Showing papers on "Cement published in 2005"

Effect of fly ash fineness on compressive strength and pore size of blended cement paste

Abstract: This paper presents an experimental investigation on the effect of fly ash fineness on compressive strength, porosity, and pore size distribution of hardened cement pastes. Class F fly ash with two fineness, an original fly ash and a classified fly ash, with median particle size of 19.1 and 6.4 μm respectively were used to partially replace portland cement at 0%, 20%, and 40% by weight. The water to binder ratio (w/b) of 0.35 was used for all the blended cement paste mixes. Test results indicated that the blended cement paste with classified fly ash produced paste with higher compressive strength than that with original fly ash. The porosity and pore size of blended cement paste was significantly affected by the replacement of fly ash and its fineness. The replacement of portland cement by original fly ash increased the porosity but decreased the average pore size of the paste. The measured gel porosity (5.7–10 nm) increased with an increase in the fly ash content. The incorporation of classified fly ash decreased the porosity and average pore size of the paste as compared to that with ordinary fly ash. The total porosity and capillary pores decreased while the gel pore increased as a result of the addition of finer fly ash at all replacement levels.

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Abstract: Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.

Stabilisation of clayey soils with high calcium fly ash and cement

Abstract: The effectiveness of using high calcium fly ash and cement in stabilising fine-grained clayey soils (CL,CH) was investigated in the laboratory. Strength tests in uniaxial compression, in indirect (splitting) tension and flexure were carried out on samples to which various percentages of fly ash and cement had been added. Modulus of elasticity was determined at 90 days with different types of load application and 90-day soaked CBR values are also reported. Pavement structures incorporating subgrades improved by in situ stabilisation with fly ash and cement were analyzed for construction traffic and for operating traffic. These pavements are compared with conventional flexible pavements without improved subgrades and the results clearly show the technical benefits of stabilising clayey soils with fly ash and cement. In addition TG–SDTA and XRD tests were carried out on certain samples in order to study the hydraulic compounds, which were formed.

Mixture Proportioning for Internal Curing

Abstract: Internal curing is the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water. An equation has been developed in a previous study for calculating how much lightweight aggregate (LWA) is needed to supply water for internal curing of any given concrete mixture. This paper presents refinements for estimating the parameters of this equation that will provide a readily recognized means of choosing the proper amount of LWA and improving mixture proportioning. The two major factors to be considered are the variation of chemical shrinkage of cement with Portland cement phase composition and curing temperature, and the relevant value for the absorption of the LWA. In order to determine the amount of LWA needed, it is recommended that the mass composition of the cement clinker be obtained from either a detailed scanning electron microscope/X-ray image analysis or the Bogue calculation. Then, the expected chemical shrinkage of the cement at 25 deg C should be calculated. If the expected average curing temperature is above 25 deg C, the calculated value should be decreased by 0.0005 per deg C above 25 deg C. If the expected average curing temperature is below 25 deg C, the calculated value should be increased by 0.0005 per deg C below 25 deg C. The desorption of the LWA from a saturated state down to a relative humidity of relevance for the internal curing of concrete should then be measured. Finally, the determined values for chemical shrinkage and absorption of LWA should be substituted in the original equation to obtain the desired mass of lightweight fine aggregate in the concrete mixture.

Heat of Hydration Models for Cementitious Materials

Abstract: Models are used to characterize the behavior of concrete exposed to in-place conditions. These models need to include methods to quantify the heat of hydration of cementitious materials. This article presents the formulation of a general hydration model for cementitious materials. The authors note that the degree of hydration characterizes the formation of hydration products as hydration progresses over time, and each concrete mixture has a unique degree of hydration development. The authors used semi-adiabatic calorimeter tests on 13 different concrete mixtures and with heat of hydration data from 20 different cement types to provide a convenient, indirect means of characterizing the formation of hydration products by measuring the heat released during hydration. Their hydration model incorporates the effect of following variables: cement chemical composition, cement fineness, supplementary cementing materials (Class F fly ash, Class C fly ash, and ground-granulated blast-furnace (GGBF) slag cement), mixture proportions, and concrete properties (density, thermal conductivity, and specific heat). The authors conclude that this model provides a reasonable and accurate representation of the heat of hydration development under different curing temperatures.

Impact strength of a few natural fibre reinforced cement mortar slabs: a comparative study

Abstract: This paper presents the experimental investigations of the resistance to impact loading of cement mortar slabs (1:3, size: 300 mm × 300 mm × 20 mm) reinforced with four natural fibres, coir, sisal, jute, hibiscus cannebinus and subjected to impact loading using a simple projectile test. Four different fibre contents (0.5%, 1.0%, 1.5% and 2.5%—by weight of cement) and three fibre lengths (20 mm, 30 mm and 40 mm) were considered. The results obtained have shown that the addition of the above natural fibres has increased the impact resistance by 3–18 times than that of the reference (i.e. plain) mortar slab. Of the four fibres, coir fibre reinforced mortar slab specimens have shown the best performance based on the set of chosen indicators, i.e. the impact resistance (Ru), residual impact strength ratio (Irs), impact crack-resistance ratio (Cr) and the condition of fibre at ultimate failure.

