit. A promising alternative are multivalent tetraallyl compounds, such as tetraallylammonium bromide, N,N,N‘,N’-tetraallylpiperazinium dibromide and N,N,N‘,N’-tetraallyltrimethylenedipiperidinedibromide. In contrast to BIS, the copolymerisation with DADMAC is statistic. However, gelation with the new tetraallyl crosslinkers is much slower than with BIS and follows the order TAPB < TAMPB < TAAB, but the differences become significantly smaller as the content increases. At low contents, all three enable the production of gels with high swelling capacities of up to 360 g/g.

T. B. Mrohs, O.Weichold
Multivalent Allylammonium-Based Cross-Linkers for the Synthesis of Homogeneous, Highly Swelling Diallyldimethylammonium Chloride Hydrogels
Gels 2022, 8, 100. https://doi.org/10.3390/gels8020100