In this paper, the influence of additions of nanosilica (nSi) and microsilica (mSi) on the behaviour of binary and ternary mixtures in chloride environments is studied. The main objective is to obtain high-performance self-compacting concrete (HPSCC) with a high durability which can meet specific demands in such aggressive environments. Ten blends were manufactured using Portland cement (CEM I 52.5 R) and additions of nSi and mSi in binary and ternary mixtures. The results of three tests frequently used to evaluate resistance to chloride penetration– electrical resistivity, migration and chloride diffusion –were studied and compared. Both binary and ternary mixtures presented significant improvements in chloride resistance, generally in proportion to the total content of the addition. In all the ternary mixtures, high resistivity is obtained, which indicates that such mixtures have a notably low chloride penetrability. Furthermore, these mixtures provided extremely low chloride diffusion coefficients even at small addition ratios.
Many studies have been carried out on self-compacting concrete (SCC) (
The advantageous workability and cohesion of SCC are mainly due to a high content in fines, a reduction in the content and size of coarse aggregates and the action of superplasticizer additives that provide the necessary flowability for casting work (
Significant strides have been made in the last two decades in improving the mechanical and resistant performance of concrete, particularly its compressive strength. Therefore, high- and ultra-high-strength concretes are relatively easily obtained now. However, making the mixture self-compacting and producing high-durability concrete for applications that require a service life of over 100 years is considerably more difficult. The main weakness in concrete durability involves the connectivity and size of the porous network, which determines the ingress of external aggressive substances that can affect the cement matrix and the steel embedded in the structural concrete. Prominent among these aggressive agents are chlorides. When chlorides reach the steel, they cause depassivation and corrosion, two frequent mechanisms of degradation in the structural elements of reinforced concrete. Therefore, when the microstructure of cement-based materials becomes denser and has a refined porous network, it becomes more resistant to aggressive agents in general and chlorides in particular.
Conventionally, pozzolanic additions have been used to densify, reduce and refine the porous structure, due to the combined effect of the pozzolanic reactions and the filling effect associated with the small size of the material. Among the most interesting pozzolanic additions is microsilica (mSi), whose incorporation in cement-based mixtures has resulted in sound performance (
HPSCC seems to be a promising material for many applications and structures. However, its performance must be studied before it is widely adopted in construction. Also, the behaviour of structural elements made with HPSCC has to be more thoroughly understood, and design provisions in step with the latest advances are needed. This paper forms part of a larger research effort into the influence of binary and ternary mixtures of mSi and nSi in different proportions on the properties of fresh- and hardened-state HPSCC (
In this study, 10 blends were designed (
Properties of Portland cement (PC) and mineral additions (nSi, mSi).
SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 | K2O | Na2O | Loss on ignition (%) | Density (g·cm−3) | Specific surface area (m2g−1) | |
---|---|---|---|---|---|---|---|---|---|---|---|
PC | 19.20 | 6.07 | 1.70 | 63.41 | 2.56 | 3.38 | 0.2 | 0.33 | 2.09 | 3.5 | 0.42 |
nSi | 99.90 | - | - | - | - | - | - | - | 0.10 | 1.29 | 200 |
mSi | 94 | - | - | - | - | - | - | - | - | 0.7 | 30 |
The aggregates and proportioning used in the mixtures were as follows: 1160 kg/m3 of siliceous sand (termed A) measuring less than 4 mm in diameter, with a fineness modulus of 3.30; 585 kg/m3 of rolled gravel (G) measuring 6 mm to 12 mm in diameter; and 100 kg/m3 of limestone filler (LF) with a granulometry in accordance with UNE 12620:2003+A1:2009 (
Additionally, two chemical additives were used: SIKA ViscoCrete® 5720, based on polycarboxylate polymers (solids content 36%, density 1.09 kg/l), as superplasticizer (SP) and SIKA Stabilizer® 4R (solids content 3–3.5%, density 1.03 kg/l) as stabilizer (MV), which controls concrete viscosity with a constant amount in mixtures of 0.15% by weight of cement (wt.%).
