The effect of delayed ettringite formation on fine grained aerated concrete mechanical properties

2.1 Materials and mixture design

For sample preparation, the cement CEM I 42.5 R was used. In order to ensure the formation of the alumina-ferric oxide-monosulphate phase (AF_m), we used fly ash with defined components and a specific surface area. The role of the fly ash was two-fold – chemical and physical, whereby the chemical components are actively involved in the pozzolanic reaction, while the physical ones work as nuclei sites and filler. We used chemically neutral standard sand with properties according to EN ISO 196-1 and ISO 697 for the aggregate. The purpose of adding a chemically neutral aggregate was to prevent an alkali silica reaction, which would result from reactive minerals in the aggregate. The most important precaution was the use of the petrographic components of aggregates that will not react with alkali [17]. The used additive – an air entraining agent – was the anion type based on abietic acid (TKK d.o.o., Srpenica, Slovenia). The components were mixed with clean drinking water.

2.2 Test procedures

Concrete mixture components were determined on the basis of a standard consistency of cement paste. We prepared three different series of samples. The composition of mixtures is shown in Table 1. Samples were mixed with a laboratory mixer to the requirements of EN 196-1. Fresh concrete was built in standard moulds with dimensions of 40/40/160 mm by a vibrating table with vibration of 5 s, frequency of 50 Hz and amplitude of 0.75 mm. Samples were tended for 28 days in a climate chamber at a temperature of 20±2°C and a relative humidity of 98±2%. Measurements of density, compressive strength and flexural strength were performed on all samples at intervals of 7, 14, 28 and 56 days. The set of samples that contained fly ash were subjected to the Duggan test after 28 days of treatment in the climatic chamber. This test is used to achieve accelerated DEF. The test consists of soaking the samples in demineralised water at 20±2°C and drying them at a temperature of 81±2°C. After the Duggan [18] test, the prisms were laboratory conditioned for 48 h in a desiccator, in-between each of the above phases, and were then once again immersed in demineralised water for 24 h in order to fill the capillaries and voids with water. In these samples, we controlled DEF, by measuring the expansion as a result of ettringite crystal growth.

At the moment when, in micrometres, expansion was no longer observed, we stopped the measurements. We considered that DEF had been completed. Measurements of density and strength were then performed on the samples. These measurements were repeated on the samples on which the Duggan test was not carried out, so we could obtain the comparisons of these values.

3.1 The results of investigations of concrete components

Cement, standard sand and air entraining agent were used as the benchmark for standard-quality results. Therefore, declared properties are not included. Fly ash contains silicates, carbonates and phosphates of calcium, magnesium, iron and aluminium and other elements. Illite-kaolinite clays, apart from illite and kaolinite minerals, also contains a-quartz, Fe₂O₃ and CaCO₃ [19]. The results of the laboratory analysis for fly ash are shown in Table 2.

3.2 Characterisation of the macrostructure

We observed significant break areas of the hydrated concrete using the optical microscope Olympus SZX. The control of dispersion bubbles of the air entraining agent showed that they were mainly uniformly distributed. The bubbles had a diameter between 25 and 50 μm. The average measured distance between the bubbles was 0.1 to 1.2 mm.

3.3 Scanning electron microscopy characterisation

The characteristic fields of air entraining bubbles were observed with a scanning electron microscope (SEM) JEOL JSM 5610 (JEOL Ltd., Tokyo, Japan) and QUANTA 200 3D (FEI, Eindhoven, Netherlands). We discovered ettringite crystals in these bubbles. This proved our assumption that the bubbles of air entraining agents are nucleation sites. The morphology of ettringite crystals is very similar. Ettringite crystals grow in bunches, and all crystals are needle-like and thin. The observation of a large number of bubbles showed that the sample labelled AI had less crystals of ettringite as shown in Figure 2. The samples that were labelled BI and CI showed many more bunches of ettringite crystals. These crystals are mainly grown on porous sites in air bubbles – see Figure 3. More microcracks in bubbles were noticed on the samples labelled BI-DT and CI-DT compared to the samples labelled BI and CI. In these microcracks, we could also observe the presence of very thin and needle-like ettrigite crystals suggesting their rapid growth – see Figure 4.

