Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.     

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.     

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.     

Compressive strength and microstructural characteristics of class C fly ash geopolymer

Geopolymers prepared from a class C fly ash (CFA) and a mixed alkali activator of sodium hydroxide and sodium silicate solution were investigated. A high compressive strength was obtained when the modulus of the activator viz., molar ratio of SiO₂/Na₂O was 1.5, and the proper content of this activator as evaluated by the mass proportion of Na₂O to CFA was 10%. The compressive strength of these samples was 63.4 MPa when they were cured at 75 °C for 8 h followed by curing at 23 °C for 28 d. In FTIR spectroscopy, the main peaks at 1036 and 1400 cm^?1 have been attributed to asymmetric stretching of Al–O/Si–O bonds, while those at 747 cm^?1 are due to the Si–O–Si/Si–O–Al bending band. The main geopolymeric gel and calcium silicate hydrate (C–S–H) gel co-exist and bond some remaining unreacted CFA spheres as observed in XRD and SEM–EXDA. The presence of gismondine (zeolite) was also observed in the XRD pattern.     

Durability of very high volume fly ash cement pastes and mortars in aggressive solutions

The resistance of very high volume fly ash cement pastes and mortars activated by Na₂SO₄ has been monitored following immersion for up to 90 d in 0.1 M HCl, 4.4% Na₂SO₄ and ASTM-compliant sea water. Changes in the compressive strengths of mortars and in crystalline phases, bond environments, and the microstructure of pastes following immersion were monitored. Experiments were repeated with a commercially available sulfate resistant cement. Both cements were found to present adequate resistance to both sea water and the Na₂SO₄ solution. However, both were severely degraded by acid immersion. Differences in potential degradation mechanisms based on the chemistry of the fly ash binder and the reference cement are discussed.     

Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume

The influence of high-calcium fly ash and silica fume as a binary and ternary blended cement on compressive strength and chloride resistance of self-compacting concrete (SCC) were investigated in this study. High-calcium fly ash (40–70%) and silica fume (0–10%) were used to replace part of cement at 50, 60 and 70 wt.%. Compressive strength, density, volume of permeable pore space (voids) and water absorption of SCC were investigated. The total charge passed in coulombs was assessed in order to determine chloride resistance of SCC. The results show that binary blended cement with high level fly ash generally reduced the compressive strength of SCC at all test ages (3, 7, 28 and 90 days). However, ternary blended cement with fly ash and silica fume gained higher compressive strength after 7 days when compared to binary blended fly ash cement at the same replacement level. The compressive strength more than 60 MPa (high strength concrete) can be obtained when using high-calcium fly ash and silica fume as ternary blended cement. Fly ash decreased the charge passed of SCC and tends to decrease with increasing fly ash content, although the volume of permeable pore space (voids) and water absorption of SCC were increased. In addition when compared to binary blended cement at the same replacement level, the charge passed of SCC that containing ternary blended cement was lower than binary blended cement with fly ash only. This indicated that fly ash and silica fume can improve chloride resistance of SCC at high volume content of Portland cement replacement.     

Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Detection of nanostructural anomalies in hydrated cement systems

Monitoring the flow of helium gas into the structure of hydrated cement systems has proven to be a useful method for following nanostructural changes in the C–S–H phase of hydrated cement systems. The method is sensitive to changes that occur on removal of structural water from the layered silicates. The helium-inflow method was applied, in this study, to normally hydrated low-water–cement ratio (w/c) Portland cement pastes (w/c < 0.38) and to low w/c autoclaved cement systems containing fly ash and elemental sulfur. Unusually, high amounts and rates of inflow were observed for these pastes. It was postulated that inflow occurred into both interlayer and other spaces in the latter. The inflow into the other or ‘trapped’ space was unexpected and considered anomalous in absence of a widely accepted explanation. The structural differences which were observed at the nanoscale for the low w/c preparations were consistent with behavioral aspects for published structural models of layered C–S–H. These include the models of Richardson and Jennings and concepts involving the existence of two types of C–S–H. Arguments for the existence of ‘trapped’ space between aggregates of C–S–H layers are advanced. Evidence for the preservation of C–S–H structures (similar to those formed during normal hydration) for the autoclaved systems containing fly ash and sulfur is presented. The evidence is compatible with the existence of ‘trapped’ space within layered agglomerates and the collapse of C–S–H structure on removal of water from interlayer space, typical of normally hydrated pastes.     

