Microstructural investigation on fiber reinforced SCC subjected to sulphuric acid attack

All self-compacting concrete mixes are increasingly being used in many applications, some of which are subject to acid attack. This work aims to investigate the resistance of a variable range of SCC to sulphuric acid attack. The main studied variables included the cementitious materials type (silica fume and highly reactive Attapulguite), limestone powder (chalk powder and Al-gubra) and the inclusion of hybrid fibers (steel, plastic and polypropylene fibers). The powder content of the mixes was kept constant (500) kg/m3. The slump flow, L-box, and V-funnel were performed for mixes in their fresh state. In the present work, the specimens were immersed in sulphuric acid solution at concentration of (0.5%) upto 289 days after normal curing for 28 days. After the concrete hardened, two types of test were performed before and after immersing in sulphuric acid solution, viz. compressive strength and microstructure tests. The results obtained from this work, showed that concrete mixes with pozolana materials only suffered high deterioration in mechanical properties as compared to concrete mixes with chalk powder only after exposure to 289 days in sulphuric acid solution in addition to, by using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS), a number of elements were formed, which were indicated the product of hydration of cement paste before chemical immersed as measured with EDS equipment, as well as hand after chemical immersed, an increase in sulphur and calcium content of test sample, as measured with EDS equipped which confirms the identification of gypsum formation.


Introduction
The Deterioration of Concrete and mortar can be affected by different attacks under environmental water conditions. There are different kinds of environmental water attack, among which the acid attack is the most common. Thus, the microscopic structure deteriorates, moreover the strength of the structure decreases, and the structures finally get destroyed [1] Different studies about the assessment of the deterioration of SCC subjected to sulphuric acid using microstructure technique have been completed. Attiogbe and Rizkalla [2] studied the microstructure degradation using SEM / EDS tests. Different concrete mixtures (Ordinary Portland Cement type Ι and type Ѵ) were immersed in 1% sulphuric acid solution for 10 weeks. A change in sulphur content of the test samples was used as the chemical indicator of the degree of deterioration. They suggested that the samples submit expand the size as a result of the sulphuric acid attack. A comparison between SEM photo for un attacked and attacked samples shows that the acid reacts with the cement leading to the formation of more porous material. In their study the EDS analysis showed that the sulphur content of the acid-attacked specimen is higher than that of the un attacked specimen.  [3] used SEM and EDS analysis to study the microstructure changes in cement paste. SEM photo, shows the precipitation of (gypsum), (needle-like crystals) in the surface cracks. They reported that, the formation of gypsum results in the degradation of mortar. EDS test showed elements such as sulphur, oxygen, and calcium, indicating the determination of gypsum formation.
Siad et al. [4] represented a study on the mechanism of deterioration of different types of concrete by using SEM and EDX analysis. The immersion period of all concrete specimens in acid solution was about 12 weeks. The SEM and EDAX data obviously showed that both calcium and sulphur were the dominant elements. Kim and Lee [5] used SEM and EDS, microstructural test to examine concrete element of concrete structure subjected to sewage water for nearly ten years. In this study specimens were taken from a sewage treatment tank. The SEM and EDS show the presence of sulphate products due to H2SO4 attack. The SEM of the samples, which was taken from the surface of the samples indicates the existence of a large amount of gypsum. They reported that formation gypsum is one of the causes of low compressive strength of the concrete. Yingfang and Haiyang [6] examined the pore characteristics of the concrete by (SEM) and (EDS) analysis. They used acid solution with different pH levels ranging from 1.0 to 2.5. From the SEM images, they observed that gradual changes occur in the microstructure of concrete samples. They reported that, with the constant exposure, the C-S-H becomes denser, more the C-H crystal takes shape, the internal micro cracks become wider.
The aim of this research is to study the deterioration of SCC subjected to sulphuric acid using microstructure technique and to study the correlation between the durability problem and the microstructure.

