Alkali activated metakaolin with high limestone contents – Statistical modeling of strength and environmental and cost analyses
Introduction
Geopolymers are alkali activated binders with a three-dimensional inorganic polymeric structure [[1], [2], [3]] formed by alkaline activation of an aluminosilicate precursor such as metakaolin (MK) [[4], [5], [6], [7]]; these can have similar or superior mechanical properties compared to Portland cement [8,9]. The purity of the precursor MK [7,[10], [11], [12], [13], [14], [15]] and the overall chemical composition of the alkaline system have strong influences on the properties [6,16,17]. Compressive strengths of 23–85 MPa have been reported for MK-based geopolymer [6,7,11,14,[17], [18], [19], [20]] for molar ratios Na/Al of 0.9–1.4 and Si/Al of 1.8–3.1; such molar ratios are commonly kept fixed by adjusting the concentration of sodium silicate and sodium hydroxide relative to the chemical composition of the MK, which resulted in the use of high amounts of alkaline activator. For example a blend of Na/Al = 1.4 and Si/Al = 3.1 can reach 72 MPa after curing at 50 °C for 24 h and then at 20 °C for 27 days [7]; the activator requirements were high, of ∼25%Na2O relative to the mass of precursor, which required about 48% anhydrous sodium silicate of module Ms = 3.27 plus 18%NaOH relative to the mass of MK.
The literature shows debate about the environmental footprint of the geopolymers [8,[21], [22], [23], [24], [25], [26]] due to the strong environmental impacts of the of sodium silicate [[27], [28], [29]] and sodium hydroxide [22,27,30,31]. Even though aluminosilicate minerals are among the most abundant materials worldwide [[32], [33], [34], [35]], kaolinitic clays need calcination at 700–850 °C to make them reactive (MK) [[36], [37], [38]]; this increases the environmental impact and cost on the final product, so the manufacture of MK-based geopolymers is sometimes questioned [26,39].
On the other hand, limestone (LS) represents an attractive raw material for the manufacture of low environmental footprint binders as it is widely and readily available and as its handling can be readily scaled using the infrastructure of the Portland cement industry [35]. LS is used as a partial substitution of Portland cement of up to 35% according to international standards [40,41], although current studies [35,[42], [43], [44]] have suggested that cements with up 50% mass LS are suitable in terms of strength, durability, practicability and environmental impact. In those studies, LS seems not to be a totally inert component since it reacted with aluminum phases, from of Portland cement or from supplementary materials, improving the microstructure, early strength and durability of the resulting product [42,43].
Most recently, LS has been reported as a supplementary precursor on MK-based geopolymer cements [[45], [46], [47], [48], [49], [50]]. The addition of 10%LS in pastes [46,47] improved strengths reaching up to 20 MPa, while higher %LS was detrimental; the authors considered the LS as an inert filler that enhanced the microstructure of the hardened binders. Another study [50] on pastes found also a favorable on the strengths by adding 20%LS reaching up to 45 MPa but higher %LS was also deleterious; the authors discarded the possibility of a simple micro-aggregate effect of the LS, as its dissolution was observed participating in the geopolymeric gel structure improving the strength. A study in pastes of MK-LS activated with NaOH [48] reported 7 MPa as the best strength with 50%LS; the authors indicated that the LS enhanced the release of Al and Si ions from MK and the leaching of Ca2+ from the LS was observed.
Furthermore, there are also reports of LS as a supplementary precursor in blends with other precursors of a chemical composition different to the aluminosilicates, i.e in blends with blast furnace slag [[51], [52], [53]], blast furnace slag-fly ash [54] and waste silica soda lime glass [55] with interesting effects. Moreover, LS has been recently reported in novel alkaline binders as the only precursor activated with waterglass [56] forming reaction products of type calcium silicate hydrates.
In all cases above mentioned for alkali activated MK, the mixtures of pastes were designed by fixing the amount of alkaline activator relative to the mass of the precursor of 100%MK; however, the amount of alkaline activator was not adjusted when the LS was added, which did not reduce the high amounts of alkalis in the binders, so the cost and environmental impact were not significantly reduced. High amounts of alkali have two sides, on the one hand, can promote a greater dissolution of MK favoring the strength, but on the other hand it can result in an excess of unreacted free alkalis that produce efflorescence and brittleness in the hardened binders [57], which compromises the durability.
The alkali activated blends of MK-LS appear to be suitable alternative cements in terms of strength, durability, practicability, scalability and environmental impact; nonetheless, those criteria must be systematically studied in greater depth in order to take them to the industrial level. The experimentation based in statistical methods, such as the response surface methodology, is a robust route to obtain systematized information to ensure the quality and the desired properties in the materials [52,58,59]. Such methodology allows to study the behavior of the independent variables as well as to optimize the response variable according to desired specifications. We hypothesized that throughout the use of statistical methods, it is possible to design high performance alkali activated blends of MK-LS using high loads of limestone, reducing the amount of alkaline activators while reducing also the environmental impact and costs. Based on previous experience in the laboratory, this work presents a systematized study in pastes of alkali activated MK-LS blends incorporating up to 80% limestone, with statistical modeling and optimization of the compressive strength. In addition, CO2 emissions, energy consumption and cost of production are comparatively analyzed for the alkali activated MK-LS blends and a reference of a blended Portland cement (BPC).
Section snippets
Materials
Pastes were prepared by alkaline activation of precursors of ground MK and LS with blends of water glass (29.85% SiO2, 9.12% Na2O and 61.03% H2O; density of 1.41 g/cm3) and NaOH flakes of 98% purity (density of 2.13 g/cm3), both of industrial grade. Table 1 presents the oxide composition obtained by X-ray fluorescence spectrometry and other relevant data of the precursors. The MK was obtained after calcination of a kaolin at 750 °C for 4 h, it was predominantly amorphous (Fig. 1a) with traces
Compressive strength data
The average and standard deviation of compressive strength as a function of time are given in Table 4. The strength ranged from 0 to 65 MPa indicating a strong effect of the chemical composition. Pastes E2, E10 and E11 did not develop good strength, while formulations like E6 and E13 pastes showed low early strength of ∼12 MPa which increased up to over time ∼20 MPa. Pastes E4 and E12 reached intermediate early strength with small increases over time. The rest of the pastes showed relatively
Discussion
This study showed that with the proper chemical composition it is possible to use high limestone loads to obtain high performance alkali activated MK-LS binders with a reduced environmental footprint and production cost. For example, a paste with 60%LS, Na/Al = 1.52 and Si/Al = 2.8 can reach 56.13 ± 1.94 and 61.46 ± 4.89 MPa after 1 and 28 days, respectively, with CO2 emissions, energy consumption and cost reduced in 61%, 11% and 15%, respectively, relative to reference BPC paste of
Conclusions
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The strength of alkali activated metakaolin pastes with high loads of limestone was modelled by the surface response method as a function of various experimental factors. The Pearson correlation among the predicted and measured strengths was of 0.99, which showed the accuracy of the model and proper consistency on the experimental work.
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The percentage of limestone, Na/Al molar ratio and Si/Al molar ratio were all found to affect the 28-day compressive strength; the effect of the factors is not
Declaration of competing interest
The authors declare no conflict of interests.
Acknowledgments
Thanks are due to the financial support of the Project 114 SEP-Cinvestav 2018 and to the Mexican National Council of Science and Technology for the scholarship to P Perez-Cortes.
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