Strength Of Blended Cement Pastes Research Articles

Co-utilization of aqueous carbonated basic oxygen furnace slag (BOFS) and carbonated filtrate in cement pastes considering reaction duration effect

It is generally agreed that aqueous carbonation of basic oxygen furnace slag (BOFS) is a promising and effective strategy to sequestrate CO2 and enhance the slag properties for use as a construction material. However, the handling of wastewater produced in the aqueous carbonation route is still a challenge. This work investigates the impact of co-utilizing carbonated BOFS as a partial cement replacement and carbonated filtrate as a mixing solution on the fresh and mechanical properties as well as the hydration and microstructural characteristics of cement paste. The carbonation duration of BOFS via the aqueous route (0, 10, 20, 30, 40, 80, and 120 min) was studied, and it was found that calcite was the dominant CaCO3 polymorph with different crystal shapes depending on the concentration of Mg ion in the slurry solution. The early-age compressive strength of blended cement pastes improved with the addition of 15% BOFS slurry carbonated for 40 min compared to as-received BOFS since the in-situ produced fine CaCO3 particles in the former could promote nucleation sites for cement hydration. However, the strength of pastes containing BOFS slurry carbonated for a longer duration was decreased due to the inhibited hydration reaction induced by the nesquehonite (MgCO3∙3H2O). At later ages of hydration, the nucleation effect was less obvious and the strength of cement paste was limited by the lower cement content. Moreover, the volume expansion issue caused commonly by f-CaO/MgO in BOFS can be improved through the precipitation of CaCO3 and nesquehonite.

Effect of Metakaolin and Lime on Strength Development of Blended Cement Paste

To develop a more reactive pozzolan for supplementary cementitious materials (SCMs), the co-calcination of kaolinite and limestone was investigated for its contribution to hydration of blended cement. Kaolinite (with ~50 wt% quartz impurity) was calcined at 700 °C, and a mixture of kaolinite and limestone was calcined at 800 °C. These activated SCMs were added to ordinary Portland cement (OPC), replacing ca. 30 wt% of the OPC. The compressive strength of these blended cement paste samples was measured after 28 and 90 days, while the hydration products and microstructural development in these blended cement pastes were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results revealed that adding free lime to OPC, together with metakaolin, led to enhanced compressive strength. The compressive strength of this new blended cement paste reached 113% and 112% of the compressed strength of pure OPC paste after 28 and 90 days of hydration, respectively. Furthermore, this study showed that the improvement was due to the increased consumption of Portlandite (CH), the formation of calcium aluminosilicate hydrate (CASH), and the reduction of porosity in the sample containing free lime and metakaolin.

Hydration development of blended cement paste with granulated copper slag modified with CaO and Al2O3

Granulated copper slag (GCS) is currently applied to partial cement replacement as a supplementary cementitious material (SCM), but with a low rate due to its low pozzlanic activity. In this contribution we introduce an approach to enhance the reactivity of GCS by modifying the mineral structure using calcium oxide (CaO) and aluminum oxide (Al2O3) both by 10 wt.%, in a molten state. Blended cement pastes were formulated using cement (70 wt.%) and the modified GCS (30 wt.%). The development of hydration heat and strength were examined using isothermal calorimetry and strength tests, respectively. The mineral composition and microstructure of hydration products for different reaction periods were examined using X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). Also, toxicity characteristic leaching procedure (TCLP) was performed on the samples. Results showed an increase in the hydration heat emission rate from the early hydration and the compressive strength of blended cement paste after curing for 28 days, indicating that the addition of CaO and Al2O3 in GCS improves pozzolanic activity. XRD and SEM-EDS analysis indicated that the modified GCS consumed more calcium hydroxide (CH) accompanied by increased generation of calcium silicate hydrate (C–S–H) gels in blended cement with a microstructure containing more gel phases and fewer pores, forming a more compact structure. Leaching of heavy metals of paste samples was lower than Environmental Protection Agency (EPA) limit. It is possible to apply the modified GCS as a sustainable material to promote cleaner production for cement and concrete industries.