Figure 2

Micro fractography of the sample AI with ettringite crystals; scanning electron microscope (SEM), secondary electron imaging (SEI).

Figure 3

Micro fractography of the sample BI with ettringite crystals; SEM, SEI.

Figure 4

Micro fractography of the sample BI-DT microcrack on the wall of air bubble with ettringite crystals; SEM, SEI.

The comparison of the microstructures of the specimens from fine grained concrete mixes AI, BI and BI-DT presented in Figures 2–7 clearly demonstrates that, as expected, similar air-entraining agent (AEA)-induced nuclei exist in the AI, BI, CI, BI-DT and CI-DT specimens. Although ettringite crystals can be found in concretes produced by using pure Portland cement, no visible ettringite crystals were detected in any of the large number of prisms from the mix AI. However, ettringite crystals did appear in specimens of all other fine grained concrete mixes. Microstructures of the specimens from the fine grained concrete mix AI show very little ettringite crystals in AEA nuclei themselves, and they were detected in microcracks. Similar to Myneni et al. [20], these crystals have thin, needle-shaped morphology and are approximately 2 μm in length, revealing rapid growth. Fly ash in fine grained concrete mix BI may well be a source of soluble calcium for ettringite formation, as reported by Solem and McCarthy [21], Zhang and Reardon [22] and Chrysochoou and Dermatas [23], because its crystals were found in greater quantities in microcracks and within the AEA-induced nuclei. Figures 2 and 3 show that ettringite crystals have thin, needle-shaped morphology, but those found in microcracks are only approximately 2 μm long as opposed to the 10 μm long crystals found in the nuclei. The microcrack that appeared on the surface of the nucleus in Figure 5 can be associated with the shrinkage of the matrix during hydration [24]. The comparison between various BI and CI specimens shows that ettringite crystals start growing wherever there is enough space for growth before further damaging concrete, which enables further growth. AEA-induced nuclei may, therefore, act as relief reservoirs enabling the growth of substances like ettringite crystals in hardened concrete with minimum or no damaging effects. Hime [25] even recommends air entrainment as a way to prevent DEF and reports on only a single incident where air-entrained concrete suffered from DEF.

Figure 5

Micro fractography of the sample BI-DT air bubble with microcracks and ettringite crystals; SEM, SEI.

Figure 6

Micro fractography of the sample CI with ettringite crystals on a porous site; SEM, SEI.

Figure 7

Micro fractography of the sample BI-DT microcrack with ettringite crystals; SEM, SEI.

3.4 Thermal gravimetric analysis

Pozzolanic materials can partially substitute Portland cement in order to enhance the properties of concrete and mortars such as durability and mechanical properties. The interaction between additions such as fly ash and Portland cement can be investigated by various techniques such as thermal analysis and X-ray diffraction. Thermal gravimetric analysis (TG), derivative thermogravimetry and differential scanning calorimetry are considered important tools to evaluate the nature of hydrated products according to different cement hydration, in addition to quantifying the different phases.

Simultaneous thermal analysis and TG analysis were performed on one sample of cement paste of concrete samples AI, BI and CI in a static air atmosphere. Measurements were performed with the device Netzsch STA 449C Jupiter (NETZSCH, Selb, Germany). We measured the change in mass of the sample at 25°C. Each measurement lasted 60 h. The maximum change in mass, which was 19.9%, was found in a sample of cement paste of the concrete sample BI, after a time of 446.8 min. The change in mass of the sample of cement paste – the cement sample AI – was lower and amounted to 19.7% at the time of 438.2 min, and it was higher for the sample CI amounting to 19.9 at the time of 462.9 min – see Figure 8. TG measurements have shown that by modifying the mixture of a concrete sample, changes with the fly ash did not transgress the setting time of concrete, which must be >360 and <540 min.

Figure 8

Mass degradation of AI, BI and CI according TGA.

From the thermogravimetric investigations performed, which are shown in the Tg curves, it can be concluded that fly ash and AEA additions have no significant effect on the hydration of cement.

