Artificial pozzolanic cement pastes containing burnt clay with and without silica fume

Pozzolanic cement blends were prepared by the partial substitution of ordinary Portland cement (OPC) with different percentages of burnt clay (BC), Libyan clay fired at 700 °C, of 10, 20, and 30%. The pastes were made using an initial water/solid ratio of 0.30 by mass of each cement blend and hydrated for 1, 3, 7, 28, and 90 days. The pozzolanic OPC–BC blend containing 30% BC was also admixed with 2.5 and 5% silica fume (SF) to improve the physicomechanical characteristics. The hardened pozzolanic cement pastes were subjected to compressive strength and hydration kinetics tests. The results of compressive strength indicated slightly higher values for the paste made of OPC–BC blend containing 10% BC The results of DSC and XRD studies indicated the formation and later the stabilization of calcium silicates hydrates (CSH) and calcium aluminosilicate hydrates (C₃ASH₄ and C₂ASH₈) as the main hydration products in addition to free calcium hydroxide (CH). Scanning electron microscopic (SEM) examination revealed that the pozzolanic cement pastes made of OPC–BC mixes possesses a denser structure than that of the neat OPC paste. Furthermore, the addition of SF resulted in a further densification of the microstructure of the hardened OPC–BC–SF pastes; this was reflected on the observed improvement in the compressive strength values at all ages of hydration.     

Improvement of acid resistance of Portland cement pastes using rice husk ash and cement kiln dust as additives

This paper represents a laboratory study on the acid resistance of hardened ordinary Portland cement (OPC) and blended OPC pastes at two different curing temperatures. The blended materials used are rice husk ash (RHA) and cement kiln dust (CKD). The blended cement pastes were prepared using a water/solid (W/S) ratio of 0.3. The effects of immersion in deionized water (pH 7) and sulfuric acid solutions (pH 1, 2 and 3) at two temperatures (20 and 50 °C) on the compressive strength and phase composition of the various hardened blended cement pastes were studied. The results of compressive strength revealed that the increase of curing temperature from 20 to 50 °C resulted in increase the reduction of compressive strength due to acid attack up 2 months, but the resistance to sulfuric acid attack increases after that time due to the formation of crystalline calcium silicate hydrates (CSH) which have higher resistance to acid attack than the amorphous CSH formed at the early ages of hydration. The presence of RHA and CKD improves the resistance to sulfuric acid attack at both curing conditions. From the results of X-ray diffraction analysis and differential scanning calorimetric technique curves, the main hydration products identified are CSH, portlandite, and calcium sulfoaluminate hydrates.     

Utilization of electric arc furnace dust as an admixture to Portland cement pastes

Electric arc furnace dust (EAFD) is termed as a hazardous waste due to its contamination with heavy metals. Inertization of such very fine dust can be occurred via stabilization and solidification process within the hydrated Portland cement matrix. In this paper, the effect of the addition of various ratios of EAFD on the properties of the hardened Portland cement paste was investigated. Compressive strength, chemically combine water and free lime contents were determined. In addition, phase composition using XRD; DTA analysis; as well as microstructure of the formed hydrates for some selected samples were investigated using SEM. The obtained results showed that the paste containing 1/mass% EAFD give the highest compressive strength values at most hydration ages, specially the later ages, compared to the neat Portland cement blank paste. Whileas, the pastes containing 3 and 5/mass% EAFD showed lower values of compressive strength compared to those of the blank paste.     

Calorimetry in the studies of cement hydration

Calorimetry was applied to an investigation of the early hydration of Portland cement (PC)–calcium aluminate cement (CAC) pastes. The heat evolution measurements were related to the strength tests on small cylindrical samples and standard mortar bars. Different heat-evolution profiles were observed, depending on the calcium aluminate cement/Portland cement ratio. The significant modification of Portland cement heat evolution profile within a few hours after mixing with water was observed generally in pastes containing up to 25% CAC. On the other hand the CAC hydration acceleration effect was also obtained with the 10% and 20% addition of Portland cement. As one could expect the compressive and flexural strength development was more or less changed—reduced in the presence of larger amount of the second component in the mixture, presumably because of the internal cracks generated by expansive calcium sulfoaluminate formation.     

