Citation: Olivieri, F.; Castaldo, R.;
Gentile, G.; Lavorgna, M. Monitoring
Water Absorption and Desorption in
Untreated and Consolidated Tuff by a
Non-Invasive Graphene-Based
Humidity Sensor. Materials 2023, 16,
1878. https://doi.org/10.3390/
ma16051878
Academic Editors: Ki-Hyun Kim and
Deepak Kukkar
Received: 3 February 2023
Revised: 14 February 2023
Accepted: 21 February 2023
Published: 24 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Monitoring Water Absorption and Desorption in Untreated and
Consolidated Tuff by a Non-Invasive Graphene-Based
Humidity Sensor
Federico Olivieri
1
, Rachele Castaldo
1
, Gennaro Gentile
1,
* and Marino Lavorgna
2
1
Institute of Polymers, Composites and Biomaterials, National Research Council of Italy, Via Campi Flegrei 34,
80078 Pozzuoli, Italy
2
Institute of Polymers, Composites and Biomaterials, National Research Council of Italy, P.le E. Fermi 1,
80055 Portici, Italy
* Correspondence: gennaro.gentile@cnr.it
Abstract:
A hybrid montmorillonite (MMT)/reduced graphene oxide (rGO) film was realised and
used as a non-invasive sensor for the monitoring of water absorption and desorption in pristine
and consolidated tuff stones. This film was obtained by casting from a water dispersion con-
taining graphene oxide (GO), montmorillonite and ascorbic acid; then the GO component was
thermo-chemically reduced and the ascorbic acid phase was removed by washing. The hybrid film
showed electrical surface conductivity that varied linearly with the relative humidity, ranging from
2.3 × 10
−3
S
in dry conditions to 5.0
×
10
−3
S at 100% RH. The sensor was applied onto tuff stone
samples through the use of a high amorphous polyvinyl alcohol layer (HAVOH) adhesive, which
guaranteed good water diffusion from the stone to the film and was tested during water capillary
absorption and drying tests. Results show that the sensor is able to monitor water content changes
in the stone, being potentially useful to evaluate the water absorption and desorption behaviour of
porous samples both in laboratory environments and in situ.
Keywords: stone; conservation; water absorption; water desorption; humidity; graphene sensor
1. Introduction
Natural stone is a sustainable building material with an intrinsically low carbon
footprint [
1
] that has high relevance in cultural heritage since mankind has always explored
and exploited stone throughout history for different purposes [
2
]. Preserving and restoring
stone-built heritage is, thus, one of the more relevant topics of conservation and, for this
reason, the understanding of stone decay phenomena has been widely investigated in
the past [
3
]. Stone decay can occur through different mechanisms, including the chemical
action of anthropic pollutants, the physico-chemical degradation, induced by water-soluble
salt migration and crystallisation, and the biodegradation [
4
]. Moreover, freezing–thaw
cycles can quickly promote the degradation of stones, especially in the presence of a high
water content [5].
Thus, water plays a crucial role in stone degradation. Water can induce degradation
of stone by promoting a loss of cohesion of its structure, mechanical stresses as a result of
the recrystallisation of soluble salts, and/or freezing–thaw cycles or chemical corrosion
by an uptake of water-dissolved pollutants [
6
]. Moreover, the kinetic and the extent of
stone biodegradation phenomena are strongly influenced by water availability [
7
]. Water is,
therefore, commonly regarded as the main factor affecting the degradation mechanisms of
stones, acting as a primary degradation agent through freeze–thaw cycles or the dissolution
of the stone binders, by acting as a solvent and transport medium for other degradation
agents, such as pollutants or water-soluble salts, or by creating high humidity conditions
necessary for biological degradation.
Materials 2023, 16, 1878. https://doi.org/10.3390/ma16051878 https://www.mdpi.com/journal/materials