3.5 XRM analysis

XRM analyses were performed with electron scanning microscope JEOL JSM 5610 on samples which are fracture sites of hydrated fine grained concrete labelled BI-DT and CI-DT. We analysed the particular field with crystals of ettringite as shown in Figure 9. Based on the results, it was suggested that in areas of air bubbles in the cement matrix, the sulphate attack is completed. The purpose of this study was to determine the chemical composition of ettringite crystals of the fine grained concrete mixes AI, BI and CI and in the samples BI-DT and CI-DT on which Duggan’s test was performed. These results confirm that the process of DEF is completed too. Chemical elements sulphur and aluminium, which are characterised by a chemical reaction, are present only in sites with ettringite crystals.

Figure 9

XRM analysis of ettringite crystal in the air bubble of AEA: (A) specimen BI-DT; (B) specimen CI-DT.

3.6 pH measurement

The fall of pH usually indicates the occurrence of reaction of fly ash. The pozzolanic activity of fly ash depends on the pH value of the pore solution. Sampling of fine grained hydrated concrete was done by breaking off a piece. This piece of concrete was crushed and sifted through a sieve with an aperture diameter of 0.02 mm. Fifty grams of sample was mixed with a laboratory stirrer with 10 ml of distilled water. The solution was filtered before being tested through a filter paper N°40. The measurement procedure was taken from the literature [26]. Measurements were performed with the pH meter Mettler Toledo S20 (METTLER TOLEDO, Greifensee, Switzerland) on three samples of each type of concrete at intervals of 1, 2, 3 and 4 min. For each sample, we performed three measurements and calculated the mean value. The results of mean values are presented in Table 3. Based on the measurements, we found that the pH of hydrated concrete with fly ash was reduced due to the Duggan test, although this value did not change significantly. It is known that the pH of concrete with fly ash is less than the pH of concretes without it. This is due to the binding of Ca(OH)₂ with reactive silicates contained in fly ash [27].

3.7 Expansion measurement

After a required 28-day curing period, six prisms of the mix BI and CI were exposed to the Duggan test in order to achieve the accelerated ettringite formation. The prisms were then placed into a standard apparatus for the determination of length change of hardened cement paste, mortar and concrete, constructed according to ASTM C490-86. During these measurements, the apparatus itself was placed in a climatic chamber with a constant temperature of 20±2°C and relative humidity of 98±2% (see Figure 10).

Figure 10

The apparatus for the measurement of expansion of hardened concrete according to ASTM C490-86 placed in a climatic chamber.

Ettringite formation was then monitored by measuring length change (expansion) with the Mahr’s MarCator 1080/12.5/0.005 mm digital micrometer (MAHR, Esslingen, Germany). The results were recorded with an analogue/digital converter connected to a workstation. Developing expansion was measured regularly at 15-min intervals with a measurement accuracy of 0.005 mm, although intervals could well be longer considering the slow pace of DEF.

Figures 11 and 12 show the change of length for six fine grained concrete prisms from the mixes BI and CI that were exposed to the Duggan test and a final 24 h immersion in demineralised water. The change of length of prisms was measured daily and stopped at prisms for the mix BI after 73 days and for the mix CI after 82 days when measurements did not show any further expansions. The results of measurements of the length change (expansion) of prismatic samples were evaluated as an average value, which is equal to 0.0125%. All samples were inspected visually, and no cracks were found.

Figure 11

The results of expansion measurements for samples labelled BI-DT after the Duggan test.

Figure 12

The results of expansion measurements for samples labelled CI-DT after the Duggan test.

3.8 Mechanical properties

The density of hardened fine grained concrete (ρ), its compressive strength (f_c) and flexural strength (f_m) were measured on six additional prisms for each of the three mixes after standard 7, 14 and 28 days and additionally after 56 and 121 days when compressive and flexural strengths should reach a plateau. The mechanical properties of fine grained concrete were examined with a universal dynamometer Zwick Roell (Zwick/Roell, Ulm, Germany) and a method according to EN 196-1.

Tables 4, 5 and 6 show measured densities (ρ), flexural (f_m) and compressive strength (f_c) with the value of standard deviation (σ) of hardened fine grained concrete prisms for mixes AI, BI, CI, BI-DT and CI-DT. The compressive strength of (BI) prisms after 121 days is 6.9% lower than that of prisms (BI-DT), and the compressive strength of (CI) prisms after 112 days is 6.8% lower than that of (CI-DT) prisms. The comparison of results showed that concrete with fly ash has a slightly lower early strength and increased final strength [28].