The effect of temperature on the hydration of composite cements containing limestone powder and fly ash

The effect of the curing temperature (5, 20 and 40°C) on the degree of hydration, amount of bound water and calcium hydroxide, porosity and the development of mechanical properties was investigated on pastes and mortars prepared with fly ash (FA)?Climestone (L) Portland composite cements. Increasing the curing temperature for ordinary Portland cement (OPC) leads to a more inhomogeneous distribution of hydration products, resulting in an increased coarse porosity and therefore a lower compressive strength after 7?days and longer. In contrast, the FA containing mortars showed higher compressive strength with increasing curing temperature up to 90?days. The reaction of the FA is increased at 40°C and strongly retarded at 5°C. At 20 and 40°C, FA reduces the porosity at later ages. The replacement of 5% of the OPC or FA by L powder did not impair the strength at 5 and 20°C, but lowered strength slightly at 40°C for the FA blended cements. The porosity appears to be the dominating factor regarding the compressive strength, independent of whether part of the OPC is replaced by FA and L powder or not.     

Influence of curing conditions on properties of high calcium fly ash geopolymer containing Portland cement as additive

This paper investigated the mechanical properties and microstructure of high calcium fly ash geopolymer containing ordinary Portland cement (OPC) as additive with different curing conditions. Fly ash (FA) was replaced with OPC at dosages of 0%, 5%, 10%, and 15% by weight of binders. Setting time and microstructure of geopolymer pastes, and flow, compressive strength, porosity and water absorption of geopolymer mortars were studied. Three curing methods viz., vapour-proof membrane curing, wet curing and temperature curing were used. The results showed that the use of OPC as additive improved the properties of high calcium fly ash geopolymer. The strength increased due to the formation of additional C–S–H and C–A–S–H gel. Curing methods also significantly affected the properties of geopolymers with OPC. Vapour-proof membrane curing and water curing resulted in additional OPC hydration and led to higher compressive strength. The temperature curing resulted in a high early compressive strength development.     

Modelling of slaked lime–metakaolin mortar engineering characteristics in terms of process variables

The purpose of the present study is to determine the effect of factors such as dosage, curing conditions and use of a superplasticiser admixture on the porosity, mechanical strength and composition of slaked lime (SL)–metakaolin (MK) mortars. Statistical correlations have been established to describe the mechanical properties as well as porosity and composition of the slaked lime–metakaolin mortars.The SL/MK ratio has a moderate effect on mortar flexural and compressive strengths. The SL + MK/sand ratio is the factor with the highest impact on all the properties studied: strength, porosity and mortar composition. As this ratio increases, strength, porosity and amount of hydration and carbonation products formed in the samples also rise. The next factor by order of importance is the presence of a superplasticiser admixture, which affects porosity, strength and the amount of calcite in the sample. The presence of this superplasticiser admixture increases strength, raises the percentage of calcite in the mortars and reduces porosity. It is particularly striking that neither curing nor open air carbonation time (in the range studied) has a significant effect on the composition or porosity of the SL–MK mortars studied, although they do have a moderate effect on mechanical strength.     

The hydration and microstructure of ultra high-strength concrete with cement–silica fume–slag binder

This study investigated the flowability, compressive strength, heat of hydration, porosity and calcium hydroxide content of ultra-high-strength concrete (UHSC) with cement–silica fume–slag binder at 20 °C. The composition of the binder was designed using seven-batch factorial design method. The relationships between the binder composition and the properties were expressed in contours. Results showed that proper silica fume content could improve the flowability and compressive strength of UHSC, reduce the porosity and calcium hydroxide content of UHSC. Slag reduced the flowability, compressive strength, porosity, and calcium hydroxide content of UHSC to certain extent. The silica fume and slag demonstrated positive synergistic effects on the flowability and 3 d compressive strength, but have negative synergistic effects on the total heat of hydration, hydration heat when the time is infinitely long(P₀), 56 d compressive strength, porosity and calcium hydroxide content of UHSC.     

