Scholarly article on topic 'The Influence of Biomass Fly Ash on the Plasticizing Effects in Cement Pastes'

The Influence of Biomass Fly Ash on the Plasticizing Effects in Cement Pastes Academic research paper on "Civil engineering"

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{"biomass fly ash" / "cement paste" / "yield stress" / viscosity / "rheological properties" / thixotropy.}

Abstract of research paper on Civil engineering, author of scientific article — Gintautas Skripkiūnas, Mindaugas Macijauskas, Džigita Nagrockienė, Aurelijus Daugėla

Abstract The aim of this work is to determine the impact of biomass fly ash (BM-FA) on the plasticizing effects in cement pastes. The rheological properties of the cement pastes have been determined using rotational viscometer with coaxial cylinders Rheotest RN4.1. These tests, using rotational viscometer, have been conducted by making the assessment of cement paste flow curves, using Bingham rheological model. The rheological properties (yield stress, viscosity and thixotropy) of cement paste were determined at different times after paste mixing: 0 min, 30 min, 60 min, 90 min and 120 min. The cement pastes with same W/C ratio equal 0.35 were tested. 0%, 5%, 10%, 20% and 30% of cement were replaced by BM-FA. The rheological behavior of the cement pastes was also studied and compared to that of a reference BM-FA free production in order to evaluate the effect of BM-FA addition on rheological properties. Results showed that having replaced 10% of cement with BM-FA the paste exhibits better rheological properties: not only lower yield stress but also lower viscosity from mixing until 120 min after. Adequate amount of BM-FA (10%, in this case) not only improves rheological properties but also helps retaining plasticizing effect for the period of 120 min, which is important in long distance concrete mixtures transportation. It was observed that having replaced up to 10% of cement with BM-FA thixotropy was also improved and the fresh concrete mixtures produced can be transported, molded, consolidated, and finished within 90 minutes from mixing without losing good workability and rigidity.

Academic research paper on topic "The Influence of Biomass Fly Ash on the Plasticizing Effects in Cement Pastes"

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Procedía Engineering 172 (2017) 1015 - 1022

Procedía Engineering

www.elsevier.com/locate/procedia

Modern Building Materials, Structures and Techniques, MBMST 2016

The influence of biomass fly ash on the plasticizing effects in

cement pastes

Gintautas Skripkiunasa, Mindaugas Macijauskasb\*, Dzigita Nagrockienec, Aurelijus

Daugelad

abcdDepartment of Building Materials and Products, Faculty of Civil Engineering, Vilnius Gediminas Technical University, Sauletekio al. 11,

LT-10223 Vilnius, Lithuania

Abstract

The aim of this work is to determine the impact of biomass fly ash (BM-FA) on the plasticizing effects in cement pastes. The rheological properties of the cement pastes have been determined using rotational viscometer with coaxial cylinders Rheotest RN4.1. These tests, using rotational viscometer, have been conducted by making the assessment of cement paste flow curves, using Bingham rheological model. The rheological properties (yield stress, viscosity and thixotropy) of cement paste were determined at different times after paste mixing: 0 min, 30 min, 60 min, 90 min and 120 min. The cement pastes with same W/C ratio equal 0.35 were tested. 0 %, 5 %, 10 %, 20 % and 30 % of cement were replaced by BM-FA. The rheological behavior of the cement pastes was also studied and compared to that of a reference BM-FA free production in order to evaluate the effect of BM-FA addition on rheological properties. Results showed that having replaced 10 % of cement with BM-FA the paste exhibits better rheological properties: not only lower yield stress but also lower viscosity from mixing until 120 min after. Adequate amount of BM-FA (10 %, in this case) not only improves rheological properties but also helps retaining plasticizing effect for the period of 120 min, which is important in long distance concrete mixtures transportation. It was observed that having replaced up to 10 % of cement with BM-FA thixotropy was also improved and the fresh concrete mixtures produced can be transported, molded, consolidated, and finished within 90 minutes from mixing without losing good workability and rigidity.

©2017Publishedby ElsevierLtd. Thisis anopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of MBMST 2016

Keywords: biomass fly ash; cement paste; yield stress; viscosity; rheological properties, thixotropy.