The effects of solvents on C–S–H as determined by thermal analysis

Un-hydrated Portland cement consists of several anhydrous and reactive phases, that when mixed with water react to form hydrates. The main hydration product of Portland cement is calcium silicate hydrate (C–S–H). It is the main binding phase in a concrete system, hence is important to construction chemists. The concrete engineer measures the compressive strength of concrete after prescribed hydration periods, typically 1, 3, 7, 28 days. It is often convenient to mimic these intervals by stopping the hydration reaction at the same times. Several techniques can be employed to stop this hydration reaction. One of which is solvent-based and involves mixing a polar solvent such as acetone or isopropyl alcohol, with the hydrated cement. This mixing should be vigorous enough to blend the free water, in the partially hydrated cement system, with the polar solvent without altering the cement system’s matrix. The solvent-water mixture has a much lower boiling point and the mixture quickly evaporates out of the system. This achieves two goals. It stops the hydration reaction at the moment of solvent mixing, and it removes free water to prevent further hydration from occurring. This procedure theoretically leaves behind a dry, chemically unaltered, partially hydrated cement paste. In this way, pastes can be analyzed after the prescribed 1, 3, 7 or 28 days of hydration. This paper uses thermogravimetric analysis (TG) results to investigate the assumption that solvents have no thermodynamic or chemical effect on the hydrated cement paste phases.     

Thermal resistance of hardened cement pastes containing vermiculite and expanded vermiculite

The durability and thermal stability of hardened Portland cement pastes containing vermiculite (V) and expanded vermiculite (EV) exposed to high temperatures were studied. Different mixtures were prepared using 2.5, 5, and 10 wt% of both types of V. Each mixture, after 28?days of hydration, was heated at 300, 600, and 800?°C for 3?h. Two modes of cooling were used; gradual cooling in air and rapid cooling in cold water. The percentage of residual strength, chemically combined water content, change in phase composition, and the thermal stability of the heated specimens were studied. The specimens cooled in water showed greater loss in strength than the air-cooled specimens. The presence of V improved the heat resistance of ordinary type I Portland cement (OPC) pastes. 5 wt% replacement revealed the best performance at all heating temperatures. The EV showed better thermal resistance than the nonexpanded one. Addition of silica fume (SF) with V in OPC pastes lead to superior performance. This can be explained as result of the combined effects of insulation properties of V and pozzolanic reactivity of SF which accounts for the notable increase in the residual strength for these mixes.     

Thermal resistance,microstructure and mechanical properties of type I Portland cement pastes containing low-cost nanoparticles

This study aimed to utilize laboratory-prepared nano-silica (NS) and nano-alumina (NA) as low-cost nano-oxides additions for improving the mechanical properties and thermal resistance of hardened ordinary Portland cement (OPC) pastes. NS was synthesized from rice husk ash in the absence of any surfactant, while NA was synthesized from AlCl₃ in the presence of CTAB as a surfactant. The average particle sizes of synthesized NS and NA were 30 and 40 nm, respectively. Nano-silica or nano-alumina was added to OPC as a single phase with different ratios of 0.5, 1, 2 and 3 by mass % of OPC. The physico-chemical characteristics of different OPC-NS and OPC-NA hardened pastes were studied after 1, 3, 7, 14, 28 and 90 days of hydration. The resistance of the hardened composites for firing was evaluated for specimens cured for 28 days under tap water and then fired at 300, 600 and 800 °C for 3 h. The fired specimens were cooled by two methods: gradual cooling and rapid cooling. The compressive strength test was performed for all mixes at each firing temperature. The compressive strength results revealed that the optimum addition of NS is 1, whereas the optimum addition of NA is 0.5 by mass % of OPC. XRD, TG/DTG and SEM results indicated that ill-crystalline and nearly amorphous C–S–H, C–A–S–H and C–A–H were the main hydration products.     

Consumption of calcium hydroxide and formation of C–S–H in cement pastes

The pozzolanic properties of the equilibrium catalyst (Ecat), an abundant waste from fluid catalytic cracking units of oil refineries, have been studied in cementitious matrices. The consumption of calcium hydroxide is usually taken as a means to assess the pozzolanicity of a material, ignoring the formation of C–S–H and other hydrates, the main product of the pozzolanic reaction. The use of thermal analysis methods allows the identification and quantification of the products of the hydration reaction, based on the loss of chemically combined water of phases present in a cementitious matrix. In the present study, TG/DTG techniques were used to quantify the amount of C–S–H + ettringite, C–A–S–H + C–A–H, and calcium hydroxide in cement–Ecat pastes, containing 0–40 % Ecat, aged for 1–28 days. The influence of the content of these phases on the mechanical properties of the resulting samples was also investigated by axial compression. The results further confirmed the pozzolanic activity of Ecat, so that the production of C–S–H and C–A–S–H increased with the aging time, but in a smaller amount than in the reference paste. The compressive strength test showed that the pastes containing 10 % Ecat, aged for 14 and 28 days, presented improved properties as compared to the corresponding references.     

