© 2022 Imperial College London.Fly ash-slag based Geopolymer cement (GPC) has demonstrated mechanical properties and environmental advantages that make it one of predominant sustainable alternatives to Portland cement (PC). Despite the fact that numerous environmental analyses about geopolymers are being published, their environmental impact after the end of service-life has barely been explored. Given that construction-waste management is a major sustainability issue, the present study is investigating the potential of recycling fly ash-slag GPC as a fine aggregate in mortar mixes. The major physical properties of the fine recycled aggregates (FRA) were tested and compared to those of PC FRA and natural sand of similar fineness. The effect of incorporating FRA in low (25%) and high (50%) percentage in PC or GPC matrix mortars was investigated. The 28day compressive and flexural strength of mortars were tested. Also the 28day water absorption and flow of mixes incorporating GPC FRA were recorded. GPC FRA exhibited properties similar to those of PC FRA and poorer than those of natural sand. The results of compressive and flexural strength proved that FRA addition had a negligible effect in all cases. The influence of the high water absorption of GPC FRA, relatively to that of natural sand, was prominent on the workability of fresh mixes and possibly affected the water absorption of mortar prisms. The effect of GPC FRA proved to be similar to that of PC FRA on compressive strength, while none of the tested mortar properties appeared to be jeopardised by the incorporation of the GPC FRA in the mixPeer reviewedFinal Published versio

Chaliasou, Napoleana-Anna

Heath, Andrew

Paine, Kevin

English

Fly ash-slag based Geopolymer cement (GPC) has demonstrated mechanical properties and environmental advantages that make it one of predominant sustainable alternatives to Portland cement (PC). Despite the fact that numerous environmental analyses about geopolymers are being published, their environmental impact after the end of service-life has barely been explored. Given that construction-waste management is a major sustainability issue, the present study is investigating the potential of recycling fly ash-slag GPC as a fine aggregate in mortar mixes. The major physical properties of the fine recycled aggregates (FRA) were tested and compared to those of PC FRA and natural sand of similar fineness. The effect of incorporating FRA in low (25%) and high (50%) percentage in PC or GPC matrix mortars was investigated. The 28day compressive and flexural strength of mortars were tested. Also the 28day water absorption and flow of mixes incorporating GPC FRA were recorded. GPC FRA exhibited properties similar to those of PC FRA and poorer than those of natural sand. The results of compressive and flexural strength proved that FRA addition had a negligible effect in all cases. The influence of the high water absorption of GPC FRA, relatively to that of natural sand, was prominent on the workability of fresh mixes and possibly affected the water absorption of mortar prisms. The effect of GPC FRA proved to be similar to that of PC FRA on compressive strength, while none of the tested mortar properties appeared to be jeopardised by the incorporation of the GPC FRA in the mi

Chaliasou, Napoleana

OPUS

Recycling of fly ash-slag Geopolymer binder in mortar mixes

© 2022 Imperial College London.Fly ash-slag based Geopolymer cement (GPC) has demonstrated mechanical properties and environmental advantages that make it one of predominant sustainable alternatives to Portland cement (PC). Despite the fact that numerous environmental analyses about geopolymers are being published, their environmental impact after the end of service-life has barely been explored. Given that construction-waste management is a major sustainability issue, the present study is investigating the potential of recycling fly ash-slag GPC as a fine aggregate in mortar mixes. The major physical properties of the fine recycled aggregates (FRA) were tested and compared to those of PC FRA and natural sand of similar fineness. The effect of incorporating FRA in low (25%) and high (50%) percentage in PC or GPC matrix mortars was investigated. The 28day compressive and flexural strength of mortars were tested. Also the 28day water absorption and flow of mixes incorporating GPC FRA were recorded. GPC FRA exhibited properties similar to those of PC FRA and poorer than those of natural sand. The results of compressive and flexural strength proved that FRA addition had a negligible effect in all cases. The influence of the high water absorption of GPC FRA, relatively to that of natural sand, was prominent on the workability of fresh mixes and possibly affected the water absorption of mortar prisms. The effect of GPC FRA proved to be similar to that of PC FRA on compressive strength, while none of the tested mortar properties appeared to be jeopardised by the incorporation of the GPC FRA in the mixPeer reviewe

