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The effect of hydrothermal treatment on the compressive strength and intrinsic air permeability of fly ash cement blended mortars.

INTRODUCTION

It is well known that Portland cement concrete is the most widely used construction material in this modern world. However, the cement industry is held responsible for some of the CO2 emissions that cause climate change. This is because the production of one ton of Portland cement emits approximately one ton of CO2 into the atmosphere. The amount of CO2 emitted is equivalent to about 7% of total greenhouse gas emission in the world [1]. This phenomenon has prompted many researches during the past decades to switch direction towards the utilization of waste materials such as fly ash, wood waste ash, slag and silica fume as partial cement replacement materials to improve durability and sustainability of the blended cement concrete while reduced the cost and the negative environmental effects related to OPC production.

Fly ash is a by-product of the combustion of pulverized coal and is a pozzolanic material. When it is mixed with Portland cement and water, it generates a product similar to that formed by cement hydration but having a denser microstructure that is less permeable [2]. Throughout the last few decades, extensive researches on the mechanical, durability and microstructure properties of fly ash-cement blended mortars and concrete have been well established [3, 4]. However, the curing regime of fly ash-cement blended mortars and concrete especially the hydrothermal treatment has not been well established. Hence it is the objective of this paper to study the effect of hydrothermal treatment on the mechanical and durability properties of fly ash-cement blended mortars.

MATERIALS AND METHODS

Materials:

ASTM Type I Portland Cement (PC) with median particle size of 3.9pm, specific surface area of 1.0432m2/g and specific gravity of 3.02 was used throughout the laboratory investigations. Pulverized fuel ash (PFA) or fly ash used throughout the experimental program was sourced from YTL Cement in Malaysia. The ash with specific gravity of 2.80 meets the general requirements of ASTM class F fly ash under the practice code ASTM Standard C618. Locally sourced quartzitic natural river sand in uncrushed form with a specific gravity of 2.69 and a maximum aggregate size of 5mm were used as fine aggregates in the present work with saturated surface dry conditions. Type F superplasticizer of sulfonated melamine formaldehyde condensates category was used throughout the experimental program. Potable water from local water supply network was used as mixing water.

Methods:

Mixture proportioning and mix design:

Portland Cement (PC) was replaced partially by fly ash at replacement levels of 5%, 10%, 15%, 20%, 25% and 30% respectively by total binder weight. The water/binder (w/b) and fine aggregates/binder ratios were maintained at 0.32 and 2.25 respectively throughout the laboratory investigation. The mix design of fly ash-cement blended mortar mixtures were shown in Table 1.

Mortar mixing and curing:

Portland cement, fly ash and fine aggregates were initially dry mixed at low mixing speed for 5 minutes to achieve the desired homogeneity. Mixing water and superplasticizer were subsequently added and the wet mixing continued for another 3 minutes before the fresh mortars were subjected to flow test and cast into various moulds for curing purpose. The mortars were then allowed to harden for 1 day before being demoulded and cured in lime-saturated water until the prescribed testing ages. Two different curing regimes were employed in this present work: (1) 1 set of mortars were subjected to cured in lime- saturated water and tested on the age of 7 days and, (2) another set undergone the same curing condition, but with the addition of hydrothermal treatment at 70oC for 24 hours after 7 days of normal water curing.

Mechanical and durability properties tests:

For the determination of compressive strength, mortars specimens with cube dimension of 50x50x50mm were fabricated, cured and tested in accordance to the procedures prescribed in ASTM Standard C109. The reported results of compressive strength were taken as the average of three specimens tested at a given ages of mortar.

For the determination of intrinsic air permeability values of hardened mortars, an air permeability cell as proposed by Cabrera and Lynsdale [5] was used.

RESULTS AND DISCUSSIONS

Compressive strength:

Compressive strength results of treated and untreated PFA-cement blended mortars at various cement replacement ratios are presented in Fig. 1. It can be noted that at all cement replacement levels, the PFA-cement blended mortar mixes yielded lower compressive strength in comparison to the control mix. This phenomenon is most probably due to the nature of pozzolanic activity of the fly ash. During the early hydration period (< 1 week), there is a slightly higher portlandite amount of PFA-cement blended mixes if compared with pure PC, as the hydration of clinker is accelerated due to the filler effect thus produced more portlandite. Therefore, the reaction of fly ash is slow as the "filler" effect of fly ash is dominant over the pozzolanic reaction of fly ash and this caused the reduction of compressive strength of all the PFA-cement blended mortars if compared to control mix.

Upon hydrothermal treatment, all the mortar mixes show an increase in the compressive strength except for mortar mixes C and PF5 which exhibit slight reduction of 4.5% and 0.96% respectively. The reduction in compressive strength of the aforementioned mixes were due to the negative effect of high temperature curing which is directly related to modifications of the microstructure of the OPC cement matrix [6, 7]. Also, significant difference between the thermal expansion coefficients of various phases of the constituent material in cementitious composite contributed further to the negative effect. The increase of compressive strength of the mixes upon hydrothermal treatment is more significant at higher cement replacement levels. PFA-cement blended mortar mixes of PF20, PF25 and PF30 exhibited increase in compressive strength by 16.8%, 17.98% and 22.06% respectively after hydrothermal treatment. According to Maltais and Marchand [7], the dissolution of fly ash particles is directly affected by the alkalinity of the pore water solution and it is found that the OH-ions increases significantly with temperature. Therefore, the hydration rate of fly ash is accelerated with higher curing temperature which culminated with a higher strength gain. The compressive strength of hydrothermal-treated mortars containing various cement replacement using fly ash was found to exceed 45 MPa which implies that the prescribed mortar mixes could be classified as high strength mortar by definition of the ASTM Standard C387.

