Keywords

1 Introduction

More and more industrial wastes are involved in the building ceramic production. Sewage sludge ashes (SSA) are promising secondary sources for building ceramics production.

SSA properties were studied in a number of works [1,2,3], but the results were not widely used in building ceramic industry, because SSA ceramic products not always have the necessary strength characteristics. It is due to the SSA chemical composition, which affects to the phase composition formation of ceramic.

Ceramics phase composition can be controlled by adjusting the ceramic mass composition using special additives. The most suitable modifiers for clay minerals structures are nanosized oxides of silicon and aluminum, which are realized in hydrosols [4].

Earlier in [5], it was shown that the drinking water treatment sludge filtrate, obtained after high-speed filters in the Novosibirsk water utility filtering station, is a sol of a silicate composition with a nanoscale particle size.

The purpose of study was to assess the drinking water treatment sludge filtrate effect on the SSA-clay ceramics phase composition and structure formation.

2 Materials and Methods of Research

Kamenskoye deposit (Novosibirsk region) montmorillonite-hydromica clay was used in this study.

Sewage sludge was obtained from Novosibirsk wastewater treatment plant (map No. 39). The sludge was burned in laboratory furnace at temperatures of 850 °C. Drinking water treatment sludge filtrate, obtained after high-speed filters in the Novosibirsk water utility filtering station and separated from the coarse fraction, was used as a modifying additive in the study.

Ceramic mixtures compositions are presented in Table 1.

Table 1 Ceramic mixtures compositions
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Weighed portions of the mixtures with different additives amounts were mixed with water or drinking water treatment sludge filtrate. Samples were dried at 110 °C. Then the SSA-clay samples were sintered at 1100 °C with holding at a maximum temperature for 1 h.

Differential thermal analysis (DTA) was performed to identify the phase formation features of the SSA-clay samples. X-ray phase analysis (XPA), which was performed on a Bruker D8 Advance diffractometer using Cu-Kα radiation, was used to determine the phase composition of SSA-clay ceramic samples. The PDF2 database with the Search-Match shell was used to identify the obtained diffraction patterns.

Element maps and microstructure images of SSA-clay ceramic samples were obtained using Hitachi TM 3000 electron microscope with an energy dispersive analyzer.

3 Results and Discussion

The endothermic effect at 50–150 °C temperature range is due to the sorbed water loss by clay minerals. In this case, there is a loss of mass—4.5%. The weight loss occurs at a much lower rate with further mixture heating. Heat absorption is caused by the release of chemically bound water by clay minerals in the 300–500 °C range. Kaolinite is initially dehydrate, turning into metakaolinite, and then decompose into oxides at 750–900 °C [6] (Fig. 1).

Fig. 1
figure 1

DTA results (composition № 4)

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The endothermic effect at 700 °C is associated with the montmorillonite crystal structure destruction. According [7], calcium carbonate dissociates in the 770–1010 °C temperature range.

Exothermic effects on the heating curve are due to new crystalline phases formation. The total weight loss for the test sample was 10.8%.

According to the XPA data (Fig. 2), for all SSA-clay samples after sintering at 1100 °C, the main identified phases are anorthite (3.10; 4.05 A0), quartz (3.35 A0) and hematite (2.7 A0).

Fig. 2
figure 2

XPA diffraction patterns of ceramic samples: a—50% SSA: 50% clay: 0% drinking water treatment sludge filtrate; b—50% SSA: 50% clay: 0.05% drinking water treatment sludge filtrate; c—50% SSA: 50% clay: 0.1% drinking water treatment sludge filtrate; d—50% SSA: 50% clay: 0.25% drinking water treatment sludge filtrate

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The anorthite (CaO · Al2O3 · SiO2) formation is due to solid-phase reaction between amorphous γAl2O3 formed in SSA from aluminum oxychloride and calcium silicate.

Another possible anorthite formation mechanism may be associated with the reaction between amorphous silica and calcium oxide. According study [8], kaolinite dehydration calcium carbonate dissociation are promoted by the anorthite crystallization in clay. The reaction equation has the form (Eq. 1):

$$ {\text{Al}}_{2} {\text{O}}_{3} \cdot 2{\text{SiO}}_{2} + {\text{CaCO}}_{3} = {\text{CaO}} \cdot {\text{Al}}_{2} {\text{O}}_{3} \cdot 2{\text{SiO}}_{2} + {\text{CO}}_{2} $$
(1)

The phase composition results (Table 2) indicate to increase anorthite content with adding drinking water treatment sludge filtrate.

Table 2 SSA-clay ceramic samples quantitative phase composition
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The microstructure of all samples is characterized by uniform pores and crystalline quartz compounds distribution. At the same time, SSA-clay samples with drinking water treatment sludge filtrate addition are still noticeably different in relative position of crystalline and amorphous phases.

The SSA-clay with drinking water treatment sludge filtrate additive samples microstructure feature is matrix microstructure. The spatially arranged matrix microstructure has phase composition differences between filler and binder, which confirmed by the electron microscopy data (Fig. 3d, f). Matrix formed from clay particles is a “binder”. “Filler” formed from quartz grains contained in sewage sludge ash. It is important to note that matrix include amorphous and crystalline anorthite phase. Anorthite crystals reinforce matrix SSA-clay samples microstructure, which provides higher strength and frost resistance.

Fig. 3
figure 3

Microstructure and elemental composition of SSA-clay ceramic samples: a, c, e—composition 1, b, d, f—composition 4

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SSA-clay samples with drinking water treatment sludge filtrate additive are characterized by increased density from 2.15 to 2.21 g/cm3 and compressive strength from 27.60 to 37.15 MPa (Table 3), which may be due to their matrix microstructure and increased anorthite phase content (from 64 to 68%).

Table 3 SSA-clay ceramic samples water absorption (W, %), density (ρ, g/cm3), compressive strength (R, MPa)
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4 Conclusions

DTA, XPA and electron microscopy results allow us to conclude that:

  1. 1.

    SSA-clay samples phase composition formation is determined by calcium ions diffusion into the metakaolinite structure and anorthite crystallization from melt enriched with aluminum, calcium and silicon oxides.

  2. 2.

    SSA-clay samples with drinking water treatment sludge filtrate additive are characterized by increased density and compressive strength, which may be due to their matrix microstructure and increased anorthite phase content.