Showing papers on "Cement published in 2018"

Characteristics of steel slags and their use in cement and concrete—A review

Abstract: Steel slags are industrial by-products of steel manufacturing, characterized as highly calcareous, siliceous and ferrous. They can be categorized into basic oxygen furnace (BOF) slag, electric arc furnace (EAF) slag, and ladle furnace (LF) slag. They are found to be useful in many fields, such as road construction, asphalt concrete, agricultural fertilizer, and soil improvement. However, better utilization for value-added purposes in cement and concrete products can be achieved. In this paper, an overview of the recent achievements and challenges of using steel slags (BOF, EAF and LF slag) as cement replacement (usually ground into powder form with the size of 400–500 m2/kg) and aggregate in cement concrete is presented. The results suggest that the cementitious ability of all steel slags in concrete is low and requires activation. For the incorporation of steel slags as aggregate in concrete, special attention needs to be paid due to the potential volumetric instability associated with the hydration of free CaO and/or MgO in the slags. Studies have indicated that adequate aging/weathering and treatments can enhance the hydrolyses of free-CaO and -MgO to mitigate the instability. Considering the environmental and economic aspects, steel slags are also considered to have a potential use as the raw meal in cement clinker production.

A review on properties of fresh and hardened geopolymer mortar

Abstract: Geopolymer mortar refers to the mortar manufactured with sand and geopolymer, which is composed by the base materials containing affluent aluminium and silicon that was activated by adopting alkaline solution to serve as a binder. The investigation of the properties and application of the geopolymer mortar has attracted more and more attention of the researchers and cement based industries because of its sustainability advantages. This study reviews the properties of the geopolymer mortars including fresh performance (workability, setting time, and temperature of fresh mortar), physical properties, mechanical properties (compressive strength, tensile strength, elastic properties, flexural performance, bonding behavior, and fracture behavior), durability properties (acid resistance, resistance to elevated temperature, frost resistance, water absorption, and shrinkage properties) and microstructure analysis. This study also reviews the properties of different types of geopolymer mortars prepared using various source materials as base materials. The current study results indicate that the geopolymer mortar has exhibited significant feasibility and application prospect to be used as an environmental friendly building material, which may be an appropriate replacement to the traditional cement mortar in the future.

Strength and durability of concrete containing recycled concrete aggregates

Abstract: Recycled concrete aggregates (RCA) sourced from waste concrete are a sustainable alternative to natural crushed stone aggregates. The strength and durability properties of concrete containing RCA were evaluated by a comprehensive experimental investigation involving nine control mixes. The variables considered in the experimental study are water cement ratio, cement content in concrete and percentage replacement of coarse aggregate. The strength properties such as compressive strength, modulus of elasticity, splitting tensile strength and flexural strength are studied. Durability properties such as water absorption, sorptivity, acid attack resistance and chloride permeability are also determined. The test results showed that up to 25% of natural crushed stone aggregates in concrete may be replaced with RCA, without significantly affecting the strength of concrete and that the partial replacement of natural aggregates with RCA can be recommended in areas of moderate exposure conditions. Mathematical models developed in the study can be used for the a priori prediction of the strength parameters of RCA concrete. A mix design methodology using the developed models is proposed to aid practicing engineers to determine the mix proportions of RCA concrete.

Nano-silica and silica fume modified cement mortar used as Surface Protection Material to enhance the impermeability

Abstract: In corrosion environment, corrosion ions can easily penetrate from the surface into the inside of the concrete due to the porous structure of the surface; in this case, concrete can inevitably suffer from the damage. In this study, an attempt to use nano SiO2 (NS) and silica fume (SF) modifying cement mortar as a Surface Protection Material (SPM) was made, in order to promote penetration resistance of the whole system. SPM was coated on the surface of matrix, and then interfacial bond strength between matrix and SPM was measured; shrinkage consistency was also considered; the chloride penetrability of the system was examined as well. To reveal the mechanism, effect of NS and SF on pore structure, Interfacial Transition Zone (ITZ), hydration process, and compressive strength of SPM were investigated. The results show that matrix coated with SPM on the surface has a good integrity, with excellent interfacial bond strength and little difference in shrinkage, and chloride diffusion coefficient of the system was considerably declined, in comparison with the matrix, showing an excellent penetration resistance. The mechanism behind is that SPM, which was modified with SF-NS, shows the excellent impermeability, and this kind of material existing on the surface can noticeably obstruct the chloride ions penetrating into the inside. In cement hydration process, SF and NS can not only consume a large amount of CH to form dense C-S-H, but also exert the grading filling effect, resulting in the decline in porosity, the increase in density, the improvement in microstructure of ITZ, and the enhancement in mechanical performance. The findings can provide useful experience for the design of the cement-based materials servicing in high corrosion environment.

