Acceleration of Intended Pozzolanic Reaction under Initial Thermal Treatment for Developing Cementless Fly Ash Based Mortar

Abstract

Without using strong alkaline solution or ordinary Portland cement, a new structural binder consisting of fly ash and hydrated lime was hardened through an intensified pozzolanic reaction. The main experimental variables are the addition of silica fume and initial thermal treatment (60 °C for 3 days). A series of experiments consisting of mechanical testing (compressive and flexural strength, modulus of elasticity), X-ray diffraction, and measurements of the heat of hydration, pore structure, and shrinkage were conducted. These tests show that this new fly ash-based mortar has a compressive strength of 15 MPa at 91 days without any silica fume addition or initial thermal treatment. The strength increased to over 50 MPa based on the acceleration of the intensified pozzolanic reaction from the silica fume addition and initial thermal treatment. This is explained by a significant synergistic effect induced by the silica fume. It intensifies the pozzolanic reaction under thermal treatment and provides a space filling effect. This improved material performance can open a new pathway to utilize the industrial by-product of fly ash in cementless construction materials.

Intensified Pozzolanic Reaction on Kaolinite Clay-Based Mortar

“…On the other hand, in the case of SF_15, the first peak of pore diameters moved from 9 to 17 nm between 28 and 91 days (Figure 5c). This movement means that the pores around 10 nm expanded in size (by approximately double), possibly due to the accelerated drying shrinkage with a fine pore structure by excessive SF addition [15,49]. Similarly, Rao has reported that the addition of SF has a significant influence on the drying shrinkage of cement mortar due to the strong pozzolanic reaction and pore size refinement mechanism, especially with 15% (by wt % of cement) SF addition.…”

“…The measured pore volume was divided into four size ranges in Figure 7, on the basis of the international union of pure and applied chemistry system [45]. However, the micropore range was modified from <1.25 nm to <4.5 nm based on the previous studies which have shown that the enhanced pozzolanic reaction considerably changes the pore structure especially below 4.5 nm range [15,47]. Thus, this modification can help to effectively explain the change of pore structure by the intensified pozzolanic reaction.…”

“…This indicates that the volume of mesopores (4.5-25 nm) of SF_15 were increased due to expansion with curing age. It has been reported that the pores finer than 10 nm mainly affect shrinkage and creep of hardened cementitious materials [46], and also the degree of capillary tension and stress for drying shrinkage is increased with a finer pore system (below 80 nm range in pore diameter) [15,48]. Thus, this distinct expansion might be attributed to the accelerated drying shrinkage of matrix and resultant micro cracks, as shown in the low W/B (0.25) cement paste with SF (20% of cement) [49].…”

“…These include high-volume fly ash [5,6] or slag [7] concrete, geopolymer concrete [8], and sustainable cement mixtures having CO 2 -sequestrated minerals [9], or fillers such as limestone or quartz powder [10][11][12][13]. In addition, materials having pozzolanic properties, such as fly ash (type F), slag, and SF are receiving attention as sustainable supplementary cementitious materials [14][15][16][17][18]. Along with the environmental and economic benefits as replacements for OPC, these materials also have other advantages, such as improving mechanical properties and durability [19].…”

Mechanical properties of various concrete after high temperature exposure Mechanical property of various types of concrete after high temperature exposure is investigated. Considering compressive strength requirement for a vertical element in high rise building, concrete specimens with various design strengths of 35, 80, 100, and 150 MPa were tested. Particularly, the influence of incorporated steel fiber on fire resistance is of interest in this study. Experimental results show that the fire resistance de

“…In addition to this, HPC or UHPC features to include silica fume in their mix proportions (see Table 1). This ultra-fine particle not only acts as a physical filler that fills sub-micro sized voids of concrete, but also contributes to making the concrete stronger by the pozzolanic reaction with water and portlandite which is formed by primary cement hydration [15,16]. The pozzolanic reaction progresses slowly over time, but it is accelerated with increasing curing temperature and eventually contributes to an additional increase in strength.…”