Fine Pumice as Pozzolanic Additive in Restoration Lime Mortars

Abstract

In order to improve the properties of fresh and hardened lime mortars, inorganic substances of hydraulic or pozzolanic character have been added to the air lime already in the ancient times. The most commonly used natural pozzolans included tuffs, tuffites, diatomaceous earth, zeolitic rocks, trass or pumice. Although the use of natural pumice in lime mortars is known from history, professional work on the influence of finely ground pumice on the properties of air lime mortars is almost non-existent. Rather, the effects of coarse natural pumice used as aggregate in lime and cement mortars are described in the literature. For this reason, the paper aims to describe the effects of partial lime replacement with finely ground natural pumice on the mechanical, microstructural, and durability properties of air lime mortars. The ground pumice showed similar pozzolanic activity to trass or natural zeolite predicting an improvement in mechanical properties and durability. As the replacement level in air lime mortars increased, the amount of mixing water needed for the same mortar consistency decreased, and the performance properties of the mortars improved. The increase in strengths of mortars was manifested mainly for the 40% lime replacement. This mortar reached at 28 days of age the compressive strength comparable with hydraulic lime-based mortars. The incorporation of finely ground pumice led to the formation of slightly denser, less water absorptive, and more frost resistant and salt crystallization resistant structure in air lime mortars. Lime-pumice mortars showed improved properties in all important aspects from the point of view of the utility of these mortars in restoration and conservation interventions on historic buildings. Based on the achieved results, the 40% lime replacement was found to be optimal.