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Unexplored alternative use of calcareous sludge from the paper-pulp industry in green geopolymer construction materials
Construction and Building Materials 246 (2020) 118457
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Unexplored alternative use of calcareous sludge from the paper-pulp industry in green geopolymer construction materials Manfredi Saeli a,⇑, Luciano Senff b, David M. Tobaldi c, João Carvalheiras c, Maria Paula Seabra c, João A. Labrincha c a b c
Department of Architecture, University of Palermo, Viale delle Scienze, Bld 8, 90128 Palermo, Italy Department of Mobility Engineering, Federal University of Santa Catarina, 89218-000 Joinville, Brazil Department of Materials and Ceramics Engineering, Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
h i g h l i g h t s Calcareous sludge is used as filler in geopolymeric mortars. Biomass fly ash is used as main alumina-silicate source (70 wt%). Used wastes derive from a Kraft pulp industry. Manufacture is simple and highly sustainable and reproducible. Novel geopolymeric mortars are suitable for construction applications.
a r t i c l e
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Article history: Received 22 March 2019 Received in revised form 7 January 2020 Accepted 13 February 2020
Keywords: Life environment Geopolymer mortar Biomass fly ash Calcareous sludge Paper-pulp industry Construction material
a b s t r a c t Calcareous sludge is an alkaline waste produced by the paper pulp industry that is commonly disposed of in land-fill. However, recent studies and the European regulations discourage such practice. This work investigates an alternative and innovative way to recycle and reuse this waste, as filler, in the production of green geopolymeric mortars intended for applications in construction. This is the first time that this calcareous sludge is used to produce novel waste-based materials, in both construction and geopolymer technology. The novel alkali-activated mortar also uses biomass fly ash – another slightly investigated waste stream – to substitute the metakaolin (70 wt% substitution) and the manufacture process is performed at ambient conditions. All of that reduces the overall process footprint. The implemented mix design is aimed at maximising the waste incorporation and improving the material properties, such as workability and mechanical performance. The main results demonstrate that 10 wt% of calcareous sludge can be efficaciously used as filler in the geopolymeric mortars, generating up to 30% improvement in the mechanical resistance. This alternative use of such wastes will contribute to increase the sustainability of the novel construction materials also granting environmental advantages and a financial surplus for the industry. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The relevant consumption of non-renewable raw materials and the massive generation of greenhouse gases and wastes make the present industrial system highly unsustainable, especially in the developing countries where construction has reached extraordinary levels of unsustainability [1,2]. The industrial development/ implementation might ideally pursue a systemic innovation with reducing the environmental impact and maintaining, at the same ⇑ Corresponding author. E-mail address: [email protected] (M. Saeli). https://doi.org/10.1016/j.conbuildmat.2020.118457 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
time, a positive economic feedback [3]. However, achieving these objectives together is quite complex and, often, undervalued. This paper focuses on the production of novel green geopolymeric (GP) mortars for applications in construction [4]. Two fairly unexplored industrial waste streams, deriving from the paper-pulp industry, are here recycled and reused: biomass fly ash (BFA) and calcareous sludge (CalS). The pulp and paper industry is presently a great consumer of biomass raw material, especially virgin wood. Nowadays, the Kraft process is the most common pulping technology [5]. This involves the digestion of lignin by an alkaline mixture of sodium hydroxide, sodium sulfide, and water. Lignin is then dissolved in a liquor,
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which is heated to melt the inorganic components while the organic fraction is burnt in a fluidized-bed combustor to generate steam, then converted into electricity. The BFA is produced in the biomass boiler employed for energy production at mill site and collected at the electrostatic precipitator as waste [6]. The molten inorganic salts are passed into a dissolution tank where a lime eraser is added to clarify the alkali residues; CalS is one of the wastes generated during this process [7,8]. This study is aimed at exploring an alternative exploitation of BFA and CalS that are commonly disposed of in landfills. Indeed, such practice should be limited for the serious risks of soil and water contamination, and consequent environmental degradation [9]. According to previous studies various alternatives were already considered to reuse some cellulose industrial residues, along with energy recovery and green construction materials production [10–12]. BFA are classified as a solid waste, according to the European List of Wastes [13] and has been rarely investigated in the scientific literature for traditional concrete applications [14,15] and, to date, their analogous use in GP manufacture is fairly recent [16,17]. In these work, BFA was reused as an alternative source of aluminium and silicon to partially replace the metakaolin (MK) in the GPbinder manufacture [18–22]. CalS is an inorganic alkali residue, physically homogeneous and mainly constituted of calcite [23], thus offering an interesting recycling potential. In this study, it was recycled and used, in various proportions (wt.%), as filler in the GP-mortars manufacture, The followed manufacture process is highly cost-effective envisaging ambient curing conditions (20 °C, 65% RH), thus avoiding any external source of energy as commonly done in GP processing [24– 26]. Additionally, it involves simple, reproducible, and low-cost procedures that contribute in reducing the process footprint. 2. Material and methods 2.1. Materials In this study, a mixture of BFA (70 wt%) and MK (30 wt%) was used as alumino-silicate source to produce the GP-paste (binder). BFA is mainly composed by a–quartz, calcite, mica group mineral (mixture of muscovite/illite), and microcline as crystalline phases. It was used as provided without any pre-treatment, such as milling or sieving, according to [27]. Benchmark MK, ArgicalTM by UnivarÒ, was used to adjust the GP-binder desired molar oxide ratios. The sum of SiO2 and Al2O3 was 53.3 wt% for BFA and 98.7 wt% for MK. Their characterization is presented in [17]. The alkaline activation was achieved by using a solution of commercial sodium silicate (H2O = 62.1 wt%; SiO2/Na2O = 3.15; Quimialmel LDA, D40 - PQ) and sodium hydroxide (ACS reagent, 97%; Honeywell). The NaOH solution (10 M) was prepared by dissolving sodium hydroxide pellets in distilled water at least 24 h prior to use. The used molarity was selected basing on the cited previous works. The mortar aggregate is constituted by commercial natural siliceous sand furnished by Saint-Gobain Weber Portugal. The CalS contains about 10 wt% of water so, prior to be mixed with sand, it was dried in a conventional oven, at 60 °C for 24 h, then ground in a mortar to dissolve lumps. The process might be implemented if CalS was naturally dried at mill site. That would reduce the overall energy employment for a greener material and manufacture. 2.2. Manufacture process GP-mortars specimens were prepared according to the European standard EN 998-2:2016 [28] adapting the common procedure for traditional cement-based mortars.
