Mechanisms of hydration reactions in high volume fly ash pastes and mortars

This paper describes investigations of high-volume fly ash (HVFA)-Portland cement (PC) binders, the physical and chemical properties of which have been characterized up to 365 days of curing. Physical investigations were made of compressive strength development, pore structure by porosimetry, and morphology by scanning electron microscopy. Chemical examination was conducted for solid phase composition and degree of hydration by X-ray diffraction and thermal analysis, and for pore-fluid composition by high pressure extraction and analysis.

Up to 365d the cement in the HVFA pastes is not fully hydrated. However, the ash participates in both early (sulpho-pozzolanic) and late (alumino-silicate) hydration reactions. In addition to the usual products of cement hydration, ettringite (AFt) has been identified as a product of the early hydration of the fly ash. It has not been possible to identify long term hydration products of fly ash which appear to be non-crystalline. A two-step mechanism for pozzolanic reaction between fly ash and Portland cement has been proposed involving: (a) depolymerization/silanolation of the glassy constituents of the ash by the highly alkaline pore fluids, followed by (b) reaction between solubilized silicate and calcium ions in solution to form C---S---H.     

An explanation for the negative effect of elevated temperature at early ages on the late-age strength of concrete

Elevated curing temperature at early ages usually has a negative effect on the late-age strength of concrete. This article aims to study the mechanism of this phenomenon. The results show that elevated curing temperature at early ages has a negative effect on the late-age strength of hardened cement paste, but it has a greater negative effect on the late-age strength of cement mortar. After elevated temperature curing at early ages, the late hydration of cement is hindered, but the late reaction of fly ash is not influenced. Owing to the continuous reaction of fly ash, the late-age pore structure of cement–fly ash paste under elevated curing temperature is finer than that under standard curing temperature, and the late-age strength of cement–fly ash paste under elevated curing temperature is higher. However, the late-age strength of cement–fly ash mortar under elevated curing temperature is lower. Apparently, there are differences between the effects of elevated curing temperature on hardened paste and mortar. It is the deterioration of transition zone between hardened paste and aggregate that makes the negative effect of elevated curing temperature on the mortar (or concrete) be greater than the hardened paste. As the water-to-binder ratio decreases, the negative effect of elevated curing temperature on the transition zone tends to be less.     

Effects of colloidal nanoSiO2 on fly ash hydration

The influences of colloidal nanoSiO₂ (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO₂ had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO₂ on cement/FA hydration at later ages were proposed.     

Effects of colloidal nanoSiO2 on fly ash hydration

The influences of colloidal nanoSiO₂ (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO₂ had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO₂ on cement/FA hydration at later ages were proposed.     

水泥-粉煤灰-硅灰基超高性能混凝土水化过程微观结构的演变规律

超高性能混凝土(UHPC)具有卓越的力学性能和耐久性能,应用前景广阔。采用扫描电镜背散射电子图像、热重法和氮气吸附法系统研究了水泥-粉煤灰-硅灰基UHPC浆体水化过程中微观结构的演变过程。结果表明:UHPC浆体在早期水泥水化较快,但7d后水化变得较为缓慢,粉煤灰在UHPC浆体中反应较为缓慢,28d时反应程度仅为7%;UHPC浆体中Ca(OH)2含量早期上升快速,由于硅灰和粉煤灰的火山灰反应逐渐消耗,3d后含量开始下降,但28d时浆体中仍存在部分Ca(OH)2;此外,在水化过程中,UHPC浆体的比表面积不断降低,孔隙率逐渐下降,水化产物变得更为致密。     

Ultrasonic characterization of the curing process of PCC fly ash–cement composites

The effects of the type of fly ash, mix proportion, and curing process on the pozzolanic reaction of fly ash–cement paste were investigated by ultrasonic techniques. Specifically, the speed of sound (SOS) and broadband ultrasonic attenuation (BUA) were used to investigate hydration activities of the fly ash–cement composite. SOS provided direct evidence of the delay in the hydration activity caused by mixing fly ash to the cement. The rapid heat of evolution during hydration activity, as indicated by a rapid increase in SOS, resulted in early stiffening of the Class C fly ash–cement composite. However, Class C fly ash–cement composite achieved a lower elastic modulus compared with Class F fly ash–cement composite. The hydration activity is observed to be highly dependent on the type of fly ash substituted for cement in the composite. The BUA provided the indirect evidence of ionic activities occurring during the hydration period and viscoelastic properties of the material.     

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.     

