Flexural behaviour of cement added geopolymer concrete

Ordinary Portland cement (OPC) is the essential binding material to produce the OPC concrete. Production of OPC is recently attaining a rate of 2.6 billion ton per year worldwide and growing 5% annually. OPC contributes at rate of 5 – 8% of human-worldwide CO2 emissions which are the greenhouse gases pollute the atmosphere. Geopolymer concrete (GPC) is a creative, sustainable, economical and eco-friendly material for construction industry, which is a suitable alternative to the OPC concrete, able to extensively curb the CO2 emissions. To prepare this kind of concrete, a combination of pozzolanic material such as fly ash (FA), and/or ground granulated blast furnace slag (GGBS) rich with silica and alumina can react with alkaline activator solution producing aluminosilicate gel, acting as a superb binding material for fine and coarse aggregates under special conditions of curing. This study highlights the recent explorations on geopolymer mortars and concrete. Effect of chemicals such as sulphuric acid, effect of fly ash partial replacement with different binding materials, effect of concentration of alkaline activator solutions and the effect of temperature and time of curing variation have been discussed on durability and mechanical properties of geopolymer concrete. Results have shown superb resistance of geopolymer concrete to the detrimental effects of sulphuric acid on weight and compressive strength. Furthermore, fly ash partial replacement with silica fume, OPC or GGBS, or nanosilica inclusion in GPC has a positive effect on the GPC properties. Finally, using high concentration of sodium hydroxide has a detrimental effect on GPC properties.

PurposeThe purpose of this paper is to investigate the effect of the replacement of natural coarse aggregate (NCA) with different percentages of recycled coarse aggregate (RCA) on properties of low calcium fly ash (FA)-based geopolymer concrete (GPC) cured at oven temperature. Further, this paper aims to study the effect of partial replacement of FA by ground granulated blast slag (GGBS) in GPC made with both NCA and RCA cured under ambient temperature curing.Design/methodology/approachM25 grade of ordinary Portland cement (OPC) concrete was designed according to IS: 10262-2019 with 100% NCA as control concrete. Since no standard guidelines are available in the literature for GPC, the same mix proportion was adopted for the GPC by replacing the OPC with 100% FA and W/C ratio by alkalinity/binder ratio. All FA-based GPC mixes were prepared with 12 M of sodium hydroxide (NaOH) and an alkalinity ratio, i.e. sodium hydroxide to sodium silicate (NaOH:Na2SiO3) of 1:1.5, subjected to 90°C temperature for 48 h of curing. The NCA were replaced with 50% and 100% RCA in both OPC and GPC mixes. Further, FA was partially replaced with 15% GGBS in GPC made with the above percentages of NCA and RCA, and they were given ambient temperature curing with the same molarity of NaOH and alkalinity ratio.FindingsThe workability, compressive strength, split tensile strength, flexural strength, water absorption, density, volume of voids and rebound hammer value of all the mixes were studied. Further, the relationship between compressive strength and other mechanical properties of GPC mixes were established and compared with the well-established relationships available for conventional concrete. From the experimental results, it is found that the compressive strength of GPC under ambient curing condition at 28 days with 100% NCA, 50% RCA and 100% RCA were, respectively, 14.8%, 12.85% and 17.76% higher than those of OPC concrete. Further, it is found that 85% FA and 15% GGBS-based GPC with RCA under ambient curing shown superior performance than OPC concrete and FA-based GPC cured under oven curing.Research limitations/implicationsThe scope of the present paper is limited to replace the FA by 15% GGBS. Further, only 50% and 100% RCA are used in place of natural aggregate. However, in future study, the replacement of FA by different amounts of GGBS (20%, 25%, 30% and 35%) may be tried to decide the optimum utilisation of GGBS so that the applications of GPC can be widely used in cast in situ applications, i.e. under ambient curing condition. Further, in the present study, the natural aggregate is replaced with only 50% and 100% RCA in GPC. However, further investigations may be carried out by considering different percentages between 50 and 100 with the optimum compositions of FA and GGBS to enhance the use of RCA in GPC applications. The present study is further limited to only the mechanical properties and a few other properties of GPC. For wider use of GPC under ambient curing conditions, the structural performance of GPC needs to be understood. Therefore, the structural performance of GPC subjected to different loadings under ambient curing with RCA to be investigated in future study.Originality/valueThe replacement percentage of natural aggregate by RCA may be further enhanced to 50% in GPC under ambient curing condition without compromising on the mechanical properties of concrete. This may be a good alternative for OPC and natural aggregate to reduce pollution and leads sustainability in the construction.

