Sulphate Attack on Concrete and Its Prevention

0
154

INTRODUCTION

  • Sulfate attack is a complex form of deterioration that has damaged concrete structures throughout the world.
  • Sulfate attack is particularly complex because the source of sulfates can be external or internal (delayed ettringite formation), the distress can be chemical in nature, due to alteration of hydration of products, or physical in nature, due to phase changes in the penetrating sulfate solution.

Portland cement concrete can be attacked by solutions containing sulfate ions, such as some natural ground waters and Soil containing sodium, potassium, magnesium, and calcium sulfate are the main sources of sulfate ions in groundwater

  • Sulfate attack leads to strength loss, expansion, spalling of surface layers and, ultimately, disintegration
  • Sulfate attack can be external or internal.
  • External: due to penetration of sulfates into the concrete from outside for example high- sulfate soils and ground waters, atmospheric or, industrial water pollution.
  • Internal: due to a soluble source being incorporated into the concrete at the time of mixing for example gypsum in the aggregate.
  • Portland cement might be over-sulfated.
  • Admixtures also can contain small amounts of sulfates .

 

Scanning electron microscope image of sulfate attack in concrete.

.Fig. 1. Biphasic model of concrete affected by sulfate attack

TYPES OF SULFATES

  • 1- chemical sulfate attack
  • 1.1 Calcium Sulfate
  • 1.2 Sodium Sulfate
  • 1.3 Magnesium Sulfate

2 – Physical Sulfate Attack

 

PREVENTING SULFATE ATTACK

  • External sulfate attack can be prevented through proper material selection and mixture proportioning
  • Concrete with a low w/cm will reduce porosity and permeability which reduces the rate of ingress of sulfate ions.
  • A significant correlation between higher permeability and greater expansion was reported with concretes exposed to 5% sodium sulfate (Khatri 1997)
  • On the outer hand , High damage due to six months of exposure to physical sulfate attack did not affect both the concrete compressive strength and modulus of elasticity since the damage was only limited to the external surface of the concrete. It is however expected that long-term exposure can lead to decreased mechanical properties (Nehdi et al 2014).

Compressive strength for the concrete cylinders partially immersed in 5% sulfate solution for up to six months: (a) w/b= 0.60, (b) w/b = 0.45, and (c) w/b = 0.30

.

  • Low w/cm

Low water cement ratio improves sulfate resistance .

  • Due to reduction in the water cement ratio, sulfate will not be able to penetrate into to the concrete.
  • Cement paste with a w/cm of 0.7 is approximately 10 times more permeable to that a comparable mixture with a 0.55 w/cm (Powers et al 1954).
  • Al-Amouti (2002( To achieve low permeability, one must not only use a low w/cm (i.e., less than 0.45( but also ensure adequate curing. the incorporation of SCMs into concrete mixtures is the most powerful method of reducing sulfate ingress
  • Even at the reduced w/cm, the mortar bars expanded rapidly in sulfate solution and none of the bars could be measured after 8 months exposure ( T Drimalas et al 2011).

Table 1 . Details of Mortar Mixes with Lower w/cm for ASTM C 1012 Tests

Three mortar mixes were produced with varying amounts of WL fly ash and a reduced w/cm = 0.40 (standard w/cm = 0.485 in ASTM C 1012).

The details of the mixes are given in Table1 and Figure 3.1 compares the expansion curves of the mixtures at 2 different w/cm.

Figure 3.1: Effect of w/cm on the Expansion of Mortar Bars with WL Fly Ash (ASTM C 1012 – 5% Na2SO4 Solution)

Sulfate Resistant Cements

  • Sulfate-resistant cement was developed to limit the C3A in the cement to prevent sulfate attack.
  • The lower C3A cement primarily reduces the amount of ettringite that can form.
  • Concrete to be exposed to sulfate-containing solutions or soils shall conform to requirements of Table 2 -or shall be concrete made with a cement that provides sulfate resistance and that has a maximum water- cementitious materials ratio and minimum compressive strength from Table .(ACI 318)
  • (ACI–318) In addition to the proper selection of sulfate resisting cement, other requirements for durable concrete exposed to concentrations of sulfate are essential, such as, low water-cementitious materials ratio, strength, adequate air entrainment, low slump, adequate consolidation, uniformity, adequate cover of reinforcement, and sufficient moist curing to develop the potential properties of the concrete.
  • Gollop and Taylor (1995) employed various analytical techniques to investigate the microstructure of cement paste cubes with ordinary portland and sulfate-resistant cements after 6 months submerged in magnesium and sodium sulfate solutions. Sulfate-resistant cement cubes still provided the same deleterious.

Supplementary Cementing Materials

  • The incorporation of SCMs into concrete mixtures is the most powerful method of reducing sulfate ingress
  • Most SCMs, when used in sufficient dosages, can be quite effective in preventing sulfate attack by lowering permeability .
  • Many studies have been conducted to show the benefits of incorporating various SCMs in concrete to improve sulfate resistance, and only a select few of these are cited herein for conciseness.
  • Ground-granulated blast furnace slag (GGBFS) is quite effective in controlling sulfate attack, with replacement levels from 25 to 50% providing moderate to severe exposure protection, respectively. Mortars containing 45 to 72% GGBFS (by mass replacement of cement) Hooton and Emery (1990)
  • ACI 201.2R-01 recommends the use of between 40 and 70 % by mass of slag to control sulfate attack (ACI 201 )
  • One potential issue with slag is that the typical 1% sulfur content in blast furnace slags occurs mainly as sulfide in glass phase and is released upon hydration as other constituents. The release of the sulfide cannot be ignored when considering the effects on sulfate attack (Taylor and Gollop 1997).
  • Shashiprakash and Thomas (2001) also performed significant research on combining high-CaO ashes with silica fume, with emphasis on mixtures between 3% and 6% percent silica fume (by mass of total cementitious materials). Figure 2.3 shows the effects of just a small dosage of silica fume (3%) on the sulfate resistance of concrete with a high-calcium fly ash.

 

 

LEAVE A REPLY

Please enter your comment!
Please enter your name here