* Corresponding author. Tel.: +370 5 274 5219; fax: +370 5 270 0112. E-mail address: mindaugas.macijauskas@vgtu.lt

1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of MBMST 2016

doi:10.1016/j.proeng.2017.02.152

1. Introduction

Environmental concerns and economic pressures are amongst the driving forces of today's industrial developments. Thus, considerable researches have been conducted worldwide on the use of waste materials to avert the increase of threats to the environment and to streamline present waste disposal and recycling methods by making them more affordable [1].

The production of the most common cement binder - Ordinary Portland Cement (OPC) - is related to the extremely high input of fuel and the great emission of CO2 into environment. The emission of CO2 by the industrial production of cement makes 5 % - 7 % of total CO2 emission. Until relatively recently, the use of additives and substitutes to OPC has been one of the most successful ways to decrease the CO2 emissions generated by cement and concrete producers, and amongst those the so called "industrial wastes" have got a wide attention [2-9].

Therefore, we have witnessed recent intensification of research into composite cement binders and their concretes. A certain proportion of OPC in these concretes could be replaced by other active additives (i.e.: silica fume, slag, fly ash, natural pozzolans), particularly by the wastes. Waste with pozzolana properties, which can improve the resistance of cement products to alkaline and sulphate corrosion, modify their hardening process, structure and increase the strength, etc. is particularly attractive [10]. One of the most promising types of waste is biomass fly ash (BM-FA).

The rapidly increasing amount of BM-FA produced all over the world leads to the need to recycle, not only due to the rising cost of the landfill disposal which, in turn, is reflected on the cost of the energy produced, but also as a consequence of the "zero-waste" objective which is often suggested as the final goal of all future human activities [11]. The use of BM-FA in cements, mortars or concretes is presently under study because their recycling in the production of cement based materials could have a beneficial impact on environment provided that it is possible to demonstrate that the resulting products have good mechanical properties, long durability and low pollution release of hazardous elements.

Based on the physical, chemical and morphological properties, it is reported that BM-FA, an industrial by-product of thermal power plants, has a substantial potential for use as a pozzolanic mineral additive and/or as an activator/binder in cement-based materials [12-16].

Fresh concrete is a concentrated suspension of particulate materials of widely differing densities, particle sizes, and chemical compositions in a solution of lime and other components. Because concrete must be mixed, placed (i.e.: frequently in a heavily reinforced formwork), molded, consolidated, and finished, it is necessary that in most cases a level of fluidity, generally termed workability, be maintained. This is determined by the rheological properties of the system, which are, in turn, influenced by all the components. Control of workability is one of the objectives of mixture proportioning.

Water-reducing admixtures and superplasticizers can be used to significantly increase the workability and consistency, or reduce the water demand of concrete, and are widely used in concrete production today. Supplementary cementing materials (SCMs) can also have a significant impact on the workability, consistency, and water demand of the concrete. The effect depends on the type of SCMs and the level of replacement, among other factors. Therefore, it is essential to understand the role of BM-FA in the rheology of fresh concrete to fully exploit the potential of BM-FA for improving concrete.

According to Owens [17], the major factor influencing the effects of ash on the workability of concrete is the proportion of coarse material (>45 ^m) in the ash. Owens has shown that, for example, substitution of 50 % of the material >45 ^m has no effect on the water requirement.

Stuart et. al. [18] studied the effect of high-range water-reducing admixtures and fly ashes on the water requirements of various mortars. The results showed that the fly ashes themselves were effective in reducing water content but differed considerably in their effectiveness as water-reducing admixtures.

Following up on Stuart's results, Helmuth [19] suggested that the water reduction caused by the fly ash was the result of an absorption and dispersion process as much like that of organic water-reducing admixtures.

Rajamma et. al. [1] studied the impact of BM-FA on fresh and hardened state properties of cement based materials. The results showed that, in general, the water demand of BM-FA/OPC blended cement pastes increases with the content of ash. Up to 10 % BM-FA the influence in the consistency of cement mortars is not visible, but

when substitution dosage of ash increase above 20 % the water demand also increases significantly. These results are in agreement with the findings of earlier researchers on wood ashes from other sources, such as saw dust [20-22].

The aim of this work is to determine the impact of biomass fly ash (BM-FA) on the plasticizing effects in cement pastes. The cement pastes with same W/C ratio equal 0.35 were tested. 0 %, 5 %, 10 %, 20 % and 30 % of cement were replaced by BM-FA. The rheological behavior of the cement pastes was also studied and compared to that of a reference BM-FA free production in order to evaluate the effect of BM-FA addition on rheological properties.

