Dehydration and Thermal Conditioning of WPE-DK UHPC High Temperature Materials

WPE-DK UHPC High Temperature materials are designed for applications in which dense mineral linings, protection layers or cast components are exposed to elevated operating temperatures. Before these materials are heated to service temperatures above approximately 400 °C, the initial dehydration process must be controlled. The reason is the internal water vapour pressure that can develop during the first heating cycle. If water vapour cannot escape fast enough, especially in dense and compact UHPC structures, surface spalling or explosive material disruption may occur.

 

This risk is highest during the first days after casting, when the material still contains physically bound or free moisture. The dense microstructure of WPE-DK UHPC is technically advantageous for strength, durability and chemical resistance, but it also means that moisture transport during first heating must be managed with a defined drying procedure.

Purpose of the Investigation

The purpose of the technical evaluation was to determine how WPE-DK UHPC High Temperature materials behave when heated to different temperature levels, and how the rate of heating influences mechanical performance and the risk of water vapour pressure build-up. The evaluation focused on compressive strength, flexural strength, erosion resistance, abrasion resistance and open porosity after exposure to high temperatures.

The results show that the mechanical properties are reduced with increasing temperature, but not critically within the investigated range. The material remains structurally intact after exposure, although binder weakening and increased porosity are measurable at higher temperatures.

Vapour Pressure and Dehydration Risk

During first heating, water inside the material evaporates and must escape through the pore structure. If the heating rate is too high, vapour pressure can build up faster than it can be released. In compact WPE-DK UHPC components with a thickness above 50 mm, this risk is higher because the vapour path is longer and the material is less able to release moisture quickly.

For this reason, drying procedures should be differentiated according to component thickness:

Layers or components below 50 mm:

A standard controlled heating procedure can generally be applied.

Layers or components above 50 mm:

A slower and more conservative dehydration procedure is recommended, because internal moisture requires more time to migrate and escape safely.

The drying process should not be treated as a simple heating operation. It is a controlled technical conditioning step that protects the material during the transition from newly cast UHPC to high-temperature service condition.

Influence of Temperature on Mechanical Properties

After heating and cooling to 20 °C, WPE-DK UHPC High Temperature materials retain substantial mechanical performance. The compressive strength decreases progressively as the exposure temperature increases, but the residual strengths indicate that the material remains functional and is not critically damaged within the tested temperature range.

Approximate residual compressive strength trend:

Exposure temperature	Approximate compressive strength after cooling
300 °C	approx. 190–200 MPa
600 °C	approx. 150–160 MPa
900 °C	approx. 120 MPa
1200 °C	approx. 75–80 MPa

The flexural strength follows the same general trend. At 300 °C, the material still shows high flexural performance. With increasing temperature, the flexural strength is reduced, but remains measurable even after exposure to 1200 °C.

Approximate residual flexural strength trend:

Exposure temperature	Approximate flexural strength after cooling
300 °C	approx. 24 MPa
600 °C	approx. 17 MPa
900 °C	approx. 15–16 MPa
1200 °C	approx. 12 MPa

These results confirm that the high-temperature exposure primarily weakens the binder matrix, while the mineral aggregate structure remains largely stable.

Influence of Heating Rate

The heating rate between 100 °C/h and 200 °C/h did not show a significant negative influence on compressive strength up to 600 °C. Within this range, the mechanical strength remained broadly stable. However, from a safety perspective, heating rates above 200 °C/h should be avoided during the first dehydration cycle, because the probability of vapour pressure build-up increases.

A practical interpretation is therefore:

Controlled heating at 100–200 °C/h can be technically acceptable for suitable geometries, but the final drying schedule must always consider component thickness, age, curing temperature, moisture content, installation conditions and the intended service temperature.

Erosion and Abrasion Behaviour

Erosive sandblasting tests showed increased binder erosion with rising temperature. This indicates that the binder matrix becomes weaker after high-temperature exposure. However, the aggregates showed no critical damage, which confirms that the mineral skeleton remains stable even at elevated temperatures.

The Böhm abrasion test showed largely unchanged abrasion resistance over the investigated temperature range. This indicates that the aggregates retain their mechanical integrity and that the binder remains strong enough to hold the aggregate particles in place during wear exposure.

This distinction is technically important: high temperature affects the binder phase more strongly than the aggregate phase. For real applications, this means that WPE-DK UHPC High Temperature materials can remain suitable for hot wear zones, provided the dehydration process is correctly performed and the system is designed for the actual thermal and mechanical load profile.

Open Porosity After Heating

During dehydration, evaporating water leaves additional voids in the material. Therefore, open porosity increases with higher exposure temperature. This increase is not only caused by dehydration but also by macroscopic air voids introduced during casting or trowelling.

Proper compaction, casting technique and finishing quality are therefore essential. Trowelled materials may contain slightly more air than cast and vibrated materials. Nevertheless, with correct processing, the total air content after casting or trowelling should remain below approximately 5 %.

Approximate increase in open porosity:

Exposure temperature	Approximate increase in open porosity
300 °C	approx. 2–3 %
600 °C	approx. 12–13 %
900 °C	approx. 15 %
1200 °C	approx. 16 %

The increase in porosity is a normal consequence of dehydration and thermal exposure, but it must be considered when evaluating long-term durability, erosion behaviour and thermal cycling resistance.

Practical Drying and Heating Recommendations

For safe use of WPE-DK UHPC High Temperature materials, the following technical principles should be applied:

The first heating cycle must be controlled and should not begin before the material has reached sufficient setting and early strength. As a practical reference, the heating process may start approximately five hours after final setting, but the actual setting time depends strongly on curing temperature.

At low curing temperatures, setting can be significantly delayed. For example, setting at 5 °C can take more than twice as long as at 20 °C. This must be considered before any thermal conditioning begins.

For compact components thicker than 50 mm, the drying cycle should be slower and more conservative. Thin layers below 50 mm can generally be dried faster, because moisture has a shorter path to the surface.

The heating rate should normally remain within the controlled range of 100–200 °C/h during the first dehydration process. Higher heating rates may be possible in selected cases, but they increase the safety risk and should only be used after technical verification.

Technical Conclusion

WPE-DK UHPC High Temperature materials retain significant mechanical performance after exposure to elevated temperatures up to 1200 °C. Compressive and flexural strengths decrease with increasing temperature, but the material remains structurally coherent and technically usable after thermal exposure. The main thermal effect is the weakening of the binder matrix and an increase in open porosity, while the aggregates remain largely stable.

The heating rate between 100 °C/h and 200 °C/h does not significantly reduce mechanical strength within the investigated range. However, the first dehydration cycle must be carefully controlled to prevent water vapour pressure build-up. This is especially important for compact components and layers thicker than 50 mm.

A correctly defined drying procedure is therefore essential for the safe and reliable use of WPE-DK UHPC High Temperature systems. When properly cured, dried and heated according to component geometry, WPE-DK UHPC provides a dense, mechanically strong and thermally resistant mineral solution for demanding high-temperature industrial applications.