Examples of facings of Roman concrete walls Roman concrete, also called opus caementicium, was the core of every Roman wall after the 2nd century bc. Mostly walls made in opus caementicium were covered with other materials to make a more robust and workable surface. Opus caementicium is a construction technique using an aggregate, water and a binding agent. The aggregate functioned as a filler like gravel, chunks of bricks or stones and rubble. The binding agent is usually called mortar like lime, gypsum or pozzolana (nowadays 'Portland' cement is used).

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Opus craticium Term both used for wattle work and walls of half-timer construction, filled in with stones and/or straw and plastered with mortar
Opus incertum Using irregular shaped and random placed uncut stones or fist-sized tufa blocks inserted in a core of opus caementicium, used from the beginning of the 2nd century bc, later superseded by opus (quasi) reticulatum. First half 2nd c BC - 1st c BC.
Opus quadratum Walls of cut stone, rectangular in form. From 4th c BC on.
Opus (quasi) reticulatum Small square tufa blocks placed diagonally to form a diamond-shaped mesh pattern, often supplemented by other materials at frames of windows and doors or at reinforcements at corners of buildings with oblong tufa blocks. Last quarter 2nd c BC - mid 1st c BC (Opus quasi reticulatum); first half 1st c BC - late 1st c AD (Opus reticulatum).
Opus testaceum / latericium Brick faced masonry - kiln-backed bricks; the dominant technique throughout the imperial period. Last quarter 1st c BC; popular during Nero's reign.
Opus (reticulatum) mixtum Masonry of reticulated material reinforced and/or intersected by brick bands or interlocked with bricks. Mid 1st c AD - early 3rd c AD.
Opus vittatum Oblong (or occasionally square) tufa blocks intersected by one or more brick bands at (ir-) regular distances. Early 4th c BC (Opus vittatum simplex); 3rd c AD - 4th c AD (Opus vittatum mixtum). Typical during the reign of Maxentius and Constatine.
Opus sectile Decoration patterns and figures at walls (and floors) with precisely cut pieces of polychrome stone, usually marble
Opus spicatum Walls (and floors) made of quite small elongated tiles, laid in a fishbone pattern
Opus signinum Waterproof floor- and wall-revetment of mortar mixed with terracotta sherds and crushed tiles or bricks. Number 12 on the drawing.