Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part I: Effect of MgO

The hydration and the microstructure of three alkali activated slags (AAS) with MgO contents between 8 and 13 wt.% are investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O (WG). Higher MgO content of the slag resulted in a faster reaction and higher compressive strengths during the first days. The formation of C(− A)–S–H and of a hydrotalcite-like phase was observed in all samples by X-ray diffraction (XRD), thermal analysis (TGA) and scanning electron microscopy (SEM) techniques. Increasing the MgO content of the slag from 8 to 13% increased the amount of hydrotalcite and lowered the Al uptake by C–S–H resulting in 9% higher volume of the hydrates and a 50 to 80% increase of the compressive strength after 28 days and longer for WG activated slag pastes. For NaOH activated slags only a slight increase of the compressive strength was measured.     

Composition, morphology and nanostructure of C-S-H in 70% white Portland cement-30% fly ash blends hydrated at 55 °C.

Outer product C-S-H had a mixture of fibrillar and foil-like morphology in a 28-day-old water-activated paste, and foil- or lath-like morphology in an alkali-activated paste. It was not possible to determine the chemical composition of C-S-H using SEM-EDX because of fine-scale intermixing with other phases; TEM-EDX was necessary. The C-S-H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water. The mean length of the aluminosilicate anions in the C-S-H was similar in both systems and increased with age; those in the Op C-S-H were likely to be shorter than those present in the Ip C-S-H with water activation, but longer (and more protonated) with alkali. The potassium in the alkali-activated paste was present either within the C-S-H structure charge balancing the substitution of Al³⁺ for Si⁴⁺, or adsorbed on the C-S-H charge balancing sulfate ions.     

Effects of dosage and modulus of water glass on early hydration of alkali-slag cements

This paper examines the early hydration of alkali-slag cements activated with water glass with different n moduli and sodium metasilicate (Na₂SiO₃·5H₂O) in solution at 25 °C. The early hydration of alkali-activated blast furnace slag cements has been studied using isothermal conduction calorimetry. The cumulative heat of hydration increases by increasing the n modulus as well as the dosage of water glass, but is still lower than that of Portland cement. The compressive strength of normal-cured water glass slag cements is higher than Portland cement mortars. Drying shrinkage of alkali-slag cements is considerably higher than that of Portland cement. Consequently, industrial use of alkali-slag cement needs better understanding of the hardening mechanism and requires further research based on presented observations and results.     

Structure and mechanical properties of aluminosilicate geopolymer composites with Portland cement and its constituent minerals

The compressive strengths and structures of composites of aluminosilicate geopolymer with the synthetic cement minerals C₃S, β-C₂S, C₃A and commercial OPC were investigated. All the composites showed lower strengths than the geopolymer and OPC paste alone. X-ray diffraction, ²⁹Si and ²⁷Al MAS NMR and SEM/EDS observations indicate that hydration of the cement minerals and OPC is hindered in the presence of geopolymer, even though sufficient water was present in the mix for hydration to occur. In the absence of SEM evidence for the formation of an impervious layer around the cement mineral grains, the poor strength development is suggested to be due to the retarded development of C-S-H because of the preferential removal from the system of available Si because geopolymer formation is more rapid than the hydration of the cement minerals. This possibility is supported by experiments in which the rate of geopolymer formation is retarded by the substitution of potassium for sodium, by the reduction of the alkali content of the geopolymer paste or by the addition of borate. In all these cases the strength of the OPC-geopolymer composite was increased, particularly by the combination of the borate additive with the potassium geopolymer, producing an OPC-geopolymer composite stronger than hydrated OPC paste alone.     

Effect of geothermal waste on strength and microstructure of alkali-activated slag cement mortars

Mortars of blast furnace slag replaced with 10% of a geothermal silica waste were cured for 90 days. The binder was activated by 6 wt.% Na₂O equivalent of NaOH and water glass. The presence of the silica enhanced the formation of hydration products as shown by nonevaporable water (NEW) results. Backscattered electron images indicated that the microstructures of blended slag had less porosity than those of neat slag mortars and the interfacial zone between aggregate and hydration products was dense and of homogeneous composition similar to the matrix of hydration products. The main hydration products were C-S-H and for NaOH a hydrotalcite type phase was found as finely intermixed with the C-S-H.     

