The compressive strength, flexural strength, porosity and electrical resistivity properties of cement mortars with nano-Fe2O3 and nano-SiO2 are studied. Amorphous silica is the main component of pozzolanic materials due to its reaction with calcium hydroxide formed from calcium silicate (C3S and C2S) hydration. The pozzolanic reaction rate is not only proportional to the amount of amorphous silica but also to the surface area available for reaction. Subsequently, fine nano-Fe2O3 and nano-SiO2 particles in mortars are expected to improve mortar performance. The experimental results showed that the compressive strength of mortars with nano-Fe2O3 and nano-SiO2 particles were lower than those obtained with the reference mortar at seven and 28 days. It was shown that the nano-particles were not able to enhance mechanical strength on every occasion. The continuous microstructural progress monitored by mercury intrusion porosimetry (MIP) measurements, pore-size distribution (PSD), total porosity and critical pore diameter also confirmed such results.
The larger specific surface of new pozzolanic materials (
Nano-SiO2 in cement-based materials usually improves mechanical strength (
Nano-oxides with a smaller grain size than silica fume are studied in construction products due to the new potential properties expected. This means that they can provide potential new applications when nano scale-size particles are added to cement-based materials. Thus, this provides a new manner to design alternative cement-based materials with improved properties in comparison with conventional grain-size ones. Relatively little published research deals with the combination of nano-Fe2O3 and nano-SiO2 in cementitious building materials (
This paper examines the possible influence of a dispersion and curing procedure as a key factor in designing these new products based on nano-SiO2 and nano-Fe2O3. Since pozzolanic and filling effects are affected by nano-oxide content, variations with nano-SiO2 and nano-Fe2O3 (from 2.35% to 6% and from 2% to 4%, respectively) were also analysed. In essence, the paper studies the influence of the combination of nano-Fe2O3 and nano-SiO2 on cement mortars prepared and cured in common conditions frequently found in practice.
The standard mortar composition involves use of the sulphate-resistant Portland cement CEM I 42.5R-SR 3 as a binder, a siliceous (more than 98% SiO2) standard CEN sand according to the European Standard EN 196-1:2016 (NORMASAND) (
Chemical composition and physical properties of the cement
Chemical composition | (%) | Physical properties of cement | |
---|---|---|---|
SiO2 | 19.30 | Specific gravity (kg/m3) | 3.10 |
Al2O3 | 3.42 | Initial setting time (min) | 173 |
Fe2O3 | 4.13 | Final setting time (min) | 252 |
CaO | 67.26 | Volume expansion (mm) | 0.68 |
MgO | 1.04 | Specific surface area (SSA), Blaine (m2/kg) | 4116 |
SO3 | 2.91 | 25 µm residue (%) | 35.8 |
K2O | 0.32 | 32 µm residue (%) | 24.5 |
Na2O | 0.16 | 63 µm residue (%) | 3.1 |
P2O5 | 0.10 | Hydration heat (J/g) | 325 |
LOI | 3.70 | Compressive strength | (MPa) |
Soluble residue |
0.49 | 2 days | 30.1 |
CI- | 0.019 | 28 days | 61.0 |
Na2CO3 method.
Technical data of the commercial nano-Fe2O3, nano-SiO2 and high-range water-reducing admixture
Commercial product | Nano-SiO2 | Nano-Fe2O3 | High-range water-reducing admixture - SIKA Viscocrete 5720 |
---|---|---|---|
Purity (%) | 40.54 | 99.721 | 40.54 |
Density (g/cm3) | 1.295 | - | 1.090 |
pH (20ºC) | 10.3 | - | 4 |
Superficial area (m2/g) | 205 | 60-120 | - |
Viscosity (m•Pa•s) | 9.21 | - | - |
The particle size of the commercial nano-SiO2 Levasil was 10-20 nm (this value was supplied by the producer). The specific surface area of the commercial Iron III oxide nano-particles (nano-Fe2O3) was 55.9 m2/g, and the mean particle size 1.95 µm, with both being experimentally measured.