University of Hertfordshire Research Archive

1  Young Researchers’ Forum III Innovation in Construction Materials  12 April 2016          Paper Number XX    Recycling of fly ash-slag Geopolymer binder in mortar mixes  N.A. Chaliasou, A. Heath and K. Paine Department of Architecture and Civil Engineering, University of Bath   ABSTRACT Fly ash-slag based Geopolymer cement (GPC) has demonstrated mechanical properties and environmental advantages that make it one of predominant “green” alternatives to Portland cement (PC). Despite the fact that numerous environmental analyses about geopolymers are being published, their environmental impact after the end of service-life has barely been explored. Given that construction-waste management is a major sustainability issue, the present study is investigating the potential of recycling fly ash-slag GPC as a fine aggregate in mortar mixes. The major physical properties of the fine recycled aggregates (FRA) were tested and compared to those of PC FRA and natural sand of similar fineness. The effect of incorporating FRA in low (25%) and high (50%) percentage in PC or GPC matrix mortars was investigated. The 28day compressive and flexural strength of mortars were tested. Also the 28day water absorption and flow of mixes incorporating GPC FRA were recorded. GPC FRA exhibited properties similar to those of PC FRA and poorer than those of natural sand. The results of compressive and flexural strength proved that FRA addition had a negligible effect in all cases. The influence of the high water absorption of GPC FRA, relatively to that natural sand, was prominent on the workability of fresh mixes and possibly affected the water absorption of mortar prisms. The effect of GPC FRA proved to be similar to that of PC FRA on compressive strength, while none of the tested mortar properties appeared to be jeopardised by the incorporation of the GPC FRA in the mix.   1. INTRODUCTION Geopolymers were popularised by Joseph Davidovits in the 1970’s and their production relies on minimally processed natural materials or industrial by-products. There are various categories of geopolymer binders (depending on the raw materials), but from a terminological point of view, geopolymer cement is a binding system that hardens at ambient temperature. Given that this relies on addition of calcium (usually ground granulated blast furnace slag - GGBS), the most appropriate type for high volume construction applications is fly ash-GGBS based geopolymer (Davidovits, 2015). With the adoption of that type of geopolymer cement (GPC), extreme high temperature kilns with large expenditure of fuel and CO2 emissions by the decomposition of calcium carbonate are avoided. Therefore a reduction of carbon emissions at the range of 40-80% is considered possible ( Davidovits, 2013). But before proceeding to its wider adoption, the engineering community has to ensure tangible environmental benefits. An increasing number of life cycle assessments and environmental evaluations of GPCs are being conducted; most of them focusing on cradle-to-gate issues relating to the availability of materials or the environmental impact of the activating solutions (Habert, et al., 2011) (Fawer, 1999) (Heath, et al., 2014). Hardly any research concerning the stage after the end of service-life has been conducted.  Disposal of construction and demolition wastes is an issue that already troubles the construction industry as far as ordinary Portland cement (PC) concrete is concerned. Therefore it should be wise to investigate potential disposal or reuse scenarios for GPC before proceeding to its wider adoption. The present study is investigating the potential of recycling fly ash-GGBS based GPC by utilising it as recycled aggregate in mortar mixes. Fine recycled aggregates (FRA) were produced and basic physical properties such as water absorption (WA) and particle density were investigated. The potential of recycling GPC in mortar mixes with PC and fly ash-slag GPC matrices was investigated. The effect of GPC FRA on the 28 day flexural and compressive strength of mortars was tested and the results were compared to those obtained by similarly produced mortars with natural sand and PC FRA. Additionally, the flow and water absorption of mortars incorporating GPC FRA were recorded. 