High Temperature Resistance of Fly Ash-Enhanced Alkali Activated Portland Cement Mortar: Microstructural Evolution and Strength Retention

A comparative study on solidification/stabilization characteristics of coal fly ash-based geopolymer and Portland cement on heavy metals in MSWI fly ash

Circulating fluidized bed combustion (CFBC) is a newer type of burner that employ a circulating process to burn fuel effectively. CFBC burning process is gaining more popularity due to its compact size, high efficiency and lower burning temperature compared to the pulverized coal combustion (PCC) burner. The CFBC burner produces fly ash with different physical properties compared to the PCC burner, i.e. the fly ash is not rounded, and required higher water content for comparable workability. The CFBC fly ash also has a high sulfur content that is detrimental for hardened concrete. Due to its drawbacks, the CFBC hardly used as cementitious material and geopolymer precursor. This study focuses on comparing variations in the concentration of NaOH solution and variations in the ratio of alkaline activators to the setting time and compressive strength of geopolymer mortars on a new class of CFBC fly ash, which have low sulfur content. The concentrations of NaOH solution were 6M, 8M, 10M, and 12M, while the alkaline activator ratios used were 3.0, 2.5, 2.0, 1.0, and 0.5. It was concluded that the low sulfur CFBC fly ash has a potential to be utilized as geopolymer precursor, however, with a shortcoming in its high water demand. The CFBC fly ash used in this study resulted in a geopolymer matrix with good compressive strength and stability. The water demand varies with the fly ash sampling time shows the challenges in the utilization of the fly ash. The highest mortar’s compressive strength, 33.4 MPa at 90 days was achieved at NaOH concentration of 8M and ratio of sodium silicate solution to sodium hydroxide solution of 2.5 with excellent stability.

Among the construction materials; concrete, steel, timber and glass, concrete has gained popularity all over the world due to its durable properties in normal environment and easiest construction procedure. Compare to others constituent, cement performs a vital role for the production of concrete. Due to continuous increasing demand and the cost of cement, recently, the utilization of supplementary cementing materials such as industrial by-product (fly ash, silica fume and slag) and agricultural wastes (rice husk ash, palm oil fuel ash, bagasse ash and ash from timber) has become an important issue for the researchers in concrete industry. Fly ash (FA), one of these valuable industrial wastes, is generated as by-product from power generating industry. The production of FA increases every year, it is disposed for landfills without any commercial gain and now becomes a trouble. It contains a non-crystalline silicon dioxide with high specific surface area and high pozzolanic reactivity. Huge researches have been carried out for the use of pozzolans, mainly waste pozzolans such as FA, slag and rice husk ash, as a supplement of ordinary portland cement (OPC). Test results of compressive strength and durability of concrete from those previous researches ensured the use of FA as a pozzolanic material for cement replacement in concrete. In this paper, a critical review on the strength development of concrete as influenced by the use of FA as a supplement of cement in concrete has been presented on the basis of available information in the published literatures of utilization of FA in blended cement and concrete. The compressive strength of mortar and concrete as varied by the percent replacement and fineness of FA is discussed here. Physical and chemical properties, pozzolanic activity, normal consistency and setting time, strength activity index, advantages and disadvantages of using FA in concrete are also pointed out. Proper consumption of FA as pozzolanic material in concrete would be a useful step for the production of cost effective and more durable concrete. Besides, utilization of FA in cement and concrete could reduce negative environmental effect, and also would be the appropriate solution for the disposal of this waste. Key words: Fly ash, mortar, concrete, compressive strength.

