Mechanical and micromechanical properties of alkali activated fly-ash cement based on nano-indentation
Introduction
Alkali activated cement (AAC) is a potential cementitious system for sustainable development [1]. Its main constituent is pozzolan, which can react with alkali activator to form binder [2]. A number of researches [2], [3], [4] reported the difference between the composition of traditional Portland cement and fundamental rock forming minerals of the earth crust. The common chemical compositions of Portland cement and fundamental rock are silica and alumina which can also be found in a number of industrial wastes. Ground industrial wastes or by-products containing aluminosilicate when mixed with rich alkalis could form a hydraulic binder which is a type of inorganic polymer called ‘geopolymer’ or alkali activated cement (AAC). Davidovits [5] classified different types of geopolymer according to Si to Al ratio in the mixtures for various industry applications. Lloyd et al. [6] reported that calcium contents in geoploymer is important for alkali mobility that may be significant to limit the durability of embedded steel reinforcement. A precaution should be taken as some of ground industrial wastes or by-products such as those containing calcium aluminate and metakaolin geopolymer have loss of strength during long-term ageing [7]. Despite extensive research in this area, the entire polymerization process of AAC is not totally understood. It is classified as a polymer because of its huge molecule formed by a number of smaller groups of molecules. AAC has superior mechanical, chemical and thermal properties compared to ordinary Portland cement (OPC) [8]. The main benefit of AAC is that the source material is not a carbonate bearing material; therefore, it does not release vast quantities of CO2 as in the case of Portland cement. Turner et al. [9] reported that the carbon emission of AAC concrete is around 9% less than comparable OPC concrete as alkali activators are high carbon footprint materials. Also, high early age strength, high chemical durability and resistance to high temperature are beneficial properties of AAC.
Normally, cementitious materials have several phases that contribute to the mechanical properties. A number of researchers [10], [11], [12], [13] studied the characterization of alkali-activated materials by a variety of experimental methods including XRD, SEM, DTA and TGA. Jennings et al. [14] and Tennis et al. [15] proposed a model for the determination of two types of calcium silicate hydrate (C-S-H) viz., high density (HD) and low density (LD) C-S-H, at different points of the specimens geometry. The content of C-S-H is determined in term of its volume fraction of the indentation grid. Constantinides et al. [16] determined the two C-S-H types, Portlandite (CH), and clinker using this nano-indentation method. The result shows that decalcification of C-S-H phases is the primary source of nanometer-scale elastic modulus degradation.
Recently, Němeček et al. [17] studied the reaction products of AAFA paste using NaOH solution with the liquid to solid ratio of 0.531 cured at 80 °C for 12 h. Nano-indentation and environmental scanning electron microscope (ESEM) were used, and four phases of reaction products were found. The four phases were identified as N-A-S-H (sodium aluminosilicate hydrate), partly-activated slag (N-A-S-H gel intermixed with slag-like particles), non-activated slag (porous non-activated slag-like particles), and non-activated compact glass (solid non-activated glass spheres or their relicts). N-A-S-H is the main reaction product which is linked to the atomic scale and nano-structure and is independent of precursor material or the temperature curing regime. N-A-S-H phase is pure and is related to the mechanical strength of AAFA matrix. The contents of Si ions in N-A-S-H can be increased by the presence of Si ions in the raw materials. It has been found that the increasing condensation degree of Si ions in N-A-S-H relates directly to the mechanical strength gain [11], [18]. The partly-activated slag phase is intermixed with the slag-like particles. The non-activated slag phase is porous and contains non-activated slag-like particles. The non-activated compact glass phase is solid, non-activated glass sphere.
The aim of this research is to investigate the mechanical and micromechanical properties of AAFA paste and mortar. The experimental program is designed according to Taguchi’s method [19], which is efficient for investigating optimum design parameters for the required performances. The investigated parameters are density and compressive strength of AAFA. Nano-indentation and statistical analysis of the test data are used for the evaluation of microporomechanics of AAFA samples. A background on nano-indentation and how to determine the indentation modulus, hardness, Poissons ratio, cohesion, friction coefficient and packing density of materials are presented in brief.
Section snippets
Principal nano-indentation
Nano-indentation test method is now well established that the response of material upon the reversal of touch loading provides access to the elastic properties of indentation materials [20]. The measurement of the hardness and elastic modulus of material can be obtained from the relationship between indentation loads (P) and depth (h) during loading and unloading [21], [22], [23], [24], [25], [26], [27]. Fig. 1 shows a typical indentation load-depth curve. The important quantities that
Experimental program
In this experimental work, class F (low calcium) fly-ash from Australia was used to prepare AAFA paste and mortar samples. The chemical composition of fly-ash is presented in Table 1. The Loss on Ignition (LOI) of fly-ash was 1.53% and the median particle size was 45 μm. The sand with specific gravity of 2.6 was used as fine aggregate. The alkali activator was prepared by dissolving NaOH pellet in water and left for 24 h and Na2SiO3 (water glass) was then added and again left for 24 h before
Density and compressive strength
Nine mixtures of AAFA samples were prepared according to Taguchi’s design of experiment. Details of the mixtures and the experimental results of density and compressive strength at 28 days (fc28) are presented in Table 3. The analysis of variance (ANOVA) is performed to observe the relative influence of parameters and their interaction to the variation of results. Results of ANOVA analysis are presented in Table 4.
The density of AAFA at 28 days is between 1521 and 1717 kg/m3. The ANOVA
Deconvolution and particle properties
The results of nano-indentation test using deconvolution technique in terms of the CDF and PDF of the tested AAFA specimens confirm the presence of the four phases in AAFA. This is similar to the previously reported results [17]. The typical indentation load depth curves are illustrated in Fig. 2. The mean indentation depth is 240 ± 7 nm in the N-A-S-H phase, 143 ± 3 nm in the partly-activated slag, 82 ± 1 nm in the non-activated slag and 54 ± 4 nm in the non-activated compact glass phases.
With the
Conclusion
The results obtained from this investigation are based on Taguchi’s experimental design approach and statistical analysis of nano-indentation results of AAFA. The results are then used to determine the effect of test factors on the compressive strength, density, and nano-indentation deconvolution results of AAFA. Based on the results of this study, the following conclusions can be drawn:
- (1)
In terms of compressive strength, the normal AAFA paste, i.e., no SF, no sand, no SP with l/s of 0.6 is the
Acknowledgements
The authors would like to acknowledge the assistance by Dr. Kornkanok Boonserm, Department of Applied Chemistry, Rajamangala University of Technology in Thailand. The third author would like to acknowledge the support of Khon Kaen University and Thailand Research Fund (TRF) under the TRD Senior Research Scholar Contract No.RTA5780004.
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