Effects of soft water attack on Portland and natural zeolite blended cements

The durability of concrete infrastructure is related to the properties of the applied concrete and the effects of the aggressive external environment on it. When concrete is directly exposed to soft water, the leaching of calcium ions from hardened cement or cement-based pastes occurs, causing reduction in strength and further deterioration of the concrete structure. This paper presents the experimental results of soft water attack effects on phase composition and compressive strength of blended cement pastes, when cement is replaced with 0, 10, 20 and 30% of natural zeolite. In order to simulate soft water attack in laboratory conditions, paste specimens were exposed to leaching in deionised water up to 180 days. The evaluation of the changes in phase composition (ettringite, portlandite, calcium silicate hydrate gel) due to calcium ion leaching was made based on X-ray diffraction analysis, Fourier-transform infrared spectroscopy and paste compressive strength tests. The presence of portlandite and ettringite after 180 days of exposure to deionised water indicates that leaching did not influence the stability of the hydration products in blended cement pastes. Moreover, blended cement paste with 10% of natural zeolite had a higher compressive strength than the reference one.

The effect of fly ash fineness on heat of hydration, microstructure, flow and compressive strength of blended cement pastes

In this paper, an experimental study on the effect of fly ash fineness on the heat of hydration, microstructure, flow and compressive strength of blended cement pastes was carried out and evaluated against control cement paste. Fly ashes with different fineness: classified fly ash, run-of-station fly ash and grounded run-of-station fly ash; with a median particle size of 17.4, 11.3 and 5.7 μm, respectively, from the same power station source in Australia were used to partially replace Portland cement at 20% and 40% by weight of cement using a fixed water-to-binder ratio of 0.40. Results of this study showed that the cumulative heat of hydration of blended cement paste decreased as fly ash content in blended cement paste was increased. For a given cement replacement level, blended cement paste containing finer fly ash released more heat of hydration when compared to coarser fly ash. Moreover, increasing the fineness of fly ash resulted in a higher consumption of calcium hydroxide at 7 and 28 days reflecting pozzolanic reactivity and, thus, a denser microstructure than blended pastes containing coarser fly ash as revealed by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and compressive strength results. In addition, the incorporation of fly ash in the blended pastes led to the introduction of an additional hydration peak in the heat evolution curve possibly due to the late activation of fly ash by calcium hydroxide renewing the C3A reaction and converting ettringite to monosulfate. The flow of the freshly blended cement pastes was also found to improve slightly with increasing fineness of the fly ash. In addition, the hardened blended cement pastes containing 20% ground run-of-station fly ash showed comparable compressive strength with the control cement pastes at both 7 and 28 days mainly due to the higher fineness of the ground run-of-station fly ash and increased reactivity compared to coarser grade fly ash.

Numerical Modelling of the Effect of Filler/Matrix Interfacial Strength on the Fracture of Cementitious Composites.

The interface between filler and hydration products can have a significant effect on the mechanical properties of the cement paste system. With different adhesion properties between filler and hydration products, the effect of microstructural features (size, shape, surface roughness), particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system is supposed to be different, as well. In order to understand the effect of the microstructural features, particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system with either strong or weak filler-matrix interface, microscale simulations with a lattice model are carried out. The results show that the strength of the filler-matrix interface plays a more important role than the microstructural features, particle distribution and area fraction of filler in the crack propagation and the strength of blended cement paste. The knowledge acquired here provides a clue, or direction, for improving the performance of existing fillers. To improve the performance of fillers in cement paste in terms of strength, priority should be given to improving the bond strength between filler particles and matrix, not to modifying the microstructural features (i.e., shape and surface roughness) of the filler.

Influence of packing density and water film thickness on early-age properties of cement pasteswith limestone filler and metakaolin

Supplementary cementitious materials (SCMs) affect the packing density and water film thickness (WFT) of blended cement paste. Herein, the effects of limestone filler (LF) and metakaolin (MK) on the packing density, WFT, flowability, heat of hydration, setting time and compressive strength of blended cement paste were studied. It was found that the addition of LF and MK would increase the packing density but whether it would increase the WFT is dependent on the change in solid surface area. Moreover, dispersion caused by finer particles stimulates the early hydration and reduces the setting time for the same SP dosage. However, the early compressive strength depends on the stimulation and dilution effects of SCM and retarding effect of SP.