Evaluation of the mechanical properties of sea sand-based geopolymer concrete and the corrosion of embedded steel bar

This study investigates the shear behavior of slender steel-reinforced geopolymer concrete (GPC) beams with the shear span to effective depth ratio (a/d) of 4.5 and 5.0. To investigate the effect of shear reinforcement, two ordinary Portland cement concrete (OPC) beams and two GPC beams without shear reinforcement, and two OPC beams and two GPC beams reinforced with shear stirrups were cast. All beams were 150 mm wide and 225 mm deep with lengths of 1770 mm (a/d=4.5) and 1950 mm (a/d=5). The beams were tested under a three-point bending test. The experimental results showed that OPC and GPC beams without and with shear reinforcements exhibited similar crack propagation and failure mechanism. The midspan deflections of GPC beams were greater than OPC beams. The normalized shear resistance of OPC and GPC beams with a/d ratio 4.5 was greater than 4% and 30%, respectively, than beams with a/d ratio 5. OPC beams showed a greater decrease in shear resistance with an increasing a/d ratio compared to GPC beams. The shear resistances computed using empirical relationships available in various OPC design codes including AC1-318-14, AC1-318-19, fib-10 and JSCE-07 underestimated the experimental shear resistance of both OPC and GPC beams. In addition, the environmental assessment of OPC and GPC beams exhibited that GPC beams emit about 34% lower embodied CO2 emissions than OPC beams.

Geopolymer is a new development in the world of concrete which cement is replaced by pozzolanic materials such as fly ash and activated by an alkali activator. Alkali activator will act as a binder in concrete mixtures. An exposure condition of seawater to the geopolymer concrete is significant to be studied since the properties of concrete will be affected especially exposure to an aggressive environment. The aim of this study is to evaluate the durability of fly ash-based geopolymer concrete in seawater exposure, then compared to ordinary Portland cement (OPC) concrete. The ratios of fly ash to alkali activator (FA/AA) and sodium silicate to sodium hydroxide (Na2SiO3/NaOH) were fixed as 2.0 and 2.5, respectively. The concentration of NaOH used was 12M. The fly ash will be mixed with alkali activator until homogeneous. After moulding process, the sample of concrete will be immersed in seawater for 7, 14, 21, 28 and 60 days. The results showed that the density of geopolymer concrete was higher than OPC concrete. The water absorption for 60 days of curing showed the geopolymer concrete was 0.9%, meanwhile OPC concrete achieved 8.3%. The different percentage of weight loss test for geopolymer concrete and OPC concrete was 1%. The compressive strength for geopolymer concrete was reported as higher than OPC concrete.

Concrete, primarily composed of cement, contributes substantially to environmental degradation due to high CO₂ emissions. With cement production increasing by approximately 7% annually, geopolymer concrete (GPC) offers a sustainable alternative. By using industrial waste ashes as a primary binder instead of cement, GPC reduces energy consumption, mitigates waste disposal issues, and significantly lowers carbon emissions. This study assessed the effects of Rice Husk Ash (RHA), Cassava Peel Ash (CPA), and Sugarcane Bagasse Ash (SCBA) as partial replacements for Fly Ash (FA) in GPC, focusing on compressive and split tensile strengths. The mix preparation involved an alkaline solution prepared in advance, combined with FA, RHA, SCBA, CPA, sharp sand, and granite in a 1:1 liquid-to-solid ratio. The mix was cast, oven-cured for 4 h, and tested at 7, 14, and 28 days. The 100% Fly Ash (FA) mix achieved the highest compressive strength of 2.3 N/mm2 at 28 days due to its low SiO₂/Al₂O₃ ratio (0.28) and high alumina content, which promote efficient geopolymerization. In terms of tensile strength, the 50% FA + 50% CPA mix exhibited the highest value of 0.65 N/mm2 at 28 days, surpassing the 100% FA mix (0.30 N/mm2) and the 50% FA + 50% SCBA mix (0.60 N/mm2). The relatively low strength values are attributed to the short 4-h curing period, which limited full geopolymerization. This study demonstrates that using agricultural waste ashes in geopolymer concrete promotes sustainability and highlights how SiO₂/Al₂O₃ ratios influence compressive and tensile strength, guiding optimized mix designs.

