Effect of heat treatment temperature on ground pumice activation in geopolymer composites

1 Introduction

Geopolymers are a family of emerging synthetic compounds that were first proposed by Joseph Davidovits in the late 1970s [1, 2]. Alkali-activated materials are produced through chemical reaction between a highly reactive aluminosilicate source and an alkaline solution. Geopolymers are amorphous three-dimensional aluminosilicate materials with ceramic-like properties that are produced and hardened at ambient temperature. Geopolymerization is realized under highly alkaline conditions; in the presence of alkali hydroxide and silicate solution, polymerization takes place when reactive aluminosilicates are rapidly dissolved and free [SiO₄] and [AlO₄] tetrahedral units are released in solution [1, 3]. The tetrahedral units are alternatively linked to polymeric precursors by sharing oxygen atoms, thus forming amorphous geopolymer. Positive ions such as K⁺ or Na⁺ that are present in geopolymer framework cavities balance the negative charge. For the chemical designation of geopolymers based on silicoaluminates, the poly(sialate) term that is an abbreviation for silicon-oxo-aluminates has been proposed. Poly(sialates) are chain and ring polymers with Si⁴⁺ and Al³⁺, and their general formula is M_n[-(SiO₂)_z-AlO₂]_n·wH₂O, where M is a monovalent cation such as K⁺ or Na⁺, n is the degree of polycondensation, and z is 1, 2, or 3. Chains and rings are formed and cross-linked together always through a sialate Si-O-Al bridge [3, 4–8]. The preparation of geopolymer cement by using pumice reduces environmental pollution. CO₂ emissions from cement production are incurred through the consumption of fossil fuels, the use of electricity, and the chemical decomposition of limestone during clinkerization, which can take place at around 1400°C [9]. Cement is formed from the calcination of limestone and silicoaluminous material according to the reaction:

5CO2+2SiO2→(3CaO, SiO2) (2CaO, SiO2)+5CO2

One tonne of cement produced directly generates 0.55 t of chemical CO₂ and requires the combustion of carbon fuels to yield an additional 0.40 t of CO₂ [1]. To simplify, 1 t of cement=1 t of CO₂.

2 Materials and methods

2.1 Pumice

Natural pumice used in the present study was obtained from Hasankale region near Erzurum located in the east of Turkey. The pozzolan was first characterized for its chemical composition, shown in Tables 1 and 2. X-ray diffractogram for the ground pumice showed a highly amorphous phase in sample textures as shown in Figure 1 [10].

All of the powdered pumice was finer than 200 μm, and 93.8% was finer than 90 μm. The specific surface according to the Blaine method was 2980 cm²/g and the density of ground pumice was 2.38 g/cm³. The pumice was ground in a Fritsch mill (Idar-Oberstein, Germany) (Figure 2). Particle size distribution has a very important effect on geopolymer cement specification. The particle size distribution for ground pumice is illustrated in Figure 3.

Table 1

Chemical composition of Hasankale ground pumice.

SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	CaO (%)	MgO (%)	K2O+Na2O (%)	Others (%)	LOI (%)
67.08	14.06	1.91	0.87	0.25	0.11	15.72	3.94

Table 2

Mineral content of Hasankale pumice from XRD analysis.

No.	Mineral name	Framework formula
1	Cristobalite, high	SiO2
2	Silicium dioxide	SiO2
3	Labrodorite	(Na0.4Ca0.6)Al1.6Si2.4O8
4	Cristobalite, low	SiO2
5	Albite, high, sodium tectosilicate	Na(AlSi3O8)

Figure 1

X-ray diffraction pattern of Hasankale pumice.

Figure 2

FRITSCH mill used for grinding the pumice.

Figure 3

Particle size distribution of ground Hasankale pumice.

