Spontaneous moss growth on concrete

Abstract

The world population will continue to grow, resulting in an overpopulated planet and increased urbanization. This creates several problems, such as heat stress in cities, reduced biodiversity and the disappearance of greenery in cities. The World Health Organization has made a list of indicators based on which the sustainability of cities can be assessed. One of these indicators is a minimum amount of 9 m2 green space per capita. Currently, few cities meet this requirement, which threatens the quality of life for city dwellers. A possible solution to obtain more green area in urban regions can be to have more green surfaces on buildings and structures. To realize this in an efficient, low maintenance, and cheap way, concrete could be designed to stimulate the growth of mosses. Spontaneous abundant moss growth on young concrete structures can actually already be observed in the build environment, however this is often unintentional and unwanted, and is seen as degradation of the concrete. A better understanding of why spontaneous moss growth occurs on some constructions while it does not on others can help to intentionally design green concrete structures, and eliminate unwanted spontaneous moss growth on concrete. It has been found in this research that the curing conditions and the cement type being used are two major influencing factors that can contribute to the presence or absence of abundant moss growth on current relative young concrete constructions. A hospitable concrete surface is needed to support the growth of a biofilm, that can subsequently develop in moss. This hospitable concrete surface is provided by the concrete microstructure, a lowered pH, and a sufficient surface roughness. To obtain concrete that is bioreceptive and will enable mosses to grow on its surface, a low-clinker content concrete must be used that is insufficiently cured. This will cause a porous and permeable outer layer of the concrete, that has been related to moss growth on concrete. Due to the low buffering capacity towards alkalinity, the pH at the cover zone of the concrete will be lower, thereby making the surface more attractive for micro-organisms to settle on. Another effect of insufficient curing is an increase in carbonation, which also leads to a more porous and permeable cover zone for low-clinker concrete. To prevent moss growth on concrete, a dense microstructure and a high pH at the outer layer of the concrete must be created. This can be obtained by using a high-clinker content concrete. From this research it can be concluded that a high-clinker mix is relatively not very vulnerable for the curing conditions. A high-clinker content concrete will provide a dense microstructure and high buffer capacity towards alkalinity, even though it has been cured insufficiently.