2. Materials and Experimental methodology The GGBS used for the GPC precursor was provided by the Hanson Heidelberg cement group from the Port Talbot works. The fly ash (FA) was CEMEX 450-S (BS EN 450 - 1 Fineness Category S; LOI Category B). The activating solution was produced by mixing sodium hydroxide pellets (NaOH, 98-100.5%) and sodium silicate solution (Na2O(SiO2)x · xH2O, Na2O, ~10.6%,SiO2, ~26.5%) with distilled water. The chemicals were both supplied by Sigma-Aldrich and the mixing of the activator solution took place 24 hours prior to casting. For the PC elements, General Purpose Portland fly ash cement/ Sulfacrete EN 197-1 –CEMII/BV 32,5R supplied by Tarmac Cement & 2  Lime Ltd was used. The natural aggregate incorporated in the mortars was standard sand complying with BS EN 196-1 specifications. The same materials were used for the fabrication of both the recycled aggregates and the mortars. Although the PC used had fly ash addition, the specimens will be referred to as PC for convenience. The FRA were produced in the laboratory by casting 100mm cubes of pure paste to establish a worst case. For the GPC binder the proportion of FA/GGBS was 1/1, water/GPC solids 0.4, the Na2O/SiO2 ratio 1 and the percentage of Na2O in the activating solution 5% by mass precursor. The water/GPC solids ratio was used and indicates that the products deriving from the dissolution of the compounds of the activating solution were taken into consideration for the calculation of the free water proportion for the mix. For the PC paste the water/cement ratio was 0.4. The cubes were demoulded 1 day after casting and then stored in sealed plastic bags at 20±1 oC for a period of 28 days before crushing. The resulting FRA was crushed and sieved to achieve fineness and particle distribution similar to that of standard natural sand. Table 1 Amount of materials used for the production of recycled aggregate mortars in kg/m3 Matrix type PC PC PC GPC GPC GPC RA 0% 25% 50% 0% 25% 50% PC 960 960 960 - - - FA - - - 480 480 480 GGBS - - - 480 480 480 NA 2160 1620 1080 2160 1620 1080 RA - 540 1080 - 540 1080 NaOH - - - 19 19 19 Silicate - - - 53 53 53 Water 451 451 451 463 463 463 The replacement percentages of natural sand in the final mortars were 25% and 50% while the reference mixes (0% replacement) incorporated standard sand only. For the GPC mortars water/GPC solids ratio was to 0.47 and the percentage of Na2O 3%, while all the other parameters were the same as for the original GPC binder. The water/cement ratio for the PC mortars was 0.47. For all mortar mixes the proportions were 2.25 parts of sand to 1 part of binder per mass. No presaturation of aggregates or water compensation took place. Mortar prisms of 40mmx40mmx160mm were fabricated and demoulded 24h after casting. Then they were kept at 20±1oC in moist conditions for 28 days. The amount materials used for the casting of the final mortars is presented in Error! Reference source not found.. The recycled aggregates were subjected to water absorption and particle density testing according to BS EN 1097-6:2013 Test for mechanical and physical properties of aggregates. The final mortars were tested according to BS EN 196-1:2005 Methods of testing cement and BS 1881-122:2011 Testing concrete. The notation used for the mixes follows the rationale Matrix type-Replacement percentage. 3. Results and discussion The results of the FRA testing are presented in  Table 2. The apparent particle density of GPC FRA is about 20% lower than that of natural sand but still at the same order of magnitude. For oven-dried and saturated surface dried condition the difference increases at about 48% and 33% accordingly. When compared to the results obtained from the testing of similarly produced PC FRA, it appears that GPC FRA are less influenced by oven drying and water immersion.  Table 2 Particle density and 24hour water absorption of natural sand, GPC and PC FRA Aggregate type Natural Sand GPC FRA PC FRA Particle density (kg/m3):      Apparent  2631 2041 2247   Oven-dried  2202 1151 895   Saturated-Surface dried  2367 1588 1497 The obtained results do not present significant divergence those reported in literature regarding PC recycled aggregates of similar fineness. The usual ranges are 1,970-2,140 kg/m3 for oven dried and 2,190-2,320kg/m3 for saturated surface dried density ( Dhir, et al., 1999) (Wai , et al., 2012) (Hansen, 1986) ( Silva, et al., 2014b). Given the nature of the origin material (binder without any aggregate) such values are considered reasonable. The most striking observation is that GPC FRA exhibited water absorption (WA) 5 times that of natural sand. The general trends reported in literature are much lower. Specifically, PC recycled aggregates are reported to demonstrate WA 3-6 times higher than that of natural aggregates, while FRA presenting values at the range of 8-12% (Akash Rao, 2007) (Hansen, 1986). Given that the tested PC FRA exhibited an exceptionally high WA as well, it can be assumed that this significant increase is due to the nature of the original material and the fineness to which the FRA were crushed. When evaluating the obtained results comparatively though, it can be stated that GPC FRA had a better performance than PC FRA. The remarkably high WA of GPC FRA was correlated with the results of the WA of mortar prisms and the flow of fresh mixes. As it can be observed by the values demonstrated in  Table 3, the WA of GPC FRA had a prominent effect on the workability of mortars. Specifically, a gradual reduction of flow was observed with increasing replacement levels for both matrix types. With incorporation of 25% GPC FRA in the mortar, a 13% reduction of flow occurred but the mixes maintained reasonable workability. With increased replacement level though, flow endured a reduction of about 50% for both matrix types and the mixes were hard to cast.  3  Table 3 Flow and 28day water absorption of mortar mixes incorporating GPC FRA. The relation to the reference mixes with 0% GPC FRA is stated as reduction % and increase %.  