Reactivity and performance of blastfurnace slags of differing origin

The performance of an Algerian blastfurnace slag has been assessed by examining the parameters influencing reactivity in a comparative study of two slags from different origins. These parameters include chemical composition, glass content, particle size distribution, heat of hydration and microstructural development. Three cement paste systems were investigated; OPC as a control, and two blended cements; 50% OPC-50% Algerian slag and 50% OPC-50% UK slag. All samples were made with a water/binder ratio of 0.4 and cured at 20 °C for up to 90 days. The specimens were tested using calorimetry, thermogravimetry, X-ray diffraction, selective dissolution, back-scattered electron microscopy and compressive strength. The chemical composition of the slag has a profound influence on strength development. The heat evolved appears to be proportional to the strength development and may be used as an important indicator of the reactivity of slag. A relationship was established between the amount of calcium silicate hydrate formed and compressive strength.     

Influence of 15 and 80 nano-SiO2 particles addition on mechanical and physical properties of ternary blended concrete incorporating rice husk ash

This study demonstrates the effects of SiO₂ nanoparticles as additives with two different sizes of 15 and 80?nm on compressive strength and porosity of rice husk ash (RHA) blended concrete. Up to 20% of ordinary Portland cement (OPC) was replaced by RHA with average particle size of 5 micron. Also, SiO₂ nanoparticles were added to the above mixture at four different weight percentages of 0.5, 1.0, 1.5 and 2.0 and cured in lime solution. The results indicated that compressive strength of Portland cement–nano SiO₂–rice husk ash (PC–NS–RHA) ternary blended concrete was considerably increased. Moreover, the total amount of porosity decreased to a minimum with respect to the control concrete. This improvement was observed at all the curing ages and replacement levels, but there was a gain in the optimal point with 20% of RHA plus 2% of 80?nm SiO₂ particles at 90 days of curing.     

Effects of light crude oil contamination on the physical and mechanical properties of geopolymer cement mortar

Fly ash and oil contaminated sand are considered as the two waste materials that may affect environment. This paper investigated the suitability of producing geopolymer cement mortar using oil contaminated sand. A comparison between physical and mechanical properties of mortar produced using geopolymer and Ordinary Portland Cement (OPC), in terms of porosity, hydration and compressive strength, was conducted. The results showed that heat curing can increase the compressive strength of geopolymer mortar up to 54% compared to ambient curing situation. The geopolymer mortar with 1% of light crude oil contamination yielded a 20% higher compressive strength than OPC mortar containing sand with a saturated surface dry condition. Furthermore, the formation of efflorescence decreased as the level of oil contamination decreased. Moreover, the heat curing method increased the kinetic energy and degree of reaction for geopolymer cement mortar, which cause an increment of the density of the pore system and improving the mechanical properties of the resulting composites. From the results of this study, it was demonstrated that geopolymer mortar has the potential of utilizing oil contaminated sand, and reducing its environmental impacts.     

Mechanical properties and durability of mortar and concrete containing natural pozzolana and limestone blended cements

The benefits of limestone filler (LF) and natural pozzolana (NP) as partial replacement of Portland cement are well established. Economic and environmental advantages by reducing CO₂ emission are well known. However, both supplementary materials have certain shortfalls. LF addition to Portland cement causes an increase of hydration at early ages inducing a high early strength, but it can reduce the later strength due to the dilution effect. On the other hand, NP contributes to hydration after 28 days improving the strength at medium and later ages. Hence, ternary blended cement (OPC–LF–NP) with better performance could be produced. In this paper, mortar prisms in which Portland cement was replaced by up to 20%LF and 30%NP were tested in flexure and compressive strength at 2, 7, 28 and 90 days. Some samples were tested under sulfate and acid solutions and for chloride ions permeability. Results show that the use of ternary blended cement improves the early age and the long-term compressive and flexural strengths. Durability was also enhanced as better sulfate, acid and chloride ions penetration resistances were proved.     