Physical and chemical studies on cement containing sugarcane molasses

Molasses is generally used as a grinding aid in cement and as a water reducer and retarder in concrete. In China, the output primarily consists of sugarcane molasses. In this paper, the effects of sugarcane molasses on the physical performance and hydration chemistry of conventional Portland cement were investigated. The setting times, the normal consistency of cement pastes, the compressive strengths and fluidities of the mortars were respectively determined according to Chinese Standard GB/T 1346, GB/T17671 and GB/T 2419. The effect of molasses on the hydration kinetics of cement was investigated using a calorimeter. The hydration products and pore size distribution of the cement pastes were analysed by X-ray powder diffraction, differential scanning calorimetry and a mercury injection apparatus. The results show that a small amount of sugarcane molasses retards the setting and hardening of cement paste and increases the fluidity of cement mortar, while excess molasses accelerates the setting and hardening. Molasses improves significantly the compressive strength at 3d due to the decrease of porosity. The addition of 1.0 % molasses accelerates the formation of ettringite, prevents the second hydration of aluminate phase and delays the hydration of C₃S.     

Utilization of waste phosphate aluminum slag for cement manufacture

The objective of this study was to investigate the feasibility of application of waste phosphate aluminum slag (PAS) for cement manufacture. To recycle waste PAS and minimize adverse effects on cement hydration induced by phosphate, NH₄OH was used to purify PAS. X-ray diffraction (XRD) analysis was used to determine to confirm the removal of harmful phosphate. The effect of PAS on the hydration product composition, heat release and compressive strength was also investigated. The results demonstrated that NH₄OH was effective in removing harmful AlPO₄ in PAS and 10% NH₄OH was considered as the optimal treatment concentration. In addition, the purification of NH₄OH alleviated the delay in cement hydration caused by AlPO₄ and the heat release curve of purified PAS (PPAS) cement tends to that of OPC. Moreover, the compressive strength of PPAS mortar at 28 days was 49.4 ​MPa, which is 18% higher than the compressive strength of PAS mortar. PAS purified by NH₄OH can be applied to cement manufacturing.     

Effect of nano-metakaolin addition on the hydration characteristics of fly ash blended cement mortar

The effect of nano-metakaolin (NMK) addition on hydration characteristics of fly ash (FA) blended cement mortar was experimentally investigated. The amorphous or glassy silica, which is the major component of a pozzolan, reacts with the calcium hydroxide liberated during calcium silicate hydration. It is believable to add FA and NMK particles in order to make high performance concrete. The physico-mechanical properties of FA blended cement mortars made with different percentages of NMK were investigated. The experimental results showed that the compressive and flexural strengths of mortars containing NMK are higher than those of FA blended cement mortar at 60 days of hydration age. It is demonstrated that the nanoparticles enhances strength than FA. In addition, the hydration process was monitored using scanning electron microscopy and thermal gravimetric analysis (TG). The results of these examinations indicate that NMK behaves not only as a filler to improve microstructure, but also as an activator to promote the pozzolanic reaction.     

Studies on the influence of spent FCC catalyst on hydration of calcium aluminate cements at ambient temperature

The influence of spent catalyst from catalytic cracking in fluidized bed (FCC) on the hydration of two kinds of calcium aluminate cements (of about 40 and 70% content of alumina) was studied. Cement pastes were prepared with constant ratio of water/binder = 0.5 and with content of 0, 5 and 25% mass of addition as replacement of cement. The samples were stored at room temperature. Thermal analysis (TG, DTG), infrared absorption (FTIR) and X-ray diffraction methods were applied to investigate changes in various periods of hydration (up to 150 days). The compressive strength of cement mortars was also examined. On the basis of presented results it was affirmed that in studied conditions spent FCC catalyst is a reactive addition in calcium aluminate cement (CAC) pastes, which probably can create a new phase type C–A–S–H. It may be an interesting alternative for limitation of the negative phenomenon of conversion of aluminate hydrates, although the degree of the influence of the mineral additive depends on the composition of CAC and of the quantity of the used waste.     

