Abstract

Flowability of cement, defined as the ease with which cement flows once it has been set in motion (Strohman 2004). This paper explains many of problems that can affect this property, and the reasons that may lead to lack of flowability of cement powder. This research proposes an action plan to address the causes of low flowability, and the possibility of improving this property based on the application of the proposed action plan. The solutions presented here can provide guidance for addressing this property in different plants.

Keywords

Flowability; Pack-set; Silo-set; Warehouse-Set; Cement.

1. Introduction

There is no single measure of cement flowability, but various empirical tests that provide information on particular aspects are widely used. The most important is the pack-set index test, in which Portland cement is placed into a flask and then consolidated on a vibrating table by a vibration force controlled by time and energy (volts). The consolidated bed is measured for relative strength by turning the flask to a horizontal position and rotating in 180° increments until the bed collapses from the bottom of the flask. The number of half turns is a relative indication of the force required to overcome the consolidation and is designated the pack-set index, or P.S.I. The subsidence of the resulting pat provides an indication of the mechanical force needed to overcome the consolidation of Portland cement (C1565 Technical Committee 2022). 

This research identifies different types of factors that affect the flowability of cement powder; and how improve this property. 

2. The main reasons for lack of flowability of Cement Powder

A large number of plants are affected by cement flow problems. These problems can be classified as follows:

1. Pack-set is a condition of hydraulic cement, exhibited in varying degrees, following silo storage or transport in bulk railcars or trucks, that causes the cement to resist flowing until considerable mechanical effort has been applied (C1565 Technical Committee 2022). Pack set may be caused by interlocking particles, mechanical compaction, or electrostatic attraction between particles (225R Committee 1999).

2. Silo-set, which is the partial hydration of cement stored for a time and exposed to atmospheric moisture (defined also as Warehouse set), or formation of flow-inhibiting complex sulfate phase like syngenite (Chatterjee 2018). 

3. Methods for estimating the flowability of cement 

3.1. ASTM C-1565 covers the determination of the pack-set index, which defined as the numerical indication of the degree of pack set a particular cement exhibits when subjected to the procedures of this test method.

This method is intended to help manufacturers determine the relative pack-set tendency of their cement(s). The test establishes a pack-set index which, when properly correlated with field performance, is useful in predicting or preventing field unloading difficulties. The test is an aid to routine control during cement production and is not suitable for specification purposes.

In general, field performance of cement flowability is satisfactory when the pack-set index as determined on freshly ground cement averages 0 to 15 and is unsatisfactory when the index exceeds 25. Any prediction of field performance of cement flowability measuring 16 to 25 is tenable (gray region). These are general ranges and the field performance of individual cements may not necessarily fall within these ranges. Additional conditions, after the cement has left the control of the manufacturing facility, can affect the apparent pack set index as well.

The pack-set index of field cement can be evaluated in terms of the pack-set index ranges of that cement as determined when freshly-ground. This comparison can aid the manufacturer in producing cement that offers the best field performance for pack-set properties.

Silo storage of cement may result in a greater amount of consolidation than this method is designed to induce, and the resulting forces required to overcome that consolidation are not measured by this test method (C1565 Technical Committee 2022).

3.2. On the other hand; If cement transferred into a silo is very hot, and if the exterior walls of the silo are cool, water transfer can occur from the hot silo interior to the cooler exterior, forming syngenite, and creating the phenomenon known as “silo set.” Syngenite (CaSO4·K2SO4·H2O) is a needle-like crystalline substance that can hinder cement flowability significantly (Bhatty and Miller 2004). 

Syngenite can be detected by X-Ray Diffraction XRD (Struble, Graf and Bhatty 2004). Differential Scanning Calorimetry (DSC) is also helpful in discovering and quantifying the syngenite in cement as shown in Fig (1). This material is formed by the transfer of water from gypsum to potassium sulfate, often occurring in cement silos (Bhatty and Miller 2004).

Fig (1): DSC plot showing gypsum, plaster, and syngenite in a finished ground cement. 

4. Ways to overcome Pack set/Silo set phenomena

4.1. The first phenomenon could be treated by using an appropriate amount of processing additions complying with ASTM C 465 (C465 Technical Committee 2022); during the finish grinding process can typically eliminate the occurrence of the phenomena. Most often, a relatively small amount of mechanical effort will overcome the resistance to flow.

