Preparation and Characterization of Ultra-Lightweight Foamed Concrete Incorporating Lightweight Aggregates

Round 1

Reviewer 1 Report

This paper details the results of a study into foamed concrete which utilize fly ash and lightweight aggregates, and describes the effects on compressive strength, thermal conductivity, and plastic shrinkage on the final product.

The study and methodology is sound, however, I have some suggestions/questions to the author which I think could be incorporated into the manuscript to strengthen it.

1) Section 3.1: Was the fresh concrete density measured? How does it compare to the hardened density?

2) Section 3.2/Figure 3: How many cubes were tested per batch? Why is the 56 day strength not reported? With 25% fly ash replacement, the pozzolanic effects of Fly Ash should be considered. 

3) Were set times measured? I would imagine that the incorporation of the lightweight aggregate and fly ash would affect set times for the material.

4) A grammar review of the manuscript is encouraged.

Author Response

Dear editor and reviewers,

The authors wish to thank the reviewer for the valuable comments and recommendations. As suggested by reviewers manuscript has been revised and entirely corrected by English native speaker. Herewith, we would like to reply to reviewers’ suggestions and discuss changes in the submitted manuscript. The authors hope our corrections will improve the quality of the paper and we hope this will satisfy all the requirements for the publication of the article in the Applied Sciences MDPI journal.

The detailed response can be found in the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

Some statements in the paper are flawed and must be corrected. Explanations of observed results are not always sound.

Here are some remarks:

line 42: "lots of"

line 44: are is

line 63: Without aggregate it is just foamed paste.

line 77: There is no correlation between shrinkage and low hydration heat although both are due to the use of SCM.

line 84: No, increasing the foam volume will increase shrinkage. Other effects are due to volumetric changes in the mix design.

line 96: Rephrase sentence

line 115: EN 206-1 is not applicable. The workability of a foamed concrtee cannot be compared with that of a dense concrete. You yourselves confirm my statement later in line 138 to 140.

line 156: This information is surplus. There is no alternative procedure!

line 161: Since Liaver was added in addition to the binder the density should increase (see Table 2). Please explain the effect. Why does the dry density decrease when perlite is added as LWA, which has an even lower particle density than Liaver. Besides, information about properties of both LWA are missing.

line 168 to 170: Is the comparison based on the same specimen size?

line 174: Tke k-value of FA is only 0.40. Thus the result is no surprise at all.

line 176 and later: None of the mixes is named 5 or 6. This holds for the following. Don't mix designation of your mixes.

line 186: You talked aboud LWA. Is the perlite raw material and not expanded? In that case it cannot be considered being a LWA.

line 187: Why should this effect be relevant since fly ash is pozzolanic material and exhibts a negative impact in your tests?

line 191: Based on this information one must consider that mix designs given in Table 2 are incorrect since you added Liaver to the pure paste mixes.

line 197: This conclusion is flawed

line 205 to 207 and Figge 4: Show the span of the results and discuss it.

line 210 to 211: rephrase the sentence

line 223 to 224: How. This sentence is a hollow phrase.

line 224 to 225: Why? What is the difference to perlite? 

one 231: I see voids and no bubbles.

line 235/236: You cannot tell from these slides. The magnifications are too high.

one 238/239: How can you tell from these images. You show one cell wall only. Such general statements need real proof.

line 241: Why don't you use identical magnification for the comparison?

Author Response

Dear editor and reviewers,

The authors wish to thank the reviewer for the valuable comments and recommendations. As suggested by reviewers manuscript has been revised and entirely corrected by English native speaker. Herewith, we would like to reply to reviewers’ suggestions and discuss changes in the submitted manuscript. The authors hope our corrections will improve the quality of the paper and we hope this will satisfy all the requirements for the publication of the article in the Applied Sciences MDPI journal.

The detailed response can be found in the attached file.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors present an interesting experimental campign focused on ultra-lightweight foamed concrete (density < 500 kg/m3) prepared with a preformed foam and lightweight aggregates. The authors point out the effects of different ingredients on the resulting density, compressive strength, thermal conductivity and shrinkage.

Below are some comments that could contribute to improving the quality and completeness of the paper.

Line 64: for the sake of completeness, the authors should quote, in addition to what already reported, the presence of studies of foamed concrete with silica fume

Gökçe, H. S., Hatungimana, D., & Ramyar, K. (2019). Effect of fly ash and silica fume on hardened properties of foam concrete. Construction and Building Materials, 194, 1-11

and with addition of bi-directional composite grid reinforcement

Falliano D., De Domenico D., Ricciardi G., Gugliandolo E., (2019), Improving the flexural capacity of extrudable foamed concrete with glass-fiber bi-directional grid reinforcement: An experimental study, Composite Structures, 209, 45-59.