Prior to testing the mortars, the mix design shown in
Mortar mixes
Mix code | M0 | M3.5Si2Fe | M2.35Si4Fe | M6Si2Fe | M4Si4Fe |
---|---|---|---|---|---|
Nano-SiO2 (%) |
0 | 3.5 | 2.35 | 6 | 4 |
Nano-Fe2O3 (%) |
0 | 2 | 4 | 2 | 4 |
Sand(g) | 1350 | 1350 | 1350 | 1350 | 1350 |
Water (g) | 225 | 202.0 | 209.5 | 185.5 | 198.7 |
Cement (g) | 450 | 450 | 450 | 450 | 450 |
Nano-Fe2O3 (g) | 0 | 9 | 18 | 9 | 18 |
Nano-SiO2 (g) | 0 | 38.8 | 26.0 | 66.5 | 44.3 |
High-range water-reducing admixture (g) | 0 | 1 | 0 | 2 | 1 |
% weight of cement
All the mortar mixes had the same water/cement ratio of 0.5. The water provided by the Nano-SiO2 solution was deducted from the mixing water of the mortar. In addition, a high-range water-reducing admixture (HRWRA) was adopted in order to obtain similar slump in all mixes.
Flexural and compressive strength tests were performed in 40x40x160mm mortars at two, seven and 28 days according to the European Standard EN 196-1:2016 (
The curing process of the specimens was carried out according to the European Standard EN 196-1:2016. This standard method allows results to be compared under the same curing conditions for all the specimens. In accordance with Sajedi and Razak, (
Flexural strength was measured on three specimens for each mortar mix. The six samples obtained after the flexural strength testing were used for compressive strength testing. Flexural strength values were calculated according to the following equation [
where Rf is flexural strength (MPa), Ff is load applied in the middle of the specimen (N), l is side of the prism (mm) and b is distance between the two steel supporting rollers (mm).
A mortar porous system (with total open porosity and pore-size distribution) was studied at 28 days by means of mercury intrusion porosimetry (MIP) in "Phi mayúscula"12x40 mm cylindrical samples, in a range of pore radius between 0.005 and 180 μm following an internal procedure based on the ASTM D4404-04. The samples were oven-dried at 40±5°C for four days and then analysed with use of a Micromeritics AutoPore IV 9599 porosimeter.
The porosity of a material affects its physical properties and, subsequently, the mechanical strength and durability performance. In particular, the total porosity and pore-size distribution (PSD) give information regarding the open porosity. Therefore, it could serve as an indirect indicator of the permeability of the sample in certain cases. In essence, porosity measurement is a suitable tool in enabling an understanding of microstructure evolution and potential use of nano-oxide-made mortars.
By measuring the volume of mercury that intrudes into the sample material with each pressure change, the volume of pores in the corresponding size interval is obtained. Then, the total porosity can be calculated according to equation [
where Pt is the total porosity (%), Vp is the porous volume (mm3) and Vm is the sample volume (mm3).
Resistivity is a non-destructive test method described extensively in the Spanish Standard UNE 83988-1 (
Two-electrode testing on mortar prism.
where R is electrical resistance, ρ is electrical resistivity, V is potential difference, I is current intensity, l is steel plate distance and A is area (
Electrical resistivity measurements give information about the open porosity of the mortar and concrete. The Nernst-Einstein equation expresses the relationship between the electrical resistivity and ion diffusivity for porous materials (
where Di is diffusivity for ion i, ρ is electrical resistivity and k is a constant value obtained from the slope of the linear correlation between the ion diffusivity and electrical conductivity, which is the inverse parameter of the electrical resistivity (
Relationship between electrical resistivity and ion diffusivity according to ASTM C1202 (
Chloride diffusivity | Charge (Coulomb)ASTM C1202 | Electrical resistivity (Ω.m)UNE 83988-1 |
---|---|---|
High | 4000 | <50 |
Moderate | 2000 a 4000 | 50 a 100 |
Low | 1000 a 2000 | 100 a 200 |
Very low | 100 a 1000 | 200 a 2000 |
Insignificant | < 100 | >2000 |
A clear relationship between corrosion rate and electrical resistivity can be found in most of the cement-based materials (
The results obtained for flexure and compressive strength at two, seven and 28 days, and their standard deviation in MPa, are shown in
Flexure strength results and standard deviation (MPa).
