Hydrothermally treated cement-based building materials. Past, present, and future

Abstract

Hydrothermally cured or autoclaved cement-based building products have provided many challenges to researchers, manufacturers, and users since their inception nearly 100 years ago. The advantages, including the development of high strength within a few hours and a reduction of drying shrinkage, of the hydrothermal curing process have resulted in a variety of building products; inevitably, the technology of their production has undergone many stages of refinement. With the advent of nonconventional starting materials for the production of modern cements, and the push to utilize renewable resources to form blended cements, the chemical and physical make-up of hydrothermally cured building materials have changed considerably in recent years and will continue to change. It is, therefore, important to understand the chemical reactions taking place in an autoclave, and the consequent phase developments, if building materials produced by this process continue to be successful in the long term. A wide range of analytical techniques exists for characterizing the phase development in cement-based materials. The purpose of this paper is to illustrate the strength of thermal methods, especially when used in combination with other analytical techniques, in the understanding of hydrothermal reactions.

Hydrothermal processing of xonotlite based compositions

“…Typical thermal analysis results (TGA and DTA) for the stoichiometric 'C 6 S 6 H' and 'C 4 physisorbed water, underwent several small transitions between 200 and 300uC, but there was no Ca(OH) 2 or quartz present. Dissociation of a small amount of calcium carbonate (probably contained in the Ca(OH) 2 ) occurred above 600uC, followed by xonotlite decomposition, just above 800uC with the formation of wollastonite (b-CS).…”

“…1,2 Much work has been conducted on blended cement systems over the last few decades which utilise waste materials such as slag and pulverised fuel ash, 3 but other low energy manufacturing routes are also being considered. 4 One such route is autoclave curing where quartz, lime and clay can be reacted, with or without cement, at temperatures of 100 to 300uC, under saturated steam pressure to provide high strength finished products. [4][5][6] Examples are sand lime bricks, the absorbent in acetylene cylinders, and fibre cement products.…”

“…4 One such route is autoclave curing where quartz, lime and clay can be reacted, with or without cement, at temperatures of 100 to 300uC, under saturated steam pressure to provide high strength finished products. [4][5][6] Examples are sand lime bricks, the absorbent in acetylene cylinders, and fibre cement products. Strength development is due to the formation of crystalline calcium silicate hydrates such as tobermorite (Ca 5 Si 6 O 16 (OH) 2 .4H 2 O, C 5 S 6 H 5 -cement nomenclature: C5CaO, S5SiO 2 , M5MgO, H5H 2 O) or xonotlite (Ca 6 Si 6 O 17 (OH) 2 , C 6 S 6 H).…”

Mechanical and microstructure characteristics development of hardened oil well cement pastes incorporating fly ash and silica fume at elevated temperatures

“…And so, more stable hardened OWC matrices (OWC-FA and OWC-SF) with enhanced mechanical characteristics under severe conditions of pressures and temperatures are attained [30]. Evidently, as previously reported by many researchers that CSH gel has an average value of C/S ratio equals to 1.5 [8] and to avoid the unsatisfactory phase transformation (amorphous CSH into crystalline α-C 2 SH or jaffeite) under the effect of high temperatures and pressures, CaO/SiO 2 (C/S) ratio has to be diminished to about 1 [9][10][11]. In our study, using SF or FA as supplements motivates the development of stable and desirable hydration products that possess lower C/S ratio (like tobermorite, Ca 5 Si 6 (OH) 2 .4H 2 O) through calcium hydroxide consumption [12,13].…”

“…It has been disclosed that CSH gel has an average value of the C/S ratio equals to 1.5 [8]. And so, the key role to hinder the unsatisfactory phase transformation (amorphous CSH into crystalline α-C 2 SH or jaffeite) was through diminishing CaO/SiO 2 (C/S) ratio to about 1 [9][10][11]. Many researches proved that various supplements as SF [12], fly ash [13], silica flour [14] and other silica-based ingredients [15][16][17][18][19][20][21][22][23][24] motivate the advancement of stable and desirable hydration products that possess lower C/S ratio (like tobermorite) through calcium hydroxide (CH) consumption.…”