An Effort Towards Moving to Net Zero Emissions and Sustainable Concretes

The global use of concrete is second only to water, currently being 20 billion cum per year and expected to grow exponentially by 2050 due to increased urbanization and infrastructure growth(carboncure webinar Nov,2021). As the demand for concrete as a construction material increases, so also the demand for Portland cement. The production of cement was only 1.39 billion tonnes way back in 1995 and the currently it is about 4.1 billion tons in 2020 and expected to grow to 8.0 to 9.0 billion tons by 2050. On the other hand, the climate change due to global warming has become a major concern. The global warming is caused by the emission of greenhouse gases, such as carbon dioxide (CO2 ), to the atmosphere by human activities. Among all the cement industry is held responsible for some of the CO2 emissions, because the production of one ton of Portland cement emits approximately 0.8 ton of CO2 into the atmosphere and cement industry alone contribute to 5 to 7% of the total global CO2 emissions.

In order to address the climate change and global warming issues concrete and construction industry which is mainly responsible for about 30% of the total GHG emissions has to adopt drastic sustainable CO2 reduction measures in all activities. These include the utilization of supplementary cementing materials such as fly ash, granulated blast furnace slag, and the development of alternative binders to Portland cement. As opposed to OPC, the manufacturer of fly ash-slag (ggbs) based geo-polymer does not consume high levels of energy, as fly ash and slags are already industrial by-products. This geo-polymer technology has the potential to reduce emissions by 80% because high temperature calcining is not required like clinker production for cement manufacturing. These also exhibit ceramic-like properties with have superior resistance to fire at elevated temperatures. Geo-polymer can be produced by combining a pozzolanic compound or alumino silicate source material with highly alkaline solutions. Fly ash, which is available abundantly worldwide from coal burning operations, is an excellent alumino-silicate source material, whereas granulated blast furnace slag is a by-product produced from steel plants.

The experimental work was conducted to obtain the residual strength of the fly ash and granulated blast furnace slag based geo-polymer concrete at different elevated temperature for different molarity. In the present experimental work, only one source of dry low-calcium fly ash (Class-F) from Raichur thermal power station in karnataka and one source of slag (GGBS) from JSW Cement Limited plant from Bellary was used. Sodium silicate solution and sodium hydroxide pellets were procured commercially from a local vendor. The tests conducted on the GPC for the present study were, compressive strength test, UPV test and water absorption test. The tests and analytical methods which are available for the Ordinary Portland Cement were used.

Material and Methods

Fly Ash and GGBS - In the present investigation Class F fly ash and GGBS are considered as binders. The physical characteristics are reported in Table 1.

Aggregate - Locally available coarse aggregates and river sand is chosen as filler, which confirm the requirements Indian standards IS 383, 2016.

Alkaline Solution - The locally available sodium silicate and sodium hydroxide solutions are used in the present investigation as alkaline solution. The sodium hydroxide is in flakes and pellet with about 98% purity. These pellets were mixed with distilled water to obtain the sodium hydroxide solution of required molarity. In the present study, 8M (8*40g=320g) & 16M (16*40g=640g) NaOH solution is considered for investigations.

The commercial grade of sodium silicate which has purity of 78% and contains 27% of water is used in the present investigation [7]. Chemical composition of sodium silicate solution reported in Table 2.

Curing of the Specimens - The study involves the specimens cured in sun-dry condition. The samples are then exposed to different elevated temperatures of 150°C, 200°C, 250°C, 350°C for a sustained duration of 2 hours. The compressive strength tests were conducted on these specimens to study the variation in compressive strength at various conditions of exposure.

Results and Discussion

Variation of Strength with Different Binder Proportions - The test results of mixes with varying fly ash and GGBS content as binders for 16M NaOH and a constant water content of 120 kg/cum are shown in Table 3. When 100 % fly ash was used, mix appeared brownish in colour, and as the percentage GGBS increased, the intensity of brown colour reduced, as observed physically.

It can be seen from Table 3, the slump increases with the decrease in percentage of fly ash (FA) and increase in percentage of GGBS, the increase is from 50 mm to 186 mm for FA and GGBS content of 60% and 40% respectively. Then on-wards, the slump follow shows a reverse trend Fig. 1 and reduce to a value of 90 mm for 100% of GGBS. This is probably because of the water is held by GGBS for hydration and hence reduction in slump. The compressive strength for 7 days shows a continuous improvement with addition of GGBS and reaches a value of 50- 60 MPa with 100% GGBS. However, it does not show significant variation at lower percentage of GGBS.

*Fig. 3: Variation of Slump for Varying the Percentage Binder (FA and GGBS)*

It is generally noted that, 20 to 40 MPa concrete constitutes 70 to 80% of major civil construction works. It appears from the Table 3, 20- 40 MPa strength can be achieved for the percentage fly ash as 80-70% and GGBS in the ratio 20-30%. Hence, an average combination of 75% fly ash and 25% GGBS is chosen for further studies.

