Showing papers on "Cement published in 2011"

Comparisons of natural and recycled aggregate concretes prepared with the addition of different mineral admixtures

Abstract: This paper presents the results of a laboratory study on the performance of natural and recycled aggregate concrete prepared with the incorporation of different mineral admixtures including silica fumes (SF), metakaolin (MK), fly ash (FA) and Ground granulated blast slag (GGBS). The compressive and splitting tensile strength, drying shrinkage, chloride ion penetration and ultrasonic pulse velocity (UPV) of the concrete mixtures were determined. The test results, in general, showed that the incorporation of mineral admixtures improved the properties of the recycled aggregate concretes. SF and MK contributed to both the short and long-term properties of the concrete, whereas FA and GGBS showed their beneficial effect only after a relatively long curing time. As far as the compressive strength is concerned, the replacement of cement by 10% of SF or 15% of MK improved both mechanical and durability performance, while the replacement of cement by 35% FA or 55% GGBS decreased the compressive strength, but improved the durability properties of the recycled aggregate concretes. Moreover, the results show that the contributions of the mineral admixtures to performance improvement of the recycled aggregate concrete are higher than that to the natural aggregate concrete.

Synergy between fly ash and limestone powder in ternary cements

Abstract: The interaction between limestone powder and fly ash in ternary composite cement is investigated. Limestone powder interacts with the AFm and AFt hydration phases, leading to the formation of carboaluminates at the expense of monosulphate and thereby stabilizing the ettringite. The effect of limestone powder on OPC may be restricted due to the limited amount of aluminate hydrates formed by the hydration of OPC. The additional aluminates brought into the system by fly ash during its pozzolanic reaction amplify the mentioned effect of limestone powder. This synergistic effect between limestone powder and fly ash in ternary cements is confirmed in this study and it translates to improved mechanical properties that persist over time.

The implementation of wood waste ash as a partial cement replacement material in the production of structural grade concrete and mortar: An overview

Abstract: The timber manufacturing and power generation industry is gradually shifting towards the use of biomass such as timber processing waste for fuel and energy production and to help supplement the electrical energy demand of national electric gridlines. Though timber processing waste is a sustainable and renewable source of fuel for energy production, the thermal process of converting the aforementioned biomass into heat energy produces significant amounts of fine wood waste ash as a by-product material which, if not managed properly, may result in serious environmental and health problems. Several current researches had been carried out to incorporate wood waste ash as a cement replacement material in the production of greener concrete material and also as a sustainable means of disposal for wood waste ash. Results of the researches have indicated that wood waste ash can be effectively used as a cement replacement material for the production of structural grade concrete of acceptable strength and durability performances. This paper presents an overview of the work carried out by the use of wood waste ash as a partial replacement of cement in mortar and concrete mixes. Several aspects such as the physical and chemical properties of wood waste ash, properties of wood waste ash/OPC blended cement pastes, rheological, mechanical and the durability properties of wood waste ash/OPC concrete mix are detailed in this paper.

Effect of nano-CaCO3 on hydration of cement containing supplementary cementitious materials

Abstract: The efficacy of the addition of nano-CaCO3 in accelerating the hydration of ordinary Portland cement (OPC) delayed by the presence of high volumes of supplementary cementitious materials including fly ash and slag was investigated. The conduction calorimetry indicated that the early hydration of OPC was significantly accelerated by the addition of the nano-CaCO3 and the higher the amount of CaCO3 addition, the greater was the accelerating effect. The thermogravimetric analysis results showed that the amounts of added CaCO3 became slightly lower as the hydration took place; however, any new reaction products were not detected by the X-ray diffractometry analysis. The engineering properties, including microhardness and modulus of elasticity, in the early stage of the hydration were remarkably improved by the addition of nano-CaCO3. It was suggested that the seeding effect of the nano-CaCO3 particles and the nucleation of C–S–H caused the enhanced strength development.