Use of biochar as carbon sequestering additive in cement mortar

Abstract: Biochar is widely considered as effective way of sequestering carbon dioxide. The possibility of using it to enhance the mechanical strength and reduce permeability of cement mortar is explored in this study. The effect of fresh biochar and biochar saturated with carbon dioxide a priori on the setting time, mechanical strength and permeability of cement mortar was evaluated. The biochar was prepared from mixed wood saw dust at 300 °C and added to mortar during mixing at 2% by weight of cement. It was found that addition of fresh biochar and saturated biochar reduce initial setting time and significantly improve early compressive strength of mortar. The experimental results suggested that biochar addition can impart ductility to mortar under flexure, although flexural strength was not significantly influenced. Water penetration and sorptivity of mortar was significantly reduced due to addition of biochar, which indicate higher impermeability in biochar added mortar. However, it is found that addition of fresh biochar offers significantly higher mechanical strength and improved permeability compared to biochar saturated with carbon dioxide. These results suggest that biochar has the potential to be successfully deployed as a carbon sequestering admixture in concrete constructions that also provides a way to waste recycling.

Characterizations of autogenous and drying shrinkage of ultra-high performance concrete (UHPC): An experimental study

Abstract: Due to the high content of binder and low water to cement ratio, ultra-high performance concrete (UHPC), exhibits higher levels of autogenous shrinkage compared to ordinary concrete. This shrinkage has been shown to lead to a reduction in strength over time as a result of the formation of thermal and shrinkage cracks. Aiming to mitigate the negative impacts associated with shrinkage, the efficacy of three different techniques to reduce the impact of shrinkage are investigated, namely: reducing the binder content; incorporating high levels of shrinkage reducing admixture; and using crushed ice to partially replace mixing water. The effects of these techniques are experimentally investigated and the underlying mechanisms of the actions are characterized. It is found that autogenous shrinkage predominates the overall shrinkage of UHPC and that the three techniques can effectively reduce shrinkage without significantly compromising its mechanical strength. The results also suggest, that from the perspective of reducing shrinkage: the optimal binder-to-sand ratio is in the range of 1–1.1; the optimal dosage rate of shrinkage reducing admixture is 1%; and replacing of mixing water by crushed ice up to 50% by weight has also induced a significant reduction in shrinkage.

Distinguishing dynamic and static yield stress of fresh cement mortars through thixotropy

Abstract: The dynamic and static yield stress of fresh cement mortar were measured in a rotational rheometer with a vane geometry using shear rate and shear stress-controlled protocols, respectively. Through a shear rate-controlled steady-state protocol, the equilibrium flow curve is measured and fitted with the Bingham model to obtain dynamic yield stress. A negative slope in the equilibrium flow curve, shear banding and stick-slip phenomena are observed and discussed. Through a stress-controlled creep-recovery protocol, viscosity bifurcation behavior is captured and static yield stress is marked as the creep stress when the bifurcation occurs. Finally, the discrepancy between dynamic and static yield stress is tied to thixotropy.

Green concrete partially comprised of rice husk ash as a supplementary cementitious material – A comprehensive review

Abstract: The production of cement depletes natural resources, consumes high energy and emits huge amounts of green house gases. It accounts for almost 7% of the global carbon dioxide emissions, as the production of one ton of ordinary Portland cement releases approximately one ton of carbon dioxide. Due to the severe environmental pollution and health hazards associated with the cement and construction industries, they are under the strict scrutiny from the governments and environmentalists. Rice husk is an agricultural waste, whose natural degradation is restricted due to the irregular abrasive surface and high siliceous composition. It is not appropriate to be used as a feed for animals due to the low nutritional values. If dumped as landfill, they can take a lot of area and become a major challenge to the environment. If they are disposed by burning, the ashes can spread to the surrounding areas, create pollution and destroy the natural beauty. One of the possible solutions for the disposal of rice husk is to convert them into rice husk ash and incorporate them into cement based materials. The partial inclusion of rice husk ash (RHA) for cement is found to be durable, environmental friendly and economically viable. This paper presents an overview of some of the published results on the successful utilization of rice husk ash as a supplementary cementitious material and the properties of such concrete at fresh and hardened stages. Studies indicate that there is a promising future for the use of rice husk ash in normal, high strength and self compacting concrete as it shows high strength, low shrinkage and permeability, high resistance to carbonation, chloride, sulfate and acidic environments. The summery and discussions provided in this paper should provide new information and knowledge on the applications of greener and sustainable rice husk ash concrete.

The effect of steel and polypropylene fibers on the chloride diffusivity and drying shrinkage of high-strength concrete

Abstract: This paper presents an experimental study that investigates the influence of the low fiber content of polypropylene and hooked-end steel fibers on the properties of high-strength concrete. The study variables include fiber types and fiber contents. The effect of combining both fibers with a total fiber content of 1.0% was also studied in some mixtures. Silica fume, as a supplementary cementitious material, was used at 10% of the cement weight in all fiber-reinforced concrete mixtures. Compressive strength, modulus of elasticity, longitudinal resonant frequency, rapid chloride migration and free drying shrinkage tests were performed for different curing ages. The results show that replacement of the cement weight with 10% silica fume improved all of the characteristics of the concrete evaluated in this research study. It was observed that the inclusion of fibers, particularly steel fibers, enhanced the mechanical properties of concrete. It was found that the incorporation of polypropylene fibers resulted in a reduction of chloride diffusivity, while introducing steel fibers significantly increased the chloride diffusivity of concrete. Finally, the results showed that hybridization of two types of fibers was an effective way to improve the properties of concrete and specifically reduce the drying shrinkage compared with that of the plain concrete.