Intrinsic air permeability:

The results of intrinsic air permeability of treated and untreated PFA-cement blended mortars at various cement replacement levels are shown in Fig. 2. At the age of 7 days, with the exception of mortars mixes PF15 and PF20, all the untreated PFA-cement mortars exhibit lower intrinsic air permeability value, K compared to the control mortar. The reduction in the value of intrinsic air permeability with the inclusion of 5, 10, 25 and 30% of fly ash as cement replacement compared with OPC mortar were 27.14, 43.1, 46.9 and 41.43% respectively. The marked decreased in the intrinsic air permeability of the aforementioned mixes were mainly attributed to the pore-refining effect of fly ash. The addition of fly ash increase the nucleation sites for the precipitation of hydration products in cement paste and caused segmentation of larger pores which in turns resulted in a lower degree of pore continuity. However, the addition of 15 and 20% of fly ash yielded higher intrinsic air permeability value than the control mix due to excessive dosage of superplasticizer.

All the hydrothermally treated PFA-cement blended mortars mixed including the control mix showed a significant decrease in the intrinsic air permeability value in comparison with their untreated counterparts. The reduction percentage were determined as 52.62, 42.81, 47.28, 42.47, 40.09, 56.99 and 56.5% for mortar mixes C, PF5, PF10, PF15, PF20, PF25 and PF30 respectively. Rise in curing temperature triggered rigorous pozzolanic reaction from fly ash and the formation of more secondary hydration products which in turns contributed to a further refinement in the pore structure of the cement paste matrix and the paste-aggregate interface. This resulted in a lower degree of continuous pores by the pore blocking effect of the secondary hydration products.

Conclusions:

The following conclusions can be derived upon the laboratory investigation, (i) upon hydrothermal treatment, PFA-cement blended mortar with 10% cement replacement level and above showed significant increase in compressive strength. Also, PFA-cement blended mortars with cement replacement up to 30% can be classified as high strength mortar by definition of the ASTM Standard C387, (ii) the optimum level of fly ash as cement replacement material to achieve the highest compressive strength gain upon hydrothermal treatment in this study is 30%, (iii) hydrothermal treatment significantly improved the intrinsic air permeability of all the mortar mixes.

ARTICLE INFO

Article history:

Received 12 October 2014

Received in revised form 26 December 2014

Accepted 1 January 2015

Available online 17 February 2015

REFERENCES

Funding from Universiti Sains Malaysia RU grant (1001/PPBGN/814211) is appreciated.

REFERENCES

[1] Gartner, E., 2004. Industrially interesting approaches to "low-CO2" cements. Cement and Concrete Research. 34(9): 1489-1498.

[2] Nath, P. and P. Sarker, 2011. Effect of Fly Ash on the Durability Properties of High Strength Concrete. Procedia Engineering, 14: 1149-1156.

[3] Deschner, F., 2012. Hydration of Portland cement with high replacement by siliceous fly ash. Cement and Concrete Research, 42(10): 1389-1400.

[4] Rajamma, R., 2009. Characterisation and use of biomass fly ash in cement- based materials. Journal of Hazardous Materials. 172(2-3): 1049-1060.

[5] Cabrera, J.G., C.J. Lynsdale, 1988. A new gas permeameter for measuring the permeability of mortar and concrete. Mag Concr Res, 144(40): 177-182.

[6] Paya, J., 2000. Mechanical treatment of fly ashes: Part IV. Strength development of ground fly ash-cement mortars cured at different temperatures. Cement and Concrete Research, 30(4): 543-551.

[7] Maltais, Y. and J. Marchand, 1997. Influence of curing temperature on cement hydration and mechanical strength development of fly ash mortars. Cement and Concrete Research, 27(7): 1009-1020.

Part Wei Ken, Mahyuddin Ramli and Cheah Chee Ban

School of Housing, Building and Planning, Universiti Sains Malaysia, 11800 Penang, Malaysia

Corresponding Author: Part Wei Ken, School of Housing, Building and Planning, Universiti Sains Malaysia, 11800 Penang, Malaysia

E-mail: part.wei.ken@hotmail.com

Table 1: Proportion of constituent
materials and rheological properties of mortar mixtures.

Mix           FA    PC           FA           Sand
designation   (%)   (kg/         (kg/         (kg/
                    [m.sup.3])   [m.sup.3])   [m.sup.3])

C             0     671          0            1510
PF 5          5     637          34           1510
PF 10         10    604          67           1510
PF 15         15    570          101          1510
PF 20         20    537          134          1510
PF 25         25    503          168          1510
PF 30         30    470          201          1510

Mix           Water        Total SP   w/b    Moitar
designation   (kg/         added(%)          flow (%)
              [m.sup.3])

C             215          0.7        0.32   62
PF 5          215          0.7        0.32   62
PF 10         215          0.7        0.32   65
PF 15         215          0.7        0.32   72
PF 20         215          0.7        0.32   74
PF 25         215          0.3        0.32   66
PF 30         215          0          0.32   66
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Author:Ken, Part Wei; Ramli, Mahyuddin; Ban, Cheah Chee
Publication:Advances in Environmental Biology
Article Type:Report
Date:Feb 1, 2015
Words:1847
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