Characterisation of recycled aggregates and mortar studies using recycled fine aggregate
Construction and demolition (C&D) waste constitutes a considerable fraction among total solid waste generated on the Earth. C&D waste is generated by construction sites, demolition of old structures, waste remains after a natural disaster etc. As the world is developing at such a rapid rate, the amount of C&D waste generated is also increasing at a huge rate. The major problem arises due to the extensive mining to satisfy the large demand of construction materials and lack of dumping places for the wastes that is otherwise illegally dumped at unauthorized palaces. Moreover, we have limited natural resources, thus a sustainable approach is required to solve the problem of both supply of raw materials and the amount of waste generated. Recycling of C&D waste provides an option to deal with the problems mentioned above up to a great extent. Recycling in a way such that proper durability and strength is maintained is also very important. So, we can say that sustainable growth is now not an option but a necessity.
The use of recycled aggregates not only reduces the usage of virgin aggregate but also provides an alternate to the problems of landfills piling up. The use of recycled aggregates could address the demand of virgin aggregates and mining of depleting natural resource also may reduce. In this way we are able to employ 3R’s of sustainability namely - reduce, reuse and recycle. By using such methods, we are reducing the amount of waste generated, reusing the material which otherwise would have been wasted and recycling some of the other materials to utilize that in a better way.
The project is based on extracting various fractions of aggregate according to their sizes and then identifying various characterization of the fraction and then utilizing it for further purpose. Basic processes follows as crushing, treating the crushed material through heat treatment, milling and then sieving accordingly and further use the obtained fraction as per our need. Obtained concrete pieces were big in size and also aggregates and mortar were stuck together. Thus, there was need to break the bigger particles into smaller particles and also remove layer of mortar present over the aggregate. Attached mortar introduces a lot of problem because it can increase porosity, weaken ITZ etc. which will affect the strength and other properties of recycled aggregate concrete.
Effect of High Temperature on Concrete
Heating as one of the major steps in the recycling process requires understanding the effect of high temperature on concrete. For this, Thermogravimetric studies were done on the hydrated cement and granite aggregate. TGA instrument measures the amount of weight change of a material, either as a function of increasing temperature, or isothermally as a function of time. Temperature typically varies from 25ºC to 1000ºC at specified heating rate. TGA measures decomposition rate of the compounds present in the sample which can further help us in quantitative analysis of the components of the sample.
Portlandite dehydroxylates at around 460ºC and calcite decomposes between 600-800ºC. CSH phases show water loss over a wide range of temperature from 50-600ºC. Similarly, for other phases at a particular temperature peak occurs due to change in weight of the total sample which helps us to identify qualitatively and quantitatively the composition of the sample.
Fig. 2 shows the TGA of hydrated cement and granite aggregate that was present the demolished concrete.
METHODOLOGY OF THERMO-MECHANICAL TREATMENT
Demolished waste concrete was obtained from a 15 year old building located at Chennai. To break the concrete chunks into smaller pieces, manual crushing was done. After breaking of larger particles into smaller ones there was need for a way to remove layer of mortar from the aggregates. To recycle the waste concrete thermo-mechanical treatment method was applied. There were many trials performed to find the best temperature range, heating time and rubbing method to remove attached mortar from the recycled aggregates. After many trials, temperature of 500ºC was used and for milling Los Angeles Abrasion machine for 500 revolutions in presence of steel balls. After treatment, the material was sieved using sieves of different sizes [(>20mm, 20-4.75mm, 4.75mm-300 microns (Sand), <300 microns (Powder)]. Each fraction has different characteristics and thus each fraction can be further tested for best utilization.
The process is optimized such that all the feed concrete is converted into recycled products. The next step was to characterized the recycled products and find out their suitability to be used in concrete and other construction applications.
CHARACTERISATION OF RECYCLED PRODUCTS
Physical and Mechanical Properties of Coarse Recycled Aggregates
The coarse aggregates were tested for major physical and mechanical properties according to the IS 2386 part 3 and 4. The results are reported in the following table:
Properties of the treated coarse recycled aggregate is similar to that of virgin aggregate and could be used in the partial or full replacement of virgin coarse aggregate.
Estimation of Attached Mortar using Acid Cleaning
Attached mortar test is the test performed on recycled coarse aggregates to know the amount of attached mortar for determining the efficiency of treatment process. The test was performed on coarse aggregates of size 4.75-20 mm.
For this test, 500g of recycled coarse aggregate was taken in a beaker. In the beaker 1 litre of 3 Molar H2SO4 solution was added. Stirring of the solution was done in an interval of 8 hours for a period of 24 hours. Stirring of the whole material is necessary so that every aggregate interact with the H2SO4 solution and reaction is completed. After 24 hours the aggregates were observed and due to remains of attached mortar over the recycled aggregate, again the test was repeated but with 2 Molar H2SO4 solution twice (Akbarnezhad et al. 2013).
