Effects of superfine zeolite on strength,flowability and cohesiveness of cementitious paste

Superfine zeolite (SFZ) is a natural zeolite ground to higher fineness than cement. Being a pozzolanic material, it can be used to replace part of the cement to reduce the cement consumption and carbon footprint of concrete production. In this study, in order to evaluate the effects of SFZ on strength and fresh properties, a total of 30 cementitious paste mixes with different SFZ contents and different W/CM ratios were produced for 7-day, 28-day, 70-day strength tests, and flowability and cohesiveness tests. And, to evaluate the effectiveness of SFZ as a superfine filler, the changes in packing density and water film thickness (WFT) due to the addition of SFZ were measured and determined. It was found that the addition of SFZ as cement replacement up to 20% slightly decreased the early strength, but slightly increased the long-term strength. Moreover, it increased the packing density and exerted its influence on the fresh properties of cementitious paste through the corresponding change in WFT. It also significantly increased the cohesiveness at the same flowability.     

Combined effects of water film thickness and paste film thickness on rheology of mortar

In the mortar portion of a concrete mix, the water must be more than sufficient to fill the voids between the solid particles of cement and fine aggregate whereas the paste volume must be more than sufficient to fill the voids between the solid particles of fine aggregate so that there will be excess water to form water films coating all the solid particles and excess paste to form paste films coating the fine aggregate particles. Hence, it may be postulated that the water film thickness (WFT) and the paste film thickness (PFT) have major effects on the rheology of mortar. In this study, the combined effects of WFT and PFT on the rheology, cohesiveness and adhesiveness of mortar were investigated by testing mortar samples with varying water, cement and aggregate contents. It was found that whilst the WFT is the single most important factor governing the rheology of mortar, the PFT also has significant effects. Particularly, the PFT has certain interesting effects on the cohesiveness and adhesiveness of mortar, which should be duly considered in mortar design.     

Packing optimization of paste and aggregate phases for sustainability and performance improvement of concrete

Previous research demonstrated that the packing density, water film thickness and paste film thickness have great effects on the performance of a concrete mix. On this basis, it is herein proposed a strategy of adding a powder waste as both paste and aggregate replacements to reduce the cement and aggregate consumptions for sustainable development and to improve the packing densities of both the paste phase and aggregate phase for performance improvement. To evaluate such strategy, 25 concrete mixes incorporating granite polishing waste (GPW) as paste and aggregate replacements were tested. The results revealed that the addition of GPW as paste replacement up to 7.5% and as aggregate replacement up to 10% would most effectively increase the packing densities of the paste phase, aggregate phase and whole concrete mix, and thereby increasing the strength of the concrete, despite reduction in cement content. Such increases in packing density would also increase the excess water and excess paste to avoid excessive reductions in the water and paste film thicknesses, which are needed to maintain workability. Last but not least, separate optimization of the paste phase and aggregate phase is an effective way of optimizing the concrete mix design.     

Ternary blending of cement with fly ash microsphere and condensed silica fume to improve the performance of mortar

The addition of condensed silica fume (CSF) to fill into the voids between cement grains would release the water entrapped there to form water films for lubrication. However, the large surface area of CSF would thin down the water film thickness (WFT). By adding also a cementitious material that is finer than cement but not as fine as CSF, such as fly ash microsphere (FAM), the water entrapped in the voids could be released without excessively increasing the surface area. This may produce a larger WFT and better flowability than adding CSF alone. In this research, ternary blending of cement with FAM and CSF was studied by testing mortar mixes with different amounts of FAM and CSF added. It was found that the WFT is the key factor governing the properties of mortar and that ternary blending of cement with both FAM and CSF does offer some advantages.     

Packing density of cementitious materials: part 2—packing and flow of OPC + PFA + CSF

The wet packing method developed in Part 1 [Wong HHC, Kwan AKH (2007) Packing density of cementitious materials: part 1 measurement using a wet packing method. Mater Struct (Paper No. MAAS3281)] has been successfully applied to measure the packing densities of cementitious materials containing ordinary Portland cement (OPC), pulverised fuel ash (PFA) and condensed silica fume (CSF). The test results for non-blended materials revealed that whilst the addition of a superplasticiser would always increase the packing densities of OPC and PFA, the addition of a polycarboxylate-based superplasticiser could decrease the packing density of CSF. On the other hand, the results for blended materials showed that the packing density could be improved by double blending OPC with either PFA or CSF, and further improved by triple blending OPC, PFA and CSF together in appropriate proportions. A maximum packing density of 0.752 has been achieved and a ternary packing density diagram for determining the mix proportions for maximum packing density has been produced. Furthermore, the positive influence of a higher packing density on cement paste rheology has been demonstrated using the mini-slump cone test. Based on these results, the concept of excess water ratio, which is the major factor governing the rheology of a paste, is introduced.     

