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RESEARCH: The Beneficial Effect of Slag Cement on Low Temperature Sulfate Resistance of Concrete Produced with Portland-Limestone Cements and Slag

Student: Reza Mohammadi Ahani, University of Toronto

Reza was presented with a 2018 Slag Cement Project of the Year Award in the Category of Research.  More information about the ceremony can be found here. 

Presentation Slides PDF: 

RezaAhaniResearchPre...

rezafadedAbstract

Portland-limestone cement with up to 15% interground limestone was included in the Canadian Standard A3000 for Hydraulic Cements in 2008. However, due to the lack of data on sulfate resistance of concrete especially the potential for thaumasite sulfate attack at low temperatures, the use of portland-limestone cement in concretes subjected to moderate or severe sulfate exposures, even when blended with supplementary cementitious materials (SCMs), such as slag, was not allowed.

In this experimental research, the sulfate resistance of fifty three concrete mixtures produced with several combinations of portland and/or portland-limestone cements and SCMs including slag, silica fume, metakaolin, and fly ash at three different water-to-cementitious materials ratios of 0.40, 0.50, and 0.70 exposed to both sodium and magnesium sulfate solutions was evaluated in both laboratory and field exposures at low temperatures by measuring changes in length, mass, and resonant frequency of concrete prisms as well as making visual inspections. Also, the causes, mechanisms, and depth of deterioration in damaged concrete mixtures having insufficient levels of slag was investigated by mineralogical analysis using X-ray diffraction and microstructural analyses using micro X-ray fluorescence spectrometry and scanning electron microscopy.

It was found that slag at sufficient replacement levels significantly improve the resistance of concrete to sulfate attack and in such mixtures there was no impact of limestone additions. The sulfate resistance of PC+slag and PLC+slag concrete mixtures is equal to or better than that of currently allowed highly and moderately sulfate-resistant portland and blended cements. Exposure to magnesium sulfate solution is more aggressive than sodium sulfate solution. The sulfate resistance of concrete prisms in the selected field exposures is better than that of prisms in the selected laboratory exposures due to more reasonable concentrations of sulfate solutions, larger prism size, and possibly including the variable temperature used in the field exposure. Since there is no standard test method for evaluating the sulfate resistance of concrete due to the potential threat from thaumasite sulfate attack at low temperature and previous research focused on mortar tests, the results of this research can be used to develop a standard test method for evaluating the sulfate resistance of concrete.

Significance of current study

The significance of the present experimental research, funded by Cement Association of Canada (CAC) and Natural Science and Engineering Research Council of Canada (NSERC), are as follows:

  • Provide data to support adoption of more sustainable and lower carbon footprint concrete by allowing wider use of portland-limestone cement and slag.
  • Develop a standard test method for evaluating sulfate resistance of concrete since at the present time, no standard test methods exist in ASTM, AASHTO and CSA standards. More specifically, there is no standard concrete test for evaluating the potential threat from the thaumasite form of sulfate attack. However, The United States Bureau of Reclamation in USBR 4908 (1992) has three test methods for length change of hardened concrete exposed to alkali sulfates. Previous research (Ramezanianpour and Hooton, 2013a; Ramezanianpour and Hooton, 2013b) focused on mortar bar expansion tests leading to development/evaluation of CSA A3004-C8 Procedure B but this rest method was withdrawn in 2017. But do these results adequately predict concrete performance?
  • Investigate the effect of slag on the sulfate resistance of concrete mixtures produced with portland and portland-limestone cements since there is little data on the performance of portland-limestone cement in combination with slag in concrete exposed to sulfates,
  • Study the performance of concrete regarding thaumasite form of sulfate attack and correlating laboratory and field test results by performing tests both at 5 ºC in the laboratory and in a novel field exposure, buried 2.5 m underground, varying annually between approximately 1.5 and 20.5ºC which simulates the exposure of foundations,
  • Investigate what cause(s) thaumasite sulfate attack and how it can be stopped or mitigated by determining the most effective replacement level of slag and investigating the rate of deterioration from thaumasite sulfate attack, and the effect of sodium and magnesium sulfate solutions at different concentrations on sulfate resistance of concrete.

Objectives and scope of the research

The main objectives of this experimental research are as follows:

  • To evaluate the sulfate resistance of concrete mixtures made with portland and portland-limestone cements and slag in both laboratory and field exposures at low temperatures where the risk of thaumasite sulfate attack is increased,
  • To investigate the causes (the formation of ettringite and/or thaumasite), mechanisms (filling air voids only or by degradation due to disruption of the paste matrix), and depth of deterioration (surface damage only or progressive damage toward the interior) in concrete mixtures exposed to sulfates.
  • To investigate the impact of slag on mitigating the sulfate attack in concrete mixtures made with portland and portland-limestone cements and slag exposed to sulfate solutions at low temperatures in both laboratory and field exposures.

