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Comparative study of geopolymer and alkali activated slag concrete comprising waste foundry sand
Construction and Building Materials 209 (2019) 555–565
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Comparative study of geopolymer and alkali activated slag concrete comprising waste foundry sand Bavita Bhardwaj ⇑, Pardeep Kumar National Institute of Technology, Hamirpur, HP 177005, India
h i g h l i g h t s Use of WFS as partial to full replacement to normal sand in GPC and AAS concrete. WFS addition beyond 40% significantly affected workability. Strength and sorptivity enhanced upto certain WFS levels. Unimodal particle size of WFS decreased strength at higher replacements. GPC and AAS concrete have considerably low EE and ECO2e than OPC concrete.
a r t i c l e
i n f o
Article history: Received 30 November 2018 Received in revised form 11 February 2019 Accepted 11 March 2019
Keywords: Geopolymer concrete Alkali activated slag concrete Low calcium fly ash GGBS Waste foundry sand
a b s t r a c t Concrete industry and current construction practices are highly unsustainable from the standpoint of energy consumption and their high dependence on natural resources. Using alternative binders to conventional cement such as geopolymers and alkali activated slags (AAS) can be one alternative. Besides, reusing industrial by-product and waste materials in construction industry can also be emphasized. These measures can substantially lower the carbon emissions and embodied energy of concrete along with solving the problem of industrial waste disposal. The current study reports the influence of inclusion of waste foundry sand (WFS), a by-product from foundry industries, on strength, permeability and microstructure of low calcium fly ash geopolymer concrete (GPC) and ground granulated blast furnace slag (GGBS) based alkali activated slag (AAS) concrete, both cured in ambient conditions. The mix proportions for both the concretes were kept same. The natural sand was replaced by WFS in the range of 0% to 100% at an interval of 20%. Tests on hardened concretes, to study, compressive strength, split tensile strength, capillary suction, SEM and EDS analysis, were conducted to assess the strength, permeability and microstructure of both types of concretes. Addition of WFS lowered the workability of GPC and AAS concretes and the effect was abrupt beyond 40% WFS replacement level. Strength and sorptivity of both concretes improved upto 60% WFS replacement in GPC, whereas, upto 20% replacement in AAS concrete mixes, besides more than 45% of strength of reference concretes (0% WFS) was achievable at 100% replacement by WFS. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Environmental issues and sustainability are the main concerns that are guiding most of the contemporary research. Concrete, being extensively used construction material in the world, has significant contribution towards global warming and climate change. All the primary constituents of concrete have different environmental impact and cause different sustainability issues. To avert the environmental effects associated with concrete and its ingredi⇑ Corresponding author. E-mail address: [email protected] (B. Bhardwaj). https://doi.org/10.1016/j.conbuildmat.2019.03.107 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ents there is need to switch to other environment friendly and sustainable substitutes. Worldwide cement production in year 2017 was estimated around 4100 million tonnes. Out of this, about 2400 million tonnes i.e. around 59% was produced by China alone, followed by India (270 million tonnes) 6.58%, USA (86 million tonnes) 2.1%, Vietnam (78 million tonnes) 1.90% and Turkey (77 million tonnes) 1.88% as top five producers [1]. This global annual cement production is anticipated to rise to 4830 million tonnes by 2030 [2]. Cement manufacturing is highly energy intensive process and the production of 1 tonne of ordinary Portland cement (OPC) releases about equal amount of CO2 in the atmosphere, making it one of the major
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contributor to global warming. To counter this issue several attempts are being made to lower the consumption of OPC and CO2 emissions, mainly categorized under two areas. One is replacing cement in concrete by supplementary materials; e.g., fly ash, silica fume, metakaolin, bottom ash, granulated slag etc. The other is to find alternative binders for concrete such as geopolymers and alkali activated slag [3]. Geopolymers and alkali-activated binders are hardened compounds which attain their strength and other characteristics by chemical reaction between alkaline solutions and aluminatesilica rich source materials. Geopolymer is inorganic aluminasilicate polymer structured around tetrahedral coordinated Si4+ and Al3+, forming a polymer chain. These geopolymer precursors chemically bond and form oligomers leading to the formation of aluminosilicate polymers [4]. Whereas, alkali-activated binders are formed by activation of calcium-rich source materials under high alkaline conditions. After the dissolution of the precursors in alkaline solution, a binder paste with hydraulic potential is build leading to simultaneous formation of C-S-H gels and aluminosilicate polymers [4]. Fig. 1 shows general difference between alkali activated binders and geopolymers [4]. Geopolymers and alkali activated binders completely replace OPC by utilizing industrial by-products like fly ash and GGBS making concrete less energy intensive and more environment friendly [4–6]. Fly ash, the by-product of coal combustion in thermal power plants, is abundantly available worldwide, but to date only limited quantity of it is being utilized. In India, 169.25 million tonne of fly ash was generated in year 2016–17 but current level of efforts resulted in utilization of only 63.28% of it [7]. The need of power is certain to increase in coming time, so is the volume of fly ash and the problem of its disposal. The utilization of fly ash in production of cement and concrete has, to some extent, solved the problem of its disposal. It is estimated that, if fully industrialized, geopolymer cement will have 80% lower embodied greenhouse gas emissions than an equivalent amount of OPC binder [8]. The CO2 emissions in geopolymer are mainly during production of alkali hydroxide and silicate from carbonates. Moreover, as it mostly uses waste and by-products with no embodied energy so it’s embodied energy is nearly half of that of Portland cement [9]. It is estimated that price of fly ash-based geopolymer concrete is estimated to falls about 10–30 percent cheaper than that of OPC concrete [10]. Furthermore, several investigations have assessed
the performance of GGBS as a cementitious material in cement production since 1939 [11]. The use of GGBS as cement replacement material in concrete has been found to reduce the CO2 emission by a great extent [12]. Over extraction of natural resources to cater the need of rising urbanisation and industrialisation is causing detrimental effects on environment. Furthermore, sand, which is the second major constituent of concrete, the restriction in its extraction from river has increased the price of sand and severely affecting the survival of the construction industry [13]. As such, it has become imperative to find alternative materials to river sand. Metal alloy casting industries worldwide produce several million tonnes of byproducts annually of which waste foundry sand (WFS) is the major by-product [14]. United States landfills about 6–10 million tonnes of waste foundry sand per year [15,16]. India, which is the third largest casting manufacturer in the world after China and USA, produced around 1.71 million tonnes of waste per annum [17,18]. Foundries use raw sand for use in their moulding and casting operations, which is high quality size-specific silica sand and is generally of a superior quality than the sand normally generally used in construction purposes. Waste foundry sand is rich in silica content owes its black colour to coating of burnt carbon over particles. Several authors have studied the use of WFS in various civil engineering applications such as sand replacement in concrete, as highway embankment filling, soil stabilization, mortar making, brick blocks, pavement blocks etc. [19]. Addition of WFS in concrete as partial substitute (20–30%) to natural sand has improved the mechanical as well durability properties of conventional as well as special concretes such as self-compacting concrete (SCC) and geopolymer concretes [18–22]. Some authors have observed strength enhancement on replacement by WFS upto 50–60% as well [20,23]. At 80% replacement level of WFS, upto 70% of strength of reference concrete (0% WFS) was achievable [24]. Most of the research on WFS in concrete is limited to conventional concrete only and that too its use as partial replacement to normal sand. The aim of current study is to assess the strength, permeability and microstructure of ambient cured GPC and AAS concretes when WFS was used as partial to full replacement to normal sand. The potential use of GPC being more sustainable and green material as a replacement of normal concrete necessitates the study of permeation properties of concrete from durability point of view. Sorptivity test, being an important and easy method to ascertain durability of concrete, was
Fig. 1. Reaction mechanism of Alkali activated binder and geopolymer [4].
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done to find out the effect on initial rate of absorption of the developed GPC and AAS concretes due to varying proportions of WFS. The impact of GPC and AAS concrete mixes with WFS on environment is also assessed by calculating its embodied energy and embodied carbon dioxide emissions.
Table 2 Chemical moduli of GGBS. Chemical Moduli
Test Result
(CaO + MgO)/SiO2 CaO/SiO2 CaO + MgO + SiO2
1.30 1.07 76.03
2. Experimental programme The investigation studies the influence of partial to full replacement of natural sand by WFS on the geopolymer concrete (GPC) prepared using low calcium fly ash and alkali activated slag (AAS) concrete prepared using GGBS. The GPC mixes developed with fly ash as source material contained 10% of 43 Grade OPC as partial replacement to total binder to accelerate setting of concrete. All the mixes were cured in dry ambient laboratory conditions. Compressive strength, split tensile strength and capillary suction tests were conducted to assess its performance. In addition, microstructure analysis of concretes was also done by undertaking scanning electron microscopy (SEM) analysis and energy dispersive X-ray spectroscopy (EDS). Cube specimens of 100 mm size were cast for compressive strength test while, 100 mm diameter and 200 mm long cylinders were cast for testing split tensile strength and capillary suction test as per recommendation of relevant standards. 2.1. Materials Ground granulated blast furnace slag (GGBS), procured from Astraa chemicals, Chennai, conforming to standard BS 6699: 1992, was used in the study [25]. Low calcium fly ash (ASTM Class F) conforming to standard IS 3812-Part-1 [26] for geopolymer concrete was procured locally from Ambuja Cement plant. Ordinary Portland cement (OPC) of 43 grade, as procured locally from ACC cement, was used conforming to standard IS 8112 [27]. Oxides composition of these materials, as provided by suppliers, is shown in Table 1. Specific gravity for fly ash and OPC was 2.23 and 3.14 whereas for GGBS it was obtained as 2.85. Specific surface area of fly ash and GGBS was 260 m2/kg and 390 m2/kg respectively. The glass content and moisture content in GGBS was 91% and 0.1% respectively. Chemical moduli of GGBS is presented in Table 2, as provided by Astraa chemicals. Fig. 2 shows the images from scanning electron microscope (SEM) analysis of fly ash and GGBS. Particles of fly ash are round, whereas, GGBS has flaky and elongated particles with smaller size as compared to fly ash, when both are observed on same magnification under SEM. For activating source materials a combination of two alkaline solutions was chosen i.e. Sodium Hydroxide (NaOH) and Sodium Silicate (Na2SiO3). Sodium Hydroxide was obtained from supplier in pellets form, 98% pure, and mixed with normal tap water to prepare solution. For both GPC and AAS concretes, NaOH solution of 14M concentration was prepared by mixing 404 parts of solids with 596 parts of water. Commercial grade, clear colourless and viscous, Sodium Silicate solution with SiO2/Na2O by mass of 2.1 and water content of 52% was used. The solutions sodium silicate and sodium hydroxide solution were used in the proportion of 2.3, constant for all the concrete mixes. For fine aggregate, locally available crushed sand (NA) was used in consort with WFS as its partial to full replacement in different GPC and AAS concrete mixes. The oxide composition of NA and WFS is shown in Table 1. Waste foundry sand was procured from one local ferrous foundry, manufacturer of automobile parts. It is black in colour due to presence of carbon content, added in foundry during foundry operations to enhance finishing of casting. The particles of WFS were comparatively round to sub-angular in shape as shown in Fig. 2. The NA and WFS conformed to zone-2 and zone-4 respectively, as per IS-383 [28]. Coarse aggregate used were locally available crushed stone aggregates, angular in shape, with maximum size 12.5 mm. Aggregates, except WFS, were used in saturated surface dry condition. Table 3 shows physical properties of fine and coarse aggregates used and their grading curves are given in Fig. 3. Normal potable tap water was used for making alkaline solutions as well as concretes.
