Microconcrete with partial replacement of Portland cement by fly ash and hydrated lime addition

The reduction in Portland cement consumption means lower CO₂ emissions. Partial replacement of Portland cement by pozzolans such as fly ash has its limitations due to the quantity of calcium hydroxide generated in the mix. In this work we have studied the contribution of the addition of hydrated lime to Portland cement + fly ash systems. We have also studied several levels of cement replacement, ranging from 15% to 75%.The best mechanical results were obtained replacing 50% of Portland cement by the same amount of fly ash plus the addition of hydrated lime (20% respect to the amount of fly ash). In these systems, an acid-base self-neutralization of the matrix has occurred through a pozzolanic reaction of fly ash with portlandite liberated in the hydration of Portland cement and the added hydrated lime. It has been identified for these mixtures a significant amount of hydrated gehlenite, typical reaction product from rich-alumina pozzolans.     

Hydration of quaternary Portland cement blends containing blast-furnace slag,siliceous fly ash and limestone powder

In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.     

Microstructural changes in alkali activated fly ash/slag geopolymers with sulfate exposure

Sulfate attack is recognized as a significant threat to many concrete structures, and often takes place in soil or marine environments. However, the understanding of the behavior of alkali-activated and geopolymer materials in sulfate-rich environments is limited. Therefore, the aim of this study is to investigate the performance of alkali silicate-activated fly ash/slag geopolymer binders subjected to different forms of sulfate exposure, specifically, immersion in 5 wt% magnesium sulfate or 5 wt% sodium sulfate solutions, for 3 months. Extensive physical deterioration of the pastes is observed during immersion in MgSO₄ solution, but not in Na₂SO₄ solution. Calcium sulfate dihydrate (gypsum) forms in pastes immersed in MgSO₄, and its expansive effects are identified as being particularly damaging to the material, but it is not observed in Na₂SO₄ environments. A lower water/binder (w/b) ratio leads to a greatly enhanced resistance to degradation by sulfate attack. Infrared spectroscopy shows some significant changes in the silicate gel bonding environment of geopolymers immersed in MgSO₄, attributed mostly to decalcification processes, but less changes upon exposure to sodium sulfate. It appears that the process of ‘sulfate attack’ on geopolymer binders is strongly dependent on the cation accompanying the sulfate, and it is suggested that a distinction should be drawn between ‘magnesium sulfate attack’ (where both Mg²⁺ and SO₄ ^2? are capable of inducing damage in the structure), and general processes related to the presence of sulfate accompanied by other, non-damaging cations. The alkali-activated fly ash/slag binders tested here are susceptible to the first of these modes of attack, but not the second.     

Strength development of concretes with ordinary Portland cement,slag or fly ash cured at different temperatures

This paper presents the results of an investigation on the effect of Portland cement replaced by fly ash or granulated blast-furnace slag on the concrete strength at different curing temperatures. Compressive strength results are analysed according to the hyperbolic strength-age function by introducing a power indexn. The regression analysis is done considering different n values andt _o (final setting times) values.     

Study on hydration of Portland cement with fly ash using electrical measurement

The electrical resistivity method was used for measuring the electrical resistivity of cement paste incorporating with fly ash. The study found that the bulk electrical resistivity ρ(t) was a function of the solution electrical resistivity ρ0(t) and porosity Φ. A two-component model was proposed, in which ρ(t) was dominated by ρ0(t) at the early period of hydration and dominated by Φ at a later period. The porosity formation curve was derived from the 2-point method. A logarithmic equation ρ(t)=K^*In(t/t₀) was proposed to express the electrical resistivity development with time after hardening, where K represents the rate of hydration, and the pastes with fly ash had a lower K. The setting times and compressive strengths were tested and the results were compared with the electrical properties.     

Mechanical properties,drying and autogenous shrinkage of blast furnace slag activated with hydrated lime and gypsum

This article reports the characteristics of blast furnace slag (BFS) pastes activated with hydrated lime (5%) and hydrated lime (2%) plus gypsum (6%) in relation to compressive strength, shrinkage (autogenous and drying) and microstructure (porosity, hydrated products). The paste mixtures were characterized using powder X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and thermogravimetric analysis (TG/DTG). BSF activated with lime and gypsum (LG) results in larger amounts of ettringite when compared with BFS activated with lime (L). Although the porosities of the L and LG mixtures were about the same, there was a greater pore refinement for the BFS activated with lime, with an increase in mesopores volume with age. The presence of ettringite and the higher volumes of macropores cause the compressive strength of BSF activated with hydrated lime plus gypsum to be smaller than that of BFS activated with lime. For both chemical activators, compressive strength developed slowly at early ages. Autogenous and drying shrinkage were greater for the BFS activated with lime, believed to result from the more refined porous structure in comparison with the mixture activated with gypsum plus lime.     

