18.1 Chemistry - R&D - PLM: under consideration for over 20 years


PLM is one of the consolidated products that C.T.S. provides for restoration in architecture field. It is available in different formulations and grades for fresco, masonry and mosaic work. Every batch is made of accurately selected raw materials and subjected to a very strict quality control. PLMs are well known among the restorers all around the world and represent the ideal mortars for injection methodology.
We would like to recall a couple of remarkable case studies when PLMs have been successfully adopted; the first one was conducted by Daniele Rossi ( look at the picture ) on two series of wall paintings (the so called ' Castellana di Vergy') of the fourteenth century in Palazzo Davanzati, Florence; and the second one carried out by Zanolini Studio on the 'Oratorio di Santo Stefano', Lentate sul Seveso, Milan [1]. In addition, we should mention the application of PLMs during the workshop by Opificio delle Pietre Dure on the modern wall paintings of the “Paese Dipinto” in Arcumeggia, Varese.

Here are the strengths that add to PLM's credit:

high fluidity when mixing with a limited amount of water (80%; good injectability even at 60%). The flow capacity makes PLM easier to be injected than hydraulic lime and slaked lime and pozzolanic based grouts.
absence of soluble salts, in comparison with other products available on the market.

The first accurate research on PLMs goes back to 1993. That was done by the University of Trento [2]; nowadays we are proud to mention another case-study conducted by the Center for conservation of Monuments and Sites, Lausanne, Switzerland. That was regarded as a preliminary assessment for the consolidation of the external masonry (made of Aquitania sandstone) of the Cathedral of Lausanne, when they run comparative tests on six grout formulations and evaluated their efficacy.The selected products belong to three categories: ethyl silicate based grouts, i.e. Funcosil and Syton; a couple of lime-based grouts mixed with inert fillers and two well known hydraulic grouts, i.e. Ledan TC1 Plus and PLM-M. Many basic parameters were monitored during the first screening. Those are viscosity, injectability, setting time, shrinkage and adhesion.

All the samples revealed low viscosity combined with good injectability. As expected for ethyl-silicate based grouts, Funcosil reacts at low rate so that it is possible to inject over and over again and top up. On the contrary, the lime based grouts show shorter time for setting . That means the entry gates of the channels harden soon and make the repeated injections difficult. In addition, the lime based formulation comprising pozzolanic fillers and micro glass bubbles produces bad results as far as shrinkage and adhesion are concerned.

The table below reports the most important parameters. For further information (including the not seasoned mortars ) we suggest to check the reference [3]. Sandstone consolidated with Wacker OH has been chosen as reference for comparison.

                                        Viscosity          Injectability          Shrinkage          Adhesion          RM          Tp         SO42-

Funcosil KSE 500 STE          +++                    +++                       -                        +                  4           42            n.d.
Funcosil KSE 300 E
Glass bubbles + green earth pigment

++                     +++                       --                       +                  5            35           n.d.
Quartz and molasse powder
Micronized silica

+                       ++                        +                       +                  1            40          n.d. 
Glass bubbles
Quartz and molasse powder

Lime and pozzolana               
+                       ++                        ---                       -                       not tested
Glass bubbles,
Local sand

Ledan                                     ++                     +++                        +                        +                 20           46        5879

PLM-M                                    ++                     +++                        +                        +                 20           40         30

Sandstone consolidated                                 n.a.                                                                        12            19       8220
with Wacker OH

Caption :
+++ very good               RM = Compressive strength (N/mm2)
++ good                        Tp = Total porosity (%)
+ fairly good                  n.a. = not applicable
- bad                             n.d. = not detected

With regard to the other parameters, they are good for all the grout mixtures. For example, the total porosity varies from 35% to 46%, much higher than the porosity of the sandstone consolidated with Wacker OH (19%). Nitrates, phosphate, chloride and fluoride contents in both PLM and Ledan are very low and not affecting the outcomes.
Concerning the concentration of SO42-, that is quite high in Ledan (5879 μg/g, that means 6 mg/g = 0.6 % !), whereas this value is almost zero in PLM-M (only 30 μg/g, that means 0.003%).
Beside these researches conducted by Bénédicte Rousset and her colleagues, we should mention the tests carried out by Maria Rosa Lanfranchi and Sara Penoni, as reported at the Conference “Scienza e Beni Culturali ” that took place in 2005 [4].
They tested and compared four different products as follow : PLM-A, Ledan SM02, and other two products; acrylic-based and expansion-based grouts.We are confident that pointing out the state of the art of the studies and research related to this matter will help restorers to make the proper choices.
Despite the fact that two decades have elapsed from the birth of PLM ( conceived during the restoration of Pantheon), scientists and restorers still take these high fluidity grouts into consideration, as recently confirmed by the Congress “Filling the Void”, set in London and managed by ICON, The Institute of Conservation, Stone & Wall Paintings Group, and by the research in progress at Getty Conservation Institute where PLM-I and other six grouts are under exam.

1. Restoration of the “Castellana di Vergy” conducted by Daniele Rossi (2006); restoration conducted in Lentate by Studio Zanolini (2007)
2. Giovanni Carturan, Gian Domenico Sorarù, Fabiano Vulcan. Relazione tecnica sulle malte PLM, Università Degli Studi di Trento, Dipartimento di Ingegneria dei Materiali (1993)
3. Bénédicte Rousset, Stéfania Gentile, Julian James, Barbara Pozzi , 2005. Injection grouts for molasse sandstones: preliminary assessments. Proceedings RILEM workshop "Repair mortars for historic masonry", Delft University of Technology, Delft, the Netherlands, 26-28 January 2005. link : http://www.conservation-science.ch/5-0-publications.html 
4. Sara Penoni, Maria Rosa Lanfranchi “Osservazioni su alcune malte da iniezione per la riadesione degli intonaci nelle pitture murali” Atti del Convegno “Scienza e Beni Culturali”, Bressanone (2005)