assumed as 2430 kg/m3 and other calculations were made based on the density of concrete as per the mix design procedure given by Junaid et al. [29] considering the target strength as 40 MPa. Trials were done before finalizing the mix proportions. The combined weight of aggregates was taken as 76%. The alkaline solution was taken as 45% of powdered binder or source material. Same mix proportions were used for AAS concrete. Extra water was kept constant as 10 kg/m3. The proportions of the concrete constituents used is shown in Table 4. Activator solutions were premixed 24 h before casting of concrete so that the solution cools down to room temperature and stabilizes before adding to mixture [30]. The aggregates along with WFS were first mixed for 2 min in concrete mixer. After that powder binder (GGBS/FA + OPC) was added in concrete and mixed for 2 min. Activator solutions with extra water and superplasticizer were premixed and added slowly to the dry contents. After mixing for 3 min, slump cone test was performed to test the workability of fresh concrete. Thereafter, concrete was poured into concrete moulds and compacted on table vibrator. After 24 h of casting, all specimens were demoulded and then kept in laboratory for dry ambient curing until testing age of 28 days. Six mixes of GPC and AAS concrete each, with varying proportions of WFS, were cast for testing. Details of all mixes is shown in Table 5. 2.3. Testing 2.3.1. Workability and strength properties It has been widely reported that application of heat to GPC helps in initial setting of GPC and greatly enhances its final strength [31], but requirement of heat limits the application of GPC. However, low calcium GPC when ambient cured, takes more than 24 h to set. So, in the current investigation 10% of OPC was added to accelerate the initial setting of GPC. The slump test was carried out to test workability of fresh concretes according to IS 1199 [32]. All tests on hardened concretes were performed at the age of 28 days. Compressive strength and split tensile strengths were performed on automatic compression testing machine (CTM) with a load capacity of 3000 kN. The load was applied gradually at the rate of 14 N/ mm2/minute for compressive strength test and at 1.8 N/mm2/min for split tensile strength test as per IS: 516 [33] and IS: 5816 [34], respectively. The strength values were determined from the average of three specimens. 2.3.2. Absorption Capillary suction test was performed as per ASTM standard C 1585–04 [35] to find the rate of absorption, i.e. sorptivity of water by hydraulic cement concrete by measuring the increase in the mass of a specimen resulting from absorption of water as a function of time when only uniaxial penetration of water is allowed (Fig. 4). The specimens used for sorptivity test were cylindrical discs of 100 mm diameter and 50 mm thickness, cut from cylinders of 100 mm diameter and 200 mm height. Results reported for each mix are obtained from the average sorptivity value of three discs of same mix. On reaching testing age all specimens were first conditioned to prepare for test. After ambient curing for 28 days, cylinders were cut to obtain discs, the discs were then marked and kept in oven at a temperature of 105 ± 5 °C for drying, until weight was stabilised i.e. not more than 0.1% weight changed over after 24 h drying period. This usually takes 3–4 days. After drying, specimens were kept in desiccator to cool down to room temperature. Before testing, specimens were coated with paraffin wax on circumferential surfaces and sealed with tightly attached plastic sheet on the top surface to allow only uniaxial (i.e. one dimensional) diffusion of water through the uncoated surface.
2.2. Mixing and casting details The concrete mixes were proportioned as such to study the influence of WFS addition, as partial to full replacement of normal sand (NA) on strength development and permeability of GPC and AAS concrete. The density of concretes was
2.3.3. Microstructure The microstructure of GPC and AAS concretes was studied using scanning electron microscopy (SEM) imaging on Quanta FEG 450. Energy dispersive X-ray spectroscopy (EDS) analysis was performed to determine the elemental chemical
Table 1 Chemical composition of fly ash, OPC and GGBS used. Material
Fly Ash OPC GGBS WFS
By weight (%)
LOI
SiO2
CaO
Al2O3
Fe2O3
MgO
Na2O
SO3
K2O
MnO
TiO2
P2 O5
(%)
59.87 20.10 33.06 78.5
5.06 61.30 35.37 2.44
23.96 6.80 22.29 5.50
3.70 4.30 0.9 4.45
0.48 2.60 7.61 2.76
0.17 0.26 – 0.79
0.16 1.30 0.38 0.22
1.20 0.23 – 0.52
– – 0.12 –
1.27 – – 0.15
0.28 – – –
2.00 1.20 0.26 –
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Fig. 2. SEM images (a) Fly ash, (b) GGBS and (c) WFS.
Table 3 Properties of fine and coarse aggregates. Property
Fine aggregate
Specific Gravity Fineness Modulus Water Absorption (%)
NA
WFS
2.65 2.50 0.55
2.18 1.94 1.5
Coarse aggregate
2.74 6.93 0.68
composition of reacted geopolymer and AAS concrete. Specimens were taken from concretes after reaching testing age of 28 days. Specimens were first staged over conductive double-side carbon tape and subsequently coated with gold and then mounted on SEM sample stage for analysis.
3. Results and discussion 3.1. Workability and strength To improve workability of mixes Sulphonated Naphthalene based superplasticizer was added in concretes. Water to solid con-
tent for all the mixes was kept constant as 0.24. Weight of total water in concrete includes weight of water in alkaline solution as well as extra water added, whereas, weight of total solid content in binder includes weight of source material (FA + OPC/GGBS) and weight of solid content of alkalis. Increase in WFS content led to rise in demand of water and subsequently reduction in slump of concrete. Superplasticizer was added in concrete beyond 40% WFS level. Mixes with upto 40% WFS content fall in medium workability range but were harsh at higher replacements, so superplasticizer dosage was increased. In case of AAS concrete at all replacement levels mixes were having workability lower than GPC concretes attributed to GGBS being finer than fly ash used. For all mixes, superplasticizer dosage was kept in the range of 0– 2% of weight of source material such as to keep the mixes in medium workability range. Slump value was recorded just after mixing was completed (within 5 min). No segregation was observed due to WFS addition. Presence of finer particles in WFS i.e. clay-type fine materials, ashes and impurities etc. in WFS can be attributed as accountable for decreasing the fluidity of fresh concrete and increasing the cohesiveness [36–38].
100 90
Coarse Aggregates
80
% Passing
70 60
Natural Sand
50 40
Waste Foundry Sand
30 20 10 0 0.1
1
Particle size
10
Fig. 3. Grading curves of coarse and fine aggregates.
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B. Bhardwaj, P. Kumar / Construction and Building Materials 209 (2019) 555–565 Table 4 Proportions of constituents of GPC and AAS concrete. Constituents of concretes
(kg/m3)
(Fly Ash + OPC)/GGBS Fine aggregate Coarse aggregate NaOH solution Na2SiO3 solution
400 644 1196 72 108
Table 5 Details of GPC and AAS concrete mixes. S.no.
Mix No.
