Bacterial Additives Improve the Water Resistance of Mortar

Abstract

The ingress of water into mortar and concrete is an ongoing problem which can reduce the lifetime of cementitious structures. Commonly used approaches that aim at preventing water ingress mainly employ an additional surface treatment after the casting process. Thus, they are time-consuming and make use of synthetic, nonsustainable additives. In contrast, it was shown recently that a biological material, i.e., a bacterial biofilm generated by B. subtilis 3610 bacteria, can be used as a bulk additive which leads to hybrid mortar with increased wetting resistance. Here, we demonstrate that a similar enhancement of the water resistance of mortar can be achieved by using different bacterial additives, i.e., wet biofilm, freeze-dried biofilm powder, and bacterial suspensions, each of which can be produced by one of three selected variants of B. subtilis bacteria. We characterize the mechanical properties of the different hybrid mortar variants regarding their setting behavior, tensile and compressive strength, and density. Our results imply that bacterial additives could be an eco-friendly and sustainable alternative to existing synthetic mortar additives.

A rotating bioreactor for the production of biofilms at the solid–air interface

“…Moreover, this strategy comes with the advantage of easily adjusting the type of nutrient source and harvesting the microbial biomass from the growth substrate. In previous research, bacterial biofilm – in addition to other bacterial additives (Ertelt et al, 2021) – has already been proven its value as a hydrophobizing agent in a mortar (Ertelt et al, 2020; Grumbein et al, 2016). However, the classical microbiological production process of biofilm, that is, its cultivation on sterile Petri dishes and the subsequent manual harvesting step is time‐consuming and not economical.…”

“…The latter application area might seem the most surprising—even though biological additives to construction materials were already explored by the ancient Egyptians and Romans and during the Qing dynasty (Dai et al, 2019; Stark & Wicht, 1998). Recently, the idea of using bacterial biofilms as additives to cementitious materials has been rediscovered, and the resulting hybrid mortar has been shown to possess highly interesting water‐repellent properties (Ertelt et al, 2020; Grumbein et al, 2016). Of course, for such a bio‐enhanced material to be used in real construction engineering efforts, larger biofilm amounts are required than can be grown in the lab using classical microbiological assays.…”

Bacterial Materials: Applications of Natural and Modified Biofilms

“…The latter can occur via rain or water splashes, or it can originate from capillary water uprise when cementitious structures are erected in moist environments. Importantly, also in this context, bacterial additives have turned out to be extremely helpful: both, the external wetting resistance of mortar and the suppression of the capillary water uptake into the material can be enhanced using fresh [ 195,196 ] or freeze‐dried bacterial biofilm, [ 197 ] bacterial solutions, [ 197 ] or bacterial spores. [ 198 ]…”

Small Pores, Big Impact—Controlling the Porosity Allows for Developing More Sustainable Construction Materials

“…Several techniques and approaches have been developed to minimize the ingress of water. Examples include hydrophobic coatings, , the integration of hydrophobic substances, − or the use of additives, which alter the microstructure or the surface roughness of the material. − Biobased examples for the latter approach employ bacterial solutions, bacterial spores, or freeze-dried bacterial biofilm (referred to as biofilm powder) as a hydrophobizing agent in mortar. In addition to affecting the surface chemistry (the exact mechanism is yet to be identified) similar to what the wax layer achieves on lotus leaves, biofilm additives were suggested to establish water repellent properties in the resulting hybrid materials by increasing the inner and outer surface roughnessboth on the micro- and nanoscale.…”

“…In addition to affecting the surface chemistry (the exact mechanism is yet to be identified) similar to what the wax layer achieves on lotus leaves, biofilm additives were suggested to establish water repellent properties in the resulting hybrid materials by increasing the inner and outer surface roughnessboth on the micro- and nanoscale. Such bacterial additives, however, can also impact the mechanical strength of hybrid mortar (HM). , …”