1   ASEC Deputy Operation Director 

2   ASEC Technical Center (formerly)

3   ASEC Technical Center

Abstract

Workability is a general, descriptive term that indicates the ease with which a concrete can be mixed, transported, placed and compacted to give a uniform material (Taylor 1990). This paper explains all the reasons which may lead to low workability of cement paste and studies a real case of cement produced from ball mill in a cement plant. During the case study, several chemical, physical, and mineralogical tests were performed on clinker, Gypsum, cement paste and concrete under the supervision of ASEC technical center. The collected data was analyzed to find the root causes of this property. This research proposes an action plan to treat the causes of low workability, and this property has been improved based on the application of this action plan. The case study presented here can provide guidance for addressing this property in different plants.

Keywords

Workability; Cement; Concrete

1. Introduction

There is no single measure of workability, but various empirical tests that provide information on particular aspects are widely used. The most important is the slump test, in which the material is moulded by lifting away a conical container in which it was placed. The subsidence of the resulting pat provides a measure of the ability of the material to flow under its own weight (Taylor 1990). 

This research treats workability of cement paste (noting that W/C ratio 0.45, type of sand and type of aggregate are constant) by analyzing the root cause of this property. Samples were collected from the cement plant and then chemical, physical, and mineralogical tests were carried out in different laboratories and with different techniques.

2. Chemical, physical and mineralogical tests

Twelve cement samples were collected from inside mill; Axial test was performed using different size of sieves (32µ, 45µ, 63µ, 90µ, 125µ and 250µ). Separator efficiency was evaluated using Tromb curve calculation depending on particle size distribution test for four samples (feeding, rejected, after filter and final product). 

XRF (ARL 9900), a wavelength dispersive X-Ray fluorescence instrument containing X-Ray tube with Rh as anode, 11 fixed channels with goniometer and a 3.6 kV generator, was used to measure SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O, TiO2, MnO and P2O5 (Rueda, et al. 2012). 

XRD (PW 3209), an X-ray diffraction instrument, was used to measure clinker, Gypsum and cement phases. The identification of the most probable phases is carried out using Panalytical X’Pert high score software 2006 with the aid of the international center of diffraction database (Ermrich and Opper 2013). 

Paste analysis were performed in accordance with ASTM C-451/18 (ASTM 2022).

3. The main reasons for low workability of Cement Paste

At the beginning of this work, the main possible factors causing this property were reviewed. The following potential reasons have been studied:

• Clinker reactivity

• High C3A (tri calcium aluminate) levels

• High fineness

• Presence of calcium sulfate hemi hydrate

• Narrow particle size distribution (Magistri and Recchi 2010).

1. Clinker reactivity

1. High C3A level

Uncontrolled hydration of C3A before the formation of surface ettringite; lead to Flash set. It is accompanied by rapid release of heat and loss of workability and is caused either by insufficient gypsum or of insufficient sulfate solubility (Alsop, et al. 2001).

In this work, Mineral composition was evaluated by:

• Studying the mineral composition of clinker, gypsum and cement using XRD.

2. High fineness

Adsorption of water from the atmosphere will be greater in finer cements. Hence, such cements will deteriorate more rapidly in storage and on exposure to the atmosphere. Finer cements will also react more strongly with alkali-reactive aggregates. On the other hand, the workability of the paste improves with fineness (in case of optimized sulfate) because of increased cohesion and reduced bleeding (Soroka 1979); In this work, fineness of cement was evaluated by:

• Blaine test according to EN 196-6 (EN 2019)

2. Presence of calcium sulfate hemi hydrate

Depending on temperature/humidity inside the mill, gypsum can lose water being converted to other forms. Among them, calcium sulfate hemihydrate is characterized by a higher immediate solubility: higher amounts of sulfate are supplied during clinker hydration. Sulfate in excess with respect to hydrating calcium aluminate precipitates as secondary gypsum, rather than producing ettringite. Secondary gypsum absorbs water reducing fluidity and increase viscosity due to the particular shape of secondary gypsum crystals (Magistri and Recchi 2010); In this work, effect of calcium sulfate was evaluated by:

• Determining Early Stiffening of cement paste according to ASTM C451-18 (ASTM 2022)

3. Narrow particle size distribution

Narrowing the particle size distribution (PSD) saves energy in grinding, but increases the amount of water needed to produce a mix of given workability (Taylor 1990); As with Portland cement concrete the cement content necessary to achieve a workable concrete increases with decreasing water/cement ratio (Hewlett 2004).

