Fly Ash from the Thermal Transformation of Sewage Sludge as an Additive to Concrete Resistant to Environmental Influences in Communication Tunnels

Round 1

Reviewer 1 Report

This article investigates the usability of fly ash from sewage sludge to make concrete for tunnels. It is common to use fly ash to replace cement, and I do not see novelty in this article. The language needs to be improved. For example, the title is too long, and it is hard to understand. This article is suggested rejected. The following are some detailed questions.

 

1. If you know the temperature at 150-300 degree C will make the concrete explosive, why would you dry the concrete and tested at 700 degree C? What is scenario that you are simulating?
2. Why does the fly ash in your case reduce the workability? Usually, fly ash can increase the flowability of concrete.
3. If you tested the compressive strength of concrete for 28d, 56d, and 720d, why not tested the carbonation depth of samples at different days?
4. What is the temperature distribution for different thermocouples placed at different locations?
5. Figure 12, if high temperature reduces the strength, could you explain why BZ and P5% samples tested at 300 degree C has the highest strength?
6. Line 498, why can the strength increase at 20 degree C with the increase of ash? Could you explain why this is different from the strength changing trend in Fig.8?
7. I do not see the meaning of Fig. 14. This figures cannot explain anything. It does not contribute to any mechanism.

 

Author Response

Thank you for your review and comments.

New Title: Fly ash from thermal transformation of sewage sludge as an additive to concrete resistant to environmental influences in communication tunnels.

 The reduction of carbon dioxide emissions introduced by the European Union
(target: emission reduction by 55% by 2030) is encouraging research into next-generation materials containing lower amounts of clinker. In Poland and Europe, siliceous fly ashes from hard coal combustion are widely used in cement technology, mainly in concrete technology. The wide use of fly ash is primarily determined by its high fineness, like cement, its chemical and phase composition, and its pozzolanic activity. The use of silica fly ash to produce concrete is possible only when the requirements specified in PN-EN 450-1: 2012 are met. According to the requirements of this standard, the total content of SiO₂, Fe₂O₃ and Al₂O₃ should be min. 65% by weight, with a reactive SiO₂ at least 25% by weight. The content of reactive CaO should not exceed 10%, MgO less than 4%, and the total alkali content, calculated as Na ₂ O (equivalent) content, should not exceed 5% by weight. The content of soluble phosphate (P₂O₅) should not exceed 100 mg / kg. Other types of fly ash are used in some European countries based on national experience and tradition. Ashes from municipal waste incineration plants and sewage sludge incinerators have been produced in an increasing amount in recent decades.

The dynamic development of sewage networks
and municipal wastewater treatment plants, which has been visible in the last twenty years, leads to the formation of an increasing amount of municipal sewage sludge. At present, the method of their management is subject to the Act of 14 December 2012 on waste (Journal of Laws 2013, item 21) as well as executive regulations and laws specific to the method of their production, processing and environmental impact. Considering the ban on the storage of sewage sludge from January 1, 2016, their management has become not only a technical, economic, but also ecological problem. The use of secondary raw materials to produce concrete - fly ashes from thermal treatment of sewage sludge, is important not only from an economic but also an environmental point of view. In the environmental aspect, the benefits of using fly ash include, among others:

• limiting the use of natural deposits, and thus - saving fossil fuels and natural resources and limiting the devastation of the land surface because of the use of aggregates,
• reducing environmental pollution by reducing the emission of harmful gases (carbon dioxide),
• reduction quantity stored waste,
• recovery of land occupied by ash dumps.

So far, no guidelines have been developed for the use of ashes from sewage sludge incineration as a mineral raw material to produce cement-based building materials. Due to too few practical applications and operational experiences
with the use of this type of ashes, obtaining additional information about the possibilities of their management seems to be fully justified. The chemical composition, pozzolanic activity and ash grain size significantly affect the consistency and workability of the concrete mix, as well as the process of setting and hardening of cement composites.

Municipal wastewater delivered to the treatment plant is a mixture of industrial and domestic wastewater, additionally supplied with rainwater and infiltrating water from the ground. The amount and composition of wastewater flowing into the wastewater treatment plant is subject to changes in the daily, weekly, monthly, and yearly cycle. And their qualitative and quantitative characteristics depend primarily on the condition and type of a given sewage system, the standard of living of the inhabitants, the amount of water used or the industrialization of the city. An important rule is that there is no typical composition and quality of municipal wastewater, and thus there is no typical composition of the fly ash generated during the thermal treatment of sewage sludge.

In the presented article, fly ash from thermal treatment of sewage sludge was used.

1. If you know that 150-300 degrees C will make concrete explosive, why would you dry the concrete and test it at 700 degrees C? What scenario are you simulating?

The aim of the research was to determine the effect of elevated and high temperatures on the strength of concrete with fly ash from thermal transformation of sediments. During these tests, the influence of ash addition on the phenomenon of thermal chipping of concrete, often also referred to as thermal spalling, was not determined. Drying the samples was to eliminate this phenomenon and to determine the residual strength after thermal loading. During a real fire, temperatures in closed spaces usually reach up to 80°C. The temperatures selected in the study refer to the temperatures occurring at different heights of the room, i.e. from the floor (where the temperature is around 30°C) to the ceiling zone. At the same time, we wanted the temperature increase over time to be close to the so-called standard temperature-time curve.

