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RESEARCH: Rapid Reactivity Testing and Effects of Varying Replacement Levels on Cement Paste Properties

 

Ground Granulated Blast Furnace Slags:

Rapid Reactivity Testing and Effects of Varying Replacement Levels on Cement Paste Properties

By Sivakumar Ramanathan and Prannoy Suraneni

Cement production accounts for approximately 5% of total man-made CO2 emissions.1 An effective way to reduce the amount of ordinary portland cement (OPC) in concrete is to replace a portion of the OPC with supplementary cementitious materials (SCMs). While many SCMs exist, a significant problem associated with them is reductions in early-age concrete strength, especially at high SCM replacement levels. Ground granulated blast furnace slag (GGBFS) can be used to replace up to 70% of the OPC, without significantly reducing early-age properties, and is an attractive SCM when considering both sustainability and durability.1,2 The availability of GGBFS is guaranteed in industrialized countries from manufacturing facilities and in developing countries and GGBFS production could significantly increase with industrialization. Thus, GGBFS appears to be a viable SCM not only at present, but also in the future.

In the current study, a reactivity test that can be used for rapid classification of GGBFS and other SCMs is shown. The reactivity of 11 different GGBFS materials is quantified and compared with other SCMs to “classify” the GGBFS. Furthermore, one GGBFS is used as replacement for OPC at levels of 20, 40, and 60% to study the effects of the GGBFS on cementitious paste properties such as heat release, compressive strength, and bulk resistivity.

Materials and Methods

The reactivity of 11 different GGBFS materials was quantified; further details are given in the supporting documents of this report.3,4 One GGBFS was tested with OPC using a water- cementitious materials ratio of 0.40 (w/cm) and replacement levels of 20%, 40%, and 60%. The compositions of these specific materials are provided in Table 1.

 

 Table 1:

Chemical composition of GGBFS and OPC (% mass)

 

CaO

 

SiO2

 

Al2O3

 

Fe2O3

 

Na2O

 

K2O

 

LOI

 

GGBFS

42.44

32.09

12.88

0.75

0.13

0.27

N/A

OPC

64.41

19.93

4.78

4.08

 

0.18

 

0.37

 

2.23

 

 

Reactivity testing

A recently developed reactivity test is used to directly study the reactivity of the GGBFS.3,4 In the test, SCM and calcium hydroxide are mixed in the ratio of 1:3 by mass and 0.5 M KOH is added to the mixture to simulate a calcium-rich pore solution in which the SCM reactions occur. The liquid-to-solids ratio is maintained at 0.9. The mixture is hand mixed for 4 minutes and then placed into an ampoule and lowered into an isothermal calorimeter conditioned at 50 ± 0.05°C. The data for the first 45 minutes is not collected due to temperature differentials. The testing is carried out for 10 days. At the end of 10 days, the sample is removed, and 35-50 mg of this sample is placed on a platinum pan and thermogravimetric analysis is carried out by ramping the temperature at 10°C/minute up to 600°C from ambient conditions in an inert nitrogen atmosphere and the calcium hydroxide consumption is calculated.

Tests on cementitious paste – Isothermal calorimetry, bulk resistivity and compressive strength

For isothermal calorimetry, GGBFS-OPC pastes were hand mixed in a plastic container using a spatula and the mixtures were then placed in an ampoule and sealed. The ampoules were lowered into the isothermal calorimeter conditioned at 23 ± 0.05°C and heat release behavior was studied for 7 days. For resistivity and strength, cementitious paste samples were mixed in a Hobart mixer according to ASTM C305 and then poured into 50 mm (2 in.) molds. The pastes were demolded at 1 day and moist cured until testing. The bulk resistivity of the pastes was monitored nondestructively at 1, 7, 28, and 56 days using a bulk resistivity meter (ASTM C1760-12) with specimens in a surface-dry condition. Corrections were applied for specimen dimensions as described in supporting documents..5 Compressive strength was then was carried out at 7, 28, and 56 days on four specimens for each paste.

Results and Discussion

Reactivity testing

Figure 1 shows the results of the reactivity testing as a plot of heat release versus calcium hydroxide consumption. For GGBFS, the heat release values are in a relatively narrow range of 377-519 J/g SCM (averaging 446 J/g SCM) and the calcium hydroxide consumption ranges from 12-47 g/100 g SCM (averaging 31 g/100 g SCM). Silica fume and fly ash, which are also shown on the figure have a higher calcium hydroxide consumption due to their more pozzolanic nature. The heat release of the GGBFS is higher than fly ash but somewhat lower than silica fume. The reactivity test developed here has promise for rapid classification of materials, including other slag materials such as steel slags. The role of fineness in reactivity testing, specifically considering GGBFS is being explored.

