Investigation of different ways of activation of fly ash–cement mixtures
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Cement industry emits large amount of CO2. One of the ways to reduce this emission is to use cement replacements, such as fly ash, in binding mixtures. Blends containing fly ash exhibit different properties compared to typical portland cement. In the case of very high amount of fly ash used as substitute of cement, setting and hardening are elongated, and early compressive strength and also often ultimate strength are reduced. Thus, such blends require activation. The aim of the work was to clarify the influence of chemical activators (Ca(OH)2 and Na2SO4 used together) on hydration/activation of fly ash–cement mixtures containing about 80% of fly ash. Activated mixtures containing inert filler instead of fly ash or cement were also investigated to better understand the influence of chemical activators on each component of the blend. The research included early hydration periods (3 and 24 h) and subsequent days (till 90th day of hydration). Several methods were used: calorimetry, TG/DTG, FTIR, X-ray diffraction and SEM microscopy.
KeywordsFly ash Cement Activation Hydration Calorimetry TG/DTG
It is well known that cement industry emits large amount of CO2, mainly as an effect of decarbonation of limestone [1, 2]. One of the ways to reduce this CO2 emission is the use of cement replacements because of which the demand for portland cement clinker should be lower. Commonly used cements containing fly ash, slag, pozzolans and other components are commercially available . Nowadays, binding “green” mixtures containing ecologically friendly non-clinker components arouse interest and they are the subject of research works, e.g., [4, 5, 6, 7, 8, 9]. Some industry by-products can be used for this purpose. Thus, additional ecological benefit can be obtained resulting in utilization of these materials.
The amount of fly ash used as replacement of cement usually does not exceed 35%. However, composites containing much higher quantity of fly ash in the binding material arouse interest [8, 10, 11, 12, 13, 14, 15]. Portland cement in low-cement mixtures acts as hydraulic component and activator for fly ash. However, in the case of very high amount of conventional fly ash (70 mass% and more) and small amount of cement, the properties of early and final hardened composite are rather not satisfactory. It happens because conventional fly ash contains very low amount of calcium compounds and, for this reason, it cannot harden in the presence of water. Such kind of fly ash exhibits pozzolanic properties. It means that active forms of silica and alumina from fly ash can react with Ca(OH)2 in the presence of water. Ca(OH)2 is generated from hydration of cement, in the discussed case. Products of pozzolanic reaction are similar to those that are formed during portland cement hydration . However, in the case of very high volume fly ash (VHVFA) blends, the amount of cement may be insufficient to fully develop fly ash activity and to obtain the required properties of the final material. In such case, setting and hardening are extended, and early compressive strength and often also ultimate strength are significantly reduced. This limits the applicability of the material.
There are several methods to activate the system and to enhance its properties . Recently, using some inorganic salts (sulfates or carbonates, e.g., Na2SO4, Na2CO3) as chemical activators was proposed [10, 12, 13, 14, 15, 18]. Their impact on fly ash grains consists of increasing pH of reaction environment. It happens because these compounds can react with Ca(OH)2 arising in cement hydration. Solid products of the reaction are precipitated (CaSO4 or CaCO3 depending on the kind of activator) and alkaline hydroxide (e.g., NaOH in the case of sodium salts) is formed. In this way, pH increases and aluminosilicate fly ash grains can faster dissolve and react. Na2SO4 is often proposed as chemical activator for VHVFA mixtures, and discussions about mechanism of its action can be found in the literature [10, 12, 14]. Other chemical compounds, including organic salts , were also investigated.
Results of our previous research show that some activating effect for VHVFA pastes can be also observed in the case of exchange of small amount of fly ash by more active aluminosilicate pozzolanic material . Influence of selected chemical activators on pozzolanic and hydraulic activities of fly ash  and on cement pastes containing typical (30%)  and higher  amount of fly ash was also presented.