Concrete Type
Description
1 2 3 4 5 6
GF0 GF20 GF40 GF60 GF80 GF100
GPC
100% NA (FA GPC Ref mix) 80% NA + 20% WFS 60% NA + 40% WFS 40% NA + 60% WFS 20% NA + 80% WFS 0% NA + 100% WFS
1 2 3 4 5 6
AS0 AS20 AS40 AS60 AS80 AS100
AAS Concrete
100% NA (AAS Ref mix) 80% NA + 20% WFS 60% NA + 40% WFS 40% NA + 60% WFS 20% NA + 80% WFS 0% NA + 100% WFS
steady increase in the strength upto 60% replacement of NA by WFS was observed, with maximum strength obtained by Mix GF60 as 48.5 MPa. Strength gain by Mix GF60 is 43% more than the mix with 0% WFS i.e. GF0. More than 60% replacement by WFS led to abrupt decrease in strength of GPC. Whereas, in case of AAS mixes the compressive strength increased only upto a replacement level of 20% and beyond that level strength decreased. With addition of 20% WFS, level of strength of concretes increased by about 7% w.r.t. mix AS0 i.e. 0% WFS. In case of AAS concrete, on replacement of natural sand by 40% and 60% of WFS, it was possible to achieve more than 77% of the 28 day strength of the reference mix (GS0). In case of both the concretes, even at 100% replacement level of WFS (GF100 and AS100) it was possible to obtain more than 45% strength of control concretes (GF0 and AS0). Normalized compressive strength of fly ash GPC and AAS mixes w.r.t. reference mixes GF0 and AS0 are shown in Fig. 6. Split tensile strength results (Fig. 7) of GPC and AAS concrete were in tune to compressive strength results. In case of GPC mixes, Mix GF60, i.e. with 60% WFS, achieved maximum strength and beyond 60% replacement level a sharp decrease in strength was noticed. In AAS concrete mixes, mix AS20 achieved maximum split tensile strength. Besides, at 40% and 60% replacement of normal sand by WFS in AAS mixes, tensile strength more than 85% of strength of reference mix was achievable. It can be observed that FA GPC showed increase in strength upto 60% WFS, whereas, in AAS mixes strength reduced beyond 20% WFS level. This could be attributed to fly ash particles being coarser than GGBS, due to which addition of finer WFS led to refinement of concrete microstructure upto higher WFS replacement level. But beyond a certain replacement level, finer particles WFS didn’t contribute to particle packing, besides, unimodal grain size of WFS [14] led to creation of more pores in the microstructure which led to decrease in strength of concretes. 3.2. Capillary suction Uptake of water (and therefore ions) by unsaturated, hardened concrete may be characterized by the sorptivity. Initial rate of absorption (IRA) of various mixes was obtained by plotting graph
Fig. 5 shows compressive strength test results of all the GPC and AAS concrete mixes as obtained in this study. Alkali activated slag concrete mixes exhibit better strengths than GPC mixes at all replacement levels of WFS. With the addition of WFS, compressive strength increased upto different replacement levels of WFS for both series of concretes. In case of geopolymer concrete mixes,
GPC mixes
50
AAS Concrete mixes
40 30 20 10 0
0
20
40
60
80
100
WFS % Fig. 5. Compressive strength results of GPC and AAS mixes at 28 day.
GPC mixes
40 20 0 GF0
GF20
GF40
GF60
GF80
GF100
-20 -40 -60 20
% Increase/Decrease in Compressive Strength
28 Day Compressive Strength, MPa
60
60
% Increase/Decrease in Compressive Strength
Fig. 4. Schematic of capillary suction test procedure.
AAS mixes
10 0 AS0
AS20
AS40
AS60
AS80
AS100
-10 -20 -30 -40 -50
Mix Fig. 6. Normalized 28 day compressive strength results of fly ash GPC and AAS mixes w.r.t. GF0 and AS0 in ambient cured conditions.
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5.5
4.5 4 3.5 GPC mixes
3
AS100
AS80
AS60
AS40
AS20
AS0
GF100
GF40
GF20
GF0
2
GF80
AAS mixes
2.5
GF60
Split Tensile Strength, MPa
5
Mixes Fig. 7. 28 day split tensile strength results of GPC and AAS mixes.
between cumulative water absorption for the first six hours against the square root of time. For all the tested specimens, data for tested duration shaped linear relationships with value of coefficients of correlation more than 0.90. The value of slopes obtained from equations of theses plots is the IRA value. Average IRA value for all the GPC and AAS concrete mixes investigated at 28 days of curing age are shown in Fig. 8. There was consistent reduction in IRA values with increase in WFS level upto 60% and lowest value was shown by mix GF60 i.e. 0.0188 mm/Sec1/2. Concrete mixes of AAS showed lower IRA values as compared to fly ash GPC mixes with mix AS20 showed minimum IRA value i.e. 0.0143 mm/Sec1/2. In GPC mixes, reduction in IRA values shown by mixes GF20, GF40, and GF60 was respectively about 10%, 40% and 48% less than mix GF0. Whereas for AAS mixes the percentage reduction in IRA for mix with 20% WFS was observed to be as 23% lower than mix with 0% WFS i.e, AS0. This decrease in absorption could be accredited to finer WFS particles which led to pore refinement in the concrete matrix. There was abrupt rise in sorptivity values of GPC mixes beyond 60% WFS level, whereas in AAS mixes a gradual rise in sorptivity value was observed beyond 20% replacement level of WFS, this could be attributed to unimodal grain size of WFS particles which caused increase in pores in concrete when finer WFS particles were increased beyond a certain level [39].
Average IRA, mm/Sec1/2
0.045 0.04 0.035 0.03 0.025 0.02
GPC mixes
0.015
AAS Concrete mixes
0.01 0.005 0
0
20
40
60
80
100
WFS % Fig. 8. Variation in 28 day IRA values of different geopolymer and AAS concrete mixes.
3.3. SEM and EDS analysis Microstructure study of concretes was done to understand the morphology of GPC and AAS concrete, along with the influence of addition of WFS on the microstructure, presence of pores and micro cracks. Besides, with the help of EDS, atomic percentages of each element present in the concrete matrix were obtained which helped in the semi-quantitative analysis of concretes. Fly ash based geopolymer principally constitutes geopolymer paste (aluminosilicate gel), which is irregular in shape, unreacted fly ash and voids [40]. The surface image of mixes GF0, GF20, GF60 and GF90, observed using SEM is shown in Fig. 9(a)–(d). A non-uniform, coarser and heterogeneous geopolymer gel matrix containing a numbers of unreacted/partially reacted fly ash particles was witnessed in the GPC microstructure. Coarser gel and microstructure indicates moderate reactivity and results in high porosity [41]. It has been reported that unreacted components make up a significant proportion of the total volume of the binder in FA-based geopolymer binder [42]. These components are composites. The strength of geopolymer matrix, these unreacted particles and the interface between them may have a substantial effect on the overall strength of the concrete. Increased silicate content could provide a denser microstructure by increasing the reactivity, however, the unreacted fly ash, as visible in the SEM images of GPC, is unavoidable even when high alkaline concentration and high temperature curing were used in the system [41]. It can be observed that mix GF60 appears to be dense as compared to other GPC mixes. SEM images and typical EDS spectra for atomic concentration analysis of mixes depicted that the major elements present in all GPC concrete specimens were Si and Al, whereas other elements present were Na, Ca, and Fe, but in much less extents. The atomic ratio Si/Al was in the range of 1.22 to 2.54 for all GPC mixes, which confirmed that the matrix of GPC mainly contains Si-Al-O. This atomic ratio Si/Al determines the structure of geopolymer [43], a ratio of Si/Al = 1 will form polysialate (-Si-O-Al-O-) (polysialate) Si/Al = 2 will form poly sialate-siloxo (-Si-O-Al-O-Si-O-). The microstructural development of AAS concretes, AS0, AS20, AS60 and AS90, is displayed in Fig. 9(e)–(h). In case of AAS concretes a fairly even gel matrix can be seen with most of the slag grains partially dissolved by the alkali solution, as visible in Fig. 9(e), forming a C-S-H gel by reacting with the silica from the alkaline activator solution. The visual inspection identified uni-
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formly distributed micro-cracking on the surface of all the AAS concrete samples, visible from naked eye. The observed microcracking is attributed to shrinkage strains [44,45]. Analysis of SEM images noted that micro-cracks also occurred within the AAS concrete matrix. Law et al. (2012) identified the micro-
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cracks as cracks formed on the surface of partially dissolved slag grains accredited to stress built up as the reaction proceeds [46]. As the reaction follows the microstructure densifies and thus the partially dissolved slag grains become more confined, leading to formation of micro-cracks, similar to results reported elsewhere
(a) GF0
(e) AS0
(b) GF20
(f) AS20
(c) GF60
(g) AS60
Fig. 9. SEM images of concrete mixes- (a) GF0, (b) GF20, (c) GF60, (d) GF100, (e) AS0, (f) AS20, (g) AS60 and (h) AS100.
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(d) GF100
(h) AS100 Fig. 9 (continued)
Table 6 Values for EE and ECO2e for all the materials/kg along with source. Material
EE (MJ/kg)
Source
ECO2e (kgCO2e/kg)
Source
OPC FA GGBS SS SH NA WFS CA W S
4.80 0.033 0.31 5.37 8.75 0.081 – 0.083 0.20 11.50
[51] [52] [53] [54] [56] [51] – [51] [51] [51]
0.93 0.004 0.052 0.71 0.46 0.0051 – 0.0048 0.0008 0.60
[51] [52] [52] [55] [55] [51] – [51] [51] [51]
Table 7 Total embodied energy for various Geopolymer and Alkali activated slag concrete mixes. Material
EE (MJ/kg)
OPC FA GGBS SS SH NA WFS CA W S Total EE
4.80 0.033 0.31 5.37 8.75 0.081 – 0.083 0.20 11.50
EE (MJ/m3) of concrete GF0
GF20
GF40
GF60
GF80
GF100
AS0
AS20
AS40
AS60
AS80
AS100
192.00 11.88 – 579.96 254.62 52.16 – 94.26 10.60 – 1195.48
192.00 11.88 – 579.96 254.62 41.71 – 94.26 10.60 – 1185.03
192.00 11.88 – 579.96 254.62 31.27 – 94.26 10.60 – 1174.59
192.00 11.88 – 579.96 254.62 20.82 – 94.26 10.60 46 1210.14
192.00 11.88 – 579.96 254.62 10.45 – 94.26 10.60 69 1222.77
192.00 11.88 – 579.96 254.62 – – 94.26 10.60 92 1235.32
– – 124 579.96 254.62 52.16 – 94.26 10.60 – 1115.6
– – 124 579.96 254.62 41.71 – 94.26 10.60 – 1105.15
– – 124 579.96 254.62 31.27 – 94.26 10.60 – 1094.71
– – 124 579.96 254.62 20.82 – 94.26 10.60 46 1130.26
– – 124 579.96 254.62 10.45 – 94.26 10.60 69 1142.89
– – 124 579.96 254.62 – – 94.26 10.60 92 1155.44
[46,47]. These micro cracks were observed to be dispersed throughout the gel matrix and mixes with higher WFS content had more cracks with greater crack widths. The microstructure of AAS concrete was more refined as compared to GPC as shown in Fig. 9(a)–(h). In AAS system the SEM images showed the partially dissolved slag grains whereas in GPC mixes unreacted spherical fly ash particles were visible. In both type of GPC mixes, an increase in micro pores was observed with the increase in WFS content. The increase in micro-pores in the microstructure of concretes, with increased WFS content, correlates with the strength and durability results. Besides, presence of micro-cracks may also have caused increased in sorptivity in all the concretes.
