I. Garcia-Lodeiro

Hydration of Hybrid Alkaline Cement Containing a Very Large Proportion of Fly Ash: A Descriptive Model

In hybrid alkaline fly ash cements, a new generation of binders, hydration, is characterized by features found in both ordinary portland cement (OPC) hydration and the alkali activation of fly ash (AAFA). Hybrid alkaline fly ash cements typically have a high fly ash (70 wt % to 80 wt %) and low clinker (20 wt % to 30 wt %) content. The clinker component favors curing at ambient temperature. A hydration mechanism is proposed based on the authors’ research on these hybrid binders over the last five years. The mechanisms for OPC hydration and FA alkaline activation are summarized by way of reference. In hybrid systems, fly ash activity is visible at very early ages, when two types of gel are formed: C–S–H from the OPC and N–A–S–H from the fly ash. In their mutual presence, these gels tend to evolve, respectively, into C–A–S–H and (N,C)–A–S–H. The use of activators with different degrees of alkalinity has a direct impact on reaction kinetics but does not modify the main final products, a mixture of C–A–S–H and (N,C)–A–S–H gels. The proportion of each gel in the mix does, however, depend on the alkalinity generated in the medium.

2 – An overview of the chemistry of alkali-activated cement-based binders

This chapter reviews the physical-chemical principles governing alkali-activation processes in both high- and low-CaO aluminosilicate materials. It addresses factors such as starting material composition, reaction mechanisms, and the nature, composition and structure of the reaction products. Models describing the microstructure of the materials and the nanostructure of the gels formed are included for readier comprehension of these processes. An effort is likewise made to explain how the gels that may form in these systems (C-A-S-H-, N-A-S-H-, (N,C)-A-S-H-, and C(N)-A-S-H-type gels) vary with the composition of the starting materials, the pH and type of activator used, and reaction time.

Stability of Synthetic Calcium Silicate Hydrate Gels in Presence of Alkalis, Aluminum, and Soluble Silica:

A study is described of the stability (chemical composition and nano-structural change) of a synthetic calcium silicate hydrate gel in the presence of Ca(OH)2, the two main products of portland cement hydration, hours after alkalis, aluminum, and soluble silica were added to the mix. The sol–gel procedure was used to synthesize the gels. Calcium nitrate and sodium silicate solutions were used as the sources of calcium and silicon, respectively. The samples were prepared to a target Ca–Si ratio of 1.9. A 10-M NaOH solution was added to hold the pH at values greater than 13. All processes were conducted in a nitrogen atmosphere. The samples were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy, nuclear magnetic resonance, transmission electron microscopy, and energy dispersive X-ray. The findings showed that addition of a high sodium hydroxide content led to silicate polymerization. The simultaneous addition of alkalis and aluminum induced a rise in the degree of silicate condensation in the gel [Q2(nAl) and possible cross-linking, Q3, via bridging tetrahedral aluminate], along with the formation of a calcium aluminosilicate hydrate similar to strätlingite. Finally, the simultaneous addition of alkalis, aluminum, and soluble silica favored the precipitation of an alkaline aluminosilicate hydrate gel with calcium replacing a high percentage of sodium and a composition clearly different from that of the other components.

Crucial insights on the mix design of alkali-activated cement-based binders

Abstract Alkaline cements consist essentially of two components: a cementitious material and an alkaline activator. Natural materials (metakaolin) or industrial by-products (such as slag and fly ash) with amorphous or vitreous structures are used as cementitious components, while the liquid or solid alkaline activators applied are generally alkaline salts or caustic solutions. This chapter discusses the most prominent characteristics and properties of both components: the [CaO]/[SiO2] and [SiO2]/[Al2O3] ratios in the cementitious materials; the best activator for each solid precursor; and the effect of anions (OH− (pH), silicates, carbonates and sulfates) and cations (Na+or K+).

Cements with a low clinker content: versatile use of raw materials

A model for alkaline activation has recently been designed to develop new cementitious systems known as hybrid alkaline cements. These multi-component systems contain a high percentage of mineral additions (fly ashes, slags (BFS), metakaolin (MK), bentonites (BT) …), low proportions (<30%) of Portland clinker and moderately alkaline solid activators. This study explored the mechanical performance of two sodium carbonate-activated hybrid cements, CEM MK (20% CK + 40% BFS + 40% MK) and CEM BT (20% CK + 40% BFS + 40% BT). The reaction kinetics of the two systems were analyzed by isothermal conduction calorimetry. The hydrated matrices were characterized with XRD, SEM/EDX and 29Si and 27Al NMR. Both systems developed high mechanical strength, for both generated a mix of (N,C)-A-S-H and C-A-S-H gels. The proportions of the gels precipitating vary with system composition and initial reactivity, however.

Development of New Cementitious Caterials by Alkaline Activating Industrial by-Products

The alkaline activation of aluminosiliceous industrial by-products such as blast furnace slag and fly ash is widely known to yield binders whose properties make them comparable to or even stronger and more durable than ordinary Portland cement. The present paper discusses activation fundamentals (such as the type and concentration of alkaline activator and curing conditions) as well as the structure of the cementitious gels formed (C-A-S-H, N-A-S-H). The durability and strength of these systems make these materials apt for use in many industrial applications, such as precast concrete elements (masonery blocks, railroad sleepers), protective coatings for materials with low fire ratings and lightweight elements.

Cements with low Clinker Content

Hybrid alkaline cements are multi-component systems containing a high percentage of mineral additions (fly ash, blast furnace slag), low proportions (<30%) of Portland clinker and scarce amounts of alkaline activators. The substantially lower amount of clinker needed to manufacture these binders in comparison to ordinary Portland cement is both economically and ecologically beneficial. Their enormous versatility in terms of the raw materials used has made them the object of considerable interest. The present study explored the mechanical strength of binary blends mixes; B1= 20% clinker (CK) + 80% fly ash (FA) and B2=20% clinker + 80% blast furnace slag (BFS), both hydrated in the presence and absence of an alkaline activator specifically designed for this purpose. The use of the activator enhanced the development of early age strength considerably. All the hydrated matrices were characterised with XRD, SEM/EDX and (29Si and 27Al) NMR. The use of the alkaline activator generated reaction products consisting primarily of a mix of gels ((N,C)-A-S-H and C-A-S-H) whose respective proportions were found to depend upon system composition and initial reactivity.