2. Research methodology and materials used

Portland cement CEM I R (PC R) according to EN 197-1 (strength class 42.5 R), produced by JSC Akmenes Cementas was used in the investigation. Clinker mineral composition (according Bogue calculation) was: C3S - 61.0 %, C2S - 13.5 %, C3A - 8.5 %, C4AF - 10.5 %, SO3 - 3.10 %, LOI - 1.43 %. Particle density is 3.11 g/cm3 and dry bulk density is 1.22 g/cm3. The average size of particle is 15.05 pm, 50 % particles are smaller than 9.94 pm. The codes of cement used and its physical properties are given in Table 1.

Chemical composition of biomass fly ash (BM-FA) used for investigation was: SiO2 - 51.8 %, AhOa - 1.93 %, Fe2O3 - 1.67 %, CaO - 26.5 %, K2O - 6.87 %, MgO - 1.91 %, P2O5 - 3.19 %, SO3 - 3.74 %, Na2O - 0.176 %, MnO - 0.834 %, Cl - 0.670 %. The average size of particle is 69.33 pm, 50 % particles are smaller than 23.97 pm. BM-FA specific surface area is 3083 cm2/g.

Superplasticizer Sika D187 (SP) based on synthetic polycarboxylate ester (PCE) was used (consistency - liquid, conc. of active material in solution - 18%, density - 1.08 g/cm3, max. chloride content < 0.10 %, max. alkali content < 0.1 %). Chemical admixture content in cement paste was constant - 0.75 % of the cement weight.

The particle size distribution of cement and biomass fly ash and their granulometry curves are presented in the Fig. 1. These parameters were determined using Cilas 1090 Liquid (France).

Table 1. Physical properties of cement (according EN 197-1).

Type of Compressive strength, MPa Code 7 days 28 days Setting time, min Standard consistence (water content in Fineness

cement initial final %) Blaine, cm2/g > 90^m, %

CEM I 42.5 R PC R 28.9 54.6 150 200 25.4 3560 1.1

— - PC R PC R

■BM-FA--BM-FA

- nartirlp niTf) rli^trihi lïinn

■ granulometry curve 1 ; y 1

fl y f L___

It yf Uffi V

0,01 0.10 1,00 10,00 100,00 1000,00 Diameter, Mm

Fig. 1. Particle size distribution of cement (PC R) and biomass fly ash (FA-BM) and their granulometry curves.

Cement paste was mixed in a hand mixer according to EN 196-1. The mixing procedure was as follows: cement and 3/4 of required water were mixed for 2 min at low speed, after that the remaining water and SP were added in and cement paste was mixed further for 1 min at high speed.

The Theological properties of cement paste were tested using rotational viscometer with coaxial cylinders Rheotest RN4.1 with gap between cylinders of 1.48 mm. The cylinder measuring system consists of the measuring cup (assembled) (1) with coupling (3) and cylinder rotor (2). The dimensions of testing cylinders are presented in the Fig. 2.

The shear rates used in the test are presented in the Fig. 3.

The cement paste is poured into a measuring cup (1), which is fixed non-movably into the equipment stand. Inside the measuring cup the cylinder rotor (2) can rotate. Because of the intrinsic friction of the layers of the cement paste (4) appearing between the measuring cup (1) and the rotating cylinder rotor (2) positioned in the measuring cup and connected to a measuring scale; the cylinder rotor (2) makes a turn and the data displayed on the

measuring scale changes.

Fig. 2. Principle schema of testing cylinders.

Fig. 3. Shear rates during testing vs. testing duration.

Rheological properties of cement paste were tested at different times after paste mixing: 0 min, 30 min, 60 min, 90 min and 120 min. The cement pastes with same W/C ratio equal 0.35 were tested. 0 %, 5 %, 10 %, 20 % and 30 % of cement were replaced by BM-FA. A test of rheology was carried out at temperature 20±2 °C.

The yield stress and viscosity of cement paste were calculated from flow curve (t and v curve) obtained after the test with rotational viscometer. The flow curve of cement paste was analyzed adopting the Bingham rheological model using linear approximation of experimental results up to 100 s-1 shear rate when flow curve of cement paste is linear character. The EXEL computer application was used for this purpose (Fig. 4).