Characterization and properties of portland cement composites incorporating zinc-iron oxide nanoparticles

The present work exposes preliminary results concerning ordinary Portland cement (OPC) blended with oxide fumes produced in steel smelting plants and known as electric arc furnace dust (EAFD). After acid treatment of the EAFD, the powder obtained was formed basically of nanometric particles of ZnFe₂O₄. The incorporation of EAFD to OPC produced a small retardation of the setting process. Nevertheless, after 7 days the compressive strength of the OPC/EAFD pastes was superior and after 28 days the extent of hydration in OPC and OPC/EAFD pastes was equivalent. The present results indicate that a compressive strength of 72 MPa can be attained after 42 days for OPC doped with 10 wt% of EAFD.     

Hydration and properties of nano-TiO2 blended cement composites

Two types of nano-TiO₂ particles were blended into cement pastes and mortars. Their effects on the hydration and properties of the hydrated cement pastes were investigated. The addition of nano-TiO₂ powders significantly accelerated the hydration rate and promoted the hydration degree of the cementitious materials at early ages. It was demonstrated that TiO₂ was inert and stable during the cement hydration process. The total porosity of the cement pastes decreased and the pore size distribution were also altered. The acceleration of hydration rate and the change of microstructure also affected the physical and mechanical properties of the cement-based materials. The initial and final setting time was shortened and more water was required to maintain a standard consistence due to the addition of the nano-TiO₂. The compressive strength of the mortar was enhanced, practically at early ages. It is concluded that the nano-TiO₂ acted as a catalyst in the cement hydration reactions.     

Micro-structural development of gap-graded blended cement pastes containing a high amount of supplementary cementitious materials

To clarify the strength improvement mechanism of gap-graded blended cements with a high amount of supplementary cementitious materials, phase composition of hardened gap-graded blended cement pastes was quantified, and compared with those of Portland cement paste and reference blended cement (prepared by co-grinding) paste. The results show that the gap-graded blended cement pastes containing only 25% cement clinker by mass have comparable amount of gel products and porosity with Portland cement paste at all tested ages. For gap-graded blended cement pastes, about 40% of the total gel products can be attributed to the hydration of fine blast furnace slag, and the main un-hydrated component is coarse fly ash, corresponding to un-hydrated cement clinker in Portland cement paste. Further, pore size refinement is much more pronounced in gap-graded blended cement pastes, attributing to high initial packing density of cement paste (grain size refinement) and significant hydration of BFS.     

环境湿度对玻璃纤维增强水泥力学性能的影响

通过开展在不同龄期、不同环境湿度下玻璃纤维增强水泥(GRC)试件的抗折强度、抗压强度试验和基体pH值测定,研究了环境湿度对掺加粉煤灰和硅灰等活性矿物掺合料的GRC试件力学性能的影响。结果表明:环境湿度对GRC试件的抗折强度有重要影响,相对湿度越大,随着龄期增加, GRC试件抗折强度降低越严重;在温度60℃、相对湿度95%条件下,经过56 d龄期后,掺有40%粉煤灰和10%硅灰的GRC试件抗折强度比未掺加粉煤灰和硅灰的GRC试件的抗折强度提高48.5%、抗压强度提高23.6%, GRC基体pH值降低6%。在相同的湿度条件下,掺有粉煤灰和硅灰试件的pH值在各个龄期都低于普通硅酸盐水泥试件,说明粉煤灰和硅灰的掺入能降低水泥水化液相的碱度,进而延缓了纤维受侵蚀的速度,显著改善了GRC试件的力学及耐久性能。通过对试验结果进行分析,利用MATLAB软件建立了GRC试件抗折强度和抗压强度与水泥砂浆基体pH值及时间的关系式。      

Sorptivity and acid resistance of ambient-cured geopolymer mortars containing nano-silica

This study investigated the effects of nano-silica on flowability, strength development, sorptivity and acid resistance properties of fly ash geopolymer mortars cured at 20 °C. The changes in mass, compressive strength and microstructure of the specimens after immersion in acid solutions for different durations were determined. The microstructures were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. It was found that addition of nano-silica in geopolymer mortars based on fly ash alone or fly ash blended with 15% GGBFS or 10% OPC improved the compactness of microstructure by reducing porosity. Thus, the nano-silica reduced sorptivity and increased compressive strength of the mixes. The average mass loss after 90 days of immersion in acid solutions reduced from 6.0% to 1.9% by addition of 2% nano-silica. Similarly, significant reduction in strength loss after immersion in acid solution was observed in the specimens by using nano-silica.     

