Utilization of Cement Kiln By-Pass dust (in construction & agriculture purpose)

By Chemist/ Safwan Darwish Elfar

As shown in page CKD recycling/utilization

  1. The maximum substitution of OPC by CKD is limited to 15%
  2. Agricultures soil amendment by CKD is limited to acidic soil in addition to  practical difficulties to handle such fine material


The achievements of this paper are:

  1. 100% substitution of OPC by CKD (zero-cement mortar) which is differ from (Cemfree product of M/s David Ball Group)
  2.  Hydroponic* agriculture nutrients (no soil is needed!)


In single process!


*Hydroponics is a subset of hydroculture and is a method of growing plants using mineral nutrient solutions, in water, without soil. Terrestrial plants may be grown with their roots in the mineral nutrient solution only or in an inert medium, such as perlite or gravel.There are 14 essential plant nutrients. Carbon and oxygen are absorbed from the air, while other nutrients including water are obtained from the soil. Plants must obtain the following mineral nutrients from the growing media:[Allen V. Barker; D. J. Pilbeam (2007). Handbook of plant nutrition]

The primary macronutrients: nitrogen (N), phosphorus (P), potassium (K)

The three secondary macronutrients: calcium (Ca), sulfur (S), magnesium (Mg)

The micronutrients/trace minerals: boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni)


Learn more about hydroponic by visit Dr. Howard Resh website



During cement manufacturing a by-product generated is known as “cement kiln by-pass dust” which is alkaline in nature and in the form of fine powder. Based on its chemical composition it was utilized for making

1– Blocks* (Zero-Cement Mortar)

2– Agriculture propose (Hydroponic Nutrients)


Ordinary Portland cement is a controlled blend of calcium silicates, aluminates and ferrate, which is ground to a fine powder with gypsum and other materials.

The cement manufacturing raw materials are: limestone, clay, Iron ore & bauxite.

The most modern cement manufacturing dry process consist of: quarrying, pre-blending, grinding, burning, cooling & finally grinding with gypsum

But The presence of volatile oxides; K2O, Na2O, SO3 & Cl- in the raw materials lead to circulation accumulation inside the kiln which effect negatively to the burning process, to overcome this negative impact a by-Pass system is required to eliminate the accumulation of such un-wanted oxides.

This generated dust is by passed and discarded as waste. The other unwanted from environmental point of view is CO2 gas


Current Handling of the dust: 

Not all of the generated dust can be recycled in in cement production due to the high concentration of chloride & alkali contents; most of the cement manufacturers are dumping this dust in the designated areas as land fill.


Safety &Health hazard: 

Even the By-Pass Dust is not classified yet as hazard material but being in the form of fine powder containing a relatively high concentration of reactive calcium oxide in addition to other volatile oxides specially Potassium ; in other term alkaline fine powder irritating the eyes, skin & the respiratory system if inhaled

A proper self-protection tools are required during the handling of this dust. 

Photo 1: a model of fort made using CKD

Hazard elimination

As previously mentioned this dust is alkaline fine powder if reacted with water these oxides react with water to form respective hydroxides increasing the pH of the water to around ~13.

A compressed CO2 gas was passed through the mixture of CKD and water under pressure to decrease pH value & precipitating the respective carbonate salts. 

Photo 2: CO2 based by-pass dust alkalinity neutralizing system

Sufficient amount of Potassium salts are dissolved in the solution. Instead of filtering & evaporating the rich potassium solution, further additives have been added to the solution to make hydroponic agriculture nutrients.

The efficiency of the nutrients had been verified in hydroponic system in both lab and also on field scale. 

Get CKD-nutrient-calculator

(4.99 $ )

About CKD-nutrient-calculator
Adobe Acrobat document [230.1 KB]
Photo 3: Lab Scale hydroponic agriculture system
Photo 4: field scale hydroponic system

The main target of this process was to neutralize the alkalinity of cement kiln by-pass dust but the consumption of water & also water crises changed the direction of the process to get hydroponic nutrient & precipitating environmental friendly carbonates.


Initial conclusion:

The dust alkalinity was neutralized, CO2 gas was fixed as carbonate & potassium salts utilized in agriculture application

But the uses of compressed CO2 gas & the electrical power required to run the reaction chamber was a concern, as isolating CO2 gas then compress will indirectly generate CO2 gas in addition to the sophisticated calculation to maintain the elements ratios required by plant to avoid any macro element deficiency as the dust chemical composition is changing according to kiln operation conditions.

So the new direction for utilizing this by-pass dust was to:

1. Neutralize the dust alkalinity

2. Extract the potassium salts


The assumption was that:

“Simply the dust is mixed with the sand & a calculated amount of water is added to dissolve the potassium salts and react with the calcium oxide to form calcium hydroxide, in the presence of the potassium salts the silica is activate and reactive with calcium hydroxide to form calcium silicate and the rest of calcium hydroxide will react with atmospheric CO2 gas to form calcium carbonate which will kick out the water along with the dissolved potassium salts.”

To verify this assumption practically the amount of water required was calculated and added to the dust/sand mix and it work!!!

After 72 hours from casting the dust sand mortar in the mold potassium chloride started to appear on the surface of block after one week of air curing white powder brushed out as Potassium Chloride. 

Photo 5: by-pass dust mortar mold
Photo 6: potassium chloride salt on the upper surface of the block
Photo 7 : an area paved using by-pass dust blocks
Photo 8 Recycled CKD into decorative blocks


Final conclusion

A mix of cement kiln by-pass dust, sand and water is used to make a doughy texture non-hydraulic mortar, which can be casted in moulds of different shapes.

The roll of water In this mix is to have saturated potassium chloride solution to activate the silica to react with calcium hydroxide, and during the air curing of the mortar the atmospheric CO2 gas will react with the rest of calcium hydroxide to form calcium carbonate which fill the gaps and kick out the water along with dissolved potassium chloride.

Finally it is the time to deal with CKD as a raw material for more beneficial and sustainable applications!

Download Summary
cement manufacturing flowchart.pdf
Adobe Acrobat document [220.7 KB]

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*These blocks are falling in NHL5, HL5 strength class as per BS EN 459-1:2001


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  • safwan Elfar (Sunday, November 15 15 02:08 pm EST)

    Thanks Nael,
    strength is varying according to the additives; from 3 to 32 N/mm2

  • Nael Shabana (Thursday, October 29 15 02:40 am EDT)

    it is really a good idea to use bypass dust in construction but what about the strength and physical properties of this mix!

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© Safwan Elfar, 2011 This site is related to sustainability; http://sustain-earth.com/ Related Site: http://www.cementkilns.co.uk/