The Building Conservation Directory 2020

132 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 C AT H E D R A L C O MM U N I C AT I O N S components to create a hardened composite. Limestone processed in a kiln has been the predominant source of binders for mortars and concrete for at least 5,000 years and it remains the backbone of construction today. Whether it is an air lime, a hydraulic lime, a natural cement or a modern Portland cement, it all starts with limestone of some description being chemically transformed in a hot kiln. Non-hydraulic limes typically contain lime and unburnt stone as well as small proportions of the hydraulic compounds C2S/C3S, as outlined in the table above. This is because some silica is always present in limestones as none is ever truly pure. Most hydraulic components interact with one another during hydration and small HYDRAULICITY REVISITED EDWARD WALKER I N RECENT years the use of NHL mortars has been called into question following new research and the popularity of hot- mixed mortars. This has shed light on properties of the binders used which many specifiers found surprising, and some are understandably concerned that we may have been inadvertently specifying repairs that were inappropriate or less appropriate than expected. We are now in a much better position to re-evaluate the benefits and the limitations of these materials scientifically, to ensure that all masonry repairs are made using the most appropriate technologies. The purpose of this article is to provide a simplified account of the materials and the often complicated chemical reactions involved. This story has been confused by centuries of differing terminology and vernacular references. The following summary covers some of the technical elements, hopefully broken down into manageable chunks, but some technical terminology and chemical notation is unavoidable. Providing a complete picture would require volumes, and even Boynton’s Chemistry and Technology of Lime and Limestone does not cover the ‘hydraulicity’ of this discussion, despite being perhaps the most definitive guide to lime production ever written. MORTAR AND ITS COMPONENTS A binder is the component of a mortar which allows stiffening or binding of separate Prompt natural cement being used for the repair of Portreath’s harbour wall: this binder is less dense and more porous than modern Portland cements and closer to the early Portland cements used in the late 19th century. (Photo: Cornish Lime Company)