Effect of Alkaline Activators on the Mechanical Properties of Geopolymer Mortar

"Geopolymerization is a process where silica and alumina rich source materials turn into excellent binding materials by the aid of alkali solutions. Materials such as fly ash are by-products in energy power plants. Fly ash is classified based on its constituent materials. Fly ash class F mainly consists of alumina and silica. Compressive strength of class F fly ash geopolymer mortar is influenced by many factors such as fluid to binder ratio, Na2SiO3/NaOH ratio, curing duration, curing temperatures and molarity of the activator solution. The present study investigates the effect of the fluid to binder ratio and Na2SiO3/NaOH ratio on the compressive strength of geopolymer mortar. The curing temperature was fixed to 80 °C. The curing durations investigated was 24h. For each combination, three cubes with dimensions of 50 x 50 x 50 mm were casted. After heat curing in the laboratory oven, the samples were tested on a universal testing machine for the compressive strength. The results showed very high early compressive strength of 66.39 MPa for samples cured at 80 °C and for a duration of 24 hr. The significance of the present study is that it will allow for establishing methods for production of high strength geopolymer mortar that can be used in civil engineering applications, in addition to the environmental advantages of using such source materials to produce binding materials with outstanding mechanical properties.

Use of steel slag in concrete mixtures

The recent unprecedented growth in the building industry in the State of Qatar has created a pressing demand for construction materials. Needs for concrete, being the major construction material has grown with the rapidly growing construction sector. Aggregate generally occupy about 70-80% of the volume of concrete used in buildings. The needs for alternative sources of aggregates are pressing. Steel slag, which is a by-product of steel manufacturing, could be an alternative solution. This paper presents the results from experimental works completed at Qatar University to check the applicability of using steel slag aggregates in concrete mixes in Qatar. Five trial concrete mixes were prepared.  One concrete mix composing of 100% gabbro coarse aggregates was designated as the control mix. Four other concrete mixes were prepared and they contained 100%, 75%, 50%, and 25% partial replacement of gabbro aggregates with slag aggregates, by weight. The concrete mix was proportioned to have a 28-day compressive strength of 35 MPa. The water-to-cement ratio was kept constant at 0.58 for all of the five mixes. All mixtures were cured for 7, 28, and 90 days.  No admixtures or additives were used in these mixes. The data indicate that steel slag has a promising potential to be used as a partial replacement for conventional aggregates in concrete. The emphasis of the work in the initial phase of this study was on the feasibility of utilizing steel slag aggregates in concrete, as a total or partial replacement of gabbro aggregate, used in construction in the State of Qatar by studying the properties of fresh and hardened concrete. Tests were conducted on concrete samples made of different aggregates to determine their acceptability for use in concrete. The different mixes were tested to determine compressive strength, splitting tensile strength, flexural strength, air content, and bulk density. Laboratory results, obtained in the six months research project, indicated that steel slag alone or in combination with other virgin raw materials such as gabbro, sand, and others could find uses in road construction applications in the State of Qatar

Dosage Optimization of Polypropylene Fiber for Strength Enhancement of Cementitious Composites

"Concrete is the most commonly used materials for construction in Qatar as well as in the world. Exposure to sever environmental conditions causes physical deterioration of concrete structures and significantly affect the concrete’s strengths and modulus of elasticity. In the last decades, many improvements had been made in concrete technology. Most of these improvements focused on the weak point of concrete, which is tensile strength enhancement. One possible method to improve the tensile strength of cementitious composites is incorporation of fibers in the mix. Polypropylene fiber is widely used for this purpose due to their corrosion resistance and relatively low cost. Polypropylene fibers are usually incorporated in cement mortar to control cracks propagation thus enhance its tensile and flexural properties. This research focuses on Polypropylene fiber dosage optimization for strength enhancement of cementitious composites. Four dosages of Polypropylene microfibers; 0%, 0.05%, 0.1%, and 0.2% by weight of cement; were added into cement mortar to explore the optimum dosage that can lead to big enhancement in mechanical strengths of cementitious composites. The mechanical strengths were investigated in terms of compressive and flexural strengths. The results revealed that adding small amount of Polypropylene microfibers could enhance the compressive and flexural strengths of cement mortar. The maximum enhancement in the compressive and flexural strengths was equal to 26% and 19% and was achieved in the case of adding 0.1% and 0.05% by weight of cement, respectively.