In the present work, Granulated Blast Furnace Slag (GBFS) and Fly ash (FA) were used as partial replacement of Natural Sand (NS) and Ordinary Portland Cement (OPC) by weight. One control mix, one with GBFS, three with FA and three with GBFS-FA combined mixes were prepared. Replacements were 50% GBFS with NS and 20%, 30% and 40% FAwith OPC. Preliminary investigation on development of compressive strength was carried out at 7, 28 and 90 days to ensure sustainability of waste materials in concrete matrix at room temperature. After 90days, thermo-mechanical study was performed on the specimen for a temperature regime of 200o-1000oC followed by furnace cooling. Weight loss, visual inspection along with colour change, residual compressive strength and microstructure analysis were performed to investigate the effect of replacement of GBFS and FA. Although adding waste mineral by-products enhanced the weight loss, their pozzolanicity and formation history at high temperature played a significant role in retaining higher residual compressive strength even up to 800oC. On detail microstructural study, it has been found that addition of FA and GBFS in concrete mix improved the density of concrete by development of extra calcium silicate gel before fire and restricts the development of micro-cracks at high temperature as well. In general, the authors are in favour of combined replacement mix in view of high volume mineral byproducts utilization as fire protection.

Concrete is a combination of cement, fine aggregate, coarse aggregate, water and admixtures. According to world coal association survey over 4.1 billion tons of Ordinary port land cement (OPC) was used across globally in 2020 and also use of OPC emits CO2 to the atmosphere.In order to overcome the problem, a search for alternative materials is the need of the hour and apart from cement, fine aggregate is also important additive to concrete. Due to the speedyconstruction rate deficiency of materials occurs reduction in natural aggregates causes problems like dredging of sand in large scale which creates environmental imbalance. The solution is utilisation of Fly ash, silica fume, Ground granulated blast furnace slag(GGBS) which are comes under Industrial by-products. The disposal of Industrial by-product should done properly, else it will cause land pollution. In this present study silica fume, fly ashes are replaced in OPC and fine aggregate with Granulated blast furnace slag(GBFS). The experimental work execute in order to determine mechanical properties such as compression, split tensile and flexural strength of concrete with age 3,7 and 28 days at various combinations of cement with fly ash varies 20%,30%,40% and constant 8%of silica fume and fine aggregate replaces with Granulated blast furnace slag of 30%,40% and 50% in M40 grade concrete with water-cement ratio 0.38. As per experimental results conclusions were drawn. As per the experimental results it is recommended that OPC is replaced with 8% of silica fume, 20% of fly ash and fine aggregate replaced with 40% of GBFS achieved optimum strength.

Life cycle assessment and cost analysis of fly ash–rice husk ash blended alkali-activated concrete

The paper examines the properties of fly ash as partial cement replacement in normal strength concrete at ambient and elevated temperature. The properties of fly ash class F was chemically investigated. Behaviour of concrete containing fly ash subjected to elevated temperature 200 ºC up to 600 ºC at temperature rate of 2.78 ºC/min is investigated mechanically and microstructurally. Physically, fly ash concrete changed colour to brown at 400 ºC and to whitish grey at 600 ºC, which is similar to normal concrete. At 600 ºC, the residual compressive strength of fly ash concrete was slightly higher than normal concrete with a strength reduction of 13.18% and 45.22%, respectively. At 400 ºC, fly ash concrete has better splitting tensile strength than normal concrete with strength reduction of 28.81% and 43.26%, respectively. Thus, the use of fly ash as cement replacement in fly ash concrete improves concrete performance after exposed to elevated temperatures. Based on Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-Ray (EDX), fly ash concrete performed better thermally proven by the formation of ettringite. In conclusion, the utilization of fly ash as a replacement for Ordinary Portland cement (OPC) at elevated temperatures is a good innovation for concrete performance.