Results of preliminary investigation of early hydration of fly ash–cement mixtures, recently published by us [17, 23], show that it is possible to activate them by mechanical or chemical way. Interesting results were obtained in the case of combined mechanical and chemical activation based on grinding together all dry components, i.e., fly ash, portland cement, Ca(OH)2 and Na2SO4 [17, 23]. All this encouraged us to take more exhaustive research on the mechanism of chemical–mechanical activation of VHVFA mixtures. For better understanding of processes of this combined activation, research on the chemical activation (no milling process) and mechanical activation alone (without additional chemical activators) was also undertaken. Such studies are important, taking into account that hydration/activation processes are long term and formed products influence properties of the materials. Moreover, recognition of these processes can be useful in modifying and developing new activation procedures to enhance properties of fly ash–cement blends.
This work is the first part of the series of publications relating to investigation of the proposed different ways of activation of VHVFA mixtures. The aim of this research was to clarify the influence of chemical activators (Ca(OH)2 and Na2SO4) on hydration/activation of fly ash–cement mixtures, including products formed on different stages of the process and microstructure of hardened material. Na2SO4 was used as known accelerator of cement hardening, providing: increase in pH (as a result of reaction with Ca(OH)2), better solubility of fly ash grains, increase in pozzolanic reactivity as well as additional amount of sulfate for further reactions. Ca(OH)2 was proposed as additional component of the mixture to make Ca2+ available earlier for activating reaction. Moreover, excess of Ca(OH)2 can react in pozzolanic reaction in further periods. Pozzolanic activity of fly ash can be developed more, and additional amount of binding phases, such as C–S–H and C–A–S–H,1 can be formed. Thus, enhancement of properties of fly ash–cement composites can be expected.
Activated mixtures containing inert filler instead of fly ash or cement were also investigated in this work to better understand the influence of chemical activators on each component of VHVFA blend.
Materials and methods
Compositions of the investigated samples
Quantity of the components/g
Portland cement (PC)
Fly ash (FA)
Pastes were closed in small polyethylene bags immediately after mixing, and then they were stored at room temperature. After 3 h, 24 h, 7, 28 and 90 days of hydration, the samples were removed from the bags. They were crushed, and hydration processes were stopped using acetone . The samples were investigated by TG/DTG, FTIR and XRD. SEM observations were carried out on small pieces of pastes. Samples subjected to calorimetric measurements were hydrated in calorimeter at 25 °C.
calorimetric measurements—BMR calorimeter constructed at the Institute of Physical Chemistry, Polish Academy of Sciences, was used, and the results were calculated using computer software ,
thermogravimetry—SDT 2960 Thermoanalyzer (TA Instruments) was used, heating rate was 10 °C min−1, nitrogen atmosphere, and the mass of sample was 15–25 mg,
infrared spectroscopy—FTIR spectrophotometer Genesis II (Mattson) was used, and the samples were prepared as KBr pellets,
X-ray diffraction—Bruker D8 Advance diffractometer, Cu-Kα radiation, was used
SEM/EDS analysis—scanning electron microscope JEOL with an X-ray microanalyzer EDS was used.
Results and discussion
Early hydration/activation periods
Introduction of Na2SO4 and Ca(OH)2 into fly ash–cement mixture (80FA/20PC-A) causes intensification of heat release rate, which indicates acceleration of early hydration processes (Fig. 5 line 2). Induction period is reduced (it ends about 5 h after the moment of water adding), and the next period of heat release is more intense compared to the result for reference (80FA/20PC). As a consequence, shorter initial setting time can be expected compared to non-activated sample. The total heat released after 48 h of measurement is also higher compared to result received for the non-activated sample. Thus, in general, tendency of changes of heat release observed previously  was confirmed in this work.
Comparison of the results registered for activated fly ash–cement paste (80FA/20PC-A) and those for samples containing sand instead of fly ash or cement (80S/20PC-A or 80FA/20S-A, respectively) disclosed that, in early period of hydration, activating effect is mainly caused by acceleration of cement hydration. However, some influence of chemical activators on fly ash reactivity is not excluded despite the short time of hydration. Na2SO4 is easily soluble in water and can react with Ca(OH)2 increasing pH of solution. Cement minerals can be more soluble in such conditions and undergo hydration earlier. Fly ash grains need more time and high pH to dissolve. It is visible that in the case of activated sample containing sand instead of fly ash (80S/20PC–A) activating effect is higher compared to result for fly ash–cement blend (80FA/20PC–A). However, starting from 36 h of hydration, total heat released for activated fly ash–cement paste is higher and increases continuously compared to result for sand–cement sample. It indicates the development of fly ash–cement activity. This conclusion is confirmed by calorimetric results registered for mixtures without cement (80FA/20S-A). In this case, fly ash slowly and with low intensity undergoes activation. The first stage of fly ash activity development takes place between 12 and 24 h, while the second, slightly more intense, after 30 h.