Line 68 and lines 71-72: the amount and type of foaming agent, as correctly reported by the authors, affect significantly the fresh state properties and the final mechanical properties of foamed concretes. Besides this, in the relevant literature it has been demonstrated that there is a strong relationship between water/cement ratio and the nature of the employed foaming agent, which affects the final properties of foamed concretes. The authors should quote the following additional study in this field:

Falliano D., De Domenico D., Ricciardi G., Gugliandolo E., (2018), Key factors affecting the compressive strength of foamed concrete, IOP Conf. Ser.: Mater. Sci. Eng., 431, 062009

Lines 75-89: the authors discuss the main problem of foamed concrete: the shrinkage phenomena. Even without going into details, it is not clearly mentioned one of the most widely used methods to overcome this problem: the use of fibers within the mix. The authors should quote the following two pertinent studies in this context:

Falliano D., De Domenico D., Ricciardi G., Gugliandolo E., (2019), Compressive and flexural strength of fiber-reinforced foamed concrete: Effect of fiber content, curing conditions and dry density, Construction and Building Materials, 198, 479-493

Bing C, Zhen W, Ning L. Experimental research on properties of high-strength foamed concrete. Journal of Materials in Civil Engineering 2011; 24(1):113-118.

Line 135: the production of foamed concrete is influenced by the mixing speed. High shear stresses in the mixing phase are desirable so as to have a uniform and homogeneous distribution of the air bubbles (which consequently would have lower size). The authors state they used the Eirich mixer with a high shear intensity. Which is the mixing speed (rpm) employed in the preparation?

Lines 188 and 189: the authors noticed that the fine liaver (FL) does not affect the compressive strength in a clear manner. However, they point out that the addition of FL gives rise to decreases of the compressive strength in comparison with both only-cement specimens and specimens with fly ash (FA) and fine perlite (FP), with values close to those of the cement with FA. Probably, this decrease is not only related to the lower density of the specimens (the decrease of density is not so pronounced). The authors are invited to add some additional motivation, to their best knowledge, to properly justify this result.

Section 3.3: the thermal conductivity characteristics are strongly influenced by the morphological properties of the air bubbles of the foamed concrete. The authors are invited to take into account this effect and, consequently, consider the presence of different dimensions of the bubbles in the different specimens, in order to draw more consistent conclusions in terms of thermal conductivity. The authors should quote the following two pertinent studies in this field:

Sang G., Zhu Y., Yang G., Zhang H., (2015), Preparation and characterization of high porosity cement-based foam material, Construction and Building Materials, 91, 133-137]

D Falliano, E Gugliandolo, D De Domenico, G Ricciardi, Experimental investigation on the mechanical strength and thermal conductivity of extrudable foamed concrete and preliminary views on its potential application in 3D printed - RILEM International Conference on Concrete and Digital Fabrication, 277-286

Author Response

Dear editor and reviewers,

The authors wish to thank the reviewer for the valuable comments and recommendations. As suggested by reviewers manuscript has been revised and entirely corrected by English native speaker. Herewith, we would like to reply to reviewers’ suggestions and discuss changes in the submitted manuscript. The authors hope our corrections will improve the quality of the paper and we hope this will satisfy all the requirements for the publication of the article in the Applied Sciences MDPI journal.

The detailed response can be found in the attached file.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

There is one last proposal for a modification where you stat:

“Fine Liaver is non-reactive material, therefore, it does not have much volumetric changes and it can restrain the development of cracking and shrinkage. However, fine perlite has a pozzolanic reactivity and it might has volumetric changes which might increase the drying shrinkage. In addition, the water absorption of fine perlite is much higher than fine Liaver which can affect drying shrinkage significantly.” See lines: 245 - 249"

To my knowledge all pozzolanic materials used as SCM reduce shrinkage. Thus your conclusion would be a surprising effect. I propose deleting it.

Author Response

The authors agreed with the reviewer’s opinion. So, the sentence has been deleted and the text has been modified as follow:

“Fine Liaver is a non-reactive material and it therefore does not undergo much volumetric change, meaning that it can restrain the development of cracking and shrinkage. The water absorption of fine perlite is much higher than that of fine Liaver, which can result in significant drying shrinkage.” See lines 301 –305

Author Response File: Author Response.docx

Reviewer 3 Report

All the technical amendments suggested in the previous review have been meet by the authors satisfactorily, while all my previous positive considerations on the novelty of this work still hold. Therefore, the paper is now recommended for publication.

Author Response

The authors appreciate the valuable recommendation of the reviewer for our paper.