Flow Reduction WA Increase PC-0% 144% - 4.9% - PC-25% 125% 13% 6.7% 37% PC-50% 68% 53% 10.1% 108% GPC-0% 145% - 6.8% - GPC-25% 126% 13% 9.8% 45% GPC-50% 77% 47% 10.6% 56% These results are in accordance with the information reported in literature. Specifically, in most studies, it is mentioned that WA of aggregates influences directly the workability of the resulting mixes, as the hardened mortar tends to absorb the available water during mixing, resulting to less free water in the mix (Akash Rao, 2007) ( Zhao, et al., 2015) ( Evangelista & de Brito, 2007)  (Pepe, et al., 2014) (Silva, et al., 2014a). Additionally, aggregate porosity and WA have been correlated with the water absorption of the resulting mortar (Zega & di Maio , 2011)  ( Evangelista & de Brito, 2010). This remark is confirmed by the results in  Table 3. PC mortars appeared to be more prone to the influence of GPC FRA addition, given that for double replacement percentage, WA was double that of the reference mix. For the GPC mortars the replacement level did not seem to have a remarkable impact as in both cases an increase of about 50% took place.   Figure 1 Results of 28day compressive strength testing of PC and GPC mortars incorporating GPC FRA In fact the high WA of FRA in combination with the hardened paste could lead to an overall modification of the nominal w/c ratio and the resulting compressive strength of the mix. This could be related to the obtained results for compressive strength presented in Figure 1. In most cases the addition of recycled aggregates is reported to lead in decrease of mortar and concrete strength (Gerardu & Hendricks, 1985) ( Zhao, et al., 2015) (Hansen, 1986).  It is observed that regardless the replacement level, the addition of GPC FRA had a minor effect on the compressive strength of PC mixes. For 25% GPC FRA a slight increase is observed, while for 50% the strength is slightly lower than that of the reference mix. For high replacement level the favourable effect was probably counterbalanced by the dryness of the mix. In the case of GPC mortars a clear enhancement of strength is observed by the GPC FRA addition. Regardless the FRA level, a strength increase at the range of 50% took place. Apart for the justification given above for the case of PC mortars, an interpretation of the effect of GPC FRA addition could be based on the fact they derived from a binder with alkalinity higher than that of the final mortar. It is possible that alkalis from the FRA leached into the new mix resulting to the observed strength increase.  Figure 2 Results of 28day compressive strength testing of PC and GPC mortars incorporating PC FRA The results obtained by corresponding mixes incorporating PC FRA are presented in Figure 2. It is observed that in the case of PC mortar the replacement of natural sand had a similar effect, irrespective of the FRA type. For GPC mortars though, it is prominent that PC FRA had the opposite effect of GPC FRA. Summarizing the compressive strength results it could be stated that GPC FRA had a similar or enhancing effect compared to PC FRA.   Figure 3 Results of 28day flexural strength testing of PC and GPC mortars incorporating GPC FRA A similar trend was recorded for the results of the 28day flexural strength of mortars ( Figure 3). 4. Conclusions GPC FRA demonstrated physical properties inferior to those of natural sand but within acceptable limits for recycled aggregates. Their water absorption was significantly higher than the natural aggregate and influenced most of the final mortar properties in both direct and indirect ways. This value was attributed 35.924.238.935.335.237.4010203040506070OPCMortar GPCMortarCompressive Strength (MPa) 0%-RA25%-RA50%-RA35.924.241.912.337.011.6010203040506070OPCMatrix GPCMatrixCompressive strength (MPa)0%-RA25%-RA50%-RA7.836.436.68 6.566.114.65024681012OPCMatrix GPCMatrixFlexural Strength (MPa)0%-RA25%-RA50%-RA4  to aggregates’ fineness and the nature of the original material. The physical properties of similarly produced PC FRA proved to be inferior, signifying that, despite the values obtained in the present case, the use GPC FRA has potential. The workability of final mixes was influenced in a way acceptable for the case of recycled aggregate mortars. With adaptation of the FRA percentage and appropriate mix design this effect could