3
Fig. 3.1 Types of building lime
Table 3.1 Types of building lime (BS EN 459-1: 2001)
Designation Notation
Calcium lime 90 CL 90
Calcium lime 80 CL 80
Calcium lime 70 CL 70
Dolomitic lime 85 DL 85
Dolomitic lime 80 DL 80
Hydraulic lime 2 HL 2
Hydraulic lime 3.5 HL 3.5
Hydraulic lime 5 HL 5
Natural hydraulic lime 2 NHL 2
Natural hydraulic lime 3.5 NHL 3.5
Natural hydraulic lime 5 NHL 5
Notes:
Air or non-hydraulic limes are also classified according to their delivery conditions—quicklime (Q), hydrated lime (S), semi-hydrated dolomitic lime [calcium hydroxide and magnesium oxide] (S1) and completely hydrated dolomitic lime [calcium hydroxide and magnesium hydroxide] (S2).
NON-HYDRAULIC OR AIR LIMES
Limestone and chalk when burnt produce quicklime or lump lime (calcium oxide). Quicklimes include cal- cium limes (CL) and dolomitic limes (DL) depending on the composition of the starting mineral as defined by BS EN 459-1: 2001. The dolomitic limes contain significant quantities of magnesium oxide.
CaCO3
calcium carbonate 950◦C
−→CaO
lime + CO2
carbon dioxide
Slaking of lime
Slaking—the addition of water to quicklime—is a highly exothermic reaction. The controlled addition of water to quicklime produces hydrated lime (S) (mainly calcium hydroxide) as a dry powder.
CaOlime +H2O
water→ Ca(OH)2
calcium hydroxide
It is suitable for use within mortars or in the manufac- ture of certain aerated concrete blocks. Generally, the addition of lime to cement mortar, render or plaster increases its water-retention properties, thus retaining workability, particularly when the material is applied to absorbent substrates such as porous brick. Lime also increases the cohesion of mortar mixes allowing them to spread more easily. Hydrated lime absorbs moisture and carbon dioxide from the air, and should therefore be stored in a cool, draught-free building and used whilst still fresh.
Lime putty
Lime putty is produced by slaking quicklime with an excess of water for a period of several weeks until a creamy texture is produced. Alternatively, it can be made by stirring hydrated lime into water, followed by conditioning for at least 24 hours. However, the traditional direct slaking of quicklime produces finer particle sizes in the slurry, and the best lime putty is produced by maturing for at least six months. Lime putty may be blended with Portland cement in mor- tars where its water-retention properties are greater than that afforded by hydrated lime. Additionally, lime putty, often mixed with sand to formcoarse stuff, is used directly as a pure lime mortar, particularly in restoration and conservation work. It sets, not by reac- tion with sand and water, but only by carbonation, and is therefore described asnon-hydraulic. Lime wash, as a traditional surface coating, is made by the addition of sufficient water to lime putty to produce a thin creamy consistency.
Carbonation
Lime hardens by the absorption of carbon dioxide from the air, which gradually reconverts calcium oxide back to calcium carbonate.
CaOlime + CO2
carbon dioxide
−→slow CaCO3
calcium carbonate
The carbonation process is slow, as it is controlled by the diffusion of carbon dioxide into the bulk of the material. When sand or stone dust aggregate is added to the lime putty to form a mortar or render, the increased porosity allows greater access of carbon dioxide and a
L I M E , C E M E N T A N D C O N C R E T E 5 7 speedier carbonation process. The maximum size of
aggregate mixed into lime mortars should not exceed half the mortar-joint width.
HYDRAULIC LIMES
Hydraulic limes are manufactured from chalk or limestone containing various proportions of clay impurities. The materials produced have some of the properties of Portland cement, and partially harden through hydration processes, rather than solely through carbonation, as happens with non-hydraulic pure calcium oxide lime. Hydraulic limes rich in clay impurities are more hydraulic and set more rapidly than those with only a low silica and alumina content.