3 Results and discussion

For the investigation of the effect of heat treatment on geopolymer activation, nine mixes were designed and prepared (Table 6). After the geopolymers were cast in the cube molds and cured at 25°C, 45°C, 65°C, and 85°C the samples’ 28-day compressive strength was measured according to ASTM C39. Then, the Na₂O content-compressive strength diagrams were obtained from the test results. These diagrams are shown in Figures 4–7 for 25°C, 45°C, 65°C, and 85°C, respectively. If silica modulus is constant (i.e., 0.52, 0.60, and 0.68), with increase in Na₂O content (i.e., 4%, 7%, and 10%) the compressive strength will also increase for all curing temperatures (25°C, 45°C, 65°C, and 85°C), and with increasing silica modulus at constant Na₂O content the cube geopolymer samples’ compressive strength also increases. The main reason for the lower compressive strength in the 4% and 7% Na₂O content is insufficient content of ions in solution that are required for dissolving ions from pumice. From the curing temperature-compressive strength diagram shown in Figure 8, we can observe that the compressive strength increases with increasing curing temperature up to 65°C, but at temperatures higher than 65°C the compressive strength decreases for all curing temperatures. We can notice differences between increasing rate in mixes from the diagram, but for all of them, increase in curing temperature causes better compressive strength until 65°C, so 65°C is considered the optimum curing temperature.

Figure 4

The effect of Na₂O on 28-day compressive strength for samples cured at 25°C and constant MS.

Figure 5

The effect of Na₂O on 28-day compressive strength for samples cured at 45°C and constant MS.

Figure 6

The effect of Na₂O on 28-day compressive strength for samples cured at 65°C and constant MS.

Figure 7

The effect of Na₂O on 28-day compressive strength for samples cured at 85°C and constant MS.

Figure 8

The effect of heat treatment temperature on 28-day compressive strength.

Additionally, the Na₂O content-compressive strength diagrams were obtained from test results for constant and varying values of water/binder (w/b). The diagrams are shown in Figures 9–12 for 25°C, 45°C, 65°C, and 85°C, respectively. With constant w/b ratio (i.e., 0.36, 0.40, and 0.44) and increasing Na₂O content (i.e., 4%, 7%, and 10%), the compressive strength increases for all curing temperatures (25°C, 45°C, 65°C, and 85°C). With increasing w/b ratios at constant Na₂O content, the cube geopolymer samples’ compressive strength decreases.

Figure 9

The effect of Na₂O on 28-day compressive strength for samples cured at 25°C and constant w/b.

Figure 10

The effect of Na₂O on 28-day compressive strength for samples cured at 45°C and constant w/b.

Figure 11

The effect of Na₂O on 28-day compressive strength for samples cured at 65°C and constant w/b.

Figure 12

The effect of Na₂O on 28-day compressive strength for samples cured at 85°C and constant w/b.

In addition to investigation of the effect of Na₂O content on geopolymer compressive strength, the effect of aging on the compressive strength gain was also investigated at 25°C, 45°C, 65°C, and 85°C curing temperature at 3, 7, 28, and 90 days. The results obtained from compressive strength tests are shown in Figures 13–16. The aging of the samples before 28 days has a significant effect on compressive strength gain, but after 28 days this effect is insignificant. Gradual drying of the samples after the first 7 days may be the main reason for the decrease in compressive strength gain.

Figure 13

The effect of aging on geopolymer compressive strength for samples cured at 25°C.

Figure 14

The effect of aging on geopolymer compressive strength for samples cured at 45°C.

Figure 15

The effect of aging on geopolymer compressive strength for samples cured at 65°C.

Figure 16

The effect of aging on geopolymer compressive strength for samples cured at 85°C.

4 Conclusion

1. If the silica modulus is constant while Na₂O content increases, the compressive strength increases for all curing temperatures, and with increasing silica modulus at constant Na₂O content, the cube geopolymer samples’ compressive strength also increases.

2. The best curing temperature for Hasankale-based geopolymer activation is 65°C.

3. With increase in Na₂O content (i.e., 4%, 7%, and 10%), the compressive strength increases for all curing temperatures (25°C, 45°C, 65°C, and 85°C), and with increasing w/b ratios at constant Na₂O content the cube geopolymer samples’ compressive strength decreases.

4. The aging of the samples before 28 days has a significant effect on compressive strength gain, but after 28 days this effect is insignificant.