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Blast furnace cements (CEM III) and alkali-activated slags are binders for concretes with several advantageous engineering properties, and their increased adoption in construction industry could contribute to reducing the CO2 emissions associated with cement production and use. However, the current knowledge about how these cements protect steel reinforcement in concretes against corrosion is very incomplete, which impedes their large-scale application. This knowledge gap is mainly due to the fact that these cements release sulfide and other reduced sulfur species into the concrete pore solution, the consequences of which for the state of the reinforcement and electrochemical measurements are not fully understood.
The present contribution first describes peculiarities of electrochemical measurements of steel in sulfide-containing cementitious materials and related solutions as reported in the literature and a recent report by EFC Working Party 11. It is demonstrated that the high sulfide concentrations in these systems lead to low open circuit potentials and low polarisation resistances, which may be incorrectly interpreted as indicating active corrosion of the steel.
Second, preliminary results of an ongoing project [funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 458297195] related to the passivation and corrosion initiation of steel in sulfide-containing solutions and mortars are presented. Eight mortars based on one alkali-activated blast furnace slag (BFS), three alkali-activated BFS/fly ash blends, one sodium sulfate-activated CEM III/C (‘hybrid cement’), one CEM III/C, one CEM III/B, and one CEM I (ordinary Portland cement, OPC) were produced, and their pore solutions expressed and analysed after 7, 14, 28, and 56 days of curing. The pH values of the solutions differed systematically, with the highest pH values recorded for the CEM I and the alkali-activated BFS/fly ash blends with a high proportion of fly ash, and the lowest pH recorded for the CEM III/B. The redox potentials of the solutions were between −500 mV and −340 mV vs. Ag/AgCl for the alkali-activated binders, approx. +10 mV vs. Ag/AgCl for the CEM I, and in between for the CEM III/B and the CEM III/C. As expected, the electrical conductivity was highest for the alkali-activated binders. These results are explained by the chemical compositions of the pore solutions of the mortars.
Finally, a test set-up to investigate the behaviour of steel in sulfide-containing solutions and the changes on subsequent oxygen and/or chloride addition is introduced. Preliminary electrochemical measurements of steel in sulfide-containing solutions are presented and discussed in the context of the above-mentioned data from the literature and the compositions of the pore solutions of the studied mortars.
Building envelopes & natural rock surfaces represent the frontier between solid material surfaces, biosphere and the atmospheric environment – and are subject to permanent change in structure and chemistry. These surfaces are a natural habitat of subaerial rock biofilms (SAB) that are an important stage of primary succession. As their metabolic processes have a great impact on the composition of the near-surface layers of the earth biofilms on natural and manmade hard substrates are a research focus of geomicrobiology. In technical systems immobilized algae biofilms are already used successfully for wastewater cleaning. Algae can eliminate bacteria from the water and at the same time increase the oxygen concentration and the pH value in the water through photosynthesis, which again allows phosphate elimination to take place. Nitrificants contained in the algae biofilm convert ammonium to nitrite and nitrate.
Mineral claddings for building facades could be engineered to sustain either natural or previously established carefully selected algae-dominated biofilms and this way help to clean the air.
For a targeted engineering of the mineral substrates and the biofilms attached to them basic knowledge is however missing. By systematic research in an interdisciplinary team of microbiologists and building material scientists the following questions shall be clarified
• How can a curated biofilm be stabilized on the surface and designed to be durable without compromising the substrate
• What exactly do the biofilms need in terms of moisture in their respective growth stages? Or how long water must be available in what form and quantity to enable growth.
• What is the influence of the substrate on the availability and quality of the (rain) water (water quality, leaching of substances (alkalis) from the building materials, leaching of aerosol components, surface tension, suction tension, thickness of the water molecule layers adsorbed in the pore spaces....)
There are several research groups working on bioreceptive building materials which emphasizes the general interest in the topic. The idea of speeding-up the process by the targeted establishment of biofilms on building façade panels is a unique approach that has so far only been researched at BAM. Architects are very interested in biofilm facades as a new co-designing method of humans and nature reflecting the changing awareness for the loss of biodiversity in cities. If the biofilms need irrigation the evaporative cooling would reduce the urban heat island effect.