Hydration Products and Reactivity of Blast-Furnace Slag Activated by Various Alkalis

Pastes of blast-furnace slag were cured for up to 90 d using sodium silicate (waterglass), NaOH, and three different mixtures of Na₂CO₃–Na₂SO₄–Ca(OH)₂ to activate reactions. The highest slag reactivity was observed for NaOH activation and the least for waterglass, although nonevaporable water indicated similar amounts of hydration products formed. The main hydration products found using X-ray diffractometry in all systems were calcium silicate hydrate (C-S-H) and a hydrotalcite-type phase. Microanalysis was performed on pastes activated using 50% Na₂CO₃·25% Na₂SO₄·25% Ca(OH)₂, NaOH, and waterglass; the chemical composition of the C-S-H in the waterglass case was different relative to the other two alkalis. For all alkaline agents used, the C-S-H seemed finely intermixed with a hydrotalcite-type phase of Mg/Al = 1.82, on average.     

Early age strength enhancement of blended cement systems by CaCl2 and diethanol-isopropanolamine

The enhancement of the 1 day strength of cementitious systems by a combination of calcium chloride (CaCl₂) and diethanol-isopropanolamine (DEIPA) was studied, particularly in blended cement systems. A combination of quantitative X-ray diffraction with Rietveld refinement (QXRD), scanning electron microscopy (SEM)/backscattered electron image analysis, thermogravimetric analysis (TGA), and isothermal calorimetry were used to investigate the mechanism of strength enhancement by the additives. The additives were found to increase the early age mortar strength by enhancing the cement hydration, with the DEIPA enhancing primarily the aluminate hydration. DEIPA also affected the morphology of portlandite which was formed as thin plates. In parallel, the calcium-to-silica ratio of the C-S-H was found to increase with the use of DEIPA, possibly because of the inclusion of microcrystalline portlandite. After 48 h DEIPA was found to directly enhance the rate of reaction of granulated blast-furnace slag and fly ash.     

Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes

The effect of temperature on the hydration products and the composition of the pore solution are investigated for two Portland cements from 5 to 50 °C. Increased temperature leads to an initially fast hydration and a high early compressive strength. At 40 and 50 °C, the formation of denser C-S-H, a more heterogeneous distribution of the hydration products, a coarser porosity, a decrease of the amount of ettringite as well as the formation of very short ettringite needles has been observed. At 50 °C, calcium monosulphoaluminate has formed at the expenses of ettringite. In addition, the amount of calcium monocarboaluminate present seems to decrease. The composition of the pore solution mirrors the faster progress of hydration at higher temperatures. After 150 days, however, the composition of the pore solution is similar for most elements at 5, 20 and 50 °C. Exceptions are the increased sulphate concentrations and the slightly lower Al and Fe concentrations at 50 °C.     

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

Early hydration of clinker–slag–metakaolin combination in steam curing conditions, relation with mechanical properties

High strength can be obtained at early ages for precast concrete elements by the use of CEMI 52.5R cement (OPC) and thermal treatment (steam curing). To compensate for the announced withdrawal of CEM I cements because of high CO₂ emissions during their production and the ecotax that this will imply, one attractive alternative is the use of composed cements resulting from the combination of clinker with mineral admixtures. In steam curing conditions, previous studies have shown an increase in the compressive strength at one day of age for mortars incorporating an OPC/blast furnace slag (GGBS)/metakaolin (MK) combination, in comparison with mortars incorporating OPC only. The present study investigates the connection between the compressive strength, at one day of age, of steam cured mortars made with various binders and the hydration of these binders. The progress of the hydration was characterised by means of XRD, thermal and microprobe analyses. The results indicate that the increase in compressive strength when MK is incorporated (OPC/MK or OPC/MK/GGBS) can be explained by an increase in the amount of C-S-H, C-A-H, C-A-S-H phases, a decrease in the amount of CH and a change in the chemical nature of the matrix (decrease in C/S ratio). The decrease in compressive strength of OPC/slag-based material can be explained by a reduction in the amount of hydrated phases (particularly C-S-H) and compactness.These are promising results for precast concrete manufacturers who are concerned about preserving the environment.     