 

2. Why does the fly ash in your case reduce the workability? Usually, fly ash can increase the flowability of concrete

 

By analogy to the fluidized combustion of coal dust, it can be concluded that the grains of this type of fly ash are characterized by a high content of grains (grain conglomerates) with high open porosity, which translates into high water demand. The degree of porosity for Kraków was 70.86%.

The additive used is characterized by high water demand and has a negative effect on the consistency of the concrete mix - the mix is non-workable after a short time. The "positive" effect of using an additive with high water demand is the reduction of the effective water content in the concrete mix (PN-EN 206 - Effective amount of water is the difference between the total amount of water in the concrete mix and the amount of water absorbed by the aggregate), which translates into a lower ratio the above-mentioned and higher level of compressive strength (especially early strength). In practice, the concrete producer, to ensure the appropriate consistency (workability) of the concrete mix in a sufficiently long time, may add larger amounts of water - which ultimately results in lower compressive strength, high concrete shrinkage and low durability, or add superplasticizers P₂O₅ was observed in the tested ashes, which is related to the type of municipal wastewater delivered to the treatment plant. In the case of ashes from coal combustion, regardless of the technology, the content of P₂O₅ is lower than 0.5 %. The addition of this ash to concrete will slow down the cement hydration process. As it is known, in the liquid phase of the slurry, PO₄^3- ions react with Ca²⁺ as a result of which a very sparingly soluble calcium phosphate is precipitated on the surface of the cement grains in the form of a fine crystalline and poorly water-permeable layer, which significantly hinders the cement hydration process. Cement containing an increased amount of soluble phosphorus compounds binds more slowly. In the tests carried out, the mixture began to bind the quickest with ash from Krakow, the setting started after 30 hours, and the end took place after 40 hours.

3. If you tested the compressive strength of concrete for 28d, 56d, and 720d, why not tested the carbonation depth of samples at different days?

The research on the possibility of using it for cement composites has been divided into stages. First, the focus was on determining the variability of the properties of ashes from thermal treatment of sewage sludge and their impact on the properties: compressive strength and frost resistance. The next step was to examine the effect of high temperature on the produced concretes with this additive. Research on carbonation is currently underway. The article presents the preliminary results of the research. Such research is currently conducted, we want to supplement it in another publication on the use of fly ash from sewage sludge.

4. What is the temperature distribution for different thermocouples placed at different locations?

After reaching the assumed temperature, the samples were annealed for another 30 minutes, which was aimed at equalizing the temperature on the outer surface of the sample and in its entire volume. An example of this type of temperature distribution is attached to the answer. The decomposition concerns the temperature of 700C. T1 of the temperatures at the surface of the sample. T2 temperature in the middle at 1/3 of the depth. T3 temperature on middle at ½ depth

5. Figure 12, if high temperature reduces the strength, could you explain why BZ and P5%  samples tested at 300 degrees C have the highest strength?

In the case of ordinary concretes with silicate or limestone aggregate, when heated at 300C, the decrease in compressive strength is up to 10% in relation to the normal temperature. The results of our tests showed that for BZ and P5% samples the strength increased. It is difficult to clearly define what factors caused this phenomenon, perhaps it is influenced by the type of aggregate used. Nevertheless, this temperature, even in relation to ordinary concrete and the above -mentioned aggregates, does not always have a negative impact on strength, as it is also dependent on the class of concrete, humidity of the tested samples, method of heating and several other secondary factors, such as the method of concrete care.

6. Line 498, why can the strength increase at 20 degree C with the increase of ash? Could you explain why this is different from the strength changing trend in Fig.8?

The addition in the form of fly ash from the thermal transformation of sediments increases the strength of the concrete even after the period of standard maturation, i.e. 28 days. It has been proven in the research that the addition of ash in the amount of up to 10% (inclusive) has a positive effect on the increase in strength over time. When increasing the amount of the additive, the increases in strength over time are not as intense. The results of these studies were also confirmed by the authors in previous publications on fly ash from thermal transformation of sludge from other wastewater treatment plants.

7. I do not see the meaning of Fig. 14. This figures cannot explain anything. It does not contribute to any mechanism.

Yes, Figure 14 adds nothing to the article. The drawing has been deleted.

Author Response File: Author Response.docx

Reviewer 2 Report

The title on the article should be updated.

Author Response

Thank you for reviewing the article.

New Title: Fly ash from thermal transformation of sewage sludge as an additive to concrete resistant to environmental influences in communication tunnels

Reviewer 3 Report

The focus of the research is to see whether fly ash from sewage sludge thermal treatment could be used as an effective addition in concretes that are resistant to environmental impacts in communication tunnels. . The topic is of interest and the study is comprehensive. The approach and experimental program as well as data analysis are sound leading to reliable conclusions. The text is well written, but language editing can improve the quality and is recommended.

The Introduction is very long. It must be shorter and more straightforward. In addition, knowledge gap should be clarified clearer. The rest of the introduction can be moved to a separate section as literature review.

 

Page 4 line 366, use another a word than " we thank you"

Author Response

Thank you for your review and comments.

The introduction has been reduced and the literature review section has been moved to the second chapter.

Round 2

Reviewer 1 Report

Please change Fig.8. The standard deviation on the figure needs to be changed.

Besides, add all the explanation  and comments to the manuscript. If you have used your previous published paper, please add the corresponding information and cite it.

Author Response

Thank you for your review and comments.

Author Response File: Author Response.pdf