Use of local discarded materials in concrete

Steel slag, a by-product of steel manufacturing, is generated in large quantities in Qatar. In fact, it is estimated that more than 400,000tons of steel slag are generated annually in the country. Gravel, resulting from washing sand, is also produced at more than 500,000tons/year in Qatar. Both materials are not efficiently used in the country and most of its aggregate (gabbro) needs are imported from neighboring countries. This paper presents the results obtained on the use of steel slag, gravel and gabbro in concrete. A total of nine concrete mixtures were prepared. One concrete mixture that contained 100% gabbro aggregate was considered as the control mix. Four concrete blends containing 100%, 75%, 50%, and 25% steel slag (by weight) were prepared as partial replacements of gabbro aggregates. Another four concrete mixtures containing 100%, 75%, 50%, and 25% gravel (by weight) were cast as partial replacements of gabbro aggregates. All samples were cured in a water tank for 7, 28 and 90days and then subjected to compressive, flexural and splitting tensile strength tests. All concrete mixtures prepared easily met the 28-day compressive strength design requirement of 28MPa. Best results were obtained for concrete prepared using 100% steel slag aggregates. Concrete cast using 100% gravel yielded lower strength results than the control mixture (100% gabbro). However, there was an increase in strength values with an increase in gabbro content in gravel/gabbro mixtures. Additional work is necessary to establish long-term performance, especially concerning what is reported in the literature about the expansive characteristics of steel slag aggregates when used in concrete. It should be noted that concrete cured for 90days in the water tank did not exhibit any reversal in strength.Slag Aggregate Producer, under Qatar University (QU) Research Grant No. QUEX-CENG-SAP-12/13-1. The Gulf Organisation for Research and Developmen

High and Ultra-high Performance Concretes: A Solution to Reinforced Concrete Durability under Harsh Climate of Arabian Gulf

Reinforced concrete (RC) infrastructure in the Arabian Gulf region deteriorates under severe environmental conditions after only short service life. To overcome this problem, it is imperative to employ high-quality concretes and reinforce them with rebars that are corrosion resistant. This paper investigates the durability performance of newly developed high performance concretes (HPC) and ultra-high performance concretes (UHPC). The HPC and UHPC were manufactured using locally available materials in Qatar without employing any special treatment. The durability characteristics of HPC and UHPC in comparison to a normal strength concrete (NSC) were determined. Durability indicators such as concrete resistivity, sorptivity, porosity and resistance to chloride permeability were evaluated in order to access the durability of these concretes. These parameters were also compared to the concrete core samples taken from 30 to 50 years old RC structures in Doha city. The electrical resistivity of HPC and UHPC was 11 and 20 times higher than NSC, respectively. Sorptivity was 2 and 3 times less than NSC, respectively for HPC and UHPC. While the porosity of HPC and UHPC was 2.45 and 1.43% respectively. These newly fabricated concretes showed higher performance in durability testing than the concretes from real structures. With such attributes, the UHPC will be a useful tool in arresting the rapid deterioration of RC structures especially under harsh-climatic conditions of the Arabian Gulf.The funding for this research was provided by the National Priorities Research Program of the Qatar National Research Fund (a member of the Qatar Foundation) under the award no. NPRP 7-410-2-169. The statements made herein are solely the responsibility of the authors and do not necessarily reflect the opinions of the Sponsor

Sustainable utilization of waste carbon black in alkali-activated mortar production