Compressive strength results and standard deviation (MPa).
All the mortars made with nano-oxides showed compressive strength results below the reference mortar made without additions. These results differ from others reported elsewhere (
Nano-SiO2 performance has customarily been reported as increasing the compressive strength of mortars (
According to Sajedi and Razak (
The earlier mentioned MIP was performed at 28 days and the results are shown in
Critical diameter of the mortars at 28 days
Mix code | M0 | M3.5Si2Fe | M2.35Si4Fe | M6Si2Fe | M4Si4Fe |
---|---|---|---|---|---|
Critical diameter (μm) at 28 days | 0.063 | 0.183 | 0.151 | 0.834 | 1.054 |
Differential mercury intrusion porosimetry (MIP) at 28 days.
Mortar M4Si4Fe had the highest critical diameter (1.054μm), followed by mortar M6Si2Fe (0.834μm). This value decreased sharply to 0.183μm for the mortar with 3.5% nano-SiO2 (M3.5Si2Fe) and to 0.151μm for the mortar with 2.35% nano-SiO2 (M2.35Si4Fe). Contrary to what was expected, the reference mortar presented the lowest critical diameter (0.063). The curves could be classified in three main groups: with about 8%, 6% or 0% of nano-oxides. The curves moved from the left to the right when nano-oxides were added, showing a pore-size increase. These results were in line with the compressive and flexural strength ones.
In agreement with the mechanical results, critical mean diameter pores determined by MIP could be ordered from the higher to lower size as follows:
M0 < M2.35Si4Fe < M3.5Si2Fe < M6Si2Fe < M4Si4Fe
Often nano-additions, as well as other additions, act by refilling the open spaces between the aggregates and cement paste. This improves the quality of the transition zone. They are located in the capillary pores and refine pore-size distribution (
Whereas
Total porosity (%) of the mortars at 28 days.
Pore-size distribution (PSD) of the mortars at 28 days.
As a result, in the PSD analyses of mortars (
In theory, nano-particles are uniformly dispersed in mortar and when hydration begins, hydrate products diffuse and cover nano-particles (
Other researchers (
Resistivity measurements have recently been used to assess the quality of cement-based materials (
Electrical resistivity of the mortars.
Whereas the first group included M2.35Si4Fe and M3.5Si2Fe and exhibited electrical resistivity of around 100 Ω.m, the second one reached values below 350 Ω.m. Consequently, these last mortars will perform better in aggressive environments than the first ones (
Summarising, electrical resistivity measurements were recorded in order to obtain an indirect durability indicator of the mortars. In this case, as the amount of nano-oxides in the mortar increased, a greater degree of durability was found.
According to Andrade and D’Andrea (
The positive expected effect in compressive and flexural strength, in addition to durability, found when nano-SiO2 or nano-Fe2O3 are added to Portland cement mortars, can be reversed if the curing conditions and/or the nano-addition dispersing procedure are inefficient.
When the nano-Fe2O3 content is kept constant, the nano-SiO2 increase leads to a compressive strength decrease for all the ages. In such a case, an increase of pores smaller than 50 nm will have also been observed.
The authors gratefully acknowledge the financial support provided by Ministry of Economy and Competitiveness of Spain by means of the Research Fund Project BIA 2016-78742-C2-2-R.