Influence of Micro/Nano Sized Fly Ash on the Properties of Cement Mortar

Influence of Micro/Nano Sized Fly Ash on the Properties of Cement Mortar

D. K. Bera , A. K. Rath

Faculty School of Civil Engineering, KIIT University, Bhubaneswar Odisha, 751024,India

S. K. S. Parashar Faculty School Applied Science, KIIT University, Bhubaneswar

Odisha, 751024, India

Abstract The interest of research work is to prepare and study the effect of micro/ nano sized fly ash (or ground fly ash) on the mechanical properties of cement mortar. The grounded fly ash was obtained by grinding raw fly ash in a mechanical ball milling up to 5 hours, this ground fly ash (GFA) was in the range of micro to nano level. The mortar was prepared with adding of GFA as partially replaced ordinary Portland cement at the rate of 10%, 20%, 30% and 40% by weight of binder. This paper investigates the consistency and setting time of blended cement paste and compressive strength, water permeability of blended cement mortar samples. The partial replacement of GFA with cement mortar shows improvement in the microstructure and mechanical properties of mortar. Compressive strength of cement mortars blended with 10 % micro/nano-flyash particles showed results by additional to 10%, 24% & 34% than normal mortar samples after 28, 56 & 90 days of curing respectively and water absorption test proves that 10% micro/ nano blended mortar specimens absorbs 21% less water than that of other samples.

Key Words: Ground Fly Ash (Micro/Nano Fly Ash), Mechanical Ball Milling, Compressive Strength, Water Absorption.

INTRODUCTION

Pozzolanic materials like fly ash is currently on of the major ingredients for high strength and most efficient construction materials. As the fly ash available from thermal power plans is much coarse in nature, grinding operation is essential to realise the high performance mortar and concrete in building and transport sectors.

Grinding of fly ash is one of the very attractive options to obtain higher compressive strength of the concrete, as has been reported extensively in the technical literatures like [11], [12], [13], [14], [15], [16] and [17]. Superior strengths of fly ash mortars are obtained by Paya et al. [17] using ground fly ash to finer sizes, where the water cement ratio was maintained constant. These literatures are however not mentioning the consistency related flow parameters for the mortars, in case grinding is changed.

Extensive studies were conducted by Kiattikomol et al. [18] for the enhanced strength of mortars using grounded fly ash of five different origins, to three different fineness. The consistency and the substitution rate were kept constant for the mortars in above study. water demand by some fine ash samples in raw form were more compared the control varieties of mortar. The irregular shape and porous nature of fly ash are the dominating attributes for the higher water requirements.

Hence the conclusion was that workability and consistency of mortar do not improve by the blending of fly ash in all cases. One of the major contributions of many significant works viz [2], [3], [4], [5], [6] states microstructure study, mechanical properties and durability properties of concrete are dependent on fine particles such as nano particles of SiO2, TiO2, Al2O3, Fe2O3, salg and fly ash as partial replacement with cement.

Partial distribution of micro/ nano in cement paste and mortar

is studied in the current research work. This paper highlights the characterization of micro/ nano fly ash by mechanical ball milling process through XRD and SEM. This also assess the effects of micro/ nano-scaled fly ash on the consistency, setting time, compressive strength and water absorption properties of cement mortar.

EXPERIMENTAL PROCEDURE

Raw Materials: Cement:

In this study, ordinary Portland cement (OPC) 43 grade with Blaine specific surface 300 m2/Kg complying with IS:12269:1987 was used. The chemical and physical compositions of cement are mentioned in Table 1.

Fine Aggregate:

In the preparation of cement mortar mixes standard sand in accordance with IS: 650:1991, was used with cement and blended cement i.e. with micro/nano-fly ash in different proportions.

Water: Water used, is free from impurities and having pH-7 for the specimen preparation.

Fly ash:

The fine particle of fly ash was collected from thermal power plant and added as a partial replacement to cement. The chemical and physical compositions or properties of fly ash are mentioned in Table 1.

Micro/nano-fly ash or Ground fly ash (GFA):

An effort was taken to prepare micro/ nano sized fly ash from raw fly ash by mechanical ball milling. The morphology of nano sized fly ash was mentioned in XRD and SEM as shown in fig 1 & 2.