Alkali-activated complex binders from class C fly ash and Ca-containing admixtures

Processes that maximize utilization of industrial solid wastes are greatly needed. Sodium hydroxide and sodium silicate solution were used to create alkali-activated complex binders (AACBs) from class C fly ash (CFA) and other Ca-containing admixtures including Portland cement (PC), flue gas desulfurization gypsum (FGDG), and water treatment residual (WTR). Specimens made only from CFA (CFA100), or the same fly ash mixed with 40 wt% PC (CFA60–PC40), with 10 wt% FGDG (CFA90–FGDG10), or with 10 wt% WTR (CFA90–WTR10) had better mechanical performance compared to binders using other mix ratios. The maximum compressive strength of specimens reached 80.0 MPa. Geopolymeric gel, sodium polysilicate zeolite, and hydrated products coexist when AACB reactions occur. Ca from CFA, PC, and WTR precipitated as Ca(OH)₂, bonded in geopolymers to obtain charge balance, or reacted with dissolved silicate and aluminate species to form calcium silicate hydrate (C-S-H) gel. However, Ca from FGDG probably reacted with dissolved silicate and aluminate species to form ettringite. Utilization of CFA and Ca-containing admixtures in AACB is feasible. These binders may be widely utilized in various applications such as in building materials and for solidification/stabilization of other wastes, thus making the wastes more environmentally benign.     

Conversion of high silicon fly ash to Na-P1 zeolite: Alkaline fusion followed by hydrothermal crystallization

In this research, we converted high silicon fly ash to a high ion-exchange capacity zeolite using a two stages conversion process. Alkaline fusion was applied to collapse the fly ash crystalline phases and release Si content. Then Si/Al ratio of the synthesis sol adjusted with appropriate amount of industrial grade materials. A synthesis solution with the molar ratio of 2.2 SiO₂:Al₂O₃:5.28 Na₂O:106 H₂O was hydrothermally crystallized to Na-P1 zeolite at 120 °C for 4 h. The as-synthesized zeolite characterized by means of X-ray diffraction, infrared spectroscopy, scanning electron microscopy and thermal analysis. Cation exchange capacity of the zeolite was determined using ammonium acetate method. The zeolitization remarkably improved the cation exchange capacity of the final product (e.g. 3.23 meq/g in comparison to the raw fly ash ～0.005–0.02 meq/g).Due to the high CEC and sufficient whiteness of the final product, we suggest that the as-synthesized zeolitic powder is a potential candidate as a detergent builder.     

Influence of starting material on the early age hydration kinetics,microstructure and composition of binding gel in alkali activated binder systems

The influence of starting material on the hydration kinetics and composition of binding gel in alkali activated binder systems was evaluated. The starting materials used were ground granulated blast furnace slag, class C fly ash and class F fly ash. All starting materials were activated using alkaline solution with a SiO₂/Na₂O ratio of 1.5. The hydration kinetics were monitored using in situ isothermal conduction calorimetry and the chemical compositions of the binder gels were determined by energy dispersive X-ray spectroscopy. In the fly ash systems, the calorimetric curves had only one peak, which occurred in the first 30 min of reaction, and lacked an induction period. Two peaks were distinguishable in slag systems, though the induction period was much shorter than that of a typical OPC system. The gel composition ratios, including Ca/Si, Na/Si, Na/Al and Al/Si, were different in each of the systems and are discussed in detail.     

Properties of low temperature belite cements made from aluminosilicate wastes by hydrothermal method

The aim of this study is the synthesis at low temperature (1000 °C) of reactive belite cement, rich in reactive C₂S phases (α′_L and/or β-C₂S), starting from aluminosilicate wastes (oil well drilling mud and hydraulic dam sludge) and hydraulic lime dust recovered from bagging workshops. A hydrothermal treatment of the raw cement mixture was performed in an alkaline solution of KOH (0.6 M), with heating at 100 °C for 4 h under atmospheric pressure and continuous agitation. The burning of the hydrothermal mixture at 1000 °C produced reactive belite cements containing between 79% and 86% of C₂S (α′_L and/or β polymorphs), the rest being C₁₂A₇ (5–8%), C₄AF (7–11%) and free lime (2%). The cements were characterized by X-ray Fluorescence (XRF), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), microprobe, and laser granulometry. The hardening evolution of belite cement pastes and mortars was followed by thermo-gravimetric analysis (TGA), setting time and compressive strength. The results showed a rapid production of hydrates (C–S–H/C–A–S–H, calcium aluminate hydrates and portlandite), with a fast setting time (reaction of C₁₂A₇) and compressive strength evolution that led to these cements being classified in the 32.5 category according to EN 197-1.     