In this work, PSD characteristics was evaluated by:

• Studying grinding characteristics by axial test

• Determine separator efficiency by Tromb curve calculation. 

4. Results & Discussion

In this part of the case study, the results of the tests hereby presented 

1. XRD was performed for Clinker, Gypsum and Cement showed that 

1. Total C3A content in clinker equal to 5.44% (Cubic phase 3.01%& Ortho phase 2.44%) is acceptable.

2.  79.5% of Gypsum Di-hydrated converted to Gypsum hemi-hydrated; This indicates an increase in temperature during the production process and could lead to a false set.

3. Cement mineral composition doesn’t show any undesirable phase.

2. Blaine test were performed for Cement shown normal range of fineness 3250±50 cm2/g.

3. Paste analysis results shown abnormal premature early stiffening and further mixing can dispel this rigidity (False Set). 

4. Axial test shown that

1. Cement mill is not able to grind fine particles.

2. Cement mill is able to grind coarse particles.

   
   

   

   
   

3. The missing capability of grinding fine particles is due to the relative big media size

5. PSD was performed for cement mill shown that:

1. separator efficiency is according to expectations for a third generation separator for sieves 45μm – 90μm.

2. Low separator efficiency for sieve 20μm

   
   

   

   
   

5. Conclusion

According to the results obtained, several reasons contribute to the occurrence of low workability of cement paste, including:

• False set which usually occurs due to some of the gypsum dehydrates as a result of contacting hot clinker or high temperatures in the grinding mill. The plant should use

• Cool clinker by water spray on clinker feeder; Or

• Use 50% Gypsum anhydride in addition to 50% Gypsum di-hydrated

• Missing capability of grinding fine particles. The plant should resort the charge in chamber 2 of cement mill by use balls as the following recommendation:

• Balls of 25 mm, 20mm and 15mm resort by 40%, 40% and 20%, respectively.

• Low separator efficiency for sieve 20μm. The plant should perform mechanical & process check for separator internal parts and operating parameters.

References

• Alsop, Philip A, Hung Chen, Arthur L Chin, Andrew J Jackura, Michael I McCabe, and Herman H Tseng. 2001. Cement plant operations handbook. Portsmouth: International Cement Review.

• ASTM , Technical committe. 2022. "Standard test method for Early stiffening of hydraulic cement." In ASTM handbook 2022, by ASTM technical committee, 1-4. united States: ASTM international.

• EN, Technical committe. 2019. "Methods of testing cement Part 6: Designation of fineness." In European Standards, by EN technical committe, 1-22. London: European Committee for Standardization.

• Ermrich, Martin, and Detlef Opper. 2013. XRD for the analyst. Almelo: Panalytical.

• Hewlett, Peter. 2004. Lea's Chemistry of Cement and Concrete. London: Elsevier Science & Technology Books.

• Magistri, Matteo, and Pietro Recchi. 2010. Strategies for the improvement of the cement workability. Research Paper, Milan: MAPEI.

• Rueda, Jairo, Henry Santamaria, Jorge Montana, Nestor Bernal, and Renan Jaimes. 2012. Analysis of clinker phases with the thermo scientific ARL 9900. Switzerland: Thermo Fisher Scientific.

• Soroka, I. 1979. Portland Cement paste and concrete. London: THE MACMILLAN PRESS LTD.

• Taylor, Harlod F.W. 1990. Cement Chemistry. London: Academic Press.