Role of water film thickness in rheology of CSF mortar

In a recent study, the authors have demonstrated that the combined effects of water content, packing density and solid surface area on the rheology of cement–sand mortar may be evaluated in terms of the water film thickness (WFT). The present study aims to extend the concept of WFT to mortar containing condensed silica fume (CSF). For the study, mortar samples with various CSF and water contents were made for packing density, flowability and rheology measurements. It was found that although the effects of adding CSF are fairly complicated, the WFT is still the single most important parameter governing the rheology of CSF mortar. However, the rheological properties are dependent also on the CSF content. Correlations of the rheological properties to both the WFT and CSF content yielded R² values of at least 0.896.     

Optimization of calcium chloride content on bioactivity and mechanical properties of white Portland cement

This research investigates the optimization of calcium chloride content on the bioactivity and mechanical properties of white Portland cement. Calcium chloride was used as an addition of White Portland cement at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% by weight. Calcium chloride was dissolved in sterile distilled water and blended with White Portland cement using a water to cement ratio of 0.5. Analysis of the bioactivity and pH of white Portland cement pastes with calcium chloride added at various amounts was carried out in simulated body fluid. Setting time, density, compressive strength and volume of permeable voids were also investigated. The characteristics of cement pastes were examined by X-ray diffractometer and scanning electron microscope linked to an energy-dispersive X-ray analyzer. The result indicated that the addition of calcium chloride could accelerate the hydration of white Portland cement, resulting in a decrease in setting time and an increase in early strength of the pastes. The compressive strength of all cement pastes with added calcium chloride was higher than that of the pure cement paste, and the addition of calcium chloride at 8 wt.% led to achieving the highest strength. Furthermore, white Portland cement pastes both with and without calcium chloride showed well-established bioactivity with respect to the formation of a hydroxyapatite layer on the material within 7 days following immersion in simulated body fluid; white Portland cement paste with added 3%CaCl₂ exhibited the best bioactivity.     

Adding limestone fines as cementitious paste replacement to improve tensile strength,stiffness and durability of concrete

The addition of a filler such as limestone fines (LF) to fill into the voids between aggregate particles can reduce the cementitious paste volume needed to produce concrete. In previous studies, it has been found that the addition of LF to reduce the cementitious paste volume would substantially increase the cube strength, and reduce the heat generation and shrinkage of the concrete produced. In this study, the authors aimed to evaluate the effects of adding LF as cementitious paste replacement on the tensile strength, stiffness and durability of concrete. For the evaluation, a series of concrete mixes with LF added to replace an equal volume of cementitious paste were tested for their workability, cube strength, tensile splitting strength, modulus of elasticity, water penetration depth and chloride permeability. The results showed that the addition of LF as cementitious paste replacement would at the same water/cement ratio, and even at the same cube strength, improve the tensile strength, stiffness and durability of concrete.     

Biomass fly ash effect on fresh and hardened state properties of cement based materials

Cement pastes and mortars were prepared by replacing ordinary Portland cement with different dosages of biomass fly ashes (0, 10, 20 and 30% BFA) whilst in dry condition. The effect of BFA on the flow behaviour (spread on table and rheology), setting time, temperature of hydration and electrical resistivity was studied in this experimental research. Increasing the amount of BFA in the compositions required extra dosage of water, as a result of particles fineness, tendency for agglomeration and retention/absorption of water molecules. As a consequence, the relative amount of free water diminishes and the flowability is poorer. The introduction of BFA also led to an increase in setting time, while the resistivity obtained from the impedance measurements tends to be lower than the reference paste (ash-free). The higher concentration of mobile species in the pore solution, namely sodium ions introduced by the ash, explains that tendency. The hydration temperature of cement pastes tends to decrease with the level of cement to ash replacement. Between the two tested ashes (from grate and fluidized sand bed furnaces), differences in particle size and shape, in the amount of residual organic matter and concentration of inorganic components define minor changes in the workability and setting behaviour. Therefore, the introduction of biomass fly ashes affects the hardened state features but do not compromise them.     

