An Experimental Study on Reasonable Content of Slag in Concrete C30

Abstract. This study used ordinary portland cement of 42.5 strength grade, mixed with various minerals and water reducer for the preparation of concrete C30 by means of four schemes, for each of which a specific mix proportion was employed. Accordingly the compressive strength of concrete with different preparations at different ages were compared. No matter which scheme was employed, single or double doping, with or without superplasticizer, the compressive strength of concrete at different ages was maximized when slag amount was 30% or maximum. Hence it can be concluded that the reasonable content of slag should be 30% for the preparation of concrete C30. It is suggested that the above results can be better applied to large-volume concrete, underground, harbor, road and bridge and civil engineering as well.

Key words: slag; fly ash; superplasticizer; compressive strength; reasonable content

1. Introduction

Slag is a kind of melt mainly made up of calcium aluminosilicate through blast furnace ironmaking and possesses potential high activity. It can be ground into powder which is equivalent replacement of part of cement and incorporated with concrete to improve its working property, compressive strength and durability and to delay the setting time (Zhang Caixia, 2004). Its preliminary strength is equal to silicate while its later strength is comparatively high. In addition, slag is a sort of industrial waste whose price is much lower than cement. Slag, together with its composite admixture, can physically improve the mechanical properties of cement in the following three ways: (1) achieving composite gel effect of admixture; (2) improving its bond strength of interface; (3) forming its own microscopic compact structure.

Accordingly slag proves to be ideal material for the preparation of concrete. High performance concrete has been attached much importance for its superior workability, mechanical properties and durability. This concrete prepared with high quality admixture, water reducing agent and low water-binder ratio has been unanimously recognized by the academic sphere. Compared with ordinary concrete, it inevitably requires more amount of mineral admixture in order to save cement or solve other technical problems. Although China is rich in mineral admixture but very little of it can be used. On the contrary, consumption of mineral admixture increases with each passing year while concrete has been commercialized. On the other hand, less cement content is required for high performance concrete, large variation of mineral admixture may cause the discreteness of its strength, which is harmful for its quality. At present, some research institutes or companies have begun to physically process the raw material, such as employing superfine grinding or air separation technology which did achieve success technologically in certain sense but inevitably increases cost because of energy consumption, low efficiency and initial high investment of equipment. Hence it cost more than it should be for the preparation of cement and there is much room for improvement. Accordingly, Wang Anling (2005) made some experiments on this to meet the requirements of durability and construction of national grand theatre and stadium. According to Wang, active mineral admixture can fill and refine the stone hole of cement and improve its density and interface structure between it and coarse aggregates.

2. Experiment

2.1 Material

2.1.1 Water reducer

It is naphthalene series FDN produced by new material factory, the 3rd Bureau of CSCEC. Outer join method is used.

2.1.2 Cement

It is ordinary portland cement of 42.5 strength grade produced by Huangshi Huaxin Cement Plant. According to the national standard in ISO (GB/T17671-1999, a group of specimens with cement sand strength are prepared and maintained until the age under the standard condition. Their mechanical properties all meet the standard required.

2.1.3 Coarse aggregate

It is gravel of good grading produced by Tianshan, Huanghsi, Hubei Province. Its packing density is= 1383 kg/and its apparent one is= 2.70.

2.1.4 Fine aggregate

It is medium sand of good grading from Xiaba River, Wuhan and meets the need of Region II with its apparent density =2.65g/c, packing density=1440kg/and fineness modulus 2.77.

2.1.5 Fly ash

It is dry discharge fly ash of grade two from Qingshan Thermal Power Plant. After grinding, its specific surface area is 6000and apparent density=2.1g/c.

2.1.6 Slag

It is granulated blast-furnace slag processed by Metallurgical Slag Plant, Wuhan Iron and Steel Corporation. Impurities are removed, the dried slag is ground in ball mil, 5% gypsum and 0.05% triethanolmine are added. After the grinding and activation, slag’s specific surface area is 4500 and apparent density=2.35g/c.

2.2 Scheme

This experiment was conducted in a lab in School of Mechanics and Civil Engineering, Huazhong University of Science and Technology. Granulated blast-furnace slag is used and mixed with grounded fly ash and water reducer by means of equivalent replacement. Various mix proportions are employed, compressive strength of different schemes are compared and thus the best amount of slag is obtained.

2.3 Result

2.3.1 Specimen preparation

This study used the above-mentioned raw material, namely ordinary portland cement of 42.5 strength grade for the preparation of C30 concrete. Its water cement ratio is W/C=0.53(or 0.42). Various schemes are used to prepare concrete of different ages such as 7d、14d、28d、60d whose results are compared.