The manufacture process involved: a) dry hand mixing for 1 min to ensure a uniform blend of a1) MK and BFA and a2) sand and CalS; b) homogenization of sodium hydroxide and silicate at 50 rpm for 5 mins; c) mixture of the alkaline solution with the solid precursors (BFA + MK) at 60 rpm for 9 mins; d) adding the aggregate mixture (sand + CalS), and mix for 1 min at the same speed to ensure homogeneity; e) pouring the fresh slurry into standard metallic moulds (standard dimensions of 40 40 160 mm) and vibrating for 2 mins on a vibrating table to grant compactness and remove any entrained air; f) seal the moulds with a plastic film and let the specimens harden for 24 h at ambient conditions (20 °C, 65% RH), and g) unsealed and demould the hardened samples, and curing at ambient conditions until testing. This procedure was previously optimised by the authors in [29]. 2.3. Materials characterization CalS mineralogical composition was evaluated by X-ray diffraction (XRD) using a Rigaku Geigerflex D/max-Series instrument (Cu Ka radiation, 5–80° 2h, 0.02° 2h step-scan and 10 s per step). Its chemical composition was evaluated by X-ray fluorescence (XRF) using a Panalytical Axios spectrometer. Particle-size distribution was determined by laser diffraction (Coulter LS230 analyser, Fraunhofer method and Polarization Intensity Differential Scattering). The microstructure was investigated by scanning electron microscopy (SEM) using a Hitachi analytical FE-SEM SU-70. The consistency (spread) of the fresh GP-mortars (slurry) was estimated by flow table test according to the European Standard EN 1015-3:1999 [30]. The (initial and final) setting time was estimated by Vicat test according to the European Standard EN 1963:2016 [31]. The reported values were calculated as the average from three rod penetrations (in different places) during the same test. The temperature evolution of the GP-paste during setting and initial stages of curing was monitored in a quasi-adiabatic calorimeter under ambient conditions. The reported values were calculated as the average from three measurements. Hardened specimens were characterised after 28 days of curing. Bulk density was calculated as the average from three specimens. The water absorption was determined by the Archimedes principle (weight variation, DP/P %) by immersing the specimens in distilled water for 24 h once they had been dried to constant mass at 60 °C in a conventional oven. The water absorption due to capillary action was evaluated according to the European Standard EN 1015-18 [32]. Three replicas were used to calculate the mean values. The mechanical performance was determined at various stages of curing (28, 60, 120, and 180 days) according to the European Standards EN 1015-11 [33] and EN 998-2 [28] by means of uniaxial compressive strength tests (UCS). A universal testing machine (Shimadzu, AG-25TA), provided with a 250 kN load cell running at a displacement rate of 0.5 mm/min, was used. The mean values were obtained from three tests randomly taken from the sample batch. The values of axial strain, displacement quotient at maximum strength and the original specimen’s length, were also calculated. 2.4. Tested formulations Mix design has been outlined to maximise the incorporation of the CalS, and improve the mortars mechanical performance. The main driver of this study is the application of these novel GPmortars in construction, according to the international relevant standards and guidelines (i.e. EN 998-2 [28], ACI 116R-00 [34], Eurocode 2 [35], and Norme Tecniche per le Costruzioni [36]). Therefore, the admissible minimum UCS value was 10 MPa (at 28 days of curing) in order to classify these novel materials in
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resistance class ‘‘M10”. That is an essential requirement for mortars intended for structural applications. The mix design of the GP-binder was optimized by the authors in [17,27] and presents the following molar oxide ratios: SiO2/Al2O3 = 5.27, Na2O/Al2O3 = 1.31, Na2O/ SiO2 = 0.25, and H2O/Na2O = 15.9. These values are in accordance to [4]. The binder/aggregate (B/A) ratio, used for the mortars production, was 1/3, based on preliminary work [29] The main characteristics of the used GP-binder and the reference (CalS-free) GP-mortar formulations are summarized in Table 1. The slurry consistency was the main indicator of the GP-mortars workability and their consequent suitability for real applications. Increasing quantities of CalS were admixed until the paste became too viscous to be worked (spread of about 10 cm). That limited the maximum admissible CalS dosage to 12.5 wt%. Subsequently, distilled water was added to increase the workability, thus maximising the waste amount (cf. Table 2). In this work, the alumino-silicate solid precursors (ratio BFA/MK), the alkaline activator solution (ratio sodium hydroxide/ sodium silicate), and the ratio B/A were kept constant. The tested GP-mortar formulations are shown in Table 2. For all the designed mixes the liquid (water)/solid (L/S) ratio is reported. The experimental plan has been formally divided into two parts, according to the discussed limit of workability and the subsequent use of water: (I) addition of CalS only (no additional water in the slurry); (II) water addition. 3. Results and discussion 3.1. CalS characterization
3
Fig. 1. CalS XRD pattern with the peaks indication of calcite (top, black), and magnesite (bottom, red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(0.52), SiO2 (0.50), Al2O3 (0.29) and a loss on ignition (LOI) of 42.33%. Fig. 2 shows the CalS microstructure that appeared made of hexagonal platelet-shaped particles assembled in elongated columns. The particle size distribution is presented in Fig. 3, along with that of sand. CalS particle size ranges between 0.04 and 160 lm, with a mean value of 25.86 lm. Sand particles size ranges between 80 lm and 1.6 mm, with a mean value of about 0.65 mm. 3.2. Mortars characterization
The XRD analysis of the CalS is presented in Fig. 1. It revealed CaCO3 as the main crystalline phase with some minor amounts of magnesite (MgCO3). Accordingly, the XRF analysis revealed, as main chemical species (wt.%), CaO (54.21), Na2O (1.00), MgO
Table 1 Main characteristics of the used GP-binder and reference (CalS-free) GP-mortar. Property
Measured value
Consistency (spread by flow table) [cm] Bulk density [Kg/m3] Sorptivity by immersion [%] Coefficient of capillarity [kg/(m2min0.5)] Compressive break point [MPa] Bending resistance [MPa]
GP-binder
GP-mortar (ref.)
>30 1307 38 0.87 22.15 ± 1.22 3.05 ± 0.37
21 1832 13 0.35 21.66 ± 0.03 4.08 ± 0.58
Table 2 Mix design of the tested GP-mortars. Experimental plan
Sample n.
CalS [wt.%]
Water [wt.%]
L/S ratio
(I)
1 (ref.) 2 3 4 5 6 7
– 1.0 2.5 5.0 7.5 10.0 12.5
–
0.196 0.194 0.191 0.186 0.181 0.176 0.171
(II)
8 9 10 11 12 13 14
12.5
1.5 2.5 3.5 3.5 4.5 5.5 4.0
0.186 0.196 0.206 0.201 0.211 0.221 0.201
15.0
17.5
3.2.1. Fresh state properties The slurry consistency, evaluated by flow table test (spread), returns the paste workability and gives an indication of the final characteristics of the mortar once set (i.e. adequate compactness, homogeneity, etc.). A spread ranging between 18 and 22 cm guarantees a good material workability and a suitable compaction and homogeneity intended for applications in construction [37,38]. A desired more fluid or pasty nature mainly depends on the mortars specific application; in any case, the consistency can be easily modified by chemicals addition (i.e. plasticizers). However, in this study their use was avoided to prevent undesired reactions with the GPbinder, with a preference to (distilled) water. Pictures of the flow table tests and spread values of the tested formulations are presented in Figs. 4 and 5, respectively. Increasing the amount of CalS the slurry becomes more pasty, until reaching unworkability (spread of about 11 cm – almost equivalent to a state of unworkability), corresponding to the dosage of 12.5 wt% (Figs. 4D and 5, left). That is caused by the very small dimensions of CalS particles (conversely a high specific surface area) that tend to adsorb the available water molecules. Then, to increase the workability of the mixture, various amounts of water were supplied. At the same time, the more water is added the more CalS can be admixed, maximising the waste re-usage (Fig. 4/bottom line and Fig. 5, right). More particularly, it is observed that for the mixture with 12.5 wt% of CalS, the workability increases from 11 cm to 19 cm when 3.5 wt% of water is added. That quantity of water was sufficient to increase the CalS addition to 15 wt%, with an acceptable consistency of about 16 cm. The original consistency (21 cm spread) is recovered by adding 5.5 wt% of water. The maximum tested amount of CalS was 17.5 wt%, with 4 wt% of water, that presented a spread of 15 cm. High amounts of water, as generally observed for common construction materials [39,40] may cause a drastic sorptivity increase or, even worse, a substantial mechanical resistance decrease. A
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Fig. 2. CalS micrographs.
Fig. 3. Particle size distribution: CalS (A) and sand (B).
Fig. 4. Flow table test photographs of some investigated formulations. Top line – influence of CalS addition: A – reference (CalS-free) GP-mortar; B CalS, D 12.5 wt% CalS. Bottom line - influence of water addition on D: E 1.5 wt% H2O; F 2.5 wt% H2O; G 3.5 wt% H2O.
good balance between amount of CalS and water addition must be carefully considered. Setting time is also fundamental to assess the material properties and its possible real applicability. According to [41], in this work the initial setting time is considered as the time when the needle penetration is 39 ± 0.5 mm; the final setting time when less than 0.5 mm of penetration is measured. Values, calculated as the average from three rod penetrations, are reported in Fig. 6A. It is observed that, increasing the CalS amount in the formulation, the initial setting time is delayed. For the produced GPmortars, the measured delay was about 1 min per wt.% of added CalS. That is caused by the lower binder percentage, which is the reactive component, along with a lower available water amount,
5 wt% CalS, C
10 wt%
that is mainly consumed to wet the small CalS particles, in accordance with [42,43]. The setting process started after 140 mins for the reference (CalS-free) GP-mortar, and can be considered finished (final setting time) after 200 mins. The mortar with 5 wt% CalS showed a longer final setting time (235 mins); the mortar with 10 wt% CalS showed a period that is comparable to that of the reference (205 mins). The short final setting time of the GPmortar with 10 wt% of CalS is due to the high CalS amount that probably subtracts much more available water from the paste making it, de facto, drier As a preliminary study, the temperature evolution of the GP-pastes during setting (initial stages of curing) was monitored in a quasi-adiabatic calorimeter under ambient conditions. This
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Fig. 5. Spread values of the produced GP-mortars: without (left) and with (right) water addition.