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.     

Effect of specimen size and shape on compressive strength of concrete containing fly ash: Application of genetic programming for design

The use of fly ash as a mineral admixture in the manufacture of concrete has received considerable attention in recent years. For this reason, several experimental studies are carried out by using fly ash at different proportions replacement of cement in concrete. In the present study, the models are developed in genetic programming for predicting the compressive strength values of cube (100 and 150 mm) and cylinder (100 × 200 and 150 × 300 mm) concrete containing fly ash at different proportions. The experimental data of different mixtures are obtained by searching 36 different literatures to predict these models. In the set of the models, the age of specimen, cement, water, sand, aggregate, superplasticizers, fly ash and CaO are entered as input parameters, while the compressive strength values of concrete containing fly ash are used as output parameter. The training, testing and validation set results of the explicit formulations obtained by the genetic programming models show that artificial intelligent methods have strong potential and can be applied for the prediction of the compressive strength of concrete containing fly ash with different specimen size and shape.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

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.     

Evolution of alkaline activated ground blast furnace slag–ultrafine palm oil fuel ash based concrete

The two locally available pozzolanic solid wastes (PMs) – ultrafine palm oil fuel ash (UPOFA) and ground blast furnace slag (GBFS) – have been used as base materials to develop high alkaline activated strength concrete. The samples were prepared with combined aggregate modulus of 3.66 and at constant GBFS/PM that varied from 0 to 0.3. The combined alkaline activators (CAA) (Na₂SiO₃ and NaOH) to PMs ratios (CAA/PMs), temperature and curing durations also varied as 0.45–0.55, 25–90 °C, and 6–24 h, respectively. The findings revealed that the strength at 3-day and 28-day were 69.13 and 71.2 MPa, respectively and the respective optimum GBFS/PM, CAA/PM, temperature and curing duration are 0.2, 0.5, 60 °C and 24 h. GBFS was found to contribute to the soluble Ca, heterogeneity, and amorphousity of the product. This eventually facilitated the formation of suspected calcium-silicate-hydrate and the geopolymer products of Ca/Na-aluminosilicate-hydrate (C/NASH) that enhanced the compressive-strength results.     

Accelerated carbonation effect on behaviour of ternary Portland cements

The carbonation of hydrated ternary Portland cement (TPC), containing thermally activated paper sludge and fly ash, was studied in a semi-dynamic atmosphere of 100% CO₂, and 65% relative humidity at 20 ± 1 °C, for a period of 30 days. The changes of the mineralogical phases and porosity before and during the carbonation were characterized by X-ray diffraction and mercury intrusion porosimetry. The kinetics of the process was evaluated from the total CaCO₃ content by means of thermogravimetric analysis. The pH of the simulated pore solution was analyzed before and after different periods of time. An equivalent study was carried out on ordinary Portland cement (OPC), whose results were compared to those of TPC one. The carbonation attack was 2.2 times faster in the case of the TPC. The results were discussed on the basis of the different porosity, portlandite content and pH of the pore-solution.     

Effect of circulating fluidized bed combustion ash on the properties of roller compacted concrete

Circulating fluidized bed combustion (CFBC) ash, which has such a high content of f-CaO and SO_3, is a waste or by-product of petroleum coke combustion power stations. The purpose of this study is to investigate the effects of CFBC ash on the properties of roller compacted concrete (RCC). CFBC ash was used to replace fine aggregate with various dosages (5%, 10% and 15%) by weight. All mixtures were designed according to ACI 211.3R and prepared for testing. During casting, cylinders were vibrated and compacted with different pressures of 25 g/cm², 50 g/cm² and 75 g/cm², respectively. Test results show that CFBC ash can increase the water absorption and effectively reduce the initial surface absorption. Meanwhile, CFBC ash has a positive effect on compressive strength, splitting tensile strength, and sulphate attack resistance of hardened RCC. SEM revealed that the main hydration products of specimens containing CFBC ash are AFt (ettringite), C–S–H (hydrated calcium silicate) and portlandite. Based on the presented observations and results, RCC with the dosage of 5% CFBC ash as fine aggregate replacement and the roller compaction pressure of 75 g/cm² is recommended.