- up to about 380 °C—dehydration of products such as: C–S–H phase, hydrated sulfoaluminates, hydrated aluminates, and, on very early hydration periods, also unreacted gypsum (Δ m1—Fig. 8);
- from about 380 °C to about 460 °C—dehydroxylation of Ca(OH)2 (Δ m2—Fig. 9);
from about 600 °C to about 680 °C—decomposition of carbonates;
above 800 °C—reduction of sulfates with non-burned carbon  (only for samples containing fly ash).
After early hydration period (3 h), the pastes bound a very low amount of water (estimated as Δm1, Fig. 8). Results registered for non-activated reference show that Ca(OH)2 is not precipitated at this early stage. Its presence as well as small increase in bound water is visible after 1st day. Two peaks on DTG (up to 150 °C) after 3 h of hydration likely indicate the presence of small amount of gypsum which undergoes reaction during the next hydration periods (Fig. 7a).
The courses of TG/DTG curves (Figs. 6, 7) of activated mixtures after 3 h of hydration are similar to those obtained for non-activated one (80FA/20PC). The obvious difference is the pronounced effect of dehydroxylation of Ca(OH)2 which was introduced as an activator (on DTG, clear peak with an extreme at about 400 °C is visible for all activated samples). Because Na2SO4 is the second component of the activating mixture, TG/DTG curves for samples 80FA/20PC-A and 80FA/20S-A (Figs. 6b, d, 7b, d) exhibit more clear mass loss above 800 °C compared to result for non-activated reference. This effect is not visible for activated sample composed without fly ash (80S/20PC-A) as this blend does not contain the carbon necessary for high-temperature reduction of sulfate.
On the 1st day of hydration, DTG curves registered for samples containing cement and chemical activators (Fig. 7b, c), i.e., 80FA/20PC-A and 80S/20PC-A, show untypical shape from about 360 °C to 460 °C. Two peaks are visible at this temperature range. Ca(OH)2 from cement hydration appears (DTG peaks with extreme at about 430 °C visible for all samples containing cement). The results evidence that Ca(OH)2, which precipitated during cement hydration, has different morphology and, resulting from this, different thermal stability compared to Ca(OH)2 which was introduced as an activator. TG results show that mass loss resulting from dehydration of hydrated silicate, aluminate and sulfoaluminate phases (Δm1) is the largest for activated fly ash–cement paste 80FA/20PC-A. Thus, this composition bound the greatest amount of water during 24 h of hydration.
TG and DTG curves for activated sample without cement (80FA/20S-A) exhibit significant reduction in the effect of decomposition of Ca(OH)2. Such results indicate that Ca(OH)2 quickly undergoes reaction in the system (reaction of Ca(OH)2 with Na2SO4 and start of pozzolanic reaction). Thus, conclusions based on the results of calorimetric measurements regarding the development of fly ash activity in the presence of chemical activators were confirmed.
For comparison, in the case of non-activated blend, FTIR spectrum collected after 1st day has almost the same shape as spectrum after 3 h. (Only insignificant broadening of the main fly ash band toward lower wavenumbers was observed.) Thus, conclusions presented above, based on calorimetric and thermogravimetric measurements, were confirmed. Chemical activators influence fly ash grains and stimulate their reactivity just after several hours after addition of water.
IR spectra collected for mixture in which fly ash was replaced by sand (80S/20PC-A, Fig. 10c) show influence of chemical activators on cement hydration. Intense bands at 1112 cm−1 and about 620 cm−1 result probably from ettringite. Bands characteristic for silicates of portland cement change position. Moreover, new band at 668 cm−1 appears. It may confirm the formation of C–S–H-type products .