The yield stress of cement paste (to) and viscosity (n) values were obtained from linear equation (Bingham rheological model):

t = t0 + nv,

where: t - shear stress, Pa;

to - yield stress of cement paste, Pa; n - viscosity of cement paste, Pas; v - shear rate, s-1.

Fig. 4. Calculation of (a) yield stress, viscosity and (b) thixotropy of cement paste from flow curve.

The thixotropy of cement paste was calculated from flow curve of cement paste obtained after testing in 300 s-1 shear rate (Fig. 4). The thixotropy value was calculated by the reduction of shear stress during the constant shear rate equal 300 s-1 by the equation:

T = (T1 - T2) / T1,

where: t1 - shear stress at 300 s-1 before retention during testing, Pa; t2 - shear stress at 300 s-1 after retention during testing, Pa.

3. Results of experimental research

3.1. Flow behaviour of cement pastes

From the determined flow curves (see Fig. 5a and b), 5 or 90 minutes after the start of mixing, we can see that cement PC R pastes with BM-FA and without it have different rheological properties.

-pcr+sptbm-fa[10%]

-PCRtSP

- PC R + SP4 RM -FA(20%} •

4m 50 60 S'lieai' rate, a"1

-PCR»SP(RM.FAp%) -PCR»SP»RMFA(30%)

-PCR+SP+limfa(10%)

-pckisp

-PCR» SP» BN FAP OK,)

-PCRtSPtBM FA(5%) -PC RtSP» ОМ РЛ(ЗОК)

40 50 60

Shear гн1е,з''

Fig. 5. Cement PC R pastes flow curves (shear stress dependency on shear rate) (a) 5 min and (b) 90 min after the mixing of the pastes.

From Fig. 5a we can see that having mixed into the PC R paste some plasticising admixture (superplasticizer SP - 0.75 % of cement mass, in this case), the rheological properties of the paste improve: while the shear rate is improving from 0 to 100 s-1, lesser shear stress is observed, compared to cement paste without SP. Seeking to determine the impact of BM-FA on plasticised paste rheological properties, we received the following results: having replaced 5 %, 20 % or 30 % of cement with BM-FA, giving increasing shear rate from 0 to 100 s-1, respectively bigger shear stress was observed, compared to paste without BM-FA. With a notable exception: having added 10 % of BM-FA, the opposite effect was observed - given increasing shear rate from 0 to 100 s-1, shear stress is lesser compared to cement PC R paste without BM-FA.

From Fig. 5b we can see that even after 90 min from the mixing of the pastes, having 10 % of cement replaced by BM-FA rheological properties did not change much compared to rheological properties of cement paste 5 min after the start of mixing. In addition to that, as the shear rate increases from 0 to 100 s-1, pastes shear stresses are lesser compared cement pastes with 5 %, 20 %, 30 % of BM-FA or without them. Thus, we can conclude that the 10 % of BM-FA, given the same contents (V/C = 0.35), retain the prescribed paste plasticizing effect from the mixing until 90 minutes after.

3.2. Yield stresses and viscosities of cement pastes

PC R+ SPi BM ■ FA(S%J 1-fA(20%)-PCR+SP*BM-FACJ<m)

-PtK+SP+BM KA(.0%)

45 W) 75 D N ration, min

-PCFMSP

-PCR»5P»BM.FA(30K)

-PCRtSPi(iM-FA(5%) -PCRtSPtRM-FA(30%)

PCR<SPtBM-FA(10%)

45 GO 75 Duration, Ellin

Fig. 6. Cement PC R pastes (a) yield stresses and (b) viscosities change within 120 min duration.

From Fig. 6a we can see that in cement PC R paste without BM-FA yield stress remains constant from the start of mixing to until 60 min after. However, from 60 min margin onwards intensive yield stress increase was observed. Having replaced 5 % or 30 % of cement with BM-FA cement paste yield stress increase marginally from the start of mixing to until 90 min after. From 90 min onwards intensive yield stress increase is evident.