Table 1. Chemical and physical properties of cem

Table 2. Effect of milling hours on particle size

Fig. 1. X-ray diffraction of 0, 5 & 10 hr ground fly ash

Fig.2(a)

Fig 2 (b)

Fig 2 (c)

Fig 2. Cumulative % of particles passing of (a) Raw fly ash (b) 5 hr grinding & (c) 10 hr grinding FA

Fig. 3(a)

Fig.3 (b)

Fig. 3. SEM image of (a) raw fly ash (b) nano-fly ash at 10 h milling

Table. 3 Consistency, initial and final setting time of blended cement pastes

Mortar specimen:

The cement was blended with micro/ nano fly ash in the proportions of 0%,10%, 20%, 30% and 40% by weight replacement of cement and the ratio was taken 1:3 (blended cement: standard sand). The nomenclature of the corresponding samples was for normal cement mortar (N) and for 10%, 20%, 30%, and 40% by weight replacements of cement mortar were named as N10, N20, N30 and N40 respectively. The mortar specimens are mentioned in the Table 3. The water was added to the blended cement as per IS code recommendation.

Consistency & Initial and final setting time tests:

The normal consistency, initial and final setting time tests were conducted with the Vicats apparatus in accordance with IS: 4031:1988 part 4 and 5 and shown in Table no-2.

Compressive strength test:

Mortar cubic specimens of 70.6 x 70.6 x 70.6 mm were made for compressive strength in accordance with IS 4031 (Part 6) 1988. After 24 hours of casting, all specimens were inserted in water for curing at room temperature (25 ± 50C) for 3, 7, 28, 56 and 90 days as per the recommendation of IS 4031-1988. The cube specimens were taken for compressive strength test using a compression testing machine under load control at 0.2 MPa/s.

Water absorption test:

Three 70.6 mm cube samples of each mortar mix were taken for water absorption test. Saturated surface dry specimens were put in an dry oven at 100°C for 48 h. After measuring the initial weight, specimens were immersed in water for 48 h. After taking out from water, the specimen was kept outside for one hour and then the final weight was measured and the absorption was calculated and reported in the Fig.6 to assess the permeability of mortar.

RESULTS AND DISCUSSION

Consistency and Setting time:

Due to increase of ground fly ash percentages the water demand increases with respect to percentage addition of ground fly ash as shown in table 3.

Structure and microstructure analysis:

It was found from XRD for the 10 h milling, the crystallite size was reduced from 15 µm to 289 nm shown in Table 1 and Fig. 1. The size, shape and texture of the fresh as well as micro/nano structured fly ash were studied using Scanning Electron Microscopy (SEM). The high intensity peak of SiO2 at angle 26.58 degree diffraction peak reduces at 10h, it shows that the crystalline size reduces while milling in to smaller scale and it also confirmed from the microstructure image shown in Fig. 1 & 3. Particle size distribution of different hours milling of fly ash was shown in Fig 2.

Compressive strength of mortar:

The compressive strength results of cube specimens at 3, 7, 28, 56 and 90 days were mentioned in the Table 3.

The results were mentioned in Table 3. The table shows the effect of ground fly ash addition with cement mortar on compressive strength of the hardened mortar.

It was observed that the strength was improved in 10% replacement of ground fly ash and as on increase in GFA the strength gets decreasing.

Table 4. Compressive strength of mortar specimens

Water Absorption Test:

The absorption values of mortars were shown in fig. 4. It was clear that presence of pozzolanic material in cement mortar decreased the water absorption value. Micro/nano-FA was more effective in reduction of permeability than that of normal cement mortar. The following two phenomena can be distinguished for decreasing of permeability caused by using nano-FA:

1. Pore filling effect of micro/ nano particles to the pores of sand, cement and also it blocks the passages connecting capillary pores and water channels in cement paste

2. Produce a large number of C-S-H gel products on hydration of micro/ nano particles and make a more homogenous distribution of C-S-H gel and hence less pore structure.

Water absorption was reduced as on adding of Micro/nano fly ash, for mortar mix N10, the percentage reduction of water absorption was 21%. Similarly for other mixes the percentage reduction of water absorption were found to be 8%, for N20 specimen.

1.6

1.45

N

N10

N20

N30

N40

0.8

0.6

0.4

0.2

0

0.95

1

1.4 1.29

1.2

1.2 1.104

%, Water absorption

Fig. 4. percentage of water absorption of mortar mixes.

CONCLUSION:

From the experimental data, the following final points are to be derived :

• After 10 hr of grinding in mechanical milling the sample reaches nearly nano level as seen in XRD and SEM Fig. 1 and 3.

• As on adding of micro/ nano fly ash the water demand increased and similarly the water demand increases with respect to addition of nano fly ash.

• The compressive strength of the N10 was found to be highest with respect to other GFA blended cement samples as mentioned in Table 4. With the addition of GFA to cement the blended mortar samples improved their strength characteristics. At 10% GFA the strength was improved by 10%, 24% & 34% than normal mortar samples after 28, 56 & 90 days of curing respectively.

• Percentage of water absorption of N10 blended mortar specimen absorbs 21% less water than that of other samples.

In higher percentage of GFA samples showed less refinement of the pore structure, which leads less strength.

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