The Use of Solid Sodium Silicate as Activator for an Amorphous Wollastonitic Hydraulic Binder

To ensure the acceptable mechanical strength of amorphous wollastonitic hydraulic binders (AWHs), activation with a sodium silicate solution is necessary. However, the use of this type of activator increases the final cost and the complexity of the product's overall use. In this work, we focus on enhancing the manufacturing of the alkaline activator by producing three Na2SiO3 powders using cost-effective raw materials. The procedure consisted of heating a mixture of NaOH pebbles with either sand, glass, or diatomite to a temperature of 330 °C for 2 h. After synthesis, the powders were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) techniques. Finally, mortars made with AWHs were activated using the synthesized powders that were added either as a solid or dissolved in an aqueous solution. The compressive strength results in these mortars show that the lab-made activators are competitive with the traditional sodium silicate activators. Furthermore, the synthetized activators can be added in either solid form or pre-dissolved in a solution. This innovative approach represents a more economical, sustainable and easy-to-use approach to enhancing the competitiveness of AWHs.

The Effect of Oyster Shell Powder on the High-Temperature-Properties of Slag-Ceramic Powder-Based Geopolymer

There is a lack of scientific understanding of adding an oyster shell powder (OSP) to geopolymer concrete. The purpose of this study is: (1) to evaluate the high-temperature resistance of the alkali-activated slag ceramic powder (CP) mixture added with OSP at different temperatures, (2) to address the lack of application of environmentally friendly building materials, and (3) to reduce solid waste of OSP pollution and protect the environment. OSP replaces granulated blast furnace slag (GBFS) and CP at 10% and 20% (based on binder), respectively. The mixture was heated to 400.0, 600.0, and 800.0 °C after curing for 180 days. The results of the experiment are summarized as follows: (1) The thermogravimetric (TG) results indicated that the OSP20 samples produced more CASH gels than the control OSP0. (2) As the temperature increased, the compressive strength and ultrasonic pulse velocity (UPV) both decreased. (3) Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) results reveal that the mixture undergoes a phase transition at 800.0 °C, and compared with the control OSP0, OSP20 undergoes a different phase transition. (4) The size change and appearance image results indicate that the mixture with added OSP inhibits shrinkage, and calcium carbonate decomposes to produce off-white CaO. To sum up, adding OSP can effectively reduce the damage of high temperatures (800.0 °C) on the properties of alkali-activated binders.

Experimental investigation of geopolymer concrete along with biomedical and bone China waste at different molarities of sodium hydroxide

In this study, geopolymer concrete (GPC) is prepared using fly ash as source material along with alkaline activators (sodium hydroxide + sodium silicate) for sustainable development. There are three different sodium hydroxide molarities: 8, 12, and 16 utilised. Incinerated biomedical waste ash (BMW) and bone China waste (BCW) are substituted for the fine aggregates in GPC at varying ratios of 10, 20, 30, 40, and 50%. The utilisation of wastes in place of fine aggregates in GPC are helpful in solving the dumping problem of wastes, saving energy, and natural resources (sand quarries). The results showed that, relative to the control mix, the density, workability, and strength increased up to 60% replacement of sand by 30% BMW and 30% BCW and beyond this, the strength and other properties decreased. In contrast to the combination of 50% BMW and 0% BCW, the mix with 50% BCW and 0% BMW demonstrated great strength. In terms of molarity, the mixes with 16 M sodium hydroxide concentration showed higher workability, density, strength and lower air content as compared to the mixes with 8 M and 12 M sodium hydroxide concentration.

Effects of the Curing Regime, Acid Exposure, Alkaline Activator Dosage, and Precursor Content on the Strength Development of Mortar with Alkali-Activated Slag and Fly Ash Binder: A Critical Review

Reductions of green gas emissions and the reuse/recycling of industrial byproducts are important for the mitigation of the environmental impact of the construction industry. The replacement of ordinary Portland cement (OPC) is a concrete binder with industrial byproducts that possess sufficient cementitious and pozzolanic properties, such as ground granulated blast furnace slag (GBS) and fly ash. This critical review analyzes the effect of some of the most critical parameters on the development of the compressive strength of concrete or mortar that consists of combinations of alkali-activated GBS and fly ash as binders. The review includes the effects of the curing environment, the proportions of GBS and fly ash in the binder, and the concentration of the alkaline activator on strength development. The article also reviews the effect of exposure as well as the age of samples at the time of exposure to acidic media on the development of concrete strength. The effect of acidic media on mechanical properties was found to depend not only on the type of acid but also on the alkaline activator solution, proportions of GBS and fly ash in the binder, and the age of the sample at the time of exposure, among other factors. As a focused review, the article pinpoints important findings such as the change in compressive strength over time when mortar/concrete is cured in an environment that permits the loss of moisture versus curing in a system that retains the alkaline solution and keeps reactants available for hydration and the development of geopolymerization products. The relative contents of slag and fly ash in blended activators have a significant impact on strength development. Research methods used include a critical review of the literature, a comparison of reported research findings, and identifying reasons for agreement or disagreement of findings.