Isothermal calorimetry, bulk resistivity, and compressive strength

Figure 2a and 2b show the heat flow and release of cementitious pastes containing GGBFS replacements from 0-60%. The peak heat flow decreases proportionally with GGBFS replacement. The (cumulative) heat release values range from 232-301 J/g cementitious material. As the replacement level increases, there is a decrease in the heat release values at all ages. These findings suggest that GGBFS could be used in mass concrete to reduce temperature gradients.2

Figure 3a shows the evolution of bulk resistivity of pastes. At 7 days, there is no significant difference in the bulk resistivity for different mixtures. However, at 28 and 56 days, the bulk resistivity increases as the GGBFS replacement level increases. These findings could indicate reduced transport through the sample and could result in better durability of the mixtures containing higher GGBFS replacements.

Figure 3b shows the evolution of compressive strength of the pastes up to 56 days. At 7 days, the GGBFS 0 paste has a marginally higher compressive strength but at 28 and 56 days, the trend is reversed. This indicates that replacement of OPC with GGBFS does not adversely affect the compressive strength of mixtures after 7 days, and unlike pozzolanic materials such as fly ash, GGBFS does not cause a significant loss of compressive strength at early ages due to its latent hydraulic nature.

Future work will focus on effects of fineness and its impacts on reactivity, study of the reactivity of unconventional slags, and the use of cement paste or mortar bulk resistivity to rapidly evaluate conventional and unconventional slags.

fig1

Fig. 1 (above): Reactivity testing results for GGBFS (average values for other SCMs shown for comparison).

 

fig21

fig22

Fig. 2 (above): Heat release characteristics of different BFS mixtures: (a) Heat flow; and (b) Heat release

 

fig31

fig32

Fig. 3 (above): Evolution of hardened properties of GGBFS pastes: (a) Bulk resistivity; and (b) Compressive strength

Conclusions

The use of GGBFS could be significant in developing countries, which experience significant industrialization and infrastructure development. This could contribute to a significant reduction in carbon footprint in terms of cement and concrete consumed and environmental concerns in terms of waste management. High-replacement levels of GGBFS without impacting the concrete properties make this material a prime candidate to address the issue of sustainable and environmentally friendly development in such regions.

The main conclusions from this study include:

  • The reactivity test shown here shows promise for rapid classification of SCMs and could be used to study conventional and unconventional slag materials;
  • High replacement levels of GGBFS reduce the heat release and therefore may reduce temperature rise. This indicates that BFS could potentially be used in mass concreting applications;
  • Higher replacement levels of GGBFS result in an increase in bulk resistivity, suggesting reduced transport of ionic species through the bulk. This test may be used to rapidly assess durability of cementitious materials; and
  • The compressive strength evolution suggests that high replacement levels of GGBFS do not adversely affect the compressive strength at early and later ages.

References

  1. Juenger, M.C.G.; Winnefeld, F.; Provis, J.L.; and Ideker, J.H., “Advances in alternative cementitious binders,” Cement and Concrete Research, V. 41, No. 12, Dec. 2011, pp. 1232-1243.
  2. Moon, H.; Ramanathan, S.; Suraneni, P.; Shon, C.-S.; Lee, C.-J; and Chung, C.-W., “Revisiting the effect of slag in reducing heat of hydration in concrete in comparison to other supplementary cementitious materials,” Materials 11 (10), Sep. 2018.
  3. Suraneni, P., and Weiss, J., “Examining the pozzolanicity of supplementary cementitious materials using isothermal calorimetry and thermogravimetric analysis,” Cement and Concrete Composites, V. 83, Oct. 2017, pp. 273-278.
  4. Suraneni, P.; Hajibabaee, A.; Ramanathan, S.; Wang, Y.; and Weiss, J., “New insights from reactivity testing of supplementary cementitious materials,” Cement and Concrete Composites, V. 103, May 2019, pp. 331-338.
  5. Spragg, R.; Villani, C.; Snyder, K.; Bentz, D.; Bullard, J.; and Weiss, J., “Factors that influence electrical resistivity measurements in cementitious systems, Transportation Research Record, V. 2342, No. 1, Jan. 2013, pp. 90-98.
  6. Wang, Y., and Suraneni, P., “Experimental methods to determine the feasibility of steel slags as supplementary cementitious materials, Construction and Building Materials, V. 204, Apr. 2019, pp. 458-467.

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