Materials
Portland cement (PC), (ASTM C150 -Type) [7] was used as a binder. The cement conforms to the Iraqi standardwas [8], silica fume of class N pozzolans [9], high reactivity Attapulguite (HRA) to ASTM 618-03 [10]. 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 [11]. Two types of limestone powder (Gubra and chalk powder) were used throughout this investigation. Table (1) illustrates the properties of the materials [12]. Three types of fibers were used for the purpose of this study. These types are steel, plastic and polypropylene fibers. The researchers used volume of fibers in order to obtain the suitable workability. Table (2) and figure (1) show the properties and the photos of the fibers. Table (3) illustrates the SCC mixes. The fresh properties of the binder conformed to rules guiding for SCC design [13] [14]. The results of fresh properties of SCC mixtures are summarized in Table (4).

Acid solution
The pH of the solution was about (2.3). The Specimens were immersed for 289 days after curing in water for 28 days. For SCC LP AT, (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 Al2O3 in HRA, and this may cause deterioration and further reaction of concrete that is exposed to sulfuric solution. This finding is also confirmed by Daczko et al. cited by Joorabchian [15], (in their work they used high reactive mitekaolin instead of AT). Mixes with limestone powder (chalk powder) (SCC LP), had the lowest compressive strength loss due to the high neutralization capacity and slower rate of reaction with acid relative to the filler. The resistance of mixes with limestone powder to acid attacks can be attributed to two important factors: [15] 1. The high percentage of (CaCO3) (68.73 %) content increased the ability of limestone powder to consume more aggressive acid, and decreased the portlandite (C-H) content which was formed by the cement.

Crushing strength of concrete
2. Gypsum can act as a surface sealing layer which can retard the deterioration process of the acid Concretes with silica fume and hybrid fibers, (M5 and M6) have a high compressive loss as shown in Fig (2) comparable with M1(without silica fume). This may be attributed to the reduction in portlandite available for reaction with the acid, because it has already reacted with the SF, producing CaSiO2. 2H2O + H2SO4 → CaSO4+ Si(OH)4 + H2O ……1 This finding is supported by other researchers [16].
There is another reason about the negative impact of silica fume, which can be clarified as follows. Inspite of densification of the pore structure of SCC by silica fume in order to achieve lower permeability; low resistance to acid attack has been established. This is due to the negative effect of poor densification creating less space to accommodate stresses caused by the growth of large gypsum crystals. This finding is also supported by other researchers [17] [18].

SEM and EDS for SCC Concrete Samples
(SEM) and EDS were carried out to identify the products formed before and after H2SO4 solution attack. The results are exhibited in figures (3) to (14), for M1, M2, M3, M4, M5, and M6, respectively.
In the present study, sulfur and calcium component percentage from the EDS are of primary interest. This is because both of these elements are also formed as a result of the reaction between sulfuric acid and cement paste. This finding is observed by another researcher [2].
From the SEM photos and EDS analyses before attack, the following was observed: 1.
A dense structure was formed for all samples before acid immersion, as shown from SEM photos, also the presence of void in some parts of samples with hybrid fibers as shown in figures (7) A and (7) B.

2.
From EDS, different elements were formed such as Ca, O, AL, Si, C, Mg and S as a result from the hydration product.
After immersing in H2SO4 for 289 days, the following was noticed: 1.
The EDS data clearly indicated that the dominant elements was calcium, sulfur and gypsum. This is agreed with observations by different researchers [4] [5] . 2.
The micrograph of concrete samples exposed to H2SO4 for 289 days indicated that these samples submit significant deterioration because it is highly porous as shown in SEM micrograph, as can be seen from a comparison of the SEM images before and after exposure to H2SO4 solution.
3. Some chemical reaction was participated one fiber as shown in figure. (11), which can reduce the bond between the cement matrix and the fibers and resulted in more porous in cement matrix.        loss after 289 days exposure to acid solution; the reduction was 26%. On the other hand, mixes with (Algubra) (M2) gave a higher compressive strength loss (47) % comparable with mixes (M1) 3. The mixes containing is not clning mixture of plastic and micro steel fibers were better than the others mixes in terms of the compressive strength. Depending on the microstructure results, the following observations can be concluded: 1. After immersion of specimens in sulfuric acid solution, the increase in sulfur and calcium content of test sample, as measured with EDS equipment, which shows the identification of gypsum formation, is a good sign to the extent of damage in concrete because of exposure to sulfuric acid. 2. The micrograph from SEM test of concrete samples immersed in H2SO4 for 289 days indicated that these concrete samples showed significant deterioration because it is highly porous, as can be seen from a comparison of the SEM images before and after exposure to H2SO4 solution.