This article investigates the potential utilization of waste carbon black (WCB) resulting from the aluminum industry as a by-product material in the fly ash-based geopolymer composites production. Experimental study was conducted to evaluate the effect of WCB on the performance of the geopolymer. Different contents of WCB including 5%, 10%, 15%, 20%, 30%, and 40%,by weight of the fly ash, have been incorporated in the geopolymer mix as either additives or fly ash replacement. Life cycle assessment (LCA) has also been conducted to evaluate the landfills utilization and the environmental impact of the WCB incorporation. The experimental results reflected that the WCB could be used as additives in small quantities (5% of fly ash weight) to the geopolymer mix without negatively affecting its performance. Adding 5% of WCB insignificantly enhanced the compressive strength of the geopolymer by 5%, increased its workability and density by 3% and 4%, respectively, and did not affect its excellent thermal stability. Scanning electron microscopic (SEM) imaging showed more unreacted fly ash particles combined with more voids and cracks within the microstructure of the geopolymer with high WCB content. Finally, incorporating WCB in the geopolymer production improved the utilization of landfills use and reduced the global warming potential, acidification potential, eutrophication potential and abiotic depletion potential

Evaluation of Bond Strength Between Carbon Fiber Reinforced Polymer (CFRP) Composites with Modified Epoxy Resins and Concrete

Rehabilitation and strengthening of concrete structures are becoming more significant in civil engineering applications. The use of externally bonded Fiber Reinforced Polymers (FRP) is one of the methods to strengthen and rehabilitate reinforced concrete members, providing noticeable improvement to their capacity in resisting load. Carbon Fiber Reinforced Polymer (CFRP) is used along with epoxy resins to evaluate the bond strength of two commercially available epoxies (EPON 828 and EPON 862) between CFRP and concrete. In addition, three new combinations that resulted from mixing the two epoxies were examined. The mechanical properties of epoxy resins are significantly weaker than this of the CFRP making the epoxy characteristics the determining factor in the quality of the bond strength. Three-point flexural test was conducted to examine the bond strength between the CFRP composites and concrete. Further, differential scanning calorimetry was conducted to examine the glass transition temperature of the resultant epoxies. The results showed that the optimum composition was a mixture of 70% of epoxy 828 and 30% of epoxy 862. Therefore, achieving better bond strength and high glass transition temperature, resulting in CFRP composite with higher fire resistance

Performance of Mild Steel and Corrosion-Resistant Steel Rebars in Chloride-Contaminated Concrete

Reinforced concrete (RC) infrastructure in Arabian Peninsula is subjected to harsh climatic conditions of high temperatures, humidity, and airborne chlorides and there is a high concentration of salts in seawater and soils. These factors instigate corrosion of reinforcing steel in RC infrastructure at the early stages of the service life. To overcome the durability issues of RC infrastructures, corrosion-resistant reinforcing bars are employed. In this study, a comparison of microcell and macrocell corrosion of mild steel (MS) and two types of corrosion-resistant rebar namely the high chromium (HC) and stainless steel (SS) was established. Nine concrete block samples of 20x10x350 mm were cast with top rows of reinforcements, the top row consists of MS, HC, or SS, and the bottom row contained only SS. Blocks were conditioned under 3.5% NaCl for 2 years and linear polarization resistance and macrocell currents were evaluated to compare the corrosion performance of mild and corrosion resistant steel rebars. It was observed that SS is the most corrosion-resistant steel rebar, where high chromium steel showed up to three times more corrosion resistance than mild steel under chloride attack

Bio-Based Self-Healing Concrete for Sustainable and Durable Concrete Infrastructure

In this study, bio-self-healing concrete was manufactured using a natural phenomenon called microbial-induced calcium carbonate precipitation (MICP). The bacillus cereus bacteria isolated from Qatari soil was used for this purpose. These bacteria have endured the harsh weather of high temperatures, humidity, and alkaline soil conditions. Hence, are a potential candidate for long-term self-healing in concrete structures that are subjected to the climate of the Middle Eastern region. The bacteria were encapsulated in sodium alginate beads then the beads were added to the cement-sand mortar. The nutrients for bacteria such as urea, calcium nitrate, yeast extract, and calcium chloride were mixed in mortar as dry constituents. After curing for 28 days, cracks were artificially induced in the prismatic samples, which were reinforced with steel rebars at the tensile side. Samples were placed in water to instigate self-healing. It was observed that the bacteria healed the cracks up to 0.70 mm. It is concluded that the used bacteria are viable in the alkaline concrete matrix and capable of producing calcium carbonate