Fig. 6. Displacement of the needle inside the fresh GP-mortars’ paste (setting time) (A) and heat release during curing (B).
analysis evaluates the influence of the CalS incorporation in the alkali activation kinetic. The calorimetric measurement values are reported for the first hours upon mixing, as the average from three different samples (Fig. 6B). It is observed that the three analysed formulations showed a decreasing single peak in the first minute after mixing (Fig. 6B, inlet), demonstrating that the geopolymerization reactions were already started [44]. Subsequently, the released heat slowly decreases till room conditions. Geopolymerization is a quite complex process where the precursors nature can strongly influence the reactions process [45,46]. The CalS addition influences the reactions decreasing the temperature of about 1.5 °C, and slowing down the reactions process. In any case, the waste usage in the GP-mortars is not detrimental to a complete geopolymerization.
3.2.2. Hardened state properties The microstructure of some specimens (0, 5, and 10 wt% of CalS) is presented in Fig. 7. The CalS addition makes the paste denser and more uniform, as fillers do. Furthermore, less and small pores are observed (less voids, higher compaction) and the particles look agglomerated in smaller structures. Consequently, as a general trend, increasing the CalS amount promotes a rise in the hardened GP-mortars bulk density, whose values are presented in Fig. 8. The effect of adding water to the mixture is, as expected, a general bulk density decrease due to a higher porosity. Analogously, the values of water absorption, both by immersion and by capillary action, are strictly dependent on the CalS and water amounts, as shown in Fig. 9. More particularly, the water
Fig. 7. Micrographs of: A – reference (CalS-free) GP-mortar; B – GP-mortar with 5 wt% CalS; C – GP-mortar with 10 wt% CalS.
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Fig. 8. Bulk density of the produced GP-mortars: without (left) and with (right) water.
Fig. 9. Sorptivity of the produced mortars: by immersion (A) and by capillary test (B).
sorptivity, in both the cases, tends to decrease by increasing the CalS amount. Indeed, the very small CalS particles tend to fill in the voids in the GP-mortars matrix preventing the water absorption. That is an unquestionable advantage for any structural applications as a high water sorptivity may cause structural damages and steel bars, if present, oxidation. This particular aspect will be object of further research. At the same time, the water addition, as commonly observed for traditional construction materials, causes a larger water absorption by immersion, following a linear dependence of L/S ratio (red line) (Fig. 9A). 3.2.3. Mechanical performance The specimens mechanical performance was assessed at various stages of curing. The values of UCS are reported in Fig. 10, the influence of water addition in Fig. 11, and the stress–strain curves at 28-curing-days in Fig. 12. Lastly, for a selection of specimens (up to 10 wt% CalS, without water addition) the mechanical performance was further investigated at 60, 120, and 180 days of curing in order to assess the material performance during the natural aging. Results are shown in Fig. 13.
Fig. 11. H2O/solid ratio vs. UCS of the investigated formulations. The red line indicates the approximated linear dependence; the green spot represents the reference (CalS-free) GP-mortar. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. UCS of the produced GP-mortars: without (left) and with (right) water.
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Fig. 12. Stress–strain curves of the tested mortar’s formulations. A – specimens without water; B – specimens with 12.5 wt% CalS without/with water; C – specimens with 15 wt% CalS with water; D – specimen with 17.5 wt% CalS with water.
Fig. 13. Selected produced GP-mortars mechanical performance at various curing steps: A – UCS; B – improvement percentage (to 28 aging days).
As a general trend, increasing the CalS amount increases the UCS value (Fig. 10). That is a consequence of an efficient filler effect, improving the compactness degree and the density of the material. The reference (CalS-free) GP-mortar shows a UCS of 21. 66 ± 0.03 MPa while a value of 28.11 ± 1.29 MPa was measured for the 10 wt% CalS addition, corresponding to about a 30% improvement. A very slightly further enhancement was observed
for the sample containing 12.5 wt% CalS (28.65 ± 1.99 MPa). The addition of CalS made the material more deformable. The calculated average strain was 2.35 ± 0.03%, against 1.8% shown by the reference (CalS-free) GP-mortar (cf. Table 3). The water addition , as expected, decreases the UCS value (Fig. 10, right) that also follows an approximated linear dependence with the H2O/solid ratio (Fig. 11). In general, the elastic regions of the compositions
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Table 3 Mechanical features and classes of resistance. Experimental plan
Sample n.
CalS [wt.%]
Water [wt.%]
UCS [MPa]
Axial strain [%]
Class of resistance [EN 998-2]
(I)
1 (ref.) 2 3 4 5 6 7
– 1.0 2.5 5.0 7.5 10.0 12.5
–
21.66 21.27 22.53 24.01 25.09 28.11 28.65
1.80 2.13 2.38 2.35 2.35 2.33 2.38
M20 M20 M20 M20 M25 M25 M25
(II)
8 9 10 11 12 13 14
12.5
1.5 2.5 3.5 3.5 4.5 5.5 4.0
23.62 21.64 18.20 18.85 16.37 14.37 17.14
2.30 2.18 2.08 2.28 2.25 2.23 2.00
M20 M20 M15 M15 M15 M10 M15
15.0
17.5
to which water was added (12.5 and 15 wt%) was not substantially altered (apart from the final peak) (Fig. 12). Ultimately, adding water determines very slight axial strain’s negative shifts. Table 3 reports the main mechanical features of the produced GP-mortars, along with the classes of resistance according to EN 998-2 [28]. The addition of up to 5 wt% of CalS does not alter the original M20 class of the reference. On the contrary, an addiction of 7.5 12.5 wt% upgrades the specimens to M25 class. The effect of adding water is detrimental for the GP-mortars mechanical performance, causing an immediate class downgrade. Finally, mechanical tests were performed at various stages of curing for the GP-doped mortars up to 10 wt% of CalS (Fig. 13). The tested stages of curing were 60, 120, and 180 days. Similarly to traditional construction materials, also the produced GPmortars show an increasing mechanical gain over time. After 120 days of curing, no significant improvements are observed. That is particularly evident for the highest CalS amounts; on the other hand the reference (CalS-free) GP-mortar, or the formulations with few amounts of CalS, shows a further margin of improvement. 4. Conclusions In this work novel GP-mortars were produced recycling and reusing wastes originated by a Portuguese Kraft pulp industry: biomass fly ash and calcareous sludge. Such wastes are commonly disposed of in land-fill and find here an alternative usage, in line with the recent European directives. The BFA is here used to partially substitute the MK in the binder formulation and the CalS as a filler. Mix design was aimed at maximizing the CalS incorporation while engineering the novel GP-material for robust applications in construction (UCS 10 MPa). Furthermore, in the pursuit of sustainability, the manufacture process was conducted at ambient conditions by a simple, reproducible, and cost-efficient process. That contributes in reducing the overall material footprint. The CalS addition increases the slurry consistency, making the material less workable, generates (small) delays in the initial setting time, and decreases the released heat, indicating a lower degree of the geopolimerisation reactions. Also the microstructure is affected becoming denser and less porous. That is directly connected to the bulk density that tends to increase, while the water sorptivity decreases. Moreover, increasing the CalS amount, the UCS tends to increase, as expected for a filler action. All these characteristics are highly recommended for applications in construction, especially if structural uses are foreseen. The CalS addition of 7.5–12.5 wt% upgrades the reference (CalS-free) mortar from class M20 to M25. Considering some issues, such as workability, the addition of 10 wt% CalS resulted the best balance.