Properties of GPC with 75% Fly Ash and 25% GGBS Exposed to Elevated Temperatures - The basic properties of GPC, namely compressive strength, ultra sonic pulse velocity and water absorption have been measured using standard procedures and their results along with the discussions are presented in subsequent sections.

Physical Observation - The following are physical observations made on geo-polymer concrete specimens with regards to colour, propagation of cracks and spalling, soon after heating the specimens to elevated temperatures.

Discoloration: The discolouration in geopolymer concretes exposed to elevated temperatures can be attributed to the changes in iron compounds present in the constituent materials such as fly ash, fine aggregates and coarse aggregates. Fly ash being present in significant quantities contributes considerable amount of iron compounds into the mix and hence, the discoloration is prominently observed in geopolymer concretes. The change in colour can be used as a tool to arrive at the probable temperature exposure in geopolymer concrete, as in the case of Portland cement concrete. However, the reaction of iron compounds occurs at temperatures beyond 5000 c. In the present study, the specimens have been exposed from 150°C to 350°C, there was no significant colour change observed. The specimens exposed to sustained temperature of 2500 C and 3500 C appears to be bright in colour as compared to initial specimens.

Cracks and Spalling: There were no cracks observed on the specimens which were subjected to sustained elevated temperature of up to 350°C. No spalling was observed in any of the specimens which were subjected to different sustained temperatures up to 350°C.

Fig. 6: Relationship b/w Compressive Strength of Mix 1 and Sustained Temperature

Compressive Strength: Exposed to elevated temperature after 28 days: The mix with lower molarity requires a longer time for polymerization. In Mix 1, the polymerization is still incomplete and hence there is increase in strength when heated to 150°C. However, in Mix 2 which is of higher molarity, the polymerization must have been completed and hence the strength reduction might have taken place at elevated temperatures.

Compressive Strength: Exposed to elevated temperature after 90 days: Mix 1 showed considerable good strength than Mix 2 after exposure to different elevated temperatures cured after 7 days, 28 days and 90 days at sun-dry condition.

There is considerable increase in strength between 28 days and 90 days in Mix 1 when compared to mix 2. This could be due to the slower rate of development of strength in lower concentration mixes as compared to higher concentrations, when sun dried. The Mix 1 also shows reduction in strength on exposure to elevated temperature indicating the completion of geopolymerization. In Mix 2, the strength development from 28 days to 90 days is negligible indicating that most of the geopolymerization is complete in 28 days. It is also seen that the rate of reduction in strength on exposure to elevated temperature is higher in Mix 2 as compared to Mix 1. In Mix 1, due to slow geopolymerization, the hydration of GGBS also occurs simultaneously, contributing to strength to a considerable extent. Hence, the matrix could be a mixture of both geoplymerization and hydration products. This might have reduced the effect of - Percentage reduction in compressive strength as compared to control concrete: The compressive strength test is the primary test to access the mechanical strength of concrete. For 7 day sun-dry curing the average compressive strength was found as 39.1 MPa for Mix 1. The specimens were subjected to elevated temperature of 150°C and 200°C for a period of 2 hours the strength noted were 59.8 MPa and 44.23 MPa respectively. There was increase of about 53% and 13% in strength of the concrete respectively. This increase is due to the polymerization process achieved after exposed to elevated temperature. When it is subjected to 250°C and 350°C there was a decrease in strength of about 16% and 30% respectively are tabulated in Table 7. This may be due to the initial water loss in the concrete and possible development of micro cracks. For 28 day sun-dry curing, the average compressive strength was found as 45.7 MPa, There was not any reduction in strength when subjected to temperature of 150°C and when it was subjected to 200°C , 250°C and 350°C there was a decrease in strength by almost 20%, 41% and 45% respectively and are tabulated in Table 7. This reduction in strength of the concrete is probably due to thermal expansion of concrete or due to micro structural changes occurred in the GPC. If the pores in the concrete are interconnected by micro cracks which enhance the discontinuity in the matrix, there will be reduction in strength of the concrete. For 90 days sun-dry curing there was a reduction of 8% strength when subjected to temperature of 150°C. When it was subjected to 200°C, 250°C and 350°C there was a decrease in strength by almost 31%, 51% and 53% respectively are tabulated in Table 7.

For Mix 2, for the 7 day sun-dry curing, average compressive strength was found as 53.1 MPa. There was not any reduction in strength when subjected to temperature of 150°C and when it was subjected to 200°C, 250°C and 350°C there was a decrease in strength by almost 43%, 65% and 69% respectively and are tabulated in Table 7.1. For 28 day sun-dry curing the average compressive strength was found as 67.5 MPa.