Micromechanical multiscale fracture model for compressive strength of blended cement pastes

The evolution of compressive strength belongs to the most fundamental properties of cement paste. Driven by an increasing demand for clinker substitution, the paper presents a new four-level micromechanical model for the prediction of compressive strength of blended cement pastes. The model assumes that the paste compressive strength is governed by apparent tensile strength of the C-S-H globule. The multiscale model takes into account the volume fractions of relevant chemical phases and encompasses a spatial gradient of C-S-H between individual grains. The presence of capillary pores, the C-S-H spatial gradient, clinker minerals, SCMs, other hydration products, and air further decrease compressive strength. Calibration on 95 experimental compressive strength values shows that the apparent tensile strength of the C-S-H globule yields approx. 320MPa. Sensitivity analysis reveals that the “C-S-H/space” ratio, followed by entrapped or entrained air and the spatial gradient of C-S-H, have the largest influence on compressive strength.

Residual Strength of Blended Cement Pastes and Mortar Exposed to Elevated Temperatures

This paper presents the outcome of a series of tests to determine properties of blended cement pastes and mortar mixtures using Taguchi’s design of experiment approach. Fly ash (FA), water to cementitious material ratio (w/c), superplasticiser (SP) and sand to cementitious material ratio (s/c) are the main parameters considered. Mixture samples were tested at varying temperatures up to 800oC. The ANOVA results and regression analysis show that an increase in FA content and w/c ratio lead to a decrease in the density of blended cement. For compressive strength development, an increase in FA content and s/c ratio decreased the compressive strength development. Within the tested levels of the parameters, the optimum mix to achieve the highest compressive strength of blended cement mixtures is 20% of FA content, 1.5 of s/c ratio, 0.35 of w/c ratio and 0.2% of SP. Increasing the content of FA and SP improved the overall residual strength, and the optimum mix design to achieve the highest residual strength is 20% of FA, 1.5 of s/c ratio, 0.35 of w/c ratio and 0.2 of SP. Based on all the test results, an empirical formula for predicting residual strength of blended cement paste and mortar is proposed.

Flow behavior and strength for fly ash blended cement paste and mortar

Abstract In this study, compressive strength and flow behavior of fly ash incorporated cement paste and mortar were investigated. For this purpose, four water to binder ratios ( w /( c + f )) such as 0.25, 0.35, 0.45 and 0.55 with three curing ages and five wide range of fly ash replacement levels ( f / c ratio) namely 0, 0.1, 0.2, 0.3 and 0.4 were introduced in the experimental scheme. The workability of the mixture was ensured through Marsh cone and flow table tests for fly ash blended cement paste and mortar, respectively. The test result showed that compressive strength increases with age as expected in all cases and an empirical relationship for compressive strength of blended cement paste and mortar with mix factors such as w /( c + f ) ratio, f / c ratio and age is also proposed. The estimated compressive strength versus w /( c + f ) ratio curves for different curing ages and f / c ratios resembles the nature of Abrams’ strength versus w / c ratio curves.

Hydration characteristics of waste catalysts used as pozzolanic materials

This investigation examines the chemical and physical behaviors of waste catalyst (WC), a byproduct of the petroleum refining industry, used as a component of cement‐based materials. WC was recycled as a pozzolanic material partially to replace cement to enhance the pozzolanic activity of pastes contained WC. Fourier transformation infrared spectroscopy (FTIR), differential thermal and thermogravimetric analysis (DTA/TGA) and 29Si magnetic angle spinning/nuclear magnetic resonance (29Si MAS/NMR) approaches were adopted to relate both the hydration products of pastes contained WC and the structural features to its cementitious activity. The cementitious activity of WC was evaluated by testing the compressive strength of blended cement pastes. The results demonstrate that the blended cements that contained 10% WC exhibited favorable cementitious activity owing to the formation of C‐S‐H C‐A‐H and C‐A‐S‐H materials in the hydration products, and an increase in densification. The substitution of cement by WC would have an additional positive environmental impact because it would permit the recycling of a byproduct of the refinery industry in Taiwan that otherwise would remain in the environment as a waste product. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 353–358, 2014

