Abrasion Resistance of Cement Paste with Granulated Blast Furnace Slag and Its Relations to Microhardness and Microstructure

CHEN Xiaorun ,HE Zhen ,CAI Xinhua ,ZHAO Rixu ,HU Lingling ,CHEN Hongren

(1.China West Construction Academy of Building Materials,Chengdu 610015,China;2.China Construction Ready Mixed Concrete Co.,Ltd.,Wuhan 430205,China;3.State Key Laboratory of Water Resources and Hydropower Engineering Science,Wuhan University,Wuhan 430072,China;4.Shandong Chunhe New Material Research Institute Co.,Ltd.,Rizhao 276800,China;5.Hubei (Wuhan) Institute of Explosion Science and Blasting Technology,Jianghan University,Wuhan 430056,China)

Abstract: The abrasion resistance of cement pastes with 30 wt%,40 wt% and 50 wt% granulated blast furnace slag (GBFS),and its relations to microhardness and microstructure like hydration products and pore structure were studied.Results indicated that GBFS decreased the abrasion resistance of paste,and among the pastes with GBFS,the paste with 40 wt% GBFS showed the highest abrasion resistance.The microhardness of GBFS was lower than that of the cement,and the microhardness of the hydration products in paste with GBFS was also lower than that of the hydration products in paste without GBFS,so that the abrasion resistance of paste decreased when GBFS was incorporated.The reason for the decrease of microhardness of pastes with GBFS was that the contents of Ca(OH)2 in pastes with GBFS was significantly lower than that in the paste without GBFS,while large amounts of calcium aluminate hydrates and hydrotalcite-like phases (HT) in pastes with GBFS were generated.Furthermore,among the pastes with GBFS,the paste with 40 wt% GBFS showed the lowest porosity which was the main reason for its highest abrasion resistance.

Key words: paste;abrasion resistance;granulated blast furnace slag;microhardness;microstructure

1 Introduction

The overflow surface of hydraulic structures and road surface often incurs severe abrasion erosion damage.The ways to improve the abrasion resistance of concrete include using the maximum amount of the hardest available coarse aggregate,improving the impact toughness of concrete by the introduction of various fibersor latexinto the matrix,and improving the compressive strength of concrete by reducing the

w/c

ratioand incorporating mineral admixtures,like silica fume (SF),granulated blast furnace slag (GBFS)and fly ash (FA).From the earliest research and application of abrasion resistance concrete,SF has been successfully used in hydraulic abrasion resistance concrete.The research by Liuindicated that for concretes with surface crack containing 5 wt% and 10 wt% SF,the gains in abrasion resistance were nearly 13% and 15%,respectively,over that of the reference concrete.In recent years,GBFS has also been used in abrasion resistance concrete and performs well as expected.The research by Kumar

et al

indicated that the abrasion resistance of concrete containing GBFS was higher than that of the reference concrete,but lower than that of the concrete containing SF.The research by He

et al

indicated that GBFS with a surface area of 719 m/kg can obviously improve the abrasion resistance of concrete.Studies about the influence of FA on the abrasion resistance of concrete indicated that the modification effect of FA was not obvious,and the abrasion resistance of concrete would even decrease when the content of FA exceeded 20 wt%.The improvement of the abrasion resistance of concrete incorporating mineral admixtures is mostly attributed to a good interfacial bond strength and a dense matrix.While,the influence and mechanism of mineral admixtures on the abrasion resistance of the paste is still unclear.Hardened cement paste is a multiphase composite material composed of unhydrated cement particles,hydrated products (including CSH,CH,Aft,

etc

) and pores,and its macroscopic mechanical properties depend on the micro-structural features (

i e

,the type,amount and distribution of solids and voids).In this paper,the abrasion resistance of pastes with GBFS at different contents and its relationships with micro-hardness,composition of hydration products and pore structure were studied to reveal the mechanism of the influence of GBFS on the abrasion resistance of paste.

2 Experimental

2.1 Materials and sample preparation

Cement (Grade 52.5,named PⅡ52.5) and GBFS(Grade 105) were used in this study.After examining their chemical compositions and specific surface areas(Table 1),GBFS was mixed with cement in 30%,40% and 50% quantity on weight replacement basis.Samples were named as:P,control sample without GBFS;FS

n

,cement with GBFS,where

n

=30,40 and 50 respectively,represents 30%,40% and 50% ratio of GBFS replacing cement.Ratio of water to binder was 0.3 in all samples.

Paste specimens (φ25 mm×50 mm) were prepared and cured in water at 20 ±2 ℃.Until 90 days,specimens were cut into two pieces of φ25 mm× 25 mm for abrasion resistance test.After abrasion resistance test,slices approximately 10 mm thick were obtained from the center of the specimens by cutting and polishing for micro-hardness test.Cubes with a size of 10 mm were impregnated under vacuum with a spectrally transparent epoxy resin,and gradually polished using cloth impregnated with diamond spray of successively finer sizes down to 0.05 μm for backscattered electron image analysis (BSE-IA).To prevent carbonation and continued hydration,the samples for micro-hardness test and BSE-IA were kept in pure alcohol.The debris from the crushed paste specimens were kept in pure alcohol to stop hydration,and then ground to a size less then 10 μm for thermogravimetric/derimetric thermo-gravimetric (TG/DTG)analysis.Cubes with a size of 5 mm were removed from the center of the crushed paste specimens and vacuum dried at 60 ℃ for 48 h for mercury intrusion porosimetry (MIP) analysis.