The hydration properties of pastes containing municipal solid waste incinerator fly ash slag

This study investigated the hydration properties of Type I, Type III and Type V cements, mixed with municipal solid waste incinerator fly ash, to produce slag-blended cement pastes. The setting time of slag-blended cement pastes that contained 40% slag showed significantly retardation the setting time compared to those with a 10% or even a 20% slag replacement. The compressive strength of slag-blended cement paste samples containing 10 and 20% of slag, varied from 95 to 110% that developed by the plain cement pastes at later stages. An increased blend ratio, due to the filling of pores by C-S-H formed during pozzolanic reaction tended to become more pronounced with time. This resulting densification and enhanced later strength was caused by the shifting of the gel pores. It was found that the degree of hydration was slow in early stages, but it increased with increasing curing time. The results indicated that it is feasible to use MSWI fly ash slag to replace up to 20% of the material with three types of ordinary Portland cement.     

脱硫石膏对大掺量粉煤灰-矿渣粉干混砂浆性能的影响

针对大掺量粉煤灰、矿渣粉导致干混砂浆早期强度和后期强度较低的问题,研究脱硫石膏对该干混砂浆性能的影响;采用X射线衍射、扫描电镜及孔结构分析等手段进行微观机理讨论。结果表明,在大掺量粉煤灰矿粉干混砂浆中掺加占胶凝材料总质量6%～8%的脱硫石膏,对和易性无不良影响,并可显著提高浆体的抗压强度及拉伸粘结强度,收缩率降低10%以上,并改善抗碳化能力,使砂浆体积更稳定;脱硫石膏对粉煤灰及矿渣粉起到激发硫酸盐和碱性的双重作用,并在一定程度上促进水泥水化;胶凝材料的水化产物改善砂浆浆体内部结构,使砂浆浆体中的孔隙大大减少。     

Effect of water curing conditions on the hydration degree and compressive strengths of fly ash–cement paste

This paper explains the effect of water curing condition on compressive strengths of fly ash–cement paste by quantitative data of hydration degree. Hydration of fly ash–cement paste was estimated by Rietveld analysis and selective dissolution. The result shows that the hydration degree of belite is affected by water curing conditions, more so than that of fly ash and alite. Fly ash still continues to hydrate even without an extra, external supply of water. The strong dependence of fly ash–cement concrete on curing conditions does not come from the hydration degree of fly ash, but rather comes from the hydration degree of cement, especially belite. When the water to binder ratio is low enough, the hydration of cement plus small hydration of fly ash are considered to be enough for adequate compressive strength at the beginning. Then, compressive strength of fly ash–cement paste becomes less sensitive to the water curing period.     

Ca(OH)2解耦法对混合水泥中C-S-H凝胶的半定量研究

基于掺合料反应程度与Ca(OH)2消耗量之间关系的建立,结合水泥水化平衡计算理论,提出混合水泥水化产物中Ca(OH)2量的解耦方法并将其用于混合水泥水化产物C-S-H凝胶半定量计算.采用该法对掺粉煤灰的混合水泥不同龄期水化产物C-S-H凝胶进行了半定量分析研究.     

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.     

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.     

Porosity of cement paste cured at 45 °C as a function of location relative to casting position

The influence of location relative to the casting position, on porosity and pore size distribution of cement pastes, was investigated. Three different pastes were prepared at a constant water/binder ratio of 0.45. The pastes were the control paste (CP) in which Portland cement was used and no cement replacement materials were added, pastes with 22% and 9% replacement (by mass) of cement with fly ash (FA) and silica fume (SF), respectively. Paste specimens were cast in cube moulds and were either cured in air at a temperature of 45 °C and relative humidity of 25% for 28 days or moist cured for 14 days after casting at 45 °C, followed by air curing at 45 °C and 25% relative humidity for further 14 days. Samples were taken from various locations of the cube specimens. Porosity and pore size distribution were conducted on the paste samples using the mercury intrusion porosimetry technique.The results show that large differences in porosity and pore size distribution exist between samples taken from different locations relative to casting positions. These differences are larger in pastes subjected to dry curing as compared to pastes subjected to some initial moist curing. The influence of sample location relative to casting position on porosity and pore size distribution of paste is compared with absorption of concrete performed in a previous investigation. The correlation between pore volume of paste and water absorption of concrete is also conducted.     

The chemical composition and microstructure of hydration products in blended cements

Pastes of neat and blended Portland cement (incorporating either 60% ground granulated blast furnace slag, or 30% pulverised fuel ash, or 22% volcanic ash) were cured for one year at temperatures ranging from 10 to 60 °C. The hydration products were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The apparent porosity of the pastes increased with increasing curing temperature. Chemical analysis data for the hydration products are presented in ternary composition diagrams, where it is noted that in the presence of the replacement materials the composition of the C–S–H shifted towards higher Si and Al contents, whereas that of Ca was lower.     

