Early and Long-Term Assessment of High-Performance Concrete Contained Nano-Silica Exposed to Sulfate Attack

This research aims to study the influence of using the Nano-silica in high-performance concrete subjected to external sulfate attack with different levels. Four concrete mixes were prepared by using two types of Portland cement: ordinary and sulfate resistant cement. From each mix, cubic concrete specimens were cast and subjected to different exposure conditions. The concrete cubes were immersed in sodium sulfate (Na2SO4), magnesium sulfate (MgSO4) and calcium sulfate (CaSO4) solutions with concentrations of (1.5, 10 and 15) %. The specimens were tested for compressive strength at (3, 7, 14, 28, 90, 180, 270, 365, and 545) days and the values of sulfate penetration in concrete samples were determined. It can be found from the results that high-performance concrete containing Nano-silica gave a relatively good sulfate resistance compared to the reference sample. Moreover, results indicated that the strength deterioration was reduced after incorporating nano-silica.


Introduction
Concrete is one of the most vastly utilized substances in the construction sector [1,2]. However, it may not withstand for long periods due to being attacked by some harmful agents. These aggressive agents can be categorized into two broad denominations, internal and external agents. The external agents include sulfate and chloride ions, carbon dioxide, freeze-thaw cycles and abrasion [3]. The main chemical attack to the concrete in contiguity with an aqueous solution is that correlated with sulfates. The major sulfate chances in ground water are sodium, calcium, magnesium and potassium. Sulfates exist in clays and other soils. The mechanisms of retrogradation rely on the transit of fluids or gasses through the concrete pore structure. The achievement of less permeable concrete is one of the fundamental purposes when trying to get durability. The essential technique for obtaining this is to fabricate a concrete with a low water/cement ratio [4]. High-performance concrete (HPC) is more durable than usual concrete because it is made with a minimizing amount of water so that its microstructure becomes denser with less porosity [5]. However, sulfate, chloride ions, acids or other types of grievous chemicals may attack HPC.
The durability of concrete relies on the microstructural characterization of the hydrated cement paste in the transition zone. In addition, it depends on the evolution of micro crakes that may result from thermal gradients, shrinkage and overloading [6]. During Portland cement hydration, sulfates can able to react with calcium aluminate and free lime causes eventual failure of the concrete. Under sulfate attach, the reaction between the hydrated cement paste and sulfate leads to macro-cracking, spalling and loss of cohesion of concrete [7]. The pores of the concrete microstructure play an important role in IOP Publishing doi:10.1088/1757-899X/1090/1/012074 2 diffusing the sulfate ions within the concrete. Therefore, the production of concrete with dense microstructure is a worthwhile approach to reduce the effect of harmful ions, including sulfates. According to the above, it can be observed that limited studies are addressed the impact of NS on sulfate resistance of HPC at later ages (more than one year). Therefore, this study is conducted to explore the effect of NS admixture as substitution of cement using two types of cement under three types of sulfate (sodium, calcium and magnesium sulfates) solutions at early and later ages.   [32] requirements with SO3 content of 0.013 and specific gravity of 2.66 was used as a coarse aggregate. Table 4 shows the grading of the gravel.

Nano-silica.
Nano-silica was incorporated into the mixes as 10% replacement of cement (by weight). Nano-silica with a specific surface area of 150000 m 2 /kg, average particle size of 14 nm and SiO2 content of ˃ 99% was utilized.

Compressive strength test
The compressive strength test was determined according to BS.1881: part 116 [33] specification. Three cubes were tested for each mix at each age.

Sulfate resistance test
Concrete sulfate resistance was found by immersing specimens in different concentrations of sulfate solutions. The concentrations of sulfate solutions that were used in previous researches ranged between 1% and 15% [34,35], so the percentages used in this study were chosen to cover the above range. The concrete specimens were immersed in (1.5, 10, 15) % of Sodium, Calcium and Magnesium sulfates solutions (MgSO4, Na2SO4 and CaSO4, respectively). Then the influence of this immersion on the compressive strength and penetration values in the concrete specimens were observed. Concrete specimens were exposed to two types of sulfate effect: 1. About 70% of the specimens' depth was continuously immersed in the sulfate solution.
2. About 70% of the specimens' depth was subjected to wetting-drying cycles weekly.

Results and Discussion
First of all, it is worthy to note that the diffused sulfate solutions into the concrete pore react with hydrated cement products. The reaction products of Calcium hydroxide and Calcium aluminate with dissolved sulfate ions have large volumes so that disruption of the paste occurs. This reaction leads to internal pressure in cement paste, the appearance of micro cracks and causing loss of the concrete strength. Generally, it was found from the results (Figures 1 to 8) that the strength of concrete specimens decreased with the increase of sulfate attack. However, in some cases, the concrete specimen's strength increased at the early ages, due to crystallization of reaction yield in the pores of the concrete. The influence of sulfate solutions on the compressive strength of different exposed mixes varied according to sulfate concentration, type of cement and Nano-silica used.     For wetting-drying cycles exposure, as presented in figures 5, 6, 7 and 8. Results showed that sulfate solution increased strength losses. The concrete specimens were immersed in salt solution gave higher compressive strength than the strength of concrete specimens exposed to the same sulfate solution with continuous wetting and drying. The use of Nano-silica reduced the strength loss of concrete specimens due to provide concrete with less porosity.    For the reaction to have happened, the sulfate ions should permeate into the concrete. The values of salt solution penetration into the concrete specimens were measured by cutting the core molds of the concrete specimens to determine the values of salt penetration by changing the color of the concrete to slightly white [37]. It can be seen from figures 9, 10, 11 and 12 that using Nano-silica with concrete mixes has greatly reduced the sulfate solution penetration in the concrete specimens for different exposure conditions.
The values of salt solution penetration in the control samples were greater than those Nano-silicabased mixtures. This due to the filling of concrete pores by the gel formed by the reaction of Nanosilica with calcium hydroxide, or filling these pores with Nano-silica materials, consequently, increasing concrete density and reducing permeability. However, it is necessary to give sufficient time for Nano-silica before concrete exposed to sulfate solution as shown in figures 9, 10, 11 and 12. Besides, it can be seen from figures 3, 4, 7, 8, 11 and 12 that Nano-silica materials were more effective when used with SRPC.

Conclusions and recommendations
The main findings of this study are: 3. HPC containing Nano-silica is more durable than that do without Nano-silica because the microstructure of the concrete becomes less porosity, more compact and controlling concrete cracks. 4. There was a significant reduction in the penetration of sulfates into samples containing Nano-silica (30-60)%. 5. The concrete specimens that exposed to wet-dry cycles exhibited more deterioration than wet-cured specimens.