The addition of water, although allowing higher CalS quantity, negatively affects the overall materials properties: lower bulk densities, higher porosity, and consequently higher water absorptions, both by immersion and by capillary action. To the maximum admissible CalS dosage of 12.5 wt% (without water), the water addition lets reaching a maximum (tested) quantity of 17.5 wt% CalS, otherwise unfeasible. Anyway, no beneficial effects are observed by adding water and improving the quantity of the used waste. Adding just 1.5 wt% of water to the 12.5 wt% CalS formulation would let using 2.5 wt% more CalS with a still suitable UCS (same class of the reference, M20, but with a slight higher UCS value). A quantity of 15 wt% CalS is admissible with 3.5 wt% of water, with a (bottom end) limit in workability and questioning results regarding the other properties. Finally, in all the tested cases, CalS admixture determines a higher deformability but the mechanical behaviour in the elastic region is not substantially altered. Curing showed to continue during time until, at least, 180 (tested) days, making the UCS increase and, consequently, granting the material higher resistances. Further works foresee the complete analysis of the curing process to better explore the geopolymerization progression in relation to the addition of various fillers. Notes M. Saeli and L. Senff designed the experimental plan and produced the specimens (2.1, 2.2, 2.3, 2.4, and 3.2.1), D.M. Tobaldi carried out the mineralogical analysis (3.1), J. Carvalheiras carried out the experiments of setting time and temperature evolution determination (3.2.1), M. Saeli carried out the physical and mechanical characterization (3.1 – SEM and particle size distribution, 3.2.2, and 3.2.3); M.P. Seabra and J.A. Labrincha were in charge of the scientific coordination. All the authors were involved in the manuscript preparation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financed by Portugal 2020 through European Regional Development Fund (ERDF) in the frame of Operational Competitiveness and Internationalization Programme (POCI) in the scope of the project PROTEUS - POCI-01-0247-FEDER-017729 and in the scope of the project CICECO - Aveiro Institute of Materi-
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als POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), co-financed by national funds through the FCT/MEC. Manfredi Saeli thanks the Italian Ministry of Education, University and Research (MIUR) for financing his work in the framework of the PON ‘‘Research and Innovation 2014-2020” section 2 ‘‘AIM: Attraction and International Mobility” with D.D. 407 of 27/02/2018 co-financed by the European Social Fund – CUP B74I19000650001 – id project AIM 1890405-3, area: ‘‘Technologies for the Environments of Life”, S.C. 08/C1, S.S.D. ICAR/10. David Maria Tobaldi is grateful to Portuguese national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. Dr. Cristina Sequeira (Department of Geoscience, University of Aveiro) is acknowledged for the XRF analysis. References [1] M. Schneider, M. Romer, M. Tschudin, H. Bolio, Sustainable cement production – present and future, Cem. Concr. Res. 41 (7) (2011) 642–650. [2] A. Kylili, P.A. Fokaides, Policy trends for the sustainability assessment of construction materials: a review, Sustain. Cities Soc. 35 (2017) 280–288. [3] C. Freeman, Technology and Economic Performance, Pinter, London, 1987. [4] J. Davidovits, Geopolymers Inorganic polymeric new materials, J. Therm. Anal. 37 (8) (1991) 1633–1656. [5] M. Ghochapon, G. Rafiqul, M. Pomthong, A. Suttichai, Integration of the biorefinery concept for the development of sustainable processes for pulp and paper industry, Comput. Chem. Eng. 119 (2018) 70–84. [6] L.A.C. Tarelho, E.R. Teixeira, D.F.R. Silva, R.C.E. Modolo, J.A. Labrincha, F. Rocha, Characteristics of distinct ash flows in a biomass thermal power plant with bubbling fluidised bed combustor, Energy 90 (2015) 387–402. [7] T.M. Grace, B. Leopold, E.W. Malcolm, Alkaline pulping, in: M.J. Kocurek, F. Stevens (Eds.), Pulp and Paper Manufacture, vol. 5, Joint Textbook Committee of the Paper Industry of the United States and Canada, Atlanta, 1991. [8] P. Bajpai, Management of Pulp and Paper Mill Waste, Springer International Publishing, 2015. [9] European Parliament, Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste. [10] L. Senff, G. Ascensão, V.M. Ferreira, M.P. Seabra, J.A. Labrincha, Development of multifunctional plaster using nano-TiO2 and distinct particle size cellulose fibers, Energy Build. 158 (2018) 721–735. [11] R.C.E. Modolo, T. Silva, L. Senff, L.A.C. Tarelho, J.A. Labrincha, V.M. Ferreira, L. Silva, Bottom ash from biomass combustion in BFB and its use in adhesivemortars, Fuel Process. Technol. 129 (2015) 192–202. [12] R.C.E. Modolo, L. Senff, J.A. Labrincha, V.M. Ferreira, L.A.C. Tarelho, Lime mud from cellulose industry as raw material in cement mortars, Mater. Constr. 64 (316) (2014) e033. [13] European Parliament, Commission Decision 2000/532/EC, List of Wastes. [14] S. Wang, L. Baxter, Comprehensive study of biomass fly ash in concrete: strength, microscopy, kinetics and durability, Fuel Process. Technol. 88 (11) (2007) 1165–1170. [15] R. Rajamma, R.J. Ball, L.A. Tarelho, G.C. Allen, J.A. Labrincha, V.M. Ferreira, Characterisation and use of biomass fly ash in cement-based materials, J. Hazard. Mater. 172 (2) (2009) 1049–1060. [16] R. Rajamma, J.A. Labrincha, V.M. Ferreira, Alkali activation of biomass fly ash– metakaolin blends, Fuel 98 (2012) 265–271. [17] M. Saeli, D.M. Tobaldi, M.P. Seabra, J.A. Labrincha, Mix design and mechanical performance of geopolymeric binders and mortars using biomass fly ash and alkaline effluent from paper-pulp industry, J. Clean. Prod. 208 (2019) 1188– 1197. [18] Part Wei Ken, Mahyuddin Ramli, Cheah Chee Ban, An overview on the influence of various factors on the properties of geopolymer concrete derived from industrial by-products, Constr. Build. Mater. 77 (2015) 370–395.
