Slag Cement Pastes Research Articles

Effect of Phosphogypsum on the Properties of Over-Sulfur Phosphogypsum–Ground Granulate Blast-Furnace Slag Cement

Over-sulfur phosphogypsum(PG)–ground granulate blast-furnace slag(GGBFS) cement paste is utilized by GGBFS, Portland cement clinker(PCC), additive, water and modificated phosphogypsum paste(MPG), produced by milling PG mixed with a certain proportion of steel slag(SS), GGBFS and water. The effect of PG on the properties of over-sulfur PG– GGBFS cement was investigated. The mechanical performances and hydration mechanism of the cement with different kinds, proportions and particle size of PG were analyzed based on setting time, volume stability, strength test, XRD and SEM analyses. The experimental results show that,the optimum mixture of PG amount in the binder was 45%. Overdose of PG may caused strength deterioration. The optimum grinding time of MPG in the binder was 20min. The soluble phosphorus content of PG in the binder was under 0.05%.

Effect of fresh water leaching on the microstructure of hardened composite binder pastes

The microstructure of cement–slag and cement–fly ash pastes leached by fresh water for 2–3years was investigated by MIP, XRD, TG and SEM. Results indicated that long-term fresh water leaching caused degradation of the pastes in terms of mass loss, increase in porosity and decrease in Ca(OH)2 content and Ca/Si ratio of C–S–H gel, but showed no decomposition of hydration products. The pastes containing a small amount of fly ash were not damaged severely, incorporation of 65% of fly ash deteriorated most severely, whereas the cement–slag pastes (⩽70%) showed high long-term fresh water leaching resistance.

Effect of blast furnace slag on self-healing of microcracks in cementitious materials

The physico-chemical process of self-healing in blast furnace slag cement paste was investigated in this paper. With a high slag content i.e., 66% in cement paste and saturated Ca(OH)2 solution as activator, it was found that the reaction products formed in cracks are composed of CSH, ettringite, hydrogarnet and OH–hydrotalcite. The fraction of CSH in the reaction products is much larger than the other minerals. Large amount of ettringite formed in cracks indicates the leaching of SO42− ions from the bulk paste and consequently the recrystallization. Self-healing proceeds fast within 50h and then slows down. According to thermodynamic modeling, when the newly formed reaction products are carbonated, the filling fraction of crack increases first and then decreases. Low soluble minerals such as silica gel, gibbsite and calcite are formed. Compared to Portland cement paste, the potential of self-healing in slag cement paste is higher when the percentage of slag is high.

C-A-S-H gels formed in alkali-activated slag cement pastes. Structure and effect on cement properties and durability

The development of alternatives to traditional Portland cement produced with more eco-efficient processes (lower energy consumption and CO 2 gas emissions) is an item on climate change and innovation agendas. Alkaline cements and concretes are an effective alternative to traditional cements.The structure of the C-S-H gel in Portland cements consists mostly of 14-nm tobermorite (with a chain length of five) and jennite (2-link chain). The mechanical properties of C-S-H gels can be explained in terms of the three types of packing found in these gels: low density (LD), high density (HD) and ultra-high density (UHD).The main reaction product in alkali-activated slag (AAS) cements is a C-A-S-H gel, which adopts different structures depending on the nature of the alkaline activator. When the activator is a NaOH solution (4 % Na 2 O by slag weight), the C-A-S-H gel formed has an intermediate structure between 14-nm tobermorite with a chain length of five links and 11-nm tobermorite with 14 links. When the activator is a waterglass solution (4 % Na 2 O by slag weight), traits characteristic of 14-nm tobermorite with 11-link chains and 11-nm tobermorite with a chain length of 14 co-exist in the structure of the C-A-S-H gel formed. This densely packed structure (with three HD states) yields excellent mechanical properties. Like the C-A-S-H gels obtained in NaOH (4% Na2O)-activated AAS paste, the C-A-S-H gels forming in AAS gels activated with waterglass have no UHD states.The structure and composition of these C-A-S-H gels determine strength development in AAS mortars and concretes as well as their resistance to aggressive chemicals.