Ten HPSCC dosages were designed. In order to obtain reference values to compare the results, an HPSCC without mineral additions was prepared (CEM I 52.5 R as the sole cementitious material), identified as HAC. Three dosages were made with 2.5%, 5% and 7.5% nSi, which were respectively identified as HAC[nSi]-2.5, HAC[nSi]-5 and HAC[nSi]-7.5. Three more were made with 2.5%, 5% and 7.5% mSi, which were identified as HAC[mSi]-2.5, HAC[mSi]-5 and HAC[mSi]-7.5. Lastly, the remaining three dosages were made by using admixtures of both nSi and mSi (ternary mixtures) in percentages, respectively, of 2.5%/2.5%, 5%/2.5% and 2.5%/5% (identified as HAC[nmSi]-2.5/2.5, HAC[nmSi]-5/2.5 and HAC[nmSi]-2.5/5).
HPSCC mixture proportions.
Material (kg/m3) (%) | HAC | HAC | HAC[nSi]-5 | HAC[nSi]-7.5 | HAC[mSi]-2.5 | HAC[mSi]-5 | HAC[mSi]-7.5 | HAC[nmSi]-2.5/2.5 | HAC[nmSi]-5/2.5 | HAC[nmSi]-2.5/5 |
---|---|---|---|---|---|---|---|---|---|---|
nSi | - | 11.25 | 22.5 | 33.75 | - | - | - | 11.25 | 22.5 | 11.25 |
2.5% | 5% | 7.5% | 2.5% | 5% | 2.5% | |||||
mSi | - | - | - | - | 11.25 | 22.5 | 33.75 | 11.25 | 11.25 | 22.5 |
2.5% | 5% | 7.5% | 2.5% | 2.5% | 5% | |||||
Water | 162 | 166 | 170.1 | 174.1 | 166 | 170.1 | 174.1 | 170.1 | 174.1 | 174.1 |
SP (%) | 2 | 3.30 | 4 | 6 | 2.30 | 2.50 | 2.70 | 3.60 | 4.80 | 3.90 |
The tests performed on the blends to determine self-compacting were as follows: slump-flow diameter [df (mm)], V‑funnel flow time [TV (s)] and L-box height ratio [Cbl] (
Results of self-compacting and compressive strength tests.
Self-compacting tests | HAC | HAC[nSi]-2.5 | HAC[nSi]-5 | HAC[nSi]-7.5 | HAC[mSi]-2.5 | HAC[mSi]-5 | HAC[mSi]-7.5 | HAC[nmSi]-2.5/2.5 | HAC[nmSi]-5/2.5 | HAC[nmSi]-2.5/5 |
---|---|---|---|---|---|---|---|---|---|---|
Slump-flow diameter df (mm) | 650 | 720 | 635 | 565 | 787 | 817 | 795 | 685 | 675 | 752 |
V-funnel flow time TV (s) | 8 | 11 | 13 | 17 | 10 | 7 | 5 | 12 | 12 | 10 |
L-box height ratio Cbl | 0.98 | 0.96 | 0.81 | 0.85 | 0.95 | 0.95 | 1 | 0.89 | 0.97 | 0.97 |
Slump-flow class | SF1 | SF2 | SF1 | SF1 | SF3 | SF3 | SF3 | SF2 | SF2 | SF2 |
Compressive strength (28 days) (MPa) | 62.46 | 63.79 | 65.01 | 68.86 | 65.73 | 68.37 | 70.35 | 82.17 | 71.15 | 69.25 |
Pore-size distribution and total porosity were evaluated by Mercury Intrusion Porosimetry (MIP), according to ASTM D4404-84 (2004), using a Micromeritics Autopore IV 9500 at a maximum pressure of 33,000 psi, with a range of 5 nm to 180 µm.
For electrical resistivity tests, cylindrical moulds 200 mm long and 100 mm in diameter were used to prepare a total of 30 specimens (three specimens per dosage). Electrical resistivity was determined by a non-destructive test carried out under room conditions on samples saturated to constant weight. Mortars were tested at 7, 28 and 365 days to analyse their evolution over time after a 28-day curing process at room temperature (20 ± 2°C) and 95% relative humidity. This test was carried out according to UNE 83988-1 (
where:
ρe: electrical resistivity (Ω m)
K: cell constant (m)
Re: electrical resistance (Ω)
The cell constant K is found by
where:
K: cell constant (m)
S: surface area of the specimen through which the electrical charge passes (m2)
L: specimen height (m)
The test entailed non-destructive testing and is an indirect measure of the connectivity and tortuosity parameters of concrete porosity, which have a significant influence on resistance against chloride penetration. The electrical resistivity value is related to chloride penetrability through the determination of the electrical charge (coulombs) that passes through a specimen in a determined time, as in ASTM C1202 (
Quality classification of concrete according to chloride penetrability.
