QUICKLIME
Lime is the binding material obtained by high-temperature annealing (1000-1200°С) 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 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 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 СаСО3) 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:
Quick-slaking |
up to 8 min |
Medium-slaking |
up to 25 min |
Slow-slaking |
at least 25 min |
Specification:
No. |
Name of Parameter |
Content |
1. |
Active CaO+MgO |
min 80% |
2. |
Active MgO |
max 3.5 |
3. |
CO2 |
max 5.0 |
4. |
Slaking point |
С 92 |
5. |
Slaking time, min |
2,5 |
6. |
Non-slaked grains, % |
max 11.0 |
7. |
Sieve residue, max No.2 No.008 |
98,6 85,3 |
Performance characteristics:
Active Cao+MgО, min - 90%
Active МgO content, max - 2.8%
СО-2 content, max - 1.5%
Maximum slaking point - 98°C
Slaking time, max - 8 min
Non-slaked grains, max - 4%