Characterization of cement stabilized pond ash using FTIR spectroscopy

https://doi.org/10.1016/j.conbuildmat.2020.120136Get rights and content

Highlights

Abstract

Pond ash is a mixture of fly ash and bottom ash obtained from thermal power plants. It has pozzolanic properties and is commonly treated with cement to impart cohesion and improve strength. The increase in strength gain in cemented pond ash is attributed to the hydration of cement and pozzolanic reactions due to silica present in the pond ash. Unconfined Compressive Strength (UCS) test is commonly used to determine the dosage of cement. While the strength gain is addressed by this test, the interaction mechanisms and the resulting compounds that are responsible for strength gain in such systems have not been identified at this point in time. In this study, an attempt is made to understand the interaction mechanisms between cement and pond ash by quantifying the influence of the dosage of cement, curing period, and curing temperature. FTIR spectroscopy is used for this purpose. Five dosages of cement (2, 4, 6, 8, and 24%), three variations in curing period (7, 14, and 28 days) and two different curing temperatures (27 °C and 50 °C) were adopted. Results showed that pond ash, used in this study, did not undergo any hydration reaction due to its low calcium content. In cemented pond ash, the hydration products formed in cement (Ca(OH)2) triggered pozzolanic reactions with silica present in pond ash to further form C-S-H, which is responsible for increased strength gain. The C-S-H formation did not vary linearly with cement content/curing period/curing temperature due to the pozzolanic reactions and the magnitude of influence exerted by each of these parameters depended on the other two parameters.

Introduction

Coal ash, a by-product from thermal power plant, is obtained due to combustion of pulverized coal. The thermal power plants produce three types of ashes namely, fly ash, bottom ash, and pond ash. Fly ash is collected by mechanical or electrostatic precipitators from the flue gases of the power plant and the bottom ash is collected from the bottom of the boilers. Pond ash is a mixture of fly ash and bottom ash transported in slurry form and deposited in lagoons. There are 132 thermal power plants in India and they generate 83.64 million tons of ash annually. However, only 56% is currently being used [1] and the remaining are disposed as waste. A number of applications are identified using the advantageous properties exhibited by each of these ashes.

Pond ash offers various advantages in filling applications due to its properties such as water insensitiveness during compaction, high coefficient of consolidation (1.76–4.90 cm2/min) and good frictional strength (angle of internal friction, ϕ = 29°–41°) [2], [3]. Hence, it can be used as an alternative to conventional materials for applications such as backfill, embankment construction and subbase or base courses in road construction. However, there are concerns related to low compacted density, lack of cohesion and erodability. In order to mitigate these issues, stabilization of pond ash is generally carried out. There are different types of stabilizers currently available for pond ash including cement, lime, soil and fibres [4], [5]. Various researchers have conducted studies on cemented ash in laboratories and found that cement stabilization increases strength in a shorter duration as compared to other stabilizers [5], [6], [7], [8].

The effect of cement stabilization depends on factors such as dosage of stabilizer and the curing period. Currently, the dosage of cement is determined based on the Unconfined Compressive Strength (UCS) test. According to the Electric Power Research Institute (EPRI) design manual, the minimum UCS value for cement stabilized fly ash base course is 2760–3100 kPa [9]. The minimum cement content to attain the required UCS value is fixed as the cement content for the design mix. While the strength gain can be estimated through such tests, the magnitude of influence exerted by the factors leading to strength development cannot be ascertained. The strength development can depend on a number of variables including the chemical composition of pond ash, the type of cement used, the dosage of cement which is determined by the interaction between cement and pond ash, the curing period and the atmospheric conditions, especially the temperature and humidity. Through strength based tests, one can only ascertain the combined influence of these variables and not the effect of individual parameters. To estimate the influence of each of these variables, it is necessary to understand the interactions developed during cement stabilization of pond ash.

Limited literature is available regarding the influence of curing variables on the cement stabilization of pond ash [10]. A number of studies are available that focus on the same for replacement of cement with fly ash in cement and concrete production [11], [12], [13], [14], [15], [16]. Though cement stabilization of pond ash involves addition of smaller quantities of cement (say, less than 10%) to pond ash, few hypotheses related to the interactions can be drawn from such literature. Also, the pond ash and fly ash are different from their storage perspective and presence of reactive silica. Pond ash has lower amount of reactive silica as the reactivity is lost by ponding [3]. However, their chemical composition is similar and hence their interactions may be assumed to be similar.

The cement treatment of pond ash is assumed to be characterized by two important chemical reactions [17]. The first one is the primary hydration of cement which results in the formation of primary cementitious products such as C-S-H, C-A-H, Ca(OH)2 and ettringite responsible for short-term strength gain. The second one is the reaction of Ca(OH)2, formed as a result of hydration of cement, with silica and alumina present in pond ash to form C-S-H of lower Ca/Si ratio. These reactions are known as pozzolanic reactions and they are responsible for long-term strength gain. The Ca(OH)2 content reduces on the onset of pozzolanic reactions and its measure is found to be a good indicator of pozzolanic activity. Several researchers have studied the decrease of Ca(OH)2 content or the increase in C-S-H content, which are indicators of pozzolanic reactions, using different techniques such as XRD, TGA and FTIR [18], [19].

