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Definitions of Cooked Limestone for Masonry

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Definitions of Cooked Limestone for Masonry DEFINITIONS OF COOKED LIMESTONE FOR MASONRY AERIAL LIME (WHICH ONLY HARDENS WHEN IN CONTACT WITH AIR) Aerial Lime (Air Lime), so called because it hardens by absorption of carbon dioxide from the air, is the product of the purest type of limestone; rocks with a high content of calcium carbonates. The first examples of Air Lime date back around the 8th century BC, but the Etruscans and the Greeks developed the final formula for Aerial Limes around 700BC. When quicklime is converted to hydrated lime, these two components from part of the product, with the remainder being calcium hydroxide. Calcium/Silica compounds are capable of a cementitious reaction with water, so that when the lime is used in mortar the mortar will set and harden “hydraulically”, that is, by reaction with water. The remainder of the hydrated lime will harden slowly by reaction to with the carbon dioxide in the atmosphere. In the specific field of construction limes, with air lime we indicate two products: • Quick Lime, consisting mainly of calcium oxide; made by burning limestone or chalk in a kiln. Quicklime (Lump Lime) cannot be used directly in the building industry as it needs to at removal from the furnace, to be transformed into hydrated lime, making it react with water (hydration). It is a very reactive material. • Hydrated Lime is created when Quick Lime is added to just enough water to slake it into a powder. It is a lesser form of lime putty and cannot be used as a binder; it consists predominantly of calcium hydroxide. Hydrated lime, used for the construction of mortars, plasters, architectural finishes, etc. It is available on the market in powder or paste form. From the firing of the limestones, that contain magnesium carbonate, is created 'quicklime' and subsequently 'hydrated lime', which will contain together with the calcium oxide / calcium hydroxide, the oxide and magnesium hydroxide. The terms Quick Lime and Slaked Lime are therefore used independently of the magnesium content or not, but when one wants to specify the presence of only calcium, or calcium and magnesium together, one can distinguish between the calcium lime or the dolomitic lime. (Photo: Soaking of Hydrated Lime after firing) HYDRAULIC LIME Hydraulic Limes are traditional building materials and constitute a milestone in the history of binders used in architecture before the advent of Portland cement. The first examples of the use of hydraulic mortars date back to the Etruscans/Greek/Romans. They obtained hydraulic compounds by mixing: • Aerial Lime and Pozzolana (the 1st ash emissions during a volcanic eruption) to create an hydraulic lime, but not the hydraulic lime as we know of them today, which are different and more recent product developments. The existence of peculiar kinds of limestone, such as Marly Limestone (Marlstone), when fired, resulting in types such as “Strong Lime”, “Moretta Lime", "Wild Lime" etc., is well documented throughout the history of architecture, but it was not until the eighteenth century that it was understood that the reaction mechanism of hydraulic lime was linked to the presence of clay impurities in the limestone. In the year 1793, J. Smeaton discovered that the cooking of limestone containing clay impurities, produced a type of lime (the hydraulic lime) with characteristics like those of the Aerial Lime-Pozzolana mixture. The adjective 'hydraulic', referring to a binder, is due to the French engineer Louis Vicat (1786 -1861), who first established in a precise manner, the proportions between limestone and clays necessary to produce materials capable of binding and hardening, even in the absence of air, i.e., submerged or in water. Vicat proposed the first, and in many ways still valid today, the classifications of hydraulic lime. With hydraulic limes we intend products derived from calcination of Marly Limestone or calcareous marl (natural mixtures with a certain content, from 6 to 22%, of clays or other hydrated aluminosilicates), are subjected to cooking at temperatures generally between 1000 and 1250°C . In such conditions calcium oxide (CaO) is formed which subsequently combines with the silica and the alumina of the clay, forming hydraulic silicates and hydraulic calcium aluminates, compounds that when chemically reacting with water, form stable and insoluble hydrates, which allow the material to harden and remain stable even under water (hydraulic action). HYDRAULIC LIME or NATURAL HYDRAULIC LIME? In recent years the meanings of the terms used to designate hydraulic binders and limes, specifically hydraulic limes have undergone important changes. This has led to considerable confusion and disorientation from lime users. According to the UNI EN 459-1: 2010 standard, what commercially is indicated as Hydraulic Lime (code HL, Hydraulic Lime) is not produced by cooking marls or mixtures of limestone and clay but it is