Magnesium sulfate attack on hardened blended cement pastes under different circumstances
Introduction
Sulfate attack on mortar specimens made with different types of cement was studied [1], [2], [3]. In addition, the effect of initial curing conditions on sulfate resistance of concrete, made with blended ordinary Portland cement (OPC), was investigated [4]. In addition, the effect of MgSO4 in the presence of MgCl2 on mortar samples was evaluated [5], [6]. Moreover, the evaluation of sulfate attack on blended cements, incorporating supplementary cementitious materials such as fly ash, slag, and silica fume (SF), has been carried out by many investigators [6], [7], [8], [9], [10], [11], [12], [13].
The attack of sodium sulfate on concrete is due to two principal reactions: the reaction of Na2SO4 and Ca(OH)2 to form gypsum and the reaction of the formed gypsum with calcium aluminate hydrates to form ettringite. In addition, it is noticed that MgSO4 reacts with all cement compounds, including C-S-H, thus decomposing cement, and subsequently forming gypsum and ettringite [14], [15]. In a comprehensive review on the sulfate attack, Mehta [16] pointed out that, in most cases, the loss of adhesion and strength, not expansion and cracking, is the primary manifestation of sulfate attack.
Concrete deterioration due to MgSO4 attack was attributed to the decalcification of C-S-H to form M-S-H, as well as the expansion caused by the formation of expansive salts. Concrete, with the addition of SF, showed a greater degree of deterioration on the surface together with an intense formation of polygonal cracks filled with gypsum [9]. The Ca lost and Mg gained by cement pastes as a result of MgSO4 attack were determined and it was found that the molar ratio of Mg/Ca was much higher than unity [17], [18]. Torii and Kawamura [11] studied the resistance of mortars containing fly ash or SF in 2% H2SO4, 10% Na2SO4, and 10% MgSO4 solutions. Fly ash and SF appeared to significantly increase the resistance, but only above a certain content, which depended on the type of sulfate solution.
Sorrentino et al. [19] demonstrated by expansion and microstructural studies that concrete of moderate strength (40 MPa) was deteriorated in MgSO4 or seawater. Large quantities of secondary ettringite and gypsum and a noncohesive M-S-H were formed. In higher strength concrete (e.g., 80 MPa), the reaction resulted in some compact layers of gypsum, aragonite, and brucite, which formed a barrier to prevent further attack. Thus, expansion was negligible even after immersion in seawater for 3 years.
The deterioration of concrete due to sulfate attack is considered a complex problem and it depends on many factors:
(a) factors related to concrete properties:
- 1.1.
type of Portland cement,
- 1.2.
W/C ratio,
- 1.3.
CH content,
- 1.4.
pore structure of the formed hydrates, and
- 1.5.
using of mineral admixtures such as fly ash, slag, SF, …, etc.
- 1.1.
(b) factors related to aggressive medium:
- 2.1.
type of salt,
- 2.2.
concentration, and
- 2.3.
presence of more than one kind of aggressive ions.
- 2.1.
(c) factors related to environmental conditions:
- 3.1.
temperature and
- 3.2.
drying–wetting cycles.
- 3.1.
Section snippets
Experimental
The materials used in this investigation were OPC, SF (Alexandria Company of Ferro-Silicon, Egypt), slag (Helwan Steel, Cairo, Egypt), and fine powder analytical reagent calcium carbonate (CaCO3, CC̄). The properties of the used materials are given in Table 1. The composition and designation of the different mixtures are indicated in Table 2. Each dry mixture was thoroughly mixed using a ball mill with a few number of balls for 3 h. The pastes were prepared using a W/S ratio of 0.3. The pastes
Compressive strength
The values of compressive strength of the different hardened cement pastes immersed in sulfate solutions, under various conditions as well as the control samples (stored in water), relative to those of 28-day samples are represented graphically versus time in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11.
Fig. 1 shows the relative compressive strength of the various hardened OPC pastes (Mix 1) compared to that of 28-day samples. It is obvious that the
Conclusion
The main conclusions could be derived from this investigation are summarized as follows:
- 1.
Partial replacement of Portland cement by SF (10–15%) did not show a significant improvement in sulfate resistance of hardened cement pastes.
- 2.
The hardened cement pastes contained 40% slag or 5% CC̄ showed a noticeable high sulfate resistance compared to the plain OPC pastes.
- 3.
Exposure to sulfate solution at 60 °C with drying–immersion cycles can be considered an accelerated method for sulfate attack.
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- et al.
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