Laboratory compaction of fly ash and fly ash with cement additions

The use of power-industry wastes as a material for earthen structures depends on its compactibility. It has been confirmed that a fly ash/bottom ash mix compacted several times in Proctor's moulds are not representative. The relationship between dry density of solid particles and water content for re-used waste samples was determined. The re-compaction effect on grain-size distribution, density of solid particles, specific surface and sand equivalent of wastes was investigated. Tests were conducted on fly ash samples compacted by the Standard and Modified Proctor methods. Another aim of the paper was to investigate the influence of cement additions on the compactibility of a fly ash/bottom ash mix. Waste samples in the natural state and with different percentages of cement additions (2, 5 and 10%) were compacted by both impact compaction methods to obtain compactibility curves rhod(w). It was found that cement addition resulted in an increased rhod max value, while wopt decreased. Linear regression relationships for changes in compaction parameters after cement stabilisation are also given.     

Characterisation of ground hydrated Portland cement-based mortar as an additive to alkali-activated slag cement

The sustainable development of cement manufacturing requires extension of the raw material base, including large-tonnage waste. Hydrated mortar waste is a promising mineral resource for the production of Portland cements and alternative binders, such as alkali-activated slag cement. The influences of ground-hydrated mortar aged for 3 months on the properties of alkali-activated slag fresh and hardened pastes were performed. The results show that the properties are dependent on the concentration (2.5–60%), cement:sand ratio (1:1–3) and fineness (200–600 m²/kg) of the ground hydrated mortar; the alkali activator (sodium carbonate and sodium silicate); and the curing conditions (normal conditions and steam curing). The fresh paste properties that we considered in this study included the water requirement and the setting time; the hardened paste properties we considered were the water absorption, the density, and the compressive strength after 2, 7, 14, 28, 180 and 360 days of ageing. The ground hydrated mortar improved the early strength and the long-term strength of the alkali-activated slag paste and replaced the slag up to 50%. The factors that affecting the strength of the alkali-activated slag cement with ground hydrated mortar as an additive were, in order of influence, alkali activator type > curing conditions > cement:sand ratio > ground-hydrated mortar fineness.     

Chemical activation of hybrid binders based on siliceous fly ash and Portland cement

The hydration of Portland cement (PC) blended with a high amount of a siliceous fly ash (70% fly ash, 30% PC) has been examined. The fly ash contributes significantly to the long-term strength development, when compared to a reference sample with quartz powder. However the long setting time and the poor early strength prevent the use of such binders. Therefore the effect of different activators (sodium carbonate, potassium sodium silicate, potassium citrate and sodium oxalate) on the setting, the hydration kinetics and the strength development of the fly ash-PC blend has been investigated.The addition of the activators increases the pH and decreases thus the calcium concentrations in the pore solution, which leads to a faster reaction of alite and thus to early setting and increased early strength. On the long term, the high alkali concentrations lower the compressive strength and lead to a (partial) destabilization of ettringite.Sodium oxalate and potassium sodium silicate accelerate both the setting of the fly ash-PC blend and increase the early compressive strength. Furthermore, they show better compressive strengths at later ages compared to the other activators. Based on these findings, they can be considered as the most suitable accelerators among the investigated activators.     

Static and cyclic properties of clay subgrade stabilised with rice husk ash and Portland slag cement

In the present study, clay soil collected from new Banda, Uttar Pradesh, India has been treated with rice husk ash (RHA) and Portland slag cement (PSC). Based on unconfined compressive strength test results, the optimum mix obtained is of 82.5%Soil+7.5%PSC+10%RHA. The increase in strength of the optimum mix is about 29.8%, 37.2% and 48.55% for a curing period of 7, 14 and 30 days, respectively. The soaked California bearing ratio (CBR) test gives about 91.75% higher values as compared to unsoaked CBR test for a curing period of 30 days. Strain-controlled cyclic triaxial tests were conducted to study the variation of degradation index, shear modulus and damping ratio of the optimum mix with number of cycles for strain amplitudes of 0.4%, 0.6%, 0.8% and 1% and for frequencies of 0.2 and 1 Hz at an effective confining pressure of 100 kPa. It is observed that the degradation index decreases at a fast rate for the first 25–50 cycles. From the study, it is concluded that the aforementioned mix may be suitable for pavement subgrade material.     

