Table 2 Batch compositions (wt%) of geopolymers prepared

———————————————————————————————————————-

Precursors                   SAGP                FSGP                  LGP                        SGP

———————————————————————————————————————-

Metakaolin                    28.4                  34.8                   34.7                        33.6

NaAlO2                        15.8

Ludox                                                                               46.9

NaOH                                                    12.3                    12.5

Fumed silica                                          19.2

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

—————————————————————————————————

a=molar ratio

Table 1 Precursors used in making geopolymers

————————————————————————————————————————————-

Precursor                 Description

————————————————————————————————————————————-

Metakaolin     Kaolin clay (Kingwhite 80, Unimin, Australia Pty. Ltd.) heated at 750°C for 15h. Clay has a molar ratio of SiO2/Al2O3=2.01 and also contains 1 wt% TiO2 quartz and hematite
NaAlO2           Solution consisting of 34 wt% NaAlO2, 9 wt% NaOH and 57 wt% water. Supplied by Redox Ltd., Australia

Ludox              Solution consisting of 40 wt% silica and 60 wt% water, Ludox AS40, Sigma Aldrich, Australia

NaOH              Ajax Fine Chemicals Ltd., Australia

Fumed silica   Silica Fume SF 98, Australian Fused Materials PL. 93 wt% silica, 4 wt% (ZrO2+HfO2)

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 [11]. 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[12]:

Table 4 lists the variation of open porosity and bulk density for LGP samples made by varying the amount of water. It is seen that by using less water, the open porosity can be reduced. However, the samples developed cracks after about 10 days for those with less water than for H2O/ Na molar ratio of <5.5. The sample with a molar ratio of 6 had much higher porosity. Although the CCS of eachsample except for molar ratio of 5.5 was not carried out, the CCS would certainly be the maximum for this sample.

There is almost a linear correlation in the mechanical properties with porosity irrespective of the precursor type.Figure1shows such a result for the CCS and Young’s modulus versus open porosity for each geopolymer composition. The properties are highly dependent on the porosity (i.e. range 20–32% porosity), and this dominates over the precursor chemistry. The lower the porosity the higher the CCS, Young’s modulus, fracture toughness and MOR, although there is a slight discrepancy with MOR and toughness of SGP and LGP which both exhibited the same level of porosity. Furthermore, there is good linear correlation between all the measured mechanical properties.The effect of sand additions on mechanical properties of FSGP and SGP materials was investigated with 0, 40 and 60 wt% sand (only for SGP) and the results are shown in Table5, along with comparative data for OPC. For SGP the addition of sand resulted in a small decrease in porosity over the material with no sand. The opposite is the case for FSGP. For SGP the MOR decreased with the addition of sand. However, the addition of sand led to improved MOR and Young’s modulus (rule of mixtures) for FSGP even though the porosity increased. There is almost no change in the CCS values for both the precursor types with and without sand. Figure2shows fracture toughness for materials FSGP, SGP and OPC with the differing level of sand addition. The addition of the coarse sand increases the fracture toughness as expected because the large-grained sand particles act as crack arrestors or bridging sites (see later Fig3b) similar to coarse-grained polycrystalline ceramics [14]. For 60 wt%sand addition to SGP the strength and fracture toughness are only slightly different to that at 40 wt%. The increase in fracture toughness is more marked for FSGP than for SGP with the 40 wt% addition of sand. The fracture toughness of OPC does not change markedly with the addition of sand, but in comparison with the GP materials the toughness is higher. These values appear reasonable given that the toughness of OPC is generally within the range 0.2–1.0 MPam^1/2. The magnitude of the toughness of the GPs is low indicative of extremely brittle materials due to the large pores and other defects in the microstructure but is rather poor compared to silica glass (=0.7 MPam^1/2) and fine-grained alumina ceramics (=3 MPam^1/2).

The CCS values are dependent on the porosity in general, but they are also dependent on the chemical composition [4]. In this work even with the molar ratios of Si/Al ~ 2 and Na/Al ~ 1 the mechanical properties are dictated by the porosity levels attained as a result of the precursors used. Clearly, the microstructure (i.e. porosity level, pore size, other macro defects) controls Young’smodulus and strength properties of the GPs. This is in accord with Duxson’s [15] findings that Young’s modulus is more sensitive to the microstructure than the chemical composition when applied to GPs.

      Optical images of SGP containing 40 wt% sand are shown in Fig.3. Figure 3a shows a large sand grain in the GP matrix, which is typical of all the compositions. The large round black spots ranging from about 40 micron to70 micron in diameter are macropores and these were quite common in all four GP materials produced. Figure 3b reveals a crack blunted and deflected by a large sand particle, indicating an improvement in fracture toughness of GPs with large aggregates (see also Fig.2).