Assessing the effect of biomass ashes with different finenesses on the compressive strength of blended cement paste

This study assesses the effect of biomass ashes with different finenesses on the compressive strength of blended cement paste. rice husk ash (RHA), palm oil fuel ash (POFA) and river sand (RS) were ground to obtain two finenesses: one was the same size as the cement, and the other was smaller than the cement. Type I Portland cement was replaced by RHA, POFA and RS at 0%, 10%, 20%, 30% and 40% by weight of binder. A water to binder ratio (W/B) of 0.35 was used for all blended cement paste mixes. The percentages of amorphous materials and the compressive strength of the pastes due to the hydration reaction, filler effect and pozzolanic reaction were investigated. The results showed that ground rice husk ash and ground palm oil fuel ash were composed of amorphous silica material. The compressive strength of the pastes due to the hydration reaction decreased with decreasing cement content. The compressive strength of the pastes due to the filler effect increased with increasing cement replacement. The compressive strengths of the pastes due to the pozzolanic reaction were nonlinear and were fit with nonlinear isotherms that increased with increasing fineness of RHA and POFA, cement replacement rate and age of the paste. In addition, the model that was proposed to predict the percentage compressive strength of the blended cement pastes on the basis of the age of the paste and the percentage replacement with biomass ash was in good agreement with the experimental results. The optimum replacement level of rice husk ash and palm oil fuel ash in pastes was 30% by weight of binder; this replacement percentage resulted in good compressive strengths.

Dry Shrinkage and Compressive Strength of Blended Cement Pastes with Fly Ash and Silica Fume

This paper reports the drying shrinkage and compressive strength results of cement pastes with fly ash and silica fume. In this study, Portland cement (PC) was used as the basic cementitious material. Fly ash (FA) and silica fume (SF) were used as cement replacement materials at levels of 0%, 5%, 10%, and 15% , 40%, 35%, 25%, and 15% by weight of the total cementitious material, respectively. The water/cement (PC + FA + SF) ratios (w/c) was 0.28 by weight. The samples produced from fresh pastes were demoulded after a day; then they were cured at 20 ±1°C with 50 ± 3% relative humidity (RH) until the samples were used for drying shrinkage and compressive strength measurement at various ages. The results show that drying shrinkage and compressive strength increase with increasing SF content, and the optimum composition of blended cement pastes is the cement paste with 30% fly ash and 10% silica fume, which possesses lower drying shrinkage values than that of plain cement paste and higher early age strength than that of blended cement pastes with fly ash. Furthermore, a linear relationship is established between compressive strength and drying shrinkage. By comparing the development of compressive strength and the drying shrinkage deformations, it appears possible to predict the drying shrinkage according to the acquired compressive strength.

Effects of densified silica fume on microstructure and compressive strength of blended cement pastes

Some experimental investigations on the microstructure and compressive strength development of silica fume blended cement pastes are presented in this paper. The silica fume replacement varies from 0% to 20% by weight and the water/binder ratio (w/b) is 0.4. The pore structure by mercury intrusion porosimetry (MIP), the micromorphology by scanning electron microscopy (SEM) and the compressive strength at 3, 7, 14, 28, 56 and 90 days have been studied. The test results indicate that the improvements on both microstructure and mechanical properties of hardened cement pastes by silica fume replacement are not effective due to the agglomeration of silica fume particles. The unreacted silica fume remained in cement pastes, the threshold diameter was not reduced and the increase in compressive strength was insignificant up to 28 days. It is suggested that the proper measures should be taken to disperse silica fume agglomeration to make it more effective on improving the properties of materials.