Surface relaxation and chemical exchange in hydrating cement pastes: a two-dimensional NMR relaxation study.

Abstract: We report the first nuclear magnetic resonance NMR two-dimensional correlation T1-T2 and T2-T2 measurements of hydrating cement pastes. A small but distinct cross peak in the two-dimensional relaxation spectrum provides the first direct evidence of chemical exchange of water between gel and capillary pores occurring over the first 14 days of hydration. A correlation of features along the line T1=4T2 provides strong supportive evidence for the surface diffusion model of 1 H nuclear spin relaxation in cements and for a multimodal discrete pore size distribution. Differences in detail of the results are reported for white cement paste and white cement paste with added silica fume. Both the method and the theory presented can be applied more widely to other high surface area materials with other reactive surface areas.

The chemical constitution and biocompatibility of accelerated Portland cement for endodontic use

Abstract: Camilleri J, Montesin FE, Di Silvio L, Pitt Ford TR. The chemical constitution and biocompatibility of accelerated Portland cement for endodontic use. International Endodontic Journal, 38, 834–842, 2005. Aim To evaluate the biocompatibility of mineral trioxide aggregate and accelerated Portland cement and their eluants by assessing cell metabolic function and proliferation. Methodology The chemical constitution of grey and white Portland cement, grey and white mineral trioxide aggregate (MTA) and accelerated Portland cement produced by excluding gypsum from the manufacturing process (Aalborg White) was determined using both energy dispersive analysis with X-ray and X-ray diffraction analysis. Biocompatibility of the materials was assessed using a direct test method where cell proliferation was measured quantitatively using Alamar Blue TM dye and an indirect test method where cells were grown on material elutions and cell proliferation was assessed using methyltetrazolium assay as recommended by the International standard guidelines, ISO 10993-Part 5 for in vitro testing. Results The chemical constitution of all the materials tested was similar. Indirect studies of the eluants showed an increase in cell activity after 24 h compared with the control in culture medium (P < 0.05). Direct cell contact with the cements resulted in a fall in cell viability for all time points studied (P < 0.001). Conclusions Biocompatibility testing of the cement eluants showed the presence of no toxic leachables from the grey or white MTA, and that the addition of bismuth oxide to the accelerated Portland cement did not interfere with biocompatibility. The new accelerated Portland cement showed similar results. Cell growth was poor when seeded in direct contact with the test cements. However, the elution made up of calcium hydroxide produced during the hydration reaction was shown to induce cell

Free, restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibres

Abstract: Many investigations are realized to establish the basic mechanical properties of vegetable fibre reinforced composites (VFRC) but not their shrinkage and creep behaviour. Some works have been realized to establish the shrinkage of cement mortar matrices reinforced with cellulose fibres, but very few results has been published with regards to shrinkage of VFRC with short sisal and coconut fibres. In this paper a concise summary of several investigations is presented to establish the influence of sisal and coconut fibres on the free and restrained plastic shrinkage, early drying shrinkage cracking, crack self-healing and long-term drying shrinkage of mortar matrices. The free and restrained shrinkage were studied by subjecting the specimens to wind speed of 0.4–0.5 m/s at 40 °C temperature for up to 280 min. The self healing of cracks of the VFRC was studied by using the same specimens as for the study of restrained shrinkage which were kept further in a controlled environment with 100% relative humidity and temperature of 21 °C for up to 40 days. Drying shrinkage tests were carried out at room temperature with about 41% relative humidity for 320 days. The influence of curing method, mix proportions and partial replacement of ordinary Portland cement (OPC) by ground granulated blast-furnace slag and silica fume on the drying shrinkage of VFRC was also investigated. Finally, based on the obtained results on drying shrinkage an equation using the recommendation of ACI model B3 was adjusted and compared well with the obtained experimental data.

Strength and modulus of marine clay-cement mixes

Abstract: This paper examines the strength and modulus of marine clay-cement mixes with high cement content. Although similar studies have been reported, many of these studies were conducted using soil-cement and water-cement ratios which are more applicable to deep cement mixing than jet grouting. The objective of this study is to investigate how the strength and modulus of cement-treated Singapore marine clay vary with cement and water contents at a range of cement contents, which is more representative of that used in jet grouting. To facilitate parametric studies that are relevant to jet grouting operations, a working range of the constituents was proposed for Singapore marine clay based on the liquid and bleeding limits of the soil-cement mixes. Comparison with data from some previous jet grouting studies and projects indicates that the liquid and bleeding limits can encompass most, if not all, of the parameter range normally used in jet grouting operations. The results of unconfined compression tests on cemen...

Carbon Dioxide Uptake during Concrete Life Cycle - State of the Art

Abstract: Carbonation results when carbonate ions from dissolved carbon dioxide react with the Ca ions of the cement paste and precipitate calcium carbonate. By time all Ca-bearing cement hydrates will decom ...