After the completion of the test, aggregates were washed on 4.75 mm sieve and allowed to dry and their final weight was taken.
Attached Mortar = Initial Weight – Final Weight
Initial weight of aggregate – 500 grams
Final weight of aggregate after completion of test – 481 grams
Attached mortar – 19 grams
Thus, percentage attached mortar is 3.8%.
The result of this test indicates the significant cleaning of recycled aggregate.
Characterization of Hydrated Cement Powder
Hydrated Cement Powder is the fraction obtained after sieving of the treated recycled concrete aggregate which is of size less than 300 microns. Hydrated cement powder could have the potential to be used as admixtures in the concrete. If the Hydrated Cement Powder has a higher content of unreactive phases it can be used as filler otherwise it can be used as reactive admixture which might improve strength and durability. For this various characterization tests are performed on Hydrated Cement Powder which includes specific gravity, acid dissolution test, X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Particle Size Distribution etc.
Specific Gravity – It helps us know how the weight of the material will vary when we use Hydrated Cement Powder as a filler.
Acid Dissolution Test – Acid dissolution tells us about how much content of Cementitious and Siliceous content is there in the composition.
X-Ray Diffraction – XRD can be performed for both qualitative and quantitative analysis of the sample.
Thermogravimetric Analysis – TGA can be used for the quantification of CSH, Portlandite, Calcite etc.
Particle Size Distribution – Particle size distribution helps in characterizing what is the composition of different sizes in the fraction and how well they can form a packed structure.
Specific gravity of hydrated cement powder using helium pycnometer
Helium pycnometer can give density of the substances by measuring the volume of the substance kept in the instrument by using the weight of sample. Helium pycnometers sometimes are also called as gas pycnometer. The gas used is chosen such as it does not react with the material whose characterization is needed to be done. Thus, inert gases like Helium are preferred for this kind of experiments.
The density of Hydrated Cement Powder obtained using gas pycnometer was 2.41 g/cc.
Acid dissolution test
Acid Dissolution Test is performed to determine the amount of Cementitious and Siliceous matter present in the sample. Cementitious part can dissolve in certain type of acidic solution while siliceous part can be retained on the filter paper. For this a known amount of sample is mixed with a fixed molarity of acid solution. After that with the use of a filter paper the solution is filtered then the filter paper is dried and weighted. Using the dry weight of the filter paper one can determine the weight of siliceous part retained on the filter paper.
Knowing the corresponding content of cementitious and siliceous part in the obtained powder is necessary as it further help us decide that whether it can be used as a filler or can it be used as reactive materials.
For our test a sample of 1g of Hydrated Cement Powder was taken in conical flask filled with 200 ml of water. In the conical flask a 30 ml of 1 Molar HCl solution was added and then it was allowed to mix for 1 hour in an automatic stirrer at a rate of 250 RPM. After 1 hour the solution was filtered and after the filtration was complete, the filter paper was allowed to completely dry in the oven. After the filter paper was all dried up its weight was taken.
A total of 4 trials was done and the corresponding results is shown below:
Average Cementitious Content - 55.2%
Average Siliceous Content - 44.8%
X-Ray diffraction (XRD) test
X-Ray powder diffraction (XRD) is used for analysis of different phases in a crystalline material. X-Rays generated are directed at the sample and the diffracted rays are detected and thereafter analysed. The intensity of the deflected X-Rays is recorded. When the geometry of the incident X-rays affecting the sample satisfies the Bragg Equation, constructive interference occurs and a peak in intensity is recorded. The output that we obtain for the XRD process is in the form of Counts per Second. XRD is a quick test for the phase identification of the material. One of the peculiar things about this test is that the observer should have some idea about the composition of the material to get better information of the composition of his material. This is because the peaks are compared with pre-known peaks and then the composition of the material is concluded. Due to many compounds having same peak, analysis could be difficult if observer have no idea of what the sample composition.
XRD can be used to detect various individual phases in the cement powder also. XRD on cements are generally performed in two ways; qualitative and quantitative analysis. In qualitative phase analysis a search/match feature is used which compares the peak of our powder with that of the peak of the material which observer has searched and match it with the peaks. If a peak is confirmed that means that the compound with searched elements is present in the powder. Thus, it gives us an idea of the composition of the powder. In our analysis of the powder we have performed qualitative phase analysis.
Following data was obtained from the XRD test
Particle size analysis
Particle size distribution of a compound is quantification of particles within a range of a size. A good distribution of particles within a range is quite beneficial for the system as it reduces the water demand of the system (considering other properties of the materials are same) and produces a system which is well packed.
Laser diffraction particle size method for the analysis was used. Basic principle behind this is that it measures particle size distribution by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. This angular data is processed and the size of the particles is calculated using the Mie theory of light scattering.
