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Lime Cements, Plasters, Mortars and Concretes

There is a plethora of terms used to describe the various products derived from calcined limestone. A definition of terms used in the context of this report are outlined below. The definitions here are based on features identifiable in hand specimen and are therefore intended for use by the field archaeologist. Consequently these definitions may differ somewhat from those applied by scientists employing microscopic and chemical techniques. (reference)


Limestone is the natural rock type from which cements and concretes are derived. A limestone is a sedimentary rock composed of carbonates, namely the minerals calcite (calcium carbonate; CaCO3) and dolomite (calcium magnesium carbonate; CaMg[CO3]2), derived either chemically or organically. Being natural materials, limestones can have a wide range of depositional environment and components and can contain varying amounts of non-carbonate material. The type of limestone calcined to produce lime for the manufacture of cements and concretes can profoundly affect the durability and properties of the material produced.


"Calx", lime in Latin provides the etymological root for calcium, calcite and calcination. Strictly speaking, lime is calcium oxide. This is acquired by burning limestone, which in simplest terms, removes the carbon from the calcium carbonate (calcite). Lime forms the base for all cements and concretes. However the composition of the limestone being variable, the true composition of the lime is also variable, and the term may be generally used to described calcined limestone in general. Lime may also referred to as 'quicklime', 'unslaked lime' or 'lump lime'.

Slaked Lime

Lime will only harden into a cement when water is added to it. The process is called ëslakingí, hence ëslaked limeí, also referred to as 'lime putty'.


"Cement" is derived from caementa, which actually referred to the aggregates mixed with the slaked lime rather than the bonding agent itself. Opus caementicium refers to the masonry constructed from concrete coursework. In the context of this work, the word cement is used exclusively in reference to the hardened binding material of any aggregate. In simplest terms, this is the slaked lime, which in the presence of air reverts to calcium carbonate. This material may also contain finely powdered admixtures, such as ash or fired ceramic. However, an alternative term for the hydrated lime binder is not put forward. In geological parlance, the term cement is used strictly to define material (often calcite, but occasionally silica) that adheres clasts in a rock, and generally the word ëcementí is accepted in non-specialised use as a glue. Consequently it is defined as the adhesive binder in this report.


An aggregate is material added to a cement. In this report, the term does not include finely powdered additives to the cement such as ash. It is usually composed of rock fragments, chosen for their strengthening properties and occasionally for decorative reasons. Carefully chosen aggregates can make a concrete or mortar resemble natural rock. Organic material, including grasses, reeds and also bones can be used as aggregates, often in combination with rock material. Aggregates can be sub classed into categories of fine aggregates - that with dimensions less than 5 mm, and coarse aggregates - that with dimensions greater than 5 mm.

Concrete and Mortar

Both concrete and mortar are materials composed of a cement plus an aggregate. The two terms are very simply defined. A concrete is a material where the majority of the aggregate has dimensions greater than 5 mm. A mortar is a material having aggregate with dimensions less than 5 mm (Prentice, 1990). "mortarí from the Latin mortarium, originally referring to the trough in which the material was mixed, is often used wholesale to describe the bonding material of concrete masonry. This is a fair use of the term as this material binding brick-sized blocks, often laid in courses rather than haphazardly poured, is composed of lime cement with fine aggregate. Concrete should be used to describe material containing coarse aggregate generally used to fill formwork.

Hydraulic Cements

Hydraulic cements are waterproof and will even set underwater. Consequently most materials used in Roman and later periods for lining structures intended to carry water and for construction in marine and riparian environments will be hydraulic. Such materials are identified by the presence of finely pulverized material added to the cement binder which will cause discoloration from white to pale browns or pinks. Common additives, are volcanic ash or crushed ceramic sherds. These materials are known as pozzolans.


A pozzolan is defined as a siliceous and/or aluminous substance that will, in the presence of water combine with lime to form cementitious compounds. Such material include clays, that are rendered active by firing (fresh geological clays would absorb too much water during the curing process, resulting in spallation and ultimately cracking of the concrete), and other substances, including waste products from blast furnaces and even rice husk ash (see Hill, et al., 1992 and references therein). However, natural pozzolans are derived primarily from volcanogenic products.


The term "plaster" is one of the most universally used, describing a multitude of products, usually those which will provide a smooth coat to a wall or other surface. Plaster of Paris is powdered gypsum (CaSO4.2H2O, derived from rocks distinctly different from limestones) which, with water added, will harden and set. Lime plaster is simply a mixture of lime, water and sand, or just lime and water in the case of whitewash. To avoid confusion, it is recommended that these terms should be used in full to define compositional types.


Stucco, like plaster, is a term of complex use. However it is entrenched in all literature as describing two distinct materials. Firstly it is used to refer to a fine white mortar composed of lime, crushed marble and glue-like binding additive (egg-white for example) typically used for making good surfaces for painting frescoes or generally for smoothing walls. Alternatively it is used to describe decorative work in plaster of Paris. In this report, stucco will be used to define the former case alone.