3.4. Environmental impact analysis Sustainable development as defined by Brutland Commision is ‘‘The development that meets the needs of present without compromising the ability of future generations to meet their own needs” [48]. In relevance to concrete industry, sustainable development may mean utilising available natural resources judiciously, not over exploiting them; using alternative construction materials, which are available locally, requires lesser energy for their production, are economical, having lower impact on environment and can be capable of producing good quality concrete, which after its full service life can be reutilised or disposed-off safely into environment without any harmful effects. The environmental impact of
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ECO2 (kgCO2e/kg)
OPC 0.93 FA 0.004 GGBS 0.052 SS 0.71 SH 0.46 NA 0.0051 WFS – CA 0.0048 W 0.0008 S 0.60 Total ECO2e
ECO2 (kgCO2e/m3) of concrete GF0
GF20
GF40
GF60
GF80
GF100
AS0
AS20
AS40
AS60
AS80
AS100
37.20 1.44 – 76.68 13.38 3.28 – 5.74 0.042 – 137.762
37.20 1.44 – 76.68 13.38 2.63 – 5.74 0.042 – 137.112
37.20 1.44 – 76.68 13.38 1.97 – 5.74 0.042 – 136.452
37.20 1.44 – 76.68 13.38 1.31 – 5.74 0.042 2.4 138.192
37.20 1.44 – 76.68 13.38 0.66 – 5.74 0.042 3.6 138.742
37.20 1.44 – 76.68 13.38 – – 5.74 0.042 4.8 139.282
– – 20.8 76.68 13.38 3.28 – 5.74 0.042 – 119.922
– – 20.8 76.68 13.38 2.63 – 5.74 0.042 – 119.272
– – 20.8 76.68 13.38 1.97 – 5.74 0.042 – 118.612
– – 20.8 76.68 13.38 1.31 – 5.74 0.042 2.4 120.352
– – 20.8 76.68 13.38 0.66 – 5.74 0.042 3.6 120.902
– – 20.8 76.68 13.38 – – 5.74 0.042 4.8 121.442
Note: OPC- Ordinary Portland Cement; FA-Fly Ash; GGBS-Ground Granulated Blast Furnace Slag; SS-Sodium Silicate solution; SH-Sodium hydroxide solids; NA-Sand; WFSWaste Foundry Sand; CA-Coarse Aggregates; SP-Super plasticizer; W-Water.
any material is generally indicated by two parameters- embodied energy and embodied CO2 emissions. Embodied energy (EE) is the total energy required, and embodied CO2 emissions (ECO2e) is the total CO2 emissions, throughout the lifecycle of a material i.e. from extraction, processing/manufacture, transportation, maintenance till demolition i.e. from cradle to grave. In this study, EE and ECO2e of all the concrete mixes are calculated by considering only extraction and manufacturing process, i.e. from cradle to gate, by taking the EE and ECO2e values of constituent materials from secondary sources. The values of EE and ECO2e for materials may vary depending upon the country, age of data and boundary conditions [49]. The values for EE and ECO2e for all the materials, per kg, along with their published source is shown in Table 6. Tables 7 and 8 shows the calculated values of EE and ECO2e for all GPC and AAS concrete mixes for 1 m3 of concrete, respectively. The EE and ECO2e values indicated in Tables 7 and 8 are for solid sodium hydroxide. Water content in Table 8 consists of extra water and water required to prepare sodium hydroxide solution only. The aim of this analysis is to indicate the impact of WFS addition in reducing the environmental analysis also the comparison of reference GPC and AAC concrete mixes with the EE and ECO2e of conventional mix (OPC + FA) of similar strength. The mix proportions for conventional concrete (40 MPa), containing 30% fly ash are taken from Arora and Singh (2016) [50]. The EE and ECO2e values of the conventional mix are calculated as 1851.58 MJ/m3 and 329.18 kgCO2e/ m3 respectively, which is much higher than EE and ECO2e values for GF0 and AS0. It can be observed that in both GPC and AAS concretes the mixes with 100% WFS has maximum EE and ECO2e values. The use of WFS in concretes lowered the EE and ECO2e upto certain WFS replacement levels but at higher replacement levels, demand of superplasticizer also increased which led to increased EE and ECO2e values. Mix GF40 and AS40 showed minimum EE value i.e. 1174.59 MJ/ m3 and 1094.71 MJ/m3 amongst GPC and AAS concrete mixes, respectively. Geopolymer mixes have higher EE and ECO2e values due to addition of OPC as 10% of total source material. In all the mixes, the alkaline solution have maximum contribution towards EE and ECO2e values followed by OPC/GGBS, coarse aggregates, fine aggregates and then superplasticizer. Alkaline solutions have a share of 68%-71% in total EE of GPC mixes whereas 72%-76% in case of AAS concrete mixes, similarly, a total share of 65–66% in total ECO2e of GPC mixes whereas 74%–76% in case of AAS concrete mixes. The EE and ECO2e of WFS is considered as null since it is an industrial by-product and was just sieved through 4.75 mm sieve and used as it was discarded from industry without any processing.
4. Conclusions Following conclusions can be drawn from the current investigation: WFS addition into fly ash GPC and AAS concrete led to decrease in workability of concretes. Beyond 60% WFS content in GPC and 40% WFS in AAS, abrupt drop in workability of mixes was observed. Presence of water absorbing finer particles i.e. claytype fine materials, impurities and ashes etc. in WFS, can be accredited to this. In GPC mixes, partial replacement upto 60% of NA by WFS led to consistent rise in 28 day compressive strength, with maximum strength (GF60) as 43% more than reference mix (GF0). Whereas, at 100% replacement (GF100) about 47% of the strength of control concrete was achieved. In AAS concretes, maximum strength gain was observed in mix with WFS replacement level of 20%. At 40 and 60% replacement level of WFS strength achieved was more than 77% of the 28 day strength of the reference mix (AS0). It was possible to achieve more than 45% of compressive strength of control concretes (GF0 and AS0) even at 100% replacement level of WFS (GF100 and AS100). Split tensile strength results were similar to compressive strength results. Sorptivity results were in correlation with strength results. In case of GPC and AAS concretes mixes, GF60 and AS20 showed minimum IRA value at 28 day curing age which about 48% and 23% of IRA value of respective reference mixes. Coarser aluminosilicate gel is observed in microstructure of GPC mixes due to presence of unreacted and partially reacted fly ash particles which further increased with rise in WFS level leading to increase in porosity and hence weaker concrete matrix. Ratios of atomic concentrations of Si and Al was found in the range of 1.2–2.54 indicating polymerisation in the matrix with formations of poly sialate and poly sialate-siloxo structures. There was visible surface micro-cracking as well as microcracks, inside the concrete matrix attributable to shrinkage strains in case of AAS concretes. Wider cracks were found in case of concretes with higher WFS percentages, leading to increase in porosity at higher replacement levels. The embodied energy (EE) and embodied carbon dioxide emissions (ECO2e) of developed GPC and AAS concrete mixes is found very low when compared with a conventional concrete mix of similar strength. Besides, WFS inclusion helped in lowering EE and ECO2e values of GPC and AAS concrete mixes with beyond
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40% replacement by WFS superplasticizer content was increased to keep the concrete within workable range, which increased the EE and ECO2e values. Conflict of interest The authors declare that they have no conflicts of interest related to this work. Acknowledgement The authors would like to greatly acknowledge the valuable support of Concrete Structures and Material Testing Laboratory’s staff at National Institute of Technology, Hamirpur, India for carrying out testing of concrete work reported in this study. Ministry of Human Resources and Development, India is also gratefully acknowledged for providing financial support. References [1] Indian Bureau of Mines, Indian Minerals Yearbook 2017 (Part- III : Mineral Reviews)-Cement, Ministry of Mines, Govt of India, 2018, pp. 1–23. [2] www.statista.com, Major countries in worldwide cement production from 2012 to 2017 (in million metric tons), (2018). https://www.statista.com/ statistics/267364/world-cement-production-by-country/ (accessed 15.11.2018). [3] B. V Rangan, Fly Ash-Based Geopolymer Concrete, Prth, Australia, 2008. [4] V.B. Thapa, D. Waldmann, A short review on alkali-activated binders and geopolymer binders, in: Vielfalt Im Massivbau - Festschrift Zum 65. Geburtstag von Prof. Dr. Ing. Jürgen Schnell Author, Co-Author : Pahn, Matthias Thiele, Catherina, Ernst & Sohn, Berlin, Germany, 2018: pp. 576–591. http://hdl. handle.net/10993/35284. [5] J. Davidovits, Properties of geopolymer cements, in: First International Conference on Alkaline Cements and Concretes, Scientific Research Institute on Binders and Materials, Kiev, Ukraine, 1994, pp. 131–149. [6] J. Davidovits, Chemistry of geopolymeric systems, terminology Saint-Quentin, France, in: Geopolymere’99 International Conference, 1999, pp. 1–2, https:// doi.org/10.1021/ie980804b. [7] Central Electricity Authority Annual Report 2017–18, Ministry of Power, Govt of India. 2018 1–248. [8] J. Davidovits, False Values on CO2 Emission for Geopolymer Cement/Concrete published in Scientific Papers, Published in Scientific Papers, Technical Paper #24, Geopolymer Institute Library. (2015) 1–9. [9] S. Kumar, R. Kumar, Geopolymer: cement for low carbon economy, Indian Concr. J. 29–37 (2014). [10] N. Lloyd, B.V. Rangan, Geopolymer Concrete: a Review of Development and Opportunities, Cipremier.Com, 2010. http://www.cipremier.com/ e107_files/downloads/Papers/100/35/100035037.pdf. [11] C. Li, H. Sun, L. Li, Cement and Concrete Research A review: the comparison between alkali-activated slag (Si + Ca) and metakaolin (Si + Al) cements, Cem. Concr. Res. 40 (2010) 1341–1349, https://doi.org/10.1016/j. cemconres.2010.03.020. [12] P. Dinakar, K.P. Sethy, U.C. Sahoo, Design of self-compacting concrete with ground granulated blast furnace slag, Mater. Des. 43 (2013) 161–169, https:// doi.org/10.1016/j.matdes.2012.06.049. [13] D.A.R. Dolage, M.G.S. Dias, C.T. Ariyawansa, Offshore sand as a fine aggregate for concrete production, Br. J. Appl. Sci. Technol. 3 (2013) 813–825. [14] B. Bhardwaj, P. Kumar, Waste foundry sand in concrete: a review, Constr. Build. Mater. 156 (2017) 661–674, https://doi.org/10.1016/ j.conbuildmat.2017.09.010. [15] E.S. Winkler, A.A. Bol’shakov, Characterization of Foundry Sand Waste, Chelsea, Massachusetts, 2000. [16] R. Siddique, G. De Schutter, A. Noumowe, Effect of used-foundry sand on the mechanical properties of concrete, Constr. Build. Mater. 23 (2009) 976–980, https://doi.org/10.1016/j.conbuildmat.2008.05.005. [17] S.P. Oudhia, An Overview of Indian Foundry Industry, Metalworld, 2015, pp. 3– 4. [18] R. Siddique, G. Singh, Utilization of waste foundry sand (WFS) in concrete manufacturing, Resour. Conserv. Recycl. 55 (2011) 885–892, https://doi.org/ 10.1016/j.resconrec.2011.05.001. [19] R. Siddique, A. Noumowe, Utilization of spent foundry sand in controlled lowstrength materials and concrete, Resour. Conserv. Recycl. 53 (2008) 27–35, https://doi.org/10.1016/j.resconrec.2008.09.007. [20] R. Siddique, Y. Aggarwal, P. Aggarwal, E.H. Kadri, R. Bennacer, Strength, durability, and micro-structural properties of concrete made with usedfoundry sand (UFS), Constr. Build. Mater. 25 (2011) 1916–1925, https://doi. org/10.1016/j.conbuildmat.2010.11.065. [21] J.S. Joy, M. Mathew, Experimental study on geopolymer concrete with partial replacement of fine aggregate with foundry sand, Int. J. Adv. Technol. Eng. Sci. 3 (2015) 559–569.