Hydration of cement with superabsorbent polymers

Hydration of cement is a complex thermodynamic system where a number of heterogeneous compounds interact with each other to form cement hydrates. Superabsorbent polymers (SAP) can be added to cement systems with many different reasons, so it is relevant that the basic knowledge of this new compound on the development of hydration is well understood. This paper reports basic research on thermal analysis of cement pastes with SAP—a suspension-polymerized poly acrylic acid–acrylamide copolymer. Several parameters were analysed: the concentration of SAP, the effect of particle size distribution and their influence on the hydration process with focus on ordinary Portland cement. The methodology included thermogravimetric analysis and differential scanning calorimetry. Combined water method was employed at different thermodynamic conditions, so the energy of activation in the different systems can be accessed. The introduction of SAP in cement-based materials significantly affects the chemical balance of ordinary Portland cement. The effect is not only in terms of the amount of hydrates, but also the type of hydrates being generated, thermodynamically favourable to precipitation of calcium hydroxide. This paper provides information relevant to hydration modelling and comprehension of cementitious materials when internal curing is active.     

Early stages hydration of high initial strength Portland cement

Thermogravimetry (TG) and derivative thermogravimetry (DTG) were used to analyze the early stages of hydration of a high-initial strength and sulphate resistant Portland cement (HS SR PC) within the first 24 h of setting. The water/cement (W/C) mass ratios used to prepare the pastes were 0.35, 0.45, and 0.55. The hydration behavior of the pastes was analyzed through TG and DTG curves obtained after different hydration times on calcined cement mass basis to have a same composition basis to compare the data. The influence of the W/C ratio on the kinetics of the hydration process was done through the quantitative analysis of the combined water of the main hydration products formed in each case. TG and DTG curves data calculated on calcined mass basis of all the results were converted to initial cement mass basis to have an easier way to analyze the influence of the W/C ratio on the free and combined water of the different main hydrated phases. The gypsum content of the pastes was totally consumed in 8 h for all cases. A significant part of the hydration process occurs within the first 14 h of setting and at 24 h the highest hydration degree, indicated by the respective content of formed calcium hydroxide, occurs in the case of the highest initial water content of the paste.     

Reactivity of fly ash in cement paste studied by means of thermogravimetry and isothermal calorimetry

Four paste mixtures with varying replacement level of the cement content by fly ash have been studied. Due to fly ash, the acceleration period decreased and a third hydration peak was noticed with isothermal calorimetry. The total heat after 7 days increased with increasing fly ash content. From 1 to 7 days, thermogravimetry showed a higher chemically bound water and Ca(OH)₂-content for the pastes with fly ash. Between 7 and 14 days the calcium hydroxide started to be depleted due to the pozzolanic reaction. A unique relation was found between calcium hydroxide and total heat development.     

Simultaneous measurements of heat of hydration and chemical shrinkage on hardening cement pastes

Isothermal calorimetry and chemical shrinkage measurements are two independent techniques used to study the development of hydration in cementitious systems. In this study, calorimetry and chemical shrinkage measurements were combined and simultaneously performed on hydrating cement paste samples. Portland cement pastes with different water to cement ratios and a cement paste containing calcium sulfoaluminate clinker and anhydrite were studied. The combined calorimetry/chemical shrinkage test showed good reproducibility and revealed the different hydration behavior of sealed samples and open samples, i.e., samples exposed to external water during hydration. Large differences between sealed and open samples were observed in a Portland cement paste with low water to cement ratio and in the calcium sulfoaluminate paste; these effects are attributed to self-desiccation of the sealed pastes. Once the setup is fully automatized, it is expected that combined calorimetry/chemical shrinkage measurements can be routinely used for investigating cement hydration.     