セメント硬化体中の塩化物イオンの拡散におけるアルカリイオン濃度とC-A-S-Hの影響

塩化物イオン拡散メカニズムの解明はセメント内塩分浸透抑制を行う上で重要である。しかしながら、塩化物イオンの拡散過程において細孔中のアルカリイオンによって形成したC-A-S-Hが与える影響について定量的な結論は得られていない。そこでC-S-HおよびC-A-S-Hの2つの吸着サイトを考慮した塩化物イオン拡散モデルを構築し、塩化物イオンの拡散実験との検証を行った。しかし、アルカリイオン濃度が増加した試料での拡散予測結果では実験結果と乖離を生じた。そこで、C-A-S-Hの構造変化が原因であると推定し、C-A-S-Hの増加に伴う構造の変化を考慮した屈曲度をモデルに組み込み、モデルの構築を行った。

Early-age strain–stress relationship and cracking behavior of slag cement mixtures subject to constant uniaxial restraint

In this study, the early-age strain–stress development of mixtures containing slag cement was measured on uniaxially restrained specimens using a specially designed testing frame starting right after casting. It was found that thermal deformation dominates early-age stress development in low w/cm and low slag cement paste systems. While for concrete, autogenous shrinkage is the major contributing factor for tensile stress development. The early-age stress development of uniaxially restrained cementitious mixture specimens is profoundly influenced by the overt early-age relaxation effect, such that the compressive stress is significantly reduced and tensile stress develops before shrinkage begins. A linear shrinkage and tensile stress relationship is found to exist in mixtures subject to such constant restraint, regardless of w/cm and slag cement contents. Slag cement has the benefit of delaying tensile stress development and cracking time because of the reduced early-age thermal effect as compared to the ordinary Portland cement mixture. The cracking of the slag cement mixture is mainly due to the greater long-term autogenous shrinkage. The cracking time is found closely related to the shrinkage rate rather than the shrinkage magnitude, a unique relationship can be used to describe such phenomenon.

Adsorption of Calcium Lignosulphonate Water Reducer on Alkali-Activated Slag Cement Pastes

This paper shows how calcium lignosulphonate（CSL）water reducer affects the adsorption and rheological properties of alkali-activated slag cement (AASC) pastes activated by NaOH. It was concluded that the adsorption of water reducer on AASC pastes depended directly on the dosage of the water reducer and the retarder. Slag particles grounded with the retarder YP-3 adsorbed one and a half times of water reducer CSL as much as pure slag particles at 1% mass of the slag, and the absolute value increment of the zeta potential of the AASC suspension containing the retarder YP-3 is 14.00mV, compared with 9.25mV in the system without YP-3. Besides, the AASC pastes activated by NaOH containing the retarder YP-3 show better rheological properties and lower fluidity loss over time.

Effect of Quartz Powder on the Strength and Shrinkage of Alkali-Activated Slag Cement

Quartz powder with different fineness were added into alkali activated slag cement to reduce the drying shringkage of the hardened cement paste. Experimental results showed that partially replacing the slag powder with quartz powder will reduce the water requirement of normal consistency, dry shrinkage and early age compressive and flexural strength of alkali-activated slag cement (AASC), however, the flexural/compressive strength ratio and the fracture toughness will be increased and so does the later strength. The fracture section of specimen of quartz powder modified alkali-activates slag cement paste has a rougher texture than that of pure alkali-activates slag cement paste.

Effect of addition of nano-magnetite on the hydration characteristics of hardened Portland cement and high slag cement pastes

In this investigation the effect of addition of magnetite nanoparticles on the hydration characteristics of both ordinary Portland cement (OPC) and high slag cement (HSC) pastes was studied. The cement pastes were prepared using a water/solid (W/S) mass ratio of 0.3 with addition of 0.05, 0.1, and 0.3 % of magnetic fluid Fe3O4 nanoparticles by mass of cement. An aqueous stable magnetic fluid containing Fe3O4 nanoparticles, with a mean diameter in the range of super-paramagnetism, was prepared via co-precipitation method from ferrous and ferric solutions. The admixed magnetite-cement pastes were examined for compressive strength, chemically combined water content, X-ray diffraction analysis, and differential scanning calorimetry. The results of compressive strength revealed that the hardened pastes made from OPC and HSC admixed with different amounts of magnetic fluid have higher compressive strength values than those of the neat cement OPC and HSC cement pastes at almost all ages of hydration. The results of chemically combined water content for the admixed cement pastes showed almost the same general trend and nearly comparable values as those of the neat cement pastes. From the XRD diffractograms obtained for the neat OPC and HSC cement pastes, the main hydration products identified are calcium silicate hydrates, portlandite, and calcium sulfoaluminate hydrates. Addition of magnetic fluid nanoparticles to both of OPC and HSC did not affect the main hydration products of the neat OPC or HSC cement in addition to one main basic difference, namely, the formation of calcium iron hydroxide silicate as a new hydration product with a reasonable hydraulic character.

