The Effect of Superplasticisers and Silica Fume on Cement Rheology

By Skyla BailyJun 14 2022

Studies of the rheology of fresh cement paste can aid in the development of novel concrete products with high water reduction, improved workability, and improved mechanical properties. The effect of silica fume on the rheology of cementitious paste over time in the combination of two kinds of superplasticizers was investigated in a study in the journal Materials Today: Proceedings. 

Study: Effect of silica fume on cement rheology properties in presence of superplasticisers. Image Credit: touch1976/Shutterstock.com

The use of polycarboxylate superplasticizers in cement with mineral addition might cause incompatibilities owing to cement-admixture and addition-admixture interactions in most circumstances. This can considerably enhance the paste’s viscosity and thixotropy, making it difficult to utilize.

The huge surface area of SF cement absorbs a lot of water and makes it difficult to work with. Superplasticizers must be employed in SFcement pastes to keep the cementitious material’s characteristics over time.

Methodology

Portland cement with a specific surface of 3250 m²/kg and a mineralogical composition of 53.10% C3S, 22.60% C2S, 10.10% C3A, and 8.29% C4AF was utilized in this project.

This research looked at two distinct superplasticizers. SIKA VISCOCRETETEMPO 12 (also known as PA) is a superplasticizer acrylic copolymer, while SIKA VISCOCRETE 3045 (denoted as PC) is a modified polycarboxylate. Sika Algeria Company provided the superplasticizers. 

Using a Haake Rheowin AR 2000 rheometer, rheological measurements were taken on paste specimens made up of mixed cement containing 2, 4, 6, 8, and 10% silica fume. The period between the start of the rheological measurement and the end of the paste mixing was 10 minutes. 

As a reference, the pastes without SF (C-PA and C-PC) were employed. The rheology of the pastes was investigated using the evolution of shear stress vs shear rate. The shear rate was increased by 0.33 Pa/s up to 500 s^–1 using the rheometer. The period between the start of the rheological measurement and the end of the paste mixing was 10 minutes.

The thixotropic test was carried out on a paste specimen containing 6% silica fume for 10, 40, and 70 minutes. The test was carried out by applying a variety of shear rates to the paste cement at rest using the same rheometer.

The thixotropy was assessed using a hysteresis loop test, in which the shear rate was raised from 0 to 500 s^–1 and then lowered to zero, and the up-curve and down-curve created a hysteresis loop.

Results

The development of shear stress vs. shear rate was investigated using ordinary Portland cement pastes containing 2, 4, 6, 8, and 10 wt% silica fumes. The Herschel-Bulkley (H-B) model might be used to fit the curves.

The flow resistance lowers with increasing SF concentration in the presence of a superplasticizer. However, the paste with PA additive had a greater viscosity rise than the paste with PC. PC has a far better dispersing capacity than PA, as seen by the low consistency found.

Many experts agreed with the benefits of the Herschel-Bulkley rheological model employed in this work and recommended utilizing it to describe the rheology of low E/C concrete.

The hysteresis loops govern the thixotropy of cement pastes. The area of the hysteresis loop in A6-admixture pastes diminishes over time, whereas it grows in B6-admixture pastes.

The use of a PC-admixture in C-SF paste was shown to be more thixotropic than the use of a PA-admixture. Given the high activity of organic admixtures that evolved over time, the thixotropy value fell.

After the paste has been set at rest, the microstructural network will begin to form again, and the internal structure will be diminished.

The thixotropy of cement materials is formed by the cement particle flocculation/coagulation, dispersion, and hydration. If the cement paste contains 6% silica fume particles and is made with acrylic copolymer superplasticizer (PA), the particles tend to rejoin and congeal.

A high thixotropy number implies that this material is difficult to work with, whereas a low thixotropy value causes segregation, which is bad for material quality. It is critical to take the material’s thixotropy into account.

Conclusions

The team drew some results from an experimental inquiry into the rheology of cement paste with varying substitution concentrations of silica fume (2% to 10%) and in the presence of two types of superplasticizers.

The availability of silica fume addition conditions the additives’ fluidizing impact on cement pastes; the rheology is significantly altered, and the consistency index and rate index both increased significantly.

Shear-thinning behavior was seen in the presence of SF, with a low rate index (n < 0.45). Furthermore, the results revealed that, despite its high yield stress, the PC-admixture has a greater influence on C-SF paste fluidity than the PA-admixture.

At 70 minutes, the cement paste containing PC-admixtures had a modest surface value. As a result, cementitious paste with PC adjuvant can be applied at 10 and 40 minutes to help with material placement. To eliminate the problem of segregation, it is advisable to use the cement paste with PA additive at 70 minutes.

Journal Reference:

Chalah, K., Mahdad, M., Benmounah, A., Kheribet, R. and Akouche, A. (2022) Effect of silica fume on cement rheology properties in presence of superplasticisers. Materials Today: Proceedings. Volume 58, Part 4 Available Online: https://www.sciencedirect.com/science/article/pii/S2214785322005703

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