A brief history and review of geopolymer technology is presented with the aim of introducing the technology and the vast categories of materials that may be synthesized by alkali-activation of aluminosilicates. The fundamental chemical and structural characteristics of geopolymers derived from metakaolin, fly ash and slag are explored in terms of the effects of raw material selection on the properties of geopolymer composites. It is shown that the raw materials and processing conditions are critical in determining the setting behavior, workability and chemical and physical properties of geopolymeric products. The structural and chemical characteristics that are common to all geopolymeric materials are presented, as well as those that are determined by the specific interactions occurring in different systems, providing the ability for tailored design of geopolymers to specific applications in terms of both technical and commercial requirements.

Geopolymer technology: the current state of the art

http://www.personal.psu.edu/faculty/g/u/gur/cement%20for%20the%20future.pdf

Alkali-activated fly ashes: A cement for the future

The Role of Inorganic Polymer Technology in the Development of ‘Green Concrete’

Understanding the relationship between geopolymer composition, microstructure and mechanical properties

Carbon dioxide equivalent (CO2-e) emissions: A comparison between geopolymer and OPC cement concrete

A study examining the complex hydration chemistry of a hybrid alkaline cement containing a high content of coal bottom ash (BA) (>70%) and a low content of portland cement clinker in the presence of an alkaline activator is presented. The use of a water reducing additive was found to significantly delay the overall hydration process, allowing an opportunity to more clearly distinguish the hydration reactions that take place. The results presented showed that both the cement clinker phases and the ash glassy phases are highly reactive for the first 3 d of hydration. In situ formed reaction products portlandite and gypsum were shown to be metastable and had disappeared within 3 d of hydration. Ettringite stability was limited in the hybrid system but unlike gypsum and portlandite, remained detectable for the first 3 d of hydration at least. SEM-EDX and subtracted Fourier transform infrared evidence suggest the development of three different gel bond environments, tentatively attributed to C–(A)–S–H, C–A–S–H, and (N,C)–A–S–H type gels.

The Early Age Hydration Reactions of a Hybrid Cement Containing a Very High Content of Coal Bottom Ash

The detection of bridging tetrahedra, or Q2(L), in the silica chains constituting C–S–H gels is controversial. While some authors maintain that 29SiMAS-NMR can be used to distinguish between Q1 and Q2 units but not between the silicon environments associated with Q2 (intermediate position) and Q2(L) (intermediate and bridging position) units, others claim the contrary. The present article addresses this issue with a report on 29SiMAS-NMR studies of a number of calcium silicate hydrate gels. The findings showed that nuclear magnetic resonance can differentiate between Q2 and Q2(L) units in highly polymerized C–S–H gels with mean chain lengths (MCL) of over 12 and Ca/Si ratios of <1. At higher Ca/Si ratios, however, the MCL declined, with the concomitant rise in Q1 units. Under these conditions, in which it was practically impossible to distinguish between Q2 and Q2(L), the presence of a single signal for all Q2 units was assumed.

C–S–H Gels: Interpretation of 29Si MAS‐NMR Spectra

Microstructure studies on Portland cement pastes obtained in highly alkaline environments

In this study a series of different gels (inorganic polymers) was synthesized with a single procedure, although varying the pH of the synthesis medium across a wide range of values. The chemicals used were as follows: a 0.25 M solution of K 2 O · SiO 2  · 9H 2 O as the source of silica, a 0.25 M solution of Al(NO 3 ) 3  · 9H 2 O as the source of aluminum; and a 0.25 M solution of KOH to regulate the pH. Gel thermal stability was also determined, up to 1200 °C. The results show that at acidic pH both Si-rich and Al-rich gels are formed, which may crystallize into cristoballite and mullite, respectively, under thermal treatment. The gels synthesized at alkaline pH, in turn, remain stable at temperatures over 1200 °C. Depending on their composition, such gels may crystallize into leucite.

Synthesis and thermal behavior of different aluminosilicate gels

This study explored the behavior of laboratory-synthesized tricalcium aluminate (C3A) in alkaline media. In all the trials, the C3A was hydrated with water containing a certain percentage of alkali activators (4% Na2CO3, 4% Na2SO4, or 8% Na2SO4 by weight of C3A). Two- and 28-day mechanical strength values were determined and the products were characterized with XRD, DTA, and NMR. The compressive strength of the C3A pastes rose in the presence of the alkaline activators used in this study. The alkalis induced the formation of calcium monocarboaluminate (C4ACH11) as the main hydration product in all cases. In addition, when Na2SO4 was added, a sodium calcium sulfoaluminate, phase U (C4ASNH16), formed. The presence of this mineral, whose formation is associated with high pH values, furnished indirect information on the alkalinity in the medium.

Ángel Palomo

Papers

The Early Age Hydration Reactions of a Hybrid Cement Containing a Very High Content of Coal Bottom Ash

C–S–H Gels: Interpretation of 29Si MAS‐NMR Spectra

Microstructure studies on Portland cement pastes obtained in highly alkaline environments

Synthesis and thermal behavior of different aluminosilicate gels

Alkaline Hydration of Tricalcium Aluminate