Electrical charge that passes (coulombs) ASTM C1202 | Electrical resistivity (Ω cm) UNE 83988-1 | Chloride penetrability |
---|---|---|
> 4000 | < 5 | High |
2000 to 4000 | 5 to 10 | Moderate |
1000 to 2000 | 10 to 20 | Low |
100 to 1000 | 20 to 200 | Very low |
< 100 | > 200 | Negligible |
With the aim of analysing the evolution of the concretes’ resistivity over time, the specimens were tested at the ages of 7, 28 and 365 days.
In order to determine the chloride migration coefficient, two slices 100 mm in diameter and 50 mm in length for each mixture were tested according to the NT BUILD 492 (1999) standard (
where:
Dnssm: non-steady-state migration coefficient (× 10−12 m2/s)
U: absolute value of the voltage applied (V)
T: average temperature of the anode solution between its initial and final value (°C)
L: specimen thickness (mm)
Xd: average depth of chloride penetration (mm)
t: test duration (hours)
The chloride diffusion test was carried out according to the CEN/TS 12390-11 standard (
The test results yielded the surface chloride concentration on the exposed surface (% mass) (Cs) and the apparent diffusion coefficient (m2/s) (Da), obtained by fitting
where:
Ci: initial chloride concentration (% mass)
Once the apparent diffusion coefficient is obtained, the chloride penetration coefficient (KCl) is calculated following
where:
The chloride penetration coefficient enables the corrosion initiation time to be predicted for a given concrete cover thickness according to
where:
The free chloride concentration of the same concrete specimens was measured experimentally following the recommendations of RILEM TC 178-TMC (
Total porosity and percentage of macro- and micropore values for all mixtures.
Total porosity (%) | Macropores (Ø > 50 nm) | Micropores (Ø < 50 nm) | ||
---|---|---|---|---|
9.24 | 24.02 | 75.98 | ||
9.31 | 16.83 | 83.17 | ||
9.91 | 23.49 | 76.51 | ||
8.10 | 13.32 | 86.68 | ||
9.03 | 8.60 | 91.40 | ||
8.34 | 13.05 | 86.95 | ||
9.98 | 8.49 | 91.51 | ||
7.43 | 11.87 | 88.13 | ||
9.00 | 15.31 | 84.69 | ||
7.15 | 21.63 | 78.37 | ||
9.09 | 15.07 | 84.93 | ||
6.97 | 27.03 | 72.97 | ||
8.36 | 18.88 | 81.12 | ||
7.34 | 16.32 | 83.68 | ||
9.18 | 15.96 | 84.04 | ||
7.36 | 21.40 | 78.60 | ||
9.65 | 10.87 | 89.13 | ||
8.61 | 14.08 | 85.92 | ||
8.83 | 9.34 | 90.66 | ||
7.13 | 12.73 | 87.27 |
As can be observed, the results show that the use of nSi and mSi may have a positive effect on improving the microstructure and reducing the internal pore structure at both ages and in all mixtures as compared to the reference mixture (HPSCC). It could be said that nSi causes a reduction in pore diameter, though it is associated with a slight decrease in total porosity. This effect might be explained by the formation of a larger amount of C-S-H gels and gel pores due to the high pozzolanic activity of the nSi addition in regard to its great surface/volume ratio, reducing pore size. In the case of mixtures with mSi, a significant reduction of total porosity is observed (which might be due to a filling effect of the addition), while pore size remains unchanged. When nSi and mSi additions are combined in ternary mixtures, an overlap of both effects is observed. In this case, the behaviour might be explained by an enhanced hydration process (due to the presence of the nSi addition) and the final packing efficiency (related to the continuity of the particle size distribution of the components).
