Quick Drying Cement on the Way to Gibe III

2011-09-29

Sep. 29, 2011 - The first shipment of 10,000tn of slag-cement mixed with fly ash out of 150,000tn to be used for the foundation work of Gibe III dam bought at a cost of around 12.3 million dollars from Italy is to arrive by the end of this month.
The cement was bought from Cementir Holdings, which produces and distributes grey cement in Italy and is being transported by the Ethiopian Shipping Lines. Grey Cement is usually composed of low aluminates tricalcic contents, with high granulated slag content and pozzolana, which have high resistance to Sulphate, low hydration heat and high water resistance.
The procurement of the cement was carried by the Public Procurement and Disposal Agency (PPDA) under the Ministry of Finance and Development (MoFED). The cement was bought at 82.1 dollars per tonne and will be transported to the project site 470Km from Addis in the lower course of the Omo River. The price of this cement stood at 88.5 dollars per tonne in the last month, according to the August publication of the Cement Review Magazine, which is published by Hanson Heidelberg, a UK based company engaged in production and consultancy of cement.
Salini Costruttori S.p.A, which is undertaking the construction of Gibe III, had been testing different mixes of cement bought here and from abroad for one year to find the right combination of materials needed for the foundation, according to Azeb Asnake, Gibe III Hydroelectric power project manager.
The cement needed was of a type that is different from ordinary cement, it needed to have a very low heat of hydration when mixed, making it dry faster and become solid.
The 243 meter high dam, the tallest in Africa, requires a strong foundation that can carry its weight and last a long time. The foundation needs five million meter cubes of concrete mix that needs to be filled in exact combination round the clock using Roller Compacted Concrete (RCC), according to Azeb.
"As layering is very important in this procedure it is necessary to use a dry system, whose ratio of water mix is very minimal compare to other mixes," she said. "Once layering has started, work must be done continuously for 24 hours; otherwise it can create weak spots that can crack easily."
However, finding the right mix for Gibe III, which is estimated to cost 1.5 billion euro proved elusive. Dongfang Electric Corporation (DEC) is building the Electro mechanical work of the power plant, with 470 million Euros.
In one of the sampling processes, Maple Cement from Pakistan was considered an option due to its high heat of hydration. It was planned to be mixed with fly ash imported from South Africa, according to the project manager.
"However the time it would take and the resources needed to import both the cement and the fly ash proved too expensive," Azeb, who oversees the dam which is expected to be finished in 2014, said. "The Maple Cement also exhibited different characteristics from batch to batch imported."
The original design done in 2006 Gibe III, which has an installed capacity to produce 1870 MW of electricity, had called for a rock fill dam headrace tunnels, manifolds, surge shafts, powerhouse, and switchyard on the left bank. However, due to difficulties with obtaining proper and sufficient insurance coverage for the Rock fill Dam, the design was changed to Roller Compacted Concrete (RCC).
Cement needed for the filling, which needs to have a higher ratio of cement for strength, is provided by Mugher Cement Factory, according to Azeb. An additional 700,000tn is needed for the completion of the dam.
"This will be completed in mid September and Salini will start the RCC work," she said.