From Fig. 6a it is also evident that having replaced 10 % or 20 % of cement with BM-FA, the yield stress of cement paste remains constant from the start of mixing until 90 min after. From 90 min marginal yield stress increase is observed. Notably, having changed 10 % of cement by BM-FA we get significantly lower yield stress measurements (8.03-9.43 Pa) compared to those of cement PC R pastes with BM-FA. Respectively: 5 % - 13.7043.10 Pa, 20 % - 13.30-17.10 Pa, 30 % - 14.70-31.30 Pa, or without them - 11.10-28.10 Pa, within 2 hour period from the start of mixing.

From Fig. 6b we can see that viscosity of cement PC R paste with BM-FA (5 % of cement being replaced by BM-FA) and without them gradually increases from the start of mixing until 60 min after. From 60 min onwards intensive increase of viscosity is observed. Having replaced 20 % or 30 % of cement with BM-FA viscosity of cement paste gradually increases from mixing until 90 min after. From 90 min margin onwards intensive increase of viscosity is observed. However, having replaced 10 % of cement with BM-FA viscosity remains the same from mixing until 60 min after. From 60 min onwards marginal increase is observed. In addition to that significantly lesser viscosity of the paste was determined (0.44-0.65 Pas) compared to cement pastes with BM-FA respectively:

5 % - 0.54-1.19 Pa s, 20 % - 0.62-1.06 Pa s, 30 % - 0.80-1.21 Pa s, or without them - 0.44-1.07 Pa s, within 2 hour period after mixing.

In conclusion we can state that having replaced 10 % of cement with BM-FA the paste exhibits better rheological properties: not only lower yield stress (see Fig. 6a) but also lower viscosity from mixing until 120 min after. Adequate amount of BM-FA (10 %, in this case) not only improves rheological properties but also helps retaining plasticizing effect for the period of 2 hours, which is important in long distance concrete mixtures transportation.

3.3. Thixotropy of cement pastes

—♦—5 lulu ■ 15 in ¡it * 30 min —~—60 mm —1—90 mm —•— I nui: O.IS (!,L4 0.12 ft 0,10 S 0,08

'.B o,o<)

0,02 0,00

Fig. 7. Cement PC R pastes thixotropy change within the period of 120 min.

In the case of many cementitious materials, a reversible evolution of the material rheological behaviour is often noted during the dormant period of the hydration reaction: the apparent static yield stress continuously evolves. As this evolution can be erased by a strong shearing of the paste and the material can be brought back to a reference state, it is often described as thixotropy [23, 24, 25].

Cement PC R (specific surface area - 3560 cm2/g) paste thixotropy is largely influenced by BM-FA (specific surface area - 3083 cm2/g) fineness, particle size distribution/granulometry (see Fig. 1) and amount in the paste.

From Fig. 7 we can see that having replaced 10 % of cement by BM-FA, thixotropy of cement paste for the period of 2 hours from the start of the mixing of the paste changes marginally (0.11-0.14). Having replaced 20 % or 30 % - thixotropy of the paste is reduced respectively: 0.07-0.11, 0.02-0.09.

We can also observe that having increased the percentage of BM-FA in the paste to 10 %, thixotropy increases within 60 min period. From 60 to 90 min - remains unchanged compared to thixotropy of the paste right at the start of mixing. Thus, we can state that having replaced up to 10 % of cement with BM-FA thixotropy is improved and the fresh concrete mixtures produced can be transported, molded, consolidated, and finished within 90 minutes from mixing without losing good workability and rigidity.

4. Conclusions

Rheological properties of cement paste depend on biomass fly ash mineral composition, its fineness, particle size distribution/granulometry and amount in the paste. In this work we determined:

1. With 10 % of cement replaced by biomass fly ash, the paste has better rheological properties: not only lower yield stress (Fig. 6a), but also lower viscosity (Fig. 6b) in the period from mixing until 120 minutes after. 10 % of biomass fly ash not only improves rheological properties, but also helps retain the plasticizing effect for the period of 120 min, which is important in long distance concrete mixtures transportation.

2. Cement paste thixotropy is largely influenced by biomass fly ash (specific surface area - 3083 cm2/g) fineness, particle size distribution/granulometry (see Fig. 1) and amount in the paste. Having replaced up to 10 % of cement with biomass fly ash thixotropy is improved and the produced fresh concrete mixtures can be transported, molded, consolidated, and finished within 90 minutes from mixing without loses in terms of workability and rigidity.

li i miKms fly.mh iinitvn t.m

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