After beta tests with cast stone and architectural precast producers, swimming pool contractors, and other white portland cement users, Lawrenceville,
After beta tests with cast stone and architectural precast producers, swimming pool contractors, and other white portland cement users, Lawrenceville, Ga.-based Vitro Minerals, Inc., announced early this year the commercial launch of two highly reactive pozzolan product grades Û VCAS-8 (5000 Blaine) and VCAS-micronHS (9000 Blaine) Û for white and high performance applications.
Derived from high-silica, low-alkali content fiberglass manufacturing byproducts, the VCAS (vitreous calcium aluminosilicate) agents impart additional calcium silicate in hardened concrete through reactivity with lime produced during cement hydration. The reactivity phase can span months, depending on moisture encountered after initial hardening, and results in a surface or structure with higher density or compressive strength, and potentially greater resistance to sulfate attack.
Target applications for the two pozzolans include cast stone, architectural precast, and masonry units containing white cement; glass fiber-reinforced concrete; swimming pool plaster; grouts; mortars; highway barriers; precast products requiring a more reflective surface; or, high-performance mixes for which engineers might otherwise specify silica fume or metakaolin. A product line that recently entered VCAS beta testing is concrete countertops.
According to Vitro Minerals, all pilot users have been receiving VCAS grades in bulk for at least six months running. Early adopters include Architectural Cast Stone of Georgia; Advanced Cast Stone of Texas; and pool contractor Magic Technologies of Florida.
THROUGH THE MILL
After primary sizing and drying, VCAS production entails proprietary shredding, grinding and classification of fiberglass byproducts. Ball mill grinding, similar to that used in the clinker-to-finished portland cement process, yields powder with gradations akin to slag cement and a little finer than Type I portland (8 micron mean diameter), and silica fume or metakoalin (3 micron mean diameter). High-efficiency classifiers produce the fine white powder with quality-assured physical properties (Figure 1). Vitro Minerals officials note that VCAS technology represents the first large-scale application of recycled glass in concrete, augmented by a consistently low-alkali content feedstock totaling upwards of 150,000 tons annually.
VCAS-8 and VCAS-micronHS are comparably priced with white cement and silica fume, respectively, and available in 50-lb. bags, 1-ton super sacks, or bulk. The low cost and color/chemistry consistency allow for as much as 40 percent cement replacement with low water demand. According to Vitro Minerals, the product’s sustainability can be readily determined: 1 ton of cement replaced by 1 ton of VCAS pozzolan results in the reduction of about 1 ton of carbon dioxide emissions and savings of about 4 million-5 million BTUs of energy and 1.5 tons of raw materials. Put another way, every 21-ton truckload used is equivalent to taking three automobiles off the road.
VCAS 8 is commonly used in systems with low water-to-cement ratio and steam-cured or warm-weather curing conditions, where the lower reactivity associated with an 8-micron median particle size is sufficient for the application. MicronHS has a much finer particle size and reactivity normally associated with silica fume and metakaolin. Both VCAS grades exhibit about 10 percent lower water demand than silica fume or metakaolin.
Much like fly ash and coal-fired power generating stations, VCAS processing is sited with fiberglass production. Vitro Minerals is processing under contract at a South Carolina grinding operation, while officials eye a dedicated ball mill, most likely in either Texas, Tennessee or North Carolina initially. In due time, the company plans on having production facilities in all three states.
Unlike silica fume, coal fly ash, blast furnace slag and other byproducts, Vitro Minerals officials conttend, VCAS pozzolans are free of iron, manganese and other undesirable color-inducing impurities. VCAS grades exhibit pozzolanic activity comparable to silica fume and metakaolin when tested in accordance with ASTM C618 and C1240. The pozzolans react with calcium hydroxide produced during the hydration of portland cement to form additional cementitious compounds such as calcium silicate and alumino-silicate hydrates, thus increasing strength, density and resistance to chemical attack, as well as control efflorescence.
Chemically, VCAS pozzolans are comprised largely of oxides of silicon, aluminum and calcium with no deleterious impurities. The CaO-SiO2-Al2O3 proportions (Figure 2), the low alkali content, and the amorphous structure are ideal for a pozzolanic additive in hydraulic concrete. The low iron content makes them particularly well suited for applications using white cement.
VCAS products meet the technical requirements of ASTM C618 for use as supplementary cementitious materials in concrete, while blended pozzolanic cements produced with VCAS exceed the requirements of ASTM C1157: Standard Performance Specifications for Hydraulic Cement. Typical strength curves at 20 percent cement replacement are shown in Figure 3. VCAS-micronHS surpassed the control before three days, making it a good choice for high performance applications where high early-strength is required. The product also meets the accelerated pozzolanic activity index, ASR control and sulfate resistance requirements of ASTM C1240 for silica fume (Figure 4).
VCAS pozzolans provide white portland cement characteristics with resistance to sulfate attack (ASTM C1012). Tests have shown excellent dimensional stability of a white cement mortar with 20 percent VCAS replacement after more than four years of exposure. Under these harsh conditions, the 100 percent white cement control mortar disintegrated in less than 200 days. VCAS also is effective at controlling expansion due to the alkali-silica reaction (ASTM C441) and reducing chloride ion penetration (ASTM C1202).
|Med Particle Size (µm)||3||8|
|Passing No. 325 Mesh (%)||99.9||97.0|
|Specific Surface Area, (cm2/g)||8,500||4,040|
|Pozzolonic Strength Index, 28d (% control)||127||104|
|Melting Point (_C)||1,200||1,200|