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Comparative study of geopolymer and alkali activated slag concrete comprising waste foundry sand Bavita Bhardwaj ⇑, Pardeep Kumar National Institute of Technology, Hamirpur, HP 177005, India
h i g h l i g h t s Use of WFS as partial to full replacement to normal sand in GPC and AAS concrete. WFS addition beyond 40% significantly affected workability. Strength and sorptivity enhanced upto certain WFS levels. Unimodal particle size of WFS decreased strength at higher replacements. GPC and AAS concrete have considerably low EE and ECO2e than OPC concrete.
a r t i c l e
i n f o
Article history: Received 30 November 2018 Received in revised form 11 February 2019 Accepted 11 March 2019
Keywords: Geopolymer concrete Alkali activated slag concrete Low calcium fly ash GGBS Waste foundry sand
a b s t r a c t Concrete industry and current construction practices are highly unsustainable from the standpoint of energy consumption and their high dependence on natural resources. Using alternative binders to conventional cement such as geopolymers and alkali activated slags (AAS) can be one alternative. Besides, reusing industrial by-product and waste materials in construction industry can also be emphasized. These measures can substantially lower the carbon emissions and embodied energy of concrete along with solving the problem of industrial waste disposal. The current study reports the influence of inclusion of waste foundry sand (WFS), a by-product from foundry industries, on strength, permeability and microstructure of low calcium fly ash geopolymer concrete (GPC) and ground granulated blast furnace slag (GGBS) based alkali activated slag (AAS) concrete, both cured in ambient conditions. The mix proportions for both the concretes were kept same. The natural sand was replaced by WFS in the range of 0% to 100% at an interval of 20%. Tests on hardened concretes, to study, compressive strength, split tensile strength, capillary suction, SEM and EDS analysis, were conducted to assess the strength, permeability and microstructure of both types of concretes. Addition of WFS lowered the workability of GPC and AAS concretes and the effect was abrupt beyond 40% WFS replacement level. Strength and sorptivity of both concretes improved upto 60% WFS replacement in GPC, whereas, upto 20% replacement in AAS concrete mixes, besides more than 45% of strength of reference concretes (0% WFS) was achievable at 100% replacement by WFS. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Environmental issues and sustainability are the main concerns that are guiding most of the contemporary research. Concrete, being extensively used construction material in the world, has significant contribution towards global warming and climate change. All the primary constituents of concrete have different environmental impact and cause different sustainability issues. To avert the environmental effects associated with concrete and its ingredi⇑ Corresponding author. E-mail address: [email protected] (B. Bhardwaj). https://doi.org/10.1016/j.conbuildmat.2019.03.107 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ents there is need to switch to other environment friendly and sustainable substitutes. Worldwide cement production in year 2017 was estimated around 4100 million tonnes. Out of this, about 2400 million tonnes i.e. around 59% was produced by China alone, followed by India (270 million tonnes) 6.58%, USA (86 million tonnes) 2.1%, Vietnam (78 million tonnes) 1.90% and Turkey (77 million tonnes) 1.88% as top five producers [1]. This global annual cement production is anticipated to rise to 4830 million tonnes by 2030 [2]. Cement manufacturing is highly energy intensive process and the production of 1 tonne of ordinary Portland cement (OPC) releases about equal amount of CO2 in the atmosphere, making it one of the major
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contributor to global warming. To counter this issue several attempts are being made to lower the consumption of OPC and CO2 emissions, mainly categorized under two areas. One is replacing cement in concrete by supplementary materials; e.g., fly ash, silica fume, metakaolin, bottom ash, granulated slag etc. The other is to find alternative binders for concrete such as geopolymers and alkali activated slag [3]. Geopolymers and alkali-activated binders are hardened compounds which attain their strength and other characteristics by chemical reaction between alkaline solutions and aluminatesilica rich source materials. Geopolymer is inorganic aluminasilicate polymer structured around tetrahedral coordinated Si4+ and Al3+, forming a polymer chain. These geopolymer precursors chemically bond and form oligomers leading to the formation of aluminosilicate polymers [4]. Whereas, alkali-activated binders are formed by activation of calcium-rich source materials under high alkaline conditions. After the dissolution of the precursors in alkaline solution, a binder paste with hydraulic potential is build leading to simultaneous formation of C-S-H gels and aluminosilicate polymers [4]. Fig. 1 shows general difference between alkali activated binders and geopolymers [4]. Geopolymers and alkali activated binders completely replace OPC by utilizing industrial by-products like fly ash and GGBS making concrete less energy intensive and more environment friendly [4–6]. Fly ash, the by-product of coal combustion in thermal power plants, is abundantly available worldwide, but to date only limited quantity of it is being utilized. In India, 169.25 million tonne of fly ash was generated in year 2016–17 but current level of efforts resulted in utilization of only 63.28% of it [7]. The need of power is certain to increase in coming time, so is the volume of fly ash and the problem of its disposal. The utilization of fly ash in production of cement and concrete has, to some extent, solved the problem of its disposal. It is estimated that, if fully industrialized, geopolymer cement will have 80% lower embodied greenhouse gas emissions than an equivalent amount of OPC binder [8]. The CO2 emissions in geopolymer are mainly during production of alkali hydroxide and silicate from carbonates. Moreover, as it mostly uses waste and by-products with no embodied energy so it’s embodied energy is nearly half of that of Portland cement [9]. It is estimated that price of fly ash-based geopolymer concrete is estimated to falls about 10–30 percent cheaper than that of OPC concrete [10]. Furthermore, several investigations have assessed
the performance of GGBS as a cementitious material in cement production since 1939 [11]. The use of GGBS as cement replacement material in concrete has been found to reduce the CO2 emission by a great extent [12]. Over extraction of natural resources to cater the need of rising urbanisation and industrialisation is causing detrimental effects on environment. Furthermore, sand, which is the second major constituent of concrete, the restriction in its extraction from river has increased the price of sand and severely affecting the survival of the construction industry [13]. As such, it has become imperative to find alternative materials to river sand. Metal alloy casting industries worldwide produce several million tonnes of byproducts annually of which waste foundry sand (WFS) is the major by-product [14]. United States landfills about 6–10 million tonnes of waste foundry sand per year [15,16]. India, which is the third largest casting manufacturer in the world after China and USA, produced around 1.71 million tonnes of waste per annum [17,18]. Foundries use raw sand for use in their moulding and casting operations, which is high quality size-specific silica sand and is generally of a superior quality than the sand normally generally used in construction purposes. Waste foundry sand is rich in silica content owes its black colour to coating of burnt carbon over particles. Several authors have studied the use of WFS in various civil engineering applications such as sand replacement in concrete, as highway embankment filling, soil stabilization, mortar making, brick blocks, pavement blocks etc. [19]. Addition of WFS in concrete as partial substitute (20–30%) to natural sand has improved the mechanical as well durability properties of conventional as well as special concretes such as self-compacting concrete (SCC) and geopolymer concretes [18–22]. Some authors have observed strength enhancement on replacement by WFS upto 50–60% as well [20,23]. At 80% replacement level of WFS, upto 70% of strength of reference concrete (0% WFS) was achievable [24]. Most of the research on WFS in concrete is limited to conventional concrete only and that too its use as partial replacement to normal sand. The aim of current study is to assess the strength, permeability and microstructure of ambient cured GPC and AAS concretes when WFS was used as partial to full replacement to normal sand. The potential use of GPC being more sustainable and green material as a replacement of normal concrete necessitates the study of permeation properties of concrete from durability point of view. Sorptivity test, being an important and easy method to ascertain durability of concrete, was
Fig. 1. Reaction mechanism of Alkali activated binder and geopolymer [4].
