Precursors SAGP FSGP LGP SGP
Metakaolin 28.4 34.8 34.7 33.6
NaOH 12.3 12.5
Sodium silicate 55.8 63.1
Water 33.7 5.9 3.3
H2O/Naa 9.4 4.5 5.5 7.2
Si/Ala 2 2 2 2
Na/Ala 1 1 1 1
Fumed silica Silica Fume SF 98, Australian Fused Materials PL. 93 wt% silica, 4 wt% (ZrO2+HfO2)
Sodium silicate Solution, consisting of 14.7 wt% Na2O, 29.4 wt% SiO2 and 55.9 wt% water, Grade D, PQ Corporation, Australia
Water Deionised water
Sand Washed beach sand, Australian Foundry Services Pty. Ltd., Australia
Two sets of OPC samples were made by mixing Aalborg cement with a water/cement ratio of 0.3, one set with 40 wt% sand and the other without. They were cast similar to the GPs (see above). They were cured at ambient for 28 days before testing.
Physical and mechanical properties
The bulk densities of all the materials were determined by measuring the mass and the volume with a Hg pycnometer.Open porosities were determined by immersing each specimen in octane (non-polar liquid) for 24 h and measuring the mass absorbed (density of octane=0.703 g/cm3). Saturation with water was considered unsuitable because of leaching in water, especially Na . This method of determining open porosity is a comparative method and not an absolute determination as the open porosity may be higher because of incomplete permeation of the liquid into the pores.
CCS specimens of 25-mm diameter and 40-mm high, were cast for each batch. Each specimen was ground ﬂat and parallel using silicon carbide paper prior to testing. The CCS tests were conducted at room temperature in a servo-electric universal mechanical testing machine (Instron8562) with a 100 kN load cell. The specimens were tested in position control at a cross head speed of 5 micron/s. A minimum of ﬁve specimens for each composition were tested.
Flexure specimens were prepared as bars 6 x 10 x 50 mm, for elastic modulus and strength evaluation. Five specimens were obtained from each batch composition. Young’s modulus of each specimen was measured using a non-destructive impulse excitation technique. The long edges of each specimen were then chamfered on the prospective tensile face to avoid edge failures. The strength or modulus of rupture (MOR) was determined using three-point bending with a support span S=40 mm on the testing machine at a cross-head speed of 5micron/s. The MOR values, sigma F, were calculated from the failure load and the specimen dimensions using the formula:
where P is the fracture load, b is the specimen width and w
is the specimen thickness.
Specimens for toughness evaluation were the same size(but now with thickness =10 mm and width = 6 mm) to those used for MOR except that a notch was made at the centre of each bar on the tensile face using a thin diamond blade. The notch depth was measured using an optical measuring device (MicroVu, Model 9050A). The notched specimens were then tested in three-point bending as before, with all failures occurring from the notch. The fracture toughness, K IC, of each specimen was then calculated [13
see formulae https://www.scribd.com/doc/72132356/Mechanical-Properties-of-Metakaolin-based-Geopolymers-With-Molar-Ratios-of-SiAl-2-and-NaAl-1
where P is the fracture load, b is the specimen width, wis the specimen thickness, a is the notch length and S is the span length. Equations 2 and 3 are valid for S=4w as used in this work (S=40 mm and w=10 mm).
All the GPs were analysed by X-ray diffraction (XRD:Model D500, Siemens, Karlsruhe, Germany) using CoKalpha radiation on crushed portions of material. Selected samples were sectioned, mounted in epoxy resin and polished to a 0.25-micron diamond ﬁnish and examined by a scanning electron microscope (SEM: Model 6400, JEOL, Tokyo,Japan) operated at 15 kV and ﬁtted with an X-ray micro-analysis system (EDS: Model: Voyager IV, TracorNorthern, Middleton, WI, USA). Some polished sampleswere also examined using optical microscopy.
Results and discussion
Physical and mechanical properties of GPs with 40 wt% sand are listed in Table3
. The mechanical property values of the SAGP are the lowest of the lot. More water was used to make this geopolymer, and this is believed to have resulted in SAGP having the highest open porosity. It is important to note that the water used (Table2
) to make a geopolymer varied widely depending on the precursor used. The highest CCS was obtained for LGP and SGP, and these also exhibited the highest Young’s modulus. Using different mixing techniques should reduce the water requirement, thus making the porosity lower.
Table 3 Physical and mechanical properties of GPs with 40 wt%sand
SAGP FSGP LGP SGP
Bulk density(g/cm3) 1.45 1.48 1.57 1.60
Open porosity (%) 32 28 20 20
CCS (MPa) 16 (2) 28 (5) 69 (5) 70 (6)
MOR (MPa) 3.2 (1.0) 7.4 (0.8) 8.8 (0.5) 7.2 (0.4)
K1c(MPa m^1/2) 0.25 (0.04) 0.43 (0.04) 0.49 (0.03) 0.56 (0.02)
E (GPa) 5.5 (0.2) 9.6 (0.4) 14.0 (0.6) 14.0 (0.6)
Standard deviations are given in parentheses
Table 4 Variation of porosity and density of LGP with water content
H2O/Na molar ratio Open porosity (%) Bulk density (g/cm3)
2 6 1.85
4 12 1.64
4.5 16 1.55
5.5 20 1.57
6 30 1.30
Notes: 1. Porosity and density determined on the seventh day after making.
2. First three samples developed cracks after 10 days