Enhancing the Shelf Life of Cement

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 28 2024

A recent article published in Construction Materials proposed novel strategies to mitigate cement prehydration using alkyl ketene dimer (AKD) wax and a mix of AKD and paraffin (AKD+PAR) wax. This method was compared to the conventional pretreatment using oleic acid.

Background

Portland cement's shelf life can be compromised by prehydration during storage. To address this, cement is often pretreated with a protective coating to slow down the prehydration process and reduce losses in concrete strength. One common pretreatment involves oleic acid, which has shown promise in delaying prehydration. However, concrete produced with oleic acid-treated cement typically exhibits decreased strength due to increased entrained air content.

As an alternative, super-plasticizing or retarding admixtures are added to the concrete mix when using prehydrated cement to counteract prehydration issues. These admixtures have proven effective, particularly with calcium aluminate/sulfoaluminate cement, which are highly prone to prehydration.

This study explores the use of hydrophobic wax pretreatments, specifically AKD and a combination of AKD and PAR, to enhance the shelf life of cement. The performance of these hydrophobic pretreatments is compared with the traditional oleic acid-based approach.

Methods

A standard HE (high early) type cement was subjected to hydrophobic treatment using additives such as AKD, AKD combined with PAR, and oleic acid. The treatment process involved mixing the cement with these additives in an attrition mill for four minutes. Both treated and untreated (control) samples of cement underwent accelerated aging to simulate conditions of equilibrium temperature and relative humidity. Sampling occurred initially (pre-aging) and at intervals of 1, 2, 4, 8, and 12 weeks post-aging.

To analyze the microstructures, the samples were prepared on carbon-coated stubs and examined using a field emission scanning electron microscope (FESEM). Additionally, the particle size distribution of the aged cement samples was measured through laser diffraction, with sonication applied to disperse the particles effectively.

To evaluate the performance of both fresh and hardened mortar, tests were conducted to measure flow, air content, and compressive strength following the guidelines of the American Society for Testing and Materials (ASTM). The mortar mixtures consisted of cement, sand, and a polycarboxylate superplasticizer powder mixed in tap water to achieve a water-to-cement ratio (w/c) of 0.38 by mass. This superplasticizer was critical to maintaining workability at a relatively low w/c ratio.

Compression strength tests were carried out on 50 mm cubic specimens, which were stored in a lime-saturated solution after varying curing periods of 7, 28, and 90 days. Additionally, isothermal calorimetry tests were conducted to assess the heat evolution in both cement pastes and mortar samples without any repetition in the testing procedure.

Results and Discussion

Both the AKD and AKD+PAR hydrophobic additives effectively delayed prehydration under warm and humid storage conditions. The FESEM studies demonstrated cement particle agglomeration (greater in treated cement than in control) even before prehydration due to the hydrophobic additives. Though sonication tends to reduce agglomeration, particle sizes for all the cement increased with aging under constant sonication.

At two weeks of aging, the AKD+PAR-treated cement was least affected by prehydration. However, at four and eight weeks, all treated samples exhibited abundant hydration products (but lesser than the control) due to the binding of neighboring cement particles with hydration phases. After 12 weeks of accelerated aging, the control cement exhibited the largest particle sizes.

Paste calorimetry results demonstrated a decrease in the main hydration peak with aging for all samples. The largest decrease was observed in the control cement paste, which also experienced consistently lower cumulative heats of hydration as compared to the treated cement. This was attributed to their slightly finer particle size distributions. Without aging, the particle size distributions for the AKD and AKD+PAR-treated cement were slightly finer than the control, while oleic acid-treated cement was coarser.

Contrasting the pastes, mortars made with aged cement demonstrated delayed hydration due to the significantly lower water-to-cement ratio (w/c) of 0.38 and the addition of a superplasticizer. While the flow of these mortars remained relatively stable up to two weeks of aging, it decreased dramatically after four weeks, though this was less pronounced in mortars containing AKD+PAR.

Further analysis of the properties of hardened mortars showed a decline in strength with cement aging, particularly for mortars made with control cement. Mortars made with cement treated with oleic acid and AKD+PAR exhibited lower strength after seven days compared to those using control cement, which was attributed to increased air content and a tendency for agglomeration due to prehydration.

Based on strength degradation observed by other researchers, the study approximated equivalent storage times at ambient conditions in terms of accelerated aging. For instance, eight weeks of accelerated aging in this study was considered equivalent to five to ten years of storage under normal conditions.

Conclusion

Overall, the AKD and AKD+PAR-treated cement exhibited enhanced resistance to prehydration reactions under elevated temperature and humidity conditions. Additionally, the strength of mortars using AKD+PAR-treated cement with accelerated aging up to 12 weeks cement was higher than the mortars made with oleic acid-treated or untreated cement.

The researchers also demonstrated the feasibility of using AKD and AKD+PAR as grinding aids during clinker milling. The size distributions for milled clinker yielded by both treatments were equivalent to those produced using a commercial grinding aid, but they were expensive. However, such hydrophobic wax pretreatments for cement are appropriate for regions like Indonesia and India with high temperatures and humidity.

Journal Reference

Ozersky, A., Khomyakov, A., Zhao, P., Herzog Bromerchenkel, L., Chernoloz, O., & Peterson, K. (2024). New Mitigation Strategies for Cement Prehydration. Construction Materials, 4(2), 444–467. https://doi.org/10.3390/constrmater4020024, https://www.mdpi.com/2673-7108/4/2/24

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