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[19] S. Ahmari, X. Ren, V. Toufigh, L. Zhang, Production of geopolymeric binder from blended waste concrete powder and fly ash, Constr. Build. Mater. 35 (2012) 718–729. [20] S. Subhashree, P.S. Suresh, Effect of synthesis parameters on compressive strength of fly ash-slag blended geopolymer, Constr. Build. Mater. 170 (2018) 225–234. [21] R.M. Novais, G. Ascensão, D.M. Tobaldi, M.P. Seabra, J.A. Labrincha, Biomass fly ash geopolymer monoliths for effective methylene blue removal from wastewaters, J. Clean. Prod. 171 (2018) 783–794. [22] R.M. Novais, J. Carvalheiras, D.M. Tobaldi, M.P. Seabra, R.C. Pullar, J.A. Labrincha, Synthesis of porous biomass fly ash-based geopolymer spheres for efficient removal of methylene blue from wastewaters, J. Clean. Prod. 207 (2019) 350–362. [23] S. Ikhmayies, et al., Characterization of Minerals, Metals, and Materials, Minerals, Metals & Materials Series, Springer, Cham, 2017. [24] J. Davidovits, Synthesis of new high temperature geo-polymers for reinforced plastics/composites, Society of Plastic Engineers, Brookfield Center, 1979, 151– 54. [25] D. Feng, J.L. Provis, J.S.J. van Deventer, Thermal activation of albite for the synthesis of one-part mix geopolymers, J. Am. Ceram. Soc. 95 (2) (2012) 565– 572. [26] K. Komnitsas, D. Zaharaki, Geopolymerisation: a review and prospects for the minerals industry, Miner. Eng. 20 (2007) 1261–1277. [27] M. Saeli, R.M. Novais, M.P. Seabra, J.A. Labrincha, Mix design and mechanical performance of geopolymer binder for sustainable construction and building material, IOP Conf. Ser. Mater. Sci. Eng. 264 (2017) 012002. [28] EN 998-2:2016, Specification for mortar for masonry - Part 2: Masonry mortar, European Committee for Standardization, 2016. [29] M. Saeli, L. Senff, M.P. Seabra, J.A. Labrincha, Alkali-activated fly ash-based mortars for green applications in architecture and civil engineering. In press in: International Journal of Engineering and Technology. [30] EN 1015-3:1999: Methods of test for mortar for masonry. Determination of consistence of fresh mortar (by flow table), European Committee for Standardization, 1999. [31] EN 196-3:2016, Methods of testing cement – Part 3: Determination of setting times and soundness. [32] EN 1015-18:2002, Methods of test for mortar for masonry – Part 18: Determination of water absorption coefficient due to capillary action of hardened mortar, European Committee for Standardization, 2002. [33] EN 1015-11:1999, Methods of test for mortar for masonary – Part 11: Determination of flexural and compressive strength of hardened mortar, European Committee for Standardization, 1999. [34] ACI 116R-00, 73 – American Concrete Institute, 2005. [35] EN 1992 Eurocode 2: Design of concrete structures, The European Union, 1992. [36] D.M. 17/01/2018, Norme Tecniche per le Costruzioni. Repubblica Italiana, Consiglio Superiore dei Lavori Pubblici. [37] R. Chudley, R. Greeno, Building Construction Handbook, Routledge, Abingdon, 2016. [38] P. Riva, A. Guadagni (Eds.), Manuale dell’Ingegnere Civile, Hoepli, Milan, 2018. [39] F.M. Lea, The Chemistry of Cement and Concrete, third., Edward Arnold Publishers Limited, London, 1970. [40] A.P. Boresi et al., Advanced Mechanics of Materials, John Wiley and Sons, New York, 1993. [41] H. Sleiman, A. Perrot, S. Amziane, A new look at the measurement of cementitious paste setting by Vicat test, Cem. Concr. Res. 40 (2010) 681–686. [42] Tao Bai et al., Influence of steel slag on the mechanical properties and curing time of metakaolin geopolymer, Ceram. Int. 44 (13) (2018) 15706–15713. [43] F. Pacheco-Torgal et al., Nanotechnology in eco-Efficient Construction: Materials, Processes and Applications, second ed., Woodhead Publishing, Cambridge, 2019. [44] Z. Zhang, J.L. Provis, H. Wang, F. Bullen, A. Reid, Quantitative kinetic and structural analysis of geopolymers. Part 2. Thermodynamics of sodium silicate activation of metakaolin, Thermochim. Acta 539 (2013) 163–171. [45] S. Chithiraputhiran, N. Neithalath, Isothermal reaction kinetics and temperature dependence of alkali activation of slag, fly ash and their blends, Constr. Build. Mater. 45 (2013) 233–242. [46] P.D. Silva, K. Sagoe-Crenstil, V. Sirivivatnanon, Kinetics of geopolymerization: role of Al2O3 and SiO2, Cem. Concr. Res. 37 (4) (2007) 512–518.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Unexplored alternative use of calcareous sludge from the paper-pulp industry in green geopolymer construction materials Manfredi Saeli a,⇑, Luciano Senff b, David M. Tobaldi c, João Carvalheiras c, Maria Paula Seabra c, João A. Labrincha c a b c
Department of Architecture, University of Palermo, Viale delle Scienze, Bld 8, 90128 Palermo, Italy Department of Mobility Engineering, Federal University of Santa Catarina, 89218-000 Joinville, Brazil Department of Materials and Ceramics Engineering, Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
h i g h l i g h t s Calcareous sludge is used as filler in geopolymeric mortars. Biomass fly ash is used as main alumina-silicate source (70 wt%). Used wastes derive from a Kraft pulp industry. Manufacture is simple and highly sustainable and reproducible. Novel geopolymeric mortars are suitable for construction applications.
a r t i c l e
i n f o
Article history: Received 22 March 2019 Received in revised form 7 January 2020 Accepted 13 February 2020
Keywords: Life environment Geopolymer mortar Biomass fly ash Calcareous sludge Paper-pulp industry Construction material
a b s t r a c t Calcareous sludge is an alkaline waste produced by the paper pulp industry that is commonly disposed of in land-fill. However, recent studies and the European regulations discourage such practice. This work investigates an alternative and innovative way to recycle and reuse this waste, as filler, in the production of green geopolymeric mortars intended for applications in construction. This is the first time that this calcareous sludge is used to produce novel waste-based materials, in both construction and geopolymer technology. The novel alkali-activated mortar also uses biomass fly ash – another slightly investigated waste stream – to substitute the metakaolin (70 wt% substitution) and the manufacture process is performed at ambient conditions. All of that reduces the overall process footprint. The implemented mix design is aimed at maximising the waste incorporation and improving the material properties, such as workability and mechanical performance. The main results demonstrate that 10 wt% of calcareous sludge can be efficaciously used as filler in the geopolymeric mortars, generating up to 30% improvement in the mechanical resistance. This alternative use of such wastes will contribute to increase the sustainability of the novel construction materials also granting environmental advantages and a financial surplus for the industry. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The relevant consumption of non-renewable raw materials and the massive generation of greenhouse gases and wastes make the present industrial system highly unsustainable, especially in the developing countries where construction has reached extraordinary levels of unsustainability [1,2]. The industrial development/ implementation might ideally pursue a systemic innovation with reducing the environmental impact and maintaining, at the same ⇑ Corresponding author. E-mail address: [email protected] (M. Saeli). https://doi.org/10.1016/j.conbuildmat.2020.118457 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
time, a positive economic feedback [3]. However, achieving these objectives together is quite complex and, often, undervalued. This paper focuses on the production of novel green geopolymeric (GP) mortars for applications in construction [4]. Two fairly unexplored industrial waste streams, deriving from the paper-pulp industry, are here recycled and reused: biomass fly ash (BFA) and calcareous sludge (CalS). The pulp and paper industry is presently a great consumer of biomass raw material, especially virgin wood. Nowadays, the Kraft process is the most common pulping technology [5]. This involves the digestion of lignin by an alkaline mixture of sodium hydroxide, sodium sulfide, and water. Lignin is then dissolved in a liquor,
2
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which is heated to melt the inorganic components while the organic fraction is burnt in a fluidized-bed combustor to generate steam, then converted into electricity. The BFA is produced in the biomass boiler employed for energy production at mill site and collected at the electrostatic precipitator as waste [6]. The molten inorganic salts are passed into a dissolution tank where a lime eraser is added to clarify the alkali residues; CalS is one of the wastes generated during this process [7,8]. This study is aimed at exploring an alternative exploitation of BFA and CalS that are commonly disposed of in landfills. Indeed, such practice should be limited for the serious risks of soil and water contamination, and consequent environmental degradation [9]. According to previous studies various alternatives were already considered to reuse some cellulose industrial residues, along with energy recovery and green construction materials production [10–12]. BFA are classified as a solid waste, according to the European List of Wastes [13] and has been rarely investigated in the scientific literature for traditional concrete applications [14,15] and, to date, their analogous use in GP manufacture is fairly recent [16,17]. In these work, BFA was reused as an alternative source of aluminium and silicon to partially replace the metakaolin (MK) in the GPbinder manufacture [18–22]. CalS is an inorganic alkali residue, physically homogeneous and mainly constituted of calcite [23], thus offering an interesting recycling potential. In this study, it was recycled and used, in various proportions (wt.%), as filler in the GP-mortars manufacture, The followed manufacture process is highly cost-effective envisaging ambient curing conditions (20 °C, 65% RH), thus avoiding any external source of energy as commonly done in GP processing [24– 26]. Additionally, it involves simple, reproducible, and low-cost procedures that contribute in reducing the process footprint. 2. Material and methods 2.1. Materials In this study, a mixture of BFA (70 wt%) and MK (30 wt%) was used as alumino-silicate source to produce the GP-paste (binder). BFA is mainly composed by a–quartz, calcite, mica group mineral (mixture of muscovite/illite), and microcline as crystalline phases. It was used as provided without any pre-treatment, such as milling or sieving, according to [27]. Benchmark MK, ArgicalTM by UnivarÒ, was used to adjust the GP-binder desired molar oxide ratios. The sum of SiO2 and Al2O3 was 53.3 wt% for BFA and 98.7 wt% for MK. Their characterization is presented in [17]. The alkaline activation was achieved by using a solution of commercial sodium silicate (H2O = 62.1 wt%; SiO2/Na2O = 3.15; Quimialmel LDA, D40 - PQ) and sodium hydroxide (ACS reagent, 97%; Honeywell). The NaOH solution (10 M) was prepared by dissolving sodium hydroxide pellets in distilled water at least 24 h prior to use. The used molarity was selected basing on the cited previous works. The mortar aggregate is constituted by commercial natural siliceous sand furnished by Saint-Gobain Weber Portugal. The CalS contains about 10 wt% of water so, prior to be mixed with sand, it was dried in a conventional oven, at 60 °C for 24 h, then ground in a mortar to dissolve lumps. The process might be implemented if CalS was naturally dried at mill site. That would reduce the overall energy employment for a greener material and manufacture. 2.2. Manufacture process GP-mortars specimens were prepared according to the European standard EN 998-2:2016 [28] adapting the common procedure for traditional cement-based mortars.