Development of alkali activated cement from mechanically activated silico-manganese (SiMn) slag

Silico-manganese (SiMn) slag has been used to develop alkali activated cement binder. The reactivity of SiMn slag was altered by mechanical activation using eccentric vibratory and attrition mill. The reaction kinetics during alkali activation of SiMn slag and structural reorganization were studied using isothermal conduction calorimetry and Fourier transform infrared spectroscopy. The particle size after milling was smaller in attrition milled samples but reaction started earlier in vibratory milled samples due to more reactivity. This observation was further supported by compressive strength which was highest in samples prepared from vibratory milled slag. The main reaction product was C–S–H (C = CaO, S = SiO₂, H = H₂O) of low crystallinity of different types with varying Si/Al and Ca/Si ratio. An attempt has been made to relate the microstructure with mechanical properties. The results obtained in this study establish technical suitability of SiMn slag as raw material for alkali activated cement.     

Utilization of ceramic waste as fine aggregate within Portland cement and fly ash concretes

The aim of this research work was to investigate the feasibility of using ceramic waste and fly ash to produce mortar and concrete. Ceramic waste fragments obtained from local industry were crushed and sieved to produce fine aggregates. The measured concrete properties demonstrate that while workability was reduced with increasing ceramic waste content for Portland cement concrete and fly ash concrete, the workability of the fly ash concrete with 100% ceramic waste as fine aggregate remained sufficient, in contrast to the Portland cement control concrete with 100% ceramic waste where close to zero slump was measured. The compressive strength of ceramic waste concrete was found to increase with ceramic waste content and was optimum at 50% for the control concrete, dropping when the ceramic waste content was increased beyond 50%. This was a direct consequence of having a less workable concrete. However, the compressive strength in the fly ash concrete increased with increasing ceramic waste content up to 100%. The benefits of using ceramic waste as fine aggregate in concrete containing fly ash were therefore verified.     

Hydrophobic and water-resisting behavior of Portland cement incorporated by oleic acid modified fly ash

The water-repellent and anti-permeability properties of cement are crucial for the durability and safety of concrete structures. In this work, we prepared a hydrophobic Portland cement by using oleic acid as a modifier for fly ash and examined the properties of the cement paste samples. Fly ash was firstly reacted with oleic acid by the dry milling method, and the modified fly ash was used to prepare the hydrophobic Portland cement. The IR spectra confirmed that the surface of fly ash was successfully capped with oleic acid, and carboxylic acid moieties were bonded with ≡SiOH and neutralized. The TG-DSC results showed that the amount of oleic acid loaded on the fly ash beads was 7.21 wt%. Fly ash dispersed evenly in the prepared cement paste samples and the distance between beads ranged in 2–10 μm. The water contact angle of the cement paste samples increased with rising content of modified fly ash, which demonstrated good water-repellent behavior. Different cement sections showed similar water-repellent behavior, which proved that the inner structure of the cement was also hydrophobic. Using the fly ash modified with oleic acid significantly decreased the water uptake and gas permeability of the prepared cement paste samples. The hydrophobic cement sample was optimal when the content of the modified fly ash in the cement was 12 wt% and after the cement was cured for 28 days.     

A method for predicting the alkali concentrations in pore solution of hydrated slag cement paste

The alkalinity of the pore liquid in hardened cement paste or concrete is important for the long-term evaluation of alkali-silica reaction (ASR) expansion and corrosion prevention of steel bar in steel reinforced structures among others. It influences the reactivity of supplementary cementitious materials as well. This paper focuses on the alkali binding in hydrated slag cement paste and a method for predicting the alkali concentrations in the pore solution is developed. The hydration of slag cement is simulated with a computer-based model CEMHYD3D. The amount of alkalis released by the cement hydration, quantities of hydration products, and volume of the pore solution are calculated from the model outputs. A large set of experimental results reported in different literatures are used to derive the alkali-binding capacities of the hydration products and practical models are proposed based on the computation results. It was found that the hydrotalcite-like phase is a major binder of alkalis in hydrated slag cement paste, and the C?CS?CH has weaker alkali-binding capacity than the C?CS?CH in hydrated Portland cement paste. The method for predicting the alkali concentrations in the pore solution of hydrated slag cement paste is used to investigate the effects of different factors on the alkalinity of pore solution in hydrated slag cement paste.     

Fractographic studies of microstructural development in hydrated Portland cement

A comprehensive study of early hydration and morphological development in freeze-dried Portland cement paste has been carried out using scanning electron microscopy in conjunction with energy dispersive X-ray analysis. The microstructure develops from its early appearance as a porous C-S-H spherulite structure containing hexagonal CH crystals, with Hadley grains and large inherent pores being significant features, to one of large CH dispersions in massive C-S-H. Some aspects of microstructural influence on mechanical properties are discussed.     

ASR in mortar bars containing silica glass in combination with high alkali and high fly ash contents

In the paper, the problem of ASR in mortar systems with high contents of alkali and fly ash is studied. The results show that the danger of ASR exists in this system which it is different from ordinary plain cement system because in these systems, serious ASR was accompanied by great expansion of the specimens studied.