Strength development of ternary blended cement with limestone filler and blast-furnace slag

The benefits of limestone filler (LF) and granulated blast-furnace slag (BFS) as partial replacement of portland cement are well established. However, both supplementary materials have certain shortfalls. LF addition to portland cement causes an increase of hydration at early ages inducing a high early strength, but it can reduce the later strength due to the dilution effect. On the other hand, BFS contributes to hydration after seven days improving the strength at medium and later ages.Mortar prisms in which portland cement was replaced by up to 20% LF and 35% BFS were tested at 1, 3, 7, 28 and 90 days. Results show that the contribution of LF to hydration degree of portland cement at 1 and 3 days increases the early strength of blended cements containing about 5–15% LF and 0–20% BFS. The later hydration of BFS is very effective in producing ternary blended cements with similar or higher compressive strength than portland cement at 28 and 90 days. Additionally, a statistical analysis is presented for the optimal strength estimation considering different proportions of LF and BFS at a given age. The use of ternary blended cements (PC–LF–BFS) provides economic and environmental advantages by reducing portland cement production and CO₂ emission, whilst also improving the early and the later compressive strength.     

Microstructure and mechanical properties of accelerated sprayed concrete

Considering the different hydration processes of concrete without accelerator, sprayed concrete with low-alkali accelerator not only presents short setting times and high early-age mechanical properties but also yields different hydration products. This study presents an analysis of the mechanical properties of concrete with and without accelerator and sprayed concrete with three water–binder (w/b) ratios and four dosages of fly ash (FA) after different curing ages. It also examines the setting time, mineral composition, thermogravimetric–differential scanning calorimetry curves and microscopic images of cement pastes with different accelerator amounts. Furthermore, the setting time and microstructure of accelerated sprayed concrete with different w/b ratios and FA contents are examined. Results show that the retarded action of gypsum disappears in the accelerated cement–accelerator–water system. C₃A is quickly hydrated to form calcium aluminate hydrate (CAH) crystals, and a mesh structure is formed by ettringite, albite and CAH. A large amount of hydration heat improves the hydration rate of the cement clinker mineral and the resulting density, thereby improving mechanical properties at early curing ages. The setting times of the pastes increase with increasing w/b ratio and FA dosage. Thus, the hydration level, microstructure and morphology of the hydration products also change. Models of mechanical properties as functions of w/b, FA and curing age, as well as the relationship between compressive strength and splitting tensile strength, are established.     

The effects of CaCl<Subscript>2</Subscript>-blended acrylic polymer emulsion on the properties of cement mortar

Although acrylic polymer emulsions have been reported to impart many desirable attributes to cement mortar; delayed hydration, excessive air entrapment and moisture induced loss of strength have been highlighted as constraints. This paper explores the utilization of hydrated calcium chloride blended-acrylic polymer emulsion (CP) as a mitigation measure to these aforementioned drawbacks. First, the effects of 0, 0.5, 1.0 and 1.5% of CP by mass of cement on the early-age cement paste hydration and mortar flow were investigated. Thereafter, the influence of CP on the hardened porosity, moist-cured compressive strength, initial rate of capillary water absorption and rapid chloride permeability (RCPT) were evaluated. Test results indicate that the addition of CP to pastes sped up the cement hydration process, accelerating the final setting time of pastes by approximately 0.5–1.5 h as the CP content of pastes increased. Moreover, CP slightly increased the flow of fresh mortar, the hardened porosity of mortar mixtures containing 0.5 and 1.0% CP were also comparable to those of the plain reference mortar. With the exception of the 1.5% CP blended mortar, the 14 days moist-cured compressive strength of 0.5–1.0% CP blended mortar mixtures were also comparable to that of the plain reference mixture. Relative to the reference mixture, the addition of CP to mortar reduced the initial rate of capillary water absorption of mortar, with the mixture containing 1.5% CP giving a maximum reduction of 23%. Conversely, RCPT results indicate that above 0.5% CP addition level, CP generally increased the electrical conductivity of mixtures.     