2.3.2 Measurement of the strength of concrete

This experiment was conducted in a laboratory of School of Mechanics and Civil Engineering, Huazhong University of Science and Technology. Since non-standard specimen 100×100×100mm was used, the coefficient 0.95 was supposed to be multiplied by the strength measured. The standard for the measurement should be Article 6.0.5 in Experiment Standard for Mechanics Performance of Ordinary Concrete (GB/T50081-2002). There are four schemes in all.

Scheme one: slag replaced cement content of 10%-50%.

Scheme two: slag replaced cement content in the range of 10%-40% and fly ash replaced cement content of 10%-40%.

Scheme three: furnace slag replaced cement content in the range of 10%-40% and the dosage of fly ash is 10%.

Scheme four: furnace slag replaced cement content in the range of 10%-40% , the dosage of fly ash was 10% and 1% water reducing agent was added. For the details of the four schemes, see Table one. For the sake of observation and analysis, the mix ratio of concrete，concrete mix per cubic metre and compressive strength of different ages are listed in Table one.

3. Result and Analysis

From Table one, it can be obviously seen that after the incorporation of composite slag admixture, there has been certain improvement for the compatibility of different cement and admixture. In addition, there will be no more loss as far as degree of fluidity is concerned and as mixing amount increases, compatibility is also becoming better and better, which is of paramount importance for the pump delivery technology of concrete. The following dwells upon each of the four schemes.

Scheme one: when slag was single doped for the concrete, from No. 1-1～1-5 it could be seen that mixing amount of blast-furnace slag was in the range of 10%～30% and the strength of concrete at different ages was increasing. In addition, When mixing amount reached 30%, the strength of concrete 7d、14d、28d、60d was maximum, 28.7 MPa、34.3 MPa、38.3MPa and 44.2 MPa respectively while strength was decreasing when mixing amount of slag was more than 30%.

Scheme two: No. 2-1～2-4 revealed that when slag and fly ash were double doped for the concrete, mixing amount of blast-furnace slag was in the range of 10%～30% and proportion of fly ash was in the range of 40%～20%，the strength of concrete at different ages was increasing. When mixing amount of slag reached 30% and fly ash amount was 20%, the strength of concrete 7d、14d、28d、60d was maximum, 23.5 MPa、27.1MPa、33.8MPa 、41.0 MPa respectively. Strength of concrete was decreasing when mixing amount of slag or fly ash was more than 30%.

Scheme three: from No. 3-1～3-7 in table one it can be seen that when slag and fly ash were double doped for the concrete, or proportion of fly ash was fixed as 10%, and mixing amount of blast-furnace slag was in the range of 10%～30%, the strength of concrete at different ages was increasing. When mixing amount of slag reached 30%, the strength of concrete 7d、14d、28d、60d was maximum, 28.8 MPa、36.8MPa、39.4MPa 、44.0 MPa respectively。

Scheme four: No. 4-1～4-6 in table one. When slag and fly ash were double doped for the concrete, 1% of water reducing agent was added and mixing amount of blast-furnace slag was in the range of 10%～30%, the strength of concrete at different ages was increasing. When mixing amount of blast-furnace slag was 30% and fly ash 10%, the strength of concrete 7d、14d、28d、60d was maximum, 41.3 MPa、43.9MPa、44.1MPa 、46.05MPa respectively. Strength of concrete was decreasing when mixing amount of slag or fly ash was more than 30%.

From the experimental data of the four schemes, it can be seen that no matter which scheme was adopted, weather it was single or double doping, with or without water reducing agent, the strength of concrete at different ages was maximum when mixing amount of blast-furnace slag was 30%. Hence it can be seen that the reasonable content of slag should be 30% for the preparation of concrete C30.

4 Conclusion and Suggestion

From the above results and discussion it can be concluded that when slag powder replaced small amount of cement, the preliminary strength of concrete increased in a sense, mainly because of slag powder’s micro filling and topography effects. When the replacement amount increased, slag powder’s micro filling and topography effects were not sufficient to make up for the damage of the preliminary strength of concrete due to the decrease of large amount of cement. And this might explain the reason why the preliminary strength of concrete was decreasing while strength of 28d increased and was maximum when mixing amount reached the maximum. The later strength of concrete increased due to the second reaction between slag powder and Ca(OH)2. Crystal concentration decreased around the interface of aggregate-cement stone, the size of crystal grain was reduced and compactness of the interface was increased so that the interface structure of aggregate-cement stone was greatly improved. Thus the following conclusions can be arrived at.