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.     

Effect of particle-size distribution and specific surface area of different binder systems on packing density and flow characteristics of cement paste

The particle-size distribution (PSD) and specific surface area (SSA) of binders significantly affect the fresh and hardened characteristics of cement-based materials. An experimental investigation was undertaken to evaluate the influence of PSD and calculated SSA of various binary and ternary binder systems on flow characteristics, packing density, and compressive strength development of cement paste. The influence of dispersion state of the binder on packing density was evaluated using the wet packing density approach to determine the optimum water demand (OWD) needed to achieve maximum wet density. The modified Andreasen and Andersen (A&A), Rosin–Rammler (RR), and power law grading models were employed to optimize the PSD of binder system to achieve maximum packing density, while maintaining relatively low water demand. The incorporation of high-range water reducing admixture (HRWRA) is shown to decrease the OWD and increase the packing density resulting from greater degree of dispersion of the binder. The combined effect of lower OWD, greater packing density, and higher SCM reactivity results in higher compressive strength. The increase in SSA from 425 to 1600 m²/kg results in an enhancement in packing density from 0.58 to 0.72, while further increase in SSA from 1600 to 2200 m²/kg reduces the packing density from 0.72 to 0.62. Binder systems using a distribution modulus between 0.21 and 0.235 determined from the A&A model exhibited 18%–40% lower minimum water demand (MWD) to initiate flow, 8%–35% higher OWD to reach maximum wet density, and 15%–25% higher packing density compared to the binder with 100% cement. Binder systems with lower A&A distribution modulus resulted in higher relative water demand (RWD) required to increase fluidity, thus reflecting greater level of robustness. Good correlations were established between the A&A distribution modulus, SSA, RR spread factor, and power law distribution exponent.     

Computer simulation of interfacial packing in concrete

Computer simulation of particle packing against an aggregate surface was undertaken to show the effects of four variables on interfacial porosity profiles. The variables in order of significance and their assumed physical meaning are: sticking probability (tendency of cement particles to flocculate), amplitude of particle motion (energy of mixing), travel distance of particle to surface (thickness of water film surrounding aggregate), and original particle density (roughly related to water/cement ratio). In all cases, simulations demonstrated that interface porosity decreased from nearly 100% directly at the interface to that of the bulk paste at two to three particle diameters. Flocculation (sticking probability) was found to be the single most-significant variable. Highly flocculated systems produced very porous interfaces. When flocculation was reduced, packing became more efficient. It was also found that energy of mixing (amplitude of motion), was not an entirely independent variable. The simulation showed that, if the tendency to flocculate was high, gentle mixing (low amplitude of motion) was found to result in better packing and a less porous interfacial zone. If, on the other hand, flocculation was low, then vigorous mixing (high amplitude of motion) promoted better packing near the interface. The thickness of the water film surrounding the aggregate (travel distance) was found to have only a minor effect on the outcome of simulations, while original packing density (w/c) resulted in no significant differences at all.     

超细矿渣粉对硅酸盐水泥性能和微观结构的影响

通过掺入不同量的超细矿渣粉,研究其对普通硅酸盐水泥凝结时间、标准稠度用水量以及水泥胶砂流动性和强度的影响。结果表明,水泥浆体的初凝、终凝时间在矿渣粉掺量为5%(质量分数,下同)时有所缩短,而随着超细矿渣粉掺量的增加,初凝时间都有所延长,在掺量为20%时初凝时间最长。然而终凝时间的变化不大,只有掺量为30%时稍有延长;水泥的标准稠度用水量先减少后增加,在掺量为20%时最小;随着超细矿渣粉掺量的增大,水泥胶砂的各龄期抗折强度、3d抗压强度不断提高,7d、28d抗压强度在掺量为20%时达到最大值,之后有所降低。掺入超细矿渣粉后,能通过填充以及与水泥水化产物氢氧化钙发生反应,使水泥中氢氧化钙含量明显降低,水泥微观结构更加密实。     

On the factors affecting strength of portland cement

This paper reports mechanical property measurements for Portland Cement paste free from fabrication artifacts (e.g. bubble-type voids), and compares them to published results both for normal and new high strength cement. Removal of large voids (above 100m) by vacuum de-airing leads to an increase of 15% in mean flexural strength and a small decrease in fracture toughness. This increase in flexural strength is predictable from the tied-crack model previously proposed to explain the notch-sensitivity behaviour of hardened cement paste, and for which direct experimental evidence was obtained. It is suggested that factors such as moisture content are at least as important as large voids in controlling mechanical properties. It is concluded that the much increased strength of the new polymer-containing cements must result from improvements to the microstructure other than the simple elimination of voids.     