As explained above, in order to measure electrical resistivity, a voltage was applied between two electrodes, and the current transmitted through the water solution within the pores was measured. This method was used mainly to determine chloride penetrability. Polder in 2001 (
The average values of electrical resistivity at 7, 28 and 365 curing days for mixtures with nSi, mSi and ternary mixtures of nmSi are shown, respectively, in
Average values and error bars of electrical resistivity at 7, 28 and 365 curing days for mixtures with nSi additions.
Average values and error bars of electrical resistivity at 7, 28 and 365 curing days for mixtures with mSi additions.
Average values and error bars of electrical resistivity at 7, 28 and 365 curing days for mixtures with ternary mixtures of nmSi.
In general, an increase in electrical resistivity was obtained with time. However, the resistivity values at 365 days of curing for the HAC[nSi]-7.5 and HAC[nmSi]-2.5/5 mixtures were lower than those obtained at 28 days. This may be due to a notably low capillary absorption of the mixtures containing a greater total amount of nanosilica addition, which would result in especially good chloride resistance .
It may also be observed that the addition of nanosilica produced a significant increase in resistivity at seven days of curing: up to 675.3% for the HAC[nSi]-7.5 mixture, in comparison with HAC alone. This result could be of significant importance in applications where low chloride penetrability at an early age is required. The percentages of increment decreased at later ages of curing, and the improvement lessened with a lower content of total addition. Even so, the smallest increase in resistivity was 42% for HAC[nSi]-2.5 at 365 days of curing.
In the case of mixtures with mSi, a significant increase in electrical resistivity from 7 to 28 days of curing was observed. The percentages of these increases in electrical resistivity with respect to the reference range from 53.01% (HAC[mSi]-2.5) to 142.73% (HAC[mSi]-7.5) at seven days of curing and vary from 193.36% (HAC[mSi]-2.5) to 905% (HAC[mSi]-7.5) at 28 days.
Among the ternary mixtures, HAC[nmSi]-5/2.5 presented the highest electrical resistivity, with 925.16 Ωm after 28 days of curing. These concretes, like the binary mixtures with the addition of mSi, experienced the highest increase in their electrical resistivity from 7 to 28 days.
The improvement in resistivity behaviour achieved with nSi at seven days may be attributed to pore refinement, a reduction in the percentage of macropores due to the pozzolanic reaction and the packing effect observed at early ages. Binary mixtures with nSi showed a pore size reduction proportional to the amount of addition. In binary mixtures with mSi, the microstructure produced had a significantly lower total porosity at 28 days due to a slower pozzolanic reaction, reaching resistivity values similar to binary mixtures with nSi at this age. The average pore size of binary mixtures with mSi was similar to that of the reference concrete. Lastly, the ternary mixtures developed a microstructure that is a result of the combination of both effects, exhibiting a small average pore size (proportional to the amount of nSi) and a lower total porosity (proportional to the amount of mSi). This may explain the resistivity results; the best values at both ages came from the ternary mixtures, possibly because of their having a continuous particle size distribution from the smallest particle size (addition) to the largest particle size (coarse aggregate) in the dosage of the mixture.
Classification of chloride penetrability at 7, 28 and 365 days of curing, according to the ASTM C1202 standard.
Mixtures | HAC | HAC[nSi]-2.5 | HAC[nSi]-5 | HAC[nSi]-7.5 | HAC[mSi]-2.5 | HAC[mSi]-5 | HAC[mSi]-7.5 | HAC[nmSi]-2.5/2.5 | HAC[nmSi]-5/2.5 | HAC[nmSi]-2.5/5 |
---|---|---|---|---|---|---|---|---|---|---|
Resistivity (kΩ cm) (7 days) | 6.62 | 19.65 | 24.40 | 51.31 | 10.15 | 10.13 | 16.06 | 23.64 | 29.44 | 45.59 |
Chloride penetrability (7 days) | Moderate | Very low | Very low | Very low | Low | Low | Low | Very low | Very low | Very low |
Resistivity (kΩ cm) (28 days) | 8.33 | 24.30 | 33.00 | 76.96 | 24.43 | 44.03 | 83.70 | 57.52 | 92.52 | 87.40 |
Chloride penetrability (28 days) | Moderate | Very low | Very low | Very low | Very low | Very low | Very low | Very low | Very low | Very low |
Resistivity (kΩ cm) (365 days) | 21.87 | 31.05 | 39.16 | 68.07 | 38.02 | 52.11 | 98.60 | 66.66 | 95.03 | 72.77 |
Chloride penetrability (365 days) | Very low | Very low | Very low | Very low | Very low | Very low | Very low | Very low | Very low | Very low |
Given these results, it is noteworthy that concretes with nanosilica are classified in the same level in both binary and ternary mixtures, providing notably low chloride penetrability from early ages.
