Performance of Self-Compacting Concrete Containing Pozzolanic Materials in Aggressive Environment

- The aims of this study is to investigate the resistance of different SCC mixtures to aggressive solutions. The investigation included the type of the cementitious materials (silica fume and high reactivity Attapulguite-HRA) and limestone powder (chalk powder and Al-gubra). The powder content of the mixes was kept constant, 500 kg/m 3 . The slump flow, L-box, and V-funnel were performed for mixes in there fresh state. In the present work, the specimens were immersed in sulfuric acid solution with a concentration of 0. 5% up to 289 days after normal curing for 28 days. After concrete has hardened, two types of test are performed. Firstly, destructive tests are conducted including (compressive strength, splitting tensile strength, and modulus of rupture). Secondly, a mass loss as non-destructive test is performed. The results obtained from this work, show that concrete mixes with chalk powder only, had the best resistance to sulfuric acid solution comparable with concrete made with Pozzolanic materials the reduction in compressive strength was 25. 9 %. In addition, concrete mixes with HRA had the worst resistance.


Introduction
Concrete can be attacked by liquids with pH value below 6.5 but the attacks are severe only at a pH below 5.5 [1]. Sulfuric acid fluids are classified as the most aggressive of natural threats to concrete structures. Generally, they arise from industrial operations, but they can be caused by urban areas activities. Acid attack is influenced by the processes of disintegration of the paste constituent. The admixtures in SCC paste can negatively or positively affect its resistance to acid aggression [2]. The deterioration of concrete in acidic environments is influenced by several factors such as the type of cement used, permeability of concrete, and the surrounding environmental conditions. Furthermore, the solubility of calcium salt produced from the acid -base reaction of the cementitious paste and attacking acid is an important factor [3]. Many studies about the possibility of improving the quality and performance of concrete have been accomplished. Dhiyaneshwaran et al. [4] used fly ash Class F at different replacement levels, 10, 20, 30, and 40% by weight of cement. After 28 curing, the specimens were immersed in aggressive solution, 1% sulfuric acid, the weight reduction and the compressive strength loss were calculated at the age of 28, 56 and 90 days. The results noticed that, as increasing in fly ash percentages, both the compressive strength loss and the weight reduction of the specimen would be reduced. Chang et al. [5] investigated six concretes mixes; contain limestone aggregates and siliceous aggregates. They also used three Pozzolanic materials, slag, fly ash and silica fume. In their work, the concrete specimens were immersed in 1% H 2 SO 4 solution. The result from the experimental program shows that, using concrete with cement containing fly ash and silica fume and limestone aggregates performed the best. Monteny et al. [6] reported that, in the case of low permeability (high pore densification), the fine pores may contribute an increase the capillary suction resulting in solution which enters deeper into the concrete. In spite of the total amount of solution taken up by the concrete is relatively small, the solution which intervention the concrete, is very aggressive and can dissociates into sulfate ions and due of pore densification, the solution will comes in contact with a larger concrete surface comparable with the same amount of aggressive solution is taking up by concrete in the case of large pores. Increase the surface area of the concrete-contact with the aggressive solution will lead to an increase in the reaction products. A study was conducted by Daczko et al. as cited by Joorabchian [7] on the performance of concrete, in their work different values of pH in the range of (1 to 7) and three admixtures MK, silica fume and an organic corrosion inhibitor (OCI) were used in this study to enhance concrete performance bars against sulfuric acid solutions. The replacement level for silica fume and Metakaolin was 8% in two separate mixes. In this work, the total period of immersion in acid solution was 100 days. The final results of their work demonstrate improved performance of OCI and silica fume concrete, but no such improvement was observed for concrete containing Metakaolin concrete Daczko et al. believed that the presence of high percentage of Al 2 O 3 in the MK may cause further reaction and corrosion of concrete that is exposed to sulfuric acid. An experimental program was carried out by Senhadji et al. [8] to study the influence of different supplementary cementitious materials (silica fume, natural Pozzolan) and limestone fine at various replacement levels. The results showed that the addition of limestone powder or natural Pozzolan could improve the acid resistance of mortar, but at different rates depending on the proportion of supplementary cementitious materials. On the other hand, mortars with silica fume are seriously damaged in the sulfuric acid environment. Girardi et al. [9] studied the deterioration of concretes mixtures with different cement composition, (Portland limestone, blast furnace slag, silica fume, Pozzolanic cement and Pozzolanic cement with silica fume) to a cycle exposure of sulfuric acid. The result indicated that mixtures containing silica fume exhibited the lowest expansion.

Research Significance
Several concrete elements are susceptible to chemical attack of sulfuric acid, including, industrial floors of chemical plants, superstructures, sewage pipe systems, etc. A comprehensive review indicates that there is lack of information on the role of high reactive Attapulguite (HRA) that can be inserted in SCC in such aggressive environment.

Materials
The binders used in Table 1 included ordinary Portland cement (OPC), (ASTM C150 -Type 1) [10]. The cement was tested and checked according to IQS 5:1984[11], silica fume (SF) to ASTM 1240-03 [12], high reactivity Attapulguite to ASTM 618-03. The used HRA was prepared by calcinations the Attapulguite powder at 750°C with soaking time of 1/2 hour then left to cool down [13]. Two types of limestone powder (gubra and chalk powder) were used throughout this investigation. The binder content was kept compatible to the guidelines for SCC mixture design, i. e. EFNARC [14]. The maximum nominal size for crushed coarse aggregate was 10 mm, with a specific gravity of 2.62. The constituents of the selected SCC mixes are given in Table 2. To improve flowability of the SCC mixtures, (SP) designed for the production of SCC (Glenium 51) with relative density of 1.1 at 20C was incorporated in all mixtures shown in Table 3. The dosages of SP was modified to maintain a slump flow of 600 -750 mm, T 50 (4 to 10) sec., L-box index (>0. 75) (3 Ǿ10 mm with 50 mm gaps) and V-funnel flow (3 to 25) sec.

