Alkaline Activation of Metakaolin: Effect of Calcium Hydroxide in the Products of Reaction

Abstract

The alkali activation of metakaolin (MK) leads to the production of high-mechanical-performance network-structure materials. Adding calcium hydroxide (Ca(OH) 2 ) to the raw MK produces a somewhat different reaction: a network structure and C-S-H gel form. In the present study, MK and (MK ؉ Ca(OH) 2 ) mixes were activated with 5M and 12M NaOH solutions and cured at 45°C. A 5M concentration, in the absence of Ca(OH) 2 , did not produce MK activation within the test time. An activator concentration of 12M resulted in complete activation and the formation of a network structure. When Ca(OH) 2 was present in the raw mix, a small amount of C-S-H gel formed.

Properties of alkali activated slag–fly ash blends with limestone addition

“…One is the high calcium system, having a C-A-S-H type gel as the main reaction product [1]. The other is the low calcium system, having N-A-S-H type gels with three-dimensional network as the major reaction product [2]. Recently, growing attention is paid to blended alkaline systems that are produced by mixing calcium enriched precursors with aluminosilicates due to several modified properties such as setting times, workability, shrinkage, mechanical properties and durability [3].…”

Factores que afectan al desarrollo inicial de resistencias a compresión en hormigones de ceniza volante activados alcalinamente (sin OPC)

“…It will be noted from the curves in Figure 5 that activated fly ash concrete gains compression strength very quickly, reaching a value of 45.8 MPa after the first day. Mechanical strength rises very quickly because the polymerisation reaction in the alkaline gel takes place very rapidly (7)(8)(9)(10)(11)(12). Contrary to other reports (13,26), however, as Figure 5 shows, contra de lo indicado por otros autores (13,26), las resistencias mecánicas continúan aumentando con el tiempo porque, aunque de forma más lenta tal y como se observa en la Figura 5, la reacción de activación sigue progresando.…”

“…En general, la activación alcalina puede dar lugar a dos tipos de materiales cementantes: compuestos basados en Si/Ca y/o basados en Si/Al. La activación de metacaolín y de la ceniza volante (FA) constituyen ejemplos típicos del último (8)(9)(10)(11).…”

“…In general, alkaline activation yields two types of cementitious materials: Si-and Ca-based or Siand Al-based compounds. The alkali activation of metakaolin and fly ash (FA) constitute typical examples of the latter (8)(9)(10)(11).…”

Investigation on the Effect of Slag and Limestone Powder Addition in Alkali Activated Metakaolin

“…The hydration process is called geopolymerization, which contains a series of chemical reactions including the dissolution of starting materials, polycondensation, condensation and finally the crystallization of new reaction products [8]. Final products can be classified into two types according to different raw material chemistry, for silicate and aluminum enriched precursors like metakaolin or fly ash, final products are amorphous alkaline aluminosilicate hydrate gels with a three-dimensional network of aluminate and silicate units that are charge balanced by alkali cations, or can be simply described as N-A-S-H type gel [9]; for calcium and silicate enriched ones like granulated blast furnace slag, final products are aluminium substituted C-S-H (I) type gel with lower Ca/Si ratio [10]. Sodium hydroxide modified sodium silicate has been widely accepted as an ideal activator in terms of strength and other beneficial properties, the alkaline activator type and dosage have a significant influence on the reaction process and comprehensive properties of the final products [11].…”