Experimental study of the interfacial transition zone (ITZ) of model rock-filled concrete (RFC)

Rock-filled concrete (RFC), a new type of concrete that was developed mainly for large scale concrete construction, has a different casting process than conventional concrete: large rocks are piled into the formwork first, then self-compacting concrete (SCC) is poured in and fill the voids of the rock skeleton under gravity due to its high flowability. One of the key issues about RFC lies in its large interfaces between the SCC and rocks. In this paper, laboratory-scale model RFC consisting of coarse aggregates (simulating rocks) and cement grout (simulating SCC) was cast to simulate RFC in construction. The effects of different factors (aggregate size, rheology of cement grout, etc.) on the properties of the interfacial transition zone (ITZ) between cement paste and aggregates of model RFC were investigated using Backscatter Electron (BSE) and nanoindentation techniques. Furthermore, by comparing the results of BSE and nanoindentation at identical regions, the relationship between porosity and elastic modulus was found to agree well with empirical formulas, bridging the microstructure with the mechanical properties of concrete.     

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

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.     

The influence of cellulose nanocrystal additions on the performance of cement paste

The influence of cellulose nanocrystals (CNCs) addition on the performance of cement paste was investigated. Our mechanical tests show an increase in the flexural strength of approximately 30% with only 0.2% volume of CNCs with respect to cement. Isothermal calorimetry (IC) and thermogravimetric analysis (TGA) show that the degree of hydration (DOH) of the cement paste is increased when CNCs are used. The first mechanism that may explain the increased hydration is the steric stabilization, which is the same mechanism by which many water reducing agents (WRAs) disperse the cement particles. Rheological, heat flow rate measurements, and microscopic imaging support this mechanism. A second mechanism also appears to support the increased hydration. The second mechanism that is proposed is referred to as short circuit diffusion. Short circuit diffusion appears to increase cement hydration by increasing the transport of water from outside the hydration product shell (i.e., through the high density CSH) on a cement grain to the unhydrated cement cores. The DOH and flexural strength were measured for cement paste with WRA and CNC to evaluate this hypothesis. Our results indicate that short circuit diffusion is more dominant than steric stabilization.     

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

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.     

Properties and interaction of cement with polymer in the hardened cement pastes added absorbent polymer

Ordinary Portland cement mixed with various amounts of absorbent polymer in the form of sodium acrylate ((–CH–)_nCOONa) have been studied. As the content of absorbent polymer increased, heat evolution of samples decreased up to 1.15 wt % of absorbent polymer addition and conversely increased over 1.75 wt %. Flexural strength of cement paste with absorbent polymer was improved more than 20%. As the content of absorbent polymer increased, the porosity values decreased and mean pore diameter shifted to small pore diameter region. Flexural strength of ordinary Portland cement paste had a linear correlation with non-evaporable water content but, that of cement paste with absorbent polymer deviated from a linear correlation with non-evaporable water content. The chemical difference between cement pastes with and without absorbent polymer was found by the inductively coupled plasma-atomic emission spectroscopy and the infrared spectroscopy. For the infrared spectra of absorbent polymer, bands at 1416 and 1560 cm^{– 1} were assigned to C–O single bond and C=O double bond respectively, namely, unidentate complex. As the curing time increased, the absorption bands near 1416 cm^{– 1} shifted to longer wave number and the absorption bands near 1560 cm^{– 1} to shorter wave number and finally bidentate complex was formed. Absorbent polymer released sodium ions to pore solution under the basic condition of pH 12.5–13.5 and became polyacrylic acid. Then some of these polyacrylic acid were crosslinked with others by calcium ions leached from cement grains. Calcium ion was regarded as a central charge connecting the negative parts in carbon-oxygen polarization of absorbent polymer' functional groups.     

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.