Slag powder can be used not only to protect the environments, improve the durability of materials made of cement and possesses significant economic benefits. The production of Cement, main raw material of concrete, consumes too much resources and energies and greatly pollutes the environments. Various engineering requires higher performance concrete, a basic and widely-used building material. Besides strength, durability of concrete is also required. Hence durability of high performance concrete becomes the main indicator of engineering design. To prepare concrete of high quality, appropriate amount of mineral admixture is required in addition to the necessities such as cement, water, aggregate and admixture. Obviously slag powder has become one of the necessary means as far as durability and working performance of concrete are concerned. It may decrease the crack caused by temperature variation because of hydration heat.

Because of the need of large amount of cement and high temperature caused by hydration heat for the preparation of high strength and performance concrete, the inner temperature may reach as high as 60-70℃ or even 95℃ with a short period of time. The difference between the internal temperature and the external one may lead to the crack of concrete. And ground slag powder can significantly decrease the temperature. Accordingly its incorporation decreases the amount and also hydration heat of cement on the condition that the total amount of cementing material with gypsum remains the same, especially when admixture increases. Although slag powder may cause volcano ashes reaction in concrete and release hydration heat, this reaction should lag behind cement hydration reaction and takes more time. With the gradual heat emission of concrete to the surroundings, hydration heat produced by the secondary cement hydration reaction contributes little to the increase of the high temperature of concrete. Research indicated that hydration of slag is low with cement hydration first and slag hydration after it. As the result, not only the temperature of concrete is decreased, but also the occurrence time for its highest temperature is delayed. Both of these two are favorable of the avoiding or decreasing of the crack of concrete.

The incorporation of slag produces hydration calcium silicate with higher strength, more durability, more amount and low alkali, which can improves the formation of hydration of gel material and the durability of concrete. For instance, with sulfate, concrete mixed with active mineral admixture can be greatly improved for its resistance against sulfate. With alkali-aggregate reaction, concrete mixed with slag powder has hardened body of smaller porosity than that of portland cement. In case that water can not enter the inner part of slag concrete, there will be serious lack of water. When freezing and thawing occurs alternately, Concrete’s resistance against freezing is also greatly strengthened. Frost resistance is directly associated with low water cement but also closely related to active mineral admixture. In addition, as far as concrete in sea water is concerned, calcium silicate in cement reacts to sulfate in sea water , produces ettringite and high pressure of crystallization and leads to the expansion, cracking and destruction of concrete. Besides, ordinary portland cement with high calcium silicate produces more Ca(OH)2 which can reacts to chloride and produces loose CaCl2 with low compressive strength. Nevertheless, incorporation of slag powder may consume large amount of Ca(OH)2. Even if sulfate invades, little calcium rock is produced.

After the incorporation of slag, the matrix hardening of slag-cement becomes compact, its seepage resistance turns to be strong and it becomes more capable of resisting infiltration, sea water and sulfate. In addition to the above mentioned two aspects of significance of the application of slag powder, it possesses very high economic value. As admixture of concrete, slag powder may improve concrete’s performance in a sense, accordingly prolongs the life of cement and decreases the large amount of cost needed to maintain and rebuild. In addition, the application of slag decreases the amount of cement in concrete and the its cost is decreased to 10一15％(Jiao Lijing, 2004). To sum up, the following are the specific characteristics of concrete C30 in this experiment.

(1) As far as the preparation of concrete is concerned, composite slag admixture makes full use of its own shape, active and micro-aggregate filling effects so as to form a super-stack effect. It overcomes the defects and negative effects of single fly ash or ground granulated blast furnace slag and proves to be economical.

(2) In certain sense composite slag admixture improves the compatibility of cement and admixture. At the same time the working properties of cement have been improved and last long enough time, which is of great significance for the actual construction.

(3) Concrete made up of composite slag admixture possesses not only high working, mechanical and durable performance but also apparently economical and social benefits.

(4) By means of using composite slag admixture for the preparation of concrete, large amount of industrial waste can be made full use of so as to protect the environments, which conforms to the national strategy of sustainable development.

The experiments results of this study indicated that if slag and fly ash replace part of cement, less cement will be needed for the preparation of concrete C30. The obvious influence factors might be water gel ratio, ground slag and fly ash which are the equivalent replacement of cement. There exists a best replacement value, namely gel, 30% and 15% of the total amount, high performance water reducing agent, 2.0% of the total mixing amount, which should be best for the improvement of the compressive strength of concrete 26d.

It is suggested that the concrete mixed with slag powder can be applied to large-volume concrete engineering, underground, harbor, road and bridge and ordinary civil engineering as well.

5. Acknowledgement

This research is supported by science foundation of Hubei Engineering University (No. z2012013)

References

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4.Jiao Lijin. Research on slag powder’s increase of strength effect in cement and concrete[D]. Huazhong University of Science and Technology, 2002: 46-89.