obtained by mixing Portland cement with a good filler content (finely ground inert material, generally of calcareous type) and small quantities of aerating additives. Hydraulic Limes (HL) are simply cements of poor resistance, with potential negative effects on durability for restoration use because of the danger to for ettringite and thaumasite (high content water-soluble salts). The products obtained by cooking natural marls or homogeneous mixtures of limestone and clay materials are referred to as Natural Hydraulic Calcium’s. The Natural Hydraulic Calcium’s are marked with the acronym NHL (Natural Hydraulic Lime) are not modified with the addition of pozzolanic or hydraulic materials (clinker, cement, ash, etc.). Legislation today provides for a further distinction, based on the mechanical resistance of a 28 day cycle. This distinction translates into the definition of three classes: NHL 2, NHL 3.5 and NHL 5. UNI EN 459-1: 2010 IMPORTANT ACRONYMS The UNI EN 459-1: 2010 standard classifies the hydraulic limbs in three sub-categories. • Natural Hydraulic Lime (NHL): derived exclusively from natural marls or siliceous limestones, that has natural impurities of clay and other minerals, and the amount of impurities within it determines how hard it will set. NHL works by setting in the presence of water, hence the term natural hydraulic lime. • Hydraulic Lime (HL): Lime consisting predominantly of Ca hydroxides, silicates and Ca aluminates, produced by mixing cement, blast furnace slag, limestone filler and other materials that harden the mortar. These additives do not have to be declared. • Formulated Lime (FL) hydraulic calcium: a new class, hydraulic lime mixed with aerated lime, and /or natural hydraulic lime with added hydraulic or pozzolanic material (of which it's obligatory declare the name and the percentage). Thanks to this rule, what is meant by natural hydraulic lime (NHL) has been defined. According to this classification, the only material that can be defined and used as such is the one signed NHL. The number that accompanies the code (NHL 2, NHL 3.5 and NHL 5) indicates the mechanical resistance of the lime, referred to as the minimum compressive strength of a mortar specimen after 28 days of maturing, expressed in MegaPascal (Mpa). The classification of the resistances also applies to the other two categories of hydraulic limes, the HL and the FL, obviously created to leave room for all those products that until yesterday occupied the field of natural hydraulic lime properly so called. LIME PRODUCTION Lime is the product of the cooking of calcareous rocks, more or less pure, at temperatures between 900 and 1200 ° C. The furnace shutdown and other manufacturing processes follow in relation to the type and quality of the final product. Today lime is produced industrially, in modern plants and with controlled processes. In the past it was the result of a craft work, heir to the ancestral knowledge of the art of building. LIME PROCESSES "There is magic in picking a stone from the earth, cooking it and demolishing it in the fire, making it plastic with water, working it according to will and regaining it in solid, thanks to the influence of air" is what is described today as the Lime Process". A process that, after a series of steps, leads the limestone to become the main binder of most of the man-made constructions in the last ten thousand years. The 'lime cycle' is divided into four fundamental moments. Selection of Limestone The mineralogical and chemical characteristics of limestones used as raw material for lime production are of fundamental importance. The most suitable limestones for the manufacture of aerated lime must have a microcrystalline structure, high content of carbonates and contain percentages of impurities, in particular a clay like nature, not higher than 5%. Cooking During cooking, the limestone is placed in the ovens and brought to a temperature close to 900 ° C. In these conditions the calcium carbonate decomposes into calcium oxide (quicklime) and carbon dioxide. The schematic reaction of the process is as follows: CaCO3 -> CaO + CO2 Lime Off The Quick Lime, put in contact with water, reacts with a strong development of heat and turns into a white powder (or a paste) called slaked lime, chemically calcium hydroxide. The schematic reaction is the following: CaO + H2O -> Ca (OH) 2 Carbonation Once in place, in the form of mortars, stuccos, paints, etc., carbonation intervenes. This process that can occur only in the presence of carbon dioxide (and water) leads to the transformation of the Lime Off into calcite, thus closing what is called the lime cycle. The schematic reaction of the carbonation is the following: Ca (OH) 2 + CO2 -> CaCO3 + H2O. Even if the previous reaction is correct from the formal point of view, the actual reaction is more complex. In fact, observing the previous reaction, the process would seem to take place between a solid Ca (OH) 2 and a CO2 gas, whereas in reality the reaction occurs in aqueous phase.
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