The structural behaviours of steel reinforced geopolymer concrete beams: An experimental and numerical investigation

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel.

Geopolymers are an emerging type of cementitious material purported to provide an environmentally friendly alternative to Portland cement-based concrete. This paper reports the results of experimental research on fracture properties (fracture energy and brittleness) of fly ash based geopolymer concrete and paste with various mix parameters. The characteristic length of the geopolymer concrete was approximately three times less than that of ordinary Portland cement (OPC) concrete, due to an increase in tensile splitting strength of about 28%, a decrease in elastic modulus of about 22% and a decrease in fracture energy of about 24%. The difference in characteristic length is similar to that reported between high-strength and normal-strength OPC concretes, indicating that the geopolymer concrete exhibits higher brittleness than its OPC counterpart. This trend was found to be consistent between pastes and concretes, implying that the difference between geopolymer and OPC concrete is due to the type of matrix formation (geopolymerisation or hydration). For geopolymer concretes made with different mix parameters, fracture properties are closely correlated to their compressive strength.

Behaviour of ambient cured prestressed and non-prestressed geopolymer concrete beams

Portland cement concrete is a world-wide used construction material. However, when Portland cement concrete is exposed to fire, its mechanical properties are deteriorated. The deterioration of concrete is generally caused by the decomposition of the Portland cement hydrate or the thermal incompatibility between cement paste and aggregate. Spalling, which is a violent or non-violent breaking off of layers or pieces of concrete from the surface of a structural element, may also occur when the concrete is exposed to rapidly rising temperatures. It is generally believed that spalling is influenced by the build-up of pore water pressure and thermal gradient in the concrete when exposed to elevated temperatures. Geopolymer is an alternative cementitious material which has ceramic-like properties. Geopolymer belongs to the family of inorganic polymers. The chemical composition of geopolymer is similar to natural zeolite, but the microstructure is amorphous. It is suggested that geopolymer processes a potential superior fire resistance due to its amorphous and ceramic-like properties. The objective of this thesis is to study the fire resistance of geopolymer material and to explore the spalling behaviour of geopolymer material when exposed to elevated temperatures. In this thesis, a method was presented to carry out spalling test in small scale specimen with exposure to rapid temperature rise using a commonly available electric furnace. Hydrocarbon fire and standard fire exposure can be simulated by manipulating the exposure location of the surface of the concrete cylinder. Ordinary Portland cement concrete cylinders with different strengths were tested. The results demonstrated that this method was an effective and convenient technique to predict the spalling risk of a concrete. The spalling behaviour of geopolymer concrete by using the surface exposure test and standard gas furnace fire test was studied. It was shown that 100% fly ash based geopolymer concrete had a better spalling resistance to rapidly rising temperature exposure than that of Portland cement concrete. The study of sorptivity test of geopolymer concretes results showed that the geopolymer concrete specimen's structure is more porous and more continuous pore structure than Portland cement concrete specimen. The more porous structure of geopolymer than OPC concrete facilitates the release of the internal steam pressure during heating. Hence, less tensile stress is imposed in the geopolymer concrete than Portland cement concrete during heating, reducing the geopolymer's risk of spalling. When slag was used as a replacement to fly ash in the geopolymer binder, geopolymer paste and concrete specimens developed considerably high strength at room temperature. It was showed that the magnitude of shrinkage of fly ash and slag based geopolymer is significantly higher than that of 100% fly ash based geopolymer and Portland cement concrete. The residual strength of fly ash based geopolymer concrete with slag replacement after exposure to elevated temperature was studied. It was found that the residual strength of 100% fly ash based geopolymer concrete after elevated temperature exposure increased in the temperature range of 200~500°C compared with OPC concretes. Fly ash based geopolymer concrete with slag replacement experienced a strength loss at the temperature range of 200~300°C, then followed by a strength gain at 300~400°C, and another strength loss after 500°C. When slag was used as an additive to fly ash based geopolymer concrete, the overall strength loss of geopolymer concretes with slag replacement after exposure to elevated temperatures ranging from 200~800°C was higher compared with 100% fly ash based geopolymer concrete, however, it was significantly lower than that of the Portland cement concrete specimens. The investigation of fire resistance property of fly ash and slag based geopolymer material when exposed to hydrocarbon fire was followed. After hydrocarbon fire, no spalling was observed on geopolymer concretes when using varying factors as binder, slag replacement, cation type of alkaline liquid activator, room temperature and elevated temperature curing. Residual strength testing of geopolymer concretes after hydrocarbon fire exposure showed a similar residual strength percentage compared with the result of Portland cement concrete. However, it is noted that high strength Portland cement concrete spalled, while high strength geopolymer concrete still had a considerably high residual strength after fire exposure.