The manufacture process involved: a) dry hand mixing for 1 min to ensure a uniform blend of a1) MK and BFA and a2) sand and CalS; b) homogenization of sodium hydroxide and silicate at 50 rpm for 5 mins; c) mixture of the alkaline solution with the solid precursors (BFA + MK) at 60 rpm for 9 mins; d) adding the aggregate mixture (sand + CalS), and mix for 1 min at the same speed to ensure homogeneity; e) pouring the fresh slurry into standard metallic moulds (standard dimensions of 40 40 160 mm) and vibrating for 2 mins on a vibrating table to grant compactness and remove any entrained air; f) seal the moulds with a plastic film and let the specimens harden for 24 h at ambient conditions (20 °C, 65% RH), and g) unsealed and demould the hardened samples, and curing at ambient conditions until testing. This procedure was previously optimised by the authors in [29]. 2.3. Materials characterization CalS mineralogical composition was evaluated by X-ray diffraction (XRD) using a Rigaku Geigerflex D/max-Series instrument (Cu Ka radiation, 5–80° 2h, 0.02° 2h step-scan and 10 s per step). Its chemical composition was evaluated by X-ray fluorescence (XRF) using a Panalytical Axios spectrometer. Particle-size distribution was determined by laser diffraction (Coulter LS230 analyser, Fraunhofer method and Polarization Intensity Differential Scattering). The microstructure was investigated by scanning electron microscopy (SEM) using a Hitachi analytical FE-SEM SU-70. The consistency (spread) of the fresh GP-mortars (slurry) was estimated by flow table test according to the European Standard EN 1015-3:1999 [30]. The (initial and final) setting time was estimated by Vicat test according to the European Standard EN 1963:2016 [31]. The reported values were calculated as the average from three rod penetrations (in different places) during the same test. The temperature evolution of the GP-paste during setting and initial stages of curing was monitored in a quasi-adiabatic calorimeter under ambient conditions. The reported values were calculated as the average from three measurements. Hardened specimens were characterised after 28 days of curing. Bulk density was calculated as the average from three specimens. The water absorption was determined by the Archimedes principle (weight variation, DP/P %) by immersing the specimens in distilled water for 24 h once they had been dried to constant mass at 60 °C in a conventional oven. The water absorption due to capillary action was evaluated according to the European Standard EN 1015-18 [32]. Three replicas were used to calculate the mean values. The mechanical performance was determined at various stages of curing (28, 60, 120, and 180 days) according to the European Standards EN 1015-11 [33] and EN 998-2 [28] by means of uniaxial compressive strength tests (UCS). A universal testing machine (Shimadzu, AG-25TA), provided with a 250 kN load cell running at a displacement rate of 0.5 mm/min, was used. The mean values were obtained from three tests randomly taken from the sample batch. The values of axial strain, displacement quotient at maximum strength and the original specimen’s length, were also calculated. 2.4. Tested formulations Mix design has been outlined to maximise the incorporation of the CalS, and improve the mortars mechanical performance. The main driver of this study is the application of these novel GPmortars in construction, according to the international relevant standards and guidelines (i.e. EN 998-2 [28], ACI 116R-00 [34], Eurocode 2 [35], and Norme Tecniche per le Costruzioni [36]). Therefore, the admissible minimum UCS value was 10 MPa (at 28 days of curing) in order to classify these novel materials in
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resistance class ‘‘M10”. That is an essential requirement for mortars intended for structural applications. The mix design of the GP-binder was optimized by the authors in [17,27] and presents the following molar oxide ratios: SiO2/Al2O3 = 5.27, Na2O/Al2O3 = 1.31, Na2O/ SiO2 = 0.25, and H2O/Na2O = 15.9. These values are in accordance to [4]. The binder/aggregate (B/A) ratio, used for the mortars production, was 1/3, based on preliminary work [29] The main characteristics of the used GP-binder and the reference (CalS-free) GP-mortar formulations are summarized in Table 1. The slurry consistency was the main indicator of the GP-mortars workability and their consequent suitability for real applications. Increasing quantities of CalS were admixed until the paste became too viscous to be worked (spread of about 10 cm). That limited the maximum admissible CalS dosage to 12.5 wt%. Subsequently, distilled water was added to increase the workability, thus maximising the waste amount (cf. Table 2). In this work, the alumino-silicate solid precursors (ratio BFA/MK), the alkaline activator solution (ratio sodium hydroxide/ sodium silicate), and the ratio B/A were kept constant. The tested GP-mortar formulations are shown in Table 2. For all the designed mixes the liquid (water)/solid (L/S) ratio is reported. The experimental plan has been formally divided into two parts, according to the discussed limit of workability and the subsequent use of water: (I) addition of CalS only (no additional water in the slurry); (II) water addition. 3. Results and discussion 3.1. CalS characterization
3
Fig. 1. CalS XRD pattern with the peaks indication of calcite (top, black), and magnesite (bottom, red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(0.52), SiO2 (0.50), Al2O3 (0.29) and a loss on ignition (LOI) of 42.33%. Fig. 2 shows the CalS microstructure that appeared made of hexagonal platelet-shaped particles assembled in elongated columns. The particle size distribution is presented in Fig. 3, along with that of sand. CalS particle size ranges between 0.04 and 160 lm, with a mean value of 25.86 lm. Sand particles size ranges between 80 lm and 1.6 mm, with a mean value of about 0.65 mm. 3.2. Mortars characterization
The XRD analysis of the CalS is presented in Fig. 1. It revealed CaCO3 as the main crystalline phase with some minor amounts of magnesite (MgCO3). Accordingly, the XRF analysis revealed, as main chemical species (wt.%), CaO (54.21), Na2O (1.00), MgO
Table 1 Main characteristics of the used GP-binder and reference (CalS-free) GP-mortar. Property
Measured value
Consistency (spread by flow table) [cm] Bulk density [Kg/m3] Sorptivity by immersion [%] Coefficient of capillarity [kg/(m2min0.5)] Compressive break point [MPa] Bending resistance [MPa]
GP-binder
GP-mortar (ref.)
>30 1307 38 0.87 22.15 ± 1.22 3.05 ± 0.37
21 1832 13 0.35 21.66 ± 0.03 4.08 ± 0.58
Table 2 Mix design of the tested GP-mortars. Experimental plan
Sample n.
CalS [wt.%]
Water [wt.%]
L/S ratio
(I)
1 (ref.) 2 3 4 5 6 7
– 1.0 2.5 5.0 7.5 10.0 12.5
–
0.196 0.194 0.191 0.186 0.181 0.176 0.171
(II)
8 9 10 11 12 13 14
12.5
1.5 2.5 3.5 3.5 4.5 5.5 4.0
0.186 0.196 0.206 0.201 0.211 0.221 0.201
15.0
17.5
3.2.1. Fresh state properties The slurry consistency, evaluated by flow table test (spread), returns the paste workability and gives an indication of the final characteristics of the mortar once set (i.e. adequate compactness, homogeneity, etc.). A spread ranging between 18 and 22 cm guarantees a good material workability and a suitable compaction and homogeneity intended for applications in construction [37,38]. A desired more fluid or pasty nature mainly depends on the mortars specific application; in any case, the consistency can be easily modified by chemicals addition (i.e. plasticizers). However, in this study their use was avoided to prevent undesired reactions with the GPbinder, with a preference to (distilled) water. Pictures of the flow table tests and spread values of the tested formulations are presented in Figs. 4 and 5, respectively. Increasing the amount of CalS the slurry becomes more pasty, until reaching unworkability (spread of about 11 cm – almost equivalent to a state of unworkability), corresponding to the dosage of 12.5 wt% (Figs. 4D and 5, left). That is caused by the very small dimensions of CalS particles (conversely a high specific surface area) that tend to adsorb the available water molecules. Then, to increase the workability of the mixture, various amounts of water were supplied. At the same time, the more water is added the more CalS can be admixed, maximising the waste re-usage (Fig. 4/bottom line and Fig. 5, right). More particularly, it is observed that for the mixture with 12.5 wt% of CalS, the workability increases from 11 cm to 19 cm when 3.5 wt% of water is added. That quantity of water was sufficient to increase the CalS addition to 15 wt%, with an acceptable consistency of about 16 cm. The original consistency (21 cm spread) is recovered by adding 5.5 wt% of water. The maximum tested amount of CalS was 17.5 wt%, with 4 wt% of water, that presented a spread of 15 cm. High amounts of water, as generally observed for common construction materials [39,40] may cause a drastic sorptivity increase or, even worse, a substantial mechanical resistance decrease. A
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Fig. 2. CalS micrographs.
