Impact of the supplementary cementitious materials on the kinetics and microstructural development of cement hydration

Supplementary cementitious materials (SCMs) are currently used to replace part of the clinker to reduce the environmental impact of cement and to favor the use of local materials. Two representative SCM have been used in this work: slag and fly ash. Each one corresponds to one of the SCM categories: blast furnace slag is a hydraulic material while siliceous fly ash is a pozzolanic material. A hydraulic material sets and hardens under water while pozzolans need calcium hydroxide (also referred as Portlandite) and water to produce hydrates. As a consequence when slag or fly ash is blended with clinker, its reaction occurs in addition to the cement reaction. This work aims to study the impact of the SCMs on cement hydration to get a generic knowledge on the changes in kinetics and microstructural development when SCMs replace a part of the Portland cement. These changes are important as they can modify the microstructure and change the long term properties. It is interesting to quantify and understand these effects to predict their performance and improve these sustainable cements. A large part of the reaction of clinker phases takes place within two days after the mixing with water. During the same period, slag and fly ash particles are not yet reacting, however they can enhance the hydration of the clinker phases. Using inert quartz powders with different fineness, our results show a clear relation between the inter-particle distance and the kinetics of the acceleration period. This can be explained by the shearing between particles which increases as the inter-particle distance decreases. The micrographs showed that reducing the distance or increasing the mixing speed produces more nucleation sites of C-S-H on the grains surfaces. The study of limestone indicates that limestone powder has an additional effect beyond the effect of increasing the shear produced by other SCMs. The nucleation density is much higher on limestone surface and the morphology of C-S-H is changed to individual needles. The arrangement of the atoms at the surface of limestone seems to explain the increased nucleation. In blended systems, the peak during the deceleration period has sometimes been considered as marking the beginning of the reaction of SCM. In this work, we showed that the peak only corresponds to the second dissolution of C3A although the SCM substitution level changes the kinetics of this reaction. It has been shown that this effect is caused by the faster depletion of sulfate ions, due to the faster reaction of clinker phases and faster formation of C-S-H. A part of the sulfate is adsorbed on the C-S-H structure. This leads to the faster depletion of sulfate in the solution and change the morphology of C-S-H. Our results showed that the adsorption of sulfate promotes the growth of C-S-H as we observed needles diverging from nucleation sites. When the sulfate desorbs the C-S-H needles grow more parallel to each other. This effect was confirmed with the observation of alite where sulfate is not present. The results from 1H NMR showed that the structure itself of C-S-H is affected: few gel pores are formed in presence of sulfate. Over the course of hydration, C-S-H fills the space available between the grains. Once the capillary pores reach a size of 6-8nm, the formation of C-S-H is slowed down and the capillary pores do not reduce further in size. The stabilization of the pore size was explained by the increase of the supersaturation index [...

Nanostructured non-adhesive surfaces for Micro- and Nanomanipulation.

International audienceAdhesion between microgripper end-effector and a nano/micro-object is a main topic for manipulation in micro- and nanoscale. Tuning this force is a great challenge. Adhesion force is directly 2 linked to the chemical composition and the surface roughness of both, the object and the gripper. Recently, we proposed a multispheres Van der Waals force model able to predict this force. The surface used was structured by an array of polystyrene spheres with radii from 35 nm to 2 mm. The experimental pull-off forces have confirmed our model. In this present work, we analyzed other innovating structure such as non-closed packed polystyrene (PS) spheres and organized Si Nanostructures, formed by chemical etching. The adhesion values of the pull-off force measured on these structures were very low (in the range of 2 to 10 nN), and suggest that these new structures have non-adhesive properties. A new model taking in account the roughness and the organization of the PS spheres and Si Nanostructures has been developed to predict these new properties

Evolution of pore structure in blended systems

In this study, the effect of SCM in the cement paste was isolated by using ternary systems combining Portland cement, quartz and SCM. The results show clear differences in how the hydrates from the reaction of clinker, slag and fly ash fill space. The reaction of slag is more efficient than that of fly ash in modifying the porosity. Our results indicate that Portland cement (PC), slag and fly ash reactions are limited at later ages by the lack of water-filled capillary pores. The higher the space available, with increasing the water/solids ratio, the later the reaction is limited. This explains the lower degree of reaction of SCM in blended systems at high replacement level

Influence of limestone on the hydration of ternary slag cement

The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cements have been investigated. Commercial CEM I 52.5 R was blended with slag and limestone; maintaining a clinker to SCM ratio of 50:50 with up to 20% slag replaced by limestone. The sulphate content was maintained at 3% in all composite systems. Hydration was followed by a combination of isothermal calorimetry, chemical shrinkage, scanning electron microscopy, and thermogravimetric analysis. The hydration of slag was followed by the implementation of QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different w/s ratios. Thus, the method was used to analyse hydrated cements without dehydrating the specimens. The results show that the presence of limestone enhanced both clinker and slag hydration. The pore volume and pore solution chemistry were further examined to clarify to the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus lowering of the aluminium concentration in the pore solution led to the improved slag hydration

Blast furnace slag-Mg(OH)(2) cements activated by sodium carbonate

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)2 was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na2O/slag ratios. Particles of Mg(OH)2 do not chemically react within these cements. Instead, Mg(OH)2 acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)2 replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)2 still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)2 cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)2-bearing wastes such as Magnox sludge

Hydration of water- and alkali-activated white Portland cement pastes and blends with low-calcium pulverized fuel ash

Pastes of white Portland cement (wPc) and wPc-pulverized fuel ash (pfa) blends were studied up to 13 years. The reaction of wPc with water was initially retarded in the presence of pfa particles but accelerated at intermediate ages. Reaction with KOH solution was rapid with or without pfa. A universal compositional relationship exists for the C-A-S-H in blends of Pc with aluminosilicate-rich SCMs. The average length of aluminosilicate anions increased with age and increasing Al/Ca and Si/Ca; greater lengthening in the blends was due to additional Al3+ at bridging sites. The morphology of outer product C-A-S-H was always foil-like with KOH solution, regardless of chemical composition, but with water it had fibrillar morphology at high Ca/(Si+Al) ratios and foil-like morphology started to appear at Ca/(Si+Al) ≈1.2-1.3, which from the literature appears to coincide with changes in the pore solution. Foil-like morphology cannot be associated with entirely T-based structure