Comparison of the Compressive Strengths of the Interground and Separately Ground Marble-

The compressive strength development of the marble-blended Portland cement mortars was investigated up to 28 days. The results are shown in Table 4.3.

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Table 4.3. Compressive strength development of the interground and separately-ground marble-containing mortar mixtures

Compressive Strength (N/mm2)

Marble

% Grinding Blaine fineness

(cm2/g) 2-day 7-day 28-day

6-I-3000 6 I 3000 17.6 32.3 41.7

15-I-3000 15 I 3000 16.4 31.1 38.3

30-I-3000 30 I 3000 13.5 26.3 34

6-I-5000 6 I 5000 27.4 41.8 49.1

15-I-5000 15 I 5000 19.4 33.5 39.7

30-I-5000 30 I 5000 18 30.9 35.4

6-S-3000 6 S 3000 20.6 35.1 43.3

15-S-3000 15 S 3000 19.1 33.8 42.4

30-S-3000 30 S 3000 16.4 31 37.7

6-S-5000 6 S 5000 31.7 46.9 53.4

15-S-5000 15 S 5000 29.6 44.3 51.5

30-S-5000 30 S 5000 24.3 38.5 44.4

CONTROL - - 3000 20.3 37.6 50.5

CONTROL - - 5000 26.7 45.4 57.8

Figures 4.17 and 4.18 show the compressive strength development of the interground and separately-ground marble blended cements.

Figure 4.17. Compressive strength development of the interground blended cements with Blaine fineness 3000 cm²/g

Figure 4.18. Compressive strength development of the separately-ground blended cements with Blaine fineness 3000 cm²/g

It can be seen that increasing marble content causes a decrease in the compressive strength values at all ages. This decrease corresponds to about 20 % for the interground mixtures and about 15 % for the separately-ground mixtures, at 28 days, for 30 % cement marble content. The 2-day strength of all

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3000 cm²/g fineness mixtures are all above ~12 MPa, even for 30 % cement marble addition. The strength development for the interground and separately-ground 5000 cm²/g Blaine mixtures is shown in Figures 4.19. and 4.20.

Figure 4.19. Compressive strength development of the interground blended cements with Blaine fineness 5000 cm²/g

Figure 4.20. Compressive strength development of the separately-ground blended cements with Blaine fineness 5000 cm²/g

Increasing the fineness of the blended cements can increase the 2-day strengths to 20 MPa. Once again, the decrease in strength at any chosen age is greater for the interground cements than for the separately-ground ones.

The most important observation made is that separately-ground cement mortars always give higher strengths than the interground cement mortars. This is no doubt due to differences in the size distribution of the cements, particularly the differences in the mean sizes of the clinker and the marble within a blended cement. In separate grinding to a selected Blaine fineness value the fineness of both the clinker and the marble are approximately the chosen fineness value. Hence, the mean or medium particle diameter is similar for both raw materials. In intergrinding however, the difference in the hardnesses of the two raw materials lead to different grinding behavior and the softer marble tends to accumulate more in the smaller sizes. The clinker tends to remain coarser than it would otherwise.

Since the marble do not contribute much to the strength of the mortar, especially at early ages, the coarser-ground clinker reacts less at early ages and less overall. The effect on ultimate strength is more difficult to interpret as it depends on the chosen fineness. As the overall cement fineness increases, the median diameters of both the marble and the clinker decrease. For very high fineness, a large fraction of even the coarser clinker particles can hydrate, given enough time, contributing to ultimate strength gain. For lower overall fineness, the part of the clinker that remains unhydrated will be greater and the ultimate strength will be low in addition to the low early strength.

When the strengths of the various mortars are compared to the control mortars containing 100 % Portland cement, it is seen that some of the separately-ground marble blended cements give higher 2-day strengths. At both 3000 and 5000 cm²/g Blaine, the strengths of the 6 % marble-containing mixtures exceed the strengths of the control mortars. This amount of exceeding is very small for the lower fineness case but greater than 15 % for the higher-fineness mortar. In fact, even the 7-day compressive strength of the 6 % marble-containing 5000 cm²/g Blaine mortar exceeds the strength of the portland cement-only control at the same age (46.9 MPa > 45.4 MPa). While all 28-day compressive strengths are lower than that of the control, the separately-ground blend mortars come within 8 %.

Another observation about strength development is that separately-ground cement mortars achieve a greater fraction of their ultimate strength sooner i.e. their 2-day strengths are a greater fraction of their 28-day strengths. There exists a 20 to 25 % difference between the 28-day strengths of the 5000 Blaine cement mortars and the 3000 Blaine cement mortars, for the separately ground mortars. A possible explanation for this could be that much of the clinker in the 3000 Blaine cements is too coarse and does not react, keeping the degree of hydration low but in the case of the 5000 Blaine cements, this coarse clinker is reduced sufficiently in size and starts to react within the 28-day period.

