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|Technical Bulletin #5 - White Cement & Swimming Pool Plastering|
Technical Bulletin #5 - White Cement & Swimming Pool Plastering
White cement meeting ASTM C150, ASTM C595, or ASTM C1157 ("white cement") is used in plaster, concrete, and masonry materials for a variety of architectural and decorative applications. White cement is specified in these applications for its color consistency, appearance and the ability to create a durable, rock-like material.
Understanding the basics of cement chemistry and other factors that affect concrete color may help reduce some misconceptions about discoloration of cementitious materials. Color is an important quality control issue in the white cement industry in order to achieve consistency in brightness and tone (see Figure 1).
The gray color that is common for most portland cement is due to the iron and manganese oxide compounds present in cement. However, to produce white cement, raw materials having very low levels of these metals are selected. In addition, the firing process is controlled to minimize how those metals are bound up in the clinker structure, which improves its whiteness, consistency and color tone.
The Chemistry of Portland Cement
While ASTM C595 and ASTM C1157 white cements are often used, the Standard Specification for Portland Cement ASTM C150 is used herein as an example because it best describes the chemical and physical characteristics of portland cement. The most common cement designation in C150 is Type I, for normal or general use. Table 1 compares the chemistry of two typical Type I portland cements, one gray and one white, which have similar characteristics. One difference between white and gray portland cements is that one of the six major oxides that describe their chemical composition is significantly lower in white cement than in gray cement. White cement typically contains 0.4% or less iron oxide (Fe2O3) rather than 2.6% for
Table 1. Chemical and Compound Composition, Color Value, and Fineness of Cements
White cements are very low in ferrite phases because they are made with raw ingredients having low iron content. Whereas the potential compound composition of C4AF is typically around 8% or even higher for gray cement, it is only 1% for white cement (see Table 1). The white color of the product is directly attributable to the low ferrite and manganese content.
Swimming Pool Plastering and Color
White cement is typically specified for pool plasters to ensure clean, bright, consistent colors, including
The color of plaster, concrete, or mortar of any cement-based material is primarily dependent on the cementitious portion (cement and any supplementary cementitious material "SCM" that might be used) as well as the color of the fine aggregate. Anyone seeking dynamic color or decorative finishes can benefit from cement-based products that exclusively use white cement. White cement has all the strength, toughness and structural characteristics of gray cement, but the added benefits, as listed above, are the principal considerations for its selection.
Cement Hydration and Color
As explained above, white cements are white because they have very low ferrite contents. In any cement the ferrite phases (C4AF) normally become lighter in color as the cement chemically reacts with water, a process called hydration. Accelerators are used to speed up hydration in concrete mixes placed during cold weather. One common accelerator, calcium chloride (CaCl2), has a retarding effect on the hydration of the ferrite-containing compound C4AF. When CaCl2 is added to gray cement concrete it will retard hydration of the ferrite phases which may remain dark colored (Greening and Landgren 1966). This effect associated with CaCl2 does not apply to white cement-based mixes. There is very little risk of discoloration due to adding calcium chloride to white cement-based materials/mixtures because white cement contains almost no ferrite. Research associated with C4AF discoloration by the Portland Cement Association has been done exclusively on gray cement and is discussed in several of their publications.
PRACTICE AND MATERIALS UNIQUE TO THE PLASTERING TRADE
Miller, F. MacGregor; Powers, Laura J.; Taylor, Peter C., Investigation of Discoloration of Concrete Slabs, SN2228, Portland Cement Association, 1999.