Natural hydraulic limes

Natural graphite refractories, 12 793-794 Natural hydraulic limes (NHL), 15 53-54, 55... 

Natural hydraulic limes, as defined in [16.60] (see section 26.9), are produced from siliceous or argillaceous limestones containing more or less silica, alumina and iron [16.61]. Typical levels in the limestone are [16.6] ... 

Special natural hydraulic limes, as defined in [16.60] are produced by intimately blending powdered natural hydraulic limes with powdered pozzolanic or hydraulic materials. 

Natural hydraulic limes are hydrated with sufficient water to convert free CaO into Ca(OH)2, without hydrating significant amounts of the calcium silicates and aluminates (see section 16.10.1). 

Over the centuries, other pozzolanic substances were blended with slaked lime to produce what are now called synthetic (or artificial) hydraulic limes. Both naturally occurring pozzolans (such as trass, found in Germany), and synthetic pozzolans (such as ground blast furnace slag) have been, and still are used. It was also found that some impure limestones, containing silica and alumina, produced slaked limes with a range of hydraulic properties. Such natural hydraulic limes were widely used in construction and building for mortar and concrete. 

Natural hydraulic limes have traditionally been classified in three categories feebly hydraulic, moderately hydraulic, and eminently hydraulic. 

The presence of added Portland cement can be detected by X-ray diffraction and studying the alite (3CaO Si02) peak at 0.176 nanometres. Portland cement is rich in alite, which is only formed at above 1250 °C, whereas natural hydraulic limes are calcined at below 1250 °C and do not contain significant amounts of the mineral [26.18]. 

It will be noted that the compressive strength requirements of the three classes of hydraulic limes overlap. This is to accommodate variations caused by changes in the composition of the limestone and in the calcining process (in the case of natural hydraulic limes) and by variations in the components of artificial hydraulic limes. 

Artificial hydraulic limes consist mainly of calcium hydroxide, calcium silicates and calcium aluminates. They are produced by blending suitable powdered materials, such as natural hydraulic limes, fully hydrated air limes and dolom-itic limes, pulverised fuel ash, volcanic ash, trass, ordinary Portland cement and blast furnace slag. 

Eminently hydraulic limes (also called Roman limes and hydraulic limes) are natural hydraulic limes, which have pronounced hydraulic properties. 

Hydraulic limes have the property of setting and hardening under water — see natural hydraulic limes, special natural hydraulic limes and artificial hydraulic limes. The term is also used to describe eminently hydraulic and Roman limes. 

Hydraulic limestone is an impure carbonate containing considerable amounts of silica and alumina. Calcination of hydraulic limestone at temperatures below 1250 °C produces natural hydraulic lime. 

Natural hydraulic limes are limes produced by burning, at below 1250 °C, of more or less argillaceous or siliceous limestones, with reduction to powder by slaking with or without grinding. They consist of calcium silicates, calcium aluminates and calcium hydroxide. 

The design of adhesive mortars was based on binders of either hydrate lime-metakaolin or natural hydraulie lime, with the aim of formulating a complex system characterized by the highest compatibility. Nowadays, both hydrate lime-metakaolin and natural hydraulic lime mortars are widely used in the field of restoration and conservation of architectural monuments, due to their capability to enhance the chemical, physical, structural and mechanical compatibility with historical building materials (stones, bricks and mortars) (Rosario 2009). This compatibility is a very critical prerequisite for the optimum performance of conservation mortars, considering the damage caused to historic monuments dming the past decades, due to the extensive use of cement-based composites. 

In this framework, in the specially designed mortars consisting of binders of either lime and metakaolin or natural hydraulic lime and fine aggregates of carbonate nature, nano-titania of anatase (90 per cent) and rutile (10 per cent) form has been added (4.5-6% w/w of binder). The aim was to study the effect of nano-titania in the hydration and carbonation of the above binders and to compare the physico-chemical properties of the nano-titania mortars with those mortars without nano-titania, used as reference. Thermal analysis (DTA-TG), infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed to investigate the evolution of carbonation, hydration and hydraulic compound formation during a six-month curing period. Furthermore, the stone-mortar interfaces, the adhesion resistance to external mechanical stress, relative to the physicochemical characteristics of the stone-mortar system and the role of the nano-titania as additive, were reported and are discussed in this chapter. 

Binders of either lime (L by CaO Hellas) with metakaolin (M Metastar 501 by Imerys), or natural hydraulic lime (NHL NHL3.5z by Lafarge), as well as nano-titanium dioxide (T nano-structured nano-titania by NanoPhos), used as filler due to its photocatalytic activity, were employed for the design of the adhesive mortars. The already established improvement of hydration and carbonation process due to the photocatalytic activity of nano-titania in anatase form (Hyeon-Cheol 2010) added to cement mortars, was taken into consideration to assess whether the adhesion performance of the studied... 

NHL natural hydraulic lime, M metakaolin, L lime, B binder. A aggregate, W water. 

Based on the physico-chemical and mechanical characterization of all the studied adhesive mortars the MLT mortars with metakaolin, lime and nano-titania and the natural hydraulic lime mortars with nano-titania (NHLT2) have been seleeted as the most appropriate for the adhesion of fragment porous stones. 

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