Below is the particle size analysis of the hydrated cement powder along with the other fractions
Characterization of Recycled Fine Aggregates
Fine Recycled Concrete Aggregate (FRCA) is the fraction of the size 4.75-0.30 mm obtained after sieving of the treated aggregate. Increasing demand of natural sand for construction is responsible for fast depleting and henceforth we are in a dire need for an alternate source. FRCA can serve as a good alternate option. FRCA could be used in many places like mortar, concrete etc. if complies the specifications mentioned in the codes.
Table 3 below shows comparison of specific gravity and water absorption of Treated Fine RCA, Crushed Fine RCA and River Sand.
|Aggregate Type||Specific Gravity||Water Absorption (%)|
|Treated Fine RCA||2.35||5.91|
|Crushed Fine RCA||2.04||9.38|
The specific gravity of treated fine RCA is greater than the crushed fine RCA and the water absorption of treated RCA is lower than that of crushed RCA. This is due to the fact that the crushed RCA has been first thermo-mechanically treated. Thermo-Mechanical treatment separates the fines hydrated cement on fine RCA which is further sieved as fraction passing 300 microns. Since we have FRCA in the range of 4.75 mm – 300 microns, the fine particle gets eliminated improving quality of the treated RCA.
MORTAR STUDIES USING FINE RECYCLED CONCRETE AGGREGATES
To investigate the suitability of recycled sand, it was replaced with natural river sand to study the mortar properties at various replacement ratios. However, there are certain specification available in the codes that are discussed below:
IS Code Specifications for Fine Recycled Concrete Aggregate
IS 383:2016 has recently included the specifications of recycled concrete aggregate in the codes. The maximum replacement percentage for different types of concrete are given as follows –
|Type of Concrete||Max Replacement Percentage|
Also, the additional requirements state that specific gravity should be in a range of 2.1 - 3.2 and the water absorption up to 10% is permitted subject to pre wetting of aggregates before batching and mixing. Both of these conditions are well satisfied by the material.
Influence of Fine Recycled Concrete Aggregate on the Mortar Properties
To study the influence of fine recycled concrete aggregate on the properties of mortar a water by cement (w/c) ratio of 0.5 was fixed and samples with a replacement percentage of 20%, 50% and 100% of river sand was prepared. The tests that were performed to study the influence of FRCA on mortar properties were workability, fresh density, compressive strength, flexural strength and shrinkage.
A volumetric mix design was chosen and shown in table 5 below:
All the weights are in grams FR-X –> X% of FRCA.
The tests results obtained are as follows
|Fresh Density (g/cc)||2.30||2.31||2.28||2.21|
3 – Day
7 – Day
7 – Day
CONCLUSION AND RECOMMENDATIONS
1. Thermo-mechanical treatment of crushed aggregate has significantly improved the quality of recycled concrete aggregate due to the removal of attached mortar. The coarse aggregate up to very large extent have properties similar to the virgin aggregates. Also the improvement in the physical properties of fine aggregates has improved because the fine fraction was separated as hydrated cement powder.
2. Results of the attached mortar test with acid cleaning indicates significant removal of attached mortar from the recycled concrete aggregate. After the treatment a minimal amount of about 3.8% of attached mortar was left.
3. Acid dissolution test results of hydrated cement powder shows an average siliceous content 44.84% and average cementitious content 55.16%. Particle size analysis results shows that D50 particle size is around 100 microns. Since it has a high amount of siliceous content and most of the particles are not very hence cannot offer high surface area or reactivity, it can be used as filler. XRD test’s result shows presence of CSH gel, portlandite and calcite in the hydrated cement powder.
4. Workability of the mortar mix prepared increases with the increase in fine percentage. This is because of the fact that due to the volumetric replacement of the sand the amount of sand reduces but the amount of water increases comparatively and also due to the water absorption correction factor. Thus, due to comparatively high-water content workability of the mix increases.
5. Initially mortar mix has almost same fresh density due to 20% replacement percentage but as we increase the replacement percentage more and more the fresh density started decreasing. This is due to the fact that the Recycled Concrete Aggregate has a specific gravity lower than that of the river sand.
6. Strength does not have a linear variation with the replacement percentage. 50% replacement strength value is quite similar to that of the 0% replacement. On 3rd day of strength test, the strength of the FR-50 was greater than that of the FR-0. But on the 7th day of testing, the strength of the FR-0 was greater than that of the FR-50. This indicates higher initial strength for the 50% replacement sample. More data of 28 & 56 days are needed to reach to a conclusion.
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IS:383-2016 - Coarse and Fine Aggregate for Concrete - Specification
I want to express my sincere gratitude to my guide, Prof. Ravindra Gettu, for providing me the privilege of working under his expert supervision. I sincerely thank Mr. Rohit Prajapati, Ph.D. research scholar for providing me knowledgeable insight and helping me throughout my internship in all sorts of work.
I am also thankful to lab staff for helping me whenever necessary. Moreover, I thank the Indian Academy of Sciences, Bangalore for giving me this opportunity to pursue a summer research fellowship at such a prestigious institute.