Silica autoclave curing

Fig. 10 shows a fractured surface of autoclave-cured PAO composite and Fig. 11 shows one of autoclave-cured PA1. There appears smooth surface of A3 silica sand or FRP powder, which may suggest the same effect to multiple cracks in the early stage of bending. 

Flexural strength of autoclave-cured specimens shows a drastic drop by including FRP powder (see Fig. 13). PA that contains only fine powder as pozzolanic materials shows the highest flexural strength. PAO that contains silica sand locates the next. 

Two types of pulp sand mortar were used as the core plate of sandwich specimens. Mortar of Type M was made by mixing high-early-strength Portland cement, silica fume and pulp sand. Mortar of Type P was made directly with pulp sand. Mix proportions of each type of mortar are shown in Table 1 and Table 2. Pressurized forming was used to produce mortar plates of each type, whose pressure was lOMPa. Then specimens were put in a moist room of 20 C for about one day. After steam curing at 50 C for 24 hours, they were autoclave-cured on condition of 180 C for 3 hours. Finally they were dried out in an electric oven of 110 C for 24 hours. The geometry of the specimen was 50x10x200 (mm). 

The specimens were subjected to different kinds of cure ordinary cure in water, six-day cure in hot water, autoclave cure alone and combined with hot water. In Figure 13.3, the compressive strength is shown as a function of the high purity silica content. A compressive strength of over 300 MPa was observed for compositions with steel particles. 

Several investigators have investigated the effect of autoclave curing on the products formed with mixtures of cement/lime, silica, and water. In an investigation, the effect of quartz (of different surface areas) added in different amounts to cements was investigated by DTA/TG techniques.The main phases identified included CH (endothermal peak at 453°C), (Z-C2SH (endothermal peak at 523°C), and C-S-H (a broad endothermal effect in the range of 100-700°C). An exothermal effect at 843-846°C indicated the crystallization of j8-wollastonite, /3-CS due to aluminous tobermorite or C-S-H of C/S ratio 0.8 to 1.0. The intensity of... 

The final stage of the production process involves curing, which can be based on room temperature treatment in a tunnel in which moist conditions are maintained, or higher temperature steam curing to accelerate the hardening process. Autoclave curing is also common in the asbestos-cement industry, and in such instances part of the cement is replaced with finely ground silica. Replacement of part of the cement with low-cost inert fillers or fly ash is also sometimes used, for economic reasons. 

The binders are silica, lime, slag, or cement. The balls are somewhat dried, if necessary, and then cured in steam autoclaves. During the hydrothermal treatment lime and silica react to form hydrosilicate gels, which act as binders. 

Fig. 11.6 Effects of partial replacement of cement by finely ground silica on the compressive strengths of mixes cured normally and in the autoclave. After Mcnzcl (MHO).

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