Effect of different high surface area silicas on the rheology of cement paste

J I Tobón,O Mendoza,M V Borrachero,J Payá,O J Restrepo

doi:10.3989/mc.2020.15719

J I Tobón, O Mendoza + Show 3 more

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https://doi.org/10.3989/mc.2020.15719

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Abstract

This work studies the effect of nanosilica (NS) on the rheology of cement paste by comparing it with two high specific surface area silicas: silica fume (SF) and pyrogenic silica (PS). Portland cement pastes were produced with different water-to-cementing material ratios and different solid substitutions of cement by silica. Water demand, setting time, and rheology tests were performed. Results showed that NS and SF decreased plastic viscosity, while PS increased it. Only PS was found to have an effect on yield stress. NS showed the most decreasing effect on viscosity, regardless of its higher water demand. It was concluded that the behavior of pastes containing NS and SF is governed by the “ball-bearing” effect from silica particles, by their agglomeration degree, and their impact on the solid volume fraction. The behavior of pastes containing PS is governed by its ability to absorb a portion of the mixing water.

Highlights

This work studies the effect of nanosilica (NS) on the rheology of cement paste by comparing it with two high specific surface area silicas: silica fume (SF) and pyrogenic silica (PS)
It was confirmed that NS, SF and PS are mainly composed of amorphous silica, finding only minor crystalline minerals in SF probably from its production process
Regarding their surface area and average particle size, high specific surface area (SSA) values were found for all silica samples, while NS presented a d50 in the scale on nanometers and SF and PS presented a d50 in the scale of micrometers

Summary

Introduction

This work studies the effect of nanosilica (NS) on the rheology of cement paste by comparing it with two high specific surface area silicas: silica fume (SF) and pyrogenic silica (PS). It was concluded that the behavior of pastes containing NS and SF is governed by the “ball-bearing” effect from silica particles, by their agglomeration degree, and their impact on the solid volume fraction. RESUMEN: Efecto de diferentes sílices de alta área superficial sobre la reología de pastas de cemento. SF particles, which are spherical, exist from the nanometric to the micrometric scale usually as agglomerates They are a condensate byproduct of high temperature processes such as the production of metallic silicon or iron-silicon alloys [3]. While the porous structure of PS particles is capable of absorbing a portion of the mixing water, worsening workability, the spherical nature of SF creates a “ball-bearing” effect that decreases the friction between particles moving past each other more [9,10,11]. The “ball-bearing” effect is not exclusive of silica particles, other spherical components, such as entrapped air bubbles [12, 13], are capable of inducing it

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