Effect of Raw and Calcined Diatomite Addition on the Microstructure of Limestone-Based Alkali-Activated Materials

In recent years, the cement industry has focused on incorporating natural materials, industrial wastes, and by-products as partial replacements for Portland cement, commonly referred to as pozzolans. These materials can undergo a pozzolanic reaction involving the interaction between their amorphous silica and the calcium hydroxide released during Portland cement hydration, leading to the formation of calcium silicate hydrate (C–S–H). The incorporation of such materials has been shown to improve the microstructure of cementitious systems, as well as their mechanical properties and durability—enhancements that are difficult to achieve using ordinary Portland cement alone [1]. This strategy not only reduces production costs and energy consumption but also significantly decreases CO2 emissions Among the pozzolanic materials commonly used as partial cement replacements are silica fume, ground granulated blast furnace slag, fly ash, and various natural pozzolans [2]. Within the latter category, diatomite stands out due to its low density, high silica content, elevated reactivity, and large specific surface area. These characteristics not only confer favorable pozzolanic behavior but also contribute to reduced thermal conductivity in concrete, thereby enhancing overall performance. Diatomite has been reported to reach its maximum pozzolanic activity when calcined at 650 °C, with progressive increases in concrete compressive strength observed as calcination time increases up to approximately 2 h [3]. This behavior is attributed to the increase in the amorphous fraction induced by thermal treatment, which enhances the availability of reactive silica. In the presence of free Ca(OH)2, this silica participates in a secondary pozzolanic reaction leading to the formation of C–S–H-type gels, thereby promoting matrix densification and strength development [4]. In alkali-activated systems based on limestone, the predominant crystalline phases include pirssonite (Na2Ca(CO3)2·2H2O) and portlandite (Ca(OH)2), in addition to residual calcite associated with unreacted fractions [5,6]. Portlandite plays a crucial role as a calcium source for C–S–H formation, whereas pirssonite formation is associated with the interaction of Ca²⁺, CO3²⁻, and Na⁺ in highly alkaline environments, particularly when reactive silica availability is limited. The Ca/Si ratio of the gel is a key parameter governing the structural stability of the material. In Portland cement–derived systems, this ratio typically ranges from 1.6 to 2.0, whereas in slag-based binders it varies between 0.8 and 1.2 [7]. Significantly lower values may result in the formation of structurally unstable silica-rich gels. Menchaca-Ballinas et al. [8] reported Ca/Si ratios as low as 0.14 in systems incorporating glass waste, which prevented the formation of a stable C–S–H network due to insufficient calcium availability. In this context, higher limestone contents promote stability in aqueous environments by increasing the availability of calcium species and modifying the molecular structure of the gel toward more stable configurations [7-9]. Based on this background, the present study evaluated the effect of diatomite calcination on its reactivity and its influence on the microstructure of a limestone-based alkali-activated material. Particular attention was given to the formation of C–S–H-type gels derived from the interaction between thermally generated amorphous silica and available calcium species within the system. To determine the optimal calcination temperature, thermal analysis was conducted using a simultaneous thermal analyzer (TA Instruments SDT Q600) at a heating rate of 20 °C/min up to 1200 °C under a dynamic argon atmosphere. The results established 650 °C as an appropriate temperature to induce the decomposition of clay minerals and maximize the reactive amorphous fraction. A calcination time of 3 h was selected. Subsequently, thermally treated diatomite was used to prepare pastes with compositions 40CD–70LS, 40RD–60LS, 30CD–70LS, and 30RD–70LS. These mixtures were alkali-activated with a 10 M NaOH solution and subjected to thermal curing during the first 24 h. The abbreviations CD and RD denote calcined and raw diatomite, respectively, while LS represents limestone. Microstructural characterization was performed at 28 days to assess cementitious phase development. Calcination reduced the specific surface area from 62.45 m²/g to 15.07 m²/g, which is attributed to dehydroxylation and the decomposition of hydrated silicates and carbonates. However, the average pore size remained essentially unchanged (1.81 nm), indicating that no significant collapse of the porous structure occurred. Despite the reduction in specific surface area, reactivity increased due to the higher amorphous content. Microstructural analysis revealed that pastes incorporating calcined diatomite exhibited a denser matrix and greater C–S–H gel formation (Ca/Si = 1.4) compared with those prepared using raw diatomite. In the latter, pirssonite formation predominated, which is attributed to the limited availability of reactive silica. This restriction reduces C–S–H formation and favors carbonate precipitation. Microstructural analysis of alkali-activated diatomite–limestone pastes prepared with raw and calcined diatomite led to the following conclusions: Calcination at 650 °C for 3 h was identified as an optimal condition for enhancing the pozzolanic reactivity of diatomite. This treatment promotes effective decomposition of clay minerals without inducing the formation of undesirable crystalline phases that could reduce pozzolanic activity. Mineralogical characterization confirmed that calcination at 650 °C for 3 h favors the dehydroxylation of clay minerals such as montmorillonite, thereby maximizing the content of reactive amorphous silica. Pastes prepared with raw diatomite exhibited a higher amount of pirssonite compared with those containing calcined diatomite. This difference is attributed to the lower availability of reactive silica in raw diatomite, which limits C–S–H gel formation and promotes carbonate precipitation, ultimately resulting in reduced mechanical performance.