Fig. 3. Particle size distribution: CalS (A) and sand (B).
Fig. 4. Flow table test photographs of some investigated formulations. Top line – influence of CalS addition: A – reference (CalS-free) GP-mortar; B CalS, D 12.5 wt% CalS. Bottom line - influence of water addition on D: E 1.5 wt% H2O; F 2.5 wt% H2O; G 3.5 wt% H2O.
good balance between amount of CalS and water addition must be carefully considered. Setting time is also fundamental to assess the material properties and its possible real applicability. According to [41], in this work the initial setting time is considered as the time when the needle penetration is 39 ± 0.5 mm; the final setting time when less than 0.5 mm of penetration is measured. Values, calculated as the average from three rod penetrations, are reported in Fig. 6A. It is observed that, increasing the CalS amount in the formulation, the initial setting time is delayed. For the produced GPmortars, the measured delay was about 1 min per wt.% of added CalS. That is caused by the lower binder percentage, which is the reactive component, along with a lower available water amount,
5 wt% CalS, C
10 wt%
that is mainly consumed to wet the small CalS particles, in accordance with [42,43]. The setting process started after 140 mins for the reference (CalS-free) GP-mortar, and can be considered finished (final setting time) after 200 mins. The mortar with 5 wt% CalS showed a longer final setting time (235 mins); the mortar with 10 wt% CalS showed a period that is comparable to that of the reference (205 mins). The short final setting time of the GPmortar with 10 wt% of CalS is due to the high CalS amount that probably subtracts much more available water from the paste making it, de facto, drier As a preliminary study, the temperature evolution of the GP-pastes during setting (initial stages of curing) was monitored in a quasi-adiabatic calorimeter under ambient conditions. This
M. Saeli et al. / Construction and Building Materials 246 (2020) 118457
5
Fig. 5. Spread values of the produced GP-mortars: without (left) and with (right) water addition.
Fig. 6. Displacement of the needle inside the fresh GP-mortars’ paste (setting time) (A) and heat release during curing (B).
analysis evaluates the influence of the CalS incorporation in the alkali activation kinetic. The calorimetric measurement values are reported for the first hours upon mixing, as the average from three different samples (Fig. 6B). It is observed that the three analysed formulations showed a decreasing single peak in the first minute after mixing (Fig. 6B, inlet), demonstrating that the geopolymerization reactions were already started [44]. Subsequently, the released heat slowly decreases till room conditions. Geopolymerization is a quite complex process where the precursors nature can strongly influence the reactions process [45,46]. The CalS addition influences the reactions decreasing the temperature of about 1.5 °C, and slowing down the reactions process. In any case, the waste usage in the GP-mortars is not detrimental to a complete geopolymerization.
3.2.2. Hardened state properties The microstructure of some specimens (0, 5, and 10 wt% of CalS) is presented in Fig. 7. The CalS addition makes the paste denser and more uniform, as fillers do. Furthermore, less and small pores are observed (less voids, higher compaction) and the particles look agglomerated in smaller structures. Consequently, as a general trend, increasing the CalS amount promotes a rise in the hardened GP-mortars bulk density, whose values are presented in Fig. 8. The effect of adding water to the mixture is, as expected, a general bulk density decrease due to a higher porosity. Analogously, the values of water absorption, both by immersion and by capillary action, are strictly dependent on the CalS and water amounts, as shown in Fig. 9. More particularly, the water
Fig. 7. Micrographs of: A – reference (CalS-free) GP-mortar; B – GP-mortar with 5 wt% CalS; C – GP-mortar with 10 wt% CalS.
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Fig. 8. Bulk density of the produced GP-mortars: without (left) and with (right) water.
Fig. 9. Sorptivity of the produced mortars: by immersion (A) and by capillary test (B).
sorptivity, in both the cases, tends to decrease by increasing the CalS amount. Indeed, the very small CalS particles tend to fill in the voids in the GP-mortars matrix preventing the water absorption. That is an unquestionable advantage for any structural applications as a high water sorptivity may cause structural damages and steel bars, if present, oxidation. This particular aspect will be object of further research. At the same time, the water addition, as commonly observed for traditional construction materials, causes a larger water absorption by immersion, following a linear dependence of L/S ratio (red line) (Fig. 9A). 3.2.3. Mechanical performance The specimens mechanical performance was assessed at various stages of curing. The values of UCS are reported in Fig. 10, the influence of water addition in Fig. 11, and the stress–strain curves at 28-curing-days in Fig. 12. Lastly, for a selection of specimens (up to 10 wt% CalS, without water addition) the mechanical performance was further investigated at 60, 120, and 180 days of curing in order to assess the material performance during the natural aging. Results are shown in Fig. 13.
Fig. 11. H2O/solid ratio vs. UCS of the investigated formulations. The red line indicates the approximated linear dependence; the green spot represents the reference (CalS-free) GP-mortar. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. UCS of the produced GP-mortars: without (left) and with (right) water.
M. Saeli et al. / Construction and Building Materials 246 (2020) 118457
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Fig. 12. Stress–strain curves of the tested mortar’s formulations. A – specimens without water; B – specimens with 12.5 wt% CalS without/with water; C – specimens with 15 wt% CalS with water; D – specimen with 17.5 wt% CalS with water.
Fig. 13. Selected produced GP-mortars mechanical performance at various curing steps: A – UCS; B – improvement percentage (to 28 aging days).
As a general trend, increasing the CalS amount increases the UCS value (Fig. 10). That is a consequence of an efficient filler effect, improving the compactness degree and the density of the material. The reference (CalS-free) GP-mortar shows a UCS of 21. 66 ± 0.03 MPa while a value of 28.11 ± 1.29 MPa was measured for the 10 wt% CalS addition, corresponding to about a 30% improvement. A very slightly further enhancement was observed
for the sample containing 12.5 wt% CalS (28.65 ± 1.99 MPa). The addition of CalS made the material more deformable. The calculated average strain was 2.35 ± 0.03%, against 1.8% shown by the reference (CalS-free) GP-mortar (cf. Table 3). The water addition , as expected, decreases the UCS value (Fig. 10, right) that also follows an approximated linear dependence with the H2O/solid ratio (Fig. 11). In general, the elastic regions of the compositions
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Table 3 Mechanical features and classes of resistance. Experimental plan
Sample n.