Influence of modulus of alkaline activator on the removal of Pb2+ by mesoporous geopolymer adsorbent

In this study, we synthesised metakaolin-based mesoporous geopolymer adsorbent and investigated the effect of alkaline activator modulus (molar ratios of SiO₂/Na₂O) on Pb²⁺ adsorption. The geopolymer prepared using 1.2 M alkaline activator performs excellent Pb²⁺ removal with a maximum adsorption capacity of 172.71 mg g^-1. The pseudo-second-order model fit the adsorption kinetics satisfactorily, indicating that the adsorption process is dominated by chemical adsorption. The adsorption data appropriately fit the Langmuir isotherm model. The contributions of adsorption methods corresponding to the total quantity adsorbed declined in the following order: EDTA extraction (formation of Pb aluminium oxide and Pb-containing amorphous materials) > residual fraction (Pb stabilisation in the tetrahedral aluminosilicate network) > ion exchange. Overall, the alkaline activator modulus significantly influenced the Pb²⁺ adsorption characteristics of the geopolymer adsorbent.

Statistical Methods for Modeling the Compressive Strength of Geopolymer Mortar

In recent years, geopolymer has been developed as an alternative to Portland cement (PC) because of the significant carbon dioxide emissions produced by the cement manufacturing industry. A wide range of source binder materials has been used to prepare geopolymers; however, fly ash (FA) is the most used binder material for creating geopolymer concrete due to its low cost, wide availability, and increased potential for geopolymer preparation. In this paper, 247 experimental datasets were obtained from the literature to develop multiscale models to predict fly-ash-based geopolymer mortar compressive strength (CS). In the modeling process, thirteen different input model parameters were considered to estimate the CS of fly-ash-based geopolymer mortar. The collected data contained various mix proportions and different curing ages (1 to 28 days), as well as different curing temperatures. The CS of all types of cementitious composites, including geopolymer mortars, is one of the most important properties; thus, developing a credible model for forecasting CS has become a priority. Therefore, in this study, three different models, namely, linear regression (LR), multinominal logistic regression (MLR), and nonlinear regression (NLR) were developed to predict the CS of geopolymer mortar. The proposed models were then evaluated using different statistical assessments, including the coefficient of determination (R²), root mean squared error (RMSE), scatter index (SI), objective function value (OBJ), and mean absolute error (MAE). It was found that the NLR model performed better than the LR and MLR models. For the NLR model, R², RMSE, SI, and OBJ were 0.933, 4.294 MPa, 0.138, 4.209, respectively. The SI value of NLR was 44 and 41% lower than the LR and MLR models’ SI values, respectively. From the sensitivity analysis result, the most effective parameters for predicting CS of geopolymer mortar were the SiO₂ percentage of the FA and the alkaline liquid-to-binder ratio of the mixture.

Durability Performance of Geopolymer Concrete: A Review

Geopolymer concrete is produced from the geopolymerization process, in which molecules known as oligomers integrate to form geopolymer networks with covalent bonding. Its production expends less thermal energy and results in a smaller carbon footprint compared to Ordinary Portland Cement (OPC) concrete. It requires only an alkaline activator to catalyze its aluminosilicate sources such as metakaolin and fly ash, to yield geopolymer binder for the geopolymerization to take place. Because of its eco-friendly technology and practical application, current research interest is mainly concentrated on the endurance of geopolymer concrete to resist heat and chemical aggressions. As such, it is pertinent for this review article to provide critical insight into the recent progress in research on the durability of geopolymer concrete. One significant outcome of the review is that the admixture of geopolymer concrete could be blended with additives such as micro-silica and fibers such as polypropylene fibers, to enhance its durability. The review on the durability aspects of geopolymer concrete showed that it had high compressive strength at an optimal elevated temperature, low to medium chloride ion penetrability, and high resistance to acid attack and abrasion. This makes geopolymer concrete a viable candidate to replace OPC concrete in the construction industry.

