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.
Amorphous silicas of high specific surface area (SSA), such as nanosilica (NS), silica fume (SF) and pyrogenic silica (PS), are a very reactive pozzolans that can be produced at industrial scale or obtained as by-products from several industrial processes (
Blending the aforementioned pozzolans in Portland cement pastes, mortars, and concretes has been linked to positive effects such as acceleration on hydration rate, refinement of pore structure, enhancement of compressive strength, and increasing the durability (
Besides high SSA, morphology of the silica particles plays an important role in determining some fresh state properties of the matrix such as water and superplasticizer demands, setting time and hydration kinetics (
It has been reported that NS increases the water demand of cement paste, worsening its workability in some addition ranges (
The materials used in this study were: Portland cement type III produced by Argos Colombia; an aqueous dispersion of nanosilica particles produced by BASF Chemicals (NS); powdered pyrogenic silica produced by Glassven (PS); and powdered silica fume produced by BASF Chemicals (SF).
Physical and chemical characterizations of the raw materials were performed to obtain their chemical composition, particle size distribution, particle morphology and specific surface area. Chemical composition was obtained by X-Ray fluorescence using an ARL 8680s Total Cement Analyzer equipment, following the ASTM C114-03 standard in B4Li2O7 pills. Specific surface area (SSA) was obtained in a Micrometrics Gemini V 2380 surface area analyzer, using N2 adsorption and the BET method. Particle size of SF and PS was measured by dynamic light scattering (DLS), using a Mastersizer equipment from Malvern. Particle size of NS was measured by laser diffraction using a Zetasizer equipment from Malvern.
Morphology of the particles was characterized using Transmission Electron Microscopy (TEM) images acquired in a TECNAI 20 Twin microscope from FEI, using an acceleration of 200 kV. Each sample was dispersed in ethanol (C2H5OH) and sonicated for 30 min in an ultrasonic bath prior to imaging. After sonication, an aliquot of the sample was dropped in a Fomvar 300 cupper mesh and coated with carbon. Electrophoretic mobility values were determined using ζ-potential measurements in a Zetasizer nanoZS equipment from Malvern. The pH of the dispersions was varied from neutral to basic using ammonium hydroxide (NH4OH) in deionized water. Each sample was placed in a disposable capillary cell stabilized at 25 °C, ζ-potential was determined 3 times for each pH value.
Cement pastes were produced using 0%, 5% and 10% mass substitutions of cement by silica. Setting time and normal consistency were performed for each paste following ASTM C191 and ASTM C187 standards respectively. Pastes for rheology were mixed manually using water-to-cementing material (w/cm) ratios of 0.45, 0.50, 0.55 and 0.60. Nomenclature and detailed composition of the studied samples are presented in
Nomenclature and composition by mass of the studied pastes.
Paste | w/cm | % Cement | % Silica |
---|---|---|---|
REF | 0.45, 0.50, 0.55 and 0.60 | 100 | 0 |
5%NS | 95 | 5 | |
10%NS | 90 | 10 | |
5% SF | 95 | 5 | |
10% SF | 90 | 10 | |
5% PS | 95 | 5 | |
10% PS | 90 | 10 |
Physical, chemical and mineralogical characterization results obtained for the three studied silica are presented in
Physical and chemical characteristics of NS, SF and PS.
Parameter | NS | SF | PS |
---|---|---|---|
SiO2 (%) | 93.56 | 92.84 | 87.21 |
Al2O3 (%) | 0.00 | 0.22 | 0.00 |
Fe2O3 (%) | 0.39 | 1.00 | 0.13 |
CaO (%) | 0.22 | 0.46 | 0.13 |
MgO (%) | 0.13 | 0.75 | 0.13 |
Na2O (%) | 0.62 | 0.26 | 1.00 |
K2O (%) | 0.02 | 0.44 | 0.01 |
SO3 (%) | 0.30 | 0.31 | 0.86 |
Cr2O5 (%) | 0.04 | 0.00 | 0.00 |
MnO (%) | 0.01 | 0.03 | 0.01 |
P2O5 (%) | 0.13 | 0.16 | 0.12 |
TiO2 (%) | 0.02 | 0.01 | 0.04 |
Loss on ignition (%) | 4.46 | 3.01 | 10.01 |
Average size - d50 (µm) | 0.098 | 83.7 | 23.6 |
Specific surface area - SSA (m2/g) | 51.4 | 28.0 | 147.9 |
Density - ρ (g/cm3) | 1.12 | 2.06 | 1.98 |
dBET (µm) | 0.104 | 0.021 | 0.104 |
Agglomeration factor - FAG | 0.94 | 1151.84 | 804.64 |
X-Ray diffraction patterns obtained for NS, SF and PS (a.u.: arbitrary units).