It is also important to mention that the classification places all concretes, including the reference concrete, in the same category at 365 days. Therefore, if only this category were considered, the use of different additions would seem irrelevant after one year. This is because the classification establishes a rather wide range for the real values of resistivity in each category and assigns chloride penetrability levels on the sole basis of whether the value lies within this wide range . This should be noted, since concretes that differ by as much as 350% in electrical resistivity (as is the case of HAC[mSi]-7.5 in comparison with the reference concrete) lie within the same category. In addition, this classification does not allow the service life of a concrete to be estimated.
Ion transport in mortar specimens was evaluated though chloride migration tests according to the NT BUILD 492 standard (
Average values and error bars of chloride migration coefficient at 28 curing days for mixtures with nSi compared with HAC.
Average values and error bars of chloride migration coefficient at 28 curing days for mixtures with mSi compared with HAC.
Average values and error bars of chloride migration coefficient at 28 curing days for mixtures with ternary mixtures of nmSi compared with HAC.
As can been observed in
In
To study how the amount of addition influences the chloride migration coefficient, three regression curves have been calculated, one for each type of concrete with additions (see
Regression curves of the influence of addition amount on the chloride migration coefficient.
Given the results, the use of addition provides HPSCC with good resistance against chloride migration, as the published literature has shown (
Other published works have identified relationships between the chloride migration coefficient and electrical resistivity (
Correlation curves between the chloride migration coefficient and electrical resistivity.
This section shows the results of chloride diffusion testing according to the CEN/TS 12390-11 standard (
Chloride concentration profiles of the concretes with a total addition of 2.5% compared with HAC: a) total chlorides, b) free chlorides, c) combined chlorides.
Chloride concentration profiles of the concretes with a total addition of 5% compared with HAC: a) total chlorides, b) free chlorides, c) combined chlorides.
Chloride concentration profiles of the concretes with a total addition of 7.5% compared with HAC: a) total chlorides, b) free chlorides, c) combined chlorides.
The penetration profiles of the concretes with a total addition of 5% are shown in
The case of the concretes with a total addition of 7.5% is shown in
Chloride diffusion coefficient for mixtures with nSi compared with HAC.
Chloride diffusion coefficient for mixtures with mSi compared with HAC.
Chloride diffusion coefficient for mixtures of nmSi compared with HAC.
The study of total, free and combined chloride penetration presented in the previous section might explain why the chloride diffusion coefficient is higher for binary concretes with nSi than for binary concretes with mSi when the percentage of addition is 2.5% or 5%. This might be caused by a greater chloride binding capacity provided by the addition of mSi to concrete. However, this trend changes when the percentage of total addition is 7.5%. Then the concretes exhibit similar behaviour with comparable degrees of total, free and combined chloride concentration, as well as similar chloride diffusion coefficients. This could mean that the behaviour of 7.5% additions does not depend on silica size.
Correlation line between the chloride diffusion coefficient and the chloride migration coefficient for the mixtures.
The figure shows a linear relationship with a fairly good fit, R2 = 0.9571, between the parameters. Therefore, given that the information provided by the migration test follows the same trend as that of the chloride diffusion test, such information could be used in studies of concrete behaviour with respect to chloride penetration. This is important due to the reduction in time, since use of the chloride migration test provides quantitative results in a maximum of five days, whereas with a natural diffusion test 90 days of exposure are necessary before proceeding to the chloride valuation to find the diffusion coefficient.
Correlation curve between resistivity and chloride diffusion coefficient for the mixtures.
These accurate fits might allow relations to be established for inferring values, although the experimental campaign would have to be extended in order to validate the correlations in HPSCC with micro- and nanosilica additions.
Structure service life may be estimated using the model described in EHE 08 (
Estimated chloride penetration by natural diffusion as a function of time for the reference concrete and binary concretes with nSi.
The estimated times of chloride penetration in binary concretes with mSi are shown in
Estimated chloride penetration by natural diffusion as a function of time for the reference concrete and binary concretes with mSi.
