Mechanical characteristics
Figures 3 and 4 show the increase of the average compressive strength and elastic modulus, respectively, versus the duration of the heat curing period at 80 °C. All measurements were performed at 28 days after casting. For compressive strength, three tests were performed for each curing condition. The elastic modulus was measured using one specimen only. Figure 3 shows that the compressive strength of the geopolymer concrete increases greatly with the increasing duration of the 80 °C curing period. The maximum average compressive strength measured was about 58 MPa and was obtained after 7 days of heat curing. Whereas the 80 °C curing for 1 day only led to an average compressive strength of about 36 MPa. However, the benefit on the compressive strength of increasing the duration of the 40 °C curing period is only moderate. The scatter of both the compressive and tension strength measurements is reasonably low for all tests (Table 3). Figure 4 shows that the elastic modulus is close to the maximum value after 1 day in the 80 °C water. There appears to be little benefit in increasing the duration of the 40 °C curing period on the elastic modulus.
Table 3 Average compressive strength f
c28 and elastic modulus E
c28 after 28 days versus heat curing duration
Drying shrinkage tests
Figure 5 compares the drying shrinkage measured on the specimens cured for 1 day at 40 °C and 1 day at 80 °C. In Fig. 5, experimental results are compared to the values calculated for an equivalent OPC based concrete using Eurocode 2 (EN 1992-1-1, [27]). Only the drying shrinkage was taken into account to calculate the time-dependent shrinkage strain for all curing regimes. The shrinkage strains measured on the specimens cured for 1 day at 40 °C were about three times the value calculated for OPC concrete in accordance with Eurocode 2 (EN 1992-1-1, [27]). However, in the case of 80 °C cure for 1 day, the drying shrinkage strain was similar or less than that specified in Eurocode 2. For the specimens cured for 3 days at 40 °C, the shrinkage strain was also similar to that specified in Eurocode 2, as shown in Fig. 6. Extending the duration of heat curing up to 7 days (Fig. 7) provides only a small benefit in terms of shrinkage reduction, even though the 28 days compressive strength of the geopolymer concrete is significantly increased (Fig. 3). It is important to note that the consistency of results over three tests is good for all curing procedures.
Figure 8 shows the average shrinkage strain obtained after 90 days for all curing conditions versus the 28 days compressive strength of the geopolymer concrete. Provided that the duration of the curing is sufficiently long and in the experimental conditions of the tests, geopolymer concrete performs well with regard to shrinkage, irrespective of the curing temperature. The effect of reduced shrinkage is related to pore size distribution as discussed in previous work by [28] where it was demonstrated that the reason for high shrinkages in alkali activated slag is related to the coarse pore size distribution. The use of fly ash in the system must have the opposite effect as compared to slag alone where the shrinkages are higher than comparable OPC concretes [28]. Indeed, Kovalchuk et al. investigated the microstructure development of heat cured geopolymer binder using low calcium fly ash and alkaline solution both very similar to the ones used in this study including the effect of thermal curing conditions on pore structure (total porosity and average pore diameter), down to a minimum pore diameter of 0.0067 μm, using a Micrometrics Autopore II 9220 porosimeter [29]. It was found that curing conditions, particularly in term of relative humidity, play an essential role in the development of a material’s microstructural characteristics (such as porosity and phase composition), kinetics and degree of reaction and their respective macroscopic properties. Large pores (10–50 μm) were observed on dried cured specimens lowering the compressive strength. Dry heat curing is not recommended for low calcium fly ash systems. On the contrary, when specimens were wet cured, the resulting material developed a very dense structure which is consistent with the results reported in this paper.
At this stage of the research, it appears that heat treatment reduces the average pore size of the low calcium fly ash geopolymer concrete resulting in reduced shrinkage in spite of the resulting increase in the capillary tension. The effect is similar to that in OPC concrete where shrinkage (and creep) is generally smaller in denser, higher strength concrete.
Creep tests
Table 4 shows the concrete compressive strength when the creep tests were started at 8 days (f
c8), for the two curing conditions. Three extra concrete cylinders were cast in order to measure the compressive strength after 8 days for each curing condition. The value of the sustained load applied to each specimen and the resulting measured instantaneous strains are also shown in Table 4.
Table 4 Compressive strength of the concretes after height days, sustained load applied during creep tests and concrete instantaneous strains
Figure 9 shows the total strains measured for all specimens including the control cylinders (not loaded) which are used to assess the drying shrinkage component of the strain. Strains measured on the control cylinders are similar for both curing conditions (3D40-curing and 7D80-curing) and is in accordance with drying shrinkage test results already discussed. The total strains measured on the creep specimens cured three days at 40 °C are much higher than those measured on the creep specimens cured for seven days at 80 °C. The total strains measured on the specimens cured seven days at 80 °C appear to be due mostly to shrinkage.
The creep coefficient can be calculated using [30, 31]:
$$\varphi (t,\;t_{0} ) = \frac{{\mathop \varepsilon \nolimits_{\text{cc}} (t,\;t_{0} )}}{{\mathop \varepsilon \nolimits_{\text{e}} }}$$
(1)
where φ(t, t
0) is the creep coefficient, t is time (in days), t
0 is the age at first loading, ε
cc(t, t
0) is the time dependant concrete strain due to creep and ε
e is the instantaneous elastic strain when the sustained loading is first applied. The time dependent experimental creep strain is calculated as the total strain minus the instantaneous strain and the time dependent shrinkage strain measured using the control specimens.
The creep coefficient as determined from Eq. 1 for the 3D40-curing and 7D80-curing tests are presented in Figs. 10 and 11, respectively and compared to Eurocode 2 (EN 1992-1-1, [27]). At early age, the creep coefficient is similar to that calculated using Eurocode 2 for specimens cured for 3 days at 40 °C. Beyond, 50 days, the creep coefficient appears to be less than that calculated using Eurocode 2. This is consistent with results in the literature [17, 20] showing that the addition of a minor quantity of GGBFS does not significantly affect creep (or shrinkage) of low calcium fly ash geopolymer concrete. After 7 days at 80 °C, the subsequent creep is very small as the creep coefficient is only about 0.2. As in the case of the shrinkage tests, the consistency of results over three tests is good for all curing procedures.
It is generally accepted that creep in OPC originates in the hardened cement paste that consists of a hydrated cement gel containing numerous capillary pores. The hydrated cement gel is made up of colloidal sheets of calcium silicate hydrates separated by spaces containing absorbed water. Creep in OPC concrete is thought to be caused by several different and complex mechanisms, including sliding of the colloidal sheets in the gel between the layers of absorbed water, expulsion and decomposition of the interlayer water within the hydrated cement gel, deformation of the aggregate and the hydrated cement gel as viscous flow and local fracture involving the breakdown (and formation) of physical bonds. The proportion of creep associated with each of these mechanisms is not yet understood despite extensive research over the last eighty years. Recent research relates the creep response of OPC to the packaging density distributions of calcium-silicate-hydrates [32]. The mechanisms of creep in fly ash geopolymer concrete are still to be determined and are likely to be different from those in OPC concrete.
Further research is required in order to investigate in detail the mechanisms such as change in surface energy or loss of disjoining pressure [33, 34] that could affect both creep and shrinkage and that could be responsible for the observed geopolymer concrete behaviour.