TEM images were used to understand the incompatibility between average particle size and surface specific area of SF and PS. One typical image of each silica studied is presented in
TEM images of (a) NS, (b) SF and (c) PS.
A morphological difference was also identified among NS, SF and PS. While NS and SF were found to be made of spherical particles, PS was found to be made of connected chain-like elements, which correspond to silica droplets of pyrogenic origin (
The tendency to agglomerate of the three silicas at different pH values was studied through ζ-potential measurements. The obtained results are presented in
ζ-potential values for NS, SF and PS at different pH values.
NS, SF and PS present a clear tendency to decrease (absolute value) their ζ-potential with the increase of pH but without reaching the isoelectric point. NS had the lowest ζ-potential, always in the stable zone for the entire pH range studied. On the other hand, ζ-potential values for PS and SF approached or surpassed -25 mV at pH values of 12 or higher. This indicates that in the typical environment generated by cement hydration, PS and SF can be considered metastable or unstable with a tendency to agglomerate. SF was found to be prone to agglomeration at a pH of 7, this helps to explain why particle size microscopy images showed particle agglomerates rather than individual ones, since both these tests were carried out in deionized water, which has a typical pH of approximately 7.
Water demand and setting time of Portland cement pastes were considered two important properties that can be modified by NS, SF and PS additions and might lead to misinterpretations of the rheology results if not taken into account. Water demand was studied through normal consistency tests, while setting time was studied using Vicat´s needle. Results are presented in
Water demand and setting time of Portland cement pastes blended with NS, SF and PS (5 and 10% solid substitutions).
Paste | w/cm for normal consistency | Initial setting time (min) | Final Setting time (min) | Setting time (min) |
---|---|---|---|---|
REF | 0.28 | 112 | 143 | 31 |
5%NS | 0.37 | 131 | 160 | 29 |
10%NS | 0.42 | 133 | 160 | 27 |
5% SF | 0.30 | 149 | 207 | 58 |
10% SF | 0.32 | 154 | 204 | 50 |
5% PS | 0.44 | 142 | 233 | 91 |
10% PS | 0.56 | 125 | 241 | 116 |
Regarding setting time, for all silica studied, the initial and final setting times increased when compared to the reference paste, this as a direct consequence of the increase of w/cm (being cm the cementing material, in this study the sum of cement and silica), necessary to attain normal consistency. Additionally, when comparing the 5% and 10% solid substitutions of NS and SF, it can be seen that higher amounts of silica led to a decrease in setting time: this is associated with nucleation effects activity (
Rheological experiments were carried out in cement pastes following the proportions presented in
(a) Typical flow curve obtained for the studied pasted showing a Bingham model fitted to the linear descending portion of the curve, (b) typical instantaneous viscosity curve obtained for the studied pastes.
All the plastic viscosity (
Plastic viscosity (
Yield stress from the Bingham model (
μ0 versus τ0 plot for all pastes studied in this work. (Solid line: effect of water content).
NS and SF were found to decrease
Regarding yield stress, NS and SF were found to have from little to no impact over
To obtain a general visualization of the effect of the studied silicas over the rheological behavior of cement paste,
The solid fraction of each paste studied was computed by dividing the volume of solids (cement + silica) by the total volume of paste. The obtained results are presented in
Solid volume fraction results of Portland cement pastes blended with NS, SF and PS.
To better visualize the effect of the solid fraction modifications on the rheological behavior of the studied pastes,
Plastic viscosity (
Yield stress from the Bingham model (
Three main factors were identified as relevant to understand the behaviors shown in
Water demand: while water demand results were found to be proportional to the SSA and amount of silica blended in each paste, as expected (
“Ball-bearing” effect and solid fraction: NS and SF, which present similar trends towards lower
ξ-potential: it is known that during the first minutes of hydration, pH of the liquid phase elevates rapidly (
Experimental results obtained in this work allow concluding that:
NS and SF particles decrease the viscosity of the paste while having little to none effect on the yield stress. This can be related to the presence of a “ball-bearing” effect induced by the particles and to their lower density compared to cement, which increases the solid volume fraction of the granular matrix.
PS particles increase both viscosity and yield stress of the paste due to their porous structure capable of absorbing an important portion of the mixing water.
Regardless of the high water demand of the studied silicas, the rheological behavior of pastes containing NS and SF is mainly governed by the geometry and dispersion degree of the spherical silica particles in the matrix, while the behavior of pastes containing PS is governed by the ability of the silica to absorb a portion of the mixing water in its porous structure.