LIME STONE
On Zanzibar, lime production has a long tradition. This craft is an important part of the local building culture and is deeply connected to the history and unique architectural style, especially in Stone Town and the surrounding areas. Coral stones, taken from fossilized coral reefs, are the main raw material for making lime. The lime is then used as plaster and building mortar in historical buildings. But how exactly are these coral stones turned into versatile lime, and what challenges come with this process?
The production starts with collecting coral stones. In the past, living coral was taken directly from the ocean, dried, and used for lime production. This was often done without thinking about the environmental damage it caused, which led to serious harm to coral reefs. These reefs are home to rich aquatic ecosystems that provide food and shelter for many marine organisms. When the damage became clear, people began using fossilized coral reefs found underground instead. Fossil coral is usually dug up by hand, using simple tools like shovels and picks. Even so, removing fossil coral impacts the environment and requires careful planning to balance its use and environmental protection.
The collected coral stones are first dried in the sun. This step takes several days or weeks to prepare the stones for burning. Next, the stones are burned in traditional open heap kilns at temperatures of 650–900 °C. The open heap kiln method consists of a circular heap of fuel; usually coconut stems or wood logs in combination with small pieces of firewood, also coconut husks and shells, traditionally used at the start point of firing. There is no standard diameter for the open heap method, but normal range is 3.5-8m and height of 1-2m. The thinner firewood is arranged on the ground following circular pattern set on site, to an average height of 0.4-0.7m, and coconut and/or wood logs placed over thinner firewood in an average of 3 to 4 layers. On the top of the logs limestone (coral stones) are laid starting with bigger sizes in average diameter 15-20cm from the center of the circular kiln, and gradually followed with various small sizes, till the conical shape of the kiln is created. One side is provided with a firing doorway, normally directed towards the wind, to act as natural blowing mechanism and pushes the fire further inside the core of the kiln. These kilns are lit when there is favorable wind and allowed to burn slowly over a period of about 24h hours.
During burning, the calcium carbonate (CaCO₃) in the stones is turned into calcium oxide (CaO), also known as quicklime, and carbon dioxide (CO₂) is released. This process, called calcination, uses a lot of energy. Wood and coal are common fuels for the kilns, raising concerns about the sustainability of energy sources. Wingate[1] experienced in East Africa in every one ton per day produced by heap kiln, using logs of firewood, the ratio of limestone to wood fuel to hydrated lime (2.5-3.0): (0.7-0.8): 1, equivalent to 250-300kg of wood fuel for 1 ton of limestone.
After it has cooled, the unburnt stone at top most of kiln (uncalcined stones) are normally removed waiting for second firing, and the remainder slaked in situ by pouring water over the heap; some lime burners, left the lime after firing exposed to weather to be air slaked, drawing-in the moisture from the atmosphere and rains. After slaking and cooling, resulting in dry hydrated lime in powder form is bagged in used cment, flour or rice sacks, normally no sieving is done before bagging, the bag weight normally range in between 25kg to 50kg, a bag of 25kg may cost between 2500-3000 Tanzanian shillings equivalent to € 1.00 – 1.20.
The quicklime is then mixed with water in a process called slaking. This creates calcium hydroxide (Ca(OH)₂), a paste that serves as the base for lime plaster. Often, laterite soil and sand is added to the paste to improve its consistency and strength. The finished plaster is applied to walls, where it reacts with carbon dioxide from the air, hardening into calcium carbonate again. This natural cycle makes lime plaster durable and breathable, which suits Zanzibar’s tropical climate.
The open heap kiln is very easy to prepare, can be affordable by the local people; less capital to start preparation. However, there is a serious, deforestation problem associated with this method of production; many woods cut down without equal replanting; 25,000 – 30,000 trees per year are used for fuel alone in Zanzibar (Holmes and Schilderman[2]). Also poor quality control leading to the production of poor quality lime; this results in poor durability and strength in building construction.
Vertical shaft kilns are relatively efficient in terms of reaching and maintaining favorable temperatures of about 950 to 1100 degrees Celsius for complete calcination of coral stones. Vertical shaft kiln in Zanzibar, which were abandoned for nearly three decades without being in use. In the 1940s, the local people with supervision of German experts constructed a vertical shaft kiln in Dunga central district about 16km from Zanzibar city. In 1964 the plant was given to National Service Department “Jeshi la Kujenga Uchumi (JKU)”. The kiln was rehabilitated 25 years ago for production but unfortunately there is no production of lime today. Another vertical shaft kiln was located at Dimani south east of island about 15km from Zanzibar city. The structure was demolished in late 2000s.
Saris Lime products is a trade-owned vertical shaft kiln, a local small-scale entrepreneurship located at Fumba, about 16km from Zanzibar city, southern west of Zanzibar. Like any other vertical shaft kiln, it is operated on a batch or continuous basis with coal as fuel. The kiln has to be loaded with alternate layers with coral stone and coal at ratio 1:7 coal to limestone. A fire is lit from the bottom through the discharging door. As the burning proceeds, a drop is realized in the height of materials, which is promptly topped up with alternate layers as before, until the kiln is completely filled to the top. The quicklime is allowed to cool for two days before off-loading commences. After burning, the quicklime is processed further. The quicklime is offloaded from the kiln through discharge doors and immediately carried and put in the huge pots “Karais” for the slaking operation. It is dissolved in water at temperature of about 300-350 degrees Celsius and continually stirred, the milky solution passes through pipes into the sieve towards through the first chamber; when completely full, and it goes to second chamber until all three chambers are completely full. The lime in form of putty, settles at the bottom of the chambers after some days. In these tanks, the material is sorted by particle size. Heavier, coarser particles settle at the bottom and are separated for other uses, while finer particles remain suspended in the water and are used to make high-quality lime plaster. The tanks not only sort the material but also clean it, removing sand or other impurities and improving the quality of the final product.
During production, several by-products are created, many of which are reused. Coarse leftovers, such as under-burnt stones and over-burnt stones, are often put back into the kilns or used as foundation materials or gravel. Fine coral fragments, including dust and small particles from crushing, are used as fillers or additives in mortar and plaster. Sediments from the settling tanks, including sand and larger coral pieces, are collected and used in construction, while finer lime is reserved for premium plaster. Even the water from the tanks is recycled to reduce waste and environmental impact.
These practices help reduce waste, but the process still has significant environmental impacts. Burning lime releases large amounts of CO₂, contributing to global warming. Using wood as fuel often leads to deforestation, increasing the strain on local resources. The extraction of fossil coral also alters coastal landscapes. While live coral reefs are not directly affected, removing fossil coral can increase coastal erosion and weaken natural barriers that protect the land. This threatens the stability of the coast and the livelihoods of communities that rely on healthy ecosystems.
Efforts are being made to make lime production more sustainable, such as using regulated mining, upgrading to efficient kiln technologies, and recycling materials. Lime production also supports the local economy by creating jobs and preserving traditional building knowledge. Lime production from coral stones on Zanzibar is a fascinating process that connects deeply to the island's history and culture. However, it faces challenges that require a careful balance between tradition and environmental conservation. The abundant availability of Portland cement at lower prices makes it more favorable to be used instead of lime. This craft reminds us of the importance of using resources responsibly and protecting local traditions for future generations.
Bibliography
[1] Wingate, M (1985, Pg 45) Small-scale lime burning. London: Intermediate Technology Publication.
[2] Holmes, S and Schilderman, T (1997), Conservation and development: A study of the production and use of lime in East Africa, Journal of Architectural Conservation, Vol. 3,
(3) 42-58.