Experimental program

In this study, the sulfate resistance of fifty three concrete mixtures produced with several combinations of portland and/or portland-limestone cements and supplementary cementitious materials exposed to both sodium and magnesium sulfate solutions was evaluated both in the laboratory and in a novel field sulfate exposures at low temperatures where the thaumasite form of sulfate attack is the main concern. The underground field sulfate exposure site was developed at St. Mary’s Cement located at Leaside in Toronto which was a trench dug to 2.5 m deep to accommodate 6 heavy duty covered polyethylene tanks.

Twelve different cements used in this project included GU cement, three types of portland-limestone cements (with 9%, 10.5%, and 15% limestone), and eight types of sulfate-resistant cements.

In order to evaluate the sulfate resistance of combinations of portland-limestone cements and slag, changes in length, mass, and resonant frequency of concrete prisms were measured as well as making visual inspections. Additionally, other properties of the concrete mixtures such as compressive strength, rapid chloride permeability, and resistivity were also evaluated. Also, the cause, mechanism, and depth of deterioration in damaged concrete mixtures having insufficient levels of slag was investigated by mineralogical analysis using X-ray diffraction and microstructural analyses using micro X-ray fluorescence spectrometry and scanning electron microscopy.

Conclusions and experimental findings

The main conclusions made from the results of current experimental research are as follows:

  1. As with room temperature exposure, at low temperatures, concrete mixtures made with 100% high C3A cements with or without limestone are not sulfate-resistant.
  2. In the field exposure, for an equal level of slag, the presence of limestone had no impact on sulfate resistance of concrete.
  3. In the laboratory, PLC15 required 50% slag for equal performance to GU cement with 40% slag. However, in the field, no extra slag was needed.
  4. The sulfate resistance of PC+slag and PLC+slag concrete mixtures at W/CM ratios of 0.40  and 0.50 is equal to or better than that of traditional highly and moderately sulfate-resistant portland and blended cements (CSA A3001 HS, HSb, MS, and MSb).
  5. For all concrete mixtures, the sulfate resistance in field exposure at annual temperature ranging from 1.5 to 20.5 ºC was better than in the laboratory at 5 ºC. This is due to more reasonable concentrations of sulfate solutions, variable temperature exposures, and the larger prism sizes used in the field exposure.
  1. As expected, exposure to magnesium sulfate solution is more aggressive than sodium sulfate solution for all concrete mixtures.
  2. In damaged concrete prisms made with or without portland-limestone cements, the initial signs of deterioration from ettringite were in the form of cracking, spalling, and loss of materials at the corners and edges of the concrete prisms. Thaumasite was only detected at later ages after initial damage caused by formation of ettringite and gypsum. Therefore, it was concluded that thaumasite was not the primary cause of deterioration.
  3. From microstructural analysis using SEM/EDX and micro XRF, the following conclusions are made based on the progression of damage:
    1. Regardless of damage, the sulfate penetration at any given time was limited to 1-2 mm. However, the depth of deterioration in magnesium sulfate exposure was more extensive, up to 5 mm below the surface, compared to sodium sulfate exposure.
    2. In concrete showing damage, after damage to the paste mainly by ettringite formation, 1- 2 mm layers progressively spalled off. However, voids were filled partially or completely with ettringite, thaumasite, gypsum, or a mixture of them.
    3. Thaumasite formation in voids occurred in all mixtures examined including HS cement. Therefore, there is no relationship to the presence of limestone in the cement used.
    4. There was no damage to surface layers of concrete mixtures with sufficient levels of slag even after 5.5 years of exposure and the ingress of sulfate ions only resulted in partial or complete filling of the air voids in the outer 1-2 mm.
    5. Below the surface layer, regardless of damage, voids were only partially filled with gypsum or calcium hydroxide.
  4. Since concrete prisms have bigger cross sections than mortar bars, deterioration mainly occurs at the surface and does not affect the interior of the prisms, thus preventing expansion. Therefore, monitoring mass change of concrete prisms in combination with visual inspection are better tools to evaluate sulfate resistance than measuring the length change.
  5. The results of this experimental research in both laboratory and field exposures can be used to develop a standard test method for evaluating the sulfate resistance of concrete, since no standard test methods currently exist in North American standards except USBR 4908-92.
  6. As the result of this research, the Canadian A3000 and A23.1 standards have been changed to allow the use of portland-limestone cement together with slag in sulfate exposures without any low-temperature testing or other restrictions.

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