碱激发剂浓度及模数对碱矿渣胶凝材料抗压性能及水化产物的影响研究

采用水玻璃(复掺氢氧化钠调整模数)激发粒化高炉矿渣活性制备碱矿渣净浆试样.采用抗压强度测试、X-射线衍射(XRD)、综合热分析(TG-DSC)等技术手段研究了激发剂碱浓度(4%、6%、8%)及模数(0.75、1.00、1.50、2.00)对碱矿渣胶凝材料抗压性能及水化产物的影响.研究结果表明:激发剂模数较低时(0.75和1.00),碱矿渣胶凝材料抗压强度随着碱浓度的增加而呈下降趋势;激发剂模数较高时(1.50和2.00),试件强度在碱浓度为6%时达到最大值.在相同碱浓度下,激发剂模数为1.50时试件抗压强度值最大.碱矿渣胶凝材料主要水化产物为C-S-H凝胶,同时伴有C-A-S-H凝胶生成.另外观测到少量斜方钙沸石(CaAl2Si2O8· 4H2O)的生成.在部分配合比中还观测到水滑石(Mg6Al2(OH)16CO3· 4H2O)的存在.碱浓度较高的碱矿渣胶凝材料中生成了较多的C-S-H水化产物.激发剂模数较高时(1.50和2.00),更有利于碱矿渣中C-S-H水化产物的生成.碱浓度/模数较低时, C-S-H产物结晶度有所提高.相较于C-S-H凝胶结晶度,其生成量对碱矿渣胶凝材料抗压强度的影响更为显著.     

Effect of alkalis on fresh C-S-H gels. FTIR analysis

The present study addresses the effect of different concentrations of Na₂O on the structure of a composition of fresh C-S-H gels. The gels were synthesized from laboratory reagents, using calcium nitrate as the source of calcium and a sodium silicate solution for the silicon. A 10-M solution of NaOH was used to maintain a pH of over 13 throughout. The synthesized gels were exposed to different amounts of 8-M NaOH to determine their chemical stability and subsequently characterized with Fourier transform IR spectroscopy. The results showed that the addition of different concentrations of sodium leads to C-S-H gel modification even in the very short term.     

The effects of nano-C-S-H with different polymer stabilizers on early cement hydration

A promising approach to accelerate cement hydration known as “seeding technology” has been discovered using nano-particles to provide additional nucleation sites for growing of C-S-H. Two different types of polymer, polycarboxylate (PCE) and polysulfonate (PSE) were used as stabilizer to synthesize nano-C-S-H via co-precipitation process. The obtained C-S-H-polymer composites were characterized by means of XRD, FTIR, thermogravimetric analysis (TGA), TEM, dynamic laser scattering (DLS), and BET. DLS measurement shows that the particle size of the obtained C-S-H-polymer suspension ranges from 82.6 to 589.9 nm. The results of DLS and BET show that the particle size of the C-S-H particles synthesized using PCE polymer as stabilizer is smaller than those synthesized with PSE polymer, and hence the specific surface area is much higher. FTIR and TGA results confirm the presence of the polymers in the obtained C-S-H composites particles. XRD results indicate that the presence of the polymers reduces the crystallinity of C-S-H due to the absence of the d002 peak at 2θ of 7°. The calorimetry results show that the main hydration peak of cement is dramatically increased by the addition of the C-S-H-polymer composites. It is interestingly found that the acceleration effect of the C-S-H-polymer composites is linearly proportional to the total surface area of the nanoparticles introduced into the cement pastes. At the same time, it is found that the secondary hydration peak, usually known as the sulfate-depletion peak, is greatly advanced by addition of the C-S-H nano-particles in comparison with the blank cement paste. The acceleration effect of the nano-C-S-H is further verified in a pure C₃S system.     

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.     

Evolution of strength, microstructure and mineralogical composition of a CKD–GGBFS binder

Characterization of a nontraditional binding material containing cement kiln dust (CKD) and ground granulated blast furnace slag (GGBFS) is discussed in this paper. Significant compressive strength was obtained for a CKD–GGBFS blend with 70% CKD and 30% GGBFS at a water-to-binder ratio of 0.40 after 2 days of curing at elevated temperature. Similar strength was also obtained for the samples subjected to normal moisture curing over a period of 28 days. The compressive strength increased with additional moist curing in both the cases. The microstructural and the mineralogical examinations show that the strength development was mainly due to the formation of calcium silicate hydrate (C-S-H). In addition to normal C-S-H, aluminum and magnesium incorporated C-S-H phases were also present in the CKD–GGBFS blends. The formation of ettringite appears to be a contributing factor in early age strength development of CKD–GGBFS binder.     

Improved evidence for the existence of an intermediate phase during hydration of tricalcium silicate