Lastly, the estimate of chloride penetration for ternary concretes is shown in
Estimated chloride penetration by natural diffusion as a function of time for the reference concrete and ternary concretes with nmSi.
Estimated corrosion initiation time for a 20-mm coating.
Amount of addition | Concrete | Corrosion initiation time (years) | Increase with respect to HAC (years) |
---|---|---|---|
0 | HAC | 116.72 | 0 |
2.5 | HAC[nSi]-2.5 | 219.31 | 102.59 |
2.5 | HAC[mSi]-2.5 | 238.13 | 121.41 |
5 | HAC[nSi]-5 | 258.79 | 142.07 |
5 | HAC[mSi]-5 | 477.22 | 360.49 |
5 | HAC[nmSi]-2.5/2.5 | 543.01 | 426.29 |
7.5 | HAC[nSi]-7.5 | 710.88 | 594.15 |
7.5 | HAC[mSi]-7.5 | 839.01 | 722.29 |
7.5 | HAC[nmSi]-5/2.5 | 918.46 | 801.74 |
7.5 | HAC[nmSi]-2.5/5 | 681.36 | 564.64 |
Adding 2.5% nano- or microsilica yields similar estimated corrosion initiation times regardless of the size of the addition. Both additions double the corrosion initiation time for HAC, although microsilica provides a slightly higher value.
In the case of concretes with a total addition of 5%, the estimated corrosion initiation time for binary concrete with mSi is 85% higher than that of binary concrete with nSi. This might be due to the difficulty of compaction of mixtures with a high nSi content. However, the longest corrosion initiation time is obtained with the ternary mixture, probably due to a packing effect and the low porosity created by a wider particle size distribution (
HAC[nmSi]-5/2.5 has the longest estimated service life of all the mixtures featuring a total addition of 7.5%. This mixture represents the best combination of nano- and microsilica addition for resisting chloride penetration, probably due to the optimisation of the packing effect, overcoming the difficulties in compaction caused by considerable percentages of nSi.
The experimental results and the estimated service life may be interpreted by taking into account that nano- and microsilica additions reduce the amount of interconnected pores, since their incorporation in cement blends leads to a refinement of the porous network and changes the ionic concentration in the pore solution (
The results shown in
Additionally, ternary mixtures show good self-compacting properties (see
HPSCC with the addition of nano- and microsilica in binary and ternary mixtures presents significant improvements in chloride-resistant behaviour. This improvement is in general proportional to the total content of addition. The use of nSi and mSi may have a positive effect on improving the microstructure and reducing the internal pore structure from early stages.
In binary mixtures the addition of microsilica presents higher values of electrical resistivity than the addition of nanosilica. However, when classified by resistivity, all mixtures, both binary and ternary, exhibit a notably low chloride penetrability.
Similarly, binary mixtures prepared with microsilica exhibit lower chloride migration coefficients than binary mixtures with nanosilica. Significantly lower values are obtained with ternary mixtures, however, even in the case of the mixture with the smallest amount of each addition.
The addition of microsilica provides the concrete with a greater capacity for chloride combination than does the addition of nanosilica for percentages less than or equal to 5%.
Binary mixtures prepared with microsilica have lower chloride diffusion coefficients than binary mixtures with nanosilica at 28 days. However, the combination of both additions in ternary mixtures results in significantly low chloride diffusion coefficients, even at small addition ratios. Further future study including additional parameters, such as chloride diffusion coefficients at different ages, is necessary in order to examine the influence of nSi and mSi on the durability of concrete mixtures, in particular at early ages, where the microstructure improvements obtained are promising.
The chloride diffusion coefficients and the concentrations of total, free and combined chlorides were similar in each concrete containing a total addition of 7.5%, regardless of the amount of silica addition.
The addition of nano- and microsilica in binary and ternary mixtures leads to notably low chloride diffusion coefficients, with a significant increase in the estimated service life according to Spanish regulations. These results might also allow the thickness of cover to reinforcement to be reduced without leading to negative consequences for structure durability.
The authors wish to express their gratitude to the Spanish Ministry of Science and Innovation (Ministerio de Ciencia e Innovación), project RTI2018-100962-B-I00, for financial support.