Effects of curing conditions on shrinkage of alkali-activated high-MgO Swedish slag concrete

This study aimed to determine the effects of curing regime on shrinkage of alkali-activated concretes produced from a Swedish high-MgO blast furnace slag. Sodium carbonate (SC), sodium silicate (SS), and their combination were used as alkali activators. The studied curing procedure included heat-treatment, no heat-treatment, sealed and non-sealed conditions. The heat curing increased the compressive strengths of the concretes activated with SS and with the combination of SS and SC. Sealed-curing applied for a period of 1 month reduced the measured drying shrinkage by up to 50% for all studied heat-treated samples. Conversely, the same curing procedure significantly increased the development of the drying shrinkage once the seal was removed after 28 days of curing in the case of the SC-activated concretes non-heat treated. Higher degree of reaction/hydration reached by the binders in these concretes was indicated as the main factor. All of the concretes studied had showed a significant microcracking of the binder matrix, with the most extensive cracking observed in the sealed lab-cured mixes. The heat-cured mixes activated with SS and combination of SC and SS showed the most homogenous microstructure and low extensive micro cracking comparing with lab-cured ones

Creep and long-term properties of alkali-activated Swedish-slag concrete

The construction of the future is moving in the direction of environmentally friendly materials and the use of various types of industrial byproducts and wastes. The use of blast furnace slag (BFS) for the production of concrete is one of those alternatives. In this study, pastes and concretes based on high-MgO BFS were alkali activated with 10% by weight sodium carbonate, sodium silicate, and a combination of both. Heat treatment and laboratory curing were applied. The results showed that heat treatment was effective at reducing the drying shrinkage of alkali-activated slag concretes and promoting high early strength. However, the sodium carbonate–activated slag concrete specimens showed a reduction in compressive strength at later ages. All concrete specimens tested exhibited high drying shrinkage; the highest values were for sodium silicate–activated concretes and the lowest were for sodium carbonate–activated concretes. All concretes tested showed very large creep, which was partly related to the small maximum aggregate size (8 mm) and the effects of carbonation. The carbonation depth after 12–24  months was significantly smaller for the heat-treated specimens and for concrete activated with sodium silicate. The carbonation process resulted in a more porous binder matrix, leading to long-term strength loss and increased creep, especially for sodium silicate–activated mixes

Alkali activation of a high MgO GGBS – fresh and hardened properties

In this study, concretes and pastes were produced from a high magnesium oxide (MgO) ground granulated blast furnace slag (magnesium oxide content 16·1 wt%) by alkali activation with various amounts and combinations of sodium carbonate and sodium silicate. Sodium carbonate activators tended to reduce slump compared to sodium silicate at the same dose, and, in contrast to the literature for many blast furnace slags with more moderate magnesium oxide, to shorten the initial and final setting times in comparison with concretes activated by sodium silicate for dosages less than 10 wt%. Higher heat curing temperatures and the use of larger dosages of alkali activators resulted in higher early-age compressive cube strength values. The X-ray diffraction analysis of 7 and 28 d old pastes activated with sodium carbonates revealed formation of gaylussite, calcite, nahcolite and calcium-aluminium-silicate-hydrate (C–A–S–H) gel. Curing at 20°C appeared to promote dissolution of gaylussite and calcite, while heat curing promoted their replacement with C–A–S–H, which also resulted in higher ultimate cube compressive strength values. Conversely, mixes activated with sodium silicate contained less crystalline phases and more disordered gel, which strengthened the binder matrix

A novel cement-based hybrid material

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are known to possess exceptional tensile strength, elastic modulus and electrical and thermal conductivity. They are promising candidates for the next-generation high-performance structural and multi-functional composite materials. However, one of the largest obstacles to creating strong, electrically or thermally conductive CNT/CNF composites is the difficulty of getting a good dispersion of the carbon nanomaterials in a matrix. Typically, time-consuming steps of purification and functionalization of the carbon nanomaterial are required. We propose a new approach to grow CNTs/CNFs directly on the surface of matrix particles. As the matrix we selected cement, the most important construction material. We synthesized in a simple one-step process a novel cement hybrid material (CHM), wherein CNTs and CNFs are attached to the cement particles. The CHM has been proven to increase 2 times the compressive strength and 40 times the electrical conductivity of the hardened paste, i.e. concrete without sand.Peer reviewe