The specimen when subjected to elevated temperature of 150°C and there is a reduction in strength of about 23%. This may be due to the initial water loss in the concrete and possible development of micro cracks, but when it was subjected to 200°C, 250°C and 350°C there was a decrease in strength by almost 65%, 71% and 75% respectively are tabulated in Table 7.1. This reduction in strength of the concrete is probably due to thermal expansion of concrete or due to micro structural changes occurred in the GPC. If the pores in the concrete are interconnected by micro cracks which enhance the discontinuity in the matrix, there will be reduction in strength of the concrete.

For 90 days sun-dry curing there was a reduction of 37% strength when subjected to temperature of 150°C. When it was subjected to 200°C, 250°C and 350°C there was a decrease in strength by almost 66%, 75% and 77% respectively are tabulated in Table 7.1. The percentage of reduction is more when subjected to different elevated temperatures after curing at sun-dry condition in this Mix 2.

Variation of Each Mixes with Different Ages in Terms of Compressive Strength, UPV and Water Absorption -

The various tests such as compressive strength, UPV and water absorption were conducted for Mix 1 (120 litre/m3 water content with molarity of 8M) after specimens subjected to elevated temperature of 1500 C, 2000 C, 2500 C and 3500 C for a sustained period of 2 hours after 7 days, 28 days and 90 days cured at sun-dry condition. The entire test can be used for characterisation. From the Fig. 6.1, 6.2 and 6.3 it is observed that the similar trend can be seen indicating interdependency for the test results of compressive strength, UPV and water absorption after 7 days, 28 days and 90 days.

The various tests such as compressive strength, UPV and water absorption are conducted for Mix 2 (120 litre/m3 water content with molarity of 16M) after specimens subjected to elevated temperature of 1500 C, 2000 C, 2500 C and 3500 C for a sustained period of 2 hours after 7 days, 28 days and 90 days cured at sun-dry condition. The entire test can be used for characterization. From the Fig. 6.4, 6.5 and 6.6 it is observed that the similar trend can be seen indicating interdependency for the test results of compressive strength, UPV and water absorption after 7 days, 28 days and 90 days.

Variations of Mix 1 after 7, 28, 90 Days b/w Characteristic Parameters & Sustained Temperature

Variations of Mix 1 After 7, 28, 90 Days b/w Characteristic Parameters & Sustained Temperature

Conclusions

The investigations have shown that using Fly ash along with GGBS as base material, it is possible to produce GPC of compressive strengths in the order of 40-70 MPa with sun-dry curing.
GGBS, as one of the base materials results in early initial strength and it makes possible to de-mould the laboratory specimens very early (4-6 hours) similar to that of cement concrete. This is important for in-situ applications of GPC in construction industry.
The change in the molarity of the solution does not play a major role on workability of GPC for a constant water content of 120 kg/m3 . Variations of Mix 1 after 7, 28, 90 Days b/w Characteristic Parameters & Sustained Temperature Variations of Mix 1 After 7, 28, 90 Days b/w Characteristic Parameters & Sustained Temperature
The specimens when exposed from 1500 C to 3500 C, there was no significant color change observed. The specimens exposed to sustained temperature of 2500 C and 3500 C appears to be bright in color as compared to initial specimens.
The specimens with 7 days exposure of sun dry, when exposed to sustained temperature of 1500 C, increase in compressive strength was observed. The polymerization appears to still in complete at this stage, gets accelerated at elevated temperature and hence strength increased when exposed to 1500 C.
The reduction in the strength when exposed to beyond 1500 C was due to the evaporation of free moisture released during geo-polymerization. At higher temperatures of 2500 C and 3500 C, further degradation occurs due to considerable difference in thermal expansion between aggregate and paste in the concrete matrix. This will result in the cracking at interface between aggregate and paste.
The specimens cured at sun-dry condition shows less water absorption before exposure to elevated temperature. After exposure to sustained temperature the percentage of water absorption increases. This is probably due to the formation of fine cracks in the specimens and interconnecting of pores inside the concrete. Good correlation between UPV values and water absorption is a testimony to the formation of cracks/voids when exposed to elevated temperature.
The early strength development of geo polymer concrete with 25% GGBS under sun-dry curing opens an excellent possibility of in-situ application of geo polymer concrete. Such concretes exhibit better strength properties with increase in molarity of alkali solutions.
This research work increases possibilities of making and developing ecofriendly geo polymers concretes for practical construction applications through out the world to reduce carbon footprint which is a major concern with the current usage ordinary Portland Cement Concretes (OPC).
Geo Polymer Concretes or Earth friendly concretes or No Cement Concretes are the futuristic solutions concrete constructions for moving towards Net Zero CO2 emissions in constructions.

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Code Reference
• IS: 10262-1982, Indian Standards, Recommended Guidelines for Concrete Mix-Design
• IS: 516-1959, Indian Standards, Methods of Tests for Strength of Concrete.
• IS: 383 – 1970, Specification for coarse And Fine Aggregates from natural Sources for Concrete.
• IS: 13311-1992 Non-Destructive testing of Concrete-Method of test.

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