QUICKLIME

QUICKLIME

 Lime is the binding material obtained by high-temperature annealing (1000-1200 °C) in the shaft or rotary furnaces of sedimentary rocks consisting mainly of calcium carbonate CaCO3 (limestones, chalk, dolomite). The following chemical reaction occurs under temperature: CaCO3 → CaO + CO2 ↑. 

However, calcium carbonate losses up to of 44% of the weight with carbon dioxide, and become light and porous. Resultant product is the lump quicklime (fine porous slices of 5-10 cm). Subsequently, lump quicklime is exposed either to slaking with the use of water or post-grinding with obtaining of powdered quicklime.

Advantage of powdered lime over lump one consists in that during mixing it behaves like gypsum binding material: firstly, it forms plastic paste, and sets after 20-40 min. This may be due to the fact that water forming a paste is partially consumed for lime slaking. However, lime paste thickens and losses plasticity. Due to less amount of free water the powdered lime-based materials are less porous and stronger. In addition, lime during slaking is warmed-up that facilitates work with it in cold weather. Water required for mixing makes of 100-150% of lime weight, depending on lime quality (to be determined experimentally). 

The most critical quality indexes of powdered quicklime are:

• Activity – percentage of oxides to be able to be slaked;
• Number of non-slaked grains (underburning or overburning)

• - Underburning (undecomposed SASO3) occurring at very low burning point; reduces lime quality, because it has not binding properties.
• - Overburning occurs at very high burning point. Overburning grains are slaked slowly and can cause cracking and rupture of already hardened material.

• Slaking time:

Lime is subdivided into the following types depending on slaking time:

Specification:

Performance characteristics:

Active Cao+MgO, min – 90%

Active MgO content, max – 2.8%

SO-2 of content, max – 1.5%

Maximum slaking point - 98 °C

Slaking time, max – 8 min

Non-slaked grains, max – 4%