Natural hydraulic limes are traditionally categorised asfeebly,moderatelyoreminently hydraulicdepending on their clay content, which is in the ranges 0–8%, 8–18% and 18–25%, respectively. These traditional grades equate approximately to the 28 day compres- sive strengths of 2, 3.5 and 5 MPa, respectively, for NHL2, NHL3.5 and NHL5. Eminently hydraulic lime mortar is used for masonry in exposed situations, moderately hydraulic lime mortar is used for most normal masonry applications and feebly hydraulic lime mortar is appropriate for conservation work and solid wall construction. Grey semi-hydraulic lime is still produced within the UK in small quantities from chalk containing a proportion of clay. It is used with very soft bricks and for conservation work. Natu- ral hydraulic limes (NHLs) are produced by burning chalk and limestone but hydraulic limes (HLs) are produced by blending the constituents in appropriate proportions.
Hydraulic lime, usually imported from France, is mainly used for the restoration of historic buildings, where the use of modern materials would be inappro- priate. It is gauged with sand only, giving a mix which develops an initial set within a few hours, but which hardens over an extended period of time. The work- able render or mortar mixes adhere well and, because the material is flexible, the risks of cracking and poor adhesion are reduced. The dried mortar is off-white in colour and contains very little alkali, which in Port- land cement mortars can cause staining, particularly on limestone. Hydraulic lime may be used for inte- rior lime washes, and also for fixing glass bricks where a flexible binding agent with minimum shrinkage is required. Unlike hydrated lime, hydraulic lime is little affected by exposure to dry air during storage.
LIME MORTAR
Various advantages of lime-based mortars over Port- land cement mortars are reported. The production of building lime consumes less energy, thus reduc- ing greenhouse emissions compared to the equivalent manufacture of Portland cement. The subsequent car- bonation process removes CO2 from the atmosphere.
Lime-based mortars remain sufficiently flexible to allow thermal and moisture movement, but addition- ally, due to the presence of uncarbonated lime, any minor cracks are subsequently healed by the action of rainwater. The recycling of bricks and blocks is easier due to the lower adherence of the mortar. Lime mortar construction is more breathable than Portland cement masonry and lime mortars are more resistant to sul- phate attack than standard Portland cement mixes due to their lower tricalcium aluminate content.
Typical lime mortars are within the range 1 : 2 and 1 : 3, lime : aggregate ratio. A 1 : 2 lime : sand mix made with NHL3.5 lime equates approximately to the Stan- dard BS 5628-1 designation (iii) class M4 mortar and a 1 : 3 mix equates roughly to a designation (iv) class M2 Portland cement mix. A well-graded sharp sand should be used. Because of the slow carbonation pro- cess, masonry lifts are limited, and the mortar must be allowed some setting time to prevent its expulsion from the joints. Little hardening occurs at tempera- tures below 5◦C.
The use of lime mortars will eventually be incor- porated into an annex for use with the Standard BS 5628-1: 2005.
HEMP LIME
Hemp is grown, particularly in France, for its fibre which is used in the manufacture of certain grades of paper. The remaining 75% of the hemp stalks, known as hemp hurd, is a lightweight absorbent mate- rial which has the appearance of fine wood chips.
When mixed with hydraulic lime it produces a cement mixture which sets within a few hours and gradually
‘petrifies’ to a lightweight solid due to the high silica content of hemp hurd. The material mix can be poured and tamped or sprayed as required and formwork may be removed after 24 hours or less. The set material, sometimes referred to as ‘hempcrete’, which has good thermal insulation properties and a texture similar to that of cork, has been used for the construction of floors, walls using plywood formwork, and also blocks for blockwork. The material is also used as a solid infill
Fig. 3.2 Manufacture of Portland cement—the wet process for timber-frame construction. In this case the com- bination of the moisture-absorbing properties of the hemp with the nature of lime, affords some protection to the timber-framing which it encloses. The moisture- absorbing properties of hemp lime give rise to higher thermal efficiencies than are calculated for the material based on conventional thermal transmittance data. To build a conventional house of hemp lime would require approximately 40 m3of the material containing 7–10 tonnes of hemp, produced from about 1 ha of land.
EXTERNAL LIME RENDERING
External lime rendering is usually applied in a two- or three-coat system, to give an overall thickness of up to 30 mm. In exposed situations, hydraulic lime is used and the thicker initial coat may be reinforced with horsehair. The final coat can be trowelled to receive a painted finish; alternatively, pebble dash or rough cast may be applied.