CalS [wt.%]
Water [wt.%]
UCS [MPa]
Axial strain [%]
Class of resistance [EN 998-2]
(I)
1 (ref.) 2 3 4 5 6 7
– 1.0 2.5 5.0 7.5 10.0 12.5
–
21.66 21.27 22.53 24.01 25.09 28.11 28.65
1.80 2.13 2.38 2.35 2.35 2.33 2.38
M20 M20 M20 M20 M25 M25 M25
(II)
8 9 10 11 12 13 14
12.5
1.5 2.5 3.5 3.5 4.5 5.5 4.0
23.62 21.64 18.20 18.85 16.37 14.37 17.14
2.30 2.18 2.08 2.28 2.25 2.23 2.00
M20 M20 M15 M15 M15 M10 M15
15.0
17.5
to which water was added (12.5 and 15 wt%) was not substantially altered (apart from the final peak) (Fig. 12). Ultimately, adding water determines very slight axial strain’s negative shifts. Table 3 reports the main mechanical features of the produced GP-mortars, along with the classes of resistance according to EN 998-2 [28]. The addition of up to 5 wt% of CalS does not alter the original M20 class of the reference. On the contrary, an addiction of 7.5 12.5 wt% upgrades the specimens to M25 class. The effect of adding water is detrimental for the GP-mortars mechanical performance, causing an immediate class downgrade. Finally, mechanical tests were performed at various stages of curing for the GP-doped mortars up to 10 wt% of CalS (Fig. 13). The tested stages of curing were 60, 120, and 180 days. Similarly to traditional construction materials, also the produced GPmortars show an increasing mechanical gain over time. After 120 days of curing, no significant improvements are observed. That is particularly evident for the highest CalS amounts; on the other hand the reference (CalS-free) GP-mortar, or the formulations with few amounts of CalS, shows a further margin of improvement. 4. Conclusions In this work novel GP-mortars were produced recycling and reusing wastes originated by a Portuguese Kraft pulp industry: biomass fly ash and calcareous sludge. Such wastes are commonly disposed of in land-fill and find here an alternative usage, in line with the recent European directives. The BFA is here used to partially substitute the MK in the binder formulation and the CalS as a filler. Mix design was aimed at maximizing the CalS incorporation while engineering the novel GP-material for robust applications in construction (UCS 10 MPa). Furthermore, in the pursuit of sustainability, the manufacture process was conducted at ambient conditions by a simple, reproducible, and cost-efficient process. That contributes in reducing the overall material footprint. The CalS addition increases the slurry consistency, making the material less workable, generates (small) delays in the initial setting time, and decreases the released heat, indicating a lower degree of the geopolimerisation reactions. Also the microstructure is affected becoming denser and less porous. That is directly connected to the bulk density that tends to increase, while the water sorptivity decreases. Moreover, increasing the CalS amount, the UCS tends to increase, as expected for a filler action. All these characteristics are highly recommended for applications in construction, especially if structural uses are foreseen. The CalS addition of 7.5–12.5 wt% upgrades the reference (CalS-free) mortar from class M20 to M25. Considering some issues, such as workability, the addition of 10 wt% CalS resulted the best balance.
The addition of water, although allowing higher CalS quantity, negatively affects the overall materials properties: lower bulk densities, higher porosity, and consequently higher water absorptions, both by immersion and by capillary action. To the maximum admissible CalS dosage of 12.5 wt% (without water), the water addition lets reaching a maximum (tested) quantity of 17.5 wt% CalS, otherwise unfeasible. Anyway, no beneficial effects are observed by adding water and improving the quantity of the used waste. Adding just 1.5 wt% of water to the 12.5 wt% CalS formulation would let using 2.5 wt% more CalS with a still suitable UCS (same class of the reference, M20, but with a slight higher UCS value). A quantity of 15 wt% CalS is admissible with 3.5 wt% of water, with a (bottom end) limit in workability and questioning results regarding the other properties. Finally, in all the tested cases, CalS admixture determines a higher deformability but the mechanical behaviour in the elastic region is not substantially altered. Curing showed to continue during time until, at least, 180 (tested) days, making the UCS increase and, consequently, granting the material higher resistances. Further works foresee the complete analysis of the curing process to better explore the geopolymerization progression in relation to the addition of various fillers. Notes M. Saeli and L. Senff designed the experimental plan and produced the specimens (2.1, 2.2, 2.3, 2.4, and 3.2.1), D.M. Tobaldi carried out the mineralogical analysis (3.1), J. Carvalheiras carried out the experiments of setting time and temperature evolution determination (3.2.1), M. Saeli carried out the physical and mechanical characterization (3.1 – SEM and particle size distribution, 3.2.2, and 3.2.3); M.P. Seabra and J.A. Labrincha were in charge of the scientific coordination. All the authors were involved in the manuscript preparation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financed by Portugal 2020 through European Regional Development Fund (ERDF) in the frame of Operational Competitiveness and Internationalization Programme (POCI) in the scope of the project PROTEUS - POCI-01-0247-FEDER-017729 and in the scope of the project CICECO - Aveiro Institute of Materi-
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als POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), co-financed by national funds through the FCT/MEC. Manfredi Saeli thanks the Italian Ministry of Education, University and Research (MIUR) for financing his work in the framework of the PON ‘‘Research and Innovation 2014-2020” section 2 ‘‘AIM: Attraction and International Mobility” with D.D. 407 of 27/02/2018 co-financed by the European Social Fund – CUP B74I19000650001 – id project AIM 1890405-3, area: ‘‘Technologies for the Environments of Life”, S.C. 08/C1, S.S.D. ICAR/10. David Maria Tobaldi is grateful to Portuguese national funds (OE), through FCT, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19. Dr. Cristina Sequeira (Department of Geoscience, University of Aveiro) is acknowledged for the XRF analysis. References [1] M. Schneider, M. Romer, M. Tschudin, H. Bolio, Sustainable cement production – present and future, Cem. Concr. Res. 41 (7) (2011) 642–650. [2] A. Kylili, P.A. Fokaides, Policy trends for the sustainability assessment of construction materials: a review, Sustain. Cities Soc. 35 (2017) 280–288. [3] C. Freeman, Technology and Economic Performance, Pinter, London, 1987. [4] J. Davidovits, Geopolymers Inorganic polymeric new materials, J. Therm. Anal. 37 (8) (1991) 1633–1656. [5] M. Ghochapon, G. Rafiqul, M. Pomthong, A. Suttichai, Integration of the biorefinery concept for the development of sustainable processes for pulp and paper industry, Comput. Chem. Eng. 119 (2018) 70–84. [6] L.A.C. Tarelho, E.R. Teixeira, D.F.R. Silva, R.C.E. Modolo, J.A. Labrincha, F. Rocha, Characteristics of distinct ash flows in a biomass thermal power plant with bubbling fluidised bed combustor, Energy 90 (2015) 387–402. [7] T.M. Grace, B. Leopold, E.W. Malcolm, Alkaline pulping, in: M.J. Kocurek, F. Stevens (Eds.), Pulp and Paper Manufacture, vol. 5, Joint Textbook Committee of the Paper Industry of the United States and Canada, Atlanta, 1991. [8] P. Bajpai, Management of Pulp and Paper Mill Waste, Springer International Publishing, 2015. [9] European Parliament, Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste. [10] L. Senff, G. Ascensão, V.M. Ferreira, M.P. Seabra, J.A. Labrincha, Development of multifunctional plaster using nano-TiO2 and distinct particle size cellulose fibers, Energy Build. 158 (2018) 721–735. [11] R.C.E. Modolo, T. Silva, L. Senff, L.A.C. Tarelho, J.A. Labrincha, V.M. Ferreira, L. Silva, Bottom ash from biomass combustion in BFB and its use in adhesivemortars, Fuel Process. Technol. 129 (2015) 192–202. [12] R.C.E. Modolo, L. Senff, J.A. Labrincha, V.M. Ferreira, L.A.C. Tarelho, Lime mud from cellulose industry as raw material in cement mortars, Mater. Constr. 64 (316) (2014) e033. [13] European Parliament, Commission Decision 2000/532/EC, List of Wastes. [14] S. Wang, L. Baxter, Comprehensive study of biomass fly ash in concrete: strength, microscopy, kinetics and durability, Fuel Process. Technol. 88 (11) (2007) 1165–1170. [15] R. Rajamma, R.J. Ball, L.A. Tarelho, G.C. Allen, J.A. Labrincha, V.M. Ferreira, Characterisation and use of biomass fly ash in cement-based materials, J. Hazard. Mater. 172 (2) (2009) 1049–1060. [16] R. Rajamma, J.A. Labrincha, V.M. Ferreira, Alkali activation of biomass fly ash– metakaolin blends, Fuel 98 (2012) 265–271. [17] M. Saeli, D.M. Tobaldi, M.P. Seabra, J.A. Labrincha, Mix design and mechanical performance of geopolymeric binders and mortars using biomass fly ash and alkaline effluent from paper-pulp industry, J. Clean. Prod. 208 (2019) 1188– 1197. [18] Part Wei Ken, Mahyuddin Ramli, Cheah Chee Ban, An overview on the influence of various factors on the properties of geopolymer concrete derived from industrial by-products, Constr. Build. Mater. 77 (2015) 370–395.
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