Evaluation of pre-coated recycled concrete aggregate for hot mix asphalt

Recycled concrete aggregate (RCA) used for pavement construction can ease aggregate shortage problem and reduce environmental pollution. The mechanical properties of hot mix asphalt (HMA) with RCA are usually inferior to those of HMA with river crushed stone (CS). In this research, slag cement paste used for pre-coated RCA (PCRCA) with coating thickness of 0.25 mm, 0.45 mm and 0.65 mm to reinforce its ability is evaluated. The result shows that the PCRCA with coating thickness of 0.25 mm has the optimum coating paste volume for HMA mixture. The indirect tensile strength (ITS) test, moisture sensitivity test and wheel-track rutting test of HMA with substitution ratios of 25%, 50%, 75% and 100% PCRCA mixture are discussed. The results indicate that the properties of PCRCA have highly pores contents, absorption of water and asphalt contents. However, the physical properties of the PCRCA used as aggregate and test of HMA with PCRCA are within the range of the specification requirements.

FT-IR Study on Early-Age Hydration of Alkali-Activated Slag Cement

The hydration process of alkali-activated slag cement, especially at early-age, was studied by FT-IR, compared with that of Portland cement. The results show that during the hydration of alkali activated slag cement, two processes have taken place, the dissolution of Al3+ from the slag and then the recombination of Al3+ ions with the silicate anions. The former associated with the break of the glass network of slag and the later with the re-polymerization of the silicate and aluminosilicate anions. The rate of break of the glass network increases with the dosage of water glass, which completed in 24 h when the dosage of the water glass solution equivalent to Na2O exceeds 6% of the slag powder. It is confirmed by the IR study that Ca(OH)2 is absent in the hardened alkali activated slag cement paste.

The influence of small particle on the fluidity of blast furnace slag cement paste containing superplasticizers

This paper presents research results of superplasticizers effect on rheological properties of cement pastes containing blast furnace slag. The percentage of slag was from 20% to 80% by mass of cement. The specific surface of slag was similar to cement according to Blaine’a but the slag contained smaller percentage of fine fraction than cement. It was found that superplasticizer addition lowers rheological parameters of cement pastes. The decrease of rheological parameters depends on fine fraction contents in cement. Various degree of fluidity of cement pastes (C3 and CEM III/A 32.5) with similar contents of slag (about 60%) results from different amount of fine fraction below 20 μm.

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–S–H has weaker alkali-binding capacity than the C–S–H 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.

Chemical and Microstructural Characterization of 20‐Month‐Old Alkali‐Activated Slag Cements

Chemical and microstructural characterization of environmentally friendly alkali‐activated slag cement pastes and fine‐limestone aggregate concretes have been carried out after aging for 20 months and compared with 55‐day‐old cement pastes. Although the production of crystalline phases slowed, the cement paste continues to react. Si moves to more ordered positions, and formulae activated by Na2CO3undergo recarbonation, producing both calcite and aragonite, of which the latter occurs mainly in the recarbonation of Ca‐rich building materials known to have high durability. The reaction continues little in samples activated by NaOH/waterglass after 55 days. Elemental analysis shows the Ca/Si ratio decreasing from the center of unreacted slag grains to the outer product (Op). Elemental analyses of the cementing phase in the cement paste as well as the cementing phase in concrete specimens were quite similar. Peak shifts observed in Fourier transform infrared spectroscopy imply greater durability due to increased polymerization of Si in C–S–H, which may contain Na. Over time, more Ca dissolves from the slag and enters the Op, which was observed to be homogeneous.

Study on Properties of Fresh and Hardened Slag Cement Paste

The paper studied effect of blast-furnace slag on rheology and hydration properties of blended cement paste. Rheology of blended cement paste with polycarboxylic series (PCS) and sulphamate series (SMS) superplasticizer is tested, appraised effect degree of the dosage and fineness of slag in blended cement by saturation dosage of superplasticizer, fluidity and gradual loss of fluidity. For hardened paste, the study tested development of strength and hydration products, activation effects of gypsum and sodium sulfate, analyzed macro mechanical behavior and micro structure of blend cement made from slag.

Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag

The morphology of outer-product (Op) C–S–H in 20-year-old slag–cement pastes appeared in most blends to be finer than at younger ages. The Ca/Si and Ca/(Si + Al) ratios of the Op C–S–H decreased with increasing slag content, and the Al/Si ratio increased. The Ca/Si ratio of C–S–H in the slag-containing pastes was lower at 20 years than at 14 months and the amount of Ca(OH) 2 was reduced indicating that additional slag must have reacted. The mean aluminosilicate chain length of the C–S–H was very long in all the samples and would be expected to have increased with age. The TEM-EDX and NMR data are consistent with nanostructural models for C–S–H. The Mg/Al ratio of the Mg–Al layered double hydroxide phase (LDH) was lower at 20 years than at 14 months in all cases except for the neat slag paste; aluminium hydroxide-based structure might be interstratified with those of the Mg–Al LDH.

Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures

The residual compressive strength behavior of alkali activated slag paste (AASP) after temperature exposures up to 1,200°C was investigated. Strength loss of approximately 60% occurred between 100 and 200°C and a further strength loss in the order of 30% at 800°C. Total loss of strength occurred at 1,200°C. Thermogravimetric studies (TGA/DTG) verified AASP contained no Ca(OH)2 which governs the chemical mechanism of strength loss for ordinary Portland cement (OPC) and blended slag cement pastes. However, the TGA results showed that AASP had a higher water loss than the other binders between 100 and 200°C and higher thermal shrinkage as indicated by the dilatometry studies. The high thermal shrinkage led to a differential thermal shrinkage gradient within the AASP and induced micro stresses and cracking which was more prominent for larger samples. Differential thermal shrinkage caused by the higher thermal shrinkage of the AAS material was concluded as the mechanism which gives lower residual strength in AASP compared to OPCP.

The influence of free water content on dielectric properties of alkali active slag cement paste

The dielectric performance of alkali activated slag (AAS) cement paste was investigated in the frequency range of 1 to 1000 MHz. The experimental results showed the unstable dielectric properties of harden paste were mostly influenced by the fraction of free water in paste or absorbed water from ambient, but not including hydration water and microstructure. The free water was completely eliminated by heat treatment at 105 °C about 4 hours, and then its dielectric loss was depressed; but with the exposure time in air increasing, the free water adsorption in ambient air made the dielectric property of harden cement paste to be bad. The temperature and relative humidity of environment was the key factors of free water adsorption; hence, if the influence of free water on dielectric constant was measured or eliminated, the cement-based materials may be applied in humidity sensitive materials or dielectric materials domains.

Modeling of frost salt scaling

This paper discusses the numerical modeling of deterioration in cement-based materials due to frost salt scaling (FSS). Several aspects of FSS are investigated such as carbonation, microstructure, mechanical properties and testing conditions. Mainly blast-furnace slag cement (henceforth slag cement) systems are of interest in this paper since several reports have been indicated that cementitious materials bearing slag-rich cement are critically vulnerable under combined attack of frost and de-icing salts. In the first part, the paper deals with the effect of carbonation on the micromechanical properties and FSS resistance of 1-year-old slag cement and ordinary Portland cement pastes with W/C 0.45. The micromechanical properties were evaluated by the nano-indentation technique and the results are used to evaluate the behavior of these pastes under frost salt attack. FSS damage on the paste samples is modeled according to the glue-spall theory with the aid of Delft Lattice Model. Additionally, the carbonated cement paste microstructures are characterized by ESEM/BSE. In the second part, parameters that are varied in the investigation are the salt concentration in the external water layer and ice-layer thickness on the surface. Again the lattice type model is used to simulate the mechanism in which the material structure is implemented using digital images of the real material. Both experiments and the simulation with the model show that the amount of scaling increases with increasing thickness of the ice layer on the surface. Furthermore it is shown that with the model the well known pessimum effect for salt concentration in the water (which causes maximum damage at 3% salt) can be reproduced. The outcome of the model indicates that glue-spall theory can successfully explain FSS.

Three-dimensional computer modeling of slag cement hydration

A newly developed version of a three-dimensional computer model for simulating the hydration and microstructure development of slag cement pastes is presented in this study. It is based on a 3-D computer model for Portland cement hydration (CEMHYD3D) which was originally developed at NIST, taken over in the authors’ group and further developed. Features like the digitized 3-D microstructure, the cellular automata (CA) algorithm for simulating the random walking, phase transformation for simulating the chemical reactions, are retained. But, the 3-D microstructure was reconstructed allowing for slag particles as binder in the system. Algorithms and rules are developed to account for the interaction between Portland cement hydration and slag reaction in the paste, of which the mechanisms were revealed in the studies by Chen and Brouwers [(2007) J Mater Sci 42(2):428; (2007) J Mater Sci 42(2):444] Methods for considering the various factors on the reactivity of slag in hydrating slag cement pastes are proposed, mainly for the oxide composition of slag and the alkalinity in the pore solution composition. A comparison between the model predictions and the experimental results in literature shows that the presented computer model can successfully predict the hydration process and the microstructure development of hydrating slag cement paste.