Experimental analysis of lime putty and pozzolan-based mortar for interventions in archaeological sites

Francesca Autiero,Marco Di Ludovico,Paulo B Lourenço,Meera Ramesh,Miguel Azenha,Andrea Prota

doi:10.1617/s11527-021-01746-2

Francesca Autiero, Marco Di Ludovico + Show 4 more

Open Access

https://doi.org/10.1617/s11527-021-01746-2

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Abstract

The study aims at defining and characterizing a specific restoration mortar for archaeological masonry structures made with traditional materials and to assess the suitability of the mixture compared to other mortars; such a goal is crucial to develop and define interventions in the archaeological sites. The mixture was defined to ensure compatibility with ancient materials and following frequently adopted recommendations at the site, specifically by using: (1) raw materials as similar as possible to the ancient ones; (2) traditional mix design. Therefore, the mixture was made with commercial lime putty CL 90-S type and natural Phlegrean pozzolan, i.e. volcanoclastic material collected from the volcanic area located in the West of Naples in Italy. The precious and limitedly available natural pozzolan used in the experiments resulted in an exclusive mortar which is very similar to the archaeological ones. The mortar has a binder to aggregate ratio 1:3 by volume, according to traditional techniques typically encountered in the ancient Roman city of Pompeii and Vesuvius surrounding area. The evolution of the flexural and compressive strength, elastic modulus, bulk density, open porosity and ultrasonic pulse velocity has been monitored for up to 200 days, based on standard procedures. Moreover, the hardening process was monitored with Differential Thermal Analysis up to 90 days, through the evaluation of phase transitions associated with dehydroxylation and decarboxylation, considering different depths from the external surface of the mortar. The achieved mechanical properties were compatible with those of lime-based mixtures for repair interventions of ancient masonry structures. Moreover, the mortar was found to be well-suited to mitigate cracking, showing a low ratio between its stiffness and load capacity compared to other typologies of mortars used for masonry restoration. Ultrasonic pulse velocity test proved to be a reasonable complementary method to monitor the evolution of the hardened properties of the mortar. Carbonation was found to be still progressing at 90 days. The data presented provide useful and reliable information to approach the complex process of restoration in archeological sites.

Highlights

Restoration interventions of archaeological masonry structures and columns are often based on the use of mortars
As a part of wide research intended for the definition and implementation of suitable investigation protocols and approaches for the structural interventions at the Pompeii site [30], the present study focuses on the analysis of a repair mortar compliant with the abovementioned indications by following a carefully controlled procedure; accurately measuring several important mechanical and physical properties and assessing their evolution with the time; analyzing, discussing and comparing the achieved results, most of which were still not available in the literature
To assess the repeatability of results obtained with the hand compaction method compared to mechanical compaction, Table 2 reports the mean values of q, V, ff and fc with the standard deviations and coefficients of variation (CoV) reported in brackets for hand compacted specimens and mechanically compacted ones, produced from the same batch of mortar

Summary

Introduction

Restoration interventions of archaeological masonry structures and columns are often based on the use of mortars. As a material for restoration interventions, a mortar should be compatible with the ancient materials, durable and its properties should be well documented [1,2,3]. The production of devised repair mortars by mixing single raw materials on-site should be defined based on an accurate knowledge of the raw materials and the craftsmanship composing the ancient ones. Several experimental studies on the production of repair mortars based on the composition of the ancient ones are available in the literature. It is known that aerial lime is one of the most ancient binders used in mortars for masonry [6, 10, 13,14,15,16,17]. The carbonation process is gradual and very slow, starting from the outer surface of the joints in the first days after the application until reaching the inner part of the masonry from few months up to more than a year depending on the porosity of the mortar and the units, the wall thickness and the environmental conditions, in particular the relative humidity [7, 18]

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