Assessing the chemical involvement of limestone powder in sodium carbonate activated slag

This study aims to investigate the effect of limestone powder (LP) on the reaction of sodium carbonate activated slag. The results show that the incorporated LP up to 30% improves the strength development, especially at advanced curing ages. A slightly accelerated reaction is observed for samples containing low amount of LP (≤5%), while mixture with 10% LP shows the optimized results with respect to the heat release and strength development. Chemical effect of incorporating LP is observed at high replacement levels (≥15%), indicated by the formation of a new phase, natron (Na2CO3·10H2O). Besides, relatively high contents of hydrotalcite-like phases are generated when increasing the dosage of limestone powder. The chemical changes, including the volume changes of generating natron and the transformation of natron to calcite, is potentially responsible for the enhanced mechanical properties

Enhancement of the pozzolanic activity of natural clays by mechanochemical activation

Replacement of cement with supplementary cementitious materials (SCMs) is a proven method to reduce clinker in cement and contribute to decreased CO2 emissions. Natural clays are commonly occurring materials that do not possess pozzolanic activity in their original state. Mechanochemical activation (MCA) can be an alternative and sustainable method to enhance their reactivity. In this study, the pozzolanic reactivity of three natural clays, originating from Sweden, was analyzed after the application of MCA in a planetary ball mill. Strength activity index (SAI), Frattini test, and conductivity test were used to evaluate the pozzolanic reactivity. All processed clays by MCA have achieved a SAI greater than 100%, while the Frattini test indicated an improved pozzolanic activity of samples containing a higher amount of clay minerals. The obtained results show that MCA could improve the pozzolanic reactivity, but the effect depends on the mineralogical composition and particle size of the clays

Porosity and microstructure characterization of building stones and concretes

The microstructure of building materials greatly influences engineering properties like permeability, strength and durability. To determine this microstructure, different techniques were developed, each with its own limitations. The purpose of this study on concrete and natural building stones was to compare and to combine data obtained by X-ray computed micro-tomography (micro-CT), water absorption under vacuum and mercury intrusion porosimetry (MIP). Pore-size distribution curves ranging from 10 nm to 1 mm and total porosity results were obtained. Furthermore, micro-CT revealed the presence of an interfacial transition zone (ITZ) and of micro-cracks inside the aggregates of the concrete samples after mercury intrusion. Micro-CT visualized mercury inside large air bubbles within the concrete samples. Both micro-CT and MIP were compared and their respective advantages and disadvantages discussed

Effects of sodium nitrate and OPC-GGBS concrete mix composition on phase transition of pore water at subzero temperatures

Abstract Lowering the freezing temperature of the mixing water is crucial for concrete works at subzero temperatures. In this study, formation of ice was examined for various pastes and concretes of OPC-GGBS based, while exposed to a constant temperature of −15 °C. Sodium nitrate antifreeze admixture was added as 0, 6, 10, 15, 20, 25, 30 wt% by the total binder amount. The ice formation and its effects on the binder matrix microstructure was studied using differential scanning calorimetry (DSC), ultrasonic pulse velocity (UPV) and Scanning Electron Microscopy — Energy Dispersive Spectrometry (SEM-EDS). Several curing procedures were applied to samples before commencing tests. Results showed that, addition of 25 wt% of the sodium nitrate caused the most extensive delay of the ice growth. Mixes containing less admixture showed an increasing amount of the forming ice which in some cases lead to the development of the false strength. The hydration rate has been the highest for the mix with 25 wt% of the sodium nitrate and tended to be limited at lower additions. The porosity of the hydrated binder matrix tended to be lower for mixes characterized by a lower amount of the forming ice. In general, application of above freezing temperature resulted in resuming of the hydration process that led to densification of the microstructure and strength increase. This trend was more pronounced for mixes having lower amounts of the formed ice