Geopolymer is an inorganic alumino-silicate product that shows good bonding properties. Geopolymer binders are used together with aggregates to produce geopolymer concrete which is an ideal building material for infrastructures. A by-product material such as fly ash is mixed together with an alkali to produce geopolymer. Current research on geopolymer concrete has shown potential of the material for construction of reinforced concrete structures. Structural performance of reinforced concrete depends on the bond between concrete and the reinforcing steel. Design provisions of reinforced concrete as a composite material are based on the bond strength between concrete and steel. Since geopolymer binder is chemically different from Ordinary Portland Cement (OPC) binder, it is necessary to understand the bond strength between geopolymer concrete and steel reinforcement for its application to reinforced concrete structures. Pull out test is commonly used to evaluate the bond strength between concrete and reinforcing steel. This paper describes the results of the pull out tests carried out to investigate the bond strength between fly ash based geopolymer concrete and steel reinforcing bars. Beam end specimens in accordance with the ASTM Standard A944 were used for the tests. In the experimental program, 24 geopolymer concrete and 24 OPC concrete specimens were tested for pull out. The concrete compressive strength varied from 25 to 55 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 500 MPa steel deformed bars of 20 mm and 24 mm diameter. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. It was found from the test results that the failure occurred by splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This suggests that the existing design equations for bond strength of OPC concrete with steel reinforcing bars can be conservatively used for calculation of bond strength of geopolymer concrete.

Durability evaluation of geopolymer and conventional concretes

The properties of concrete using fly ash based geopolymer as the binder were shown in recent studies. However, most of the previous studies focused on the properties of geopolymer concrete samples cured at high temperature. In this study, fly ash based geopolymer concrete suitable for curing at ambient temperature was designed and some durability properties were investigated. Geopolymer mixtures were prepared with fly ash as the primary binder which was activated by a mixture of sodium silicate and sodium hydroxide solutions. Ground granulated blast furnace slag (GGBFS) was added as 0%, 10% and 20 % of the total binder. Samples were also cast from an ordinary Portland cement (OPC) concrete mixture in order to compare with the properties of geopolymer and OPC concretes. All the concrete samples were ambient-cured (15-20°C) after casting until tested. The tests conducted include compressive strength, drying shrinkage, sorptivity and volume of permeable voids (VPV) test. The strength of the geopolymer concretes enhanced from the early age and continued to develop in similar trend as OPC concrete. Strength increased with the increase of slag in the mixture. The geopolymer concretes showed drying shrinkage, sorptivity and VPV values comparable to those of the control OPC concrete. In general, the results show that it is possible to design fly ash and slag blended geopolymer concrete suitable for ambient curing with similar or better durability properties of conventional OPC concrete.

Abstract. The manufacturing of cement for use in construction sites all over the world has resulted in tonnes of carbon dioxide being released. It is better to replace an alternative material such as geopolymer which contributes less carbon footprint than traditional Portland cement. Concrete is the most versatile building material, yet it has drawbacks in mechanical and physical properties, such as limited ductility, high water absorption, and low compressive strength. This study aims to determine the effect of the addition of composite fibers on the mechanical and physical properties of geopolymer concrete. In this research, the physical and mechanical properties of geopolymer concrete were investigated by mixing Class F fly ash with an alkaline activator consisting of sodium hydroxide and sodium silicate. Steel fiber and polypropylene cut into 2 mm fiber were added into the geopolymer concrete as reinforcement. Various volume percentages ranging from 0% to 2% are used. Density, water absorption, workability, and compression testing were performed on all geopolymer concrete reinforced with steel and polypropylene fiber with varying volume percentages. The density of geopolymer concrete is similar to that of Ordinary Portland Cement (OPC), which is around 2400 kg/m3, and it has gradually increased with the inclusion of steel fiber, while more polypropylene fiber causes lesser density. With an increment of steel fiber and a decrement of polypropylene fiber, the result of water absorption percentage shows a decrement. Besides, the inclusion of steel fibers reduces the workability of geopolymer concrete. However, the addition of polypropylene fibers shows a higher workability. Plus, the inclusion of steel fiber and the reduction of polypropylene fiber improve the compressive strength.