A conduction calorimetric study of early hydration of ordinary Portland cement/high alumina cement pastes

Conduction calorimetry was applied to an investigation of the early hydration of ordinary Portland cement (OPC)/high alumina cement (HAC) pastes. Three different rate of heat-evolution profiles were observed, depending on the HAC/OPC ratio. Relevant processes affecting heat development include ettringite formation, HAC and OPC hydration. Results from SEM examination and X-ray diffraction studies are also presented. An acceleration of OPC hydration was observed in pastes containing less than 12.5% HAC. A similar acceleration effect on HAC hydration was also obtained with the addition of OPC. A large amount of ettringite was formed and OPC hydration delayed in the pastes containing 15%–30% HAC. The latter could be one of the factors attributed to poor strength development in these HAC/OPC systems. Early hydration mechanisms of OPC/HAC systems are also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Creep analysis of concrete containing rice husk ash

This paper presents an experimental study on the mechanical properties of concrete added with rice husk ash (RHA) as a supplementary cementitious material. The compressive strength, modulus of elasticity and creep were obtained experimentally from specimens with different RHA contents (0%, 10%, 15% and 20% of binder). The results show that the addition of RHA in concrete can improve both the compressive strength and modulus of elasticity and reduce the creep of concrete. The examination of pore micro-structure of hardened concrete using both the mercury intrusion porosimetry and scanning electron microscope techniques demonstrates that RHA particles can react with calcium hydroxide originated from cement hydration to produce additional C-S-H, which can fill voids and large pores and thus reduces the porosity related to capillary pores and voids. In addition, the release of absorbed water, which is retained in the small pores of RHA particles at early days, can improve cement hydration and thus reduce the porosity related to gel pores.     

Effects of CuO nanoparticles on compressive strength of self-compacting concrete

In the present study, the compressive strength, thermal properties and microstructure of self-compacting concrete with different amounts of CuO nanoparticles have been investigated. CuO nanoparticles with an average particle size of 15 nm were added to self-compacting concrete and various properties of the specimens were measured. The results indicate that CuO nanoparticles are able to improve the compressive strength of self-compacting concrete and reverse the negative effects of superplasticizer on compressive strength of the specimens. CuO nanoparticles as a partial replacement of cement up to 4 wt.% could accelerate C–S–H gel formation as a result of the increased crystalline Ca(OH)₂ amount at the early ages of hydration. Increasing CuO nanoparticle content to more than 4 wt.%, causes reduced compressive strength because of unsuitable dispersion of nanoparticles in the concrete matrix. Accelerated peak appearance in conduction calorimetry tests, more weight loss in thermogravimetric analysis and more rapid appearance of peaks related to hydrated products in X-ray diffraction results, all indicate that CuO nanoparticles up to 4 wt.% could improve the mechanical and physical properties of the specimens. Finally, CuO nanoparticles improved the pore structure of concrete and caused shifting of the distributed pores from harmless to low harm.     

Pore size and pore volume effects on alumina and TCP ceramic scaffolds

Controlled porosity alumina and β-tricalcium phosphate ceramic scaffolds with pore sizes in the range of 300–500 μm and pore volumes in the range of 25–45% were processed using the indirect fused deposition process. Samples having different pore sizes with constant volume fraction porosity and different volume fractions porosity with a constant pore size were fabricated to understand the influence of porosity parameters on mechanical and biological properties. In vitro cell proliferation studies were carried out with OPC1 human osteoblast cell line for 28 days with different scaffolds. Variation in pore size did not show any conclusive differences, but samples with higher volume fraction porosity showed some evidence of increased cell growth. Volume fraction porosity also showed a stronger influence on the mechanical properties under uniaxial compression loading. Compression strength dropped significantly for samples with higher volume fraction porosity, but changed marginally when only the pore size was varied.