2.2 Test methods

2.2.1 Abrasion resistance

Fig.1 Simplified schematic diagram of the abrasion test

The simplified schematic diagram of the abrasion test is shown in Fig.1.The control head and the grinding disc rotated in opposite directions at a fixed speed,and for each group of the specimens,six pieces were rested in the control head for abrasion test.The abraded surface was a cutting surface with a size of φ25 mm.The rotation speeds of grinding disc (

R

) and control head (

R

) were both 50 r/min,the normal load(

P

) was 30 N,and the abrasive particles were silicon carbide particles of 22 μm inlaid in sandpaper.During the test,the debris was washed away with tap water,and the test pieces under saturated dry condition were weighed before and after the abrasion test using an electronic balance with an accuracy of 0.01 g.The test was repeated 3 times with a single test time of 5 min and a new sandpaper of the same specification for each test.The abrasion resistance of the paste was calculated using Eq.(1):

The parameter

Wr

is the abrasion resistance,namely,the minutes required for the unit mass of abrasion,min/g.The parameter

i

is the test times.The parameter

T

is the duration for one test,5 min.The parameters

G

and

G

are the average mass before andafter the abrasion test,g,respectively.

2.2.2 Microhardness

The micro-hardness of paste was determined by an HXS-1000 digital intelligent micro-hardness tester.The test load was 100 gN with a contact time of 10 s.Ten points at equal distances in the straight direction of the sample surface were taken for the micro-hardness measurement.

2.2.3 BSE-IA

SIRION TMP field emission scanning electron microscope was used to observe the hydrated cement pasted in backscattering mode,and quantitative analysis of each component was carried out according to the gray characteristics.

2.2.4 TG/DTG

SDT-Q600 thermal analyzer was used to analyze the content of components in the paste with thermal decomposition characteristics.The temperature range was from room temperature to 1 000 ℃,the heating rate was 10 ℃/min,and 100 mL/min of nitrogen was passed to prevent sample carbonation.The total mass fraction of CH (

W

) was calculated using Eq.(2):

The parameters Landare the mass losses attributed to CH and CaCO,respectively.The parameters

M

,andare the molar masses of CH,HO and CO,respectively.The formed CaCOwas assumed to be from the CH carbonation during the grinding process and sample preparation.The bond water content from C-S-H and the aluminiferous hydration products (

BW

) was calculated using Eq.(3):

The parameter

L

is the total mass loss between 105 and 1 000 ℃.The mass loss before 105 ℃ was due to the loss of evaporable water.

2.2.5 MIP

The pore structure was tested using a Autopore V9600 mercury intrusion meter with a pressure range of 0.50-61 000 Pa and a corresponding pore size measuring range of 0.003-360 μm.

3 Results and discussion

3.1 Abrasion resistance of paste

Table 2 shows the abrasion resistance of P,FS30,FS40 and FS50,which are 3.50,2.86,3.01 and 2.63 min/g,respectively.From the above results,the incorporation of 30 wt%,40 wt% and 50 wt% GBFS lead to a decrease in the abrasion resistance of paste by 18%,14% and 25%,respectively,and among the pastes with GBFS,the paste with 40 wt% GBFS shows the highest abrasion resistance.Therefore,when cementitious materials with abrasion resistance requirements are mixed with GBFS,40% of the amount is a better choice.

*The date in bracket is the relative value of the abrasion resistance of each group of concrete to that of P.

3.2 Relation of abrasion resistance to weighted average microhardness of paste

3.2.1 Microhardness of paste

The measured values of microhardness of paste are shown in Table 3.The value 441.3 kg/mmin P and the value 310.3 kg/mmin FS50 are significantly higher than other values.According to the composition of paste and the position of the indentation corresponding to these values,it can be inferred that the values of 441.3 and 310.3 kg/mmare the microhardness of cement and GBFS,respectively.In order to compare the microhardness of the hydration products (including solid products and pores),the values excluding 441.3 and 310.3 kg/mmare selected for analysis,as shown in Fig.2.It can be seen from Fig.2 that the average micro-hardness of the hydration products in P is 75.2 kg/mm,which is higher than those of the hydration products in FS30,FS40 and FS50 being 62.1,63.3and 59.2 kg/mm,respectively.The reason may be that the hydration of GBFS consumes a large amount of Ca(OH)which have a higher elastic modulus than C-S-H.Meanwhile,the microhardness value distribution of P is less uniform than those of the pastes with GBFS,which may be due to the reason that the incorporation of GBFS makes the external hydration products of the paste increase,and the size of Ca(OH)is refined and uniformly filled in C-S-H.

