PSI - Issue 78

Francesco Bianco et al. / Procedia Structural Integrity 78 (2026) 41–48

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Fig. 5. Pictures of bioreactors placed in an oven at 38 °C.

3.2. Bio-H 2 production and calcium dissolution Fig. 6 shows the cumulative bio-H 2 production after 45 days, which was approximately 982, 1512, 550, 598, 1055, and 458 mL H 2 in C, CP, F, FR, ALL, and Control, respectively. No significant differences were obtained in F and FR compared to the control sample (i.e., digestate + substrate), which shown a similar biogas generation. On the contrary, microbial activity was stimulated in C, CP and ALL, likely due to the buffer effect of dissolved calcium hydroxide.

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Fig. 6. Cumulative biohydrogen production obtained during bioleaching experiments.

Finally, a mass balance of the samples was carried out by comparing the weight before the experiment with the weight after the experiment to check whether indeed the samples were leached during the biological process (Tab .1). Mass balances on the samples indicate that C, CP, and ALL were effectively subjected to bioleaching due to DF, with lower final weights than the average initial value. The calcium present in the concrete, due to bioleaching, tends to pass into solution, leading to an increased pH. Indeed, throughout the experimental campaign, the pH in C was found to be consistently higher (i.e., on average 4.8 – 4.9, data not shown) than in the other case studies, where lower pH values were recorded (i.e., 3.9 – 4.4, data not shown). Errors in the mass balance could also be associated with the removal/alteration of the primer and resin on the concrete surface or the loss of inert material. The recorded pH can be attributed to the production of VFAs, such as butyric acid via the anaerobic metabolism of glucose (i.e., 2 mols of bio-H 2 per 1 mol of butyric acid). The produced VFA likely reacted with calcium-bearing phases such as calcium hydroxide and calcium carbonate. Therefore, the cement matrix was chemically degraded (Tab. 1), leading to the disintegration of its structure. From the table it emerges an increase in weight for samples F and FR. These effect primarily reflects biomass accumulation rather than fiber degradation. Indeed, similar to other biofilm ‐ based systems such as AnMBBRs,