Tricalcium silicate (Ca₃SiO₅) with a very small particle size of approximately 50 nm has been prepared and hydrated for a very short time (5 min) by two different modes in a paste experiment, using a water/solid-ratio of 1.20, and by hydration as a suspension employing a water/solid-ratio of 4000. A phase containing uncondensed silicate monomers close to hydrogen atoms (either hydroxyl groups or water molecules) was formed in both experiments. This phase is distinct from anhydrous tricalcium silicate and from the calcium-silicate-hydrate (C-S-H) phase, commonly identified as the hydration product of tricalcium silicate. In the paste experiment, approximately 79% of silicon atoms were present in the hydrated phase containing silicate monomers as determined from ²⁹Si{¹H} CP/MAS NMR. This result is used to show that the hydrated silicate monomers are part of a separate phase and that they cannot be attributed to a hydroxylated surface of tricalcium silicate after contact with water. The phase containing hydrated silicate monomers is metastable with respect to the C-S-H phase since it transforms into the latter in a half saturated calcium hydroxide solution. These data is used to emphasize that the hydration of tricalcium silicate proceeds in two consecutive steps. In the first reaction, an intermediate phase containing hydrated silicate monomers is formed which is subsequently transformed into C-S-H as the final hydration product in the second step. The introduction of an intermediate phase in calculations of the early hydration of tricalcium silicate can explain the presence of the induction period. It is shown that heterogeneous nucleation on appropriate crystal surfaces is able to reduce the length of the induction period and thus to accelerate the reaction of tricalcium silicate with water.     

Insights into early hydration of Portland limestone cement from infrared spectroscopy and isothermal calorimetry

Isothermal calorimetry and diffuse reflectance infrared DR-FTIR spectroscopy are combined to correlate evolutions of spectroscopic signatures with rates of chemical reactions as reflected in the rate of heat emitted during the first 38 h of cement hydration. Portland limestone cement mortar is employed and the analysis is repeated for two different mixing procedures. Intensive blender mixing with quartz sand is found to cause activation of the cement resulting in a faster hydration process. At early stages of hydration, two types of C-S-H are formed. The spectral intensity of the earlier C-S-H is found to saturate, while that of the later form continues to acquire intensity throughout the 38 h of the experiment. Evidences are presented which support the interpretation that the two forms differ mainly in morphology and water content. Simultaneously with the saturation of the early C-S-H, a transient species is observed with DR-FTIR. This species correlates with the observed thermogram fine-structure.     

Nanogranular packing of C-S-H at substochiometric conditions

Herein, we present a comprehensive nanoindentation investigation of cement pastes prepared at substoichiometric water-to-cement (w/c) mass ratios between 0.15 and 0.4 with and without heat treatment. Based on a statistical indentation technique, we provide strong evidence of the existence of a statistically significant third hydrated mechanical phase in addition to the already known Low-Density (LD) and High-Density (HD) C-S-H phases. The nanomechanical properties of this third phase are found to follow similar packing density scaling relations as LD C-S-H and HD C-S-H, while being significantly greater. This third phase, whose nano-packing density is measured at 0.83 ± 0.01, is therefore termed Ultra-High-Density (UHD) phase. All three phases are present in concrete materials in different volume proportions: LD dominates cement-based materials prepared at high w/c mass ratios; HD and UHD control the microstructure of low w/c ratio materials. In addition, heat treatment favors the formation of HD and UHD. The insight thus gained into the link between composition, processing and microstructure makes it possible to monitor packing density distributions of the hydration products at the nanoscale.     

Hydration in high-performance cementitious systems containing vitreous calcium aluminosilicate or silica fume

The influence of a relatively new high-performance cement replacement material—vitreous calcium aluminosilicate (VCAS)—on the hydration behavior in cementitious systems, and its comparison to silica fume (SF) are presented in this paper. VCAS is shown to have no cementitious qualities, but exhibits significant pozzolanicity, which has been quantified using strength activity index and electrical conductivity change. VCAS modified pastes are found to consume more water during hydration than the corresponding SF modified pastes. Based on a normalized calcium hydroxide content defined in this paper, it is seen that the pozzolanic reaction of VCAS does not happen until 7 days while that of SF occurs as early as the first day. The degrees of hydration of the modified pastes are predicted using a model that employs the change in non-evaporable water resulting from the use of these replacement materials. VCAS modified pastes show lower later age porosities as compared to the plain and SF modified pastes. However, at equal degrees of hydration, SF modified pastes show the lowest porosity.     

Effect on fresh C-S-H gels of the simultaneous addition of alkali and aluminium

Reducing Portland cement content in cementitious binders offers a means to address the adverse environmental impacts of Portland cement manufacture. This paper investigates the impacts on hydration product chemistry of partially replacing Portland cement with alkali-activated aluminosilicates. Here, short-term effects of soluble alkali and aluminium, likely to be available in an alkali-activated system, on the structure of synthetic C-S-H gels are assessed. .C-S-H gels (synthesized at pH values of over 13) were mixed with different concentrations of aluminium nitrate and sodium hydroxide. The gels were characterized by FTIR, TEM/EDX and XRD 72 h later. The results showed that both alkali and aluminium increased the degree of silicate polymerisation in the C-S-H gels and precipitated a crystalline calcium aluminosilicate phase.