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done to find out the effect on initial rate of absorption of the developed GPC and AAS concretes due to varying proportions of WFS. The impact of GPC and AAS concrete mixes with WFS on environment is also assessed by calculating its embodied energy and embodied carbon dioxide emissions.
Table 2 Chemical moduli of GGBS. Chemical Moduli
Test Result
(CaO + MgO)/SiO2 CaO/SiO2 CaO + MgO + SiO2
1.30 1.07 76.03
2. Experimental programme The investigation studies the influence of partial to full replacement of natural sand by WFS on the geopolymer concrete (GPC) prepared using low calcium fly ash and alkali activated slag (AAS) concrete prepared using GGBS. The GPC mixes developed with fly ash as source material contained 10% of 43 Grade OPC as partial replacement to total binder to accelerate setting of concrete. All the mixes were cured in dry ambient laboratory conditions. Compressive strength, split tensile strength and capillary suction tests were conducted to assess its performance. In addition, microstructure analysis of concretes was also done by undertaking scanning electron microscopy (SEM) analysis and energy dispersive X-ray spectroscopy (EDS). Cube specimens of 100 mm size were cast for compressive strength test while, 100 mm diameter and 200 mm long cylinders were cast for testing split tensile strength and capillary suction test as per recommendation of relevant standards. 2.1. Materials Ground granulated blast furnace slag (GGBS), procured from Astraa chemicals, Chennai, conforming to standard BS 6699: 1992, was used in the study [25]. Low calcium fly ash (ASTM Class F) conforming to standard IS 3812-Part-1 [26] for geopolymer concrete was procured locally from Ambuja Cement plant. Ordinary Portland cement (OPC) of 43 grade, as procured locally from ACC cement, was used conforming to standard IS 8112 [27]. Oxides composition of these materials, as provided by suppliers, is shown in Table 1. Specific gravity for fly ash and OPC was 2.23 and 3.14 whereas for GGBS it was obtained as 2.85. Specific surface area of fly ash and GGBS was 260 m2/kg and 390 m2/kg respectively. The glass content and moisture content in GGBS was 91% and 0.1% respectively. Chemical moduli of GGBS is presented in Table 2, as provided by Astraa chemicals. Fig. 2 shows the images from scanning electron microscope (SEM) analysis of fly ash and GGBS. Particles of fly ash are round, whereas, GGBS has flaky and elongated particles with smaller size as compared to fly ash, when both are observed on same magnification under SEM. For activating source materials a combination of two alkaline solutions was chosen i.e. Sodium Hydroxide (NaOH) and Sodium Silicate (Na2SiO3). Sodium Hydroxide was obtained from supplier in pellets form, 98% pure, and mixed with normal tap water to prepare solution. For both GPC and AAS concretes, NaOH solution of 14M concentration was prepared by mixing 404 parts of solids with 596 parts of water. Commercial grade, clear colourless and viscous, Sodium Silicate solution with SiO2/Na2O by mass of 2.1 and water content of 52% was used. The solutions sodium silicate and sodium hydroxide solution were used in the proportion of 2.3, constant for all the concrete mixes. For fine aggregate, locally available crushed sand (NA) was used in consort with WFS as its partial to full replacement in different GPC and AAS concrete mixes. The oxide composition of NA and WFS is shown in Table 1. Waste foundry sand was procured from one local ferrous foundry, manufacturer of automobile parts. It is black in colour due to presence of carbon content, added in foundry during foundry operations to enhance finishing of casting. The particles of WFS were comparatively round to sub-angular in shape as shown in Fig. 2. The NA and WFS conformed to zone-2 and zone-4 respectively, as per IS-383 [28]. Coarse aggregate used were locally available crushed stone aggregates, angular in shape, with maximum size 12.5 mm. Aggregates, except WFS, were used in saturated surface dry condition. Table 3 shows physical properties of fine and coarse aggregates used and their grading curves are given in Fig. 3. Normal potable tap water was used for making alkaline solutions as well as concretes.
assumed as 2430 kg/m3 and other calculations were made based on the density of concrete as per the mix design procedure given by Junaid et al. [29] considering the target strength as 40 MPa. Trials were done before finalizing the mix proportions. The combined weight of aggregates was taken as 76%. The alkaline solution was taken as 45% of powdered binder or source material. Same mix proportions were used for AAS concrete. Extra water was kept constant as 10 kg/m3. The proportions of the concrete constituents used is shown in Table 4. Activator solutions were premixed 24 h before casting of concrete so that the solution cools down to room temperature and stabilizes before adding to mixture [30]. The aggregates along with WFS were first mixed for 2 min in concrete mixer. After that powder binder (GGBS/FA + OPC) was added in concrete and mixed for 2 min. Activator solutions with extra water and superplasticizer were premixed and added slowly to the dry contents. After mixing for 3 min, slump cone test was performed to test the workability of fresh concrete. Thereafter, concrete was poured into concrete moulds and compacted on table vibrator. After 24 h of casting, all specimens were demoulded and then kept in laboratory for dry ambient curing until testing age of 28 days. Six mixes of GPC and AAS concrete each, with varying proportions of WFS, were cast for testing. Details of all mixes is shown in Table 5. 2.3. Testing 2.3.1. Workability and strength properties It has been widely reported that application of heat to GPC helps in initial setting of GPC and greatly enhances its final strength [31], but requirement of heat limits the application of GPC. However, low calcium GPC when ambient cured, takes more than 24 h to set. So, in the current investigation 10% of OPC was added to accelerate the initial setting of GPC. The slump test was carried out to test workability of fresh concretes according to IS 1199 [32]. All tests on hardened concretes were performed at the age of 28 days. Compressive strength and split tensile strengths were performed on automatic compression testing machine (CTM) with a load capacity of 3000 kN. The load was applied gradually at the rate of 14 N/ mm2/minute for compressive strength test and at 1.8 N/mm2/min for split tensile strength test as per IS: 516 [33] and IS: 5816 [34], respectively. The strength values were determined from the average of three specimens. 2.3.2. Absorption Capillary suction test was performed as per ASTM standard C 1585–04 [35] to find the rate of absorption, i.e. sorptivity of water by hydraulic cement concrete by measuring the increase in the mass of a specimen resulting from absorption of water as a function of time when only uniaxial penetration of water is allowed (Fig. 4). The specimens used for sorptivity test were cylindrical discs of 100 mm diameter and 50 mm thickness, cut from cylinders of 100 mm diameter and 200 mm height. Results reported for each mix are obtained from the average sorptivity value of three discs of same mix. On reaching testing age all specimens were first conditioned to prepare for test. After ambient curing for 28 days, cylinders were cut to obtain discs, the discs were then marked and kept in oven at a temperature of 105 ± 5 °C for drying, until weight was stabilised i.e. not more than 0.1% weight changed over after 24 h drying period. This usually takes 3–4 days. After drying, specimens were kept in desiccator to cool down to room temperature. Before testing, specimens were coated with paraffin wax on circumferential surfaces and sealed with tightly attached plastic sheet on the top surface to allow only uniaxial (i.e. one dimensional) diffusion of water through the uncoated surface.
2.2. Mixing and casting details The concrete mixes were proportioned as such to study the influence of WFS addition, as partial to full replacement of normal sand (NA) on strength development and permeability of GPC and AAS concrete. The density of concretes was
2.3.3. Microstructure The microstructure of GPC and AAS concretes was studied using scanning electron microscopy (SEM) imaging on Quanta FEG 450. Energy dispersive X-ray spectroscopy (EDS) analysis was performed to determine the elemental chemical
Table 1 Chemical composition of fly ash, OPC and GGBS used. Material
Fly Ash OPC GGBS WFS
By weight (%)
LOI
SiO2
CaO
Al2O3
Fe2O3
MgO
Na2O
SO3
K2O
MnO
TiO2
P2 O5
(%)
59.87 20.10 33.06 78.5
5.06 61.30 35.37 2.44
23.96 6.80 22.29 5.50
3.70 4.30 0.9 4.45
0.48 2.60 7.61 2.76
0.17 0.26 – 0.79
0.16 1.30 0.38 0.22
1.20 0.23 – 0.52
– – 0.12 –
1.27 – – 0.15
0.28 – – –
2.00 1.20 0.26 –
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Fig. 2. SEM images (a) Fly ash, (b) GGBS and (c) WFS.
Table 3 Properties of fine and coarse aggregates. Property
Fine aggregate
Specific Gravity Fineness Modulus Water Absorption (%)
NA
WFS
2.65 2.50 0.55
2.18 1.94 1.5
Coarse aggregate
2.74 6.93 0.68
composition of reacted geopolymer and AAS concrete. Specimens were taken from concretes after reaching testing age of 28 days. Specimens were first staged over conductive double-side carbon tape and subsequently coated with gold and then mounted on SEM sample stage for analysis.
3. Results and discussion 3.1. Workability and strength To improve workability of mixes Sulphonated Naphthalene based superplasticizer was added in concretes. Water to solid con-
tent for all the mixes was kept constant as 0.24. Weight of total water in concrete includes weight of water in alkaline solution as well as extra water added, whereas, weight of total solid content in binder includes weight of source material (FA + OPC/GGBS) and weight of solid content of alkalis. Increase in WFS content led to rise in demand of water and subsequently reduction in slump of concrete. Superplasticizer was added in concrete beyond 40% WFS level. Mixes with upto 40% WFS content fall in medium workability range but were harsh at higher replacements, so superplasticizer dosage was increased. In case of AAS concrete at all replacement levels mixes were having workability lower than GPC concretes attributed to GGBS being finer than fly ash used. For all mixes, superplasticizer dosage was kept in the range of 0– 2% of weight of source material such as to keep the mixes in medium workability range. Slump value was recorded just after mixing was completed (within 5 min). No segregation was observed due to WFS addition. Presence of finer particles in WFS i.e. clay-type fine materials, ashes and impurities etc. in WFS can be attributed as accountable for decreasing the fluidity of fresh concrete and increasing the cohesiveness [36–38].
