Consolidant particle transport in limestone, concrete and bone

Abstract

The use of chemically compatible nano and fine particle colloidal consolidants is a new development within the field of cultural heritage conservation and applied most widely so far to the historic built environment. The ability to introduce a significantly higher quantity of chemically compatible consolidant to a substrate in fewer treatments with the possibility for greater penetration and fewer possible side-effects compared to more established consolidants is a significant advantage.

This fundamental scientific study examines the effects of a colloidal calcium hydroxide (nanolime) consolidant on medieval and quarried limestone and autoclaved aerated concrete and the efficacy of a colloidal hydroxyapatite treatment on archaeological human bone. Both calcium hydroxide and hydroxyapatite were synthesised. Characterisation of both compounds was performed by X-ray diffraction spectroscopy and particle morphology was confirmed by electron microscopy.

Particle size was determined by laser diffraction and particle tracking analysis techniques, used together to study these particle systems for the first time, and electron microscopy. The location of particles within treated substrates was established by electron and optical microscopy whilst effects on water transport were determined by imbibition experiments and numerical modelling. For the first time a modified sharp front model was applied to [particle-material]-material composites to aid the understanding of water transport in such materials. Mechanical testing was used to identify differences in material strength depending on treatment layer thickness and mercury intrusion porosimetry suggested extent of pore blocking.

It was found that non-classical effects occur in the calcium hydroxide system synthesised in this study and that particle stability can be influenced by reagent concentration. For the first time material sorptivity properties, modality and pore size distribution of Lincoln stone and archaeological bone are reported. The application of a nanolime consolidant to autoclaved aerated concrete allowed the nature of the particle transport through a highly complex material to be determined, showing that the particle concentration decreases with increasing penetration depth. Shallow nanolime particle penetration into limestone appeared ineffective on compressive strength. In a novel study the prospects of a hydroxyapatite consolidant treatment for bone were also evaluated, finding the results to be inconclusive in this small study.

For all consolidants a small reduction in material water sorptivity after treatment demonstrated the permeable nature of the treatment layer and suggests the avoidance of damage mechanisms due to highly restricted water transport. Knowledge of the efficacy and location of treatment particles and their affect on water movement, particularly in weathered material, within limestones and archaeological bone is important and was determined for all materials used in this study. This work adds to the understanding of such treatments and their capabilities and the nature of the porous materials used herein.