Microbial Concrete: Way to Enhance the Durability of Building Structures

Abstract: Natural processes, such as weathering, faults, land subsidence, earthquakes, and human activities, create fractures and fissures in concrete structures that can reduce the service life of the structures. A novel strategy to restore or remediate such structures is biomineralization of calcium carbonate using microbes, such as those in the genus of the Bacillus species. The present study investigated the effects of Bacillus sp. CT-5, isolated from cement, on compressive strength and water-absorption tests. The results showed a 36% increase in compressive strength of cement mortar with the addition of bacterial cells. Treated cubes absorbed six times less water than control cubes as a result of microbial calcite deposition. The current work demonstrated that production of "microbial concrete" by Bacillus sp. on constructed facilities could enhance the durability of building materials.

The hornification of vegetable fibers to improve the durability of cement mortar composites

Abstract: The use of vegetable fibers to reinforce a cement-based matrix has its weakest point in terms of durability. The alkalinity of the matrix and the volumetric instability of the fibers are the main causes of the loss of resistance of vegetable fiber-reinforced cement mortar composites. The aim of this study was to determine the effects of the previous hornification of the vegetable fibers on the mechanical performance and durability of softwood kraft pulp and cotton linters cement mortar composites. For this purpose, composites containing 4 wt.% of the hornificated fibers and the untreated ones were prepared with both fiber types. The mechanical performance of these composites was tested after 28 days of cure treatment and after aging with four wet–dry cycles. Results indicated that the previous treatment of fibers had beneficial effects on the mechanical performance and durability of the resulting cementitious composites.

Evaluation of sustainable high-volume fly ash concretes

Abstract: This article presents experimental research work oriented toward developing practical design tools for industrial application, and illustrates the potential benefits of the synergistic effect of an ASTM C 618 Class F fly ash (FA) and a high-range polycarboxylate superplasticizer (SP) in the production of conventional concrete. The different concretes considered in this study were produced with mass substitutions of cement by FA between 15% and 75%, and a target slump of 200 mm ± 20 mm. The total water content was minimized through the use of an optimum SP dosage that resulted in water reductions of 18%, 15% and 11% respectively for the reference mixtures of w/b = 0.5, w/b = 0.55, and w/b = 0.6, which leads to the same percentage reductions of cement. Heat release and heat flow were analyzed through isothermal and semi-adiabatic calorimetry, illustrating that heat release per unit mass of cement is independent of w/b, contrasting with the time of setting results that vary by several hours between the three different w/b ratios. The paper highlights the beneficial effect of the SP in terms of cement reduction and slump retention. Correlations between the FA substitution and slump loss, setting times, compressive strength and static modulus of elasticity (E) were established and they represent very useful tools for the practical applications of the results. Compressive strength developments up to an age of 56 d are also reported, as well as correlations between the modulus of rupture and compressive strength or splitting tensile strength at an age of 28 d.

Workability and Mechanical Performance of Steel Fiber-Reinforced Self-Compacting Concrete with Fly Ash

Abstract: Steel fibers change the properties of hardened concrete significantly. However, addition of fibers to fresh concrete results in a loss of workability. Self-compacting concrete (SCC) is an innovative concrete that is able to flow under its own weight, completely filling formwork and achieving full compaction without vibration. We have studied composites of SCC with steel fibers for further property enhancement. Water/cement ratio and cement, fly ash and superplasticizer contents were kept constant at 0.40, 400, 120 and 6 kg/m 3 , respectively. The fiber amounts were 15, 30, 45 and 60 kg/m 3 . Slump flow, J-ring and V-funnel tests were conducted for evaluating the fluidity, filling ability and segregation risk of the fresh concretes. There were no problems with mixing or workability while the fiber distribution was uniform. Steel fibers can significantly enhance toughness of SCC and inhibit the initiation and growth of cracks. © Koninklijke Brill NV, Leiden, 2011