be easily overcome. The 28day water absorption of PC mixes appeared to increase proportionally to the replacement level, while in GPC mixes the increase of FRA percentage did not result to significant fluctuations. Increase of mortar Water absorption is generally accepted to occur with FRA use. In both matrices though the increase for low replacement level was not extremely high. The compressive strength of both mortar times exhibited slight enhancement with 25% GPC FRA addition. For high replacement level the effect was less favorable but still resulted to values almost equal to those of the reference mixes. The same trend was observed for flexural strength. The effect of GPC FRA on compressive strength proved to be similar or more favorable than that of PC FRA in all cases. The overall conclusion is that the present research did not reveal any indication that could be considered as preventive for the use of GPCs as recycled in mortar mixes. Their effect on PC and GPC mortars was similar, while when compared to PC FRA they did not appear to be inferior in any aspect. For low replacement levels all the investigated properties present acceptable deviations from the reference mix. Given that in most real life cases the percentage of recycled aggregates in mixes does not exceed 30-35%, with adaptation of the mix design their use appears to be feasible. Despite the encouraging results though, it should be taken under consideration that the present study was limited in the investigation of specific physical and mechanical properties on mortars alone. Therefore the obtained results are not considered as conclusive. Although geopolymer cement does present mechanical properties similar to those of Portland, its intrinsic structure and chemistry are completely different. Therefore this research has to be expanded in order to take into account chemical aspects and microstructural parameters. Lastly, the investigation of the potential of recycling GPC mortars and concretes with various matrix types has to be conducted.  References Davidovits, J., 2013. Geopolymer Cement:A Review, France,Saint-Quentin: Institute Geopolymere. Dhir, R. K., Limbachiya, M. C. & Leelawat, T., 1999. Suitability of recycled concrete for use in BS 5328 designated mixes. Proc. Instn Civ.Engrs Structs & Bldgs,, Augoust , pp. 257 - 274. Evangelista, L. & de Brito, J., 2007. Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cement & Concrete Composites 29, p. 397–401. Evangelista, L. & de Brito, J., 2010. Durability performance of concrete made with fine recycled concrete aggregates. Cement & Concrete Composites 32 , pp. 9-14. Silva, R., de Brito, J. & R.K. Dhir, 2014a. Properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production. Construction and Building Materials 65, pp. 201-217. Zhao, Z., Remond, S., Damidot, D. & Xu, W., 2015. Influence of fine recycled concrete aggregates on the properties of mortars. Construction and Building Materials 81, p. 179–186. Akash Rao, K. N. J. ,. S. M., 2007. Use of aggregates from recycled construction and demolition waste in concrete. Resources, Conservation and Recycling 50, p. 71–81. Davidovits, J., 2015. Environmental implications of Geopolymers. [Online]  Available at: http://bit.ly/21Z1byQ [Accessed 15 January 2016]. Fawer, M. C. M. R. W., 1999. Life cycle inventories for the production of sodium silicates. The International Journal of Life Cycle Assessment, 4(4). Gerardu , J. & Hendricks, C., 1985. Recycling of road pavement materials in Netherlands.  Habert, G. d. L. J. B. N., 2011. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. Journal of Cleaner Production, 19(11). Hansen, T. C., 1986. Recycled aggregates and recycled aggregate concrete second state-of-the-art report developments 1945-1985. Materials and Structures, 19(3), pp. 201-246. Heath, A. P. K. M. M., 2014. Minimising the global warming potential of clay based geopolymers. Journal of Cleaner Production, Volume 78. Pepe, M., Koendersb, E. A., Faellaa, C. & Martinellia, E., 2014. Structural concrete made with recycled aggregates: Hydration process and compressive strength models. Mechanics Research Communications 58, p. 139–145. Silva, R., de Brito, J. & Dhir, R., 2014a. The influence of the use of recycled aggregates on the compressive strength of concrete: a review. European Journal of Environmental and Civil Engineering, p. DOI 10.1080/19648189.2014.974831. Wai , H. K., Mahyuddin , R., Kenn , J. K. & Mohd , Z. S., 2012. Influence of the amount of recycled coarse aggregate in concrete design. Construction and Building Materials,, Volume 26, p. 565–573. Zega, C. & di Maio , A., 2011. Use of recycled fine aggegate in concretes with durable requirements. Waste Management 31, pp. 2336-2340.  

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Recycling of fly ash-slag Geopolymer binder in mortar mixes

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