Influence of fly ash blending on hydration and physical behavior of belite–alite–ye’elimite cements

A cement powder, composed of belite, alite and ye’elimite, was blended with 0, 15 and 30 wt% of fly ash and the resulting blended cements were further characterized. During hydration, the presence of fly ash caused the partial inhibition of both AFt degradation and belite reactivity, even after 180 days. The compressive strength of the corresponding mortars increased by increasing the fly ash content (68, 73 and 82 MPa for mortars with 0, 15 and 30 wt% of fly ash, respectively, at 180 curing days), mainly due to the diminishing porosity and pore size values. Although pozzolanic reaction has not been directly proved there are indirect evidences.     

Studies on mechanics of development of physical and mechanical properties of high-volume fly ash-cement pastes

High-volume fly ash concrete for structural applications was developed at CANMET. In this concrete fly ash to ‘total cementitious material’ was maintained over 55%. The purpose of this work was to investigate, by the use of similar paste mixtures of the same fly ash and cement, the mechanism by which the mechanical properties were developed. Mechanical property-porosity relations, pore size distribution, permeability, degree of hydration and Ca(OH)₂ content measurements were made. It was observed that the fly ash-cememt reaction occurred relatively early at 3 to 7 days and it was concluded that the cement matrix and residual unreacted fly ash form a good mechanical bond.     

Pozzolanicity of finely ground lightweight aggregates

This paper examines the pozzolanic behavior of finely ground lightweight aggregates with a mean particle size between 4 and 26 μm. Cement pastes are made with a 20% mass replacement of cement with finely ground lightweight aggregates, fly ash, quartz, and limestone in addition to a control paste with no cement replacement. Isothermal calorimetry, thermogravimetric analysis, and compressive strength testing as well as thermodynamic calculations are performed on these pastes. Isothermal calorimetry and compressive strength testing are shown to not be able to clearly distinguish and quantify the pozzolanic response of the finely ground lightweight aggregates, fly ash, quartz, and limestone when they are used in cement pastes. However, thermogravimetric analysis and thermodynamic calculations clearly show that the finely ground lightweight aggregates are pozzolanic through the consumption of calcium hydroxide. A pozzolanic reactivity test based on isothermal calorimetry also confirms that the finely ground lightweight aggregates are pozzolanic. These results indicate that finely ground lightweight aggregates are pozzolanic and could be used in concreting applications.     

Cement-stabilized fly ash base courses

Various demonstration projects have been carried out in The Netherlands with cement-stabilized fly ash as a base course. Usually these courses were made of 100 parts by mass of fly ash; 10 parts by mass of cement; 20 to 30 parts by mass of water. However, the projects were not quite successful since delamination was observed, and long-term strength, after a period of six years of performance, appears to be much smaller than expected on the basis of preliminary laboratory research. A model for pozzolanic reaction of fly ash recently developed by Fraay and Bijen pointed out that the reactivity of fly ash is influenced greatly by the pH value of the pore water. A pH of at least 13 is required to initiate fly ash pozzolanic reaction in a Portland cement environment. Pore water extraction measurements showed that the pH of cement-stabilized fly ash often has a substantially lower value. In this high-volume fly ash application the effect of the acidity of fly ash is much greater than in ordinary concrete with cement replaced by fly ash up to 30%. By addition of NaOH and/or sodium silicate to the mixing water, the pH value can be increased above the ‘threshold’ value.

Tests were carried out with different types of class-F fly ashes and with different NaOH concentrations in the mixing water. The results show an increase in compressive strength of up to 300% depending on the type of fly ash, and a substantial decrease in the rate of water absorption.     

Internal curing of Class-F fly-ash concrete using high-volume roof-tile waste aggregate

The aim of this study was to investigate the effect of a high volume of roof-tile waste coarse aggregate (5–13 mm) as an internal curing agent on the compressive strength, modulus of elasticity, pore structure, and hydration and pozzolanic reactions in paste of fly-ash concrete with a low water-to-binder ratio of 0.30. The fly-ash concrete specimens in which the replacement ratio of cement by Class-F fly ash was 40% by mass and that of normal coarse aggregate by roof-tile waste aggregate was 40% by volume, were cured up to 728 days. Internal curing with roof-tile waste aggregate increased the compressive strength of the fly-ash concrete by 8.4–16.5% and decreased the modulus of elasticity by 4.9–12.8%. The use of a high volume of waste aggregate decreased the volume of the capillary pores in the 0.01–10 µm range and the volume proportion of the 0.02–0.33-µm pores after 28 days, but increased the volume proportion of 0.003–0.02-µm pores slightly at 7 days and significantly up to 728 days, and the consumption of Ca(OH)₂ in the fly-ash concrete. This roof-tile waste aggregate can be used as an internal water reservoir to increase the compressive strength and to improve the pore structure of concrete with a high-volume (40%) replacement of Class-F fly ash.