The generally accepted explanation of pack set is that the surfaces created during grinding of Portland cement clinker have areas with unsatisfied electrical forces. The active surfaces cause inter particle attraction resulting in agglomeration and pack set (225R Committee 1999). 

The grinding aids have strongly adsorbed by the ground particles, so that surface energy requirements are satisfied and no bond remain to attract other particles and cause agglomeration.

By elimination of the surface energy forces which normally cause interparticle attraction, grinding aids improve cement flowability after grinding. The grinding aids are added either with the mill feed or injected directly into the mill (Duda 1976).

4.2. The mechanism of pack set is different from that of Silo set, which, as mentioned previously, is a loss of flowability caused by partial hydration of cement.

Hard lumps are evidence of reaction with moisture. This condition is often referred to as warehouse set. If the lumps are screened out, the remaining cement is normally satisfactory for use. Measures for minimizing the likelihood of warehouse set of packaged cement include the following:

• Use stock on a first-in, first-out basis.

• Keep storage areas dry.

• Store bags on pallets above ground.

• Store bags under a cover that will protect them from moisture.

Soft lumps may occur in the lower bags in a high stack simply from the pressure of the bags above. Rolling the bags a few times normally breaks up these lumps. Measures for minimizing the formation of hard lumps during bulk storage and during transit include the following:

• Periodically inspect the loading hatches of bulk carriers for watertightness.

• Keep loading hatches closed when not in use.

• Compressed-air transit systems should have water traps and, in areas of extremely high humidity, the air lines should be equipped with air dryer.

Storage bins should be inspected periodically for possible water leaks (that is, roof, hatch covers, and welded seams).

Warehouse set of a different type can occur with fresh cement in storage at the manufacturing plant. This type of warehouse set, more appropriately called partial hydration, is characterized by soft lumps (lumps that break under light finger pressure) and reduced flowability. This condition can develop within a few days after production and is caused by chemical reaction of cement components during storage.

Once the flow has been started and the rigidity of the bulk material broken, however, the potential for reoccurrence of the flow problems is practically nil. The tendency of cements to undergo prehydration depends on several factors, including chemical composition, storage temperature, grinding temperature, and the moisture available during grinding. The effect of storage on the quality of cement is generally negligible, but it can cause false setting and a slight loss in strength development (225R Committee 1999).

5. Conclusion

The author explained the reasons that lead to pack set/silo set phenomenon and how to detect it. Then, based on the above, proposals were made to overcome it.

These proposals focused on using appropriate grinding aids as well as maintaining appropriate operating and storage environment

References

• ACI 225R Committee. 1999. Guide to the Selection and Use of Hydraulic Cements. Michigan: American Concrete Institute.

• ASTM C1565 Technical Committee. 2022. "Standard test method for Determination of Pack-Set Index of Portland Cement." In ASTM handbook 2022, by ASTM Technical Committe, 1-5. united States: ASTM international.

• ASTM C465 Technical Committee. 2022. “Standard Specification for Processing Additions for Use in the Manufacture of Hydraulic Cements." In ASTM Handbook 2022, by Technical Committe ASTM, 1-4. United States: ASTM International.

• Bhatty, Javed I., and F. M. Miller. 2004. "Application of Thermal Analysis in Cement Manufacturing." In Innovations in Portland Cement Manufacturing, by Javed I. Bhatty, F. MacGregor Miller and Steven H. Kosmatka, 1037-1068. Ilinois: Portland Cement Association.

• Chatterjee, Anjan Kumar. 2018. Cement Production Technology: Principles and Practice. Florida: CRC Press.

• Duda, Walter H. 1976. Cement Data Book. Berlin: Macdonald & Evans London.

• Strohman, Nile R. 2004. "Finish Milling and Grinding." In Innovations in Portland Cement Manufacturing, by F. MacGregor Miller, Steven H. Kosmatka Javed I. Bhatty, 499-517. llinois: Portland Cement Association.

• Struble, Leslie J., Luis A. Graf, and Javed I. Bhatty. 2004. "X-Ray Diffraction Analysis." In Innovations in Portland Cement Manufacturing, by Javed I. Bhatty, F. MacGregor Miller and Steven H. Kosmatka, 959-1010. Ilinois: Portland cement Association.