100 90
Coarse Aggregates
80
% Passing
70 60
Natural Sand
50 40
Waste Foundry Sand
30 20 10 0 0.1
1
Particle size
10
Fig. 3. Grading curves of coarse and fine aggregates.
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(kg/m3)
(Fly Ash + OPC)/GGBS Fine aggregate Coarse aggregate NaOH solution Na2SiO3 solution
400 644 1196 72 108
Table 5 Details of GPC and AAS concrete mixes. S.no.
Mix No.
Concrete Type
Description
1 2 3 4 5 6
GF0 GF20 GF40 GF60 GF80 GF100
GPC
100% NA (FA GPC Ref mix) 80% NA + 20% WFS 60% NA + 40% WFS 40% NA + 60% WFS 20% NA + 80% WFS 0% NA + 100% WFS
1 2 3 4 5 6
AS0 AS20 AS40 AS60 AS80 AS100
AAS Concrete
100% NA (AAS Ref mix) 80% NA + 20% WFS 60% NA + 40% WFS 40% NA + 60% WFS 20% NA + 80% WFS 0% NA + 100% WFS
steady increase in the strength upto 60% replacement of NA by WFS was observed, with maximum strength obtained by Mix GF60 as 48.5 MPa. Strength gain by Mix GF60 is 43% more than the mix with 0% WFS i.e. GF0. More than 60% replacement by WFS led to abrupt decrease in strength of GPC. Whereas, in case of AAS mixes the compressive strength increased only upto a replacement level of 20% and beyond that level strength decreased. With addition of 20% WFS, level of strength of concretes increased by about 7% w.r.t. mix AS0 i.e. 0% WFS. In case of AAS concrete, on replacement of natural sand by 40% and 60% of WFS, it was possible to achieve more than 77% of the 28 day strength of the reference mix (GS0). In case of both the concretes, even at 100% replacement level of WFS (GF100 and AS100) it was possible to obtain more than 45% strength of control concretes (GF0 and AS0). Normalized compressive strength of fly ash GPC and AAS mixes w.r.t. reference mixes GF0 and AS0 are shown in Fig. 6. Split tensile strength results (Fig. 7) of GPC and AAS concrete were in tune to compressive strength results. In case of GPC mixes, Mix GF60, i.e. with 60% WFS, achieved maximum strength and beyond 60% replacement level a sharp decrease in strength was noticed. In AAS concrete mixes, mix AS20 achieved maximum split tensile strength. Besides, at 40% and 60% replacement of normal sand by WFS in AAS mixes, tensile strength more than 85% of strength of reference mix was achievable. It can be observed that FA GPC showed increase in strength upto 60% WFS, whereas, in AAS mixes strength reduced beyond 20% WFS level. This could be attributed to fly ash particles being coarser than GGBS, due to which addition of finer WFS led to refinement of concrete microstructure upto higher WFS replacement level. But beyond a certain replacement level, finer particles WFS didn’t contribute to particle packing, besides, unimodal grain size of WFS [14] led to creation of more pores in the microstructure which led to decrease in strength of concretes. 3.2. Capillary suction Uptake of water (and therefore ions) by unsaturated, hardened concrete may be characterized by the sorptivity. Initial rate of absorption (IRA) of various mixes was obtained by plotting graph
Fig. 5 shows compressive strength test results of all the GPC and AAS concrete mixes as obtained in this study. Alkali activated slag concrete mixes exhibit better strengths than GPC mixes at all replacement levels of WFS. With the addition of WFS, compressive strength increased upto different replacement levels of WFS for both series of concretes. In case of geopolymer concrete mixes,
GPC mixes
50
AAS Concrete mixes
40 30 20 10 0
0
20
40
60
80
100
WFS % Fig. 5. Compressive strength results of GPC and AAS mixes at 28 day.
GPC mixes
40 20 0 GF0
GF20
GF40
GF60
GF80
GF100
-20 -40 -60 20
% Increase/Decrease in Compressive Strength
28 Day Compressive Strength, MPa
60
60
% Increase/Decrease in Compressive Strength
Fig. 4. Schematic of capillary suction test procedure.
AAS mixes
10 0 AS0
AS20
AS40
AS60
AS80
AS100
-10 -20 -30 -40 -50
Mix Fig. 6. Normalized 28 day compressive strength results of fly ash GPC and AAS mixes w.r.t. GF0 and AS0 in ambient cured conditions.
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5.5
4.5 4 3.5 GPC mixes
3
AS100
AS80
AS60
AS40
AS20
AS0
GF100
GF40
GF20
GF0
2
GF80
AAS mixes
2.5
GF60
Split Tensile Strength, MPa
5
Mixes Fig. 7. 28 day split tensile strength results of GPC and AAS mixes.
between cumulative water absorption for the first six hours against the square root of time. For all the tested specimens, data for tested duration shaped linear relationships with value of coefficients of correlation more than 0.90. The value of slopes obtained from equations of theses plots is the IRA value. Average IRA value for all the GPC and AAS concrete mixes investigated at 28 days of curing age are shown in Fig. 8. There was consistent reduction in IRA values with increase in WFS level upto 60% and lowest value was shown by mix GF60 i.e. 0.0188 mm/Sec1/2. Concrete mixes of AAS showed lower IRA values as compared to fly ash GPC mixes with mix AS20 showed minimum IRA value i.e. 0.0143 mm/Sec1/2. In GPC mixes, reduction in IRA values shown by mixes GF20, GF40, and GF60 was respectively about 10%, 40% and 48% less than mix GF0. Whereas for AAS mixes the percentage reduction in IRA for mix with 20% WFS was observed to be as 23% lower than mix with 0% WFS i.e, AS0. This decrease in absorption could be accredited to finer WFS particles which led to pore refinement in the concrete matrix. There was abrupt rise in sorptivity values of GPC mixes beyond 60% WFS level, whereas in AAS mixes a gradual rise in sorptivity value was observed beyond 20% replacement level of WFS, this could be attributed to unimodal grain size of WFS particles which caused increase in pores in concrete when finer WFS particles were increased beyond a certain level [39].
Average IRA, mm/Sec1/2
0.045 0.04 0.035 0.03 0.025 0.02
GPC mixes
0.015
AAS Concrete mixes
0.01 0.005 0
0
20
40
60
80
100
WFS % Fig. 8. Variation in 28 day IRA values of different geopolymer and AAS concrete mixes.
3.3. SEM and EDS analysis Microstructure study of concretes was done to understand the morphology of GPC and AAS concrete, along with the influence of addition of WFS on the microstructure, presence of pores and micro cracks. Besides, with the help of EDS, atomic percentages of each element present in the concrete matrix were obtained which helped in the semi-quantitative analysis of concretes. Fly ash based geopolymer principally constitutes geopolymer paste (aluminosilicate gel), which is irregular in shape, unreacted fly ash and voids [40]. The surface image of mixes GF0, GF20, GF60 and GF90, observed using SEM is shown in Fig. 9(a)–(d). A non-uniform, coarser and heterogeneous geopolymer gel matrix containing a numbers of unreacted/partially reacted fly ash particles was witnessed in the GPC microstructure. Coarser gel and microstructure indicates moderate reactivity and results in high porosity [41]. It has been reported that unreacted components make up a significant proportion of the total volume of the binder in FA-based geopolymer binder [42]. These components are composites. The strength of geopolymer matrix, these unreacted particles and the interface between them may have a substantial effect on the overall strength of the concrete. Increased silicate content could provide a denser microstructure by increasing the reactivity, however, the unreacted fly ash, as visible in the SEM images of GPC, is unavoidable even when high alkaline concentration and high temperature curing were used in the system [41]. It can be observed that mix GF60 appears to be dense as compared to other GPC mixes. SEM images and typical EDS spectra for atomic concentration analysis of mixes depicted that the major elements present in all GPC concrete specimens were Si and Al, whereas other elements present were Na, Ca, and Fe, but in much less extents. The atomic ratio Si/Al was in the range of 1.22 to 2.54 for all GPC mixes, which confirmed that the matrix of GPC mainly contains Si-Al-O. This atomic ratio Si/Al determines the structure of geopolymer [43], a ratio of Si/Al = 1 will form polysialate (-Si-O-Al-O-) (polysialate) Si/Al = 2 will form poly sialate-siloxo (-Si-O-Al-O-Si-O-). The microstructural development of AAS concretes, AS0, AS20, AS60 and AS90, is displayed in Fig. 9(e)–(h). In case of AAS concretes a fairly even gel matrix can be seen with most of the slag grains partially dissolved by the alkali solution, as visible in Fig. 9(e), forming a C-S-H gel by reacting with the silica from the alkaline activator solution. The visual inspection identified uni-
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formly distributed micro-cracking on the surface of all the AAS concrete samples, visible from naked eye. The observed microcracking is attributed to shrinkage strains [44,45]. Analysis of SEM images noted that micro-cracks also occurred within the AAS concrete matrix. Law et al. (2012) identified the micro-
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cracks as cracks formed on the surface of partially dissolved slag grains accredited to stress built up as the reaction proceeds [46]. As the reaction follows the microstructure densifies and thus the partially dissolved slag grains become more confined, leading to formation of micro-cracks, similar to results reported elsewhere
(a) GF0
(e) AS0
(b) GF20
(f) AS20
(c) GF60
(g) AS60
Fig. 9. SEM images of concrete mixes- (a) GF0, (b) GF20, (c) GF60, (d) GF100, (e) AS0, (f) AS20, (g) AS60 and (h) AS100.