Yet another observation is that the drop in strength with increasing marble content is steeper for the interground blends than it is for the separately-ground blends.

The setting behaviors and of the different blended cements are presented in Table 4.4. Also shown, are the water requirements of the mortar mixtures i.e. the amount of water in percentage of the mass of powder materials, needed to achieve “normal consistency”.

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Table 4.4. Initial and final setting times, and normal consistency water requirements of the interground and separately-ground marble-containing mortar mixtures

In accordance with their lower compressive strengths, the interground cement mortars set slower than their separately-ground counterparts. The difference between mortars with identical marble contents is 30 to 40 minutes, both for initial setting time and for final setting time. Both initial and final setting correspond to some mass-based degree of hydration completion and the coarser clinker in the interground cases achieves these critical hydration completion percentages later.

Increasing fineness results in earlier setting. From 3000 cm²/g to 5000 cm²/g, both initial and final setting time shorten by about one hour. As for the influence of increasing marble content on setting time, it is insignificant. The uncertainty in determining the setting time values is probably greater than the actual differences observed between the different cases.

Set retardation due to marble use has been attributed to the retardation of the hydration of C3A. It is believed that the CaCO3 increases the set retardation efficiency of gypsum by altering the surface of C3A by forming carboaluminates [Ramachandran and Chun-mei, 1986].

CHAPTER 5

CONCLUSIONS

5.1 Summary

This study investigated the use of industrial waste marble cuttings as a source of CaCO3 in marble-containing blended cement production. The blended cements were of the types CEM II/A-L and CEM II/B-L according to TS EN 197-1. Two different grinding methods were employed: separate grinding of the marble and the clinker/gypsum, and intergrinding. The gypsum-to-clinker ratio was kept constant for all cements while the marble-to-clinker ratio was varied. Two different ultimate cement finenesses were targeted and the ease of grinding of the different cases were compared. Then, mortar prism samples were prepared with the cements produced and some of their fresh and hardened properties were compared up to 28 days.

5.2 Conclusions

The following conclusions were reached as a result of this thesis study:

1) The marble is softer than the clinker and is therefore easier to grind. Mixtures of marble and clinker offer intermediate resistance as they are softer than the clinker but harder than the marble. Surprisingly, however, the marble content does not influence the overall cement grindability very much probably due to the hygroscopic marble hindering the grinding by coating the charge and insides of the mill.

2) The 5000 cm²/g Blaine fineness cements have broader particle size distribution peaks than the 3000 cm²/g cements. The D50 (median) values of the interground cements are significantly higher than those of the separately-ground cements.

3) Increasing marble content causes a decrease in the compressive strength of the mortars at all ages. This difference is ~20 % for the interground mixtures and ~15 % for the separately-ground mixtures, as 28 days for a marble content of 30 %. Despite this, the ultimate strengths of even the 30 % marble mixtures are acceptable.

4) Increasing the fineness of the cements increases the achieved compressive strength.

5) Separately ground cements always give higher strengths than interground ones with the same fineness and mineral additive content. This is attributed to differences in the resulting relative sizes of the clinker and additive in the two grinding methods. In addition, separately-ground cement achieve a greater percentage of their ultimate strength, sooner, meaning their 2-day to 28-day compressive strength ratios are higher than for the interground cements.

6) The early-strength of the mortars up to 7 days is not reduced much and even improved for the separate grinding case for a replacement level of 6 %. At higher replacement levels, both early and ultimate strengths are diminished. The drop in strength with increasing marble content is steeper for the interground cements.

7) The setting times, both initial and final, of the interground cements are longer than their separately-ground counterparts. This difference is slightly more than half an hour for both

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setting times. This is in accordance with the strength development shown by the cements.

Increasing fineness leads to shorter setting times, as expected. Marble content, however, does not seem to affect setting time noticeably.

5.3 Recommendations for Future Studies

Based on the results of this study, the following recommendations can be made for research along the same lines:

 The hydration and mechanical property development of CaCO3-containing blended cements can be supported by x-ray diffraction studies performed on hydrating samples taken at closely-spaced intervals to observe the formation and disappearance of crystalline phases.

 Thin section analysis of the marble used can be done.

 Scanning electron microscopy and x-ray techniques can be employed to distinguish the particle size distributions of the mineral additive, the clinker, and the gypsum following intergrinding.

 The heat evolution of interground and separately-ground cements of the same Blaine fineness further sieved into several size fractions can be measured to assess the contribution of different sized particles to the overall property development and to relate size with composition.

 A greater number of calcareous mineral additives can be used and their performances compared.

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