Influence of Liquid-to-Solid and Alkaline Activator (Sodium Silicate to Sodium Hydroxide) Ratios on Fresh and Hardened Properties of Alkali-Activated Palm Oil Fuel Ash Geopolymer

Malaysia is one of the largest palm oil producers in the world and its palm oil industry is predicted to generate a large amount of waste, which increases the need to modify it for sustainable reuse. The green geopolymers produced from industrial waste can be a potential substitute for cementitious binders. This type of polymer helps reduce dependency on cement, a material that causes environmental problems due to its high carbon emissions. Palm oil fuel ash (POFA) geopolymer has been widely investigated for its use as a sustainable construction material. However, there is still uncertainty regarding the total replacement of cement with POFA geopolymer as a binder. In this study, we examined the effects of different material design parameters on the performance of a POFA-based geopolymer as a building material product through iterations of mixture optimisation. The material assessed was a single raw precursor material (POFA) activated by an alkaline activator (a combination of sodium hydroxide and sodium silicate with constant concentration) and homogenised. We conducted a physical property test, compressive strength test, and chemical composition and microstructural analyses to evaluate the performance of the alkali-activated POFA geopolymer at 7 and 28 days. According to the results, the optimum parameters for the production of alkali-activated POFA paste binder are 0.6 liquid-to-solid ratio and 2.5 alkaline activator ratio. Our results show that the use of alkali-activated POFA geopolymer is technically feasible, offering a sustainable and environmentally friendly alternative for POFA disposal.

A Review of Durability and Strength Characteristics of Alkali-Activated Slag Concrete

Alkali-activated slag (AAS) is a promising alternative to ordinary Portland cement (OPC) as sole binder for reinforced concrete structures. OPC is reportedly responsible for over 5% of the global CO₂ emission. In addition, slag is an industrial by-product that must be land-filled if not re-used. Therefore, it has been studied by many investigators as environmentally friendly replacement of OPC. In addition to recycling, AAS offers favorable properties to concrete such as rapid development of compressive strength and high resistance to sulfate attack. Some of the potential shortcomings of AAS include high shrinkage, short setting time, and high rate of carbonation. Using ground granulated blast furnace slag (GGBS) as an alternative to OPC requires its activation with high alkalinity compounds such as sodium hydroxide (NaOH), sodium sulfate (Na₂SO₃), sodium carbonate (Na₂CO₃), or combination of these compounds such as NaOH and Na₂SO₃. The mechanism of alkali-activation is still not fully understood and further research is required. This paper overviews the properties, advantages, and potential shortcomings of AAS concrete.

On the Utilization of Pozzolanic Wastes as an Alternative Resource of Cement

Recently, as a supplement of cement, the utilization of pozzolanic materials in cement and concrete manufacturing has increased significantly. This study investigates the scope to use pozzolanic wastes (slag, palm oil fuel ash and rice husk ash) as an alkali activated binder (AAB) that can be used as an alternative to cement. To activate these materials, sodium hydroxide solution was used at 1.0, 2.5 and 5.0 molar concentration added into the mortar, separately. The required solution was used to maintain the flow of mortar at 110% ± 5%. The consistency and setting time of the AAB-paste were determined. Mortar was tested for its flow, compressive strength, porosity, water absorption and thermal resistance (heating at 700 °C) and investigated by scanning electron microscopy. The experimental results reveal that AAB-mortar exhibits less flow than that of ordinary Portland cement (OPC). Surprisingly, AAB-mortars (with 2.5 molar solution) achieved a compressive strength of 34.3 MPa at 28 days, while OPC shows that of 43.9 MPa under the same conditions. Although water absorption and porosity of the AAB-mortar are slightly high, it shows excellent thermal resistance compared to OPC. Therefore, based on the test results, it can be concluded that in the presence of a chemical activator, the aforementioned pozzolans can be used as an alternative material for cement.

Fly ash-based geopolymer lightweight concrete using foaming agent

In this paper, we report the results of our investigation on the possibility of producing foam concrete by using a geopolymer system. Class C fly ash was mixed with an alkaline activator solution (a mixture of sodium silicate and NaOH), and foam was added to the geopolymeric mixture to produce lightweight concrete. The NaOH solution was prepared by dilute NaOH pellets with distilled water. The reactives were mixed to produce a homogeneous mixture, which was placed into a 50 mm mold and cured at two different curing temperatures (60 °C and room temperature), for 24 hours. After the curing process, the strengths of the samples were tested on days 1, 7, and 28. The water absorption, porosity, chemical composition, microstructure, XRD and FTIR analyses were studied. The results showed that the sample which was cured at 60 °C (LW2) produced the maximum compressive strength for all tests, (11.03 MPa, 17.59 MPa, and 18.19 MPa) for days 1, 7, and 28, respectively. Also, the water absorption and porosity of LW2 were reduced by 6.78% and 1.22% after 28 days, respectively. The SEM showed that the LW2 sample had a denser matrix than LW1. This was because LW2 was heat cured, which caused the geopolymerization rate to increase, producing a denser matrix. However for LW1, microcracks were present on the surface, which reduced the compressive strength and increased water absorption and porosity.