Enhancing the Durability Properties of Sustainable Geopolymer Concrete Using Recycled Coarse Aggregate and Ultrafine Slag at Ambient Curing

Round 1

Reviewer 1 Report

The authors have performed all the relevant tests with respect to mechanical as well as durability properties of the concerned specimens but have not substantiated them with microstructural tests eg. SEM,EDAX etc. The SEM images have been taken from a previous study whose mix proportions do not match exactly with the specifications of the specimens used in the current study. Further clarity regarding the nature of the geopolymeric gel structure would have strengthened their argument. Tests such as FTIR would have given a better idea regarding the nature of the binding gel. EDAX analysis was also needed as a proof to their statement that the generation of C-S-H and C-A-S-H gel leads to the increase in compressive strength.

Author Response

The authors are in agreement with the suggestions provided by the reviewer; however, SEM, EDS and FTIR analysis was not in the scope of this research. We would like to consider these suggestions for our future research work.

Reviewer 2 Report

The manuscript presents the results of an experimental campaign conducted to investigate the effects of recycled coarse aggregate on fly ash-based alkali activated concrete and the ability of ultrafine slag to counteract its deleterious effects on the performance of concrete mixtures. Ultrafine slag was used both as substitute of fly ash or as admixture, and several mechanical and durability trials were conducted to evaluate its effect at progressive increased dosages of recycled coarse aggregate. Increase the valorization/upcycling rates of construction and demolition waste is urgently needed and develop novel processing and manufacturing technologies to improve the technical quality of recycled commodities is a relevant research topic in the view of sustainability and development of circular practices in construction. Therefore, manuscript is well aligned with journal scope. The experimental campaign is adequate and manuscript structured accordingly. However, the deleterious effects replacing natural aggregates by recycled ones on strength development and durability of concrete (alkali activated or otherwise) are well-known and the effects of ultrafine Ca-rich slag on fly ash-based alkali activated systems are abundantly described in extant literature – including in previous works of the authors. Therefore, the main scientific findings and the novelty of the manuscript are not sufficiently explicit in the view of the reviewer. In general, the results are introduced very extensively but in a scientifically sound manner. The manuscript will greatly benefit if additional efforts could be made to increase the incisiveness of the discussion and support some of the claims with relevant literature or additional evidence (e.g. Line 258 et seq.- support with data or bibliographic references the benefits of accelerating the reaction kinetics of alkali activated systems; Line 450 et seq./point 2 of conclusion- demonstrate that ultrafine slag acts primarily as a filler rather than an active precursor, and describe the reaction products formed, if any). Additional details should also be provided regarding some of the mix design decisions (for instance why 75% recycled coarse aggregate was not consider), materials and equipment used (all specifications should be provided) figures (for example declare all sources- Figure 3 reproduced for reference 17; improve the quality and include scale in all SEM figures), and data presented (include standard deviation or other measure of dispersion in all figures). The authors are also encouraged to enrich the discussion section by enlarging the list of references with scientific works other than their own.

Overall, the manuscript reports results of practical value to researchers and practitioners, and a chance to resubmit the manuscript after revision should be given so that authors can highlight contributions made to the current state of the art.

Author Response

The authors are thankful to the reviewer for encouraging comments on the manuscript. All the claims made in the manuscript have been cited as per the reviewer’s suggestions in the revised manuscript. Additional details related to mix design have also been provided. The authors added the best available figures in the manuscript.