Later hydration/activation periods
During the next days of hydration, fly ash develops its activity further and cement undergoes further hydration. As a result, the amount of bound water (presented as the mass loss Δm1, Fig. 8) increases with time. In the case of non-activated reference paste (80FA/20PC), Ca(OH)2 content rises till 7th day which confirms that hydration of cement predominates over pozzolanic reaction. After the 7th day, amount of Ca(OH)2 decreases (gradual reduction in mass loss at 410–460 °C over time, Fig. 9)2 and it is completely invisible after the 90th day. It affirms that pozzolanic activity in non-activated fly ash–cement blend develops after 7 days of hydration. Thus, previous results  obtained for fly ash–cement mixture were confirmed. Changes of the DTG shape (Fig. 7a) at temperature range up to 200 °C indicate that C–S–H is the main product of reference paste on early days of hydration, while, starting from 7th day, hydrated aluminates and aluminosilicates are also present. They are visible by the presence of DTG peak at about 140 °C. A broad peak of small intensity at about 300 °C is also observed. Intensities of these effects increase with time starting from 7th day of hydration.
TG/DTG results show that the form of Ca(OH)2 which has lower thermal stability can be relatively fast bound in other compounds. It is especially visible in the case of sample in which the only source of Ca(OH)2 is the one introduced as an activator (80FA/20S-A). On the other hand, blend containing sand (instead of fly ash) and cement, 80S/20PC-A, shows the presence of both forms of Ca(OH)2 without its reduction over time. This is because the composition does not contain pozzolan which can react with Ca(OH)2. Comparison of the TG/DTG results obtained for activated fly ash–cement pastes (Figs. 6b and 7b) with those for samples without cement (Figs. 6d and 7d) show that introduced Ca(OH)2 was bound up to 90th day of hydration and C–S–H and ettringite are the main products of hydration/activation processes. In the case of 80FA/20S-A sample, hydrated aluminates are probably not formed or only in a small degree, similarly as in the case of sample without fly ash (Figs. 6c and 7c).
Transformation of silica and aluminosilicate components of fly ash over time toward new silicate and aluminosilicate products is also confirmed in IR spectra (Fig. 10). In the case of non-activated fly ash–cement paste (80FA/20PC, Fig. 10a), the main fly ash band widens and gradually forms a new extreme at lower wavenumbers. The extreme visible at about 970 cm−1 on 28th day indicates formation of C–S–H phase. Moreover, a new band at about 425 cm−1 appears on 7th day of hydration. For activated fly ash–cement blend (80FA/20PC-A, Fig. 10b), the bands confirming formation of new aluminosilicate phases are visible earlier and more clearly. For example, on 28th day of hydration the band at 1025 cm−1, clear and intense band at 962 cm−1 and 730 cm−1 are visible.
Figures 12 and 13 present microstructure of activated fly ash–cement binder after 28th day of hydration. There are visible products of hydration/activation formed around grains of fly ash. Figure 13 shows different components which can be found in the microstructure: region enriched in calcium (points 1 and 3), non-reacted aluminosilicate fly ash grains (point 2), C–A–S–H product (point 4) and aluminosilicate product enriched in sulfur (point 5).
Chemical compounds, Na2SO4 and Ca(OH)2, can activate fly ash–cement mixture. It is evidenced, compared to non-activated fly ash–cement blend, by: shortening of induction period and intensification of period related to precipitation of C–S–H, increasing total heat released after 48 h of hydration, higher amount of bound water, reduction of Ca(OH)2 and quicker precipitation of hydrated products resulting from faster development of pozzolanic activity.
Ettringite is one of the hydration products, formed in higher amount in chemically activated mix as an effect of introduction of sulfate.
Chemical activators accelerate cement hydration and enhance reactivity of fly ash grains. In activated fly ash–cement binder, synergic effect takes place.
In early hydration hours, the presence of cement is mainly responsible for activating effect of fly ash–cement mixture. During 24 h of hydration, fly ash starts to react.
Two kinds of Ca(OH)2 can be present in activated fly ash–cement system: hydroxide introduced as component of activating mixture and the one precipitated as cement hydration product.
The knowledge about kinetics of chemical and physical processes of hydration/activation and products that are formed is key factor to develop new more ecological binders which could replace cement in the future. Results of investigation on other ways of activation of such systems (i.e., very high volume fly ash mixtures) will be discussed in next works.
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