Degree of hydration in cement paste and C<Subscript>3</Subscript>A-sodium carbonate-water systems

This research provides a fundamental understanding of the early stage hydration of Portland cement paste, tricalcium aluminate (C₃A) paste at water to cement ratio of 0.5 and C₃A suspension at water to cement ratio of 5.0 modified by 2 or 4 mass% of sodium carbonate. A high conversion of unreacted clinker minerals to gel-like hydration products in the cement-Na₂CO₃ pastes takes place rapidly between 1^st to 24^th h. Contrary the Ca(OH)₂ formation within the same time interval is retarded in the excess of CO₃²⁻ ions due to intensive rise and growth of CaCO₃ crystals in hydrated cement. Later, the conversion of clinker minerals to the hydrate phase is reduced and higher contents of calcite and vaterite relative to that of Ca(OH)₂ in comparison with those found in the Portland cement paste are observed. As a consequence a decrease in strength and an increase in porosity between hardened Portland cement paste without sodium carbonate and those modified by Na₂CO₃ are observed. C₃A hydrates very quickly with sodium carbonate between 1^st and 24^th h forming hydration products rich in bound water and characterized also by complex salts of (x)C₃A·(y)CO₂·(zH₂O type, whereas C₃A-H₂O system offers C₃AH₆ as the main hydration product. Higher content of the formed calcium aluminate hydrates in C₃A-Na₂CO₃-H₂O system also contributes to early strength increase of Portland cement paste.     

Hydration of Portland cement with alkanolamines by thermal analysis

Four types of alkanolamines (i.e., traditional alkanolamines represented by TEA and TIPA and new alkanolamines represented by DEIPA and EDIPA) were added to Portland cement as chemical additives, and their effects on the cement properties and hydration process were investigated. An isothermal calorimeter was used to track the hydration heat flow of the cement pastes with or without alkanolamines. Thermogravimetric analyses were performed to measure the degree of hydration over the course of 28 days. In addition, X-ray diffraction, MIP analysis and SEM were used as auxiliary tests. The results indicated that alkanolamines improved the compressive strength of the cement mortars. It was found that TEA increased the rate of the second hydration of C₃A, and TIPA accelerated the hydration of C₄AF. DEIPA and EDIPA promoted the hydration of both the aluminum and ferrite phases as well as catalyzed the conversion of AFt to the AFm phase. By contrast, the new alkanolamines represented by DEIPA and EDIPA expressed more superior properties.     

Identification of Composite Cement Hydration Products by Means of X-Ray Diffraction

 In this paper the effect of limestone, fly ash, slag and natural pozzolana on the cement hydration products is studied. Four composite cements containing limestone, natural pozzolana from the Milos Island, slag and fly ash have been produced by intergrinding clinker (85%), the above main constituent (15%) and gypsum. The grinding process was designed in order to produce cements of the same 28d compressive strength. The hydrated products, formed after 1–28 days, were studied by means of X-ray diffraction. Unhydrated calcium silicate compounds of clinker and hydration products such as C*H, C*S*H and ettringite are clearly observed. Although there is not significant differentiation among samples hydrated for the same period of time, modifications of calcium aluminate hydrates as well as sulfoaluminate hydrates, are indicated by the XRD patterns. In samples of limestone cement, monocarboaluminate is formed in the first 24 hours and is still present after 28 days.     

Hydration process of cement in the presence of a cellulosic additive. A calorimetric investigation

In the cement industry, the extrusion technique is used to produce flat shapes with improved resistance to compression. Extrusion is a plastic-forming process that consists of forcing a highly viscous plastic mixture through a shaped die. The material should be fluid enough to be mixed and to pass through the die, and on the other hand, the extruded specimen should be stiff enough to be handled without changing in shape or cracking. These characteristics are industrially obtained by adding cellulosic polymers to the mixture. The aim of this work is to understand the action mechanism of these additives on the major pure phases constituting a typical Portland cement: tricalcium silicate (C(3)S), dicalcium silicate (C(2)S), tricalcium aluminate (C(3)A), and tetracalcium iron-aluminate (C(4)AF). In particular, a methylhydroxyethyl cellulose (MHEC) was selected from the best-performing polymers for further study. The effect of this additive on the hydration kinetics (rate constants, activation energies, and diffusional constants) was evaluated by means of differential scanning calorimetry (DSC) while the hydration products were studied by using thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). MHEC addition in calcium silicate pastes produces an increase in the induction time without affecting the nucleation-and-growth period. A less dense CSH gel was deduced from the diffusional constants in the presence of MHEC. Moreover, CSH laminar features and poorly structured hydrates were noted during the first hours of hydration. In the case of the aluminous phases, the additive inhibits the growth of stable cubic hydrated phases (C(3)AH(6)), with the advantage of the metastable hexagonal phases being formed in the earliest minutes of hydration.