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(d) GF100
(h) AS100 Fig. 9 (continued)
Table 6 Values for EE and ECO2e for all the materials/kg along with source. Material
EE (MJ/kg)
Source
ECO2e (kgCO2e/kg)
Source
OPC FA GGBS SS SH NA WFS CA W S
4.80 0.033 0.31 5.37 8.75 0.081 – 0.083 0.20 11.50
[51] [52] [53] [54] [56] [51] – [51] [51] [51]
0.93 0.004 0.052 0.71 0.46 0.0051 – 0.0048 0.0008 0.60
[51] [52] [52] [55] [55] [51] – [51] [51] [51]
Table 7 Total embodied energy for various Geopolymer and Alkali activated slag concrete mixes. Material
EE (MJ/kg)
OPC FA GGBS SS SH NA WFS CA W S Total EE
4.80 0.033 0.31 5.37 8.75 0.081 – 0.083 0.20 11.50
EE (MJ/m3) of concrete GF0
GF20
GF40
GF60
GF80
GF100
AS0
AS20
AS40
AS60
AS80
AS100
192.00 11.88 – 579.96 254.62 52.16 – 94.26 10.60 – 1195.48
192.00 11.88 – 579.96 254.62 41.71 – 94.26 10.60 – 1185.03
192.00 11.88 – 579.96 254.62 31.27 – 94.26 10.60 – 1174.59
192.00 11.88 – 579.96 254.62 20.82 – 94.26 10.60 46 1210.14
192.00 11.88 – 579.96 254.62 10.45 – 94.26 10.60 69 1222.77
192.00 11.88 – 579.96 254.62 – – 94.26 10.60 92 1235.32
– – 124 579.96 254.62 52.16 – 94.26 10.60 – 1115.6
– – 124 579.96 254.62 41.71 – 94.26 10.60 – 1105.15
– – 124 579.96 254.62 31.27 – 94.26 10.60 – 1094.71
– – 124 579.96 254.62 20.82 – 94.26 10.60 46 1130.26
– – 124 579.96 254.62 10.45 – 94.26 10.60 69 1142.89
– – 124 579.96 254.62 – – 94.26 10.60 92 1155.44
[46,47]. These micro cracks were observed to be dispersed throughout the gel matrix and mixes with higher WFS content had more cracks with greater crack widths. The microstructure of AAS concrete was more refined as compared to GPC as shown in Fig. 9(a)–(h). In AAS system the SEM images showed the partially dissolved slag grains whereas in GPC mixes unreacted spherical fly ash particles were visible. In both type of GPC mixes, an increase in micro pores was observed with the increase in WFS content. The increase in micro-pores in the microstructure of concretes, with increased WFS content, correlates with the strength and durability results. Besides, presence of micro-cracks may also have caused increased in sorptivity in all the concretes.
3.4. Environmental impact analysis Sustainable development as defined by Brutland Commision is ‘‘The development that meets the needs of present without compromising the ability of future generations to meet their own needs” [48]. In relevance to concrete industry, sustainable development may mean utilising available natural resources judiciously, not over exploiting them; using alternative construction materials, which are available locally, requires lesser energy for their production, are economical, having lower impact on environment and can be capable of producing good quality concrete, which after its full service life can be reutilised or disposed-off safely into environment without any harmful effects. The environmental impact of
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ECO2 (kgCO2e/kg)
OPC 0.93 FA 0.004 GGBS 0.052 SS 0.71 SH 0.46 NA 0.0051 WFS – CA 0.0048 W 0.0008 S 0.60 Total ECO2e
ECO2 (kgCO2e/m3) of concrete GF0
GF20
GF40
GF60
GF80
GF100
AS0
AS20
AS40
AS60
AS80
AS100
37.20 1.44 – 76.68 13.38 3.28 – 5.74 0.042 – 137.762
37.20 1.44 – 76.68 13.38 2.63 – 5.74 0.042 – 137.112
37.20 1.44 – 76.68 13.38 1.97 – 5.74 0.042 – 136.452
37.20 1.44 – 76.68 13.38 1.31 – 5.74 0.042 2.4 138.192
37.20 1.44 – 76.68 13.38 0.66 – 5.74 0.042 3.6 138.742
37.20 1.44 – 76.68 13.38 – – 5.74 0.042 4.8 139.282
– – 20.8 76.68 13.38 3.28 – 5.74 0.042 – 119.922
– – 20.8 76.68 13.38 2.63 – 5.74 0.042 – 119.272
– – 20.8 76.68 13.38 1.97 – 5.74 0.042 – 118.612
– – 20.8 76.68 13.38 1.31 – 5.74 0.042 2.4 120.352
– – 20.8 76.68 13.38 0.66 – 5.74 0.042 3.6 120.902
– – 20.8 76.68 13.38 – – 5.74 0.042 4.8 121.442
Note: OPC- Ordinary Portland Cement; FA-Fly Ash; GGBS-Ground Granulated Blast Furnace Slag; SS-Sodium Silicate solution; SH-Sodium hydroxide solids; NA-Sand; WFSWaste Foundry Sand; CA-Coarse Aggregates; SP-Super plasticizer; W-Water.
any material is generally indicated by two parameters- embodied energy and embodied CO2 emissions. Embodied energy (EE) is the total energy required, and embodied CO2 emissions (ECO2e) is the total CO2 emissions, throughout the lifecycle of a material i.e. from extraction, processing/manufacture, transportation, maintenance till demolition i.e. from cradle to grave. In this study, EE and ECO2e of all the concrete mixes are calculated by considering only extraction and manufacturing process, i.e. from cradle to gate, by taking the EE and ECO2e values of constituent materials from secondary sources. The values of EE and ECO2e for materials may vary depending upon the country, age of data and boundary conditions [49]. The values for EE and ECO2e for all the materials, per kg, along with their published source is shown in Table 6. Tables 7 and 8 shows the calculated values of EE and ECO2e for all GPC and AAS concrete mixes for 1 m3 of concrete, respectively. The EE and ECO2e values indicated in Tables 7 and 8 are for solid sodium hydroxide. Water content in Table 8 consists of extra water and water required to prepare sodium hydroxide solution only. The aim of this analysis is to indicate the impact of WFS addition in reducing the environmental analysis also the comparison of reference GPC and AAC concrete mixes with the EE and ECO2e of conventional mix (OPC + FA) of similar strength. The mix proportions for conventional concrete (40 MPa), containing 30% fly ash are taken from Arora and Singh (2016) [50]. The EE and ECO2e values of the conventional mix are calculated as 1851.58 MJ/m3 and 329.18 kgCO2e/ m3 respectively, which is much higher than EE and ECO2e values for GF0 and AS0. It can be observed that in both GPC and AAS concretes the mixes with 100% WFS has maximum EE and ECO2e values. The use of WFS in concretes lowered the EE and ECO2e upto certain WFS replacement levels but at higher replacement levels, demand of superplasticizer also increased which led to increased EE and ECO2e values. Mix GF40 and AS40 showed minimum EE value i.e. 1174.59 MJ/ m3 and 1094.71 MJ/m3 amongst GPC and AAS concrete mixes, respectively. Geopolymer mixes have higher EE and ECO2e values due to addition of OPC as 10% of total source material. In all the mixes, the alkaline solution have maximum contribution towards EE and ECO2e values followed by OPC/GGBS, coarse aggregates, fine aggregates and then superplasticizer. Alkaline solutions have a share of 68%-71% in total EE of GPC mixes whereas 72%-76% in case of AAS concrete mixes, similarly, a total share of 65–66% in total ECO2e of GPC mixes whereas 74%–76% in case of AAS concrete mixes. The EE and ECO2e of WFS is considered as null since it is an industrial by-product and was just sieved through 4.75 mm sieve and used as it was discarded from industry without any processing.
4. Conclusions Following conclusions can be drawn from the current investigation: WFS addition into fly ash GPC and AAS concrete led to decrease in workability of concretes. Beyond 60% WFS content in GPC and 40% WFS in AAS, abrupt drop in workability of mixes was observed. Presence of water absorbing finer particles i.e. claytype fine materials, impurities and ashes etc. in WFS, can be accredited to this. In GPC mixes, partial replacement upto 60% of NA by WFS led to consistent rise in 28 day compressive strength, with maximum strength (GF60) as 43% more than reference mix (GF0). Whereas, at 100% replacement (GF100) about 47% of the strength of control concrete was achieved. In AAS concretes, maximum strength gain was observed in mix with WFS replacement level of 20%. At 40 and 60% replacement level of WFS strength achieved was more than 77% of the 28 day strength of the reference mix (AS0). It was possible to achieve more than 45% of compressive strength of control concretes (GF0 and AS0) even at 100% replacement level of WFS (GF100 and AS100). Split tensile strength results were similar to compressive strength results. Sorptivity results were in correlation with strength results. In case of GPC and AAS concretes mixes, GF60 and AS20 showed minimum IRA value at 28 day curing age which about 48% and 23% of IRA value of respective reference mixes. Coarser aluminosilicate gel is observed in microstructure of GPC mixes due to presence of unreacted and partially reacted fly ash particles which further increased with rise in WFS level leading to increase in porosity and hence weaker concrete matrix. Ratios of atomic concentrations of Si and Al was found in the range of 1.2–2.54 indicating polymerisation in the matrix with formations of poly sialate and poly sialate-siloxo structures. There was visible surface micro-cracking as well as microcracks, inside the concrete matrix attributable to shrinkage strains in case of AAS concretes. Wider cracks were found in case of concretes with higher WFS percentages, leading to increase in porosity at higher replacement levels. The embodied energy (EE) and embodied carbon dioxide emissions (ECO2e) of developed GPC and AAS concrete mixes is found very low when compared with a conventional concrete mix of similar strength. Besides, WFS inclusion helped in lowering EE and ECO2e values of GPC and AAS concrete mixes with beyond
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