Role of lime stone in cement?

Hello Raju,

There is still some controversy regarding the effectiveness of replacing of gypsum with limestone.

Several researchers (eg. Bensted and Kantro) have found that while limestone cannot be used as a total replacement for gypsum, it is possible to replace a portion of the gypsum with limestone without significant problems with cement performance. The exact amount of substitution depends on the cement and the fineness of the limestone.

 They suggest that, due to the mitigating effects of the carboaluminate formed by the reaction of carbonates with C3A, up to 25% of the gypsum could be replaced with limestone without any significant change to setting times or strengths. It is thought that carboaluminate behaves similarly to the sulphoaluminate formed by the reaction of sulphates with C3A.

On the other hand, some researchers (eg. Sprung and Siebel) believe that limestone participates only to a small extent, if at all, in the hydration reactions,and should be considered an inert material.

Therefore, to see if it works in your situation, you would have to do plant trials by incrementally increasing the limestone content and decreasing the gypsum content in small steps to see how far you could take this replacement in your cement.

Regards,

Ted.

References:

1. "Limestone Substitutes for Gypsum as a Cement Ingredient", G.S. Bobrowski, J.L. Wilson  & K.E. Daugherty, Rock Products, Vol.60. No.2, 1977 pp. 64-57.

2. "Some Hydration Investigations Involving Portland Cement - Effect of Calcium Carbonate Substitution of Gypsum", J. Benstead, World Cement Technology, Vol.11, No.8, 1980, pp.395-406

3. "Calcium Carbonante Additions", D.L. Kantro, 1978 Cement Chemists' Seminar, Portland Cement Association, Skokie, IL 1980.

4. Sprung, S., and Siebel, E., “Assessment of the Suitability of Limestone for Producing Portland Limestone Cement (PKZ), Zement-Kalk-Gips, vol. 44, no. 1, January 1991, pages 1 to 11.

Dear 

What Ted has explained is absolutely right. Limestone is no more considered an inert material. World wide studies have shown that limestone takes part in hydration reaction and early strength development in cement. On the addition of limestone powder to cement, a reaction between CaCO₃ of limestone and aluminate phase of cement (C₃A) takes place, which is quite similar to gypsum-C₃A reaction.

3.CaO.Al₂O₃ + 3.CaCO₃ + 32 H₂O  --->  3. CaO.  Al₂O₃ .3 CaCO₃. 3 H₂O 

                                                        Calcium carbo aluminate hydrate

 3.CaO. Al₂O₃.3 CaCO₃ .3 H₂O  + 2 (3 CaO.Al₂O₃) +H₂O     ----->

3 ( 3 CaO.  Al₂O₃ . CaCO₃ .11 H₂O  )

Calcium mono carbo aluminate hydrate

 Calcium carbo aluminate or carbonate ettringite is similar to sulphate ettringite which is formed during the reaction of CaSO₄ and C₃A. Due to this reason calcium carbonate upto some extent replaces the sulphates and help in removing the difficiency of sulphates in OPC.

 Lime stone addition to cement have several functions. (1)Completion of granulometric curve of different fine grain cement.(2)Obstruction of capillary pores. (3)Hampering water percolation.

 The soluability of CaCO₃ is very low compared to soluability of CaSO₄.

thanks chari it is useful information