Volume 4, Issue 1 (January 2007)
Laboratory Study of Time-of-Tooling Effects on Mortar Joint Color
Early tooling results in laitance at tooled surfaces. The laitance consists of smooth, soft, and optically continuous high water-cementitious materials ratio (w/cm) paste that contains fines that bled to the surface from the cementitious materials and aggregate. The cement is extensively hydrated. Late tooling results in discontinuous, fine, chatter-marked surfaces. These discontinuities create surface shadows. Portland cement hydration is very poor due to a low w/cm. Normal tooling results in surface textures between those from early and late tooling along with cement hydration close to that associated with late tooling. In normal- and late-tooled joints, the portland cement evidences restricted hydration due to water “squeezed” from the relatively stiff surfaces, which creates a low w/cm. Significant color differences from light to dark result when joints are, respectively, struck and tooled early, normal, and late. Overall lower w/cm results because masonry units absorb some of the mix water. Wetted bricks extend curing. Affecting the time of mortar stiffening, which controls the time of tooling, is loss of water from joint surfaces and water absorbed by masonry units. The rate of water loss from joint surfaces is a function of the water retention properties of the mortar and ambient atmospheric conditions. Pigment properties of the mortar-making materials, and particularly the portland cement, plus surface textures, establish the color and color tone of joints. A primary contributor to darker surfaces is the degree of cement hydration: the poorer the hydration, the darker the color and color tone. Affecting the degree of cement hydration is the w/cm and the curing period: higher w/cm and longer curing increases hydration and lightens color. A secondary contributor to color tone is surface texture: the greater the number of surface discontinuities the darker the color tone. Cementitious materials components that generally contribute to lighter color tones include hydrated lime and the inert material in masonry cements.