The pozzolanic reactions may not take place immediately as cement is added to pond ash. Pond ash acts as an inert material in the initial period (say, 2–7 days) after the addition of cement and no significant hydration occurs [6]. The formation of C-S-H and C-A-H gel occurs only when the glassy phase in the pond ash (for instance, alumina-silica-glass from bituminous coal based pond ash) goes into the solution. Once the glass network breaks and the glassy phase gets into the solution, pond ash acts as nucleation sites for Ca(OH)2, C-S-H and C-A-H gel formation. The onset of pozzolanic reactions depends on several factors, such as the amount of pond ash and cement, water content and the curing temperature [14]. Feldman et al. [20] conducted studies on fly ash–cement paste (60:40) and concluded that the reaction of fly ash with Ca(OH)2 starts between 3 and 7 days, but a large quantity of fly ash and Ca(OH)2 remain unreacted even after 90 days. Usually, the reduction in Ca(OH)2 content commences by 7 days, but in individual cases it may vary from 3 to 28 days. However, in no case, the pozzolanic reactions start before 2 days of hydration [16].

Studies show that the pozzolanic reactions exhibit direct correlations with the curing temperature and the dosage of cement [14], [21]. The pozzolanic reaction of fly ash in hardened cement paste begins only at a later stage (28 days) when mixed at 20 °C , whereas at 40°C the reactions begin much earlier at 7 days [14]. Also, as the cement content is higher (above 90% as in replacement of cement with fly ash), the reaction starts at about 10–12 h, similar to a cement paste. As the cement content decreases, the reaction products formed are less but the reactions could be accelerated using higher curing temperatures [11].

There have been attempts to correlate the macrostructural strength behaviour of cemented soil to the reaction products formed as a result of chemical reactions. Studies have used TGA or XRD analysis to quantify C-S-H content and to estimate its effect on strength gain in cement stabilized soils [22], [23]. Such techniques cannot be used for pond ash as the amorphous content in pond ash may not be captured using XRD analysis. Few other techniques such as SEM, FTIR and NMR have been used successfully to study the interactions developing between cement and fly ash [15], [21], [24], [25]. Among these techniques, FTIR spectroscopy is considered to be advantageous as it can identify the individual hydration products and also estimate the relative variation in magnitude of these hydration products under different conditions.

In this study, an attempt is made to identify the hydration products formed in the case of cemented pond ash as such information is scarce for these systems. FTIR spectroscopy is used for this purpose and using this technique, the hydration products formed are delineated and quantified. It is hypothesized here that, formation of C-S-H is responsible for the strength gain in cemented pond ash. The pozzolanic reactions between cement and pond ash increases the C-S-H content and decreases Ca(OH)2 which can be captured by FTIR spectroscopy. This hypothesis is proposed based on the observed interactions between cement and fly ash. The relative variation of the hydration products under the influence of parameters such as dosage of cement, curing period and curing temperature are estimated. The increase in strength gain at different cement contents and curing periods is also verified through the UCS test.

Section snippets

Materials used

The pond ash used in this study was collected from the ash pond of Ennore Thermal Power plant, Chennai, India and brought to the laboratory in a truck. The ash was in a dry condition. It was further air-dried in the laboratory and stored in bins. The natural moisture content after air drying was found to be about 3%. The average room temperature is about 30 ± 8 °C. The material is classified as non-plastic silty sand with 65% of sand sized particles. The specific gravity of pond ash was

Unconfined compression strength test results

The results of UCS tests for various cement contents and curing periods are shown in Table 2. The peak axial stress is defined as the UCS value. The varying curing periods did not affect the UCS values of the untreated pond ash. The UCS values were found to increase with the cement dosage and curing period for the cemented pond ash samples.

FTIR test results

Fig. 1 shows a sample spectra of the cement and pond ash considered in this study. To compare the changes in FTIR spectra of these two materials, four regions namely, O-H stretching, O-H bending, C-O stretching and Si-O/S-O stretching vibrations are considered. The wavenumbers, 3000–3750 cm−1 and 1570–1750 cm−1 represent the O-H stretching and O-H bending regions, respectively, for both the materials [31], [32]. The C-O stretching is observed from 1350–1570 cm−1 for cement and this region is

Influence of hydration variables

In this section, the cemented pond ash with 2, 4, 6, 8 and 24% cement is studied at curing periods of 7, 14 and 28 days and at various curing temperatures. To quantify the influence of hydration variables, six peaks are taken into consideration: three peaks at 1175, 1099 and 1036 cm−1 capturing Si-O stretching in pond ash; two peaks at 1009 and 957 cm−1 capturing Si-O stretching in cement and two peaks at 785, 734 cm−1 capturing Al-O bond in alumina. It is seen that few peaks are occurring as

Summary and conclusions

The interactions between cement and pond ash were investigated in this study using FTIR spectroscopy. The effect of cement content, curing period, and curing temperature were studied for five cement contents (2, 4, 6, 8, and 24%), three curing periods (7, 14, and 28 days) and two curing temperatures (27 °C and 50 °C). The following conclusions are obtained from this study.

  • 1.

    It is known that the Class F fly ash/ pond ash does not undergo any hydration reactions due to its lower calcium content.

CRediT authorship contribution statement

Anu Jose: Investigation, Formal analysis, Software, Writing - original draft, Data curation, Visualization. M.R. Nivitha: Formal analysis, Validation, Visualization. J. Murali Krishnan: Methodology, Validation, Writing - review & editing. R.G. Robinson: Conceptualization, Resources, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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