This paper reports on the result of an experimental investigation carried out to study the compressive strength and sorptivity properties of blended cement concrete exposed to 5% and 10% MgSO4 solution using fly ash (FA) and silpozz. Usually in sulphate environment the minimum grade of concrete is M30 and the mix design is done for target mean strength of 39 MPa. Silpozz is manufactured by burning of agro-waste rice husk in designed furnace in between 600o to 700oC which is one of the main agricultural residues obtained from the outer covering of rice grains during the milling process. There are four mix series taken with control mix. The control mix made 0% replacement of FA and silpozz with Ordinary Portland Cement (OPC). The first mix series made 0% FA and 10-30% replacement of silpozz with OPC. The second mix series made with 10% FA and 10-40% replacement of silpozz with OPC. The third mix series made 20% FA and 10-30% replacement of silpozz with OPC and the fourth mix series made 30% FA and 10-20% silpozz replaced with OPC. The samples (cubes) are prepared and cured in normal water and 5% and 10% MgSO4 solution for 7, 28 and 90 days. The studied parameters are compressive strength and strength deterioration factor (SDF) for 7, 28 and 90 days. The water absorption and sorptivity tests have been done after 28 days of normal water and magnesium sulphate solution curing. The investigation reflects that the blended cement concrete incorporating FA and silpozz showing better resistance against MgSO4 solution when compared to normal water curing (NWC) samples.

Mechanical behaviour at high temperature of alkali-activated aluminosilicates (geopolymers)

Utilization of OPC and FA to enhance reclaimed lime-fly ash macadam based geopolymers cured at ambient temperature

In this study, utilization of coal fly ash with higher loss on ignition (LOI) for geopolymer mortar was investigated. The fly ash with approximately 9% of LOI was compared with Class F fly ash. Relationship between heat curing condition and strength was clarified. As the results, although compressive strength of geopolymer mortar with higher LOI was 30-50% smaller, it was available for geopolymer mortar as an alumina silicate material. The higher temperature and the longer period for initial curing, the higher strength was obtained. In order to decrease drying shrinkage, the higher temperature and the longer period for heat curing were required.

Cement kiln dust produced in a local cement production plant in Saudi Arabia, along with fly ash resulting from combustion of heavy fuel oil in a local power generation plant were utilized as waste materials blended with ordinary Portland cement at various ratios. These blends were tested for their water requirements for normal consistency, initial setting times, and compression and tensile strengths, and were compared to those of Portland cement. Test results show that satisfactory mechanical strength (a minimum of 94% of compression strength of ordinary Portland cement) can still be achieved in blends utilizing 90% cement and not more than 4% fly ash. Adequate mechanical strengths (a minimum of 80% of compression strength of Portland cement) were achieved in blends utilizing as little as 70% cement when only kiln dust was blended. Hundred thousands of tons of cement-kiln dust (CKD) are generated annually from cement plants in the Kingdom of Saudi Arabia. The bulk of this dust, mostly with high alkali contents, is land filled with a significant financial loss to the local cement industry in terms of the value of raw materials, processing, and energy consumption during pyroprocessing, dust collection, and disposal. This fine dust is emitted from cement kilns to prevent the build up of excessive salts in the cement product. The alkali salts in the dust are derived from the clay raw

2 - Carbon dioxide sequestration by direct mineralization of fly ash

Microstructure Development and Transport Properties of Portland Cement-fly Ash Binary Systems : In view of service life predictions

This article reports the utilization of fly ash (FA) waste product from industry and silpozz which is an agro-waste from agriculture as an environmental friendly material in construction industry. The evaluation of strength and durability study was observed using FA and silpozz as a partial replacement of Ordinary Portland Cement (OPC). The studied parameters are compressive strength, flexural strength, split tensile strength and bond strength as well as the durability study involves the acid soluble chloride (ASC), water soluble chloride (WSC), water absorption and sorptivity. Scanning electron microscopy (SEM) and XRD of selected samples are also done. It reveals from the test results that the deterioration factor (DF) in compressive strength is 4% at 365 days. The DF of split tensile strength and flexural strength is 0.96% and 0.6% at 90 days respectively. The minimum slip is 1mm and 1.1mm after 28 days of testing bond strength for NWC and SWC sample respectively. The percentage decrease in bond strength is 10.35% for 28 days SWC samples. The pre-cast blended concrete samples performed better to chloride diffusion. Modulus of elasticity of SWC samples are also studied.The water absorption and sorptivity tests are conducted after 28 days of curing.