Table 3 Microhardness of paste/(kg/mm)

Fig.2 Microhardness of paste

3.2.2 Volume fraction of each component in paste

Fig.3 BSE image of paste

The backscattered electron images of pastes are shown in Fig.3.According to the characteristics of the gray value of the backscatter images,it can be seen that the holes and cracks,hydration products,unhydrated GBFS and unhydrated cement are in order from dark to light.Because the gray value range of GBFS overlaps with that of the unhydrated cement and CH,the GBFS in FS30,FS40 and FS50 was manually selected and statistically analyzed according to its typical angular shape.Due to the limitated resolution of BSE image,the smallest pore that can be resolved is 0.17 μm,so that the hydration products and pores were counted as a whole.After software processing,the area percentages of cement,GBFS and hydration products were obtained,which were the volume percentage of different phases,as shown in Table 4.It can be seen from Table 4 that the unhydrated phase in P is cement,while that in FS30,FS40 and FS50 is mainly GBFS,and in all pastes,the volume percentage of hydration products exceeds 65%.

3.2.3 Weighted average microhardness of paste

The weighted average hardness of paste was calculated according to the above results of microhardness and volume fraction of each components in paste,as shown in Table 5.And the relation of abrasion resistance to weighted average hardness of paste is shown in Fig.4.It can be seen that there is positive correlation between the abrasion resistance and weighted average hardness of paste.Therefore,the decrease in abrasion resistance of paste with GBFS was caused by two reasons:firstly,the micro-hardness of GBFS was lower than that of the cement;secondly,the micro-hardness of hydration products in paste with GBFS was also lower than that of the hydration products in paste without GBFS.At the same time,the reason for the abrasion resistance of FS40 being higher than that of FS30 and FS50 was attributed to the higher micro-hardness of the hydration products in FS40 than that of the hydration products in FS30 and FS50.

Table 5 Weighted average hardness of paste/(kg/mm)

Fig.4 Relation of abrasion resistance to weighted average hardness of paste

3.3 Relation of abrasion resistance to microstructure of paste

3.3.1 Hydration products

The weight loss and derivative weight loss curves of the paste are shown in Fig.5.The DTG curves in Fig.5 have four peaks.The first peak (within 200 ℃)is mainly caused by the dehydration from C-S-H and a small amount from Aft/Afm.The second peak(250-380 ℃) is mainly caused by the dehydration from CAHand hydrotalcite.The third peak (380-450 ℃) is mainly caused by the dehydration from Ca(OH).The fourth peak (500-700 ℃) is mainly caused by the decarbonation from CaCO.

Fig.5 TG/DTG curves of paste

Table 6 The and of paste/%

The measured weight loss values were substituted into Eqs.(2) and (3) to calculate the values of

W

and

BW

listed in Table 6.The contents of CH in FS30,FS40 and FS50 were significantly lower than that in P,while the water contents dehydrated from C-S-H and the aluminiferous hydration products were higher than that in P,which can be attributed to the increase in the content of calcium aluminate hydrates and hydrotalcite-like phases (HT) in pastes with GBFS.The relations of abrasion resistance to

W

and

BW

of paste is shown in Fig.6(a) and (b),respectively.From Fig.6(a),the abrasion resistance of paste positively correlated to the content of CH,except for FS40,which may be due to the other microstructural factors like pore structure.While,from Fig.6(b),the abrasion resistance of paste negatively correlated to the content of bond water from C-S-H and aluminiferous hydration products.The above results indicate that since CH is the hardest component in the hydration products,the significant decrease in the content of CH is an important reason for the decrease in the micro-hardness of the hydration products and the abrasion resistance of the paste.

Fig.6 Relations of abrasion resistance to WCH and BW of paste

3.3.2 Pore structure

Fig.7 Pore size distribution curves of paste

The pore size distribution curves of paste are shown in Fig.7.And the statistically calculated porosity and average pore size of paste are shown in Table 7.It can be seen that the incorporation of GBFS decreased the porosity and average pore size of pastes,which was attributed to the formation of a large amount of HT in the hydration products.In addition,the porosity of FS40 was lower than that of FS30 and FS50,which may be the main reason that the abrasion resistance of FS40 being higher than that of FS30 and FS50.

4 Conclusions

The following conclusions were drawn based on the abrasion resistance of cement paste with different content of GBFS and its relations to microhardness and microstructure:

a) The incorporation of 30 wt%,40 wt% and 50 wt% GBFS reduced the abrasion resistance of paste by 18%,14% and 25%,respectively.And the abrasion resistance of paste with 40 wt% GBFS was higher than those of the pastes with 30 wt% and 50 wt% GBFS.

b) The decrease in abrasion resistance of paste with GBFS was caused by two reasons:firstly,the micro-hardness of GBFS was lower than that of the cement;secondly,the micro-hardness of hydration products in paste with GBFS was also lower than that of the hydration products in paste without GBFS.

c) The content of CH in the hydration products decreased with the incorporation of GBFS,which was the main reason for the decrease in the microhardness of hydration products.

d) The incorporation of GBFS reduced the porosity and refined the pore size of paste,and the paste with 40 wt% GBFS had a lowest porosity,which was the main reason for its higher abrasion resistance than the pastes with 30 wt% and 50 wt% GBFS.