Preparation of Acid Solution
All concrete specimens were cured in water for 28 days, after which they were immersed in acid solution. The initial pH (2.3) of the solution increased quickly A digital portable pH meter was used for monitoring the pH levels of the sulfuric acid solutions. Specimens were fully immersed for 41 weeks.   [14], ** The European Guidelines for SCC [15]

Hardened Concrete Properties
I. Compressive strength of concrete specimens before and after sulfuric acid attack Table 4 and Figure 1 present the result of compressive strength of concrete cubes after 90, 182 and 289 days immersion in sulfuric acid solution. From Figure 1, the relative change of compressive strength after 289 was (80, 47, 25. 9, 67. 2 and 36) % compressive strength loss for M1, M2, M3, M4 and M5 respectively, it was observed that M1 (with high cement content) has a significant loss of strength, the reason for this anomaly is probably largely due to the fact that volume fraction of cement for concrete made with cement only was higher than that for concrete made with cement and other admixtures, since the acid attack cement only, the rate of loss in compressive with higher cement content was grater. For M4, (with HRA) the percentage loss of compressive strength was also high, comparable with other mixes without HRA , this may be attributed to the increases in the percentage of Al 2 O 3 in HRA, and this may cause further reaction and corrosion of concrete that is exposed to sulfuric acid solution. This finding is also confirmed by Daczko et al. cited by Joorabchian [7], in his work he used HRM instead of HRA. Mixes with limestone powder (chalk powder) (M3), had the lowest compressive strength loss due to the high neutralization capacity and slower rate of reaction with acid relative to the filler. The better resistance of cements with limestone replacement to acid attacks can be attributed to four important factors: (i) the higher fineness of chalk powder effects positively on the resisting of SCC to acid solution. It filled the micro pores in cement paste and the ability of mortar to resist sulfuric acids attack was improved by the reduced permeability and porosity (ii) the existence of a high calcium carbonate (CaCO3) (68. 73 %) content increased the capacity of limestone powder to consume more aggressive acid, and (iii) the decreased proportion of cement reduced the portlandite (C-H) content (iiii) in the case of sulfuric acid attack, the calcium salt (gypsum) formed has very low solubility in water (0. 22 g/100 g water at 20 °C), a dense layer of gypsum was formed, which was capable of retarding the deterioration process by acting as a surface sealing layer [8].
In concretes with silica fume,(M5) has a higher compressive loss comparable with M3 (without silica fume) this may be attributed to the reduce in Portlandite available for reaction with the acid C 3 S 2 H 3 +H 2 SO 4 →CaSO 4 .2H 2 O+ ‫‬ C 2 S 2 H 2 (1) This finding is supported by other researchers [8].
In the case of mixes with limestone powder M2 (gubra) According to the results, due to the high fineness (2.48 m 2 /g) of the filler the reaction with sulfuric acid may also accelerate the deterioration of SCC and increase the loss of strength [8].   Table 5 presents the results of density at 28 days for each concrete as well as the density of concrete cubes after 90, 182 and 289 days immersion in sulfuric solution. After immersion in H 2 SO 4 solution, the relative change of mass was 18, 1.85, 2.7, 6.9, 11.76 % mass loss for M1, M2, M3, M4, and M5 respectively as shown in Figure 2. It was observed that at the age of 90 days some specimens exhibit loss of mass as shown in M3, while in mixes M1,M2,M4 and M5 there was an increase in concrete mass. The mass loss at this stage probably due to the micro cracks occurs between the matrix content after the diffusion of sulfuric acid. The mass gain can be attributed to saturation of the specimens or that is, in the initial conditioning age, the porosity in the concrete decreases which results in the mass increase, shown in Figure 2. At the end of the test, concretes made with silica fume,(M5, and Mixes with high reactive Attapulguite, these mixes have a higher mass loss comparable -with M3 (without Pozzolanic material) This finding is supported by other researchers [8,9].  Figure 3. It was observed at the age of 90 days all mixes exhibit loss of splitting tensile strength as shown in M1, M2, M3and M5, while in mixer M4 there was an increase in concrete tensile strength. The tensile strength loss at this stage is probably due to the micro cracks occur between the matrix content after the diffusion of sulfuric acid. The tensile strength gain can be attributed to saturation of the specimens or that is, in the initial conditioning age, the porosity in the concrete decreases which results in the tensile strength increase. As mentioned previously. With continued exposure, most of the specimens exhibit tensile strength loss, at the end of the test (282 days), it was observed. Moreover, mix with silica fume M5 has the lowest resistance to sulfuric solution. I t can be inferred that the use of SF affects the (f cu ) in the same manner as (f t ).  M1  2577  2595  2518  2510  2450  M2  2519  2615  2528  2743  2473  M3  2534  2530  2600  2726  2465  M4  2482  2550  2520  2640  2310  M5  2516  2535  2505 2448 2220   and international conferences and has two patents in the field of building materials. At present Dr. Frayyeh is an assistant professor of civil engineering in the Building and Construction Engineering Department, University of Technology, Iraq.

Dr. Luma A. Zghair was born in
Baghdad in 1971. She received her BSc in building and construction sciences, 1994, University of Technology, Iraq and received her PhD in building materials in 2016 University of Technology, Iraq. She published more than ten papers in different journals and international conferences. At present Dr. Luma is a lecture in Civil Engineering Department, College of Engineering, University of Al-Mustansiriyah, Iraq.